2.2 –  Responses to Public Comments on AoR and Corrective Action

This file contains responses to public comments on area of review (AoR) and corrective

action (comment codes 3200 to 3700).  This section includes comments on delineation

of the AoR, use of computational models, identifying and addressing wells that need

corrective action, reevaluating the AoR, phased corrective action, and corrective action

plans.

The table below presents all of the EPA‐assigned comment codes that are included in

this section of the comment and response summary.  Use the bookmarks in the PDF to

move among the comment codes.   

Topics included in this summary and associated comment codes

Topic Code

2.2 Proposed Area of Review and Corrective Action Requirements (40 CFR 146.84) 3200

2.2.1 Delineating the AoR/Modeling CO2 movement 3300

2.2.1.1 Comments in favor of requiring computational models to delineate the AoR   3310

2.2.1.2 Comments opposed to requiring computational models to delineate the AoR 3320

2.2.1.3 Applicability /adequacy of computational models   3330

2.2.1.4 Specific delineation models 3340

2.2.1.5 Use of proprietary models for GS sites   3350

2.2.2 Corrective Action 3400

2.2.2.1 Scope of the inventory process (e.g., reliance on publicly available information,

depth of wells)

3420

2.2.2.2 Evaluation of wells in the AoR (e.g., types of data to be reviewed) 3440

2.2.2.3 Acceptable corrective action techniques for GS sites (e.g., cements) 3460

2.2.3 Area of Review Reevaluation   3500

2.2.3.1 AoR reevaluation frequency   3530

2.2.3.2 Conditions that merit reevaluation of the AoR   3560

2.2.4 Phased Corrective Action (No comments were submitted for this topic.) 3600

2.2.4.1 Comments in favor of allowing phased corrective action 3630

2.2.4.2 Comments opposed to allowing phased corrective action 3660

2.2.5 Comments on the requirement for an AoR and corrective action plan 3700

Every comment on these topic areas and the associated response is included in this

summary; there is some repetition where multiple commenters made the same

comment.

Responses to letters from individual commenters or organizations are also available in

the rulemaking docket.

  
Comment Code: 3200 2.2 Proposed Area of Review and Corrective Action Requirements (40 CFR

146.84) 

Individual Comments

Commenter Name: John Hanger Letter ID: 0130

Commenter Organization: Pennsylvania Dept. of Env. Protection Comment ID: 21503

Comment: Proposed Area of Review and Corrective Action Requirements 

-PA needs to be assured that the designated "Area of Review" is sufficient and 

inclusive to provide for sound analysis of the impacts and control of potential 

environmental impacts to groundwater, water supplies, and surface water. State 

complications may arise from property and mineral rights related to permitting 

and operations of the proposed Class VI regulations based on this definition and 

its application. 

-Area of Review should include a "buffer zone" surrounding the Area of Review 

boundary that is adequate to address risk of improper assessment. The size of 

the boundary should be left to the applicant but should be subject to the 

Director’s approval. 

Response: EPA agrees that the unique aspects of GS merit the use of sophisticated computational

modeling for delineation of the AoR, based on site-specific features and geology, including the

presence of faults, fractures, and artificial penetrations to ensure protection of USDWs. EPA believes

that computational modeling of the AoR prior to injection will provide the best obtainable estimates

of the extent of the CO2 plume over several years of active injection. EPA has strengthened the

minimum frequency for the AoR reevaluation from ten years to 5 years (146.84(e)). EPA believes

that this revised frequency addresses commenters' concerns about the inherent uncertainties in

modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring

activities, and the unique nature of CO2 GS.

EPA disagrees that incorporating a buffer zone into the definition of the AoR is necessary, because

EPA believes that the AoR definition incorporates all areas that may be affected by the injection

activity, and the iterative AoR reevaluation process required at 146.84(e) further accounts for

uncertainty in the original modeling. The Agency has carefully considered all comments on the AoR

definition at 146.84(a) and modified it to read as follows: The region surrounding the geologic

sequestration project where USDWs may be endangered by the injection activity. The area of review

is delineated using computational modeling that accounts for the physical and chemical properties of

all phases of the injected CO2 stream and is based on available site characterization, monitoring, and

operational data.

Commenter Name: Anonymous2 Letter

ID: 0133

Commenter Organization: Sierra Club National Global Warming and Energy

Committee and Sierra Club California-Nevada Regional Conservation Committee on

Energy and Climate Change

Comment

ID: 21578

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: I. Technical Definitions 

1
Area of Review and Fluid Movement: Adopt the specific language used by the adhoc CO2 workgroup in its Recommendation 3.1 for determining the parameters of 

each Area of Review, i.e. -- "based on generally accepted and site relevant 

techniques, such as modeling, that define, in three dimensions, the maximum 

extent of the modeled CO2 plume [inclusive of free phase and water- dissolved 

CO2]and associated pressure front." 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The definition in

the final rule will continue to do so. 

The UIC regulations, including the Class VI regulations, are designed to ensure protection of

USDWs from endangerment. The Agency believes that the AoR needs to incorporate not only the

CO2 itself—regardless of saturation or phase—but also the area of increased pressure ahead of the

CO2 plume, which can potentially displace saline, native fluids (i.e., naturally occurring salty water)

into USDWs. This pressure front would typically encompass an area larger than the plume itself,

including any areas of small scale fingering or heterogeneities or where CO2 saturation is less than

100 percent.

Commenter Name: Anonymous2 Letter

ID: 0133

Commenter Organization: Sierra Club National Global Warming and Energy

Committee and Sierra Club California-Nevada Regional Conservation Committee on

Energy and Climate Change

Comment

ID: 21602

Comment: Technical Definitions 

A. Area of Review and Fluid Movement 

Recommendation A: Adopt the specific language used by the ad-hoc CO2 workgroup 

in its Recommendation 3.1 for determining the parameters of each Area of Review, 

i.e. -- "based on generally accepted and site relevant techniques, such as 

modeling, that define, in three dimensions, the maximum extent of the modeled 

CO2 plume [inclusive of free phase and water- dissolved CO2]and associated 

pressure front." 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The definition in

the final rule will continue to do so. 

2
The UIC regulations, including the Class VI regulations, are designed to ensure protection of

USDWs from endangerment. The Agency believes that the AoR needs to incorporate not only the

CO2 itself—regardless of saturation or phase—but also the area of increased pressure ahead of the

CO2 plume, which can potentially displace saline, native fluids (i.e., naturally occurring salty water)

into USDWs. This pressure front would typically encompass an area larger than the plume itself,

including any areas of small scale fingering or heterogeneities or where CO2 saturation is less than

100 percent.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22106

Other EPA Comment Codes and Descriptions: 1610 1.3.1 Risks to USDWs 

Comment: 6. EPA appears to be limiting its protection of waters to only those 

used/potentially used for public water supplies. It should be made clear that 

private use or fewer numbers of uses than those qualifying as public water 

supplies within the aquifer are also to be protected. The requirements to 

identify all perforations into the USDW should also require identification of 

all perforations and water uses (springs) within the Area of Review. These sites, 

then, should be included in all analyses and modeling. 

Response: EPA disagrees that only aquifers currently providing public water supplies will be

protected by the GS regulation. The UIC regulations, promulgated under Section 1421(d)(2) of the

SDWA, including the Class VI regulations, require that injection operations protect against

endangerment for drinking water sources if such injection may result in 1) non-compliance with

national primary drinking water regulations or 2) may otherwise adversely affect the health of

persons in USDWs which supply or can reasonably be expected to supply any public water system.

The definition of a USDW is based on the total dissolved solids content, not the users of the supply

(i.e., public versus private). The applicable portion of the definition of USDW reads: "...an aquifer or

any portion of an aquifer that contains fewer than 10,000 mg/l total dissolved solids and is not an

exempted aquifer," as defined at 144.3. Aquifers supplying private wells that may reasonably be

expected to supply any public water system and meeting these criteria are considered USDWs that

warrant protection from endangerment by GS operations.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24142

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rule Provision: 146.81(d) [Definition - Area of review] 

API Comments: Area of review means the region surrounding the geologic 

Sequestration project that may be impacted by the injection activity. The area 

of review is based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream.

SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE CARBON DIOXIDE PLUME,

ASSOCIATED 

PRESSURE FRONT, AS WELL AS THE OVERLYING FORMATIONS AND SURFACE AREA

ABOVE THAT 

DELINEATED REGION. 

3
Rationale: Defining the AOR based on the extent of the CO2 plume and its 

associated pressure front is a more direct approach, rather than the approach 

offered by the EPA. The EPA definition suggests that the computational approach 

defines the AOR rather than the potential risks of the injection activity. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure in three dimensions

as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23487

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Corrective action - As a threshold matter, USWAG believes that the terminology 

used in the Proposal is problematic; use of the phrase "corrective action" to 

describe well plugging and repair risks confusion with the concept of corrective 

action under either RCRA or CERCLA. As the GAO Report cautions, public 

perception is critical to the development of a commercially viable CO2 GS 

program. GAO Report at 49. To avoid any unintended and unwarranted negative 

associations with the phrase "corrective action," USWAG suggests the Final Rule 

abandon this phrase and instead use a phrase such as "well plugging and repair." 

Response: EPA notes that the term "corrective action" at 146.84(d) has been used to describe the

process of addressing improperly plugged wells in the AoR of injection wells since the inception of

the UIC Program, e.g., at 146.7 and 146.64. While EPA is aware that the term has a different

meaning under RCRA and CERCLA, this has not caused confusion in the past. The alternative term,

"well plugging," suggested by the commenter is reserved for injection well plugging (as indicated at

146.92).

Commenter Name: Robert J. Finley Letter ID: 0180

Commenter Organization: Illinois State Geological Survey Comment ID: 23626

Other EPA Comment Codes and Descriptions: 3460 2.2.2.3 Acceptable corrective action

techniques for GS sites (e.g., cements) 

Comment: 146.84 

(c) 

4
Who determines what corrective action is required in the area of review? If a 

Class II well has been plugged in conformance with Class II requirements is that 

presumed to be adequate? Different state agencies (in a primacy state) may be 

responsible for Class II and Class VI wells, potentially complicating oversight 

of corrective action. 

Response: The GS regulation does not specify the corrective action methods to be used on wells

within the AoR to meet the requirements of §146.84(d) and instead allows flexibility to ensure that

all corrective action is appropriate to the well, the CO2 injection, and other site-specific conditions.

All corrective action must be appropriate to the GS site to ensure that the well does not allow CO2

or other fluids to move into or endanger USDWs and must be approved by the Director. EPA

recognizes that wells plugged in accordance with Class II requirements may also be appropriately

plugged for a CO2 GS project. EPA adds that all corrective action must be performed pursuant to

the approved AoR and corrective action plan, which affords the Director the discretion to require

any additional information needed to ensure that all necessary corrective action is performed. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23732

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Part 146.84(c)(2) AWWA recommends revising the first sentence to read: 

Using methods approved by the Director, identify all penetrations, including 

active and abandoned wells and underground mines, in the area of review that may 

penetrate any portion of the confining zone. 

Response: EPA clarifies that the mapping and cross sections required to be submitted under

146.82(a)(3) will define the limits of the confining zone, which would be accounted for in the AoR

delineation and corrective action evaluation required at 146.84(c). EPA adds that owners or

operators must also, under 146.82(a)(2), submit a map of the AoR that shows all injection wells,

producing wells, abandoned wells, plugged wells or dry holes, deep stratigraphic boreholes, State or

EPA approved subsurface cleanup sites, surface bodies of water, springs, mines (surface and

subsurface), quarries, water wells and other pertinent surface features including structures intended

for human occupancy and roads. EPA believes that these requirements address the commenter's

concerns. 

EPA adds that any well that penetrates the confining zone, even if it does not extend all the way

through the confining formation, is to be included in the tabulation. EPA is issuing guidance on how

to identify all wells that require corrective action. The guidance describes EPA's recommended

approaches for the data to be considered in an initial AoR delineation, how to apply computational

modeling to delineate the AoR, performing corrective action at GS sites, and, when appropriate,

5
completing AoR reevaluations. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24400

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rule Provision: 146.81(d) [Definition - Area of review] 

Proposed CCS Group Recommendations: Area of review means the region 

surrounding the geologic sequestration project that may be impacted by the 

injection activity. The area of review is based on computational modeling that 

accounts for the physical and chemical properties of all phases of the injected 

carbon dioxide stream. SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE CARBON 

DIOXIDE PLUME, ASSOCIATED PRESSURE FRONT, AS WELL AS THE OVERLYING

FORMATIONS 

AND SURFACE AREA ABOVE THAT DELINEATED REGION. 

Basis for CCS Group Comments: Defining the AOR based on the extent of the CO2 

plume and its associated pressure front is a more direct approach, rather than 

the approach offered by the EPA. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions,

as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24633

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rule Provision: 146.81(d) [Definition - Area of review] 

Anadarko Petroleum Corporation (APC) Comments: Area of review means the region 

surrounding the geologic sequestration project that may be impacted by the 

injection activity. The area of review is based on computational modeling that 

accounts for the physical and chemical properties of all phases of the injected 

carbon dioxide stream. SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE CARBON 

DIOXIDE PLUME, ASSOCIATED PRESSURE FRONT, AS WELL AS THE OVERLYING

FORMATIONS 

6
AND SURFACE AREA ABOVE THAT DELINEATED REGION. 

Rationale: Defining the AOR based on the extent of the CO2 plume and its 

associated pressure front is a more direct approach, rather than the approach 

offered by the EPA. The EPA definition suggests that the computational approach 

defines the AOR rather than the potential risks of the injection activity. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions

as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22432

Comment: Area of Monitoring and Area of Review . . .. . . . . .. . . . . Page 4 

EPA should define two regulatory regions surrounding the geologic sequestration 

project and provide specific definitions for those regions: 1) Area of 

Monitoring (AoM) and 2) Area of Review (AoR); where the AoM is used to define 

the spatial distribution of monitoring wells and the AoR is used to define the 

limit of corrective actions 

Response: EPA believes that, while the commenter offers different terminology for the AoR and the

area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or

operator is required (under 146.90) to develop and implement a testing and monitoring plan for each

GS project that must consider the delineated AoR that includes both the CO2 plume and associated

pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies

that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical

properties of all phases of the injected CO2 stream, based on available site characterization,

monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the

testing and monitoring plan, consistent with the commenter's assertion that the AoR may vary over

time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at

least every 5 years, and also requires that the owner or operator periodically review the AoR and

corrective action, testing and monitoring, and emergency and remedial response plans within one

year of an AoR reevaluation (as well as following significant changes to the facility, or when

required by the Director). The relationship between the AoR reevaluation and the testing and

monitoring plan should allow for considerations suggested by the commenter. 

EPA believes that by linking development and review of the testing and monitoring plan to the AoR

7
evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's

considerations for an area of monitoring.

Commenter Name: Richard Guinn Letter ID: 0210

Commenter Organization: Eastman Chemical Company Comment ID: 22722

Comment: §146.84 Area of review and corrective action. (c) (2) "Using methods approved by 

the Director, identify all penetrations, including active and abandoned wells 

and underground mines, in the area of review that may penetrate the confining 

zone. Provide a description of each well’s type, construction, date drilled, 

location, depth, record of plugging and/or completion, and any additional 

information the Director may require; and..." 

We recommend striking out "approved" and replacing with "prescribed". We 

recommend deleting the second sentence and replacing with "Where such 

information is available or can be obtained, provide a description of each 

well’s type, construction, date drilled, location, depth, record of plugging 

and/or completion, and any additional information prescribed by the director, 

and..." Acceptable methods for identifying deep wells into confining zones 

within the AoR should be prescribed by the EPA as part of the regulation. 

Accurate information is difficult to obtain for older wells within the AoR. 

Examples of other information that could be required by the Director should be 

clearly stated. 

Response: The final rule retains the language in 146.84(c)(2) as proposed. EPA disagrees that the

Director should prescribe the corrective action methods to be used. Instead, all corrective action

must be performed pursuant to the approved AoR and corrective action plan, which affords the

Director the discretion to review and approve planned corrective action techniques and require any

additional information needed to ensure that all necessary corrective action is performed (e.g.,

information that is not publically available). EPA is issuing guidance on how to identify all wells that

require corrective action. The guidance describes EPA's recommended approaches for the data to be

considered in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA clarifies that any well that penetrates the confining zone, even if it does not extend all the way

through the confining formation, is to be included in the tabulation. EPA is issuing guidance on how

to identify all wells that require corrective action. EPA adds that the Director has the discretion to

require the owner or operator to supplement a data records search, including requiring ground or

airborne surveys for wells, if there is reason to believe, based on knowledge of the site or of

available data bases, that the incompleteness of available information could compromise USDW

protection from endangerment.

Commenter Name: Olon Plunk Letter ID: 0217

Commenter Organization: Xcel Energy, Inc. Comment ID: 21558

Comment: Corrective Actions/AoR 

Xcel Energy is concerned that the term "corrective action" as used broadly in 

8
the proposed rule infers to the public that a problem has occurred. We concur 

with USWAG's comment that it would be better to refer specifically to "well 

plugging and repair" to avoid stigmatizing projects for what are in fact 

deliberate and proactive preventive measures. In addition, Xcel Energy advocates 

Response: EPA notes that the term "corrective action" at 146.84(d) has been used to describe the

process of addressing improperly plugged wells in the AoR of injection wells since the inception of

the UIC Program, e.g., at 146.7 and 146.64. While EPA is aware that the term has a different

meaning under RCRA and CERCLA, this has not caused confusion in the past. The alternative term,

"well plugging," suggested by the commenter is reserved for injection well plugging (as indicated at

146.92).

Commenter Name: Tiffany Rau Letter ID: 0224

Commenter Organization: Hydrogen Energy International Comment ID: 21678

Comment: Existing wells within the AoR may be ideal candidates for conversion to 

monitoring or injection wells during the life of the project and, therefore, 

should not be plugged and abandoned prior to injection. Existing wells within 

the likely plume area that are not plugged and abandoned should have diagnostic 

testing to ensure there is minimal risk of vertical leakage prior to conversion 

to monitoring or injection status. 

Response: EPA recognizes that operators may choose to convert existing wells in the AoR for use as

monitoring wells or injection wells. An operator seeking to use an existing well as a monitoring well

would need to describe that well in the proposed testing and monitoring plan to demonstrate that is

appropriately constructed for use as a monitoring well and compatible with the GS injection

operation and injectate. The Director would take the condition of the well into consideration in

approving the proposed testing and monitoring plan.

Commenter Name: Brian Williams Letter ID: 0225

Commenter Organization: BP Alternative Energy North America, Inc. (BP) Comment ID: 21728

Comment: Existing wells within the AoR may be ideal candidates for conversion to 

monitoring or injection wells during the life of the project and, therefore, 

should not be plugged and abandoned prior to injection. Existing wells within 

the likely plume area that are not plugged and abandoned should have diagnostic 

testing to ensure there is minimal risk of vertical leakage prior to conversion 

to monitoring or injection status. 

Response: EPA recognizes that operators may choose to convert existing wells in the AoR for use as

monitoring wells or injection wells. An operator seeking to use an existing well as a monitoring well

would need to describe that well in the proposed testing and monitoring plan to demonstrate that is

appropriately constructed for use as a monitoring well and compatible with the GS injection

operation and injectate. The Director would take the condition of the well into consideration in

approving the proposed testing and monitoring plan.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24020

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

9
Comment: §146.84 Area of review and corrective action. 

Proposed Rule: (a) The area of review is the region surrounding the geologic 

sequestration project that may be impacted by the injection activity. The area 

of review is based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream. 

Schlumberger Suggested Revision: (a) The area of review is based on 

computational modeling that accounts for physical and the chemical properties of 

all phases of the injected carbon dioxide stream, the associated pressure 

front, and displaced brine. 

Schlumberger Rationale: - 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The definition in

the final rule will continue to do so. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Neeraj Gupta Letter ID: 0229

Commenter Organization: Battelle Energy Technology (BET) Comment ID: 21832

Comment: Page 72, Corrective Action/Abandoned Wells - We would appreciate more clarity on 

whether it will be required to plug and abandon wells in zones above confining 

layers. At our sites, it appears the regulators have expected all inactive 

wells in the AOR to be plugged, even if they are not penetrating the injection 

zones. 

Response: EPA clarifies that wells requiring corrective action are those in the AoR and wells that

may penetrate the confining zone per 146.84(c)(2). Any well that penetrates the confining zone, even

if it does not extend all the way through the confining formation, is to be included in the tabulation.

EPA is issuing guidance on how to identify all wells that require corrective action. The guidance

describes EPA's recommended approaches for the data to be considered in an initial AoR

delineation, how to apply computational modeling to delineate the AoR, performing corrective action

at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be available on

EPA's web site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Corrective action must be performed on wells determined to need corrective action in order to

10
prevent movement of fluid into or between USDWs, therefore some wells above the confining zone

may not be required to undergo corrective action; however, such a determination will be based on

site-specific information.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22294

Comment: Class VI applicants should be required to identify all known wells penetrating 

formations affected by increased pressure 

Response: EPA clarifies that under 146.84(c)(2)operators must identify all penetrations of the

confining zone in the AoR for the GS project. This area would encompass both the CO2 plume and

areas of increased pressure associated with CO2 injection, regardless of which is larger.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24854

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rule Provision: 146.81(d) Definitions 

EPA Proposal: (d) Definitions. The following definitions apply to this 

subpart. To the extent that these definitions conflict with those in § 146.3 

these definitions govern: 

Area of review means the region surrounding the geologic sequestration project 

that may be impacted by the injection activity. The area of review is based on 

computational modeling that accounts for the physical and chemical properties of 

all phases of the injected carbon dioxide stream. 

GWPC Recommendations: Area of review means the subsurface three-dimensional 

extent of the carbon dioxide stream plume associated pressure front, and 

displaced brine, as well as the overlying formations and surface area above that 

delineated region. 

Rationale: Suggest removal of the second sentence and replacement with : "It 

includes the subsurface three-dimensional extent of the carbon dioxide plume, 

associated pressure front, and displaced brine, as well as the surface area 

above that delineated region." 

Comment on the inclusion of the word "surrounding" because the AoR should be coextensive with the area defined by the Carbon Sequestration Project. Inclusion 

of "surrounding" makes this definition open-ended and ambiguous. 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The definition in

11
the final rule will continue to do so. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Letter ID: 0263

Commenter Organization: Interstate Oil & Gas Compact Commission Comment ID: 22796

Comment: III A. 2 Proposed Area of Review and Corrective Action Requirements 

IOGCC General Comment for Section lIl. A. 2: The IOGCC believes use of traditional 

well specific AoR is problematic for comprehensive site characterization and 

licensing. The IOGCC has recommended in its report a site characterization and 

licensing process that includes elements such as geologic and engineering 

evaluation, modeling, amalgamation of property rights and other appropriate 

elements that we believe exceeds the capacity of the traditional AoR to accomplish. 

Response: EPA agrees that the AoR for a GS project should incorporate site characterization include

elements such as geologic and engineering evaluation, modeling, and other appropriate elements,

EPA clarifies that the area of review must be delineated using computational modeling that is based

on available site characterization, monitoring, and operational data. EPA is developing guidance on

AoR and corrective action at GS Class VI sites to assist owners/operators and Directors in evaluating

requirements and proposed AoR and corrective action plans under 146.84. The guidance describes

EPA's recommended approaches for the data to be considered in an initial AoR delineation, how to

apply computational modeling to delineate the AoR, performing corrective action at GS sites, and,

when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site,

at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA clarifies that the purpose of the GS regulation is to ensure the safe injection of CO2 through GS

wells in order to protect USDWs from endangerment. Property, water, or mineral rights possibly

affected by a GS project are outside the scope of this rule and therefore are not incorporated into the

AoR definition at 146.84(a).

Commenter Name: Mark Vickery Letter ID: 0280

Commenter Organization: Texas Commission on Environmental Quality Comment ID: 24100

Comment: (b) Concern exists that the proposed rules would require the operator to have 

surface access to large areas of land possibly under separate ownership. This 

concern would apply to the proposed requirements for performance of necessary 

corrective action on unplugged wells, data acquisition such as in seismic 

surveys, and location and sampling of monitor wells or other monitoring stations. 

In TCEQ's experience, adoption of a rule requirement for 3-D seismic surveys 

over the area of review in permit applications for Class I well waste disposal 

in engineered salt caverns illustrates this concern. Permit applicants were not 

able to obtain permission from surface owners to run these surveys in all areas 

required by the rules. Off-site access needed for compliance with the proposed 

12
CO2-GS rules may therefore be problematic. 

Response: EPA clarifies that the purpose of the corrective action requirement at 146.84(d) is to

ensure that all wells in the AoR that have the potential to cause the movement of CO2 or other fluids

into or between USDWs receive appropriate corrective action. In general, to meet this requirement,

the owner or operator will most likely need to obtain the right of access to wells that may require

corrective action, and demonstrate they have done this in their proposed AoR and corrective action

plan. In the AoR and corrective action plan, the owner or operator must describe how site access will

be guaranteed for future corrective action.

Commenter Name: Scott Anderson Letter ID: 0510

Commenter Organization: Environmental Defense Fund Comment ID: 21221

Comment: We're pleased that 146.84 requires modeling both of the CO2 plume and of the 

extent of formation fluid and movement. 

Response: EPA acknowledges the comment; the final rule retains this provision.

Commenter Name: Scott Anderson Letter ID: 0510

Commenter Organization: Environmental Defense Fund Comment ID: 21228

Comment: We, three more things, we think the rule needs to assure that AoRs are 

calculated as intended and updated as necessary, we think a 10-year minimum is 

too short. 

And we think that there's just an unintentional flaw in the definition of area 

of review, it doesn't seem to specifically require the consideration of the 

pressure front and would refer you to the second sentence of the definition of 

geologic sequestration project for the kind of language we think needs to be 

imported into the AoR definition. 

Response: EPA agrees that the AoR definition should account for the impact of the CO2 plume on

USDWs. The Agency has carefully considered all comments on the AoR definition at 146.84(a) and

modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The computational models used for this purpose are required to predict the fate of injected CO2 and

changes in formation pressure. EPA believes that the requirements at 146.84(c)(1) to delineate the

AoR using computational modeling are appropriate and encompass the significant processes that

could affect the location of the CO2 plume and associated pressure front. 

EPA agrees with commenters that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at 146.84(e) from 10 years to 5 years in the final GS

rule. EPA believes that this revised frequency addresses commenters' concerns about the inherent

uncertainties in modeling CO2 movement, the emerging nature of GS technology, risk-based

targeting of monitoring activities, and the unique nature of CO2 GS. 

13
Commenter Name: Talib Syed Letter ID: 0537

Comment ID: 21470

Comment: I just want to give a few minor, just highlight maybe two or three points that 

in my opinion are important. The first thing is I think we have to pay a little 

bit of attention to just transposing the area of review concept that they used 

to with the UIC program to the CO2 geologic sequestration program. And the 

reason is, it is not in my opinion, the same concept that we had when we were 

disposing of water, etcetera. So we have got buoyancy affects to regard to take 

into consideration and that may be a bigger impact n what the zone of 

endangering influence is. And I believe that has not been taken into 

consideration at all and just transposing the zone of endangering influence as 

we are used to with the water etcetera in the UIC program, without incorporating 

buoyancy affects, that is an issue that needs to be very carefully taken into 

consideration. 

Response: EPA agrees that the buoyancy of CO2 must be accounted for in delineating the AoR for

GS projects. Buoyancy is one of the characteristics of CO2 injected for GS that served as a catalyst

for the development of tailored UIC requirements for GS. The Agency clarifies that employing

multiphase computational modeling to determine the AoR as required at 146.84(c) will result in a

more accurate prediction of the fate of CO2 and changes in formation pressure. Computational

models used to delineate the AoR must consider the buoyant nature and specific properties of

separate phases of the injected CO2 and native fluids within the injection zone.

Comment Code: 3300 2.2.1 Delineating the AoR/Modeling CO2 movement 

Individual Comments

Commenter Name: Michael H. Cochran Letter ID:

0108

Commenter Organization: Bureau of Water, Division of Environment, Kansas

Department of Health & Environment (KDHE)

Comment

ID: 21961

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: For the definition of AoR it needs to be realized that the AoR could be quite 

sizeable potentially covering many square miles. We suggest using the criteria 

for the AoR as found in existing 146.6 and for the Pressure Front definition. 

Otherwise, the AoR as defined could make it impractical to accomplish the work 

required. 

For Geologic Sequestration Project definition, our comments for the AoR 

definition apply also. 

Response: The Agency has carefully considered all comments on the AoR definition at 146.84(a)

and modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

14
Commenter Name: Michael H. Cochran Letter ID:

0108

Commenter Organization: Bureau of Water, Division of Environment, Kansas

Department of Health & Environment (KDHE)

Comment

ID: 21970

Comment: 146.84 (c)(1): The CO2 plume will never cease moving. Even deep formations have 

a natural flow gradient. There will also likely be movement due to density 

gradients and diffusion. 

Response: EPA agrees with the commenter that there may be cases where movement of the CO2

plume will never cease due to gradient, diffusion, and density gradients; however, EPA believes that

plume movement may stabilize at a point where movement of the CO2 plume slows or pressures in

the injection zone sufficiently decline to the point that they no longer pose an endangerment to

USDWs. EPA believes that in all cases, these criteria will be met at some point after the cessation of

injection. As discussed in the preamble to the proposed GS regulation and the associated background

document, "Computational Modeling of Underground Injection of Carbon Dioxide for Determination

of Area-of-Review and Potential Risk to Underground Sources of Drinking Water" (cited in the

proposed rule preamble as USEPA, 2008d), research has shown that CO2 may continue to migrate

for a period of time after injection ceases.

For this reason, the GS regulation does not specify a timeframe over which the AoR delineation

model should run. Rather, the model should be used to predict the ultimate plume movement up to

the point the plume movement ceases or pressures in the injection zone sufficiently decline to the

point that they no longer pose an endangerment to USDWs. 

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24770

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: §146.84 Area of review and corrective action. 

(a) The area of review is the region surrounding the geologic sequestration 

project that may be impacted by the injection activity. The area of review is 

based on computational modeling that accounts for the physical and chemical 

properties of all phases of the injected carbon dioxide stream. 

{Redundant - same as the definition of "area of review."} 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24774

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (1) Predict, using computational modeling, the projected lateral and vertical 

15
migration of the carbon dioxide plume and formation fluids in the subsurface 

from the commencement of injection activities until the plume movement ceases, 

pressure differentials sufficient to cause the movement of injected fluids or 

formation fluids into a USDW are no longer present, or FOR after a fixed 

time period as determined by the Director. The modelINGmust: 

Response: EPA has clarified this language in the final rule. It now reads, “Predict, using existing

site characterization, monitoring and operational data, and computational modeling, the projected

lateral and vertical migration of the carbon dioxide plume and formation fluids in the subsurface

from the commencement of injection activities until the plume movement ceases, until pressure

differentials sufficient to cause the movement of injected fluids or formation fluids into a USDW are

no longer present, or until the end of a fixed time period as determined by the Director.”

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24775

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (i) Be based on detailed geologic data collected to characterize the injection 

zone AND, confining zone and any additional zones; and anticipated 

operating data, including injection pressures, rates and total volumes over the 

proposed life of the geologic sequestration project; 

(ii) Take into account RELEVANT any geologic heterogeneities AND, data 

quality, and their possible impact on model predictions; and 

(iii) Consider potential migration through KNOWN faults, fractures, and 

artificial penetrations. 

Response: The final rule retains this requirement as proposed, except that 146.84(c)(1)(ii) also

requires owners or operators to take into account other discontinuities in addition to geologic

heterogeneities. 

EPA disagrees that only the injection and confining zones should be included in AoR modeling of

GS sites. EPA believes that the modeling should also take into account the possibility of displaced

fluids or brines caused by a lateral and/or vertical pressure plume, and therefore it may be necessary

to include additional zones along with the confining/injection zones to ensure the integrity of the

confining zone and ultimately protect a USDW from endangerment.

Commenter Name: Anonymous2 Letter

ID: 0133

Commenter Organization: Sierra Club National Global Warming and Energy

Committee and Sierra Club California-Nevada Regional Conservation Committee on

Energy and Climate Change

Comment

ID: 21603

Comment: In contradistinction to the workgroup's Recommendation 3.1, neither the proposed 

definition of the Area of Review (AOR) contained in 40 CFR 146.81(d) nor the 

Required Class VI Permit Information for initially permitting the AOR under 40 

CFR 146.82, stipulate that the AOR be determined using --among other techniques 

-- modeling along three dimensions. The three dimensional modeling requirement 

only appears in the proposed definition for the phrase Geologic Sequestration 

Project at 40 CFR 146.81(d). Likewise, Recommendation 3.1's inclusion of "the 

16
associated pressure front" as a required modeling target along with the CO2 

plume -- neither appears in the Area of Review definition nor its permitting 

language under 40 CFR 146.82 [Required Class IV Permit Information], but does 

appear in the definition of the Geologic Sequestration Project at 146.81(d). 

Conversely, the Geologic Sequestration Project is not named as the targeted area 

for the initial permitting information required for Class VI injection wells at 

section 146.82, and is only mentioned later in the protocols at section 146.84 

(a) [Area of Review and Corrective Action]. These discrepancies and omissions 

may lead to confusion regarding the nature and extent of the modeling required 

at the outset for determining each AOR and its continuing viability, and could 

be corrected by inserting the workgroup's recommended AOR language in the 

Definitions and the Required Class IV Permit Information sections. 

Response: EPA disagrees that the term “three dimensional” that is in the definition of the Geologic

Sequestration Project must necessarily be repeated in the AoR requirement for modeling for that

modeling to be implicitly three-dimensional. The intent of the AoR, a long-standing concept within

the UIC Program, is to delineate the surface area and all subsurface formations below it that may

potentially be impacted by injection activity. Acceptable delineation models will provide predictions

of CO2 fate and transport, as well as changes in formation pressure, in three dimensions as a

function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggestions, the Agency has carefully considered all comments

on the AoR definition at 146.84(a) and modified it to read as follows: The region surrounding the

geologic sequestration project where USDWs may be endangered by the injection activity. The area

of review is delineated using computational modeling that accounts for the physical and chemical

properties of all phases of the injected CO2 stream and is based on available site characterization,

monitoring, and operational data. 

EPA believes that this definition is sufficiently broad and describes both the intent of the

computational modeling and the relationship of the AoR to areas that are subject to the risks

associated with injection.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22551

Other EPA Comment Codes and Descriptions: 3400 2.2.2 Corrective Action 

Comment: Area of Review and Corrective Action 

In the context of carbon storage, NGWA thinks it is important to distinguish 

between the CO2 plume footprint and the zone of elevated pressure. The rationale 

is given in the next section, but the following conditions should be met: 

- No fixed radius can be used, a zone of endangering influence (ZEI) must be 

determined through modeling (but cannot be smaller than 1⁄4 mile in its smallest 

horizontal dimension) 

- In the volume generated by the CO2 plume footprint (plus appropriate buffer) 

(this volume is defined as the "primary AOR"): all penetrations between the base 

of the injection formation and the base of the USDW should be reviewed and any 

necessary corrective action should be taken before start of project. 

- In the elevated pressure area but outside of the CO2 plume footprint (defined 

17
as the "secondary AOR"), all current penetrations through the primary seal and 

the injection formation should be reviewed. However the applicant should be 

allowed to perform corrective action in a pseudo-concentric phased manner ahead 

of and according to the development of the pressure plume. 

- Secondary AOR/zone of elevated pressure cannot be computed independently of 

other injection projects in the vicinity. The impact of other sequestration 

projects must be included in determining injection rate / period of injection. 

- Primary and secondary AOR must be periodically reviewed to ensure that 

modeling predictions are met. If they are not, the primary and secondary AOR 

should be revisited. The operator should keep track of any new penetration in 

the primary AOR volume and secondary AOR (for example, for oil and gas 

industry). 

"Primary AOR" - Why "Volume of Review" 

Because CO2 is strongly buoyant, it is not sufficient only to investigate 

penetrations in the injection formation and primary seal. The whole volume 

potentially contacted by the CO2 plume should be reviewed. The review and 

corrective actions should be performed before the start of injection because 

there is no stopping the CO2 migration once started. The greatest degree of 

effort should be on the deeper wells in the injection zone and primary 

containment then on the secondary containment and seal (if any) and then on 

shallower wells but as many as possible of the shallower wells should be 

compiled too. 

Delayed Corrective Action for "Secondary AOR" 

A simple time-dependent secondary AOR that expands as the volume of injection 

increases and the CO2 migrates is not acceptable. The secondary AOR should be 

agreed upon and reviewed before injection starts but corrective action can be 

somewhat delayed. The secondary AOR consists of only the injection formation and 

seal as used in the current rules because only brine/formation water leakage is 

an issue in the elevated pressure area. The likely presence of a large elevated 

pressure area does not make realistic imposing corrective action on all wells 

before the start of a project. There is no need to provide corrective action to 

wells before the pressure significantly rises. If another problem (e.g., CO2 

leakage) occurs earlier at another location and injection stops, pressure will 

not keep rising and will drop. In addition, delaying corrective action will 

allow taking advantage of technical improvements that will undoubtedly occur in 

the next decades. 

Response: EPA agrees that the AoR for a GS project should incorporate both the CO2 plume and

the pressure front, and clarifies that the requirements for AoR delineation in the final rule

incorporate both the CO2 plume and associated pressure front. This area would be modeled based on

anticipated CO2 volumes to be injected and other site-specific information that would impact

pressure changes in the subsurface. EPA agrees that both of these should be taken into account in

determining corrective action needs and developing a testing and monitoring plan, and that the

requirement to delineate the AoR at §146.84(c) addresses the potential risks raised by the

18
commenters. EPA believes that the final GS requirements for delineating the AoR provide sufficient

protection to USDWs, without introducing an overly complex regulatory process. 

EPA acknowledges the commenter’s support of phased corrective action and agrees that phased

corrective action may provide many benefits to a project including, but not limited to, spreading the

burden of cost, avoiding delays in project start-up, allowing for use of future, improved corrective

action techniques and addressing unanticipated changes in the movement of the CO2 plume or

pressure front. EPA believes that the phased corrective action approach is appropriate for GS

projects, and retained it at 146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22595

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: 146.84(c)(1)(i) 

Proposed revision: (i) Be based on detailed geologic data collected to 

characterize the injection zone, confining zone(s), transmissive monitoring 

zones and any additional zones; and anticipated operating data, including 

injection pressures, rates and total volumes over the proposed life of the 

geological sequestration project; 

Rationale: NGWA proposes that if there are secondary confining units that they 

should be included in the site model along with any transmissive zones between 

the primary and secondary confining units. 

Response: EPA agrees that additional confining and containment zones should be accounted for in

the AoR delineation if site-specific circumstances indicate a need for this. 146.84 (c)(1)(i) specifies

that computational modeling should be based on detailed geologic data collected to characterize the

injection zone and confining zone, as well as any additional zones.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22122

Comment: 144.55: MDEQ suggests that this rule require that all perforations within the 

area of review be identified and used in modeling. Limiting the information only 

to those perforations that penetrate the injection zone may limit the 

understanding of impacts of plume movement. 

Response: The final rule leaves 144.55 unchanged; i.e., Class VI wells are not addressed in this

provision.

Commenter Name: Michael Mudd Letter ID: 0167

Commenter Organization: FutureGen Alliance (FGIA) Comment ID: 22458

Other EPA Comment Codes and Descriptions: 4600 2.6 Proposed Plume and Pressure Front

Monitoring Requirements (40 CFR 146.90) 

Comment: Area of Review (AoR) and Area of Monitoring (AoM) 

19
p. 43493, c. 2 The Proposed Rule defines Area of Review (AoR) as "The region 

surrounding the geologic sequestration project that may be impacted by the 

injection activity". The area of review is based on computational modeling that 

accounts for the physical and chemical properties of all phases of the injected 

carbon dioxide stream. 

The proposed regulation uses the Area of Review (AoR) for defining both 

monitoring requirements and corrective actions: 

p. 43499, c. 1 Area of Review (AoR) and Corrective Action: The Agency requires 

examination of both the vertical and horizontal extent of the area that will 

potentially be influenced by injection and storage activities and identification 

of all artificial penetrations in the area that may act as conduits for fluid 

movement into USDWs (e.g., active and abandoned wells) and, as needed, perform 

corrective action to these open wells (i.e., artificial penetrations). 

p. 43539, c. 3 (d)(2) The monitoring frequency and spatial distribution of 

monitoring wells must be based on baseline geochemical data that has been 

collected under §146.82(a)(6) and any modeling results in the area of review 

evaluation required by §146.84(b); 

Comment: 

- EPA should define two regulatory regions surrounding the geologic 

sequestration project and provide specific definitions for those regions; 1) 

Area of Monitoring (AoM) and 2) Area of Review; where the AoM is used to define 

the spatial distribution of monitoring wells and the AoR is used to define the 

limit of Corrective Action: 

- Area of Monitoring (AoM): The region surrounding the geologic sequestration 

project that contains 95% of the carbon dioxide mass. The area of monitoring is 

based on monitoring data and computational modeling that accounts for the 

physical and chemical properties of all phases of the injected carbon dioxide 

stream. In the absence of monitoring data, the area of monitoring is based on 

computational modeling. 

- Area of Review (AoR): The region surrounding the geologic sequestration 

reservoir that has or is indicated to develop pressures that are greater than 

10% different from the pre-injection formation fluid pressure, but no less than 

the Area of Monitoring. The area of review is based on monitoring data and 

computational modeling that accounts for the physical and chemical properties of 

all phases of the injected carbon dioxide stream. In the absence of monitoring 

data, the area of review is based on computational modeling. 

• The proposed definitions of AoM and AoR have several important components: 

- The AoM has a mass-based definition. 

- The AoR has a pressure-differential-based definition. 

20
- The AoR is never smaller than the AoM. 

- The AoR and AoM may be asymmetrical around the injection well(s). 

- The AoR and AoM may vary over time. 

- The carbon dioxide mass should be determined using a combination of monitoring 

data and computational modeling. 

- Pressure differentials are based on equilibrium hydrostatic conditions. 

Relative to existing UIC regulations governing waste disposal, EPA is 

introducing fundamentally new concepts for defining the area of review (AoR) for 

CO2 injection. The newly proposed concept is flawed and will pose an 

unjustifiable barrier to CO2 injection projects unless the approach for defining 

the AoR is addressed in the regulation. It is neither possible, nor practical, 

to predict where every molecule of CO2 will migrate; therefore, a threshold 

levels are required. In the case of large-scale injection projects, changes in 

formation pressures could be expected dozens to hundreds of miles from the 

injection well. It would not be practical for the AoR to comprise the entire 

area experiencing changes in formation pressure. A reasonable threshold for 

establishing the AoM might be where pressure changes are greater than 10% of the 

standard formation pressure. 

The proposed regulation defines the Area of Review (AoR) as being the region 

that may be impacted by the injection activity, without defining or quantifying 

impact. To be useful the bounding areas for corrective actions and monitoring 

activities must have specific definitions. Under deep geologic sequestration 

conditions the injected fluid (i.e., supercritical CO2) has low compressibility, 

the displaced fluid (i.e., brine) has low compressibility, and the reservoir 

formation (e.g., sandstone) generally has low compressibility, all of which 

suggest that the impact (i.e., pressure change) of the injection activity could 

be detected at great distances from the injection well(s). 

Injection of CO2 into deep saline formations is driven by differences between 

the well injection pressure and formation pressure. To avoid inducing fractures 

or opening existing fractures in the formation and caprock, well injection 

pressure will be limited to some fraction below the fracturing pressure. As 

CO2 migrates into the formation the formation pressure increases around the 

injection well. The pressure distribution in the formation around the injection 

well depends on the formation properties, structure, and injection rate. 

Injection formations will be required to be confined vertically and/or 

horizontally to prevent the vertical migration of injected fluids and saline 

reservoir fluids into USDWs. 

Confined formations (e.g., a caprock overlying a horizontally extensive 

formation) allow the formation fluid to be displaced laterally by the injected 

fluid. Injecting CO2 will result in elevated pressures both inside and beyond 

the CO2 plume. Elevated pressures within the CO2 plume have the potential to 

force CO2 and formation fluid through permeable wells, fractures, or faults that 

21
penetrate the formation. Elevated pressures outside the CO2 plume have the 

potential to force formation fluid through permeable wells, fractures or faults 

that penetrate the formation. The higher the formation pressure is above the 

ambient formation pressure, the greater the potential for CO2 and formation 

fluid to leak through permeable wells, fractures, and faults. The lateral 

pressure distribution is a function of the injection rate and the lateral 

distribution of formation permeability. When CO2 injection is halted, elevated 

pressures decay over time as the injected and formation fluids migrate away from 

the injection well(s). 

The proposed regulations define an AoR and Corrective Action as being an area 

within which artificial penetrations may act as conduits for fluid movement into 

USDWs (e.g., active and abandoned wells) and, as needed, require the applicant 

to perform corrective action to seal these open wells. The potential for fluid 

movement into USDWs from the injection of CO2 into the injection formation is 

correlated with the area of elevated formation pressure, which is usually larger 

than the area that contains the injected CO2 mass. For confined formations, this 

area is defined by the lateral pressure distribution around the injection 

well(s). 

Response: EPA believes that, while the commenter offers different terminology for the AoR and the

area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or

operator is required (under 146.90) to develop and implement a testing and monitoring plan for each

GS project that must consider the delineated AoR that includes both the CO2 plume and associated

pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies

that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical

properties of all phases of the injected CO2 stream, based on available site characterization,

monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the

testing and monitoring plan, consistent with the commenter's assertion that the AoR may vary over

time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at

least every 5 years, and also requires that the owner or operator review the AoR and corrective

action, testing and monitoring, and emergency and remedial response plans within one year of an

AoR reevaluation (as well as following significant changes to the facility, or when required by the

Director). The relationship between the AoR reevaluation and the testing and monitoring plan should

allow for considerations suggested by the commenter. 

EPA believes that by linking development and review of the testing and monitoring plan to the AoR

evaluation and reevaluation process, the GS rule, in effect, incorporates and addresses many of the

commenter's considerations for an area of monitoring.

Commenter Name: Michael J. Miller Letter ID: 0168

Commenter Organization: Electric Power Research Institute (EPRI) Comment ID: 22492

Other EPA Comment Codes and Descriptions: 4650 2.6.5 CO2 plume and pressure front tracking

(seeking comment) 

Comment: Preamble, page 43505: "The confining system should be of sufficient regional 

thickness and lateral extent to contain the entire CO2 plume and associated 

pressure front under the confining system following the plume’s maximum lateral 

22
expansion." 

EPRI COMMENT #2: The criteria for defining the leading edge of the plume and 

associated pressure front are not specified in the rule, nor is the basis for 

determining if these criteria are met -- physical measurement or numerical 

computation. In either case, what minimum change in pressure from background 

will EPA use to define the location of the pressure front? Will the lateral 

extent of the CO2 plume be defined by the leading edge of the dissolved plume or 

free-phase portion of the plume? Given the difficulties in defining and 

measuring the leading edge of the plume and pressure front, EPRI believes that 

the technical focus should be on demonstrating caprock containment, rather than 

tracking fronts. 

Response: EPA agrees that demonstrating cap rock containment is an important technical focal point

for permitting Class VI wells, however the Agency also believes that it is important to know the

extent of the CO2 plume and pressure front to ensure cap rock integrity and protection of USDWs. 

EPA clarifies that the pressure front (defined at §146.81) is the zone of elevated pressure that is

created by the injection of CO2 into the subsurface. The pressure front of a CO2 plume refers to a

zone where there is a pressure differential sufficient to cause the movement of injected fluids or

formation fluids into a USDW. The pressure front includes all areas wherein pressure is great

enough to cause fluid movement into a USDW if a conduit were to exist.

The calculation of the threshold pressure that defines the edge of the pressure front is dependent on

properties of the injection and confining zones, including permeability, porosity, and compressibility,

the distance between the injection zone and the lowermost USDW, and operational conditions

including injection pressure and rate. For these reasons, the definition of the pressure front is siteand project-specific, and a single pressure value can not be provided for all sites. EPA is developing

guidance on AoR and corrective action at GS Class VI sites to assist owners or operators and

Directors in evaluating requirements and proposed AoR and corrective action plans under 146.84.

The guidance describes EPA's recommended approaches for the data to be considered in an initial

AoR delineation, how to apply computational modeling to delineate the AoR, performing corrective

action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be

available on EPA's web site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA is also developing guidance on testing and monitoring to assist owners or operators and

Directors in evaluating requirements under §146.90, which will include discussion of identifying the

extent of the CO2 plume. The guidance describes available technologies and recommended

approaches for implementing a testing and monitoring plan, including pre-operational geophysical

logging and testing of injection wells, MITs, injection well testing during operation, groundwater

quality monitoring, tracking the CO2 plume and pressure front, and soil gas and surface air

monitoring which may be required by the UIC Director for specific GS projects. The guidance can

be found on EPA’s website at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm. In

addition, in January 2009, DOE published a best practices manual entitled "Monitoring, Verification,

and Accounting of CO2 Stored in Deep Geologic Formations" (DOE/NETL-311/081508); this

manual is included in the docket for the for the final GS rule.

Commenter Name: Theresa Pugh Letter ID: 0169

Commenter Organization: American Public Power Association Comment ID: 23429

23
Comment: to use geologic sequestration in a very responsible manner. First, many states 

have not conducted modeling on groundwater use and availabity in decades, and 

the earlier modeling may have used outdated models and information. In non-oil 

and gas states (or perhaps some mining states) there is little known about the 

subsurface geology. While APPA greatly appreciates the maps and placement 

analysis modeling conducted by U.S. Geological Service or "USGS", Batelle, 

NETL/DOE and others. We do not believe that we have preliminary 

characterizations or pre-commercial data as to underground formations suitable 

for injection and long-term storage of CO2. Nor do we know what the storage 

Response: EPA acknowledges that some states do not have much geological and hydrogeologic

data. For this reason, it is important that owners or operators gather new site-specific data and

conduct new modeling for each GS project. The Agency believes that AoR delineation using

computational modeling that accounts for the multiphase flow of CO2 and native fluids (i.e.,

naturally occurring salty water), including density-driven flow (i.e., buoyancy), is necessary to

ensure protection of USDWs from endangerment. The computational models used for this purpose

are required to predict the fate of injected CO2 and changes in formation pressure. EPA believes that

the requirements at 146.84(c)(1) to delineate the AoR using computational modeling are appropriate

and encompass the significant processes that could affect the location of the CO2 plume and

associated pressure front. Models can predict CO2 fate and transport, as well as changes in

formation pressure, in three dimensions, as a function of time in order to delineate the subsurface

storage site and the AoR. Models can account for site-specific and predicted operating conditions as

well as geologic heterogeneities and potential migration through faults, fractures, and artificial

penetrations. 

EPA has evaluated technical documents regarding several computational models that may be used

for GS sites. Based on this review (as documented in the AoR section of the preamble to the

proposed GS regulation and the background document "Computational Modeling of Underground

Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk to

Underground Sources of Drinking Water"), EPA believes that several models meet the minimum

requirements for models in the GS regulation.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24130

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Definition of Geological Sequestration Project (GSP) and Area of Review (AoR) 

EPA's definitions of GSP and AoR require alignment. 

Currently the GSP and AoR definitions confusingly use different wording to 

describe similar concepts. It is API's view that these definitions should be 

essentially the same. API offers languages in our detailed edits that achieve 

this. 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

24
CO2, including free-phase and water-dissolved CO2. The definition in the final rule will continue to

do so. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24158

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(a) 

API Comments: (a) The area of review is the region surrounding the geologic 

sequestration project that may be impacted by the injection activity. The area 

of review is based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream. THE 

SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE CARBON DIOXIDE PLUME,

ASSOCIATED 

PRESSURE FRONT, AS WELL AS THE OVERLYING FORMATIONS AND SURFACE AREA

ABOVE THAT 

DELINEATED REGION. 

Rationale: Edits make this provision consistent with the definition of "Area of 

Review", which was revised. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions

as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24161

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

25
language 

Comment: Rule Provision: 146.84(c)(1) 

API Comments: (1) Predict, using computational modeling, the projected lateral 

and vertical migration of the carbon dioxide plume and formation fluids in the 

subsurface from the commencement of injection activities until the plume 

movement ceases, pressure differentials sufficient to cause the movement of 

injected fluids or formation fluids into a USDW are no longer present, or 

after FOR a fixed time period as determined by the Director. The model

MODELING must: 

Rationale: Clarification. 

Rule Provision: 146.84(c)(1) 

API Comments: (i) Be based on detailed geologic data collected AVAILABLE to 

characterize the injection zone, confining zone and any additional zones; 

and anticipated operating data, including injection pressures, rates and total 

volumes over the proposed life of the geological sequestration project THE 

DURATION OF INJECTION; (ii) Take into account any RELEVANT geologic 

heterogeneities, AND data quality, and their possible impact on model 

predictions; and 

Rationale: Allows the use of all available data, not just data collected 

specifically for this permit. 

Response: EPA has clarified the language in §146.84(c)(1) for the final rule. It now reads, “Predict,

using existing site characterization, monitoring and operational data, and computational modeling,

the projected lateral and vertical migration of the carbon dioxide plume and formation fluids in the

subsurface from the commencement of injection activities until the plume movement ceases, until

pressure differentials sufficient to cause the movement of injected fluids or formation fluids into a

USDW are no longer present, or until the end of a fixed time period as determined by the Director.”

The final rule retains §146.84(c)(1)(ii) as proposed, except that it now requires owners or operators

to take into account other discontinuities in addition to geologic heterogeneities when determining

the AoR. 

EPA disagrees that only the injection and confining zones should be included in AoR modeling of

GS sites. EPA believes that the modeling should also take into account the possibility of displaced

fluids or brines caused by a lateral and/or vertical pressure plume, and therefore it may be necessary

to include additional zones along with the confining/injection zones to ensure the integrity of the

confining zone and ultimately protect a USDW from endangerment.

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23465

Other EPA Comment Codes and Descriptions: 3140 2.1.4 Comments that the proposed geologic

data requirements are too stringent 

Comment: • The Final Rule must not make the application process overly burdensome or 

unrealistic by requiring detailed information that may not be available at the 

26
time of the initial application; instead, the Final Rule should allow for 

submission of updated information regarding a CO2 GS well and its AoR as that 

information becomes available. 

Response: EPA agrees that not all of the information listed at §146.82 of the proposed regulation

will be available at the time the owner or operator develops their initial permit application. In

response to comments and based on an evaluation of the proposed requirements at §146.82, the

Agency revised this subpart so that the final regulation specifies separate requirements for

information to be submitted with the permit application and before well construction is approved (at

§146.82(a)) and information to be submitted before operation of the well is authorized (at

§146.82(c)). 

EPA believes that this revision of §146.82 addresses commenters' concerns regarding the timing of

information to be submitted. In addition, §146.82 in the final rule is similar to the permit application

requirements for other injection well classes, which could help avoid confusion about how to meet

the permitting requirements for Class VI wells. The requirements at §146.82(c) for information to be

submitted before operation of the well is authorized are very similar to those for Class I wells, with

modifications and additions that address the unique challenges and types of data collected for GS

wells.

Commenter Name: Frederick R. Eames Letter ID: 0178

Commenter Organization: CCS Alliance Comment ID: 23874

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: EPA defines "area of review" as "the region surrounding the geologic 

sequestration project that may be impacted by the injection activity. The area 

of review is based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream." The 

phrase "may be impacted" is too vague, The AoR delineation should be based on or 

limited by certain risk criteria; otherwise, an AoR could cover an 

inappropriately large area that could be expensive to review and characterize. 

Response: EPA agrees with commenters that further clarity is needed with regard to the boundaries

of the AoR. After considering the public comments on the proposed GS regulation, EPA has

augmented the definition of AoR to better encompass the area that may be impacted by the injection

activity (i.e., the "surrounding area" to which the commenter refers). The Agency has modified the

AoR definition at 146.84(a) to read as follows: The region surrounding the geologic sequestration

project where USDWs may be endangered by the injection activity. The area of review is delineated

using computational modeling that accounts for the physical and chemical properties of all phases of

the injected CO2 stream and is based on available site characterization, monitoring, and operational

data. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

27
Commenter Name: Robert J. Finley Letter ID: 0180

Commenter Organization: Illinois State Geological Survey Comment ID: 23628

Comment: (c)(1)(iii) 

What does this mean, "Consider potential migration"? Does this require modeling 

faults as though they were transmissive even though there may be no evidence of 

such? 

Response: EPA anticipates that the modeling would consider any potential fluid migration that may

exist at the site. If site-specific geologic data indicate that faults at the site are non-transmissive, then

the model need not account for transmissive faults.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23652

Comment: In its comments, AWWA makes several recommendations for the following specific 

items to be included in the final regulation: 

- Development of a standard computation flow model to delineate the area of 

review; 

Response: EPA believes that operators should have the flexibility to select models with which they

are familiar and that are most appropriate to the site conditions and available data types and

amounts. EPA also recognizes that a standardized model for AoR delineations could be effective as

long as the model accounts for the expected variations in site conditions, and adds that a state or

regional UIC Director has the discretion to select one standard model if it took site-specific

variations into account. 

EPA does not envision that members of the public would evaluate the AoR delineation models

themselves. The models and their inputs will, by necessity, entail an extensive amount of technical

information that may not be easily understood by the public. EPA believes that it is the Director's

responsibility, as a steward of public health and the environment, to review models, model inputs,

and results and to determine whether the model is appropriate for predicting the AoR, and to

distribute information that may be easily understood by the public (e.g., in the form of a fact sheet).

However, the models must be made available to the public. 

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23690

Other EPA Comment Codes and Descriptions: 3350 2.2.1.5 Use of proprietary models for GS

sites (seeking comment) 

Comment: Category: Modeling 

EPA Request for Comment: EPA proposes that the owners or operators of GS wells 

be required to delineate the AoR for CO2 GS sites using computational fluid flow 

models designed for the specific site conditions and injection regime. EPA seeks 

comment on the use of modeling for AoR delineation. EPA Notice 43506. 

When using a proprietary model, owners or operators should clearly disclose the 

code assumptions, relevant equations, and scientific basis. EPA seeks comment on 

28
allowing the use of proprietary models for GS sites. EPA Notice 43507. 

AWWA Response: The proposed rule depends heavily on computer modeling to track 

the carbon dioxide plume and demonstrate that it is contained within the 

confining zone. As stated by EPA, some of these models are proprietary and the 

rule is unclear how or if confirmation of the model results will be provided. 

Additionally, each of the individual proprietary models can show a different 

answer to the problem. Owner/operators need to ensure that their basis for the 

model is static, and that the basis does not change once more details are known 

about the movement of the plume. For example, the operation of the geologic 

sequestration system should be adjusted based on modeling results, and the model 

shouldn't be revised because the plume is moving in a direction not originally 

predicted. 

AWWA recommends that EPA standardize on one specific model that is transparent 

and available for outside stakeholders to review, including the public. The 

other option is for EPA to develop a standard model that would be used for all 

geologic sequestration projects. If EPA were to develop a standard model, a 

pilot program should be part of the model development so that the modeling 

results could be validated with real-life monitoring data. In either case, 

clearly stating the model assumptions made during model set-up is critical, and 

these assumptions must be made available to stakeholders when they are reviewing 

the model results. 

Response: The final rule does not require use of a standardized model. As described above, EPA

believes that operators should have the flexibility to select models with which they are familiar and

that are most appropriate to the site conditions and available data types and amounts. EPA also

recognizes that a standardized model for AoR delineations could be effective as long as the model

accounts for the expected variations in site conditions, and adds that a state or regional UIC Director

could select one standard model if it took site-specific variations into account. 

EPA does not envision that members of the public will evaluate the AoR delineation models

themselves. The models and their inputs will, by necessity, entail an extensive amount of technical

information that may not be easily understood by the public. EPA believes that it is the Director's

responsibility, as a steward of public health and the environment, to review models, model inputs,

and results and to determine whether the model is appropriate for predicting the AoR, and to

distribute information that may be easily understood by the public (e.g., in the form of a fact sheet).

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23730

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Part 146.84 It is important to note that the carbon dioxide plume will never 

cease moving. Even deep formations have a natural flow gradient, and it is 

likely that there will always be some movement due to changes in density 

gradients and diffusion. This should be considered when defining the AoR for 

each project. 

29
Response: EPA agrees with the commenter that there may be cases where movement of the CO2

plume will never cease due to gradient, diffusion, and density gradients, but disagrees that the AoR

definition at 146.84(a) should describe a never-ending timeframe of CO2 plume movement. The GS

regulation recognizes that plume movement may stabilize to a point where movement of the CO2

plume slows or pressures in the injection zone sufficiently decline to the point that they no longer

pose an endangerment to USDWs, although movement does not completely cease. EPA believes that

in all cases, these criteria will be met at some point after the cessation of injection. As discussed in

the preamble to the proposed GS regulation and the associated background document

"Computational Modeling of Underground Injection of Carbon Dioxide for Determination of Areaof-Review and Potential Risk to Underground Sources of Drinking Water" (cited in the proposed

rule preamble as USEPA, 2008d), research has shown that CO2 may continue to migrate for a period

of time after injection ceases.

For this reason, the GS regulation does not specify a timeframe over which the AoR delineation

model should run. Rather, the model should be used to predict the ultimate plume movement up to

the point the plume movement ceases or pressures in the injection zone sufficiently decline to the

point that they no longer pose an endangerment to USDWs. 

EPA believes that an iterative process of AoR reevaluation is appropriate to verify that the CO2

plume and pressure front are moving through the subsurface as predicted, verify the accuracy of the

delineation model (or identify needed changes), and refine testing and monitoring activities to ensure

that USDWs are protected from endangerment and that there would be adequate warning of potential

problems at the site before any endangerment could occur. EPA adds that the requirement at

146.93(b) for post-injection site care further ensures that any continued movement of the CO2 plume

and pressure front will be addressed until it can be demonstrated that the GS operation no longer

poses an endangerment risk to USDWs. 

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23731

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Part 146.84(c)(1)(iii) AWWA recommends revising this part to read: 

Consider potential migration through faults, fractures, other discontinuities 

and artificial penetrations. 

Response: EPA did not change 146.84(c)(1)(iii) but instead changed the text at 146.84(c)(1)(ii) to

read as follows: "Take into account any geologic heterogeneities, other discontinuities, data quality,

and their possible impact on model predictions" to address the water association commenter's

concern regarding the potential for migration of CO2 at GS sites.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25180

Comment: III A. 2 Proposed Area of Review and Corrective Action Requirements 

NYSDEC-DMN General Comment for Section III. A. 2: NYSDEC-DMN believes use of 

30
traditional well-specific AoR is problematic for comprehensive and adequate site 

characterization, and that EPA’s proposed approach is appropriate. 

Response: EPA agrees that the unique aspects of GS merit the use of sophisticated computational

modeling for delineation of the AoR, based on site-specific features and geology, including the

presence of faults, fractures, and artificial penetrations. EPA believes that computational modeling of

the AoR prior to injection will provide the best obtainable estimates of the extent of the CO2 plume

over several years of active injection. EPA has strengthened the minimum frequency for the AoR

reevaluation from ten years to 5 years (146.84(e)). EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24334

Other EPA Comment Codes and Descriptions: 3310 2.2.1.1 Comments in favor of requiring

computational models to delineate the AoR (seeking comment) 

Comment: 1. AOR Definition 

The proposed rule vaguely defines the AOR as "the region surrounding" the GS 

project that "may be impacted by the injection activity." 72 Fed. Reg. at 43535 

(proposed 40 C.F.R. 146.81(d)). The definition goes on to suggest that the AOR 

is to be "based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream." Id. 

There are three issues. 

First, the proposed definition suggests the need for a more complex modeling 

approach than may typically be necessary or appropriate. As experience with 

hazardous waste injection "no migration" petitions shows, it is rarely necessary 

to attempt to model chemical changes in the injectate plume; instead 

conservative modeling of transport should typically be sufficient by itself to 

show that USDWs will not be endangered. Where this is the case, it is 

containment of the injectate plume - rather than the precise composition of the 

plume - that matters and there is no need to model chemical changes. This is a 

critical point, because it is generally far easier to model transport than 

chemical changes occurring in the subsurface. In fact, it would be 

extraordinarily burdensome to have to model "the chemical properties of all 

phases of the injected carbon dioxide stream" as the proposed rule suggests. 

Such modeling should be required only to the extent that it is truly necessary 

to ensure that USDWs will be protected. 

Second, the preamble to the proposal suggests that the "best" (most 

sophisticated?) and most "accurate" modeling should be preferred or even 

required. 73 Fed. Reg. at 43507. However, fine precision generally isn't 

necessary, The appropriate goal for modeling is to ensure protection of USDWs, 

and conservative (i.e., less precise) modeling may be sufficient and even 

preferable for this purpose. A quest for greater precision would not guarantee 

greater confidence in modeling results, and could effectively require a less 

conservative - and thus potentially less protective - modeling approach. 

31
Third, the functional purpose of the AOR is to define the area within which 

impacts of the injection activity could cause endangerment of a USDW; 

specifically, the AOR defines the area within which corrective action for 

abandoned boreholes may be necessary to prevent endangerment of USDWs. The 

language of the proposed rule (i.e., "may be impacted by the injection 

activity") is not clearly limited to impacts of the injection activity that 

could cause endangerment of USDW’s. It should be. 

Response: EPA believes that predicting the complex, multiphase, buoyant flow of the CO2, coinjectates, and compounds that may be mobilized due to injection requires the sophistication of

computational models, as required at 146.84(c)(1). Models can predict CO2 fate and transport, as

well as changes in formation pressure, in three dimensions, as a function of time in order to delineate

the subsurface storage site and the AoR. Models can account for site-specific and predicted

operating conditions as well as geologic heterogeneities and potential migration through faults,

fractures, and artificial penetrations. 

EPA acknowledges that fine precision may not always be necessary and that many factors determine

ultimate confidence in modeling results. However, EPA does not believe that more precision would

result in a less protective modeling approach. EPA anticipates that the owner or operator, in

consultation with the Director, will choose the model that is most appropriate for a particular GS

project, and that this dialogue is a key factor in assuring that the GS site is modeled (and eventually

operated and tracked) in a manner that protects USDWs from endangerment. EPA encourages and

expects owners or operators to use the best multiphase, computational models available. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24424

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(a) 

Proposed CCS Group Recommendations: (a) The area of review is the region 

surrounding the geologic sequestration project that may be impacted by the 

injection activity. The area of review is based on computational modeling that 

accounts for the physical and chemical properties of all phases of the injected 

carbon dioxide stream. THE SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE

CARBON 

DIOXIDE PLUME, ASSOCIATED PRESSURE FRONT, AS WELL AS THE OVERLYING

FORMATIONS 

AND SURFACE AREA ABOVE THAT DELINEATED REGION. 

Basis for CCS Group Comments: Edits make this provision consistent with the 

definition of "Area of Review", which was revised. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions,

32
as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24429

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(1) 

Proposed CCS Group Recommendations: (1) Predict, using computational modeling, 

the projected lateral and vertical migration of the carbon dioxide plume and 

formation fluids in the subsurface from the commencement of injection activities 

until INJECTION INDUCED plume movement ceases, pressure differentials sufficient 

to cause the movement of injected fluids or formation fluids into a USDW are no 

longer present, or after FOR a fixed time period as determined by the 

Director. The model MODELING must: 

Basis for CCS Group Comments: Clarification 

Response: Based on an evaluation of all comments received, EPA has clarified this language in the

final rule at 146.84(c)(1). It now reads, “Predict, using existing site characterization, monitoring and

operational data, and computational modeling, the projected lateral and vertical migration of the

carbon dioxide plume and formation fluids in the subsurface from the commencement of injection

activities until the plume movement ceases, until pressure differentials sufficient to cause the

movement of injected fluids or formation fluids into a USDW are no longer present, or until the end

of a fixed time period as determined by the Director.”

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24430

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(1)(i) & (ii) 

Proposed CCS Group Recommendations: (i) Be based on detailed geologic data 

collected AVAILABLE to characterize the injection zone, confining zone and 

any additional zones; and anticipated operating data, including injection 

pressures, rates and total volumes over the proposed life of the geological 

sequestration project THE DURATION OF INJECTION; 

(ii) Take into account any RELEVANT geologic heterogeneities, AND data 

quality, and their possible impact on model predictions; and 

33
Basis for CCS Group Comments: Allows the use of all available data, not just 

data collected specifically for this permit. 

Response: EPA agrees that detailed geologic data is invaluable to modeling an accurate AoR. EPA

clarifies that owners or operators may use existing data as well as data collected during site

characterization to predict the AoR. The final rule retains the language in §146.84(c)(1)(i) as

proposed, and modifies §146.84(c)(1)(ii) slightly to require owners or operators to take into account

“other discontinuities” in addition to geologic heterogeneities when predicting the AoR. 

EPA disagrees that only the injection and confining zones should be included in AoR modeling of

GS sites. EPA believes that the modeling should also take into account the possibility of displaced

fluids or brines caused by a lateral and/or vertical pressure plume, and therefore it may be necessary

to include additional zones along with the confining/injection zones to ensure the integrity of the

confining zone and ultimately protect a USDW from endangerment. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24431

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(1)(iii) 

Proposed CCS Group Recommendations: Support the proposed language. 

Basis for CCS Group Comments: Agree with EPA’s rationale. 

Response: EPA acknowledges the commenter’s support; the final GS rule retains the provision in

§146.84(c)(1)(iii) as proposed.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24438

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(g) 

Proposed CCS Group Recommendations: (g) The emergency and remedial response plan 

(as required by § 146.94) and a demonstration of financial responsibility (as 

described by § 146.85) must account for the entire area of review AS MODIFIED 

PURSUANT TO § 146.94(F), regardless of whether or not corrective action in the 

area of review is phased. 

Basis for CCS Group Comments: Need to add phrase to account for the periodic 

reevaluations and possible revisions that will be occurring. 

Response: EPA agrees with the commenter and has modified the final rule at §146.84(f) to require

the emergency and remedial response plan and the demonstration of financial responsibility to take

into account the most recently evaluated AoR.

34
Commenter Name: John R. Baza Letter ID: 0193

Commenter Organization: Utah Division of Oil, Gas, and Mining Comment ID: 23912

Comment: Area of Review ("AOR") Determination, Monitoring and Verification 

The EPA seeks comment on the use of modeling for AOR delineation, the use of 

proprietary models for GS sites, and the frequency of AOR reevaluation. By 

letter dated December 23, 2008, the Interstate Oil and Gas Compact Commission 

("IOGCC") has submitted several comments to the EPA on these issues with respect 

to the proposed rule. As a regulatory agency within a member state of IOGCC, the 

Division concurs with the IOGCC comments and recommendations related to AOR. 

Response: Based on its evaluation of all public comments and input on the proposed rule and

NODA and Request for Comment, EPA has made many revisions to the GS Rule. See the final rule

and preamble for further discussion on EPA’s final approach.

Commenter Name: Andy Horn Letter ID: 0196

Commenter Organization: Colorado Ground Water Association (CGWA) Comment ID: 23929

Comment: Issue 9. Use of models to assess area of review is only briefly discussed in 

the proposed regulation. 

The regulation specifies the use of mathematical models to assess the AoR and 

potential impacts. Prior to beginning modeling to evaluate the suitability of a 

potential reservoir, applicants should be required to collect sufficient field 

data from the reservoir and over-and underlying formations to evaluate the 

effects of the project on not only the area of review but also nearby locations 

where groundwater resources are present, even if not used widely. In modeling 

of proposed GS projects, multi-phase geologic models should be used coupled with 

solute transport modeling to evaluate the anticipated impacts. Additionally, 

models which can evaluate the geomechanical stresses upon the geologic materials 

within the AoR should be used to determine the potential for induced seismicity. 

Prior to modeling, site characterization should be performed with an adequate 

number test boreholes and wells to allow a statistically-valid characterization 

of issues within the AoR. In addition, the models should simulate the entire 

project lifespan and a substantial amount of time in the post-closure phase to 

provide insight into, support mitigation of, potential failure mechanisms. 

Issues to be evaluated in reservoir modeling should include but not be limited 

to: 

* Rock mechanics and susceptibility to fracture/failure, 

* Water quality within the AoR including strata above and lateral to the 

proposed GS reservoir, 

* Reservoir and aquifer properties, 

* Potential for CO2 migration into overlying strata via faults and improperly 

sealed boreholes, 

* Potential water quality impacts on USDWs due to the pressure field developed 

under project scale injection, 

* Mechanical and hydraulic properties of low-permeability units anticipated to 

serve as barriers to migration of the CO2 and, 

35
* The effects of fluids that will be displaced from zones between the GS 

reservoir and USDWs. 

Ongoing collection of verification data for these models is important and should 

be performed on an interval that will be sufficient to allow identification and 

mitigation of potential failure mechanisms. Use of any model must be combined 

with calibration, where field observations should be replicated in the model. 

Any models that are used must be fully documented, with assumptions and 

methodology provided. A 5-year review interval is suggested over the course of 

the project life. Models alone, without sufficient field data collected prior 

to and during operation, are not protective of the public safety or resources. 

Response: EPA agrees that detailed geologic data, including the types of data to which the

commenter refers, is invaluable to modeling an accurate AoR. The Agency believes that the AoR

delineation requirements in 146.84 (c)(1)(i), which specify using "detailed geologic data collected to

characterize the injection zone, confining zone and any additional zones; and anticipated operating

data...," and (c)(iii) to "consider potential migration through faults, fractures and artificial

penetrations," provide the necessary information needed for appropriate site modeling, subsequent

site evaluation, and protection of USDWs from endangerment. EPA adds that the extensive

requirements for data that is to be included in the permit application at 146.82 incorporate these same

types of information. The Director will review the AoR models in the context of the extensive site

data submitted with the permit application. EPA recognizes that in some instances, the Director may

require additional types of data to be included in the AoR delineation (146.84(c)(1)).

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24654

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(a) 

Anadarko Petroleum Corporation (APC) Comments: (a) The area of review is the 

region surrounding the geologic sequestration project that may be impacted by 

the injection activity. The area of review is based on computational modeling 

that accounts for the physical and chemical properties of all phases of the 

injected carbon dioxide stream. SUBSURFACE THREE-DIMENSIONAL EXTENT OF THE 

CARBON DIOXIDE PLUME, ASSOCIATED PRESSURE FRONT, AS WELL AS THE

OVERLYING 

FORMATIONS AND SURFACE AREA ABOVE THAT DELINEATED REGION. 

Rationale: Edits make this provision consistent with the definition of "Area of 

Review", which was revised. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions

as a function of time that can be used to delineate the subsurface storage site and the AoR. Although

EPA disagrees with the commenter's suggested change, the Agency has carefully considered all

36
comments on the AoR definition at 146.84(a) and modified it to read as follows: The region

surrounding the geologic sequestration project where USDWs may be endangered by the injection

activity. The area of review is delineated using computational modeling that accounts for the physical

and chemical properties of all phases of the injected CO2 stream and is based on available site

characterization, monitoring, and operational data. EPA believes that this definition is sufficiently

broad and describes both the intent of the computational modeling and the relationship of the AoR to

areas that are subject to the risks associated with injection.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24658

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(1) 

Anadarko Petroleum Corporation (APC) Comments: (1) Predict, using computational 

modeling, the projected lateral and vertical migration of the carbon dioxide 

plume and formation fluids in the subsurface from the commencement of injection 

activities until the plume movement ceases, pressure differentials sufficient to 

cause the movement of injected fluids or formation fluids into a USDW are no 

longer present, or after FOR a fixed time period as determined by the 

Director. The model MODELING must: 

Rationale: Clarification 

Response: Based on an evaluation of all comments received, EPA has clarified this language in the

final rule at 146.84(c)(1). It now reads, “Predict, using existing site characterization, monitoring and

operational data, and computational modeling, the projected lateral and vertical migration of the

carbon dioxide plume and formation fluids in the subsurface from the commencement of injection

activities until the plume movement ceases, until pressure differentials sufficient to cause the

movement of injected fluids or formation fluids into a USDW are no longer present, or until the end

of a fixed time period as determined by the Director.”

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24659

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(1)(i) & (ii) 

Anadarko Petroleum Corporation (APC) Comments: (i) Be based on detailed geologic 

data collected to characterize the injection zone, confining zone and any 

additional zones; and anticipated operating data, including injection 

pressures, rates and total volumes over the proposed life of the geological 

sequestration project THE DURATION OF INJECTION; 

(ii) Take into account any RELEVANT geologic heterogeneities, AND data 

quality, and their possible impact on model predictions; and 

Rationale: Clarification. 

37
Response: EPA agrees that detailed geologic data is invaluable to modeling an accurate AoR. EPA

clarifies that owners or operators may use existing data as well as data collected during site

characterization to predict the AoR. The final rule retains the language in §146.84(c)(1)(i) as

proposed, and modifies §146.84(c)(1)(ii) slightly to require owners or operators to take into account

“other discontinuities” in addition to geologic heterogeneities when predicting the AoR. 

EPA disagrees that only the injection and confining zones should be included in AoR modeling of

GS sites. EPA believes that the modeling should also take into account the possibility of displaced

fluids or brines caused by a lateral and/or vertical pressure plume, and therefore it may be necessary

to include additional zones along with the confining/injection zones to ensure the integrity of the

confining zone and ultimately protect a USDW from endangerment. 

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22435

Comment: Uncertainty Associated with Modeling COZ Movement and Reservoir Pressure . . . . . . .

Page 8 

A formal process for uncertainty assessment of the C02 plume footprint should be 

incorporated in the Proposed Rule. This uncertainty assessment could be based on 

a bounding approach, a sensitivity analysis, Monte Carlo simulation or other 

method, provided the range of probable circumstances are considered. 

Response: EPA acknowledges that there will be some uncertainty involved in AoR modeling. An

EPA-prepared technical background document, "Computational Modeling of Underground Injection

of Carbon Dioxide for Determination of Area-of-Review and Potential Risk to Underground Sources

of Drinking Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the proposed

GS rule. This document discusses, for example, uncertainty analysis in detail. This document was

provided as a background to clarify what information EPA analyzed in order to draft the modeling

requirements at 146.84(c)(1), as well as what actions EPA expects owners or operators and Directors

to take in this area. For example, the document states: "The requirement for an a-priori AoR

delineation based on modeling predictions stresses the need for uncertainty and sensitivity analyses

for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool for

characterizing the most and least important sources of uncertainty in environmental models."

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22440

Comment: AREA OF MONITORING (AoM) AND AREA OF REVIEW (AoR) 

PROPOSED REGULATION 

p. 43493, c. 2 Area of Review (AoR): The region surrounding the geologic 

sequestration project that may be impacted by the injection activity. The area 

of review is based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream. 

The proposed regulation uses the Area of Review (AoR) for defining both 

corrective actions and monitoring requirements: 

38
p. 43499, c. 1 Area of Review (AoR) and Corrective Action: The Agency requires 

examination of both the vertical and horizontal extent of the area that will 

potentially be influenced by injection and storage activities and identification 

of all artificial penetrations in the area that may act as conduits for fluid 

movement into USDWs (e.g., active and abandoned wells) and, as needed, perform 

corrective action to these open wells (i.e., artificial penetrations). 

p. 43539, c. 3 (2) The monitoring frequency and spatial distribution of 

monitoring wells must be based on baseline geochemical data that has been 

collected under §146.82(a)(6) and any modeling results in the area of review 

evaluation required by § 146.84(b); 

We agree with the concept of defining bounding areas for corrective actions and 

monitoring activities, but the proposed regulation uses a single area, Area of 

Review (AoR), for both and fails to address how to quantify this area. The 

proposed regulation defines the Area of Review (AoR) as being the region that 

may be impacted by the injection activity, without defining or quantifying 

impact. To be useful the bounding areas for corrective actions and monitoring 

activities must have specific definitions. Under deep geologic sequestration 

conditions the injected fluid (i.e., supercritical CO2) has low compressibility, 

the displaced fluid (i.e., brine) has low compressibility, and the reservoir 

formation (e.g., sandstone) generally has low compressibility, all of which 

suggest that the impact (i.e., pressure change) of the injection activity could 

be detected at great distances from the injection well(s). 

Response: EPA believes that, while the commenter offers different terminology for the AoR and the

area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or

operator is required (under 146.90) to develop and implement a testing and monitoring plan for each

GS project that must consider the delineated AoR that includes both the CO2 plume and associated

pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies

that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical

properties of all phases of the injected CO2 stream, based on available site characterization,

monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the

testing and monitoring plan, consistent with the commenter's assertion that the AoR may vary over

time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at

least every 5 years, and also requires that the owner or operator periodically review the AoR and

corrective action, testing and monitoring, and emergency and remedial response plans within one

year of an AoR reevaluation (as well as following significant changes to the facility, or when

required by the Director). The relationship between the AoR reevaluation and the testing and

monitoring plan should allow for considerations suggested by the commenter. 

EPA believes that by linking development and review of the testing and monitoring plan to the AoR

evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's

considerations for an area of monitoring.

EPA did not specifically define the term "impacted" in this context because of the wide range of

potential geologic and other site-specific conditions that may be vary from site to site. EPA believes

that each GS site will be unique and that the affected area where USDWs may be at risk should be

39
defined based on computational modeling that accounts for available site characterization,

monitoring, and operational data. 

EPA anticipates that the delineated AoR will serve as the foundation for identifying areas in which

corrective action and monitoring associated with the site would occur, and that the AoR and

corrective action plan and the testing and monitoring plan would be based on the delineated AoR.

EPA is developing guidance for owners or operators on developing project-specific plans; the

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA believes it is appropriate to allow the owner or operator the flexibility to develop a site-specific

AoR and corrective action plan that takes into account the unique conditions of the site, subject to

the approval of the Director. Within the UIC Program, impacts associated with injection would

include anything that could endanger a USDW, including, but not limited to, the CO2 plume and

pressure front, displaced saline fluids, co-contaminants, and mobilized minerals.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22443

Other EPA Comment Codes and Descriptions: 4600 2.6 Proposed Plume and Pressure Front

Monitoring Requirements (40 CFR 146.90) 

Comment: Injection of CO2 into deep saline formations is driven by differences between 

the well injection pressure and formation pressure. To avoid inducing fractures 

or opening existing fractures in the formation and caprock, well injection 

pressure will be limited to some fraction below the fracturing pressure. As CO2 

migrates into the formation the formation pressure increases around the 

injection well. The pressure distribution in the formation around the injection 

well depends on the formation properties, structure, and injection rate. 

Injection formations will be required to be confined vertically and/or 

horizontally to prevent the vertical migration of injected fluids and saline 

reservoir fluids into USDWs. 

Confined formations (e.g., a caprock overlying a horizontally extensive 

formation) allow the formation fluid to be displaced laterally by the injected 

fluid. Injecting CO2 results in elevated pressures both inside and beyond the 

CO2 plume. Elevated pressures within the CO2 plume have the potential to force 

CO2 and formation fluid through permeable wells, fractures, or faults that 

penetrate the formation. Elevated pressures outside the CO2 plume have the 

potential to force formation fluid through permeable wells, fractures or faults 

that penetrate the formation. The higher the formation pressure is above the 

ambient formation pressure, the greater the potential for CO2 and formation 

fluid to leak through permeable wells, fractures, and faults. The lateral 

pressure distribution is a function of the injection rate and the lateral 

distribution of formation permeability. When CO2 injection is halted, elevated 

pressures decay over time as the injected and formation fluids migrate away from 

the injection well(s). 

The proposed regulations define an AoR and Corrective Action as being an area 

within which artificial penetrations may act as conduits for fluid movement into 

USDWs (e.g., active and abandoned wells) and, as needed, require the applicant 

40
to perform corrective action to seal these open wells. The potential for fluid 

movement into USDWs from the injection of CO2 into the injection formation is 

correlated with the area of elevated formation pressure, which is usually larger 

than the area that contains the injected CO2 mass. For confined formations, this 

area is defined by the lateral pressure distribution around the injection 

well(s). 

PNNL RECOMMENDATION 

EPA should define two regulatory regions surrounding the geologic sequestration 

project and provide specific definitions for those regions; 1) Area of 

Monitoring (AoM) and 2) Area of Review; where the AoM is used to define the 

spatial distribution of monitoring wells and the AoR is used to define the limit 

of corrective actions: 

Area of Monitoring (AoM): The region surrounding the geologic sequestration 

project that The area of monitoring is based on monitoring data contains 95% of 

the carbon dioxide mass. and computational modeling that accounts for the 

physical and chemical properties of all phases of the injected carbon dioxide 

stream. In the absence of monitoring data, the area of monitoring is based on 

computational modeling. 

Area of Review (AoR): The region surrounding the geologic sequestration 

reservoir that has or is indicated to develop pressures that are greater than 

10% different from the original hydrostatic formation fluid pressure, but no 

less than the Area of Monitoring. The area of review is based on monitoring data 

and computational modeling that accounts for the physical and chemical 

properties of all phases of the injected carbon dioxide stream. In the absence 

of monitoring data, the area of review is based on computational modeling. 

The proposed definitions of AoM and AoR have several important components: 

- The AoMhas a mass-based definition. - The AoR has a pressure-differential-based 

definition. - The AoR is never smaller than the AoM. - The AoR and AoM may be 

asymmetrical around the injection well(s). - The AoR and AoM may vary over time. - 

The carbon dioxide mass should be determined using a combination of monitoring 

data and computational modeling. - Pressure differentials are based on 

equilibrium hydrostatic conditions. 

Response: EPA believes that, while the commenter offers different terminology for the AoR and the

area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or

operator is required (under 146.90) to develop and implement a testing and monitoring plan for each

GS project that must consider the delineated AoR that includes both the CO2 plume and associated

pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies

that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical

properties of all phases of the injected CO2 stream, based on available site characterization,

monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the

testing and monitoring plan, consistent with the commenter's assertion that the AoR may vary over

41
time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at

least every 5 years, and also requires that the owner or operator periodically review the AoR and

corrective action, testing and monitoring, and emergency and remedial response plans within one

year of an AoR reevaluation (as well as following significant changes to the facility, or when

required by the Director). The relationship between the AoR reevaluation and the testing and

monitoring plan should allow for considerations suggested by the commenter. 

EPA believes that by linking development and review of the testing and monitoring plan to the AoR

evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's

considerations for an area of monitoring.

Commenter Name: Michael Stroben Letter ID: 0209

Commenter Organization: Duke Energy Comment ID: 22684

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: REEVALUATION OF AREA OF REVIEW ("AoR") 

Current UIC regulations require the owner/operator of a proposed injection site 

to determine the AoR based on the area necessary to include the calculated cone 

of pressure influence. The amount of CO2 that will be injected at a GS site over 

the life of a project will be tens if not hundreds of millions of tons. This 

quantity of CO2 along with uncertain dispersion characteristics makes it next to 

impossible to determine the correct AoR utilizing a fixed radius or simple 

mathematical method. 

Acceptable methods for determining the AoR should include but not be limited to 

multiphase fluid flow models. It is anticipated that modeling will improve over 

time as experience with CO2 sequestration increases. Demonstration projects 

should not be constrained by allowing only the use of current models. Any model 

based on accepted science that has been validated should be acceptable. Models 

should be calibrated using actual site data once it becomes available. 

The AoR for a GS site for CO2 will be dynamic and in all probability irregular 

in shape. Based on the quantity of CO2 to be sequestered at a site, the AoR 

could be hundreds of square miles. A significant amount of time and money would 

be needed to complete an up-front evaluation of the life of a project AoR. A 

large portion of this effort could be wasted since the injection plumb may not 

travel as anticipated. As a result of this uncertainty, Duke Energy recommends a 

phased evaluation for the AoR. The AoR should be reviewed periodically (every 

ten years or as agreed upon by the Director). This would allow any necessary 

corrective actions to be focused where needed. This approach would allow 

projects to proceed more expeditiously, reduce start-up costs and because of the 

periodic review, provide greater protection to USDW. 

Response: EPA clarifies that the AoR must be delineated as part of the permit application, as

required at §§146.82(a)(2) and 146.82(c)(1). The Agency believes that AoR delineation using

computational modeling that accounts for the multiphase flow of CO2 and native fluids (i.e.,

naturally occurring salty water), including density-driven flow (i.e., buoyancy), is necessary to

ensure protection of USDWs from endangerment. The computational models used for this purpose

42
are required to predict the fate of injected CO2 and changes in formation pressure. EPA believes that

the requirements at 146.84(c)(1) to delineate the AoR using computational modeling are appropriate

and encompass the significant processes that could affect the location of the CO2 plume and

associated pressure front. 

EPA agrees that modeling technology is likely to improve over time, and EPA believes that future

AoR reevaluations, as required at 146.84(e), should incorporate the best modeling available. Periodic

reevaluations also ensure that a deliberative effort is made to verify, based on monitoring and

operational data, that the CO2 plume is moving within the subsurface as predicted. However, EPA

clarifies that the AoR delineation process will not be phased. Owners or operators must delineate the

entire AoR of the project prior to receiving an operating permit. This evaluation must include the

entire region surrounding the geologic sequestration project where USDWs may be endangered by

the injection activity.

EPA agrees that phased corrective action may provide many benefits to a project including, but not

limited to, spreading the burden of cost, avoiding delays in project start-up, allowing for use of

future, improved corrective action techniques and addressing unanticipated changes in the movement

of the CO2 plume or pressure front. 

Commenter Name: John King Letter ID: 0211

Commenter Organization: Shell Exploration and Production Company

(SEPCo)

Comment ID:

22767

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: 1. AOR Definition 

The proposed rule vaguely defines the AOR as "the region surrounding" the GS 

project that "may be impacted by the injection activity." 72 Fed. Reg. at 43535 

(proposed 40 C.F.R. 146.81(d)). The definition goes on to suggest that the AOR 

is to be "based on computation modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream." Id. 

There are two issues. 

First, the proposed definition suggests the need for a more complex modeling 

approach than may typically be necessary or appropriate. As experience with 

hazardous waste injection shows, it is rarely necessary to attempt to model 

chemical changes in the injectate plume. Indeed, it is typically far easier to 

model transport than chemical fate, and conservative modeling of transport 

should typically be sufficient by itself to show that USDWs will not be 

endangered. Where this is the case, it is containment of the plume -- rather 

than the precise composition of the plume -- that matters, and modeling of 

chemical changes would be unnecessary. Similarly, the preamble suggests that the 

"most sophisticated" and most "accurate" modeling should be preferred or even 

required. 73 Fed. Reg. at 43507. Again, however, the appropriate goal is to 

ensure protection of USDWs, and simpler, more conservative modeling may often be 

sufficient (and even preferable) for this purpose. 

Second, the functional purpose of the AOR is to define the area within which 

impacts of the injection activity could cause endangerment of a USDW; 

specifically, the AOR defines the area within which corrective action for 

43
abandoned boreholes may be necessary to prevent endangerment of USDWs. The 

language of the proposed rule (i.e., "may be impacted by the injection 

activity") is not clearly limited to impacts of the injection activity that 

could cause endangerment of USDW’s, and it should be. 

Response: EPA believes that predicting the complex multiphase, buoyant flow of the CO2, coinjectates, and compounds that may be mobilized due to injection requires the sophistication of

computational models, as required at 146.84(c)(1). Models can predict CO2 fate and transport, as

well as changes in formation pressure, in three dimensions, as a function of time in order to delineate

the subsurface storage site and the AoR. Models can account for site-specific and predicted

operating conditions as well as geologic heterogeneities and potential migration through faults,

fractures, and artificial penetrations. 

EPA acknowledges that fine precision may not always be necessary and that many factors determine

ultimate confidence in modeling results. However, EPA does not believe that more precision would

result in a less protective modeling approach. EPA anticipates that the owner or operator, in

consultation with the Director, will choose the model that is most appropriate for a particular GS

project, and that this dialogue is a key factor in assuring that the GS site is modeled (and eventually

operated and tracked) in a manner that protects USDWs from endangerment. EPA encourages and

expects owners or operators to use the best multiphase, computational models available. 

Commenter Name: Diane VanDe Hei Letter ID: 0213

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22935

Comment: o Groundwater modeling is improving and variable density flow models for deeper 

aquifers can be used with reasonable accuracy for understanding subsurface flow 

and quality. However these models must take variables such as heat transport 

into account to better translate to a CO2 geosequestration scenario. 

Response: EPA agrees that heat transport may be an important process in modeling of the AoR. The

GS regulation stipulates, at 146.84(a), that the physical and chemical properties of all phases of the

injected CO2 stream must be accounted for in the model. Heat transport is one of several processes

that describe the geologic site, the injection scenario, and the fluids present, all of which are the

foundation for the multiphase, AoR model development.

Commenter Name: Edward Steadman Letter ID: 0220

Commenter Organization: The Energy and Environmental Research Center at the

University of North Dakota

Comment ID:

21590

Other EPA Comment Codes and Descriptions: 1810 1.4.1 Research on identifying risks to

USDWs 

Comment: Page 43499. One would caution rule makers on putting too much faith into any one 

institution or any one method when studying potential impacts. Should not such 

information be provided by various technical sources? While the utility of 

models and laboratory data is recognized, their results should not supersede 

real-world experience, field-based data, and local knowledge. 

Response: EPA considered a wide range of information from laboratory, modeling, and field

research as it developed the requirements for GS wells. In addition to laboratory studies and

44
modeling research that provide relevant information on GS, the Agency is also involved in collecting

and analyzing GS project data and permit information from a variety of sources. Most notably, EPA

has evaluated UIC permits and reviewed operational data and information associated with the seven

regional carbon sequestration partnerships funded by the U.S. Department of Energy and coordinated

through the National Energy Technology Laboratory (NETL), and several international projects. In

addition to the information cited in the preamble to the proposed rule, EPA published a Notice of

Data Availability and Request for Comment (NODA) on August 31, 2009 (74 FR 44802), in which

the Agency described the preliminary results of research at three DOE partnership sites: the

Escatawpa, Mississippi project; the Aneth Field project in Paradox Basin, Southeast Utah; and the

Pump Canyon Site, New Mexico. 

Regarding the suggestion that EPA give more weight to field data and experiential knowledge, EPA

notes that it continues to track ongoing field and laboratory research into all aspects of GS. For

example, EPA is tracking the operations and technical developments of more than three dozen smalland large-scale GS injection projects in the US and abroad. Also, since field experience and well

construction, injection, and monitoring information relevant to GS comes from operations that inject

acid gas or inject CO2 for ER, the Agency has conducted evaluations regarding these commercial

field operations and continues to track GS-relevant developments in these commercial operations.

The Agency will continue to track GS project developments and evaluate new and ongoing research

and information to ensure USDW protection. EPA is using an adaptive approach to regulating GS

under the UIC Program. Even after finalization of the GS regulation, the Agency plans to collect and

review data every six years on GS projects to determine whether the appropriate amount and types of

information and appropriate documentation are being collected, and to determine if modifications to

the finalized requirements are appropriate or necessary.

EPA agrees that AoR modeling should be based on actual data collected on the site, including

detailed geologic data collected to characterize the injection zone, confining zone and any additional

zones, as required at 146.84(c)(1)(i). The Agency believes that the integration of site specific

information into a predictive multiphase computational model will ensure the best possible AoR

delineation. EPA also recognizes that modeling technology will continue to improve, and encourages

and expects owners or operators to use the best available multiphase computational models to

determine the AoR. Reliance on improved models will likely increase the accuracy and quality of the

AoR characterization, resulting in increased protection of USDWs from endangerment.

Commenter Name: Edward Steadman Letter ID: 0220

Commenter Organization: The Energy and Environmental Research Center at the

University of North Dakota

Comment ID:

21592

Other EPA Comment Codes and Descriptions: 1810 1.4.1 Research on identifying risks to

USDWs 

Comment: Page 43500. Regarding the basis of some of the rules related to modeling Much 

has happened in the world of modeling CO2 injection and long-term storage since 

2005. Many of the key messages from the workshop upon which some of the 

statements on this topic are based may be long out of date. 

Response: EPA continues to review new and ongoing modeling studies, including those conducted

at national research laboratories, such as Lawrence Berkeley National Laboratories as well as data

from the DOE Partnership GS pilot projects where some of the most current models are being

developed and/or used. EPA published a Notice of Data Availability and Request for Comment on

45
August 31, 2009 (74 FR 44802), in which the Agency described two recent modeling studies by the

Lawrence Berkeley National Laboratory on the potential effects of CO2 injection on USDWs and

human health in the event that a GS project is not properly sited, operated, or managed. These

studies are in the rulemaking docket and on LBNL's website at

http://esd.lbl.gov/GCS/projects/CO2/index_CO2.html. Additionally, EPA continues to fund research

on ground water that will integrate experimental and modeling efforts with the objective of

evaluating the potential consequences of CO2 leaks from GS operations into ground water resources.

EPA is using an adaptive approach to regulating GS under the UIC Program. Even after finalization

of the GS regulation, EPA plans to collect and review data every six years on GS projects to

determine whether the appropriate amount and types of information and appropriate documentation

are being collected, and to determine if modifications to the requirements are appropriate or

necessary.

As described above, EPA recognizes that modeling technology will continue to improve, and

encourages and expects owners or operators to use the best available multiphase computational

models to determine the AoR. Reliance on improved models will likely increase the accuracy and

quality of the AoR characterization, resulting in increased protection of USDWs from endangerment.

Commenter Name: Edward Mehnert Letter ID: 0221

Commenter Organization: Illinois State Geological Survey Comment ID: 21637

Other EPA Comment Codes and Descriptions: 4600 2.6 Proposed Plume and Pressure Front

Monitoring Requirements (40 CFR 146.90) 

Comment: p.43506, IIIA2 Proposed Area of Review: Does the area of review need to be one 

contiguous area? For a deep saline aquifer with no structural trap, it may be 

beneficial to have two separate areas of review--one for the CO2 plume and 

another for the pressure front at a distant USDW boundary. In this setting, the 

pressure front may move brine upgradient into a portion of the aquifer that 

contains fresher water (<10,000 mg/L TDS). The distance between the injection 

well and the "freshwater" section of the aquifer may be more than 100 miles. 

Response: EPA clarifies that the requirements for AoR delineation incorporate both the CO2 plume

and associated pressure front. EPA agrees that both of these should be taken into account in

determining corrective action needs and developing a testing and monitoring plan, and that the

requirement to delineate the AoR at 146.84(c) addresses the potential risks raised by the

commenters. EPA believes that the final GS requirements for delineating the AoR provide sufficient

protection to USDWs, without introducing an overly complex regulatory process.

Commenter Name: Edward Mehnert Letter ID: 0221

Commenter Organization: Illinois State Geological Survey Comment ID: 21638

Comment: p.43506, IIIA2 Proposed Area of Review: I agree that models offer the best 

approach to predict the potential effect of CO2 injection into an aquifer. What 

happens when multiple injection wells are operated in the same aquifer such that 

well interference is an issue? How do you manage this situation if different 

models are utilized? As an alternative to many stand-alone models, can a 

regional model with common boundary and initial conditions be developed and then 

a local model with detailed site data would be added to this regional model? 

46
The Twin Cities Metropolitian Groundwater Flow model 

(http://www.pca.state.mn.us/water/groundwater/metromodel.html) is an example of 

this concept. 

Response: EPA agrees that the impacts of multiple operators injecting into hydraulically connected

formations must be accounted for in the AoR modeling. EPA adds that the owner or operator must

demonstrate in the proposed AoR and corrective action plan to the satisfaction of the Director that

the modeled AoR will be based on detailed site-specific geologic data collected during the site

characterization process including consideration of existing conditions (e.g., current injection

activities). EPA anticipates that, where multiple operators are involved, the Director would not

approve modeling that does not account for changes that may result from all injection operations in a

formation or a series of hydraulically connected formations. EPA acknowledges the commenter's

reference to the Twin Cities Metropolitan Groundwater Flow model. The Agency anticipates that

many such examples will be developed as GS projects are deployed and new modeling products are

developed to address this new application. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Ben Yamagata Letter ID: 0227

Commenter Organization: Coal Utilization Research Council Comment ID: 21787

Other EPA Comment Codes and Descriptions: 3330 2.2.1.3 Applicability /adequacy of

computational models (seeking comment) 

Comment: Area of Review (AoR) 

The proposed regulation does not actually define the AoR, it merely states that 

the area will be determined by mathematical models. One of the concerns with 

the use of such complex models is that the site may not be fully characterized 

with respect to hydro-geologic properties. There may be significant uncertainty 

in one or more hydraulic process/parameters. Therefore, it is possible to 

obtain a wide-range of model outcomes from different model parameters. The 

treatment of uncertainty in model parameters and sensitivity of model results to 

parameter selection should be addressed. Given that the AoR will be 

periodically revisited, we recommend that the final rule make a pragmatic 

determination of what will be necessary to protect groundwater, given the 

requirement for reevaluation over time, and that the Agency focus the 

requirements to rely upon the mathematical models with the inherent differences 

noted above. 

Response: EPA agrees that modeling results are prone to varied degrees of uncertainty. As discussed

in the preamble to the proposed GS regulation and the associated background document

"Computational Modeling of Underground Injection of Carbon Dioxide for Determination of Areaof-Review and Potential Risk to Underground Sources of Drinking Water" (cited in the proposed

rule preamble as USEPA, 2008d), model predictions depend largely on the values of key parameters.

47
Often these parameter values are estimated or averaged from several data sources. EPA also agrees

that sensitivity analyses and "worst case scenario" modeling runs should be presented by the owner

or operator and evaluated by the Director. If during the permit review, the Director determines that

modeling results are too wide-ranging, further information will be required of the owner or operator. 

EPA clarifies that models are tools to help predict the movement of a CO2 plume over long time

frames. However, EPA recognizes that it cannot be expected that modeling something as complex as

a CO2 plume can be done with complete certainty; therefore EPA is also requiring that the AoR be

reevaluated to verify the CO2 plume is moving as predicted and to allow for model calibration and

necessary revisions to the required GS project plans (e.g., AoR and corrective action, testing and

monitoring, injection well plugging, post-injection site care and site closure, and emergency and

remedial response). EPA believes that near-term predictions (e.g., over the subsequent 5 years)

would have the highest degree of certainty. Periodic model calibration would allow refinement to the

AoR delineation based on real-world conditions and operating data and monitoring results, and thus

increase confidence in the modeled predictions. EPA adds that the final GS regulation includes a

requirement at 146.84(e) that the AoR be reevaluated every 5 years (an adjustment from the ten year

interval in the proposal), which should further address the commenter’s concerns regarding model

uncertainty.

Commenter Name: Neeraj Gupta Letter ID: 0229

Commenter Organization: Battelle Energy Technology (BET) Comment ID: 21831

Comment: Page 64, Area of Review Determination -More clarity on determining the AOR is 

requested. Currently it is not clear whether AOR will be based on CO2 phase 

spreading, dissolved CO2 plume, or pressure footprint. Models provide a large 

amount of information such as CO2 gas saturation, pressure, and aqueous 

concentrations. They also involve a fair amount of uncertainty. Our experience 

is that small portion of the CO2 may extend or "finger" much further than the 

bulk of the injected CO2. For example would EPA accept an AOR with a radius 

containing 90% of the CO2 as supercritical gas saturation, so that the 

applicants do not have to account for small-scale fingering or heterogeneities? 

Response: EPA disagrees with defining the AoR relative to the percentage of CO2 saturation. The

UIC regulations, including the Class VI regulations, are designed to ensure protection of USDWs

from endangerment. The Agency believes that the AoR needs to incorporate not only the CO2 itself

—regardless of saturation or phase—but also the area of increased pressure ahead of the CO2 plume,

which can potentially displace saline, native fluids (i.e., naturally occurring salty water) into

USDWs. This pressure front would typically encompass an area larger than the plume itself,

including any areas of small scale fingering or heterogeneities or where CO2 saturation is less than

100 percent. The Agency has carefully considered all comments on the AoR definition at 146.84(a)

and modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

EPA clarifies that the definition of AoR includes all areas that may potentially be endangered by the

injection activity. EPA recognizes that modeled AoRs may involve some uncertainty or “fingering,”

as the commenter mentions. Any needed corrective action must be done on wells in the modeled

AoR, which would include these areas to ensure that they address any area that would be impacted

by the CO2 plume and pressure front.

48
Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23328

Comment: VIII. The Definition of the Area of Review Should Include Monitoring and 

Modeling. 

The definition of Area of Review (AOR), Fed. Reg 43536, CFR §146.84, Part (a) 

states that the Area of Review is "based on computational modeling". It is not 

appropriate for the Area of Review to be based solely on computational modeling. 

Instead, it should be based on computational modeling, monitoring and 

operational data and updated until closure is approved. All references to 

computational modeling in relation to the Area of Review in this section, as 

well as in the definition of Area of Review in §146.81 should be amended as 

appropriate to read "based on computational modeling, monitoring and operational 

data and updated until closure is approved". 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

The definition in the proposed rule accounted for the varying physical and chemical properties of the

CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The definition in

the final rule will continue to do so. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA clarifies that the initial AoR delineation required under 146.84(c) is performed prior to

injection, and therefore no monitoring or operational data would yet exist. The AoR delineation

models will incorporate all available site data and anticipated operating procedures (e.g., injection

rates and volumes). 

The AoR reevaluation process required at 146.84(e) requires owners or operators to incorporate

monitoring or operational data and make any needed adjustments to corrective action or monitoring

at the site. EPA believes that the iterative process of AoR reevaluation is appropriate to verify that

the CO2 plume and pressure front are moving through the subsurface as predicted, verify the

accuracy of the delineation model (or identify needed changes), and refine testing and monitoring

activities to ensure that USDWs are protected from endangerment and that there would be adequate

warning of potential problems at the site before any endangerment could occur. 

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22306

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

49
Comment: The area of review provisions should be revised in several respects 

Surprisingly, the definition of "area of review" (AOR) in §146.81(d) fails to 

mention that elevated pressure a critical aspect of what it means for an area to 

be "impacted" by injection activity. Our Environmental NGO Group recommends 

borrowing language from the proposed definition of "geologic sequestration 

project" and reworking the "area of review" definition. 

Response: Although EPA disagrees with the commenter's suggested change, the Agency has

carefully considered all comments on the AoR definition at 146.84(a) and modified it to read as

follows: The region surrounding the geologic sequestration project where USDWs may be

endangered by the injection activity. The area of review is delineated using computational modeling

that accounts for the physical and chemical properties of all phases of the injected CO2 stream and is

based on available site characterization, monitoring, and operational data. EPA believes that this

definition is sufficiently broad and describes both the intent of the computational modeling and the

relationship of the AoR to areas that are subject to the risks associated with injection.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22307

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: An additional concern is that both §146.81 and §146.84(a) state that the AOR is 

"based on computational modeling". This could imply that the Area of Review 

could be based solely on computational modeling. Instead, the AOR should be 

based on computational modeling, monitoring and operational data (and kept 

updated until closure is approved). All references to computational modeling in 

relation to the Area of Review in this section, as well as in the definition of 

Area of Review in §146.81 should be amended as appropriate to read "based on 

computational modeling, as well as monitoring and other operational data." We 

urge that the definition be revised as follows: 

[In the paragraph below the text 

in all CAPS denotes added text. See also EPA-HQ-OW-0390-0250.pdf to see the 

original formatting.] 

"Area of review means the AREA that may be impacted by the injection activity. 

The area of review is based on computational modeling, as well as MONITORING AND 

OTHER OPERATIONAL DATA, that account for the physical and chemical properties of 

all phases of the injected carbon dioxide stream. IT INCLUDES THE SUBSURFACE 

THREE-DIMENSIONAL EXTENT, THROUGHOUT THE OPERATING LIFE OF THE

FACILITY, OF THE 

CARBON DIOXIDE STREAM PLUME, ASSOCIATED PRESSURE FRONT, AND DISPLACED

FORMATION 

FLUIDS, AS WELL AS THE OVERLYING FORMATIONS AND SURFACE AREA ABOVE

THAT 

DELINEATED REGION." 

Response: Based on review of all public comments on the AoR definition at 146.84(a), the Agency

has modified it to read as follows: "The region surrounding the geologic sequestration project where

50
USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational

data." The definition in the proposed rule accounted for the varying physical and chemical properties

of the CO2 including free-phase and water-dissolved CO2, as the commenter suggests. The

definition in the final rule will continue to do so.

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under 146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA clarifies that the initial AoR delineation required under 146.84(c) is performed prior to

injection, and therefore no monitoring or operational data would yet exist. The AoR delineation

models will incorporate all available site data and anticipated operating procedures (e.g., injection

rates and volumes). 

The AoR reevaluation process required at 146.84(e) requires owners or operators to incorporate

monitoring or operational data and make any needed adjustments to corrective action or monitoring

at the site. EPA believes that the iterative process of AoR reevaluation is appropriate to verify that

the CO2 plume and pressure front are moving through the subsurface as predicted, verify the

accuracy of the delineation model (or identify needed changes), and refine testing and monitoring

activities to ensure that USDWs are protected from endangerment and that there would be adequate

warning of potential problems at the site before any endangerment could occur.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24856

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rationale: Insert the definition "Computational modeling means a mathematical 

model that is appropriate to the site geometry and takes into account the 

differences between the formation and injection fluids." 

Response: The final rule does not define computational modeling. The preamble to the proposed GS

regulation and the modeling background document "Computational Modeling of Underground

Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk to

Underground Sources of Drinking Water" (cited in the proposed rule preamble as USEPA, 2008d

include FR citation) discuss, in detail, what EPA believes constitutes a computational model.

Appropriate models may incorporate numerical, analytical, or semi-analytical approaches. These

models solve a series of governing equations to predict the composition and volumetric fraction of

each phase state, as well as fluid pressures, as a function of location and time for a particular set of

conditions. EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist

owners or operators and Directors in evaluating requirements and proposed AoR and corrective

action plans under 146.84. The guidance describes the data that EPA recommends to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

51
http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24881

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84 

EPA Proposal: 146.84. Area of review and corrective action 

(a) The area of review is the region surrounding the geologic sequestration 

project that may be impacted by the injection activity. The area of review is 

based on computational modeling that accounts for the physical and chemical 

properties of all phases of the injected carbon dioxide stream. 

GWPC Recommendations: (a) The area of review is based on computational modeling 

that accounts for the physical and chemical properties of all phases of the 

injected carbon dioxide stream. 

Rationale: This is the same as the definition of "Area of review" that was 

revised by the GWPC .to delete the first sentence. 

Comments on many provisions are predicated on proposed changes in definitions 

Response: The Agency has carefully considered all comments on the AoR definition at 146.84(a)

and modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24885

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c) 

(c) Owners or operators of Class VI wells must perform the following actions to 

delineate the area of review, identify all wells that require corrective action, 

and perform corrective action on those wells: 

(1) Predict, using computational modeling, the projected lateral and vertical 

migration of the carbon dioxide plume and formation fluids in the subsurface 

from the commencement of injection activities until the plume movement ceases, 

pressure differentials sufficient to cause the movement of injected fluids or 

formation fluids into a USDW are no longer present, or after a fixed time period 

as determined by the Director. The model must: 

GWPC Recommendations: (c) Owners or operators of Class VI wells must perform the 

52
following actions to delineate the area of review, identify all wells that 

require corrective action, and perform corrective action on those wells: 

(1) Predict, using computational modeling, the projected lateral and vertical 

migration of the carbon dioxide plume and formation fluids in the subsurface 

from the commencement of injection activities until the plume movement ceases, 

pressure differentials sufficient to cause the movement of injected fluids or 

formation fluids into a USDW are no longer present, or for a fixed time period 

as determined by the Director. The model must: 

Rationale: Should say "for" not "after." 

Response: Based on its evaluation of all public comments and input on the proposed rule, EPA has

clarified this language in the final rule. It now reads, “Predict, using existing site characterization,

monitoring and operational data, and computational modeling, the projected lateral and vertical

migration of the carbon dioxide plume and formation fluids in the subsurface from the

commencement of injection activities until the plume movement ceases, until pressure differentials

sufficient to cause the movement of injected fluids or formation fluids into a USDW are no longer

present, or until the end of a fixed time period as determined by the Director.”

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24886

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(i) 

EPA Proposal: (i) Be based on detailed geologic data collected to characterize 

the injection zone, confining zone and any additional zones; and anticipated 

operating data, including injection pressures, rates and total volumes over the 

proposed life of the geological sequestration project; 

(ii) Take into account any geologic heterogeneities, data quality, and their 

possible impact on model predictions; and 

(iii) Consider potential migration through faults, fractures, and artificial 

penetrations. 

GWPC Recommendations: (i) Be based on detailed geologic data collected to 

characterize the injection zone, confining zone; and anticipated operating data, 

including injection pressures, rates and total volumes over the duration of 

injection; 

(ii) Take into account relevant geologic heterogeneities, and data quality, and 

their possible impact on model predictions; and 

(iii) Consider potential migration through faults, fractures, and artificial 

penetrations. 

Rationale: Suggest deletion of "any additional zones". You do not need to 

53
include any additional zones if you use the current CFR definition of confining 

zone since it includes multiple zones. 

Should be geologic not geological for consistency with other provisions because 

geologic sequestration project is defined. 

Suggest "duration of injection" instead of "life of the geological sequestration 

project" 

Suggest "relevant" instead of any in (ii) 

Insert "and" between "heterogeneities" and "data" 

Response: EPA agrees that detailed geologic data is invaluable to modeling an accurate AoR. The

Agency believes that the AoR delineation requirements in 146.84 (c)(1)(i), which specify using

"detailed geologic data collected to characterize the injection zone, confining zone and any

additional zones; and anticipated operating data...," and (c)(iii) to "consider potential migration

through faults, fractures and artificial penetrations," provide the necessary information needed for

appropriate site modeling, subsequent site evaluation, and protection of USDWs from endangerment.

EPA adds that the extensive requirements for data that is to be included in the permit application at

146.82 incorporate these same types of information. The Director will review the AoR models in the

context of the extensive site data submitted with the permit application. EPA recognizes that in some

instances, the Director may require additional types of data to be included in the AoR delineation

(146.84(c)(1)). Based on its evaluation of all public comments and input on the proposed rule and

NODA and Request for Comment EPA has made many revisions to the GS Rule. See the final rule

and preamble for further discussion on EPA's final approach.

EPA disagrees that only the injection and confining zones should be included in AoR modeling of

GS sites. EPA believes that the modeling should also take into account the possibility of displaced

fluids or brines caused by a lateral and/or vertical pressure plume, and therefore it may be necessary

to include additional zones along with the confining/injection zones to ensure the integrity of the

confining zone and ultimately protect a USDW from endangerment. 

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24892

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(g) 

(g) The emergency and remedial response plan (as required by § 146.94) and a 

demonstration of financial responsibility (as described by § 146.85) must 

account for the entire area of review, regardless of whether or not corrective 

action in the area of review is phased. 

GWPC Recommendations: (g) The emergency and remedial response plan (as required 

by § 146.94) and a demonstration of financial responsibility (as described by § 

146.85) must account for the entire area of review [as modified], regardless of 

whether or not corrective action in the area of review is phased. 

54
Rationale: Need to add phrase to account for the periodic reevaluations and 

possible revisions that will be occurring. Addition of the term [as modified] 

addresses this concern. 

Response: EPA agrees with the commenter and has modified the final rule at §146.84(f) to require

the owner or operator to periodically review the emergency and remedial response plan to take into

account the most recent AoR. Owners or operators must also periodically update the demonstration

of financial responsibility to account for any such changes.

Commenter Name: John McManus Letter ID: 0294

Commenter Organization: American Electric Power Comment ID: 25313

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: 5. Area of review definition and basis 

We also suggest that the second sentence of the proposed definition of GS 

project be transferred to the definition of area of review as recommended below. 

This language also reflects a change from "brine" to "formation fluids." 

"Area of review means the subsurface three-dimensional extent of the carbon 

dioxide stream plume and the associated pressure front, as well as the overlying 

formations, [any USDWs underlying an injection zone along with any intervening 

formations,] [FN2] and the surface area above that delineated region." 

[FN2] The bracketed language should be included to the extent that the 

regulations authorize GS above USDWs, an issue on which the signatories have 

expressed differing views in our respective comments on the proposed rule. See 

footnote 1 above. 

In addition, we recommend that the second sentence included both in EPA’s 

proposed definition of area of review and in section 146.84(a) be dropped from 

the definition and retained in section 146.84(a) but modified to read as 

follows: 

"(a) The area of review is based on computational modeling, as well as 

monitoring and other operational data, that account for the physical and 

chemical properties of all phases of the injected carbon dioxide stream." 

The first sentence of section 146.84(a), which duplicates the definition of area 

of review, should be deleted from section 146.84(a) and retained in the 

definition with the modifications presented above. 

Response: EPA disagrees that the AoR definition needs to specifically include the overlying

formations and surface area above the delineated region. The intent of the AoR, a long-standing

concept within the UIC Program, is to delineate the surface area and all subsurface formations below

it that may potentially be impacted by injection activity. Acceptable delineation models will provide

predictions of CO2 fate and transport, as well as changes in formation pressure, in three dimensions

as a function of time that can be used to delineate the subsurface storage site and the AoR. 

Although EPA disagrees with the commenter's suggested change, the Agency has carefully

55
considered all comments on the AoR definition at 146.84(a) and modified it to read as follows: The

region surrounding the geologic sequestration project where USDWs may be endangered by the

injection activity. The area of review is delineated using computational modeling that accounts for

the physical and chemical properties of all phases of the injected CO2 stream and is based on

available site characterization, monitoring, and operational data. EPA believes that this definition is

sufficiently broad and describes both the intent of the computational modeling and the relationship

of the AoR to areas that are subject to the risks associated with injection. 

Commenter Name: Scott Anderson Letter ID: 0520

Commenter Organization: Environmental Defense Fund Comment ID: 21430

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: We think the AOR's area of reviews need to be calculated as intended and updated 

as necessary. We think ten year updates is far too infrequent and we think that 

there are some technical problems with the way the AOR is defined in the rule. 

We believe the EPA's intention is to use elevated pressure as the main guide to 

AOR delineation, but in our judgment the definition just does not say that. 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at 146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. 

EPA agrees that the AoR definition should account for the impact of the CO2 plume on USDWs.

The Agency has carefully considered all comments on the AoR definition at 146.84(a) and modified

it to read as follows: The region surrounding the geologic sequestration project where USDWs may

be endangered by the injection activity. The area of review is delineated using computational

modeling that accounts for the physical and chemical properties of all phases of the injected CO2

stream and is based on available site characterization, monitoring, and operational data. 

Comment Code: 3310 2.2.1.1 Comments in favor of requiring computational models to delineate

the AoR (seeking comment) 

Individual Comments

Commenter Name: Doug Louis Letter ID: 0121

Commenter Organization: Kansas Corporation Commission Comment ID: 22078

Comment: KCC Staff agrees the most accurate and realistic method of determining AOR uses 

computational models, which are updated regularly in conjunction with monitoring 

of physical parameters. A prescribed fixed radius surrounding individual wells 

would not be an appropriate determination of AOR for a complex dynamic CO2 

storage facility. 

56
Response: EPA agrees that predicting the complex multiphase, buoyant flow of the CO2, coinjectates, and compounds that may be mobilized due to injection requires the sophistication of

computational models, as required at §146.84(c)(1). Models can predict CO2 fate and transport, as

well as changes in formation pressure, in three dimensions, as a function of time in order to delineate

the subsurface storage site and the AoR. Models can account for site-specific and predicted

operating conditions as well as geologic heterogeneities and potential migration through faults,

fractures, and artificial penetrations.

Commenter Name: John Stormon Letter ID: 0123

Commenter Organization: Washington State Dept. of Ecology Comment ID: 23002

Other EPA Comment Codes and Descriptions: 4600 2.6 Proposed Plume and Pressure Front

Monitoring Requirements (40 CFR 146.90) 

Comment: Area of Review: The proposed standards correctly require not just modeling, but 

tracking of the actual CO2 plume and pressure front, with periodic reevaluation 

of the modeled behavior as additional experience is gained at a sequestration 

project. Modeling is a good method of predicting the behavior of injected CO2 

but should not be relied upon without monitoring to verify the accuracy of the 

modeled predictions. The model will be continually refined throughout the life a 

sequestration project. 

Response: EPA acknowledges the commenter’s support of the modeling approach, and agrees that

reevaluation of the AoR as required at 146.84(e) must incorporate monitoring and operational data

gathered since the last model run. 

EPA agrees with commenters on the importance of monitoring at GS sites and emphasizes that

monitoring of injection activities will be an important component of every GS project. Monitoring

associated with injection wells and the movement of the CO2 plume is necessary to verify that the

injectate is confined in the target formations and does not endanger USDWs. 

EPA agrees that monitoring data should be used to review model assumptions, verify computational

model projections, and, when necessary, provide input for model refinements. As data are collected

at an injection site, model parameters can be adjusted to match observations (i.e., for model

calibration), which in turn improves the predictive capability of the model. As a GS project proceeds,

both monitoring and modeling will evolve, with monitoring results supporting effective AoR

reevaluations and leading to improved model predictions.

Commenter Name: Dwight Lockwood, PE, QEP Letter ID: 0141

Commenter Organization: Global Energy, Inc. Comment ID: 23107

Comment: Modeling 

1.We support computational fluid flow modeling during injection well site 

selection, as well as conformational modeling to confirm performance from time 

to time. This is related to the periodic re-evaluation of the Area of Review. 

Response: EPA acknowledges the commenter’s support of the modeling approach, and agrees that

reevaluation of the AoR as required at 146.84(e) must incorporate monitoring and operational data

gathered since the last model run. This will serve to verify that the CO2 plume and pressure front are

moving through the subsurface as predicted, that the delineation model is accurate (or identify

needed changes), and refine testing and monitoring activities to ensure that USDWs are protected

57
from endangerment and that there would be adequate warning of potential problems at the site before

any endangerment could occur.

Commenter Name: Dwight Lockwood, PE, QEP Letter ID: 0141

Commenter Organization: Global Energy, Inc. Comment ID: 23109

Comment: 2.There is an increasing body of knowledge that computational fluid flow 

modeling is feasible and can inform an injection operation about performance, 

and point toward changes that may be appropriate. We are familiar with those 

involved with the Weyburn Field in Canada, mentioned by EPA. 

Response: EPA also agrees that numerical computation, as required at 146.84(c)(1), may be

necessary to calculate the complex AoR for a GS project. EPA believes that predicting the complex

multiphase buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models and in some cases may specifically

require certain numerical models. EPA also agrees that physical measurement, (i.e. monitoring) is an

important component to protecting USDWs from endangerment and will serve to calibrate

computational model predictions during the AoR reevaluation to ensure that the AoR adequately

represents the extent of the CO2 plume and pressure front.

Commenter Name: Brianne Metzger Letter ID: 0151

Commenter Organization: Spectra Energy Corp Comment ID: 21819

Comment: B. The Area of Review Should Be Determined Based On Computational Fluid Flow 

Models And Reevaluation Should Occur At A Fixed Frequency Not To Exceed 10 

Years. 

We agree with the Agency’s current position that the application of 

computational fluid flow models to predict the complex multi-phase buoyant flow 

of CO2, co-injectates, and compounds is an acceptable and appropriate approach 

to mapping out the Area of Review (AoR). Such computational models are 

currently being used by industry with success. Further, industry continues to 

refine the accuracy of these models as the data and science are improved. 

Response: EPA acknowledges the commenter’s support and believes that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at 146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22111

Comment: 11. EPA seeks comment on the use of models for area of review delineation. MDEQ 

agrees that such modeling is an appropriate approach. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

58
injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22128

Comment: 146.84(2)(i): MDEQ suggests a change in timeframe. See General Comment 11 . 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at 146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS.

Commenter Name: James Dooley Letter ID: 0156

Commenter Organization: Joint Global Change Research Institute, Pacific

Northwest National Laboratory

Comment ID:

22616

Comment: Proposed Requirement of Computational Models to Support Permit Is Extremely 

Appropriate Requirement 

The USEPA should be commended for requiring the use of computational models as a 

part of the permitting process and for the renewal of permits as well as a means 

of demonstrating compliance with the permit. These computational models are 

sufficiently well developed at this point in time that they can provide a robust 

set of technical information that will be needed for issuing Class VI permits. 

While the use of these models is a highly technical enterprise and will most 

likely be something that permitees will contract out, the cost of using these 

models is trivial compared to the capital cost of a complete end to end CCS 

system or the cost of purchasing enough emissions offsets to cover the CO2 

emissions from something like a large power plant over its multidecade lifetime 

if that facility chose not to implement CCS. It is extremely appropriate to 

require the use of these sophisticated state of the art modeling tools. 

Page 43506. The USEPA should also be commended for explicitly ruling out the 

use of simple "fixed radius or simple mathematical computations" to compute the 

AOR for large scale GS projects. Sophisticated computational tools exists which 

can do AOR calculations while taking into account a large number of key variable 

and processes. These tools and not a back of the envelope calculation should be 

required. It is appropriate for EPA to explicitly forbid the use of these 

simple methodologies for Class VI AOR. By disallowing these simple methods, the 

USEPA is not materially increasing the cost of employing CCS. While this 

argument might be put forward by others as a reason to keep these simple AOR 

tools, it is extremely hard to imagine a private sector firm risking billions to 

59
tens of billions of dollars of capital needed to construct and operate a CCSenabled power plant or industrial facility without employing sophisticated 

geochemical/geomechanical tools to assess a prospective GS reservoir. So the 

USEPA is simply asking for access to data that will most likely already exists 

with the applicant and is therefore not creating a new cost for CCS deployment. 

Response: EPA acknowledges the commenter’s support and believes that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at 146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Michael J. Miller Letter ID: 0168

Commenter Organization: Electric Power Research Institute (EPRI) Comment ID: 22499

Comment: Preamble, page 43507: "EPA seeks comment on the applicability of computational 

fluid flow models for delineating the AoR of GS sites." 

EPRI COMMENT #6: We believe that the use of computer models as a tool to predict 

the size of the area of review (AoR) is appropriate and provides a flexible, 

phased approach to assessing the number of abandoned wells requiring mitigation. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Frederick R. Eames Letter ID: 0178

Commenter Organization: CCS Alliance Comment ID: 23854

Comment: 1. Area of Review 

The CCS Alliance agrees that traditional AoR delineation methods such as a fixed 

radius or simple mathematical computations would not he sufficient to predict 

the extent of CO2 plume movement. The CCS Alliance therefore supports the use of 

computational fluid flow modeling designed for the specific site conditions and 

injection regime to delineate the AoR for CO2 GS sites. Such modeling will 

provide the level of sophistication necessary to predict the complex multi-phase 

buoyant flow of the CO2, injectates, and mobilizing compounds. 

Response: EPA acknowledges the commenter’s support and believes that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at 146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

60
for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24333

Comment: B. Area of Review and Corrective Action Requirements 

We support the use of computational models for determination of the appropriate 

area of review ("AOR") for GS purposes. Modeling of the pressure effects 

associated with injection - together with conservative modeling to bound the 

potential extent of CO2 plume movement - are appropriate aids to the evaluation 

of geologic siting and corrective action to address risks presented by 

artificial penetrations such as abandoned boreholes. The use of such modeling is 

already well-established in the context of oil and gas exploration and 

production and has been used in the UIC program for "no-migration" petitions for 

hazardous waste injection. Similar modeling can reasonably be adopted for use in 

the GS context as well. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex,

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). that may

be mobilized due to injection requires the sophistication of computational models. Models can

predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as a

function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24334

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: 1. AOR Definition 

The proposed rule vaguely defines the AOR as "the region surrounding" the GS 

project that "may be impacted by the injection activity." 72 Fed. Reg. at 43535 

(proposed 40 C.F.R. 146.81(d)). The definition goes on to suggest that the AOR 

is to be "based on computational modeling that accounts for the physical and 

chemical properties of all phases of the injected carbon dioxide stream." Id. 

There are three issues. 

First, the proposed definition suggests the need for a more complex modeling 

approach than may typically be necessary or appropriate. As experience with 

hazardous waste injection "no migration" petitions shows, it is rarely necessary 

to attempt to model chemical changes in the injectate plume; instead 

conservative modeling of transport should typically be sufficient by itself to 

show that USDWs will not be endangered. Where this is the case, it is 

containment of the injectate plume - rather than the precise composition of the 

plume - that matters and there is no need to model chemical changes. This is a 

61
critical point, because it is generally far easier to model transport than 

chemical changes occurring in the subsurface. In fact, it would be 

extraordinarily burdensome to have to model "the chemical properties of all 

phases of the injected carbon dioxide stream" as the proposed rule suggests. 

Such modeling should be required only to the extent that it is truly necessary 

to ensure that USDWs will be protected. 

Second, the preamble to the proposal suggests that the "best" (most 

sophisticated?) and most "accurate" modeling should be preferred or even 

required. 73 Fed. Reg. at 43507. However, fine precision generally isn't 

necessary, The appropriate goal for modeling is to ensure protection of USDWs, 

and conservative (i.e., less precise) modeling may be sufficient and even 

preferable for this purpose. A quest for greater precision would not guarantee 

greater confidence in modeling results, and could effectively require a less 

conservative - and thus potentially less protective - modeling approach. 

Third, the functional purpose of the AOR is to define the area within which 

impacts of the injection activity could cause endangerment of a USDW; 

specifically, the AOR defines the area within which corrective action for 

abandoned boreholes may be necessary to prevent endangerment of USDWs. The 

language of the proposed rule (i.e., "may be impacted by the injection 

activity") is not clearly limited to impacts of the injection activity that 

could cause endangerment of USDW’s. It should be. 

Response: EPA believes that predicting the complex, multiphase, buoyant flow of the CO2, coinjectates, and compounds that may be mobilized due to injection requires the sophistication of

computational models, as required at 146.84(c)(1). Models can predict CO2 fate and transport, as

well as changes in formation pressure, in three dimensions, as a function of time in order to delineate

the subsurface storage site and the AoR. Models can account for site-specific and predicted

operating conditions as well as geologic heterogeneities and potential migration through faults,

fractures, and artificial penetrations. 

EPA acknowledges that fine precision may not always be necessary and that many factors determine

ultimate confidence in modeling results. However, EPA does not believe that more precision would

result in a less protective modeling approach. EPA anticipates that the owner or operator, in

consultation with the Director, will choose the model that is most appropriate for a particular GS

project, and that this dialogue is a key factor in assuring that the GS site is modeled (and eventually

operated and tracked) in a manner that protects USDWs from endangerment. EPA encourages and

expects owners or operators to use the best multiphase, computational models available. 

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22434

Comment: Using Multiphase Computational Models to Delineate the AoR. . . . . . . Page 7 

We support the use of sophisticated, multiphase, computational models designed 

for site-specific conditions for AoR delineation, as specified by EPA in the 

proposed regulations. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

62
injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22445

Comment: USING MULTIPHASE COMPUTATIONAL MODELS TO DELINEATE THE AoR 

PROPOSED REGULATION 

p. 43506, c. 1 Current UIC regulations require that the owner or operator define 

the Area of Review (AoR), within which the owner or operator must identify all 

penetrations (regardless of property ownership) in the confining zone and the 

injection zone and determine whether they have been properly completed or 

plugged. 

p. 43506, c. 2 EPA believes that predicting the complex multi-phase buoyant flow 

of the CO2, coinjectates, and compounds that may be mobilized due to injection 

requires the sophistication of multiphase computational models. EPA proposes that 

the owners or operators of GS wells delineate the AoR for CO2 GS sites using 

computational fluid flow models designed for the specific site conditions 

and injection regime. 

DISCUSSION 

The use of multiphase computational models for prediction of plume areas has been 

supported by several groups, including the Intergovernmental Panel on Climate 

Change (IPCC, (2005), which suggested the use of modeling for evaluation of the 

long-term distribution of CO2 in the subsurface. This modeling could be used as 

the basis for identification of the AoR for a given site. The IPCC report makes 

clear that the models used for these efforts should be multiphase models that 

incorporate the important physical, chemical, and geomechanical processes 

present at a given site (IPCC 2005). 

Whereas a prescribed radius or simple mathematical computation, as included in 

the current UIC regulations, may be adequate for some UIC sites, research and 

experience in modeling CO2 sequestration leads us to conclude that these 

simplified approaches are clearly not sufficient to predict the extent of an 

injected CO2 plume. EPA should be commended for requiring the use of multiphase 

computational models for delineating the AoR. We know that initial models of the 

Sleipner site suggested relatively simple and predictable migration of CO2. 

However, time lapse seismic monitoring of the injected plume revealed the 

importance of an internal shale formation within the injection reservoir that 

has significantly altered the direction and extent of CO2 migration from those 

early predictions (Chadwick et al. 2004). This finding emphasizes the 

importance of computational fluid flow modeling capable of handling complex 

internal heterogeneity for prediction of CO2 plume geometries, which are 

63
unlikely to be cylindrical or closely predicted using the approaches that have 

been previously used in UIC applications. 

PNNL RECOMMENDATION 

We support the use of sophisticated, multiphase, computational models designed 

for site-specific conditions for AoRdelineation, as specified by EPA in the 

proposed regulations. 

REFERENCES 

R.A. Chadwick, P. Zweigel, U. Gregersen, G.A. Kirby, S. Holloway and P.N. 

Johannessen. 2004. Geological reservoir characterization of a CO2 storage site : 

the Utsira Sand, Sleipner, Northern North Sea, Energy 29:1371-1381. 

IPCC. 2005. IPCC Special Report: Carbon Dioxide Capture and Storage. Summary for 

Policy Makers and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. 

Loos, and L. Meyer (eds.). CambridgeUniversity Press, NewYork. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: John King Letter ID: 0211

Commenter Organization: Shell Exploration and Production Company

(SEPCo)

Comment ID:

22766

Comment: B. Area of Review and Corrective Action Requirements 

We support the use of computational models for determination of the appropriate 

area of review ("AOR") for GS purposes. Modeling of the pressure effects 

associated with injection -- together with conservative modeling to bound the 

potential extent of CO2 plume movement -- are appropriate aids to the evaluation 

of geologic siting and corrective action to address risks presented by 

penetrations such as abandoned boreholes. The use of such modeling is already 

well established in the context of oil and gas exploration and production and 

has been used in the UIC program for "no-migration" petitions for hazardous 

waste injection. Similar modeling can reasonably be adopted for use in the GS 

context as well. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

64
migration through faults, fractures, and artificial penetrations.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24024

Comment: Proposed Rule: (1) Predict, using computational modeling, the projected lateral 

and vertical migration of the carbon dioxide plume and formation fluids in the 

subsurface from the commencement of injection activities until the plume 

movement ceases, pressure differentials sufficient to cause the movement of 

injected fluids or formation fluids into a USDW are no longer present, 

Schlumberger Suggested Revision: - 

Schlumberger Rationale: We support this as written 

Response: EPA acknowledges the commenter’s support; the final GS rule retains the provision with

minor modifications to require the use of existing data in predicting the AoR.

Commenter Name: Neeraj Gupta Letter ID: 0229

Commenter Organization: Battelle Energy Technology (BET) Comment ID: 21830

Comment: Page 64, Area of Review Determination/Modeling - We agree that multiphase 

numerical models are the best option for determining area of review (AOR) for 

complex, full-scale injection sites. Simpler analytical models may be used for 

smaller-scale, short-term tests. Multiphase computer simulations, supported 

with field data, should be primary tool for managing the storage sites through 

out the life cycle from site selection, permitting, characterization, 

operations, to post operational monitoring and closure. However, modeling 

should not be restricted to a single code or type of code. This will stifle 

innovation, because many more modeling codes are likely to be developed and 

existing commercial, academic, and national laboratory codes are incorporating 

new processes and improvements based on lessons learned from field projects. 

The incorporation of multiphase processes is likely to be a key requirement for 

models. However, we do not believe that fully coupled model with CO2 flow, 

transport, geochemical, geomechanical, and other processes in a single 

simulation for the entire facility and life cycle is currently feasible. Such 

fully-coupled codes require extensive computational power and are more 

applicable to evaluating basic science questions. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

EPA agrees that operators should have the flexibility to select models with which they are familiar

and that are most appropriate to the site conditions and available data types and amounts. EPA has

evaluated technical documents regarding several computational models that may be used for GS

65
sites. Based on this review (as documented in the preamble to the proposed GS regulation and the

background document "Computational Modeling of Underground Injection of Carbon Dioxide for

Determination of Area-of-Review and Potential Risk to Underground Sources of Drinking Water"),

EPA believes that several models meet the minimum requirements for models in the GS regulation.

Commenter Name: Steven B. Corneli Letter ID: 0234

Commenter Organization: NRG Energy, Inc. Comment ID: 23162

Comment: 1. Use of Computational Models for Area of Review 

The EPA requested comment on the proposed rule regarding the use of modeling for 

area of review (AoR). NRG supports the use of computational models for 

determination of the AoR for geologic sequestration projects. The use of such 

models is already being used in the UIC program for "no-migration" petitions for 

Class I injection wells. We believe the use of modeling for determining the 

pressure effects associated with the underground injection process is an 

appropriate confirming process for safe site selection and corrective action to 

address potential risk presented by GS. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Mark E. Fesmire Letter ID: 0237

Commenter Organization: New Mexico Oil Conservation Division

(NMOCD) Comment ID: 23209

Comment: B. The OCD applauds the inclusion of computational modeling to establish the 

Area of Review (AOR) as much more representative of reservoir conditions than 

fixed radius AOR’s currently in use in the UIC program. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

Commenter Name: Walter L. Baker Letter ID: 0253

Commenter Organization: Utah Division of Water Quality Comment ID: 22347

Comment: COMPUTATIONAL MODELING 

We support the use of multiphase models in predicting the movement of the CO2 

plume and injection zone fluids given the multiphase nature of the injectate 

particularly in the early stage of the CCGS project. However, we also emphasis 

66
the importance of the use of geochemical modeling in addressing the reactivity 

of CO2 in the presence of water and other reactive components in the matrix and 

fluids of the injection and confining zones. 

Response: EPA acknowledges the commenter’s support and agrees that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

EPA disagrees that geochemical modeling (i.e., reactive transport modeling) will be necessary for

delineation of the AoR in all cases. EPA clarifies that the GS regulation requires that AoR modeling

consider the chemical and physical properties of the injected stream and native fluids, but does not

specifically require reactive transport modeling (146.84(c)(1)). As discussed in the preamble to the

proposed GS regulation, EPA expects that AoR modeling will account for multiphase fluid flow

processes, including density-driven flow (e.g., buoyancy). EPA believes that the purpose of AoR

modeling is to predict the regions wherein fluid pressures increase and/or to which CO2 may be

transported. Based on a review of the technical literature, EPA believes that, in many cases, reactive

transport modeling will not be necessary to determine whether USDWs would be endangered. Any

areas of geochemical disturbance would be within the modeled AoR as defined at 146.84(c). EPA

adds that the Director has discretion to require additional modeling, including geochemical

modeling, if he or she determines this to be necessary. EPA adds that the purpose of modeling is to

target areas where geochemical monitoring should be performed. 

Commenter Name: Scott Anderson Letter ID: 0520

Commenter Organization: Environmental Defense Fund Comment ID: 21420

Other EPA Comment Codes and Descriptions: 1420 1.2.1.1 Support the GS rulemaking; 4660

2.6.6 Geochemical monitoring 

Comment: We think the rule, the proposed rule has done a very good job of keeping the 

focus on both CO2 leakage risks and also the risk of problems from displaced 

brine. The confining zone in the rule is required to contain both CO2 and 

displaced formation fluids. The zone of elevated pressure as defined in the rule 

focuses on both CO2 and formation fluids that might be driven out of a US -- 

into a USDW. The modeling requirements are modeling both of the CO2 plume extent 

and formation fluid movement. There is the requirement for periodic monitoring 

of groundwater quality and geochemical changes above the confining zone which 

would help detail whether either CO2 or brine is leaking and there are adequate 

response provisions. 

Response: EPA acknowledges the commenter’s support and believes that predicting the complex

multiphase, buoyant flow of the CO2, co-injectates, and compounds that may be mobilized due to

injection requires the sophistication of computational models, as required at §146.84(c)(1). Models

can predict CO2 fate and transport, as well as changes in formation pressure, in three dimensions, as

a function of time in order to delineate the subsurface storage site and the AoR. Models can account

for site-specific and predicted operating conditions as well as geologic heterogeneities and potential

migration through faults, fractures, and artificial penetrations.

67
EPA believes that the creation of a new injection well class (VI) under the existing UIC Program is

necessary to enable the use of GS as a climate change mitigation technology in a manner that

protects USDWs from endangerment. EPA agrees that the UIC Program, under the authority of the

SDWA, provides a strong framework for regulating injection of CO2 for GS and adds that the GS

regulation tailors the UIC requirements to address the unique issues associated with large-scale GS

including large injection volumes, the relative buoyancy of CO2, its corrosivity in the presence of

water, the potential presence of impurities in captured CO2, and its mobility within subsurface

geologic formations. For additional information on well construction, siting, and testing and

monitoring requirements, please also see the discussion of EPA’s Final Regulatory Approach in the

preamble to the final GS rule.

Commenter Name: Doug Louis Letter ID: 0530

Commenter Organization: Kansas Corporation Commission Comment ID: 21341

Comment: KCC staff agrees that the most accurate and realistic method of determining AOR 

is computational models which are updated regularly in conjunction with modeling 

monitoring of physical trailers. A prescribed fixed radius surrounding 

individual wells would not be an appropriate determination of AOR for a complex 

dynamic CO2 storage facility. 

Response: EPA acknowledges the commenter’s support and agrees that the unique aspects of GS

merit the use of sophisticated computational modeling for delineation of the AoR, based on sitespecific features and geology, including the presence of faults, fractures, and artificial penetrations.

EPA believes that computational modeling of the AoR prior to injection will provide the best

obtainable estimates of the extent of the CO2 plume over several years of active injection. EPA has

strengthened the minimum frequency for the AoR reevaluation from ten years to at least every 5

years (146.84(e)). EPA believes that this revised frequency addresses commenters' concerns about

the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS.

Comment Code: 3320 2.2.1.2 Comments opposed to requiring computational models to delineate

the AoR 

Individual Comments

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23485

Other EPA Comment Codes and Descriptions: 3350 2.2.1.5 Use of proprietary models for GS

sites (seeking comment) 

Comment: 8. Modeling 

As most models now are proprietary, USWAG supports the use of proprietary models 

for use with CO2 GS sites. As EPA recognizes, many popular proprietary codes in 

the petroleum-reservoir engineering discipline have been used in peer-reviewed 

studies to model CO2 sequestration. 73 Fed. Reg. at 43507. These and other 

proprietary models should be available for use in modeling for Class VI CO2 GS 

wells, provided that they are based on accepted science and have been validated, 

68
and as long as the Owners or Operators using proprietary models disclose, as 

necessary, the code assumptions, relevant equations, and scientific basis of 

each model. USWAG believes that the use of computational fluid flow models for 

delineating the AoR of GS sites is acceptable but should not be required. Id. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites and will allow their use for GS projects. EPA acknowledges that

several proprietary models have been used by petroleum-reservoir engineering professionals as well

as in peer-reviewed studies to model CO2 sequestration. EPA plans to allow their use in meeting the

requirements for the GS regulation, but clarifies that GS well operators using proprietary models are

required to disclose, to the Director's satisfaction, the code assumptions, relevant equations, and

scientific basis. EPA adds that its Confidential Business Information requirements at 40 CFR 144.5

will apply to the use of proprietary models.

EPA clarifies that the final rule will require the use of computational models. EPA believes that the

unique aspects of GS merit the use of sophisticated computational modeling for delineation of the

AoR, based on site-specific features and geology, including the presence of faults, fractures, and

artificial penetrations. EPA believes that computational modeling of the AoR prior to injection will

provide the best obtainable estimates of the extent of the CO2 plume over several years of active

injection.

Commenter Name: Chris M. Hobson Letter ID: 0201

Commenter Organization: Southern Company Comment ID: 22168

Comment: model. Id. at 43507. Based on the valuable experience Southern Company has 

gained through our C02 GS pilot project, we also support the use of 

computational fluid flow models for AoR delineation, provided that these models 

are available for use at the option of the Owner or Operator and are not 

required for all CO2 GS projects. See id. 

Response: EPA clarifies that the final rule requires the use of computational modeling at

146.84(c)(1). EPA believes that the use of computational modeling, rather than reliance on a fixed

radius or simple computation, is necessary to account for the complexity of GS projects. EPA

believes that predicting the complex multiphase, buoyant flow of the CO2, co-injectates, and

compounds that may be mobilized due to injection requires the sophistication of computational

models. Models can predict CO2 fate and transport, as well as changes in formation pressure, in

three dimensions, as a function of time in order to delineate the subsurface storage site and the AoR.

Models can account for site-specific and predicted operating conditions as well as geologic

heterogeneities and potential migration through faults, fractures, and artificial penetrations.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23231

Other EPA Comment Codes and Descriptions: 3330 2.2.1.3 Applicability /adequacy of

computational models (seeking comment) 

Comment: 11. EPA believes that predicting the complex multi-phase buoyant flow of the 

CO2, "co-injectates," and compounds that may be mobilized due to injection 

requires the sophistication of computational models. EPA proposes that the 

owners or operators of GS wells be required to delineate the AoR for CO2 GS 

sites using computational fluid flow models designed for the specific site 

conditions and injection regime. EPA seeks comment on the use of modeling for 

69
AoR delineation. EPA Notice 43506. 

There is concern about the lack of information necessary to construct a valid 

model in areas where the sequestration zone has not been extensively studied. 

This lack of information may make modeling difficult or not very useful. Any 

modeling used should be re-examined and recalibrated over time with additional 

data gathered. Modeling should not be solely relied upon where real data offers 

a different view. Seismic and offset well data can help delineate the AOR. 

Response: The Agency believes that AoR delineation using computational modeling that accounts

for the multiphase flow of CO2 and native fluids (i.e., naturally occurring salty water), including

density-driven flow (i.e., buoyancy), is necessary to ensure protection of USDWs from

endangerment. The computational models used for this purpose are required to predict the fate of

injected CO2 and changes in formation pressure. EPA believes that the requirements at 146.84(c)(1)

to delineate the AoR using computational modeling are appropriate and encompass the significant

processes that could affect the location of the CO2 plume and associated pressure front and has

retained this in the final rule. 

EPA agrees that the modeling should be re-examined and recalibrated over time. The GS regulation

at 146.84(e) also requires owners or operators to periodically reevaluate the AoR to incorporate

monitoring and operational data. This process provides additional opportunities to verify that the

models have accurately predicted movement of the CO2 plume and to make any needed adjustments

to corrective action or monitoring at the site.

Letter ID: 0263

Commenter Organization: Interstate Oil & Gas Compact Commission Comment ID: 22797

Comment: III A. 2 Proposed Area of Review and Corrective Action Requirements 

L. EPA seeks comment on the use of modeling for AoR delineation. 

IOGCC Response: See General Comment for Section III. A. 2 above. The collection 

of physical data (i.e. well bore data) maybe necessary to fully utilize modeling 

to determine the ultimate extent of the proposed CO2 storage volume. Models should 

only be used when they are appropriate. The Director needs the flexibility to be 

able to supplement or substitute other data elements as appropriate. 

Response: EPA agrees that it is appropriate to allow operators the flexibility to select modeling, site

characterization, and monitoring techniques that are most appropriate to site conditions and available

types and amounts of data. However, EPA believes that modeling is necessary to delineate the AoR

for GS projects, and the final rule retains this requirement. EPA notes that the final rule at

146.84(c)(1) requires the AoR to be predicted using existing site characterization, monitoring, and

operational data, along with computational modeling, but that it does not specify the particular data

inputs. EPA adds that the final rule at 146.82(c)(1) requires the owner or operator to revise the

proposed AoR submitted under 146.82(a)(2) as necessary to address new data collected during

logging and testing.

Comment Code: 3330 2.2.1.3 Applicability /adequacy of computational models (seeking comment) 

70
Individual Comments

Commenter Name: John Hanger Letter ID: 0130

Commenter Organization: Pennsylvania Dept. of Env. Protection Comment ID: 21504

Comment: -The proposed rule discusses multiphase modeling and considers them appropriate 

for hydraulically connected formations being used by different operators. Where 

there are multiple operators injecting into a formation that is hydraulically 

connected, rules should require an evaluation of the chemical reaction of the 

different injections, unless they are chemically the same, as well as their 

interaction with native fluids. 

Response: EPA agrees that the impacts of multiple operators injecting into hydraulically connected

formations should be accounted for in the AoR modeling. EPA adds that the owner or operator must

demonstrate in the proposed AoR and corrective action plan to the satisfaction of the Director that

the modeled AoR will be based on detailed site-specific geologic data collected during the site

characterization process including consideration of existing conditions (e.g., current injection

activities). EPA anticipates that, where multiple operators are involved, the Director would not

approve modeling that does not account for changes that may result from all injection operations in a

formation or a series of hydraulically connected formations. EPA is developing guidance on AoR

and corrective action at GS Class VI sites to assist owners or operators and Directors in evaluating

requirements and proposed AoR and corrective action plans under 146.84. The guidance describes

EPA's recommended approaches for the data to be considered in an initial AoR delineation, how to

apply computational modeling to delineate the AoR, performing corrective action at GS sites, and,

when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site,

at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22548

Comment: General Comments on the Use of Modeling for Ground-Water Protection Related to 

CO2 Sequestration 

Quantitative modeling must play an important role in the assessment of groundwater protection related to CO2 sequestration projects. A ‘quantitative model’ 

is defined here as a quantitative mathematical analog of the real subsurface 

system. 

Three overall purposes for quantitative modeling are generally recognized: 

1. Generic: Model used in a hypothetical sense 

2. Interpretive: Model used to understand system dynamics 

3. Predictive: Model used to predict future behavior of the subsurface 

system. 

All three purposes are generally needed for a CO2 sequestration project. For 

example, generic models are used to determine the important physical or 

geochemical mechanisms that dominate transport or trapping of CO2. An 

interpretive model is applied to test specific hypotheses regarding these 

behaviors. Predictive modeling provides the basis for design of the final 

71
injection scheme and establishment of potential risk to drinking water. 

Quantitative models may range from simple analytic calculations to complex 

multi-component multi-phase coupled three-dimensional numerical simulations. The 

choice of quantitative model (analytic or numerical) depends in large part on 

the ultimate objective of the modeling study, and on practical constraints 

including availability of data and computational limitations. Which type of 

model or combination of models is most appropriate for a specific situation 

depends on many factors. In general, using a combination of modeling and 

analysis techniques is recommended, because different levels of complexity are 

needed at various stages of a project and for different objectives, and 

application of multiple modeling techniques can be used for "cross-checking" and 

overall quality assurance. 

Numerical models are widely available and usually represent a relatively wide 

range of physical and chemical processes, spatial heterogeneity of material 

properties, and nonlinear interactions among different processes. While these 

kinds of models allow for substantial flexibility and can provide detailed 

predictions of system behavior, they also require large amounts of data, much of 

which may be difficult to obtain or estimate. As such, specific choices of 

models must be made with care, and some situations may most effectively be 

addressed by a combination of different kinds of models. 

Quantitative modeling, whether conducted with analytical or numerical modeling 

tools, provides significant insights into system behavior. The advantages of 

such models are that they: 

1. provide a framework for data collection centered about a specific set 

of transport and geochemical behaviors. 

2. allow communication and the interpretation of these data in the 

context of scientific principles and facilitate collaboration among 

engineers, scientists, and regulators. 

3. may be extended far into the future using multiple design scenarios 

4. can be compared to measured data which allow hydraulic and 

geochemical parameters to be estimated. 

5. Can be used to access the performance of the system in a long 

performance period for which field and laboratory data are not 

available. 

It is important to recognize that all quantitative mathematical models are based 

upon a "conceptual model" which defines the important fluid and chemical 

interactions as well as the spatial and temporal extent of the model. Because 

the conceptualization is a critical part of the modeling activity, it is 

important to explore multiple conceptual models during the modeling process to 

assure that important scenarios are not excluded from the analysis. 

The selection of which quantitative model to use relies on the identification of 

the physical/chemical parameters and functions that must be represented. Because 

the injection and migration of CO2 into the subsurface is hydraulically and 

chemically complex, comprehensive quantitative modeling would have to include 

72
many parameters and factors -- including the flow of miscible and immiscible 

phases in all potential states as well as chemical reactions among fluids and 

with the geologic media and necessary heat transfer. The geologic medium may 

also deform or be chemically altered in response to the presence of CO2. The 

complexity of these coupled processes, in the presence of complex 

hydrostratigraphy, lead to models that are at the limit of the current 

capabilities of available computer hardware and software. It is likely, 

therefore, that multiple models may be required to provide the necessary 

insights into the important physical and chemical processes. Ultimately, sound 

scientific and engineering judgments should be applied to determine which 

physical or chemical processes are most important for a given analysis, and to 

make assumptions that are consistent with those judgments. For example, during 

the injection phase of a CO2 operation, multi-phase flow physics tend to 

dominate other processes such as interphase mass transfer and geochemical 

reactions. This separation of time scales can lead to important simplifications 

for the purpose of risk assessment. Similar analysis of spatial scales can 

provide additional simplifications, leading to practical modeling tools for the 

purpose of risk assessment. It is, therefore. imperative that modeling experts 

participate and manage all phases of the modeling process, in concert and 

constant communication with the project team and the appropriate regulatory 

reviewers, to ensure that appropriate and comprehensive quantitative modeling 

tools are applied successfully." 

The process of comparing simulated and measured predictions for the purpose of 

developing a more accurate depiction of a subsurface flow system is generally 

referred to as "model calibration" in the ground-water literature. (This same 

process is referred to as "inverse modeling" in exploration geophysics and 

"history matching" in petroleum reservoir simulation.) Model calibration 

involves varying the parameters of the quantitative model to achieve the minimum 

difference between measured and predicted responses. This is usually achieved 

through an automated process using non-linear regression or a heuristic 

algorithm to optimize the problem (e.g. as implemented in the PEST or UCODE 

software). In the case of ground-water modeling, the predicted and measured 

data are usually pressures, discharge rates, or concentrations of fluids or 

dissolved species. These data may not be available during the design stage of 

a CO2 project unless wells already exist in the target formation. As wells are 

drilled and geologic and fluid information are collected, however, the model 

should be refined and calibrated. Most automated calibration procedures allow 

for statistical estimation of confidence in the estimated parameters, providing 

a basis for quantifying the sensitivity of the model to changes in its 

parameters, and for performing quality control of model predictions. 

The design, construction, and calibration of analytic or numerical models should 

follow a quality assurance and quality control procedure. These procedures 

should include benchmarking of the mathematical predictions against known 

results, a method for assuring input data quality and lineage, and a statistical 

analysis of the predictive results against available data and/or sensitivity 

analyses. ASTM guides are available for ground-water modeling and code 

benchmarking (e.g. Guides D 6025, D 5447, D 5490, D 5611, D 5609, D 5610, and D 

5718). 

73
Response: EPA agrees that complex modeling techniques are necessary to calculate the AoR while

adequately accounting for the complexities of a GS site and all phases of a CO2 plume. EPA

believes, based on the research that supported the modeling requirements in the GS regulation (as

described in the background document, "Computational Modeling of Underground Injection of

Carbon Dioxide for Determination of Area-of-Review and Potential Risk to Underground Sources of

Drinking Water" cited in the proposed rule preamble as USEPA, 2008d), that technology has

advanced to the point that a CO2 injection project can be appropriately modeled to predict and track

a CO2 plume (and pressure front). Each AoR calculation will be reviewed based on site-specific

geologic conditions, planned operating conditions, and available modeling and computational

technology. EPA agrees that appropriate data quality review should be conducted during AoR

modeling. Methods for assuring input data quality and lineage, as well as statistical analysis of

predictive results may be used during evaluation of a permit application, at the discretion of the

Director. 

EPA anticipates that modeling technology will continue to improve, and encourages and expects

owners or operators to use the best available multiphase computational models to determine the

AoR. Reliance on improved models will likely increase the accuracy and quality of the AoR

characterization, resulting in increased protection of USDWs from endangerment. 

EPA adds that, to address uncertainty in modeled results, particularly over the long term, it is

requiring that the AoR be reevaluated (under 146.84(e)) to verify the CO2 plume is behaving as

predicted and to allow for necessary revisions to corrective action or monitoring. The final GS

regulation includes a requirement that the AoR be reevaluated at least every 5 years (an adjustment

from the ten year interval in the proposal), which should further address the commenters' concerns.

This frequency will provide the Agency with information on AoR reevaluations to support

subsequent regulatory evaluations that are part of the Agency's adaptive approach. The Agency plans,

every six years, to collect and review data on GS projects to determine whether the appropriate

amount and types of information and appropriate documentation are being collected, and to

determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22552

Comment: Modeling is Needed: When considering tens of miles, trying to use 

conservatively the analytical solution as provided in 40 CFR §146.06 will likely 

generate unrealistically large AORs. Parameters such as formation thickness and 

heterogeneity, storativity and permeability values will likely vary in the zone 

of elevated pressure suggesting that model results should be accepted as valid. 

How to define the edge of the AOR? If the CO2 plume is relatively easy to 

define in terms of modeling, this is much less true for the pressure plume. 

Limits to the secondary AOR can be defined more or less as it is currently done 

by the contour line of the additional pressure that would push the injection 

formation brine beyond some elevation threshold (for example, 100 ft below the 

base of the USDW). Time-varying area where corrective action needs to take place 

as injection progresses can be defined in a similar fashion. 

AORs can be initially defined solely on the basis of modeling. However, NGWA 

strongly supports that a satisfactory match between prediction and actual field 

74
measurements needs to be assessed as the project progresses. 

Response: EPA agrees that monitoring and operational data should be used to verify that the CO2

plume and pressure front are moving as predicted in AoR calculation models (146.84(e)). EPA

believes that the requirements for periodic AoR reevaluation can also help address uncertainties

inherent in predictive modeling and allow for refinement of model predictions based on monitoring,

operational, and other site-specific data.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22581

Comment: Preamble Questions: EPA seeks comment on the applicability of computational 

fluid flow models for delineating the AoR of GS sites. EPA Notice 69. 

NGWA Response: See NGWA General Comment Document Section titled "General 

Comments on Use of Modeling for Ground-Water Protection Related to CO2 

Sequestration". 

Response: As described above, EPA agrees that complex modeling techniques may be required to

calculate the AoR while adequately accounting for the complexities of a GS site and all phases of a

CO2 plume.

Commenter Name: James Childress Letter ID: 0158

Commenter Organization: Gasification Technologies Council (GTC) Comment ID: 23771

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: §146.84 Area of review and corrective action. (c) (1) "Predict, using 

computational modeling, the projected lateral and vertical migration of the 

carbon dioxide plume and formation fluids in the subsurface from the 

commencement of injection activities until the plume movement ceases, pressure 

differentials sufficient to cause the movement of injected fluids or formation 

fluids into a USDW are no longer present, or after a fixed time period as 

determined by the Director." 

We recommend striking out "Predict, using computational modeling, the protected 

lateral and vertical migration of the carbon dioxide plume" and replacing with 

"Using computational modeling, provide protected potential ranges of lateral and 

vertical migration of the carbon dioxide plume" The use of multiphase 

groundwater flow and transport models is to predict the composition and movement 

of each phase state (e.g., liquid, gas ,supercritical fluid) of CO2 in the 

subsurface, as well as fluid pressures, to define the AoR. One of the concerns 

with the use of such complex models is that the site may not be fully 

characterized with respect to hydro-geologic properties. There may be 

significant uncertainty in one or more hydraulic processes/parameters. 

Therefore, it is possible to obtain a wide-range of model outcomes from 

different model parameters. The uncertainty in model parameters and sensitivity 

of model results to parameter selection should be addressed. 

Response: EPA has clarified this language in the final rule. It now reads, “Predict, using existing

75
site characterization, monitoring and operational data, and computational modeling, the projected

lateral and vertical migration of the carbon dioxide plume and formation fluids in the subsurface

from the commencement of injection activities until the plume movement ceases, until pressure

differentials sufficient to cause the movement of injected fluids or formation fluids into a USDW are

no longer present, or until the end of a fixed time period as determined by the Director.”

EPA agrees that modeling results are prone to varied degrees of uncertainty. As discussed in the

preamble to the proposed GS regulation and the associated background document "Computational

Modeling of Underground Injection of Carbon Dioxide for Determination of Area-of-Review and

Potential Risk to Underground Sources of Drinking Water" (cited in the proposed rule preamble as

USEPA, 2008d), model predictions depend largely on the values of key parameters. Often these

parameter values are estimated or averaged from several data sources. EPA also agrees that

sensitivity analyses and "worst case scenario" modeling runs should be presented by the owner or

operator and evaluated by the Director. If during the permit review, the Director determines that

modeling results are too wide-ranging, further information will be required of the owner or operator. 

EPA clarifies that models are tools to help predict the movement of a CO2 plume over long time

frames. However, EPA recognizes that it cannot be expected that modeling something as complex as

a CO2 plume can be done with complete certainty; therefore EPA is also requiring that the AoR be

reevaluated to verify the CO2 plume is moving as predicted and to allow for model calibration and

necessary revisions to the required GS project plans (e.g., AoR and corrective action, testing and

monitoring, injection well plugging, post-injection site care and site closure, and emergency and

remedial response). EPA believes that near-term predictions (e.g., over the subsequent 5 years) will

have the highest degree of certainty. Periodic model calibration will allow refinement to the AoR

delineation based on real-world conditions and operating data and monitoring results, and thus

increase confidence in the modeled predictions. EPA adds that the final GS regulation includes a

requirement at §146.84(e) that the AoR be reevaluated at least every 5 years (an adjustment from the

ten year interval in the proposal), which should further address the commenters' concerns regarding

model uncertainty.

Commenter Name: James Childress Letter ID: 0158

Commenter Organization: Gasification Technologies Council (GTC) Comment ID: 23779

Comment: EPA seeks comment on the applicability of computational fluid flow models for 

delineating the AoR of GS sites. EPA Notice 69. 

Computational fluid flow models should be permitted for delineating the AoR of 

GS sites provided also that data are provided supporting their validation in 

similar complex structures and flow regimes 

Response: EPA agrees that a detailed geologic assessment is essential to evaluating the presence and

adequacy of a proposed site to receive and confine the CO2. EPA encourages owners and operators

to submit (and Directors to review) any available data on the site, such as seismic or off-set well

data. The AoR and corrective action plan required at §146.84(b)must describe the site

characterization data on which the model will be based. The Director has discretion to require the

use of existing data if it is not initially included. Furthermore, the owner or operator's testing and

monitoring plan required at §146.90 should provide for site-specific data (which may include seismic

and direct measurements) that can be used to inform updates to the model and support AoR

reevaluations.

76
The Agency believes that predicting the complex multiphase buoyant flow of the CO2, co-injectates,

and compounds that may be mobilized due to injection requires the sophistication of computational

models. Models can predict CO2 fate and transport, as well as changes in formation pressure, in

three dimensions, as a function of time in order to delineate the subsurface storage site and the AoR.

Models can account for site-specific and predicted operating conditions as well as geologic

heterogeneities and potential migration through faults, fractures, and artificial penetrations.

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24223

Comment: Rule Provisions: 146.82(c), 146.84, 146.90(d)(2) 

EPA Preamble Question: 11. EPA believes that predicting the complex multi-phase 

buoyant flow of the CO2, "co-injectates," and compounds that may be mobilized 

due to injection requires the sophistication of computational models. EPA 

proposes that the owners or operators of GS wells be required to delineate the 

AoR for CO2 GS sites using computational fluid flow models designed for the 

specific site conditions and injection regime. EPA seeks comment on the use of 

modeling for AoR delineation. EPA Notice 43506. 

API Response: Modeling is well developed technology that can readily simulate 

complex behavior of injected CO2. It should be one of the techniques used to 

delineate the AoR. Modeling alone though should not be the sole basis for 

determining the AoR. Other technologies such as seismic, use of off-set wells, 

should also be considered when delineating the proposed AoR. 

The oil and gas industry has a great deal of experience that can be generally 

applied in the delineation of the AoR. 

The most complex model is not always necessary to adequately delineate the AoR. 

Some CO2 EOR projects have successfully operated for decades using only 

production data and pressure mapping, which has in effect defined AoR. 

Response: EPA agrees that the AoR should not be based exclusively on modeling. EPA has clarified

the AoR modeling requirement in §146.84(c)(1) for the final rule. It now reads, “Predict, using

existing site characterization, monitoring and operational data, and computational modeling, the

projected lateral and vertical migration of the carbon dioxide plume and formation fluids in the

subsurface from the commencement of injection activities until the plume movement ceases, until

pressure differentials sufficient to cause the movement of injected fluids or formation fluids into a

USDW are no longer present, or until the end of a fixed time period as determined by the Director.”

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82. This

would include any seismic or off-set well data collected to meet the requirements of §146.82(a).

77
Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24225

Comment: Rule Provisions: 146.84 

EPA Preamble Question: 13. EPA seeks comment on the applicability of 

computational fluid flow models for delineating the AoR of GS sites. EPA Notice 

43507. 

API Response: While computational fluid flow models can inform the delineation 

of the AoR, modeling shouldn’t be the sole criteria for determining AoR. (See 

Comment # above also.) 

Response: As described above, EPA agrees that the AoR should not be based exclusively on

modeling. EPA has clarified the AoR modeling requirement in §146.84(c)(1) for the final rule,

specifying that site characterization, monitoring, and operational data must be used along with

modeling.

Commenter Name: Robert J. Finley Letter ID: 0180

Commenter Organization: Illinois State Geological Survey Comment ID: 23627

Comment: (c)(1) 

It is unclear to this reviewer whether computational modeling can address the 

lateral and vertical movement of all "formation fluids in the subsurface". This 

needs to be restricted to the receiving reservoir. Further, it is unclear if 

current modeling can adequately address both pressure driven fluid flow and 

density driven flow at plume boundaries that carries saturated brine away from 

free phase supercritical CO2 at the plume margins. Also, in a fully equilibrated 

column of reservoir rock at hydrostatic pressure, a 1 psi differential in a 

receiving reservoir could potentially be sufficient to cause fluids to migrate. 

However, the capillary entry pressure of the confining layer may prevent this. 

As stated here, however, there is no accounting for capillary entry pressure and 

its role in confining the injectate. 

Response: EPA disagrees that modeling of only the receiving reservoir (i.e., the injection zone) is

adequate in all cases to predict the potential movement of the CO2 plume and pressure front and

ensure that USDWs will not be endangered. EPA believes, based on research and technical expert

consultations, that available complex computational models can effectively simulate CO2 transport,

fate, and changes in formation pressure in three dimensions. The Agency believes that these

multiphase models will be able to incorporate factors such as pressure driven fluid flow, density

driven flow, hydrostatic pressure and capillary entry pressure.

Multiphase models have been used by States and industry for predicting the movement of water and

solutes in soil, the behavior of non-aqueous phase liquid contaminants (e.g., trichloroethene) at

hazardous waste sites, the recovery of oil and gas from petroleum-bearing formations, and more

recently, CO2 in the subsurface. 

78
Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25181

Comment: J. EPA seeks comment on the use of modeling for AoR delineation. 

NYSDEC-DMN Response: See General Comment for Section III. A. 2 above. AoR 

delineation should be based on best available technology and available data. The 

collection of physical data (i.e., well bore data) may be necessary to fully use 

modeling to determine the ultimate extent of the proposed CO2 storage volume and 

reservoir pressures. The Director should have the flexibility to be able to 

supplement or substitute other data elements as appropriate ahead of such 

drilling and data gathering with confirmation of the model once additional 

information is available. 

Response: EPA agrees that it is appropriate to allow operators the flexibility to select modeling, site

characterization, and monitoring techniques that are most appropriate to site conditions and available

types and amounts of data. The owner or operator must demonstrate, in the proposed AoR and

corrective action plan required at §146.84(b), to the satisfaction of the Director, that the modeled

AoR will be based on detailed geologic data collected on the site. EPA anticipates that any model

that does this in a satisfactory manner will be approved by the Director. EPA acknowledges that

modeling technology is expected to continuously improve, and encourages and expects owners or

operators to use the best available multiphase computational models to determine the AoR. Reliance

on improved models will likely increase the accuracy and quality of the AoR characterization,

resulting in better protection of USDWs from endangerment.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25183

Comment: L. EPA seeks comment on the applicability of computational fluid flow models for 

delineating the AoR of GS sites. 

NYSDEC-DMN Response: See General Comment for Section III. A. 2 above. AoR 

delineation should be based on best available technology and available data. The 

collection of physical data (i.e., well bore data) may be necessary to fully use 

modeling to determine the ultimate extent of the proposed CO2 storage volume and 

reservoir pressures. The Director should have the flexibility to be able to 

supplement or substitute other data elements as appropriate ahead of such 

drilling and data gathering with confirmation of the model once additional 

information is available. 

Response: As described above, EPA agrees that it is appropriate to allow operators the flexibility to

select modeling, site characterization, and monitoring techniques that are most appropriate to site

conditions and available types and amounts of data. The owner or operator must demonstrate, in the

proposed AoR and corrective action plan required at §146.84(b), to the satisfaction of the Director,

that the modeled AoR will be based on detailed geologic data collected on the site. EPA adds that

the Director has authority to request supplemental information if needed to ensure that the site will

be protective of USDWs.

79
Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24515

Comment: Rule Provisions or Federal Register Cite: 146.82(c), 146.84, 146.90(d)(2) 

EPA Preamble Question: 11. EPA proposes that the owners or operators of GS wells 

be required to delineate the AoR for CO2 GS sites using computational fluid flow 

models designed for the specific site conditions and injection regime. EPA seeks 

comment on the use of modeling for AoR delineation. EPA Notice 43506. 

CSC Response: Modeling is well developed technology that can readily simulate 

complex behavior of injected CO2. It should be one of the techniques used to 

delineate the AoR. 

The oil and gas industry has a great deal of experience that can be generally 

applied in the delineation of the AoR. 

The most complex model is not always necessary to adequately delineate the AoR. 

Some CO2 EOR projects have successfully operated for decades using only 

production data and pressure mapping, which has in effect defined AoR. 

Response: EPA agrees that the oil and gas industry has contributed to the state of modeling science,

and anticipates that some industry-developed modeling products will be acceptable for models in the

AoR for GS sites, as long as GS well operators clearly disclose, to the Director's satisfaction, the

code assumptions, relevant equations, and scientific basis.

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

EPA encourages and expects owners or operators to use the most appropriate multiphase

computational models available to determine the AoR. However, EPA does not believe that the most

"complex" model will, in all cases, be the best available AoR delineation model. The requirements at

§146.84(c)(1) are designed to allow the owner or operator to choose, in consultation with the

Director, the modeling platform that is most appropriate for a particular GS project. 

EPA anticipates that modeling technology will continue to improve, and encourages and expects

owners or operators to use the best available multiphase computational models to determine the

AoR. Reliance on improved models will likely increase the accuracy and quality of the AoR

characterization, resulting in increased protection of USDWs from endangerment. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24517

Comment: Rule Provisions or Federal Register Cite: 146.84 

EPA Preamble Question: 13. EPA seeks comment on the applicability of 

computational fluid flow models for delineating the AoR of GS sites. EPA Notice 

43507. 

CSC Response: Computational fluid flow models can inform the delineation of the 

80
AoR. (See also Comment on Question 11 above.) 

Response: EPA encourages and expects owners or operators to use the most appropriate multiphase

computational models available to determine the AoR. The requirements at §146.84(c)(1) are

designed to allow the owner or operator to choose, in consultation with the Director, the modeling

platform that is most appropriate for a particular GS project. EPA adds that its Confidential Business

Information requirements at 40 CFR 144.5 will apply to the use of proprietary models. 

EPA anticipates that modeling technology will continue to improve, and encourages and expects

owners or operators to use the best available multiphase computational models to determine the

AoR. Reliance on improved models will likely increase the accuracy and quality of the AoR

characterization, resulting in increased protection of USDWs from endangerment. 

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24578

Comment: Rule Provisions: 146.82(c), 146.84, 146.90(d)(2) 

EPA Rule Issue: 11. EPA believes that predicting the complex multi-phase buoyant 

flow of the CO2, "co-injectates," and compounds that may be mobilized due to 

injection requires the sophistication of computational models. EPA proposes that 

the owners or operators of GS wells be required to delineate the AoR for CO2 GS 

sites using computational fluid flow models designed for the specific site 

conditions and injection regime. EPA seeks comment on the use of modeling for 

AoR delineation. EPA Notice 43506. 

Anadarko Petroleum Corporation (APC) Response: Modeling is a well developed 

technology that can readily simulate complex behavior of injected CO2. It should 

be one of the techniques used to delineate the AoR. Modeling alone though should 

not be the sole basis for determining the AoR. Other technologies such as 

seismic and/or use of off-set wells, should also be considered when delineating 

the proposed AoR. 

The oil and gas industry has a great deal of experience that can be generally 

applied in the delineation of the AoR. 

The most complex model is not always necessary to adequately delineate the AoR. 

Nor is it necessary to specify only one model as this would hinder the 

development of future models and techniques. Therefore, APC recommends that EPA 

provide flexibility in the rule to accommodate the use of future technologies 

and refrain from unnecessarily prescriptive measures. 

Response: EPA agrees that the oil and gas industry has contributed to the state of modeling science,

and anticipates that some industry-developed modeling products will be acceptable for models in the

AoR for GS sites, as long as GS well operators clearly disclose, to the Director's satisfaction, the

code assumptions, relevant equations, and scientific basis.

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

81
for all available monitoring and operational data, as required at §146.84(e).

EPA encourages and expects owners or operators to use the most appropriate multiphase

computational models available to determine the AoR. However, EPA does not believe that the most

"complex" model will, in all cases, be the best available AoR delineation model. The requirements at

§146.84(c)(1) are designed to allow the owner or operator to choose, in consultation with the

Director, the modeling platform that is most appropriate for a particular GS project. 

EPA anticipates that modeling technology will continue to improve, and encourages and expects

owners or operators to use the best available multiphase computational models to determine the

AoR. Reliance on improved models will likely increase the accuracy and quality of the AoR

characterization, resulting in increased protection of USDWs from endangerment. 

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24580

Comment: Rule Provisions: 146.84 

EPA Rule Issue: 13. EPA seeks comment on the applicability of computational 

fluid flow models for delineating the AoR of GS sites. EPA Notice 43507. 

Anadarko Petroleum Corporation (APC) Response: While computational fluid flow 

models can inform the delineation of the AoR, modeling shouldn’t be the sole 

criteria for determining AoR. (See Comments to EPA Notice 43506 and 43507 above 

also.) 

Response: EPA agrees that the AoR should not be based exclusively on modeling. EPA has clarified

the AoR modeling requirement in §146.84(c)(1) for the final rule. It now reads, “Predict, using

existing site characterization, monitoring and operational data, and computational modeling, the

projected lateral and vertical migration of the carbon dioxide plume and formation fluids in the

subsurface from the commencement of injection activities until the plume movement ceases, until

pressure differentials sufficient to cause the movement of injected fluids or formation fluids into a

USDW are no longer present, or until the end of a fixed time period as determined by the Director.”

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82. This

would include any seismic or off-set well data collected to meet the requirements of §146.82(a).

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22436

Comment: Validation of Multiphase Computational Models . . . . . Page 10 

EPA should establish a suite of test problems with specified parameters that 

could be used by the applicant to demonstrate the suitability of the 

computational code to be used for analysis of the proposed site. 

Response: EPA notes that the Agency will not perform independent "tests" or validation of

individual modeling products. EPA believes that operators should have the flexibility to select

82
models with which they are familiar and that are most appropriate to site conditions and available

data types and amounts. 

EPA clarifies that an EPA-prepared technical background document, "Computational Modeling of

Underground Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk

to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided in the

regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty

analysis in detail. This document was provided as a background to clarify what information EPA

analyzed in order to draft the modeling requirements at §146.84(c)(1), as well as what actions EPA

expects owners or operators and Directors to take in this area. For example, the document states:

"The requirement for an a-priori AoR delineation based on modeling predictions stresses the need

for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended

as the principal evaluation tool for characterizing the most and least important sources of uncertainty

in environmental models."

EPA based this background document in part on guidance drafted by the National Research Council

and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA refers

interested commenters to the document "Guidance on the Development, Evaluation and Application

of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22446

Comment: Uncertainty Associated with Modeling CO2 Movement and Reservoir Pressure 

PROPOSED REGULATION 

p. 43506, c. 3 In suggesting the use of computational flow models for 

delineating the AoR of GS sites, EPA notes their potential "to predict CO2 

movement in response to varying conditions or scenarios, such as changing 

injection rates, or the presence or absence of fractures or faults in confining 

layers." However, the Proposed Rule does not explicitly discuss uncertainty 

assessment in the use of modeling for AoR delineation. 

DISCUSSION 

Although we agree with the EPA that computational multiphase fluid flow models 

are appropriate for delineating the AoR of GS sites, the Proposed Rule does not 

adequately address how uncertainty should be managed in regards to modeling GS 

sites, and we consider this to be an important aspect. By not explicitly 

addressing uncertainty, the Proposed Rule infers that a single deterministic 

83
model result would be appropriate for determining the AoR and/or for site review 

and reevaluation. This ignores the fact that there is always a certain level of 

ncertainty in model results due to the cumulative variability in input data and 

uncertainty in model parameters. 

The carbon sequestration roadmap produced by the National Energy Technology 

Laboratory (DOE 2007) notes that one of the goals of field tests conducted during 

the Validation Phase is to "Validate and refine current CO2 reservoir models for 

various geologic sequestration sites." Validation does not prove that a given 

model is correct, but it does provide additional constraints on the model 

parameters. However, in the case of saline formation storage, history matching 

is generally not feasible for constraining uncertainties, due to a lack of 

underground data for comparison (IPCC 2005). Therefore the IPCC (2005) 

recommended that systematic parameter variation Routines and statistical 

functions should be included in future coupled simulators to allow uncertainty 

estimates for numerical reservoir simulation results. 

Uncertainty analysis is a valuable analytical tool and is critical for providing 

confidence in model results. Several approaches can be taken to capture the 

uncertainty present in any site modeling exercise (e.g., Neuman and Wierenga 

2003). Three commonly used alternatives are Monte Carlo simulation, the bounding 

approach, and sensitivity analysis. The type of uncertainty analysis appropriate 

to a particular problem depends on the complexity of the problem and the 

available data. 

Probabilistic approaches like those suggested by the IPCC (2005) and Neuman and 

Wierenga(2003) would provide useful assessments of the uncertainty associated 

with the predicted CO2 plume footprint, and could be used to evaluate the risk 

associated with various choices of theAoR. 

PNNL RECOMMENDATION 

A formal process for uncertainty assessment of the CO2 plume footprint should be 

incorporated in the Proposed Rule. This uncertainty assessment could be based on 

a bounding approach, a sensitivity analysis, Monte Carlo simulation or other 

method, provided the range of probable circumstances are considered. This would 

allow for a better assessment of the AoR that addresses the inherent uncertainty 

in computational fluid flow modeling of the injection process. 

REFERENCES 

IPCC. 2005. IPCC Special Report: Carbon Dioxide Capture and Storage. Summary for 

PolicyMakers and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. 

Loos, and L. Meyer (eds.). Cambridge University Press, NewYork. 

Neuman, SP, and PJ Wierenga. 2003, A Comprehensive Strategy of Hydrogeologic 

Modeling and Uncertainty Analysis for Nuclear Facilities and Sites. Technical 

Rpt. NUREG/CR-6805, U. S. Nuclear Regulatory Commission, Washington DC. 

U.S . DOE. 2007. Carbon Sequestration Technology and Program Plan Roadmap. 

84
NationalEnergy Technology Laboratory. 

Response: EPA acknowledges that there will be some uncertainty involved in AoR modeling. An

EPA-prepared technical background document, "Computational Modeling of Underground Injection

of Carbon Dioxide for Determination of Area-of-Review and Potential Risk to Underground Sources

of Drinking Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the proposed

GS rule. This document discusses, for example, uncertainty analysis in detail. This document was

provided as a background to clarify what information EPA analyzed in order to draft the modeling

requirements at 146.84(c)(1), as well as what actions EPA expects owners or operators and Directors

to take in this area. For example, the document states: "The requirement for an a-priori AoR

delineation based on modeling predictions stresses the need for uncertainty and sensitivity analyses

for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool for

characterizing the most and least important sources of uncertainty in environmental models."

EPA based this background document in part on guidance drafted by the National Research Council

and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA refers

interested commenters to the document "Guidance on the Development, Evaluation and Application

of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA agrees that in some cases, results from test-cases may be appropriate. In the technical

background document (USEPA, 2008d), EPA provided several questions that should be raised

during evaluation of models. One of these questions was "If using an independently developed or

unverified code, are results from test-problems and comparisons to widely used codes presented?"

Another was "Was relevant site characterization data gathered at a sufficiently high spatial resolution

to inform model development? Was this data used in the model in a way that considers uncertainty?"

Regarding the suggestion that EPA establish a suite of test problems to demonstrate the suitability of

the computational code, EPA notes that the Agency will not perform independent "tests" or

validation of individual modeling products. EPA believes that operators should have the flexibility to

select models with which they are familiar and that are most appropriate to site conditions and

available data types and amounts. 

EPA based this background document in part on guidance drafted by the National Research Council

and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA refers

interested commenters to the document "Guidance on the Development, Evaluation and Application

of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22447

Other EPA Comment Codes and Descriptions: 3350 2.2.1.5 Use of proprietary models for GS

sites (seeking comment) 

85
Comment: VALIDATION OF MULTIPHASE COMPUTATIONAL MODELS 

PROPOSED REGULATION 

p. 43507, c. 1 Models used for GS sites should be based on accepted science and 

should be validated. In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA is 

aware that the use of proprietary codes may prevent full evaluation of model 

results (e.g ., NRC, 2007). Several popular codes in the petroleum-reservoir 

engineering discipline are proprietary and owners or operators of particular 

sites may prefer to use these codes as they have previous experience with them, 

and they have been used in peer-reviewed studies to model CO2 sequestration. 

When using a proprietary model, owners or operators should clearly disclose the 

code assumptions, relevant equations, and scientific basis. 

DISCUSSION 

We agree that computational models used for GS sites should be based on accepted 

science and should be validated. However, the proposed rule does not define 

validation. We believe that these models must be validated in terms of their 

appropriateness for simulating the complexities of CO2 sequestration under the 

conditions present at the specific site being evaluated. The fact that a code 

has been validated and previously used to model CO2 sequestration (even if peerreviewed) does not ensure that it is appropriate for use at all sites and under 

all conditions. For example, it is possible that, for a specific site, certain 

physical and/or geochemical processes must be considered that are not available 

in a given computational code. Whereas that code may have reasonably and 

successfully modeled CO2 sequestration at a dissimilar site, it may not be 

appropriate for the site being considered. 

It may be appropriate for EPA to establish a suite of test problems that span 

the breadth of GS reservoir types (i.e., deep saline formations, depleted oil 

and gas reservoirs, faulted or fractured reservoirs, unmineable coal seams, 

basalts, salt domes, reactive carbonates, and shales), injection surface 

climatic extremes, injectate composition, single versus multiple wells, 

conditions (i.e., pressure reduction wells, and horizontal versus vertical 

injection wells), and physical/ geochemical processes deemed important in 

modeling CO2 sequestration (i.e. ., changes of state and chemical reactions). We 

suggest that this suite of problems be defined by experts in the field of CO2 

sequestration modeling and code development, and that a variety of computational 

codes used in modeling GS sites be benchmarked using this suite. A subset of the 

suite of problems may be sufficient to validate a code for use at a particular 

site, however this subset must be rigorously defined (by EPA or the State, if it 

has established primacy for UIC regulation) based on the conditions specific to 

the site in question to ensure that the model being validated is appropriate for 

use at the site. Efforts are already underway to evaluate and compare 

computational codes being used to model GS sites. For example, several code 

intercomparison studies have been conducted to evaluate the capabilities and 

accuracy of numerical simulators for geological storage of green house gases 

(Pruess et al., 2004 and Law et al., 2003). In addition, the Inter-Partnership 

86
Simulation and Risk Working Group within the DOE Regional Partnerships is 

embarking on an effort to compare overall modeling methodologies on a jointly 

accessible data set. This will be more inclusive than a code intercomparison 

effort as the methodologies and conceptual models used may vary widely. 

Model results that should be used to determine if a code is valid for solving a 

specified problem include plume area and maximum plume radius (as defined by 95% 

of the CO2 mass) at particular time planes, injected mass over the time period of 

simulation, pressure distribution, and CO2 phase). Based on the code benchmarks 

and expert knowledge, a tolerance must be defined that determines what 

constitutes acceptable results. This suite of test problems could be modified or 

augmented as the state-of -the-science progresses in modeling, ensuring that any 

computational code being used at a particular site is current with regard to 

accepted scientific principles and rigor. 

In addition to validating particular computational codes using a set of standard 

benchmarks, the implementation of the conceptual model should also be validated 

by comparing model results to observed data, and by using model calibration and 

history matching to improve the model results. EPA understands the importance of 

history-matching of model simulations with monitoring as stated on p. 43507, c. 

1. 

PNNL RECOMMENDATION 

EPA should establish a suite of test problems with specified parameters that 

could be used by the applicant to demonstrate the suitability of the 

computational code to be used for analysis of the proposed site. As 

computational approaches and methods improve, this suite of problems should be 

modified and/or updated to incorporate the state-of -the-knowledge in GS 

modeling. 

We agree that proprietary models can be used, provided they can demonstrate they 

produce results within the defined acceptable tolerance, in addition to 

disclosing the code assumptions, relevant equations, and scientific basis. These 

same requirements must, of course, be met by all computational codes being used 

at GS sites. 

REFERENCES: 

Law, D.H.-S., L.G.H. vander Meer andW.D. Gunter, 2003: Comparison of numerical 

simulators for greenhouse gas storage in coal beds, Part II: Flue gas injection. 

Proceedings of the 6th International Conference on Greenhouse Gas Control 

Technologies (GHGT-6), J. Gale and Y. Kaya (eds.), 1-4 October 2002, Kyoto, 

Japan, Pergamon, v.I, 563-568. Geologischen Gesellschaft,149,3,449-464. 

Pruess K, J Garcia, TKovscek, C Oldenburg, J Rutqvist, C Steefel, and TXu. 2002. 

Intercomparison of Numerical Simulation Codes for Geologic Disposal of CO2. 

LBNL-51813, Lawrence Berkeley National Laboratory, Berkeley, California. 

Response: EPA clarifies that an EPA-prepared technical background document, "Computational

87
Modeling of Underground Injection of Carbon Dioxide for Determination of Area-of-Review and

Potential Risk to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided

in the regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty

analysis in detail. This document was provided as a background to clarify what information EPA

analyzed in order to draft the modeling requirements at §146.84(c)(1), as well as what actions EPA

expects owners or operators and Directors to take in this area. For example, the document states:

"The requirement for an a-priori AoR delineation based on modeling predictions stresses the need

for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended

as the principal evaluation tool for characterizing the most and least important sources of uncertainty

in environmental models."

EPA agrees that in some cases, results from test-cases may be appropriate. In the technical

background document (USEPA, 2008d), EPA provided several questions that should be raised

during evaluation of models. One of these questions was "If using an independently developed or

unverified code, are results from test-problems and comparisons to widely used codes presented?"

Another was "Was relevant site characterization data gathered at a sufficiently high spatial resolution

to inform model development? Was this data used in the model in a way that considers uncertainty?"

Regarding the suggestion that EPA establish a suite of test problems to demonstrate the suitability of

the computational code, EPA notes that the Agency will not perform independent "tests" or

validation of individual modeling products. EPA believes that operators should have the flexibility to

select models with which they are familiar and that are most appropriate to site conditions and

available data types and amounts. 

EPA based this background document in part on guidance drafted by the National Research Council

and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA refers

interested commenters to the document "Guidance on the Development, Evaluation and Application

of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Richard Guinn Letter ID: 0210

Commenter Organization: Eastman Chemical Company Comment ID: 22721

Comment: §146.84 Area of review and corrective action. (c) (1) "Predict, using 

computational modeling, the projected lateral and vertical migration of the 

carbon dioxide plume and formation fluids in the subsurface from the 

commencement of injection activities until the plume movement ceases, pressure 

differentials sufficient to cause the movement of injected fluids or formation 

fluids into a USDW are no longer present, or after a fixed time period as 

determined by the Director." 

We recommend striking out "Predict, using computational modeling, the projected 

lateral and vertical migration of the carbon dioxide plume" and replacing with 

88
"Using computational modeling, provide projected potential ranges of lateral and 

vertical migration of the carbon dioxide plume" The use of multiphase 

groundwater flow and transport models is to predict the composition and movement 

of each phase state (e.g., liquid, gas ,supercritical fluid) of CO2 in the 

subsurface, as well as fluid pressures, to define the AoR. One of the concerns 

with the use of such complex models is that the site may not be fully 

characterized with respect to hydro-geologic properties. There may be 

significant uncertainty in one or more hydraulic processes/parameters. 

Therefore, it is possible to obtain a wide-range of model outcomes from 

different model parameters. The uncertainty in model parameters and sensitivity 

of model results to parameter selection should be addressed. 

Response: EPA has clarified this language in the final rule. It now reads, “Predict, using existing

site characterization, monitoring and operational data, and computational modeling, the projected

lateral and vertical migration of the carbon dioxide plume and formation fluids in the subsurface

from the commencement of injection activities until the plume movement ceases, until pressure

differentials sufficient to cause the movement of injected fluids or formation fluids into a USDW are

no longer present, or until the end of a fixed time period as determined by the Director.”

EPA agrees that modeling results are prone to varied degrees of uncertainty. As discussed in the

preamble to the proposed GS regulation and the associated background document "Computational

Modeling of Underground Injection of Carbon Dioxide for Determination of Area-of-Review and

Potential Risk to Underground Sources of Drinking Water" (cited in the proposed rule preamble as

USEPA, 2008d), model predictions depend largely on the values of key parameters. Often these

parameter values are estimated or averaged from several data sources. EPA also agrees that

sensitivity analyses and "worst case scenario" modeling runs should be presented by the owner or

operator and evaluated by the Director. If during the permit review, the Director determines that

modeling results are too wide-ranging, further information will be required of the owner or operator. 

EPA clarifies that models are tools to help predict the movement of a CO2 plume over long time

frames. However, EPA recognizes that it cannot be expected that modeling something as complex as

a CO2 plume can be done with complete certainty; therefore EPA is also requiring that the AoR be

reevaluated to verify the CO2 plume is moving as predicted and to allow for model calibration and

necessary revisions to the required GS project plans (e.g., AoR and corrective action, testing and

monitoring, injection well plugging, post-injection site care and site closure, and emergency and

remedial response). EPA believes that near-term predictions (e.g., over the subsequent 5 years) will

have the highest degree of certainty. Periodic model calibration will allow refinement to the AoR

delineation based on real-world conditions and operating data and monitoring results, and thus

increase confidence in the modeled predictions. EPA adds that the final GS regulation includes a

requirement at §146.84(e) that the AoR be reevaluated at least every 5 years (an adjustment from the

ten year interval in the proposal), which should further address the commenters' concerns regarding

model uncertainty.

Commenter Name: Richard Guinn Letter ID: 0210

Commenter Organization: Eastman Chemical Company Comment ID: 22729

Comment: EPA seeks comment on the applicability of computational fluid flow models for 

delineating the AoR of GS sites. EPA Notice 69. 

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Computational fluid flow models should be permitted for delineating the AoR of 

GS sites provided also that data are provided supporting their validation in 

similar complex structures and flow regimes 

Response: EPA agrees that a detailed geologic assessment is essential to evaluating the presence and

adequacy of a proposed site to receive and confine the CO2. The AoR and corrective action plan

required at §146.84(b) must describe the site characterization data on which the model will be based.

The Director has discretion to require the use of existing data if it is not initially included. 

The Agency believes that predicting the complex multiphase buoyant flow of the CO2, co-injectates,

and compounds that may be mobilized due to injection requires the sophistication of computational

models. Models can predict CO2 fate and transport, as well as changes in formation pressure, in

three dimensions, as a function of time in order to delineate the subsurface storage site and the AoR.

Models can account for site-specific and predicted operating conditions as well as geologic

heterogeneities and potential migration through faults, fractures, and artificial penetrations.

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

Commenter Name: Ronald Evans Letter ID: 0215

Commenter Organization: Denbury Resources, Inc. (DRI) Comment ID: 24291

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: B. §146.84 Area of review and corrective action. 

EPA’s proposed rule allows each applicant to define the Area of Review ("AoR") 

associated with the sequestration project. Denbury has no objection to this 

flexible approach as it allows the size and shape of the AoR to reflect the 

unique geology of each CO2 sequestration project. However, under §146.84 (a), to 

define the AoR, an applicant must use a model capable of considering the 

physical and chemical properties of all phases of the injected carbon dioxide 

stream. Denbury is uncertain that there is a model that can satisfy this 

requirement. Accordingly, Denbury suggests that the proposed rule be amended as 

follows: 

§146.84 Area of review and corrective action. 

(a) The area of review is the region surrounding the geologic 

sequestration project that may be impacted by the injection activity. 

The area of review is based on computational modeling that, TO THE 

EXTENT PRACTICABLE AND TO THE EXTENT DETERMINED TO BE NECESSARY BY

THE 

DIRECTOR, accounts for the MATERIAL physical and chemical properties of 

all CRITICAL phases of the injected carbon dioxide stream. 

Response: EPA disagrees that available multiphase models cannot consider the physical and

chemical properties of all phases of the injected CO2 stream. Research, as discussed in the preamble

to the proposed GS regulation and the associated background document "Computational Modeling of

90
Underground Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk

to Underground Sources of Drinking Water" (cited in the proposed rule preamble as USEPA,

2008d), substantiates the modeling requirements in the GS regulation and indicates that technology

has advanced to the point that a CO2 injection project can be adequately modeled to support both

prediction and tracking the movement of the CO2 plume and pressure front. EPA believes the

Director should have discretion to evaluate each plan based on current, state of the art modeling

science because there are large numbers and types of available models and because advances in the

field of geologic modeling continue. 

Based on review of all public comments on the AoR definition at 146.84(a), the Agency has

modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

Commenter Name: Darrick Eugene Letter ID: 0223

Commenter Organization: Texas Carbon Capture and Storage Association Comment ID: 23970

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: 3. §146.84 Area of review and corrective action. 

EPA’s proposed rule allows each applicant to define the Area of Review ("AoR") 

associated with the sequestration project. TxCCSA has no objection to this 

flexible approach as it allows the size and shape of the AoR to reflect the 

unique geology of each CO2 sequestration project. However, under §146.84 (a), to 

define the AoR, an applicant must use a model capable of considering the 

physical and chemical properties of all phases of the injected carbon dioxide 

stream. TxCCSA is uncertain that there is a model that can satisfy this 

requirement. Accordingly, TxCCSA suggests that the proposed rule be amended as 

follows: 

§146.84 Area of review and corrective action. (a) The area of review is 

the region surrounding the geologic sequestration project that may be 

impacted by the injection activity. The area of review is based on 

computational modeling that, TO THE EXTENT PRACTICABLE AND TO THE EXTENT 

DETERMINED TO BE NECESSARY BY THE DIRECTOR, accounts for the MATERIAL 

physical and chemical properties of all CRITICAL phases of the 

injected carbon dioxide stream. 

Response: EPA disagrees that available multiphase models cannot consider the physical and

chemical properties of all phases of the injected CO2 stream. Research, as discussed in the preamble

to the proposed GS regulation and the associated background document "Computational Modeling of

Underground Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk

to Underground Sources of Drinking Water" (cited in the proposed rule preamble as USEPA,

2008d), substantiates the modeling requirements in the GS regulation and indicates that technology

has advanced to the point that a CO2 injection project can be adequately modeled to support both

prediction and tracking the movement of the CO2 plume and pressure front. EPA believes the

Director should have discretion to evaluate each plan based on current, state of the art modeling

91
science because there are large numbers and types of available models and because advances in the

field of geologic modeling continue. 

Based on review of all public comments on the AoR definition at 146.84(a), the Agency has

modified it to read as follows: The region surrounding the geologic sequestration project where

USDWs may be endangered by the injection activity. The area of review is delineated using

computational modeling that accounts for the physical and chemical properties of all phases of the

injected CO2 stream and is based on available site characterization, monitoring, and operational data.

Commenter Name: Ben Yamagata Letter ID: 0227

Commenter Organization: Coal Utilization Research Council Comment ID: 21787

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: Area of Review (AoR) 

The proposed regulation does not actually define the AoR, it merely states that 

the area will be determined by mathematical models. One of the concerns with 

the use of such complex models is that the site may not be fully characterized 

with respect to hydro-geologic properties. There may be significant uncertainty 

in one or more hydraulic process/parameters. Therefore, it is possible to 

obtain a wide-range of model outcomes from different model parameters. The 

treatment of uncertainty in model parameters and sensitivity of model results to 

parameter selection should be addressed. Given that the AoR will be 

periodically revisited, we recommend that the final rule make a pragmatic 

determination of what will be necessary to protect groundwater, given the 

requirement for reevaluation over time, and that the Agency focus the 

requirements to rely upon the mathematical models with the inherent differences 

noted above. 

Response: EPA agrees that modeling results are prone to varied degrees of uncertainty. As discussed

in the preamble to the proposed GS regulation and the associated background document

"Computational Modeling of Underground Injection of Carbon Dioxide for Determination of Areaof-Review and Potential Risk to Underground Sources of Drinking Water" (cited in the proposed

rule preamble as USEPA, 2008d), model predictions depend largely on the values of key parameters.

Often these parameter values are estimated or averaged from several data sources. EPA also agrees

that sensitivity analyses and "worst case scenario" modeling runs should be presented by the owner

or operator and evaluated by the Director. If during the permit review, the Director determines that

modeling results are too wide-ranging, further information will be required of the owner or operator. 

EPA clarifies that models are tools to help predict the movement of a CO2 plume over long time

frames. However, EPA recognizes that it cannot be expected that modeling something as complex as

a CO2 plume can be done with complete certainty; therefore EPA is also requiring that the AoR be

reevaluated to verify the CO2 plume is moving as predicted and to allow for model calibration and

necessary revisions to the required GS project plans (e.g., AoR and corrective action, testing and

monitoring, injection well plugging, post-injection site care and site closure, and emergency and

remedial response). EPA believes that near-term predictions (e.g., over the subsequent 5 years)

would have the highest degree of certainty. Periodic model calibration would allow refinement to the

AoR delineation based on real-world conditions and operating data and monitoring results, and thus

92
increase confidence in the modeled predictions. EPA adds that the final GS regulation includes a

requirement at 146.84(e) that the AoR be reevaluated every 5 years (an adjustment from the ten year

interval in the proposal), which should further address the commenter’s concerns regarding model

uncertainty.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23231

Other EPA Comment Codes and Descriptions: 3320 2.2.1.2 Comments opposed to requiring

computational models to delineate the AoR 

Comment: 11. EPA believes that predicting the complex multi-phase buoyant flow of the 

CO2, "co-injectates," and compounds that may be mobilized due to injection 

requires the sophistication of computational models. EPA proposes that the 

owners or operators of GS wells be required to delineate the AoR for CO2 GS 

sites using computational fluid flow models designed for the specific site 

conditions and injection regime. EPA seeks comment on the use of modeling for 

AoR delineation. EPA Notice 43506. 

There is concern about the lack of information necessary to construct a valid 

model in areas where the sequestration zone has not been extensively studied. 

This lack of information may make modeling difficult or not very useful. Any 

modeling used should be re-examined and recalibrated over time with additional 

data gathered. Modeling should not be solely relied upon where real data offers 

a different view. Seismic and offset well data can help delineate the AOR. 

Response: The Agency believes that AoR delineation using computational modeling that accounts

for the multiphase flow of CO2 and native fluids (i.e., naturally occurring salty water), including

density-driven flow (i.e., buoyancy), is necessary to ensure protection of USDWs from

endangerment. The computational models used for this purpose are required to predict the fate of

injected CO2 and changes in formation pressure. EPA believes that the requirements at 146.84(c)(1)

to delineate the AoR using computational modeling are appropriate and encompass the significant

processes that could affect the location of the CO2 plume and associated pressure front and has

retained this in the final rule. 

EPA agrees that the modeling should be re-examined and recalibrated over time. The GS regulation

at 146.84(e) also requires owners or operators to periodically reevaluate the AoR to incorporate

monitoring and operational data. This process provides additional opportunities to verify that the

models have accurately predicted movement of the CO2 plume and to make any needed adjustments

to corrective action or monitoring at the site.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23233

Comment: 13. EPA seeks comment on the applicability of computational fluid flow models 

for delineating the AoR of GS sites. EPA Notice 43507. 

The use of computational flow models is appropriate, however; modeling should 

not be the sole criteria to determine the AOR if actual measured data exists. 

Response: EPA agrees that a detailed geologic assessment is essential to evaluating the presence and

93
adequacy of a proposed site to receive and confine the CO2. The AoR and corrective action plan

required at §146.84(b) must describe the site characterization data on which the model will be based.

The Director has discretion to require the use of existing data if it is not initially included. 

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

The Agency believes that predicting the complex multiphase buoyant flow of the CO2, co-injectates,

and compounds that may be mobilized due to injection requires the sophistication of computational

models. Models can predict CO2 fate and transport, as well as changes in formation pressure, in

three dimensions, as a function of time in order to delineate the subsurface storage site and the AoR.

Models can account for site-specific and predicted operating conditions as well as geologic

heterogeneities and potential migration through faults, fractures, and artificial penetrations.

Letter ID: 0263

Commenter Organization: Interstate Oil & Gas Compact Commission Comment ID: 22799

Comment: III A. 2 Proposed Area of Review and Corrective Action Requirements 

N. EPA seeks comment on the applicability of computational fluid flow models for 

delineating the AoR ofGS sites. 

IOGCC Response: See General Comment for Section III. A. 2 above. The collection 

of physical data (i.e. well bore data) maybe necessary to fully utilize modeling 

to determine the ultimate extent of the proposed CO2 storage volume. Models should 

only be used when they are appropriate. The regulator of a GS project should not 

be required to rely fully on models. As noted above, states need the flexibility 

to be able to go beyond models. 

Response: EPA agrees that a detailed geologic assessment is essential to evaluating the presence and

adequacy of a proposed site to receive and confine the CO2. The AoR and corrective action plan

required at §146.84(b) must describe the site characterization data on which the model will be based.

The Director has discretion to require the use of existing data if it is not initially included. 

EPA clarifies that the model inputs must include actual geologic information, data collected during

the site characterization process (e.g., information about artificial penetrations), and anticipated

operating parameters (e.g., injection rates) submitted with the permit application under §146.82.

Models will be revised and recalibrated during subsequent AoR reevaluations, which must account

for all available monitoring and operational data, as required at §146.84(e).

The Agency believes that predicting the complex multiphase buoyant flow of the CO2, co-injectates,

and compounds that may be mobilized due to injection requires the sophistication of computational

models. Models can predict CO2 fate and transport, as well as changes in formation pressure, in

three dimensions, as a function of time in order to delineate the subsurface storage site and the AoR.

Models can account for site-specific and predicted operating conditions as well as geologic

heterogeneities and potential migration through faults, fractures, and artificial penetrations.

94
EPA agrees that it is appropriate to allow operators the flexibility to select modeling, site

characterization, and monitoring techniques that are most appropriate to site conditions and available

types and amounts of data. The owner or operator must demonstrate, in the proposed AoR and

corrective action plan at §146.84(b), to the satisfaction of the Director that the modeled AoR will be

based on detailed geologic data collected on the site. 

Comment Code: 3340 2.2.1.4 Specific delineation models 

Individual Comments

Commenter Name: Dag Nummedal Letter ID: 0533

Commenter Organization: Colorado Energy Resource Institute Comment ID: 21478

Comment: California and the Lawrence Berkeley lab on the Berkeley campus. These labs have 

over the last five to ten years developed some codes called NUFT and Tough-react 

numerical codes worry about those things. That really have advanced our ability 

as scientists to model ahead of time what will happen to minerals and 

precipitation and dissolution when this CO2 charge fluids go through the rocks. 

Then the model predictions and the American Model Predictions will then be 

tested against lab experiments. One of the things that we are very proud of at 

the Colorado School of Mines is how dedicated the university is to laboratory 

work. There is only so much you can do in the area of modeling. You have to test 

the models in laboratory before you can feel comfortable using the models 

afterwards to predict what would happen in the subsurface. 

Response: EPA acknowledges the range and capabilities of numerical modeling codes available to

GS operators and other modelers and acknowledges the comment.

Comment Code: 3350 2.2.1.5 Use of proprietary models for GS sites (seeking comment) 

Individual Comments

Commenter Name: John Hanger Letter ID: 0130

Commenter Organization: Pennsylvania Dept. of Env. Protection Comment ID: 21505

Comment: -Proprietary models should not be acceptable for use because of the complex 

nature of the activity and the many unknowns. Through a transparent process, 

EPA should maintain minimum modeling standards and assumptions required for 

approval. 

Response: EPA disagrees that the use of proprietary models should be disallowed. EPA

acknowledges that several proprietary models are currently, successfully used by petroleum-reservoir

engineering professionals as well as in peer-reviewed studies to model CO2 sequestration. 

95
EPA does not believe that the Agency should develop a standards and assumptions for modeling the

AoR for GS sites. Instead, EPA believes that operators should have the flexibility to select the most

appropriate models for the conditions at the GS site and based on the types and amount of data

available given the wide range of site-specific conditions present at GS sites. EPA is developing

guidance on AoR and corrective action at GS Class VI sites to assist owners or operators and

Directors in evaluating requirements and proposed AoR and corrective action plans under §146.84.

The guidance describes EPA's recommended approaches for the data to be considered in an initial

AoR delineation, how to apply computational modeling to delineate the AoR, performing corrective

action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be

available on EPA's web site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

To ensure that all predictive models used for AoR calculations are meeting the Agency's

expectations, EPA plans, every six years, to collect and review data on GS projects to determine

whether the appropriate amount and types of information and appropriate documentation are being

collected, and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Carl Carlsson Letter ID: 0131

Commenter Organization: SUEZ Energy Generation NA, LLC Comment ID: 21529

Comment: Use of Proprietary Models to Delineate the Area of Review 

SEGNA agrees with EPA that models used for GS analysis should be based on 

accepted science and should be validated. In addition to requiring operators to 

disclose code assumptions, relevant equations and the scientific basis for 

proprietary models, SEGNA suggests that EPA consider establishing a validation 

process for such models using empirical data to ensure such models are sound. 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA plans to allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

In response to comments suggesting that history matching or validation of these models is needed,

EPA believes this is true for any model. EPA notes that the GS rule requires at §146.84(e) that the

AoR be reevaluated to verify that the CO2 plume is moving as predicted and to allow for necessary

revisions to the AoR and corrective action, testing and monitoring, injection well plugging, postinjection site care and site closure, and emergency and remedial response plans. EPA clarifies,

however, that the Agency will not perform independent validation of individual modeling products.

The Agency plans, every six years, to collect and review data on GS projects to determine whether

the appropriate amount and types of information and appropriate documentation are being collected,

and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22550

96
Comment: Use of commercial proprietary codes should be authorized. Proprietary doesn't 

mean secret. A proprietary code largely vetted by the industry can be the best 

application in many circumstances. To promote peer review and benchmarking, the 

assumptions and theories for all numerical models should be publicly available 

and the solution of any analytic equations used in the predictive process should 

be fully elaborated. 

Response: EPA agrees that available proprietary models may meet the complex needs for

calculating the AoRs of GS sites, and shares their concern about the need to evaluate the inputs and

assumptions used. EPA will allow the use of proprietary models in meeting the requirements for the

GS regulation, as long as GS well operators using a proprietary model clearly disclose, to the

Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA adds that

its Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of

proprietary models.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22580

Comment: Preamble Questions: In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA 

says it is aware that the use of proprietary codes may prevent full evaluation 

of model results (e.g., NRC, 2007). Several popular codes in the petroleumreservoir engineering discipline are proprietary and owners or operators of 

particular sites may prefer to use these codes as they have previous experience 

with them, and they have been used in peer-reviewed studies to model CO2 

sequestration. When using a proprietary model, owners or operators should 

clearly disclose the code assumptions, relevant equations, and scientific basis. 

EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 68. 

NGWA Response: Yes, use of commercial proprietary codes should be authorized. 

Proprietary doesn't mean secret. A proprietary code largely vetted by the 

industry can be the best application in many circumstances. To promote peer 

review and benchmarking, the assumptions and theories for all numerical models 

should be publicly available and the solution of any analytic equations used in 

the predictive process should be fully elaborated. 

Response: As described above, EPA agrees that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: Brianne Metzger Letter ID: 0151

Commenter Organization: Spectra Energy Corp Comment ID: 21818

Comment: A. EPA Should Allow The Use Of Proprietary Models For Modeling CO2 Movement

and 

97
Reservoir Pressure. 

EPA correctly acknowledges that some companies use proprietary models for their 

current GS operations, and has requested comment on allowing the use of such 

models for GS sites. Spectra strongly agrees with the continued use of 

proprietary models. While Spectra would be willing to provide information 

pertaining to the underlying assumptions and scientific basis, subject to 

protection as confidential business information ("CBI"), we would object to any 

effort by the Agency to compel further disclosure regarding such models. 

Although we recognize that the Agency is required by law to take certain steps 

to protect CBI, we are mindful that such protections are not always fail-safe 

and inadvertent disclosures of CBI are not uncommon. Such disclosures in the 

case of the critical infrastructure we operate or may operate in the future 

could be extremely detrimental and costly. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models. Information including model assumptions and equations are used by the Director to inform

his/her decision to issue the permit and ensure that the requirements are met and the GS well will not

endanger USDWs. EPA adds that its Confidential Business Information requirements at 40 CFR

144.5 will apply to the use of proprietary models.

Commenter Name: James Dooley Letter ID: 0156

Commenter Organization: Joint Global Change Research Institute, Pacific

Northwest National Laboratory

Comment ID:

22617

Comment: Page 43507. The USEPA specifically asks for comments on the appropriateness of 

allowing proprietary models to be used in support of a Class VI permit. There 

is nothing wrong with using proprietary models. The test as to whether it is 

appropriate to use proprietary models (or "open community" models) should be 

whether these models are seen by the USEPA and the technical community as 

providing robust, accurate, and useful results. It is much more important to 

understand a model’s track record in this regard than to ask for documentation 

of "code assumptions, relevant equations, and scientific basis." Given the 

inherent complexity of these models, asking for "code assumptions, relevant 

equations, and scientific basis" could result in the submittal of an extremely 

long technical document that is so dense it will not be read or the submittal of 

a very superficial treatment of the model. The USEPA should -- without delay -- 

begin deliberations on methodologies that could be used to transparently 

establish the robustness of these models and their results/outputs. This could 

be done - for example - through formal model intercomparisons using a set of 

test cases and repeating this process every couple of years. It will be far 

more useful to invest the time, thinking and if need be resources now to help 

establish which models are useful and for what applications than to request that 

each Class VI permit application include a detailed 100-1000 page description of 

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the model used and how key geomechanical/geophysical processes were 

parameterized. USEPA is strongly encouraged to change the focus here from a 

process oriented assessment of how these models work to a more outcome oriented 

measure of the perceived usefulness and robustness of these models by the 

technical community and other stakeholders (e.g., Wall Street) as useful tools 

for GS. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models.

EPA does not plan to initiate a formal review of models or modeling methodologies, as the

commenter suggests. Instead, EPA believes that it is appropriate to allow operators the flexibility to

select modeling techniques that are most appropriate to site conditions and available types and

amounts of data. The owner or operator must demonstrate, in the proposed AoR and corrective

action plan required at §146.84(b), to the satisfaction of the Director, that the modeled AoR will be

based on detailed geologic data collected on the site. EPA anticipates that any model that does this in

a satisfactory manner will be approved by the Director. EPA acknowledges that modeling technology

is expected to continuously improve, and encourages and expects owners or operators to use the best

available multiphase computational models to determine the AoR. Reliance on improved models will

likely increase the accuracy and quality of the AoR characterization, resulting in better protection of

USDWs from endangerment. 

Commenter Name: James Childress Letter ID: 0158

Commenter Organization: Gasification Technologies Council (GTC) Comment ID: 23778

Comment: EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 68. 

Proprietary models should be acceptable provided that supporting data for 

validation are submitted. 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: Theresa Pugh Letter ID: 0169

Commenter Organization: American Public Power Association Comment ID: 23401

Comment: Use of proprietary models: EPA asked about AoR and proprietary models. APPA is 

not expert on models but has some general discomfort with relying solely on 

99
proprietary models owned/operated by a third party-particularly when there are 

only approximately five companies in the United States that have such 

sophisticated models for AoR modeling. Proprietary models are de rigueur in the 

oil and gas extractive industry. We would not seek to change that for such a 

critical industry to our economy and to our own natural resource needs. However, 

it would not lead to an easy future for power plant owner/operators to not know 

how site selection and AoR decisions are made because the owner/operator of the 

AoR work would use confidential the proprietary models from even the power plant 

owner/operator. It would be rare to have such a significant portion of the 

operational costs where the power plant’s management could not review the model 

or run a cross check proprietary models to affirm site selection or to do double 

checks after operations had begun. This is a tricky question for APPA. The AoR 

decisions will be fundamental to the entire geologic sequestration decision 

tree. Yet, if proprietary models are used, the power plant management will have 

little say over the geosequestration operations. Additionally, relying upon 

proprietary models for such a significant portion of the geologic sequestration 

decision making will be anti-competitive. It is not easy for power plants 

owner/operators to easily know what the costs of an operation will be for the 

consumers if they are relying upon proprietary AoR models at the beginning of 

the geologic sequestration process and relying upon these same models during the 

40-50 year life of the plant. APPA would prefer that the use of a replicatable 

model is used. This may need considerable work by the U. S. EPA to ensure that 

the proprietary nature of these seismic modeling products and services are not 

exposed in a way that harms the oil field services industry. Perhaps the EPA can 

review what confidential business information methods are undertaken in the Food 

and Drug Administration or Homeland Security Administration as those agencies 

review very confidential drug testing or security data which much be 

confidential but still usable and able to be reviewed. 

APPA respects that the true value in the oil and gas extractive industry is in 

the quality of its seismic readings technology and models that make it the preeminent in the world. No one at APPA wants to tamper with that. We wonder if 

some sort of proprietary method can be arranged for the AoR models to be run off 

of proprietary models where the model can at least be reviewed or replicated for 

quality purposes by the state director and the power plant owner/operator. 

Without this ability to have a handle on operational costs and decisions made by 

the contractors to the power sector, many public power communities would be 

reticent to commit to a technology where the proprietary model sets in motion 

most major decisions without a way to review them or make mid-course 

corrections. 

Response: EPA agrees that available proprietary models may meet the complex needs for

calculating the AoRs of GS sites, and shares their concern about the need to evaluate the inputs and

assumptions used. EPA notes that several proprietary models are currently, successfully used by

petroleum-reservoir engineering professionals as well as in peer-reviewed studies to model CO2

sequestration. EPA adds that its Confidential Business Information requirements at 40 CFR 144.5

will apply to the use of proprietary models.

EPA clarifies that owners or operators have flexibility to select the most appropriate models for the

conditions at the GS site and based on the types and amount of data available given the wide range

100
of site-specific conditions present at GS sites. 

To ensure that all predictive models used for AoR calculations are meeting the Agency's

expectations, EPA plans, every six years, to collect and review data on GS projects to determine

whether the appropriate amount and types of information and appropriate documentation are being

collected, and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24224

Comment: Rule Provisions: 146.84 

EPA Preamble Question: 12. In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA 

says it is aware that the use of proprietary codes may prevent full evaluation 

of model results (e.g., NRC, 2007). Several popular codes in the petroleumreservoir engineering discipline are proprietary and owners or operators of 

particular sites may prefer to use these codes as they have previous experience 

with them, and they have been used in peer-reviewed studies to model CO2 

sequestration. When using a proprietary model, owners or operators should 

clearly disclose the code assumptions, relevant equations, and scientific basis. 

EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 43507. 

API Response: Use of proprietary models should be allowed. EPA’s Confidential 

Business Information requirements should apply to any proprietary model used. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models.

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23485

Other EPA Comment Codes and Descriptions: 3320 2.2.1.2 Comments opposed to requiring

computational models to delineate the AoR 

Comment: 8. Modeling 

As most models now are proprietary, USWAG supports the use of proprietary models 

for use with CO2 GS sites. As EPA recognizes, many popular proprietary codes in 

the petroleum-reservoir engineering discipline have been used in peer-reviewed 

studies to model CO2 sequestration. 73 Fed. Reg. at 43507. These and other 

proprietary models should be available for use in modeling for Class VI CO2 GS 

wells, provided that they are based on accepted science and have been validated, 

and as long as the Owners or Operators using proprietary models disclose, as 

necessary, the code assumptions, relevant equations, and scientific basis of 

101
each model. USWAG believes that the use of computational fluid flow models for 

delineating the AoR of GS sites is acceptable but should not be required. Id. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites and will allow their use for GS projects. EPA acknowledges that

several proprietary models have been used by petroleum-reservoir engineering professionals as well

as in peer-reviewed studies to model CO2 sequestration. EPA plans to allow their use in meeting the

requirements for the GS regulation, but clarifies that GS well operators using proprietary models are

required to disclose, to the Director's satisfaction, the code assumptions, relevant equations, and

scientific basis. EPA adds that its Confidential Business Information requirements at 40 CFR 144.5

will apply to the use of proprietary models.

EPA clarifies that the final rule will require the use of computational models. EPA believes that the

unique aspects of GS merit the use of sophisticated computational modeling for delineation of the

AoR, based on site-specific features and geology, including the presence of faults, fractures, and

artificial penetrations. EPA believes that computational modeling of the AoR prior to injection will

provide the best obtainable estimates of the extent of the CO2 plume over several years of active

injection.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23690

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: Category: Modeling 

EPA Request for Comment: EPA proposes that the owners or operators of GS wells 

be required to delineate the AoR for CO2 GS sites using computational fluid flow 

models designed for the specific site conditions and injection regime. EPA seeks 

comment on the use of modeling for AoR delineation. EPA Notice 43506. 

When using a proprietary model, owners or operators should clearly disclose the 

code assumptions, relevant equations, and scientific basis. EPA seeks comment on 

allowing the use of proprietary models for GS sites. EPA Notice 43507. 

AWWA Response: The proposed rule depends heavily on computer modeling to track 

the carbon dioxide plume and demonstrate that it is contained within the 

confining zone. As stated by EPA, some of these models are proprietary and the 

rule is unclear how or if confirmation of the model results will be provided. 

Additionally, each of the individual proprietary models can show a different 

answer to the problem. Owner/operators need to ensure that their basis for the 

model is static, and that the basis does not change once more details are known 

about the movement of the plume. For example, the operation of the geologic 

sequestration system should be adjusted based on modeling results, and the model 

shouldn't be revised because the plume is moving in a direction not originally 

predicted. 

AWWA recommends that EPA standardize on one specific model that is transparent 

and available for outside stakeholders to review, including the public. The 

other option is for EPA to develop a standard model that would be used for all 

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geologic sequestration projects. If EPA were to develop a standard model, a 

pilot program should be part of the model development so that the modeling 

results could be validated with real-life monitoring data. In either case, 

clearly stating the model assumptions made during model set-up is critical, and 

these assumptions must be made available to stakeholders when they are reviewing 

the model results. 

Response: The final rule does not require use of a standardized model. As described above, EPA

believes that operators should have the flexibility to select models with which they are familiar and

that are most appropriate to the site conditions and available data types and amounts. EPA also

recognizes that a standardized model for AoR delineations could be effective as long as the model

accounts for the expected variations in site conditions, and adds that a state or regional UIC Director

could select one standard model if it took site-specific variations into account. 

EPA does not envision that members of the public will evaluate the AoR delineation models

themselves. The models and their inputs will, by necessity, entail an extensive amount of technical

information that may not be easily understood by the public. EPA believes that it is the Director's

responsibility, as a steward of public health and the environment, to review models, model inputs,

and results and to determine whether the model is appropriate for predicting the AoR, and to

distribute information that may be easily understood by the public (e.g., in the form of a fact sheet).

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25182

Comment: K. EPA seeks comment on allowing the use of proprietary models for GS sites. 

NYSDEC-DMN Response: NYSDEC-DMN believes proprietary models should be allowed 

provided the Director has discretion to evaluate the accuracy of the modeling 

methodology. It should be noted that confidentiality of proprietary models will 

be subject to various jurisdictions and statutory limitations. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. Information

including model assumptions and equations are used by the Director to inform his/her decision to

issue the permit and ensure that the requirements are met and the GS well will not endanger USDWs.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24516

Comment: Rule Provisions or Federal Register Cite: 146.84 

EPA Preamble Question: 12. In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA 

says it is aware that the use of proprietary codes may prevent full evaluation 

103
of model results (e.g., NRC, 2007). Several popular codes in the petroleumreservoir engineering discipline are proprietary and owners or operators of 

particular sites may prefer to use these codes as they have previous experience 

with them, and they have been used in peer-reviewed studies to model CO2 

sequestration. When using a proprietary model, owners or operators should 

clearly disclose the code assumptions, relevant equations, and scientific basis. 

EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 43507. 

CSC Response: Use of proprietary models should be allowed. EPA’s Confidential 

Business Information requirements should apply to any proprietary model used. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models. Information including model assumptions and equations are used by the Director to inform

his/her decision to issue the permit and ensure that the requirements are met and the GS well will not

endanger USDWs. EPA adds that its Confidential Business Information requirements at 40 CFR

144.5 will apply to the use of proprietary models.

Commenter Name: Paul Pike Letter ID: 0198

Commenter Organization: Ameren Corporation Comment ID: 23946

Comment: MODELING 

Ameren believes that any model used in evaluating Class VI CO2 GS project is 

acceptable provided that it is based on accepted science and has been validated, 

and as long as we if are using a proprietary model disclose, as necessary, the 

code assumptions, relevant equations, and scientific basis of the model. 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

In response to comments suggesting that history matching or validation of these models is needed,

EPA believes this is true for any model. EPA notes that the GS rule requires at §146.84(e) that the

AoR be reevaluated to verify the CO2 plume is moving as predicted and to allow for necessary

revisions to the AoR and corrective action, testing and monitoring, injection well plugging, postinjection site care and site closure, and emergency and remedial response plans. EPA clarifies,

however, that the Agency will not perform independent validation of individual modeling products.

The Agency plans, every six years, to collect and review data on GS projects to determine whether

104
the appropriate amount and types of information and appropriate documentation are being collected,

and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24579

Comment: Rule Provisions: 146.84 

EPA Rule Issue: 12. In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA 

says it is aware that the use of proprietary codes may prevent full evaluation 

of model results (e.g., NRC, 2007). Several popular codes in the petroleumreservoir engineering discipline are proprietary and owners or operators of 

particular sites may prefer to use these codes as they have previous experience 

with them, and they have been used in peerreviewed studies to model CO2 

sequestration. When using a proprietary model, owners or operators should 

clearly disclose the code assumptions, relevant equations, and scientific basis. 

EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 43507. 

Anadarko Petroleum Corporation (APC) Response: It is critical to allow 

proprietary models to be used. EPA’s Confidential Business Information 

requirements should apply to any proprietary model used. APC recommends EPA 

allow the use of proprietary models and clearly state that CBI requirements will 

apply. 

While there has been concern raised on the current model’s ability to predict 

fluid flow in saline reservoirs, the development of technologies in this area 

will only progress when there is an economic driver that proprietary models will 

provide. In addition, being able to see the assumptions and equations of the 

model will provide the regulating authority the ability to assess the models 

accuracy. The currently proposed regulations also contain requirements to 

revisit these models when the actual site data no longer supports the models 

conclusions, or at a minimum of every 10 years, which APC believes is sufficient 

to ensure the viability of the models and data generated from their use. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models. Information including model assumptions and equations are used by the Director to inform

his/her decision to issue the permit and ensure that the requirements are met and the GS well will not

endanger USDWs. EPA adds that its Confidential Business Information requirements at 40 CFR

144.5 will apply to the use of proprietary models. 

EPA clarifies that the GS rule requires that the AoR be reevaluated (§146.84(e)) to verify that the

CO2 plume is moving as predicted and to allow for necessary revisions to the AoR and corrective

action plan. This process will also serve to validate the modeling approach, as the commenter

105
suggests.

Regarding the commenter’s concern about current modeling ability to predict fluid flow in saline

reservoirs, EPA adds that throughout development of the proposed GS regulation, EPA conducted a

number of technical workshops inviting expert advice regarding modeling, and evaluated peerreviewed journal articles and performed critical reviews pertaining to computational modeling of

CO2 injection. Information collected from these efforts assured the Agency that available complex

computational models can effectively simulate CO2 transport, fate and changes in formation

pressure in three dimensions as a function of time. However, EPA agrees with the commenter that

modeling technology will evolve and improve over the coming years. The Agency plans, every six

years, to collect and review data on GS projects to determine whether the appropriate amount and

types of information and appropriate documentation are being collected, and to determine if

modifications to the requirements are appropriate or necessary.

Commenter Name: Chris M. Hobson Letter ID: 0201

Commenter Organization: Southern Company Comment ID: 22167

Comment: Modeling- Southern Company supports EPA's suggestion to allow for use of 

proprietary models with CO2 GS sites, provided that the permit applicant 

discloses the code assumptions, relevant equations, and scientific basis of the 

model. Id. at 43507. Based on the valuable experience Southern Company has 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

Commenter Name: John Corra Letter ID: 0205

Commenter Organization: Wyoming Department of Environmental Quality

(DEQ)

Comment ID:

22202

Comment: 6. Various issues where we agree with US EPA that the Director should have the 

discretion to determine, or require: 

c. Allowing for the use of proprietary models. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. Information including

model assumptions and equations are used by the Director to inform his/her decision to issue the

permit and ensure that the requirements are met and the GS well will not endanger USDWs. EPA

adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the use

of proprietary models.

106
Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22447

Other EPA Comment Codes and Descriptions: 3330 2.2.1.3 Applicability /adequacy of

computational models (seeking comment) 

Comment: VALIDATION OF MULTIPHASE COMPUTATIONAL MODELS 

PROPOSED REGULATION 

p. 43507, c. 1 Models used for GS sites should be based on accepted science and 

should be validated. In some cases, owners or operators may choose to use 

proprietary models (i.e., not available for free to the general public). EPA is 

aware that the use of proprietary codes may prevent full evaluation of model 

results (e.g ., NRC, 2007). Several popular codes in the petroleum-reservoir 

engineering discipline are proprietary and owners or operators of particular 

sites may prefer to use these codes as they have previous experience with them, 

and they have been used in peer-reviewed studies to model CO2 sequestration. 

When using a proprietary model, owners or operators should clearly disclose the 

code assumptions, relevant equations, and scientific basis. 

DISCUSSION 

We agree that computational models used for GS sites should be based on accepted 

science and should be validated. However, the proposed rule does not define 

validation. We believe that these models must be validated in terms of their 

appropriateness for simulating the complexities of CO2 sequestration under the 

conditions present at the specific site being evaluated. The fact that a code 

has been validated and previously used to model CO2 sequestration (even if peerreviewed) does not ensure that it is appropriate for use at all sites and under 

all conditions. For example, it is possible that, for a specific site, certain 

physical and/or geochemical processes must be considered that are not available 

in a given computational code. Whereas that code may have reasonably and 

successfully modeled CO2 sequestration at a dissimilar site, it may not be 

appropriate for the site being considered. 

It may be appropriate for EPA to establish a suite of test problems that span 

the breadth of GS reservoir types (i.e., deep saline formations, depleted oil 

and gas reservoirs, faulted or fractured reservoirs, unmineable coal seams, 

basalts, salt domes, reactive carbonates, and shales), injection surface 

climatic extremes, injectate composition, single versus multiple wells, 

conditions (i.e., pressure reduction wells, and horizontal versus vertical 

injection wells), and physical/ geochemical processes deemed important in 

modeling CO2 sequestration (i.e. ., changes of state and chemical reactions). We 

suggest that this suite of problems be defined by experts in the field of CO2 

sequestration modeling and code development, and that a variety of computational 

codes used in modeling GS sites be benchmarked using this suite. A subset of the 

suite of problems may be sufficient to validate a code for use at a particular 

site, however this subset must be rigorously defined (by EPA or the State, if it 

has established primacy for UIC regulation) based on the conditions specific to 

the site in question to ensure that the model being validated is appropriate for 

use at the site. Efforts are already underway to evaluate and compare 

107
computational codes being used to model GS sites. For example, several code 

intercomparison studies have been conducted to evaluate the capabilities and 

accuracy of numerical simulators for geological storage of green house gases 

(Pruess et al., 2004 and Law et al., 2003). In addition, the Inter-Partnership 

Simulation and Risk Working Group within the DOE Regional Partnerships is 

embarking on an effort to compare overall modeling methodologies on a jointly 

accessible data set. This will be more inclusive than a code intercomparison 

effort as the methodologies and conceptual models used may vary widely. 

Model results that should be used to determine if a code is valid for solving a 

specified problem include plume area and maximum plume radius (as defined by 95% 

of the CO2 mass) at particular time planes, injected mass over the time period of 

simulation, pressure distribution, and CO2 phase). Based on the code benchmarks 

and expert knowledge, a tolerance must be defined that determines what 

constitutes acceptable results. This suite of test problems could be modified or 

augmented as the state-of -the-science progresses in modeling, ensuring that any 

computational code being used at a particular site is current with regard to 

accepted scientific principles and rigor. 

In addition to validating particular computational codes using a set of standard 

benchmarks, the implementation of the conceptual model should also be validated 

by comparing model results to observed data, and by using model calibration and 

history matching to improve the model results. EPA understands the importance of 

history-matching of model simulations with monitoring as stated on p. 43507, c. 

1. 

PNNL RECOMMENDATION 

EPA should establish a suite of test problems with specified parameters that 

could be used by the applicant to demonstrate the suitability of the 

computational code to be used for analysis of the proposed site. As 

computational approaches and methods improve, this suite of problems should be 

modified and/or updated to incorporate the state-of -the-knowledge in GS 

modeling. 

We agree that proprietary models can be used, provided they can demonstrate they 

produce results within the defined acceptable tolerance, in addition to 

disclosing the code assumptions, relevant equations, and scientific basis. These 

same requirements must, of course, be met by all computational codes being used 

at GS sites. 

REFERENCES: 

Law, D.H.-S., L.G.H. vander Meer andW.D. Gunter, 2003: Comparison of numerical 

simulators for greenhouse gas storage in coal beds, Part II: Flue gas injection. 

Proceedings of the 6th International Conference on Greenhouse Gas Control 

Technologies (GHGT-6), J. Gale and Y. Kaya (eds.), 1-4 October 2002, Kyoto, 

Japan, Pergamon, v.I, 563-568. Geologischen Gesellschaft,149,3,449-464. 

Pruess K, J Garcia, TKovscek, C Oldenburg, J Rutqvist, C Steefel, and TXu. 2002. 

108
Intercomparison of Numerical Simulation Codes for Geologic Disposal of CO2. 

LBNL-51813, Lawrence Berkeley National Laboratory, Berkeley, California. 

Response: EPA clarifies that an EPA-prepared technical background document, "Computational

Modeling of Underground Injection of Carbon Dioxide for Determination of Area-of-Review and

Potential Risk to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided

in the regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty

analysis in detail. This document was provided as a background to clarify what information EPA

analyzed in order to draft the modeling requirements at §146.84(c)(1), as well as what actions EPA

expects owners or operators and Directors to take in this area. For example, the document states:

"The requirement for an a-priori AoR delineation based on modeling predictions stresses the need

for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended

as the principal evaluation tool for characterizing the most and least important sources of uncertainty

in environmental models."

EPA agrees that in some cases, results from test-cases may be appropriate. In the technical

background document (USEPA, 2008d), EPA provided several questions that should be raised

during evaluation of models. One of these questions was "If using an independently developed or

unverified code, are results from test-problems and comparisons to widely used codes presented?"

Another was "Was relevant site characterization data gathered at a sufficiently high spatial resolution

to inform model development? Was this data used in the model in a way that considers uncertainty?"

Regarding the suggestion that EPA establish a suite of test problems to demonstrate the suitability of

the computational code, EPA notes that the Agency will not perform independent "tests" or

validation of individual modeling products. EPA believes that operators should have the flexibility to

select models with which they are familiar and that are most appropriate to site conditions and

available data types and amounts. 

EPA based this background document in part on guidance drafted by the National Research Council

and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA refers

interested commenters to the document "Guidance on the Development, Evaluation and Application

of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Richard Guinn Letter ID: 0210

Commenter Organization: Eastman Chemical Company Comment ID: 22728

Comment: EPA seeks comment on allowing the use of proprietary models for GS sites. EPA 

Notice 68. 

Proprietary models should be acceptable provided that supporting data for 

validation are submitted. 

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Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

In response to comments suggesting that history matching or validation of these models is needed,

EPA believes this is true for any model. EPA notes that the GS rule requires at §146.84(e) that the

AoR be reevaluated to verify the CO2 plume is moving as predicted and to allow for necessary

revisions to the AoR and corrective action, testing and monitoring, injection well plugging, postinjection site care and site closure, and emergency and remedial response plans. EPA clarifies,

however, that the Agency will not perform independent validation of individual modeling products.

The Agency plans, every six years, to collect and review data on GS projects to determine whether

the appropriate amount and types of information and appropriate documentation are being collected,

and to determine if modifications to the requirements are appropriate or necessary. 

Commenter Name: Edward Steadman Letter ID: 0220

Commenter Organization: The Energy and Environmental Research Center at the

University of North Dakota

Comment ID:

21613

Comment: Page 43507. Regarding the use of proprietary models: The use of proprietary 

models absolutely should be allowable. In many cases, such software packages are 

the only reliable models that can be applied with any real confidence in their 

results. That being said, the use of such models should not be required because, 

in some cases, especially those where adequate data exist, nonproprietary models 

can serve the purpose in a more cost-effective manner. 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites. EPA will allow, but not require, the use of proprietary

models in meeting the requirements for the GS regulation, as long as GS well operators using a

proprietary model clearly disclose, to the Director's satisfaction, the code assumptions, relevant

equations, and scientific basis. EPA adds that its Confidential Business Information requirements at

40 CFR 144.5 will apply to the use of proprietary models.

Commenter Name: M. Granger Morgan Letter ID: 0233

Commenter Organization: Carnegie Mellon University on behalf of the CCS

Reg Project

Comment ID:

21916

Comment: 24. Use of proprietary models: Use of proprietary models has the potential to 

exaggerate information asymmetry among project operators, regulators and the 

public. If the project operator uses proprietary software for modeling, then 

they should be required to provide all key modeling assumptions, run sensitivity 

analyses on important inputs, or provide additional model runs of the data in a 

publicly available model acceptable to EPA. 

Response: EPA shares the commenter’s concern about the need to evaluate the inputs and

assumptions used. EPA will allow the use of proprietary models in meeting the requirements for the

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GS regulation, as long as GS well operators using a proprietary model clearly disclose, to the

Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA also

agrees that sensitivity analyses should be presented by the owner or operator and evaluated by the

Director. If during the permit review, the Director determines that modeling results are too wideranging, further information will be required of the owner or operator. EPA adds that its Confidential

Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary models.

The Agency plans, every six years, to collect and review data on GS projects to determine whether

the appropriate amount and types of information and appropriate documentation are being collected,

and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23232

Comment: 12 In some cases, owners or operators may choose to use proprietary models 

(i.e., not available for free to the general public). EPA says it is aware that 

the use of proprietary codes may prevent full evaluation of model results (e.g., 

NRC, 2007). Several popular codes in the petroleum-reservoir engineering 

discipline are proprietary and owners or operators of particular sites may 

prefer to use these codes as they have previous experience with them, and they 

have been used in peer-reviewed studies to model CO2 sequestration. When using 

a proprietary model, owners or operators should clearly disclose the code 

assumptions, relevant equations, and scientific basis. EPA seeks comment on 

allowing the use of proprietary models for GS sites. EPA Notice 43507. 

The use of proprietary models is widely used and accepted in oil and gas 

reservoir simulation. These models are a logical fit to simulate carbon dioxide 

sequestration projects. These models are verified using a history-match. The 

regulatory agency responsible for monitoring carbon dioxide sequestration does 

not need access to the code in order to conduct or evaluate a history-match. A 

regulatory agency could conduct reservoir modeling using its own modeling 

software to compare. Elimination of these widely used and understood reservoir 

models would be unwise. The assumptions and equations used in the modeling 

along with a valid history match will provide the regulating authority the 

ability to assess the model’s accuracy. 

Response: EPA agrees with commenters that available proprietary models may meet the complex

needs for calculating the AoRs of GS sites, and shares their concern about the need to evaluate the

inputs and assumptions used. EPA will allow the use of proprietary models in meeting the

requirements for the GS regulation, as long as GS well operators using a proprietary model clearly

disclose, to the Director's satisfaction, the code assumptions, relevant equations, and scientific basis.

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models.

EPA acknowledges that several proprietary models are currently, successfully used by petroleumreservoir engineering professionals as well as in peer-reviewed studies to model CO2 sequestration. 

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23336

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Other EPA Comment Codes and Descriptions: 5540 2.10.2 Need for additional public

participation requirements/ use of new information technology (seeking comment) 

Comment: 2. The proposed rule allows an applicant to use proprietary modeling software to 

submit predictions of plume behavior and movement, storage capacity, and 

injection rates. Use of proprietary software would hinder public participation 

because the public could not test applicant’s modeling predictions. To ensure 

transparency in the application process, publicly available modeling programs 

must be required. 

Response: EPA expects that permitting authorities would make some information about the model

available to the public, such as information about the model used and the parameters modeled.

However, EPA does not expect that members of the public would evaluate the AoR delineation

models themselves. The models and their inputs will, by necessity, entail an extensive amount of

technical information that may not be easily understood by the public. EPA believes that it is the

Director's responsibility, as a steward of public health and the environment, to review models, model

inputs, and results and to determine whether the model is appropriate for predicting the AoR, and to

distribute information that may be easily understood by the public (e.g., in the form of a fact sheet). 

In response to comments about the need for additional public participation and the use of new

information technology, EPA shares the commenter’s concern about the need to evaluate the inputs

and assumptions used, but disagrees that the use of proprietary models should be disallowed. EPA

intends to allow the use of proprietary models in meeting the requirements for the GS regulation, as

long as GS well operators using a proprietary model clearly disclose, to the Director's satisfaction,

the code assumptions, relevant equations, and scientific basis. 

EPA believes that operators should have the flexibility to select models with which they are familiar

and that are most appropriate to the site conditions and available data types and amounts. EPA also

recognizes that a standardized model for AoR delineations could be effective as long as the model

accounts for the expected variations in site conditions, and adds that a state or regional UIC Director

could select one standard model if it took site-specific variations into account. EPA has evaluated

technical documents regarding several computational models that may be used for GS sites. Based

on this review (as documented in the preamble to the proposed GS regulation and the background

document "Computational Modeling of Underground Injection of Carbon Dioxide for Determination

of Area-of-Review and Potential Risk to Underground Sources of Drinking Water"), EPA believes

that several models meet the minimum requirements for models in the GS regulation. EPA adds that

its Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of

proprietary models.

Commenter Name: Walter L. Baker Letter ID: 0253

Commenter Organization: Utah Division of Water Quality Comment ID: 22348

Comment: PROPRIETARY MODELING 

We do not support the use of proprietary models for modeling CCGS projects. 

There should be complete transparency in all aspects of CCGS allowing for peer 

review of the models and assumptions associated with them. This process would 

be hindered by the use of proprietary models. Furthermore, there are numerous 

powerful modeling codes available in the public domain that have already been 

used in predicting fluid movement and geochemical reaction for CCGS. 

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Response: EPA disagrees that the use of proprietary models should be disallowed. EPA

acknowledges the commenter’s concerns about the need to evaluate the inputs and assumptions used

in these models, and EPA intends to allow the use of proprietary models in meeting the requirements

for the GS regulation. EPA clarifies, however, that GS well operators using proprietary models are

required to disclose, to the Director's satisfaction, the code assumptions, relevant equations, and

scientific basis. Information including model assumptions and equations are used by the Director to

inform his/her decision to issue the permit and ensure that the requirements are met and the GS well

will not endanger USDWs. EPA adds that its Confidential Business Information requirements

(144.5) will apply to the use of proprietary models. 

EPA clarifies, however, that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. Information

including model assumptions and equations are used by the Director to inform his/her decision to

issue the permit and ensure that the requirements are met and the GS well will not endanger USDWs.

Letter ID: 0263

Commenter Organization: Interstate Oil & Gas Compact Commission Comment ID: 22798

Comment: III A. 2 Proposed Area ofReview and Corrective Action Requirements 

M. EPA seeks comment on allowing the use of proprietary models for GS sites. 

IOGCC Response: Proprietary models should be allowed provided the Director has 

discretion to evaluate the accuracy of the modeling methodology. It should be 

noted that confidentiality of proprietary models will be subject to statutory 

limitation in various jurisdictions. 

Response: EPA will allow the use of proprietary models in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. 

EPA adds that its Confidential Business Information requirements at 40 CFR 144.5 will apply to the

use of proprietary models. Information including model assumptions and equations are used by the

Director to inform his/her decision to issue the permit and ensure that the requirements are met and

the GS well will not endanger USDWs. 

The Agency plans, every six years, to collect and review data on GS projects to determine whether

the appropriate amount and types of information and appropriate documentation are being collected,

and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Mark Singer Letter ID: 0504

Commenter Organization: Green America Project, LLC Comment ID: 21247

Comment: There's a lot of discussion about modeling as a key to how they're going to 

analyze whether this is sufficient, and trying to predict injectate behavior 

based on modeling, getting data and then analyzing and trying to adapt. 

The proposal does not sufficiently address transparency. Importantly, as this 

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data comes in it should be made available to the public so that scientific and 

technical experts can analyze that. 

Response: In response to the comment that the proposal does not sufficiently address transparency,

GS well operators are required to disclose, to the Director's satisfaction, the code assumptions,

relevant equations, and scientific basis. EPA adds that its Confidential Business Information

requirements at 40 CFR 144.5 will apply to the use of proprietary models. Information including

model assumptions and equations are used by the Director to inform his/her decision to issue the

permit and ensure that the requirements are met and the GS well will not endanger USDWs.

Commenter Name: Becky English Letter ID: 0524

Commenter Organization: Sierra Club Comment ID: 21483

Comment: Secondly I wanted to point out that permit applications should not be able, 

applicants rather, should not be able to use proprietary software if they are 

not willing to disclose their modeling data. And their operating assumptions and 

explain their research methods more fully for a transparent, for transparency 

with the public; people such as myself and Sierra Club. We need complete 

transparency in order to give adequate public input. That is just obvious. 

Response: EPA disagrees that the use of proprietary models should be disallowed. EPA will allow

the use of proprietary models in meeting the requirements for the GS regulation, but clarifies that GS

well operators using proprietary models are required to disclose, to the Director's satisfaction, the

code assumptions, relevant equations, and scientific basis. 

In response to the comment that the proposal does not sufficiently address transparency, EPA

clarifies that GS well operators are required to disclose, to the Director's satisfaction, the code

assumptions, relevant equations, and scientific basis. EPA adds that its Confidential Business

Information requirements at 40 CFR 144.5 will apply to the use of proprietary models. Information

including model assumptions and equations are used by the Director to inform his/her decision to

issue the permit and ensure that the requirements are met and the GS well will not endanger USDWs.

EPA acknowledges commenters' concerns about the need to evaluate the inputs and assumptions

used in these models, and will allow the use of proprietary models in meeting the requirements for

the GS regulation. EPA clarifies, however, that GS well operators using proprietary models are

required to disclose, to the Director's satisfaction, the code assumptions, relevant equations, and

scientific basis. Information including model assumptions and equations are used by the Director to

inform his/her decision to issue the permit and ensure that the requirements are met and the GS well

will not endanger USDWs. EPA adds that its Confidential Business Information requirements at 40

CFR 144.5 will apply to the use of proprietary models.

To ensure that all predictive models used for AoR calculations are meeting the Agency's

expectations, EPA plans, every six years, to collect and review data on GS projects to determine

whether the appropriate amount and types of information and appropriate documentation are being

collected, and to determine if modifications to the requirements are appropriate or necessary.

Commenter Name: Kevin Frederick Letter ID: 0525

Commenter Organization: Wyoming Department of Environmental Quality Comment ID: 21400

Comment: Discretion to allow the use of proprietary models. Wyoming, in particular, has 

114
options available for applying for confidential treatment of proprietary models 

and we believe that should be allowed at the discretion of the director as well. 

Response: EPA agrees that some of the available proprietary models may meet the needs for

calculating the AoRs of GS sites. EPA will allow their use in meeting the requirements for the GS

regulation, but clarifies that GS well operators using proprietary models are required to disclose, to

the Director's satisfaction, the code assumptions, relevant equations, and scientific basis. EPA

acknowledges that several proprietary models have been used by petroleum-reservoir engineering

professionals as well as in peer-reviewed studies to model CO2 sequestration. EPA adds that its

Confidential Business Information requirements at 40 CFR 144.5 will apply to the use of proprietary

models. Information including model assumptions and equations are used by the Director to inform

his/her decision to issue the permit and ensure that the requirements are met and the GS well will not

endanger USDWs.

Comment Code: 3400 2.2.2 Corrective Action 

Individual Comments

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24773

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: (II)(iii) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

(iv) How corrective action will be conducted to meet the requirements of 

paragraph (d) of this section, including what corrective action will be 

performed prior to injection and what, if any, portions of the area of review 

will have corrective action addressed on a phased basis and how the phasing will 

be determined; how corrective action will be adjusted if there are changes in 

the area of review; and how THE site WILL BE accessed will be guaranteed for 

future corrective action. 

{Word Choice. Secure rather than guarantee. } 

Response: The purpose of the corrective action requirement at §146.84(d) is to ensure that all wells

in the AoR that have the potential to cause the movement of CO2 or other fluids into or between

USDWs receive appropriate corrective action. In general, to meet this requirement, the owner or

operator will mostly likely need to obtain the right of access to wells that may require corrective

action, and demonstrate they have done this in their proposed AoR and corrective action plan. For

this reason, the owner or operator must describe in the AoR and corrective action plan how site

access will be guaranteed for future corrective action, as specified in §146.84(b)(2)(iv). The final

rule retains the requirement as proposed.

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24777

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

115
language 

Comment: (3) Determine which abandoned wells in the AOR have been plugged in a manner 

that prevents the movement of CO2 or FORMATION associated fluids that may 

endanger USDWs. 

Response: EPA clarifies that the term "associated fluids" as proposed in §146.84(c)(3) refers to

formation fluids, other injectates, etc. that may be present in the subsurface and affect movement of

the CO2 plume and/or pressure front. EPA believes this terminology would incorporate any fluids in

the subsurface that could potentially move through artificial penetrations and affect USDWs. EPA

has revised the language at §146.84(c)(3) to read, "... carbon dioxide or other fluids that may

endanger USDWs..."

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22551

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: Area of Review and Corrective Action 

In the context of carbon storage, NGWA thinks it is important to distinguish 

between the CO2 plume footprint and the zone of elevated pressure. The rationale 

is given in the next section, but the following conditions should be met: 

- No fixed radius can be used, a zone of endangering influence (ZEI) must be 

determined through modeling (but cannot be smaller than 1⁄4 mile in its smallest 

horizontal dimension) 

- In the volume generated by the CO2 plume footprint (plus appropriate buffer) 

(this volume is defined as the "primary AOR"): all penetrations between the base 

of the injection formation and the base of the USDW should be reviewed and any 

necessary corrective action should be taken before start of project. 

- In the elevated pressure area but outside of the CO2 plume footprint (defined 

as the "secondary AOR"), all current penetrations through the primary seal and 

the injection formation should be reviewed. However the applicant should be 

allowed to perform corrective action in a pseudo-concentric phased manner ahead 

of and according to the development of the pressure plume. 

- Secondary AOR/zone of elevated pressure cannot be computed independently of 

other injection projects in the vicinity. The impact of other sequestration 

projects must be included in determining injection rate / period of injection. 

- Primary and secondary AOR must be periodically reviewed to ensure that 

modeling predictions are met. If they are not, the primary and secondary AOR 

should be revisited. The operator should keep track of any new penetration in 

the primary AOR volume and secondary AOR (for example, for oil and gas 

industry). 

"Primary AOR" - Why "Volume of Review" 

Because CO2 is strongly buoyant, it is not sufficient only to investigate 

116
penetrations in the injection formation and primary seal. The whole volume 

potentially contacted by the CO2 plume should be reviewed. The review and 

corrective actions should be performed before the start of injection because 

there is no stopping the CO2 migration once started. The greatest degree of 

effort should be on the deeper wells in the injection zone and primary 

containment then on the secondary containment and seal (if any) and then on 

shallower wells but as many as possible of the shallower wells should be 

compiled too. 

Delayed Corrective Action for "Secondary AOR" 

A simple time-dependent secondary AOR that expands as the volume of injection 

increases and the CO2 migrates is not acceptable. The secondary AOR should be 

agreed upon and reviewed before injection starts but corrective action can be 

somewhat delayed. The secondary AOR consists of only the injection formation and 

seal as used in the current rules because only brine/formation water leakage is 

an issue in the elevated pressure area. The likely presence of a large elevated 

pressure area does not make realistic imposing corrective action on all wells 

before the start of a project. There is no need to provide corrective action to 

wells before the pressure significantly rises. If another problem (e.g., CO2 

leakage) occurs earlier at another location and injection stops, pressure will 

not keep rising and will drop. In addition, delaying corrective action will 

allow taking advantage of technical improvements that will undoubtedly occur in 

the next decades. 

Response: EPA agrees that the AoR for a GS project should incorporate both the CO2 plume and

the pressure front, and clarifies that the requirements for AoR delineation in the final rule

incorporate both the CO2 plume and associated pressure front. This area would be modeled based on

anticipated CO2 volumes to be injected and other site-specific information that would impact

pressure changes in the subsurface. EPA agrees that both of these should be taken into account in

determining corrective action needs and developing a testing and monitoring plan, and that the

requirement to delineate the AoR at §146.84(c) addresses the potential risks raised by the

commenters. EPA believes that the final GS requirements for delineating the AoR provide sufficient

protection to USDWs, without introducing an overly complex regulatory process. 

EPA acknowledges the commenter’s support of phased corrective action and agrees that phased

corrective action may provide many benefits to a project including, but not limited to, spreading the

burden of cost, avoiding delays in project start-up, allowing for use of future, improved corrective

action techniques and addressing unanticipated changes in the movement of the CO2 plume or

pressure front. EPA believes that the phased corrective action approach is appropriate for GS

projects, and retained it at 146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24163

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(3) 

API Comments: (3) Determine which abandoned wells IDENTIFIED IN 146.84 (C) (2) 

117
in the area of review have been plugged (as required by §146.92) in a manner 

that prevents the movement of carbon dioxide or associated FORMATION fluids 

that may endanger USDWs. 

Rationale: Remove as redundant. 

Response: EPA clarifies that the term "associated fluids" as proposed in §146.84(c)(3) refers to

formation fluids, other injectates, etc. that may be present in the subsurface and affect movement of

the CO2 plume and/or pressure front. EPA believes this terminology would incorporate any fluids in

the subsurface that could potentially move through artificial penetrations and affect USDWs. EPA

has revised the language at §146.84(c)(3) to read, "... carbon dioxide or other fluids that may

endanger USDWs..." EPA disagrees that deleting the term area of review is necessary and has

retained the term in the final rule language.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24165

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(e) 

API Comments: (e) If monitoring data indicate an endangerment to USDWs, the 

owner or operator must notify the Director and cease operations as required by § 

146.94.

Rationale: This is not needed since §146.94 already specifies the notification 

and cessation of injection. API suggests the removal of this language as 

unnecessarily duplicative. 

Response: Based on consideration of all public comments and other information, EPA deleted this

provision from §146.84. The final rule now contains a provision in §146.91(c) that requires the

owner or operator to report within 24 hours any evidence that the carbon dioxide stream or

associated pressure front may cause an endangerment to a USDW. EPA believes this change

consolidates reporting requirements in one place and provides additional clarity.

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23489

Comment: On a related topic, USWAG would like to ensure that the obligation of Class VI 

Owners or Operators to plug or repair a pre-existing well within the AoR will 

not be viewed as excusing prior well owners or operators from fulfilling their 

own pre-existing legal obligations under applicable federal or state law to have 

properly closed the well in the first instance. Well plugging and repair 

obligations imposed on Class VI Owners or Operators should not supersede any 

pre-existing legal obligations that have yet to be fulfilled by previous owners 

or operators. To prevent this from occurring, the Final Rule should make clear 

that, in those circumstances where pre-existing wells in an AoR have not been 

properly closed and are now subject to well plugging and repair under a Class VI 

UIC permit, EPA and/or the applicable state regulatory agency should first look 

to see whether the responsible party for such closure is still available to 

118
properly close the well as legally required before imposing such obligations on 

the Owner or Operator of the Class VI UIC permit. 

Response: EPA clarifies that the purpose of the requirement to perform corrective action at

§146.84(d) is to ensure that all wells in the AoR that have the potential to cause movement of CO2

or other fluids into or between USDWs receive appropriate corrective action. All needed corrective

action must be performed by the GS well owner or operator as described in the approved AoR and

corrective action plan, regardless of ownership of the wells in the AoR. Failure to do so would either

preclude authorization to inject or, in the case of phased corrective action, be considered a violation

of the permit and subject to applicable civil, criminal, or administrative penalties. EPA clarifies that

UIC program authority is limited to the proper operation and plugging of injection wells, and that it

does not have authority to require plugging of other types of wells.

Commenter Name: Frederick R. Eames Letter ID: 0178

Commenter Organization: CCS Alliance Comment ID: 23857

Comment: EPA should address gaining access to third party wells that may require 

corrective action. Most states require corrective action to be performed prior 

to injection and the "right of access" is taken into consideration during the 

adequacy determination of the corrective action plan. Some states, such as 

Texas, require corrective action prior to injection unless the owner/operator, 

after best efforts, is unable to get permission to access the property of a 

third party.[FN50] SDWA does not provide EPA the authority to require third 

parties to provide access to an owner/operator for the purposes of conducting 

corrective action. EPA should clarify how owner/operators and states will be 

expected to implement corrective action. 

[FN 50] 30 TEX. ADMIN Code $ 305.151 (2008) 

Prior to denying a permit application, the regulations should provide the owner 

or operator the opportunity to demonstrate that a project can maintain its 

safety and security, despite an uncorrected well. Otherwise, a potential GS site 

could go unutilized, even if the uncorrected well creates no actual risk of harm 

to USDWs. Moreover, well remediation should be applicable only to those wells 

that are likely to be impacted (meaning exposed to corrosive fluid and/or 

pressure above likely capillary entry pressure) early to mid phases of operation 

(e.g., one to 20 years). 

Response: The purpose of the corrective action requirement at §146.84(d) is to ensure that all wells

in the AoR that have the potential to cause the movement of CO2 or other fluids into or between

USDWs receive appropriate corrective action. In general, to meet this requirement, the owner or

operator most likely will need to obtain the right of access to wells that may require corrective

action, and demonstrate they have done this in their proposed AoR and corrective action plan. In the

AoR and corrective action plan, the owner or operator must describe how site access will be

guaranteed for future corrective action, as specified in §146.84(b)(2)(iv). EPA adds that, in some

circumstances, monitoring in the injection zone may be an appropriate corrective action technique.

These types of determinations would need to be made by the Director on a site-specific basis.

Commenter Name: Daniel Plath Letter ID: 0188

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Commenter Organization: NiSource, Inc. Comment ID: 25229

Other EPA Comment Codes and Descriptions: 1610 1.3.1 Risks to USDWs; 3800 2.3 Proposed

Injection Well Construction Requirements (40 CFR 146.86); 5100 2.8.1 Injection well plugging

activities (seeking comment) (40 CFR 146.92) 

Comment: aspects. The following points summarize four primary lessons learned through 

operation of natural gas storage that would be applicable to GS: 

1. History has shown that the one component in the system that has caused the 

most safety issues is the wells; specifically the operator's wells, plugged and 

abandoned (P&A) wells and third party operating wells (e.g. oil and gas 

production wells). The casing and cement in the storage well is the "first line 

of defense" in the containment of high pressure gas and a properly designed, 

constructed and maintained well is critical. Even after several decades of 

operations over a four State area, NiSource subsidiaries have never experienced 

loss of containment from cap rock leakage, migration up faults, re-activation of 

faults, earthquakes, or the other potential leakage pathways that EPA discusses 

in the Proposed Rule. While history will ultimately reveal the root causes of 

future problems, if any, with GS, NiSource submits that the predominant cause 

will likely be identified as being related to an active well, abandoned well or 

third party well if not properly addressed in the Final Rule. This is based on 

the belief that the rigorous site selection process will preclude the other 

sites that would have had other issues, e.g. cap rock leakage, from even being 

activated in the first place. 

Response: EPA agrees that proper site characterization should minimize leakage through faults,

breaks in the seal of the confining layer, and other geologic pathways. EPA believes that the

construction, operating, and monitoring requirements in the GS rule will prevent leakage from

occurring through active wells. Appropriately designed and constructed wells are necessary to

prevent endangerment to USDWs and maintain integrity of the well throughout the lifetime of a GS

project. This is the foundation of the requirements for construction and pre-operational testing of

Class VI wells at §§146.86 and 146.87. 

EPA also agrees that the AoR and corrective action process should identify both abandoned wells

and third party wells that could serve as leakage pathways. The UIC Program's long-standing

approach to corrective action, tailored to the potential corrosivity of and impurities in the CO2

stream, can address the potential risk associated with improperly plugged wells in the AoR.

EPA notes that the final rule, at 40 CFR §124.10(c)(1)(xi), requires the notification of state oil and

gas agencies, state mineral exploration agencies, and state public water supply supervision directors

about the availability of the draft permit . EPA expects that such notification can help with further

identifying wells in the area of review.

Commenter Name: Daniel Plath Letter ID: 0188

Commenter Organization: NiSource, Inc. Comment ID: 25233

Other EPA Comment Codes and Descriptions: 3800 2.3 Proposed Injection Well Construction

Requirements (40 CFR 146.86) 

Comment: The Proposed Rule adequately addresses the issues of well construction 

requirements and the review of existing plugged and abandoned wells. 

Specifically, Section 146.86 would have sufficient number of cemented strings of 

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pipe in the well while Section 146.84 would ensure that abandoned wells are 

properly plugged. 

Response: EPA acknowledges the comment and agrees that the requirement for long-string casing

of Class VI wells at §146.86(b) will offer the best protection against fluid movement behind the

casing, between formations, and above the injection zone and will ensure competency of Class VI

wells and protect USDWs from endangerment. The buoyant nature of CO2 dictates deep well

construction procedures that address movement of fluid behind the casing and provide necessary

barriers to CO2 leakage outside of the injection zone. In developing the construction requirements

for Class VI wells, EPA reviewed the work cited by Carey et al. (2007) and Crow et al. (2008). EPA

believes that they offer important justification for the establishment of tailored requirements. 

EPA agrees that corrective action as required at §146.84(d) is an important part of preventing

endangerment of USDWs from CO2 or associated fluids. EPA believes that the UIC Program's longstanding approach to corrective action, tailored to the potential corrosivity, and impurities in the CO2

stream, can address the potential risk associated with improperly plugged wells in the AoR.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24403

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: Rule Provision: 146.81(d) [Definition - Corrective action] 

Proposed CCS Group Recommendations: Support the proposed language. 

Basis for CCS Group Comments: Agree with EPA’s rationale. 

Response: EPA acknowledges the commenter’s support; the final GS rule retains the definition.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24428

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(b)(2)(iv) 

Proposed CCS Group Recommendations: iv (II) How corrective action will be 

conducted to meet the requirements of paragraph (d) of this section, including 

what corrective action will be performed prior to injection and what, if any, 

portions of the area of review will have corrective action addressed on a phased 

basis and how the phasing will be determined; how corrective action will be 

adjusted if there are changes in the area of review; and how site access will be 

guaranteed ARRANGED for future corrective action. 

Basis for CCS Group Comments: Future access cannot be guaranteed for an area in 

advance that may not even be within the boundaries of the AOR/ project. 

Response: EPA is retaining this requirement as proposed. The purpose of the corrective action

requirement at §146.84(d) is to ensure that all wells in the AoR that have the potential to cause the

movement of CO2 or other fluids into or between USDWs receive appropriate corrective action. In

121
general, to meet this requirement, the owner or operator will most likely need to obtain the right of

access to wells that may require corrective action, and demonstrate they have done this in their

proposed AoR and corrective action plan. In the AoR and corrective action plan, the owner or

operator must describe how site access will be guaranteed for future corrective action as specified in

§146.84(b)(2)(iv).

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24433

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(3) 

Proposed CCS Group Recommendations: (3) Determine which abandoned wells 

IDENTIFIED PURSUANT TO 146.84 (C) (2) in the area of review have been 

plugged (as required by §146.92) in a manner that prevents the movement of 

carbon dioxide or associated FORMATION fluids that may endanger USDWs. 

Basis for CCS Group Comments: Remove "in the area of review" as redundant. 

Response: EPA clarifies that the term "associated fluids," as proposed in §146.84(c)(3), refers to

formation fluids, other injectates, etc., that may be present in the subsurface and affect movement of

the CO2 plume and/or pressure front. EPA believes this terminology would incorporate any fluids in

the subsurface that could potentially move through artificial penetrations and affect USDWs. EPA

has revised the language at §146.84(c)(3) to read, "... carbon dioxide or other fluids that may

endanger USDWs..." EPA disagrees that deleting the term area of review is necessary and has

retained the term in the final rule language.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24435

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(e) 

Proposed CCS Group Recommendations: (e) If monitoring data indicate an 

endangerment to USDWs, the owner or operator must notify the Director and cease 

operations as required by § 146.94.

Basis for CCS Group Comments: This is not needed since §146.94 already specifies 

the notification and cessation of injection. We suggest deletion of this 

language as unnecessarily duplicative. 

Response: Based on consideration of all public comments and other information, EPA deleted this

provision from §146.84. The final rule now contains a provision in §146.91(c) that requires the

owner or operator to report within 24 hours any evidence that the carbon dioxide stream or

associated pressure front may cause an endangerment to a USDW. EPA believes this change

consolidates reporting requirements in one place and provides additional clarity.

Commenter Name: Paul Pike Letter ID: 0198

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Commenter Organization: Ameren Corporation Comment ID: 23948

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: We believe that use of the phrase "corrective action" to describe well plugging 

and repair risks confusion with the concept of corrective action under either 

RCRA or CERCLA. This potential confusion can lead to unwarranted negative 

associations with the phrase "corrective action," and we would suggest 

abandoning this phrase and instead use a phrase such as "well plugging and 

repair." We would then also support a phased approach to well plugging and 

repair ("corrective action") for CO2 GS wells as contemplated in the Proposal 

and it should be available to all Owners or Operators of CO2 GS wells rather 

than be available only at the discretion of the Director. 73 Fed. Reg. at 

43507-08. 

Response: EPA notes that the term "corrective action" at §146.84(d) has been used to describe the

process of addressing improperly plugged wells in the AoR of injection wells since the inception of

the UIC Program, e.g., at §146.7 and §146.64. While EPA is aware that the term has a different

meaning under RCRA and CERCLA, this has not caused confusion in the past. The alternative term,

"well plugging," suggested by the commenter is reserved for injection well plugging as indicated at

§146.92.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24657

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2)(iv) 

Anadarko Petroleum Corporation (APC) Comments: (iv II) How corrective action 

will be conducted to meet the requirements of paragraph (d) of this section, 

including what corrective action will be performed prior to injection and what, 

if any, portions of the area of review will have corrective action addressed on 

a phased basis and how the phasing will be determined; how corrective action 

will be adjusted if there are changes in the area of review; and how site access 

will be guaranteed ARRANGED for future corrective action. 

Rationale: Future access cannot be guaranteed for an area in advance that may 

not even be within the boundaries of the AOR/ project. 

Response: The purpose of the corrective action requirement at §146.84(d) is to ensure that all wells

in the AoR that have the potential to cause the movement of CO2 or other fluids into or between

USDWs receive appropriate corrective action. In general, to meet this requirement, the owner or

operator will most likely need to obtain the right of access to wells that may require corrective

action, and demonstrate they have done this in their proposed AoR and corrective action plan. The

final rule retains the requirement as proposed.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24661

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

123
language 

Comment: Rule Provision: 146.84(c)(3) 

Anadarko Petroleum Corporation (APC) Comments: (3) Determine which abandoned 

wells IDENTIFIED IN 146.84 (C) (2) in the area of review have been plugged 

(as required by §146.92) in a manner that prevents the movement of carbon 

dioxide or associated FORMATION fluids that may endanger USDWs. 

Rationale: Remove as redundant. 

Response: EPA clarifies that the term "associated fluids" as proposed in §146.84(c)(3) refers to

formation fluids, other injectates, etc. that may be present in the subsurface and affect movement of

the CO2 plume and/or pressure front. EPA believes this terminology would incorporate any fluids in

the subsurface that could potentially move through artificial penetrations and affect USDWs. EPA

has revised the language at §146.84(c)(3) to read, "... carbon dioxide or other fluids that may

endanger USDWs..." EPA disagrees that deleting the term area of review is necessary and has

retained the term in the final rule language.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24663

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(e) 

Anadarko Petroleum Corporation (APC) Comments: (e) If monitoring data indicate 

an endangerment to USDWs, the owner or operator must notify the Director and 

cease operations as required by § 146.94.

Rationale: This is not needed if §146.94 already specifies the notification and 

cessation of injection. APC suggests the removal of this language as 

unnecessarily duplicative. 

Response: Based on consideration of all public comments and other information, EPA deleted this

provision from §146.84. The final rule now contains a provision in §146.91(c) that requires the

owner or operator to report within 24 hours any evidence that the carbon dioxide stream or

associated pressure front may cause an endangerment to a USDW. EPA believes this change

consolidates reporting requirements in one place and provides additional clarity.

Commenter Name: John King Letter ID: 0211

Commenter Organization: Shell Exploration and Production Company

(SEPCo)

Comment ID:

22768

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: 2.Corrective Action 

Corrective action is a familiar UIC program requirement. Given the potential 

scale of GS operations, EPA has proposed requirements for a corrective action 

plan contemplating periodic reassessment of the AOR and a phased implementation 

124
of corrective action. In general, this approach is prudent and practical, 

providing what will ultimately be a rigorous approach to corrective action while 

reducing regulatory burdens by permitting a phased implementation of corrective 

action requirements. However, SEPCo does not believe that a minimum frequency 

for reevaluation of the AOR should be required. 

Recommendation: 

The need for reevaluation of the AOR should be required -- if at all -- when 

there is site specific evidence that there is actually a need to reevaluate the 

AOR. Where monitoring results remain consistent with modeling results, a 

requirement to reevaluate the AOR should not be required. 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations or planning reevaluations only when a need

is demonstrated may not be sufficient to appropriately model plume and pressure front movement,

establish conditions for corrective action, and ensure protection of USDWs from endangerment. EPA

agrees that site-specific factors should be considered in the reevaluation schedule, and notes that the

AoR and corrective action plan must specify the site-specific monitoring and operational conditions

that warrant reevaluation, as specified in §146.84(b)(2)(ii).

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24021

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3530 2.2.3.1 AoR reevaluation frequency (seeking comment) 

Comment: Proposed Rule: (b) The owner or operator of a Class VI well must prepare, 

maintain, and comply with a plan to delineate the area of review for a proposed 

geologic sequestration project, periodically reevaluate the delineation, and 

perform corrective action that meets the requirements of this section and is 

acceptable to the Director. As a part of the permit application for approval by 

the Director, the owner or operator must submit an area of review and corrective 

action plan that includes the following information: 

Schlumberger Suggested Revision: (b) The owner or operator of a Class VI well 

must prepare, maintain, and comply with a plan to delineate the area of review 

for a proposed geologic sequestration project, reevaluate the delineation, and 

perform corrective action that meets the requirements of this section in a 

manner and at a frequency acceptable to the Director. As a part of the permit 

application, the owner or operator must submit an area of review and corrective 

action plan that includes the following information: 

Schlumberger Rationale: - 

125
Response: EPA disagrees with the proposed changes and retains the provisions as proposed. For

additional information on EPA's approach to AoR and corrective action, please see the final rule and

preamble.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24023

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: (iv) How corrective action will be conducted to meet the 

requirements of paragraph (d) of this section, including what corrective action 

will be performed prior to injection and what, if any, portions of the area of 

review will have corrective action addressed on a phased basis and how the 

phasing will be determined; how corrective action will be adjusted if there are 

changes in the area of review; and how site access will be guaranteed for future 

corrective action. 

Schlumberger Suggested Revision: (iv) How corrective action will be conducted to 

meet the requirements of paragraph (d) of this section, including what 

corrective action will be performed prior to injection and what, if any, 

portions of the area of review will have corrective action addressed on a phased 

basis and how the phasing will be determined; how corrective action will be 

adjusted if there are changes in the area of review; and how site access will be 

arranged for future corrective action. 

Schlumberger Rationale: Future access cannot be fully guaranteed for an area in 

advance that may not even be within the boundaries of the initial AOR/ project. 

Response: EPA is retaining this requirement as proposed. The purpose of the corrective action

requirement at §146.84(d) is to ensure that all wells in the AoR that have the potential to cause the

movement of CO2 or other fluids into or between USDWs receive appropriate corrective action. In

general, to meet this requirement, the owner or operator will most likely need to obtain the right of

access to wells that may require corrective action, and demonstrate they have done this in their

proposed AoR and corrective action plan. In the AoR and corrective action plan, the owner or

operator must describe how site access will be guaranteed for future corrective action.

§146.84(b)(2)(iv).

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24027

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 5100 2.8.1 Injection well plugging activities (seeking comment) (40 CFR 146.92) 

Comment: Proposed Rule: (3) Determine which abandoned wells in the area of review have 

been plugged (as required by §146.92) in a manner that prevents the movement of 

carbon dioxide or associated fluids that may endanger USDWs. 

Schlumberger Suggested Revision: (3) Determine which abandoned wells identified 

in 146.84 (c) (2) have been plugged (as required by §146.92) in a manner that 

prevents the movement of carbon dioxide or formation fluids that may endanger 

126
USDWs. 

Schlumberger Rationale: - 

Response: EPA acknowledges the comment. Based on its evaluation of all public comments and

input on the proposed rule and NODA and Request for Comment, EPA has made many revisions to

the GS Rule. See the final rule and preamble for further discussion on EPA's final approach.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24028

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: (d) Owners or operators of Class VI wells must perform corrective 

action on all wells in the area of review that are determined to need corrective 

action using methods necessary to prevent the movement of fluid into or between 

USDWs including use of corrosion resistant materials, where appropriate. 

Schlumberger Suggested Revision: Owners or operators of Class VI wells must 

perform corrective action using corrosion resistant materials, when appropriate, 

on all wells identified in 146.84(c)(2) that are determined to need corrective 

action". 

Schlumberger Rationale: - 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream" Instead of corrosion resistant materials. EPA believes this language

change addresses commenters' concerns and is also consistent with other language regarding

materials requirements in the GS rule. EPA did not make the other suggested changes. See the final

rule and preamble for further discussion on EPA's final approach.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24858

Other EPA Comment Codes and Descriptions: 1220 1.1.1.1 Terminology and definitions 

Comment: EPA Proposal: Corrective action means the use of Director approved methods to 

assure that wells within the area of review do not serve as conduits for the 

movement of fluids into underground sources of drinking water (USDW). 

GWPC Recommendations: Support the proposed EPA language. 

Rationale: Agree with EPA’s rationale. 

Response: EPA acknowledges the commenter’s support; the final GS rule retains the definition.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24884

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal: (iv) How corrective action will be conducted to meet the 

127
requirements of paragraph (d) of this section, including what corrective action 

will be performed prior to injection and what, if any, portions of the area of 

review will have corrective action addressed on a phased basis and how the 

phasing will be determined; how corrective action will be adjusted if there are 

changes in the area of review; and how site access will be guaranteed for future 

corrective action. 

GWPC Recommendations: (iv) How corrective action will be conducted to meet the 

requirements of paragraph (d) of this section, including what corrective action 

will be performed prior to injection and what, if any, portions of the area of 

review will have corrective action addressed on a phased basis and how the 

phasing will be determined; how corrective action will be adjusted if there are 

changes in the area of review; and how site access will be assured for future 

corrective action. 

Rationale: Replace "guaranteed" with "assured" Future access cannot be 

absolutely guaranteed for an area in advance that may not even be within the 

boundaries of the AOR/ project. However, assurance of access is a standard that 

is less onerous than guaranteed, it is firmer than another standard such as 

"arranged" 

Response: EPA is retaining this requirement as proposed. The purpose of the corrective action

requirement at §146.84(d) is to ensure that all wells in the AoR that have the potential to cause the

movement of CO2 or other fluids into or between USDWs receive appropriate corrective action. In

general, to meet this requirement, the owner or operator will most likely need to obtain the right of

access to wells that may require corrective action, and demonstrate they have done this in their

proposed AoR and corrective action plan. In the AoR and corrective action plan, the owner or

operator must describe how site access will be guaranteed for future corrective action, as specified in

§146.84(b)(2)(iv).

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24887

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal: (2) Using methods approved by the Director, identify all 

penetrations, including active and abandoned wells and underground mines, in the 

area of review that may penetrate the confining zone. Provide a description of 

each well’s type, construction, date drilled, location, depth, record of 

plugging and/or completion, and any additional information the Director may 

require; and 

GWPC Recommendations: (2) Using methods approved by the Director, identify all 

known artificial penetrations into the confining zone, including active and 

abandoned wells and underground mines, in the area of review that may penetrate 

the confining zone. Provide a description of each well’s type, construction, 

date drilled, location, depth, record of plugging and/or completion, and any 

additional information the Director may require; and 

Rationale: Insert "known artificial penetrations into the confining zone" 

128
EPA Proposal: (3) Determine which abandoned wells in the area of review have 

been plugged (as required by § 146.92) in a manner that prevents the movement of 

carbon dioxide or associated fluids that may endanger USDWs 

GWPC Recommendations: (3) Determine which abandoned wells identified in 146.84 

(c) (2) have been plugged (as required by §146.92) in a manner that prevents the 

movement of carbon dioxide or displaced formation fluids that may endanger 

USDWs. 

Rationale: Add identified in 146.84 (c) (2) 

Remove "area of review" as redundant. 

Suggest change "associated fluids" to "displaced formation fluids." Associated 

fluids is not defined so if EPA means displaced formation fluids the regulation 

should just use this term. Otherwise, you need to define associated fluids. 

Response: EPA is retaining the requirement in 146.84(c)(2) as proposed. EPA agrees with

commenters that, for most GS sites, relying on database and other public records searches (i.e., of

"known" artificial penetrations) will be sufficient to meet the requirements at §146.84(c)(2).

However, the Agency recognizes that there may be situations where additional surveys or research to

locate wells may be appropriate. For example, there may be cases where the Director may determine

that ground or airborne surveys for wells are appropriate, e.g., if there is reason to believe, based on

knowledge of the site or of available data, that the incompleteness of information on known wells

could compromise USDW protection from endangerment. In order to preserve the Director's

discretion to require an operator to supplement a records search, EPA did not make the suggested

change.

EPA clarifies that the term "associated fluids" as proposed in §146.84(c)(3) refers to formation

fluids, other injectates, etc that may be present in the subsurface and affect movement of the CO2

plume and/or pressure front. EPA believes this terminology would incorporate any fluids in the

subsurface that could potentially move through artificial penetrations and affect USDWs. EPA has

revised the language at §146.84(c)(3) to read, "... carbon dioxide or other fluids that may endanger

USDWs..."

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24889

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal: (e) If monitoring data indicate an endangerment to USDWs, the 

owner or operator must notify the Director and cease operations as required by § 

146.94. 

GWPC Recommendations: Delete 

Rationale: Why is this needed if 146.94 already specifies the notification and 

cessation of injection? 

129
Response: Based on consideration of all public comments and other information, EPA deleted this

provision from §146.84. The final rule now contains a provision in §146.91(c) that requires the

owner or operator to report within 24 hours any evidence that the carbon dioxide stream or

associated pressure front may cause an endangerment to a USDW. EPA believes this change

consolidates reporting requirements in one place and provides additional clarity.

Comment Code: 3420 2.2.2.1 Scope of the inventory process (e.g., reliance on publicly available

information, depth of wells) 

Individual Comments

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24776

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (2) Using methods approved by the Director, identify all KNOWN ARTIFICIAL 

penetrations THROUGH THE CONFINING ZONE, including active and abandoned wells 

and underground mines, in the area of review that may penetrate the confining 

zone. Provide a description of each well's type, construction, date drilled, 

location, depth, record of plugging and/or completion, and any additional 

information the Director may require; and 

Response: EPA is retaining this requirement as proposed. EPA agrees with commenters that, for

most GS sites, relying on database and other public records searches (i.e., of "known" artificial

penetrations) will be sufficient to meet the requirements at §146.84(c)(2). However, the Agency

recognizes that there may be situations where additional surveys or research to locate wells may be

appropriate. For example, there may be cases where the Director may determine that ground or

airborne surveys for wells are appropriate, e.g., if there is reason to believe, based on knowledge of

the site or of available data, that the incompleteness of information on known wells could

compromise USDW protection from endangerment. In order to preserve the Director's discretion to

require an operator to supplement a records search, EPA did not make the suggested change.

Commenter Name: James Dooley Letter ID: 0156

Commenter Organization: Joint Global Change Research Institute, Pacific

Northwest National Laboratory

Comment ID:

22636

Other EPA Comment Codes and Descriptions: 4710 2.7.1 Proposed submissions required for

permit applications (seeking comment) 

Comment: All Wells and "All Known Wells" are not the Same Page 43507 and 43517. The 

proposed Class VI rule needs to be much more precise in its use of language in a 

couple of key areas. The rule requires an applicant for a Class VI permit to 

"identify all artificial penetrations." What exactly does this mean? Is it all 

known artificial penetrations (the rule speaks to the operator "review(ing) 

available information")? What is the test for what is a known well or available 

information (does it require detailed field surveys or does one only need to do 

a quick search of a database of wells drilled in the area)? If on the other 

hand it means literally all human penetrations, then one must ask if this is 

technologically possible (e.g., are there technologies which would allow for the 

130
identification of a small diameter non-steel walled well that has no visible 

surface features)? Is there a danger in asking for something that can not 

actually be delivered? One can guess what was likely meant by "identify all 

artificial penetrations" in terms of drafting this rule but it is not at all 

clear how this would be opperationalized at actual prospective GS sites. 

More importantly on Page 43535 under "144.55 Corrective Action", the rule says 

that owners/operators need to identify the location of "all known wells" which 

seems to be a lesser standard. In the actual text of the rule (page 43526), the 

applicant is told "Only information of public record is required to be included 

on this map." Given how many times the rule and its preamble talk about old 

wells being a potential conduit for CO2 release from the storage formation, this 

seems like an overly lax threshold. It would seem appropriate to require the 

applicant survey the actual site itself for artificial penetrations that might 

not be in the public record (the survey could include a number of different 

methods / tools). Further on in this section, the applicant is asked to submit 

a "tabulation of all wells within the area of review which penetrate the 

injection or confining zones." Again, "all wells" and "all known wells" and how 

much work does one need to do to determine if they have tried to find "all the 

wells" must be better articulated in the final rule. This same kind of language 

shows up on page 43537 within the discussion of the Area of Review and 

Corrective Action. 

Lastly, given the large amounts of money involved in a commercial CO2 storage 

project, I would think that the electric production, oil and gas, or other large 

industrial company contemplating a large CCS-enabled facility would want (or be 

required) to go beyond simply collecting existing data as a way of decreasing 

risk and uncertainty. In this context, the proposed "requirement" for going 

beyond existing data could be imposed by entities other than state or federal 

environmental regulators and could be imposed by shareholders, a public utility 

commission, or a firm’s own board of directors. In terms of abandoned wells, it 

seems likely that commercial firms would see it to be in their own best interest 

to do some kind of physical search / assessment of whether there are any known 

or unknown wells within the area of review given the relatively minor cost 

associated with something like an aerial magnetic survey versus the costs 

associated with loss of CO2 and any fines, the cost associated with any required 

post-leakage activities, or simply the cost of litigation and or loss of good 

will associated with such sloppy management of a critical, capital intensive 

pollution abatement facility. 

Response: EPA believes that, for the most part, data base and other public records searches will be

sufficient to identify artificial penetrations in the AoR. This inventory and review process is similar

to what is required of Class I and Class II injection well operators, that that the AoR and corrective

action process for these well types has been successful. 

EPA is developing guidance on AoR and corrective action for GS sites. The guidance describes

EPA's recommended approaches for the data to be considered in an initial AoR delineation, how to

apply computational modeling to delineate the AoR, performing corrective action at GS sites, and,

when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site,

at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

131
EPA agrees with commenters that, for most GS sites, relying on database and other public records

searches (i.e., of "known" artificial penetrations) will be sufficient to meet the requirements at

§146.84(c)(2). However, the Agency recognizes that there may be situations where additional

surveys or research to locate wells may be appropriate. For example, there may be cases where the

Director may determine that ground or airborne surveys for wells are appropriate, e.g., if there is

reason to believe, based on knowledge of the site or of available data, that the incompleteness of

information on known wells could compromise USDW protection from endangerment. In order to

preserve the Director's discretion to require an operator to supplement a records search, EPA did not

make the suggested change. EPA encourages owners or operators to expand their searches, as the

commenter suggests. EPA adds that all corrective action must be performed pursuant to the approved

AoR and corrective action plan, which affords the Director the discretion to require any additional

information needed (including information that is not publically available) to ensure that all

necessary corrective action is performed. 

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24162

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(2) 

API Comments: (2) Using methods approved by the Director, Identify all KNOWN 

ARTIFICIAL penetrations INTO THE CONFINING ZONE AVAILABLE IN THE PUBLIC

RECORD, 

including active and abandoned wells and underground mines, in the area of 

review that may penetrate REACH the confining zone. Provide a description of 

each well‘s type, construction, date drilled, location, depth, record of 

plugging and/or completion, and any additional information the Director may 

require; and 

Rationale: Changed for consistency with earlier language. 

Response: EPA is retaining this requirement as proposed. EPA agrees with commenters that, for

most GS sites, relying on database and other public records searches (i.e., of "known" artificial

penetrations) will be sufficient to meet the requirements at §146.84(c)(2). However, the Agency

recognizes that there may be situations where additional surveys or research to locate wells may be

appropriate. For example, there may be cases where the Director may determine that ground or

airborne surveys for wells are appropriate, e.g., if there is reason to believe, based on knowledge of

the site or of available data, that the incompleteness of information on known wells could

compromise USDW protection from endangerment. In order to preserve the Director's discretion to

require an operator to supplement a records search, EPA did not make the suggested change.

EPA clarifies that any well that penetrates the confining zone, even if it does not extend all the way

through the confining formation, is to be included in the tabulation. The guidance describes EPA's

recommended approaches for the data to be considered in an initial AoR delineation, how to apply

computational modeling to delineate the AoR, performing corrective action at GS sites, and, when

appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

132
Commenter Name: James Taft Letter ID: 0172

Commenter Organization: Association of State Drinking Water Administrators

(ASDWA)

Comment ID:

23453

Comment: Drill Holes and Abandoned Wells: 

EPA should consider options to ensure that mineral exploration drill holes and 

abandoned wells are accounted for on the AOR maps. States have serious concerns 

about the ability to account for all of these potential holes and open shear 

zones or faults that may allow migration from the injection zone to the fresh 

water aquifers, as well as leakage out of the geo- sequestration site and back 

into the atmosphere. The AOR should also be periodically reevaluated, 

Response: EPA agrees with the commenter that many factors about a GS site—including mineral

exploration drilling, abandoned wells, and open shear zones or faults—can affect containment. EPA

believes that the requirement that the AoR and corrective action plan at §146.84(b) be based on

detailed geologic data collected at the site (including the types of information cited by the

commenter) will ensure that all potential risks are considered in GS permitting decisions.

EPA adds that the Director has the discretion to require the owner or operator to supplement a data

records search, including requiring ground or airborne surveys for wells if there is reason to believe,

based on knowledge of the site or of available databases, that the incompleteness of available

information could compromise USDW protection. EPA adds that all corrective action must be

performed pursuant to the approved AoR and corrective action plan, which affords the Director the

discretion to require any additional information needed to ensure that all necessary corrective action

is performed. 

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 24375

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Part 146.84(d) AWWA recommends revising this part to read: 

Owners or operators of Class VI wells must perform corrective action on all 

wells and other artificial penetrations in the area of review that are 

determined to need corrective action using methods necessary to prevent the 

movement of fluid into or between USDWs and between the injection zones(s) and 

USDWs, including use of corrosion resistant materials, where appropriate. 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule. EPA

believes that meeting the requirement for preventing "movement of fluid into or between USDWs"

will prevent movement between the injection zone and USDWs; therefore, it did not make the

suggested change.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24432

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

133
language 

Comment: Rule Provision: 146.84(c)(2) 

Proposed CCS Group Recommendations: (2) Using methods approved by the 

Director, Identify all KNOWN ARTIFICIAL penetrations INTO THE CONFINING ZONE 

AVAILABLE IN THE PUBLIC RECORD, including active and abandoned wells and 

underground mines, in the area of review that may penetrate REACH the 

confining zone. Provide a description of each well’s type, construction, date 

drilled, location, depth, record of plugging and/or completion, and any 

additional information the Director may require; and 

Basis for CCS Group Comments: Changed for consistency with earlier language. 

Response: EPA is retaining the requirement as proposed. EPA agrees with commenters that, for

most GS sites, relying on database and other public records searches (i.e., of "known" artificial

penetrations) will be sufficient to meet the requirements at §146.84(c)(2). However, the Agency

recognizes that there may be situations where additional surveys or research to locate wells may be

appropriate. For example, there may be cases where the Director may determine that ground or

airborne surveys for wells are appropriate, e.g., if there is reason to believe, based on knowledge of

the site or of available data, that the incompleteness of information on known wells could

compromise USDW protection. In order to preserve the Director's discretion to require an operator

to supplement a records search, EPA did not make the suggested change.

EPA adds that all corrective action must be performed pursuant to the approved AoR and corrective

action plan, which affords the Director the discretion to require any additional information needed

(including information that is not publically available) to ensure that all necessary corrective action is

performed. 

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24660

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(c)(2) 

Anadarko Petroleum Corporation (APC) Comments: (2) Using methods approved by the 

Director, identify all KNOWN ARTIFICIAL penetrations INTO THE CONFINING ZONE, 

including active and abandoned wells and underground mines, in the area of 

review that may penetrate the confining zone. Provide a description of each 

well’s type, construction, date drilled, location, depth, record of plugging 

and/or completion, and any additional information the Director may require; and 

Rationale: Changed for consistency with earlier language. 

Response: EPA is retaining the requirement as proposed. EPA agrees with commenters that, for

most GS sites, relying on database and other public records searches (i.e., of "known" artificial

penetrations) will be sufficient to meet the requirements at §146.84(c)(2). However, the Agency

recognizes that there may be situations where additional surveys or research to locate wells may be

appropriate. For example, there may be cases where the Director may determine that ground or

airborne surveys for wells are appropriate, e.g., if there is reason to believe, based on knowledge of

134
the site or of available data, that the incompleteness of information on known wells could

compromise USDW protection from endangerment. In order to preserve the Director's discretion to

require an operator to supplement a records search, EPA did not make the suggested change. EPA

adds that all corrective action must be performed pursuant to the approved AoR and corrective

action plan, which affords the Director the discretion to require any additional information needed

(including information that is not publically available) to ensure that all necessary corrective action is

performed.

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23330

Comment: IX. Additional Well Identification Methods that do not rely on the Public Record 

Should be Required in Certain Provisions 

The proposed rule at CFR§146.82(b) requires a map to be submitted with the 

permit that shows wells within the area of review, but only information of 

public record is required to be included on this map. CFR§146.82(e) also 

requires a tabulation of all wells within the area of review which penetrate the 

injection or confining zone(s), which includes information on the well’s type, 

construction, date drilled, location, depth, record of plugging and/or 

completion and any additional information that the Director may require. 

Relying on information of public record in (b) is sufficient, but additional 

diligence is needed when it comes to wells that penetrate the injection or 

confining zone in (e). Information of public record is often incomplete and/or 

inaccurate. There are numerous orphaned or abandoned wells that are not listed 

on the public record that could affect the integrity of the confining zone. 

While it is possible to intercept some of these problems during operation, there 

are methods available today to identify wells not listed on the public record 

[FN40], which could identify potentially problematic wells in advance. Such a 

requirement should be included in the rule, and be made more general than 

"tabulation" to allow the Director discretion for other data formats as needed: 

[FN40: NETL , for example, has developed an electromagnetic system that can be 

used for aerial or ground-based identification of wells (casings).] 

Response: EPA acknowledges that public records may not contain data on every artificial

penetration in the AoR (or all of the data on each well that is required for the tabulation), particularly

in areas with many older wells. However, EPA notes that the inventory and review process for GS

wells required at §146.84(c)(2) is similar to what is required of Class I and Class II injection well

operators, and that the AoR and corrective action process for these well types has been successful for

many years.

In response to comments suggesting that areas with extensive oil and gas exploration may be the

most likely to have older, undocumented wells, EPA notes that states with long histories of

hydrocarbon production tend to have invested the most resources into their well databases and, as a

result, have detailed information about location and construction of wells within hydrocarbon fields.

In addition, older wells, while likely to have the least sophisticated construction, also tend to be

relatively shallow and would therefore be less likely to penetrate the confining zone.

135
EPA adds that the Director has the discretion to require the owner or operator to supplement a data

records search, including requiring ground or airborne surveys for wells, if there is reason to believe,

based on knowledge of the site or of available data bases, that the incompleteness of available

information could compromise USDW protection. EPA adds that all corrective action must be

performed pursuant to the approved AoR and corrective action plan, which affords the Director the

discretion to require any additional information needed (including information that is not publically

available) to ensure that all necessary corrective action is performed. 

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23331

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: [In the paragraph below the text in 

all CAPS denotes added text.] 

§146.82(e): "A tabulation COMPILATION of all wells within the area of review 

which penetrate the injection or confining zone(s). TO SUPPLEMENT AND/OR VERIFY 

INFORMATION OF PUBLIC RECORD, OPERATORS SHALL USE REASONABLE METHODS

TO IDENTIFY 

THE PRESENCE WELLS WITHIN THE AREA OF REVIEW THAT PENETRATE THE

INJECTION OR 

CONFINING ZONE(S). Such data must include a description [...]" 

Response: EPA is retaining the requirement, which is now found in §146.82(a)(4), as proposed.

EPA notes that the Director has the discretion in §146.84(c)(2) to require the owner or operator to

supplement a data records search—including the use of ground, airborne, or geophysical (e.g.,

magnetic) surveys—if there is reason to believe that the incompleteness of available information

could compromise USDW protection from endangerment. EPA adds that all corrective action must

be performed pursuant to the approved AoR and corrective action plan, which affords the Director

the discretion to require any additional information needed (including information that is not

publically available) to ensure that all necessary corrective action is performed. EPA is developing

guidance on AoR and corrective action at GS Class VI sites to assist owners or operators and

Directors in evaluating requirements and proposed AoR and corrective action plans under 146.84.

The guidance describes EPA's recommended approaches for the data to be considered in an initial

AoR delineation, how to apply computational modeling to delineate the AoR, performing corrective

action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be

available on EPA's web site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23332

Comment: Additionally, to delineate the Area of Review, CFR§146.84(c)(2) requires 

operators to "...identify all penetrations, including active and abandoned wells 

and underground mines, in the area of review that may penetrate the confining 

zone". Again, the public record may not be sufficient to provide accurate 

information in all cases. Problematic wells may not be limited to the confining 

zone, as would be the case if confinement were to be breached, or if pressure 

effects caused by the injection caused fluid movement outside the defined 

confining zone. We recommend the following modification: 

136
Response: The requirements at §146.84(c)(2) are designed to afford owners or operators flexibility

to identify and use appropriate resources to identify artificial penetrations in the AoR. EPA notes

that the Director has the discretion to require the owner or operator to supplement a data records

search—including the use of ground, airborne, or geophysical (e.g., magnetic) surveys—if there is

reason to believe that the incompleteness of available information could compromise USDW

protection from endangerment. Regarding the concern about identifying wells other than those that

penetrate the confining zone, EPA adds that the Director the discretion to require any additional

information needed to ensure that all necessary corrective action is performed, including information

on wells that are above the confining zone, if site-specific information indicates such a need to

ensure USDW protection.

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23333

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: [In the paragraph below the text in 

all CAPS denotes added text.] 

"Using methods approved by the Director, identify all penetrations, including 

active and abandoned wells and underground mines, in the area of review that may 

penetrate the confining zone OR OTHERWISE ACT AS CONDUITS FOR FLUID MOVEMENT 

INSIDE OR OUTSIDE THE CONFINING ZONE". 

Response: For the reasons described in the response above, EPA is retaining the requirement as

proposed.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22302

Comment: Additional well identification methods that do not rely on the public record 

should be required in certain provisions 

§146.82(b) requires a map to be submitted with the permit that shows wells 

within the area of review - only information of public record is required to be 

included on this map. §146.82(e) requires a tabulation of all wells within the 

area of review which penetrate the injection or confining zone(s), including 

information on the well’s type, construction, date drilled, location, depth, 

record of plugging and/or completion and any additional information that the 

Director may require. 

While we are comfortable with relying on information of public record in 

subsection (b), we believe that additional diligence is needed when it comes to 

wells that penetrate the injection or confining zone in subsection (e). 

Information of public record is often incomplete and/or inaccurate. There are 

numerous orphaned or abandoned wells that are not listed on the public record 

that could potentially affect the integrity of the confining zone. While 

generally it is possible to intercept these problems during operations, there 

are methods available today to identify wells not listed on the public record 

[FN22], which in many cases could identify potentially problematic wells in 

advance. A requirement to supplement public record information with field 

137
measurements should be included in the rule, and described with a term that is 

broader than "tabulation" in order to allow the Director to require other data 

formats as needed: 

[FN22: NETL , for example, has developed an electromagnetic 

system that can be used for areal or ground-based identification of wells 

(casings).] 

Response: EPA acknowledges that public records may not contain data on every artificial

penetration in the AoR (or all of the data on each well that is required for the tabulation), particularly

in areas with many older wells. However, EPA notes that the inventory and review process for GS

wells required at §146.84(c)(2) is similar to what is required of Class I and Class II injection well

operators, that the AoR and corrective action process for these well types has been successful for

many years.

EPA adds that the Director has the discretion to require the owner or operator to supplement a data

records search, including requiring ground or airborne surveys for wells, if there is reason to believe,

based on knowledge of the site or of available data bases, that the incompleteness of available

information could compromise USDW protection from endangerment. EPA adds that all corrective

action must be performed pursuant to the approved AoR and corrective action plan, which affords

the Director the discretion to require any additional information needed (including information that is

not publically available) to ensure that all necessary corrective action is performed.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 25256

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: [In the paragraph below the text in 

all CAPS denotes added text. See also EPA-HQ-OW-0390-0250.pdf to see the 

original formatting.] 

§146.82(e): "A tabulation COMPILATION of all wells within the area of review 

which penetrate the injection or confining zone(s). TO SUPPLEMENT AND/OR VERIFY 

INFORMATION OF PUBLIC RECORD, OPERATORS SHALL USE REASONABLE METHODS

TO IDENTIFY 

THE PRESENCE WELLS WITHIN THE AREA OF REVIEW THAT PENETRATE THE

INJECTION OR 

CONFINING ZONE(S). Such data must include a description [...]" 

[FN22: NETL , for example, has developed an electromagnetic 

system that can be used for areal or ground-based identification of wells 

(casings).] 

Additionally, we believe that problematic wells may not be limited to the 

confining zone, as would be the case if confinement were to be breached, or if 

pressure effects caused by the injection caused fluid movement outside the 

defined confining zone. We recommend the following modification in §146.84(c) 

(2): 

138
[In the paragraph below the text in 

all CAPS denotes added text. See also EPA-HQ-OW-0390-0250.pdf to see the 

original formatting.] 

"Using methods approved by the Director, identify all penetrations, including 

active and abandoned wells and underground mines, in the area of review that may 

penetrate the confining zone OR OTHERWISE ACT AS CONDUITS FOR FLUID MOVEMENT 

INSIDE OR OUTSIDE THE CONFINING ZONE". 

Response: EPA is retaining the requirement at §146.82(b), which is now found in §146.82(a)(4), as

proposed. EPA adds that all corrective action must be performed pursuant to the approved AoR and

corrective action plan, which affords the Director the discretion to require any additional information

needed (including information that is not publically available) to ensure that all necessary corrective

action is performed. EPA notes that the Director has the discretion at §146.84(c)(2) to require the

owner or operator to supplement a data records search—including the use of ground, airborne, or

geophysical (e.g., magnetic) surveys—if there is reason to believe that the incompleteness of

available information could compromise USDW protection from endangerment. For these reasons,

EPA is retaining the requirement at §146.84(c)(2) as proposed.

Regarding the concern about identifying wells other than those that penetrate the confining zone,

EPA adds that the Director the discretion to require any additional information needed to ensure that

all necessary corrective action is performed, including information on wells that are above the

confining zone, if site-specific information indicates such a need to ensure USDW protection.

Commenter Name: John McManus Letter ID: 0294

Commenter Organization: American Electric Power Comment ID: 25314

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: 6. Area of review and corrective action requirements 

In proposed §144.55(a) the rule would require Class I, II (other than existing), 

III or VI permit applicants to identify the location "of all known wells within 

the injection well’s area of review which penetrate the injection zone" or, in 

the case of Class II wells operating over the fracture pressure of the injection 

formation, "all known wells within the area of review penetrating formations 

affected by the increase in pressure." 

Since injection in deep saline formations will result in an area of elevated 

pressure (above the original reservoir pressure) that will be larger than the 

CO2 plume and could conceivably affect overlying or underlying formations, we 

have agreed that the requirement for Class II wells operating above the fracture 

pressure should also apply to Class VI wells. This would help prevent the 

unwanted migration of reservoir fluids from the injection zone as a result of 

the pressure increase. We therefore recommend that §144.55(a) be revised to read 

as follows, with revisions shown in blue: 

"(a) Coverage. Applicants for Class I, II, (other than existing), or III, 

injection well permits shall identify the location of all known wells within the 

injection well's area of review which penetrate the injection zone. In the case 

139
of Class II wells that operate over the fracture pressure of the injection 

formation, applicants shall also identify the location of all known wells within 

the area of review penetrating formations affected by the increase in pressure. 

For such wells which are improperly sealed, completed, or abandoned, the 

applicant shall also submit a plan consisting of such steps or modifications as 

are necessary to prevent movement of fluid into underground sources of drinking 

water (‘‘corrective action’’). Where the plan is adequate, the Director shall 

incorporate it into the permit as a condition. Where the Director’s review of an 

application indicates that the permittee’s plan is inadequate (based on the 

factors in §146.07), the Director shall require the applicant to revise the 

plan, prescribe a plan for corrective action as a condition of the permit under 

paragraph (b) of this section, or deny the application. The Director may 

disregard the provisions of §146.06 (Area of Review) and §146.07 (Corrective 

Action) when reviewing an application to permit an existing Class II well. 

Applicants for permits for Class VI wells [or for wells in other classes that 

are used for geologic sequestration and do not meet the criteria for §144.6(b) 

(4)][FN3] shall identify the location of all known wells within the area of review 

penetrating the injection zone and all known wells within the area of review 

which penetrate formations affected by the pressure front and shall perform 

corrective action as specified in §146.84." 

3/ The undersigned, except NRDC which does not support the Group’s proposed 

bright line definition and language for §144.6(b)(4) (see MSG letter of Dec. 23, 

2008), believe that this clause in brackets should be included. 

Response: The final rule leaves §144.55 unchanged from the existing UIC requirements; i.e., Class

VI wells are not addressed in this provision. Based on its evaluation of all public comments and

input on the proposed rule and NODA and Request for Comment, EPA has made many revisions to

the GS Rule. See the final rule and preamble for further discussion on EPA's final approach.

Comment Code: 3440 2.2.2.2 Evaluation of wells in the AoR (e.g., types of data to be reviewed) 

Individual Comments

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22587

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: 144.55(a) 

Proposed Revision: (a) Coverage. Applicants for Class I, II, (other than 

existing), III or VI injection well permits shall identify the location of all 

known wells within the injection well's area of review which penetrate the 

injection zone, or in the case of Class II wells operating over the fracture 

pressure of the injection formation, all known wells within the area of review 

penetrating formations affected by the increase in pressure. Applicants for 

Class VI shall perform mapping with sufficient resolution as to make a 

140
comprehensive effort to identify wells that are not in the public record using 

aerial photography, aerial survey, or other methods acceptable to the director 

corrective action as specified in §146.84 if needed 

Rationale: Concern about the use of the term known wells. Public records 

search may not be adequate for older oil and gas wells, for example. Some 

language should be added so that A more proactive approach is used, e.g. 

physically walk site or do aerial review. 

Response: The final rule leaves §144.55 unchanged from the existing UIC requirements; i.e., Class

VI wells are not addressed in this provision. Based on its evaluation of all public comments and

input on the proposed rule and NODA and Request for Comment, EPA has made many revisions to

the GS Rule. See the final rule and preamble for further discussion on EPA's final approach.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22596

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: 146.84(c)(2) 

Proposed revision: (2) Using methods approved by the Director, identify all 

penetrations, including active and abandoned wells and underground mines, in the 

area whose total depth is greater than the depth to the bottom of the lowermost 

USDW, including those penetrating the confining zone. Provide a description of 

each well’s type, construction, date drilled, location, depth, record of 

plugging and/or completion, and any additional information the Director may 

require; and 

Rationale: This makes this consistent with NGWA's proposed section 146(a)(8) 

Response: EPA clarifies that any well that penetrates the confining zone, even if it does not extend

all the way through the confining formation, is to be included in the tabulation. EPA is issuing

guidance on. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR, how to

identify all wells that require corrective action, performing corrective action at GS sites, and, when

appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

The final rule retains the language in §146.84(c)(2) as proposed. EPA clarifies that the mapping and

cross sections required to be submitted under §146.82(a)(3) will define the limits of the confining

zone, which would be accounted for in the AoR delineation and corrective action evaluation required

at §146.84(c). EPA adds that owners or operators must also, under §146.82(a)(2), submit a map of

the AoR that shows all injection wells, producing wells, abandoned wells, plugged wells or dry

holes, deep stratigraphic boreholes, State or EPA approved subsurface cleanup sites, surface bodies

of water, springs, mines (surface and subsurface), quarries, water wells and other pertinent surface

features including structures intended for human occupancy and roads. EPA believes that these

requirements address the commenter's concerns.

Commenter Name: Brianne Metzger Letter ID: 0151

141
Commenter Organization: Spectra Energy Corp Comment ID: 21826

Comment: EPA has proposed requiring the applicant submit "the status of corrective 

action on wells in the area of review." 40 C.F.R. § 146.82(q). We believe it 

is unnecessary to require the inclusion of all wells within the AoR regardless 

of whether or not they penetrate the injection zone. There could be many 

shallow wells within the AoR located hundreds or thousands of feet above the 

CO2 injection. Therefore, EPA should limit this requirement to the injection 

zone, injection pool, and only those wells that either intersect or penetrate 

the proposed reservoir, the cap rock, and possibly the porous strata directly 

above the injection zone, or wells that penetrate or are in close proximity to 

a gas storage reservoir. 

The Agency has stated that only information of public record pertaining to all 

types of wells (injection, producing, abandoned) be included on the AoR. Data 

availability and use raises a question regarding the possible existence of 

other data which might be private, proprietary and/or not publicly available. 

Such non-publicly available data may or may not be relevant to the application 

process. However, EPA should clarify to what extent an applicant would be 

expected by EPA to develop or use information that is not contained in the 

public record. 

Response: EPA clarifies that, under 146.84(c)(2), operators are required to identify all penetrations

in the AoR that penetrate the confining zone, but that corrective action is needed only for those wells

that are determined to need corrective action in order to prevent movement of fluid into or between

USDWs. The well inventory/identification process is more inclusive than the corrective action

process to allow the Director to ensure that all wells in the AoR are identified and those that need

corrective action receive it.

Commenter Name: James Childress Letter ID: 0158

Commenter Organization: Gasification Technologies Council (GTC) Comment ID: 23772

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: §146.84 Area of review and corrective action. (c) (2) "Using methods approved by 

the Director, identify all penetrations, including active and abandoned wells 

and underground mines, in the area of review that may penetrate the confining 

zone. Provide a description of each well's type, construction, date drilled, 

location, depth, record of plugging and/or completion, and any additional 

information the Director may require; and ..." 

We recommend striking out "approved" and replacing with "prescribed". We 

recommend deleting the second sentence and replacing with "Where such 

information is available or can be obtained, provide a description of each 

well's type, construction, date drilled, location depth record of plugging 

and/or completion and any additional information prescribed by the director and 

..." Acceptable methods for identifying deep wells into confining zones within 

the AoR should be prescribed by the EPA as part of the regulation. Accurate 

information is difficult to obtain for older wells within the AoR. Examples of 

other information that could be required by the Director should be clearly 

stated. 

142
Response: EPA is retaining the language in §146.84(c)(2) as proposed. Based on its evaluation of all

public comments and input on the proposed rule and NODA and Request for Comment, EPA has

made many other revisions to the GS Rule. See the final rule and preamble for further discussion on

EPA's final approach. 

EPA acknowledges that public records may not contain data on every artificial penetration in the

AoR (or all of the data on each well that is required for the tabulation), particularly in areas with

many older wells. However, the Agency recognizes that there may be situations where additional

surveys or research to locate information on wells may be appropriate. For example, there may be

cases where the Director may determine that ground or airborne surveys for wells are appropriate,

e.g., if there is reason to believe that the incompleteness of information on known wells could

compromise USDW protection from endangerment. In order to preserve the Director's discretion to

require an operator to supplement a records search, EPA did not make the suggested change. EPA

adds that all corrective action must be performed pursuant to the approved AoR and corrective

action plan, which affords the Director the discretion to require any additional information needed

(including information that may not be publically available) to ensure that all necessary corrective

action is performed. The guidance describes EPA's recommended approaches for the data to be

considered in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24026

Comment: Proposed Rule: (2) Using methods approved by the Director, identify all 

penetrations, including active and abandoned wells and underground mines, in the 

area of review that may penetrate the confining zone. Provide a description of 

each well’s type, construction, date drilled, location, depth, record of 

plugging and/or completion, and any additional information the Director may 

require; and 

Schlumberger Suggested Revision: - 

Schlumberger Rationale: We feel that there needs to be some effort to identify 

unknown wells using aerial surveys or other methods. We support this language as 

long as it is interpreted to mean more than just a record search in areas that 

that have been explored or produced oil and gas or have had other deep wells. 

Response: For most GS sites, relying on database and other public records searches (i.e., of

"known" artificial penetrations) will be sufficient to meet the requirements at §146.84(c)(2).

However, the Agency recognizes that there may be situations where additional surveys or research to

locate wells may be appropriate. For example, the Director may determine that ground or airborne

surveys for wells are appropriate, e.g., if there is reason to believe that the incompleteness of

information on known wells could compromise USDW protection from endangerment. EPA adds

that under §146.84(c)(2), the Director has the discretion to require the owner or operator to conduct

additional searches. EPA adds that all corrective action must be performed pursuant to the approved

AoR and corrective action plan, which affords the Director the discretion to require any additional

information needed to ensure that all necessary corrective action is performed. EPA is issuing

143
guidance on how to identify all wells that require corrective action; the guidance also describes the

data that EPA recommends to be considered in an initial AoR delineation, how to apply

computational modeling to delineate the AoR, performing corrective action at GS sites, and, when

appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Mark E. Fesmire Letter ID: 0237

Commenter Organization: New Mexico Oil Conservation Division

(NMOCD) Comment ID: 23211

Comment: A. Section 144.55 "Corrective Action" - The question arises whether the analysis 

of the wells in the area of review should include those wells that not only 

penetrate the injection zone, but also those that penetrate the confining zone 

also. 

Response: The final rule leaves §144.55 unchanged from the existing UIC requirements; i.e., Class

VI wells are not addressed in this provision. Based on its evaluation of all public comments and

input on the proposed rule and NODA and Request for Comment, EPA has made many revisions to

the GS Rule. See the final rule and preamble for further discussion on EPA's final approach.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22295

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: In proposed §144.55(a) the rule would require Class I, II (other than existing), 

III or VI permit applicants to identify the location "of all known wells within 

the injection well’s area of review which penetrate the injection zone" or, in 

the case of Class II wells operating over the fracture pressure of the injection 

formation, "all known wells within the area of review penetrating formations 

affected by the increase in pressure". 

Since injection in deep saline formations will result in an area of elevated 

pressure (above the original reservoir pressure) that will be larger than the 

CO2 plume and could affect overlying or underlying formations, we believe that 

the latter requirement for Class II wells operating above the fracture pressure 

should also apply to Class VI wells. This would help prevent the unwanted 

migration of reservoir fluids from outside the injection zone through existing 

wells in the confining zone, as a result of the pressure increase. We therefore 

suggest that §144.55(a) be revised to read as follows: 

[In the paragraph below the text in 

all CAPS denotes added text. See also EPA-HQ-OW-0390-0250.pdf to see the 

original formatting.] 

"Coverage. Applicants for Class I, II, (other than existing), or III, or VI

injection well permits shall identify the location of all known wells within the 

injection well's area of review which penetrate the injection zone. In the case 

of Class II wells operating over the fracture pressure of the injection 

formation, applicants shall identify the location of all known wells within the 

144
area of review penetrating formations affected by the increase in pressure. 

Applicants for Class VI SHALL IDENTIFY THE LOCATION OF ALL KNOWN WELLS

WITHIN 

THE AREA OF REVIEW PENETRATING THE INJECTION ZONE AND ALL KNOWN WELLS

WHICH 

PENETRATE FORMATIONS AFFECTED BY THE INCREASE IN PRESSURE AND shall

perform 

corrective action as specified in Sec. 146.84." 

Response: The final rule leaves §144.55 unchanged; i.e., Class VI wells are not addressed in this

provision. Based on its evaluation of all public comments and input on the proposed rule and NODA

and Request for Comment, EPA has made many revisions to the GS Rule. See the final rule and

preamble for further discussion on EPA's final approach.

Comment Code: 3460 2.2.2.3 Acceptable corrective action techniques for GS sites (e.g., cements) 

Individual Comments

Commenter Name: Homer Spencer Letter ID: 0120

Commenter Organization: Canitron Systems Inc. Comment ID: 22071

Other EPA Comment Codes and Descriptions: 1310 1.1.2.4 Comments on other "out of scope"

issues, e.g., pore space ownership; 1350 1.1.3.3 Comments on the need for GS technology 

Comment: For the past four years, our company has been field testing our 

proprietary systems and methods for utilizing a low melting point fusible metal 

alloy to mold in situ plugs in wells to repair gas leakage problems. Our focus 

has been on repair of vent gas flows that occur because of channels that exist 

within the cement surrounding production casing in wells. Our field tests have 

been carried out with permission of the Alberta Energy Resources Control Board 

("ERCB") and under their scrutiny. We presently have a proposal under 

consideration by the ERCB and the Canadian Association of Petroleum Producers 

(CAPP) to add our methods to the approved well abandonment protocols presently 

under review in Alberta. 

We believe our methods are directly applicable to the problem of long term 

leakage of sequestered GHG from wells that penetrate a reservoir used for the 

sequestration. Obviously, it makes neither economic nor environmental sense to 

capture, concentrate, compress, deliver, and inject CO2 into depleted reservoirs 

or saltwater formations if there is not a reliable sealing method used on 

existing wells in that reservoir to prevent the CO2 leaking back into the 

atmosphere. We believe that our plugging methods can assure long term success in 

preventing this leakage. 

We use an alloy of bismuth and tin for our sealing medium. This alloy is nontoxic and will not corrode or cause corrosion in the relevant well environment 

from either chemical or galvanic processes. We believe we can seal a well 

against gas flow even if the well casing has been entirely corroded away. The 

important properties of the alloy that cause it to be effective are: 

145
1) it expands about 1% by volume when solidifying from the liquid phase; 2) it 

is impermeable to gas flow; 3) it seals effectively in dirty oily casing; 4) it 

has a high specific gravity to facilitate displacement of other fluids present 

during the deployment process; and 5) it has a very low viscosity and surface 

tension such that it will enter small fissures, and absorb into and wet 

permeable rocks such as sandstone. 

Only small amounts of the alloy are required to form a seal with the 

differential pressure strength required to seal GHG sequestration wells. We set 

the plugs using an electric heater deployed on a wire line. The procedure is 

both fast and inexpensive. We have data to back the above contentions, including 

corrosion tests carried out in a CO2/saltwater environment with a pH of 3 that 

indicate plug life in steel casing in excess of 10,000 years. 

Response: EPA thanks the commenters for the information on available well materials. EPA

believes that the regulation affords owners or operators flexibility to use various materials and

techniques to perform corrective action, provided they meet with the Director's approval.

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24778

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (d) Owners or operators of Class VI wells must perform corrective action USING 

CORROSION RESISTANT MATERIAL, WHEN APPROPRIATE, on all wells IDENTIFIED IN 

§146.84(C)(2) in the AOR that are determined to need corrective action using 

methods necessary to prevent the movement of fluid into or between USDWs 

including use of corrosion resistant materials, where appropriate.

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22114

Comment: 13 . EPA suggests deferring corrective actions to seal perforations as the plume 

moves toward it. MDEQ agrees with this approach and the consideration that such 

an approach would allow for improved technology to be implemented over time. 

Response: EPA agrees that phased corrective action may provide many benefits to a project

including, but not limited to, spreading the burden of cost, avoiding delays in project start-up,

allowing for use of future, improved corrective action techniques and addressing unanticipated

changes in the movement of the CO2 plume or pressure front. EPA believes that the phased

corrective action approach is appropriate for GS projects, and retained it at §146.84(b)(2)(iv) in the

final GS regulation.

Commenter Name: Kyle Isakower Letter ID: 0170

146
Commenter Organization: American Petroleum Institute Comment ID: 24164

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(d) 

API Comments: (d) Owners or operators of Class VI wells must perform corrective 

action USING MATERIALS SUITABLE FOR USE WITH THE CARBON DIOXIDE STREAM

AND 

FORMATION FLUIDS, on all wells in the area of review IDENTIFIED IN 146.84(C) 

(2) that are determined to need corrective action". using methods necessary to 

prevent the movement of fluid into or between USDWs including use of corrosion 

resistant materials, where appropriate.

Rationale: Experience with CO2 based EOR strongly supports the use of many 

conventional materials that might not meet the technical definition of 

"corrosion resistant". 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule.

Commenter Name: Robert J. Finley Letter ID: 0180

Commenter Organization: Illinois State Geological Survey Comment ID: 23626

Other EPA Comment Codes and Descriptions: 3200 2.2 Proposed Area of Review and Corrective

Action Requirements (40 CFR 146.84) 

Comment: 146.84 

(c) 

Who determines what corrective action is required in the area of review? If a 

Class II well has been plugged in conformance with Class II requirements is that 

presumed to be adequate? Different state agencies (in a primacy state) may be 

responsible for Class II and Class VI wells, potentially complicating oversight 

of corrective action. 

Response: The GS regulation does not specify the corrective action methods to be used on wells

within the AoR to meet the requirements of §146.84(d) and instead allows flexibility to ensure that

all corrective action is appropriate to the well, the CO2 injection, and other site-specific conditions.

All corrective action must be appropriate to the GS site to ensure that the well does not allow CO2

or other fluids to move into or endanger USDWs and must be approved by the Director. EPA

recognizes that wells plugged in accordance with Class II requirements may also be appropriately

plugged for a CO2 GS project. EPA adds that all corrective action must be performed pursuant to

the approved AoR and corrective action plan, which affords the Director the discretion to require

any additional information needed to ensure that all necessary corrective action is performed. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

147
performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24434

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(d) 

Proposed CCS Group Recommendations: Owners or operators of Class VI wells must 

perform corrective action USING MATERIALS SUITABLE FOR USE WITH THE CARBON 

DIOXIDE STREAM AND FORMATION FLUIDS, on all wells in the area of review

IDENTIFIED IN 146.84(C)(2) that are determined to need corrective action". 

using methods necessary to prevent the movement of fluid into or between USDWs 

including use of corrosion resistant materials, where appropriate.

Basis for CCS Group Comments: Experience with CO2 based EOR strongly supports 

the use of many conventional materials that might not meet the technical 

definition of "corrosion resistant". 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24662

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(d) 

Anadarko Petroleum Corporation (APC) Comments: (d) Owners or operators of Class 

VI wells must perform corrective action USING MATERIALS SUITABLE FOR USE WITH 

THE CARBON DIOXIDE STREAM AND FORMATION FLUIDS, on all wells in the area of 

review IDENTIFIED IN 146.84(C)(2) that are determined to need corrective 

action". using methods necessary to prevent the movement of fluid into or 

between USDWs including use of corrosion resistant materials, where 

appropriate.

Rationale: Experience with CO2 based EOR strongly supports the use of many 

conventional materials that might not meet the technical definition of 

"corrosion resistant". 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule.

Commenter Name: Michel Paque Letter ID: 0259

148
Commenter Organization: Ground Water Protection Council Comment ID: 24888

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(d) 

EPA Proposal: (d) Owners or operators of Class VI wells must perform corrective 

action on all wells in the area of review that are determined to need corrective 

action using methods necessary to prevent the movement of fluid into or between 

USDWs including use of corrosion resistant materials, where appropriate. 

GWPC Recommendations: (d) Owners or operators of Class VI wells must perform 

corrective action using materials suitable for use with carbon dioxide streams, 

on all wells identified in 146.84(c)(2) that are determined to need corrective 

action". 

Rationale: Re-write as follows "Owners or operators of Class VI wells must 

perform corrective action using corrosion resistant materials, when appropriate, 

on all wells identified in 146.84(c)(2) that are determined to need corrective 

action". 

Response: EPA revised the text at §146.84(d) to require the use of materials that are "compatible

with the carbon dioxide stream." EPA believes this language change addresses commenters' concerns

and is also consistent with other language regarding materials requirements in the GS rule.

Comment Code: 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Individual Comments

Commenter Name: Paul Pike Letter ID: 0111

Commenter Organization: Ameren Corporation on behalf of USWAG Comment ID: 22405

Other EPA Comment Codes and Descriptions: 1250 1.1.1.4 Flexibility is needed (performancebased standards); 5000 2.8 Proposed Well Plugging and Post-Injection Site Care Requirements 

Comment: 6. Because of the widely varied characteristics and limitations of different 

carbon dioxide sequestration projects, it is essential to incorporate 

performance-based standards into the Final Rule, leading to a site-specific 

approach to requirements for Area of Review reevaluation and post-injection site 

care and closure periods. 

Response: The post-injection site care requirements in the final GS rule, as specified in §146.93(b),

incorporate a combination of a fixed timeframe and a performance standard approach. The GS rule

offers owners or operators flexibility regarding the duration of the PISC timeframe by: (1) allowing

the Director discretion to shorten or lengthen the PISC timeframe during the PISC period based on

site-specific data, pursuant to requirements at §146.93(b); or (2) affording the Director discretion to

allow a Class VI well owner or operator to demonstrate, based on substantial data during the

permitting process, that an alternative PISC timeframe may be appropriate if it ensures nonendangerment of USDWs pursuant to requirements at §146.93(c).

EPA believes that a default post-injection site care timeframe of 50 years, with flexibility to adjust

149
the timeframe for a limited number of GS projects during the permitting process where substantial

data exists to demonstrate that an alternative timeframe would be protective of USDWs, or based on

data collected during the PISC period, is appropriate to address the range of sites where GS is

anticipated to occur, to accommodate site-specific circumstances and various geologic conditions,

and addresses commenters’ concerns, while ensuring USDW protection. Both approaches will utilize

site-specific data, and be based on pressure monitoring and USDW impacts.

EPA is developing guidance to assist owners or operators in evaluating well plugging, post-injection

site care, and site closure requirements of §146.92 and §146.93. Additionally, the guidance will

assist Directors in evaluating proposed well plugging plans and post-injection site care and site

closure plans under §146.92 and §146.93. The guidance describes recommended approaches for how

an owner or operator could plug injection and monitoring wells; perform post-injection monitoring;

demonstrate the appropriateness of an alternative post-injection site care time frame; demonstrate

non endangerment; and perform site closure activities. The guidance will be available on EPA's web

site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

In response to comments about AoR evaluation, EPA disagrees that the GS regulation should include

performance-based standards for AoR calculation. However, EPA believes that consideration of sitespecific conditions is appropriate for the AoR calculation and reevaluation process. EPA believes

that the flexibility incorporated into the AoR and corrective action plans and the provision for AoR

reevaluation as required at §146.84(e) provide for an ongoing review of the site and a dialogue

between the Director and the owner or operator to ensure that the most appropriate modeling and

corrective action technologies are used to protect USDWs from endangerment. 

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24779

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (f) At the minimum fixed frequency, not to exceed 10 years, as AS specified 

in the AOR and corrective action plan, or when monitoring and operational 

conditions warrant, the owner or operator must: 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Based on the comments received (including those by water associations, an energy company,

associations, industries, states, and environmental groups), EPA has changed the minimum

frequency for the AoR reevaluation from 10 years to at least every 5 years. EPA believes that this

revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS.

150
Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24780

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (4) Submit an amended AOR and corrective action plan or demonstrate to the 

Director through monitoring data and modeling results that no CHANGE amendment 

to the AOR and corrective action plan is needed. 

{Appropriate frequency of determining AoR might not be "fixed." Owner or 

operator might need to reevaluate AoR frequently at first, particularly with 

deep brine formations about which there is little history, and then at expanding 

intervals as more information is known.} 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from 10 years to at least every 5 years in the final GS rule. EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS.

Commenter Name: Theresa Pugh Letter ID: 0169

Commenter Organization: American Public Power Association Comment ID: 23399

Comment: Area of Review: APPA has nominal background in Area of Review, but we do believe 

that AoR sites require periodic review. Given the fact that carbon sequestration 

technology is still developing, and the widely varied characteristics and 

limitations of different CO2 GS projects, it is essential to incorporate 

performance-based standards into the Final Rule, with site-tailored approaches 

to requirements for Area of Review ("AoR") reevaluation and post-injection site 

care and closure periods. APPA believes that the AoR should be periodically 

reviewed at least every five years for the first 10 years. At the conclusion of 

the tenth year APPA believes that a less stringent periodic review should be 

allowed by the director if he so chooses. However, APPA believes that the 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. EPA disagrees that a

longer reevaluation frequency in the later years of the project is appropriate, however. EPA believes

that the same justifications for reevaluations will continue to apply. EPA adds that if site monitoring

data agrees with the existing AoR delineation, the AoR reevaluation may consist of a demonstration

151
that the current AoR delineation is adequate based on site monitoring data.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24160

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2) 

API Comments: (2) A description of: 

(i) The minimum fixed frequency, not to exceed 10 years, the owner or operator 

proposes to reevaluate the area of review;

(ii) The monitoring and operational conditions that would warrant a 

reevaluation of the area of review prior to the next scheduled reevaluation as 

determined by the minimum fixed frequency established in paragraph (b)(2)(i) of 

this section.

(iii)(I) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

No rationale provided 

Rule Provision: 146.84(b)(2) 

API Comments: (iv II) How corrective action will be conducted to meet the 

requirements of paragraph (d) of this section, including what corrective action 

will be performed prior to injection and what, if any, portions of the area of 

review will have corrective action addressed on a phased basis and how the 

phasing will be determined; how corrective action will be adjusted if there are 

changes in the area of review; and how site access will be guaranteed

ARRANGED for future corrective action. 

Rationale: Future access cannot be guaranteed for an area in advance that may 

not even be within the boundaries of the AOR/ project. 

Response: EPA disagrees with the suggested changes. Regarding reevaluation of the AoR, EPA

believes that periodic AoR reevaluation at a set frequency of at least every five years is necessary to

ensure protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

To the comment on site access, EPA clarifies that the purpose of the corrective action requirement at

§146.84(d) is to ensure that all wells in the AoR that have the potential to cause the movement of

CO2 or other fluids into or between USDWs receive appropriate corrective action. In general, to

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meet this requirement, the owner or operator will most likely need to obtain the right of access to

wells that may require corrective action, and demonstrate they have done this in their proposed AoR

and corrective action plan. The final rule retains the requirement as proposed.

Commenter Name: James Taft Letter ID: 0172

Commenter Organization: Association of State Drinking Water Administrators

(ASDWA)

Comment ID:

23454

Comment: into the atmosphere. The AOR should also be periodically reevaluated, 

particularly as improved predictive models and new data become available. 

Response: EPA agrees with the commenter that periodic AoR reevaluations are appropriate. This

final rule retains this provision.

Commenter Name: Quinlan J. Shea Letter ID: 0177

Commenter Organization: Edison Electric Institute Comment ID: 23588

Comment: As with a phased approach to corrective action, a phased approach to evaluation 

and reevaluation of the AOR may prove more practical for operators while 

affording the necessary protection for USDW. As noted above, some projects may 

implicate relatively constrained areas in terms of the size of the receiving 

formation, allowing operators to conduct a broader investigation at the outset 

more efficiently; for larger projects, it may make sense to consider allowing a 

phased plan for AOR evaluation and reevaluation that would provide for a certain 

amount of investigation at the outset with more detailed evaluations to follow 

over the life of a project. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from 10 years to at least every 5 years in the final GS rule. EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS. In addition, the Director has discretion to require more frequent AoR

reevaluation if he or she has reason to believe, based on site-specific information, that a five-year

reevaluation schedule is not sufficiently protective of USDWs.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24426

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2)(i) & (ii) 

Proposed CCS Group Recommendations: (i) The minimum fixed frequency, not to 

exceed 10 years, the owner or operator proposes to reevaluate the area of 

review; (ii) The monitoring and operational conditions that would warrant a 

153
reevaluation of the area of review prior to the next scheduled reevaluation as 

determined by the minimum fixed frequency established in paragraph (b)(2)(i) of 

this section.

Basis for CCS Group Comments: These provisions in (i) and (ii) are rendered 

unnecessary by the revisions to section 146.84(b). 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every five

years is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24427

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2)(iii) 

Proposed CCS Group Recommendations: iii (I) How monitoring and operational 

data (e.g., injection rate and pressure) will be used to inform an area of 

review reevaluation; and 

Basis for CCS Group Comments: Agree with EPA’s rationale. 

Response: EPA acknowledges the comment and the final rule retains this provision.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24436

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f) 

Proposed CCS Group Recommendations: (f) At the minimum fixed frequency, not to 

exceed 10 years, as specified in the approved area of review and corrective 

action plan, or When monitoring and operational conditions warrant, owners or 

operators must: 

Basis for CCS Group Comments: Rather than a fixed time period, we recommend 

following a performance based approach that would be based on significant 

changes in injection parameters or anomalous monitoring results. 

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Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from ten years to at least every five years in the final GS rule. EPA believes that this revised

frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24656

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2) 

Anadarko Petroleum Corporation (APC) Comments: (i) The minimum fixed 

frequency, not to exceed 10 years, the owner or operator proposes to reevaluate 

the area of review; 

(ii) The monitoring and operational conditions that would warrant a reevaluation 

of the area of review prior to the next scheduled reevaluation as determined by 

the minimum fixed frequency established in paragraph (b)(2)(i) of this 

section.

(iii)(i) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

No rationale provided 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every five

years is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Paul L. Carpinone Letter ID: 0212

Commenter Organization: Tampa Electric Company Comment ID: 22906

Other EPA Comment Codes and Descriptions: 1250 1.1.1.4 Flexibility is needed (performancebased standards); 5200 2.8.2 Post injection site care (seeking comment) (40 CFR 146.93) 

155
Comment: - Because carbon sequestration technology is still developing and the 

characteristics and limitations of CO2 GS projects will vary greatly from one 

site to another it is essential to incorporate performance-based standards into 

the Final Rule, leading to a site-tailored approach to requirements for Area of 

Review ("AoR") reevaluation and post-injection site care and closure periods. 

Response: The PISC requirements in the final GS rule, as specified in §146.93(b), incorporate a

combination of a fixed timeframe and a performance standard approach. EPA believes that a default

PISC timeframe of 50 years, with Director's discretion to shorten or lengthen this timeframe,

combined with a site-specific Director’s discretion to create an alternative PISC time frame, ensures

that USDWs are protected. The GS rule offers owners or operators flexibility regarding the duration

of the PISC timeframe by: (1) allowing the Director discretion to shorten or lengthen the PISC

timeframe during the PISC period based on site-specific data, pursuant to requirements at

§146.93(b); or (2) affording the Director discretion to allow a Class VI well owner or operator to

demonstrate, based on substantial data during the permitting process, that an alternative PISC

timeframe may be appropriate if it ensures non-endangerment of USDWs pursuant to requirements at

§146.93(c). EPA expects that the Director will consider the types of information the commenter

describes in determining whether the plume and pressure front no longer pose a risk of

endangerment to USDWs and post-injection site care monitoring may be discontinued.

EPA believes that a default post-injection site care timeframe of 50 years, with flexibility to adjust

the timeframe for a limited number of GS projects during the permitting process where substantial

data exists to demonstrate that an alternative timeframe would be protective of USDWs, or based on

data collected during the PISC period, is appropriate to address the range of sites where GS is

anticipated to occur, to accommodate site-specific circumstances and various geologic conditions,

and addresses commenters’ concerns, while ensuring USDW protection. Both approaches will utilize

site-specific data, and be based on pressure monitoring and USDW impacts.

EPA disagrees that the GS regulation should include performance-based standards for AoR

calculation. However, EPA believes that consideration of site-specific conditions is appropriate for

the AoR calculation and reevaluation process. EPA believes that the flexibility incorporated into the

AoR and corrective action plans and the provision for AoR reevaluation as required at §146.84(e)

provide for an ongoing review of the site and a dialogue between the Director and the owner or

operator to ensure that the most appropriate modeling and corrective action technologies are used to

protect USDWs from endangerment. However, EPA believes that setting a minimum reevaluation

frequency is appropriate. In the final rule, the Agency changed the minimum frequency for the AoR

reevaluation at §146.84(e) from 10 years to at least every 5 years in the final GS rule. EPA believes

that this revised frequency addresses commenters' concerns about the inherent uncertainties in

modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring

activities, and the unique nature of CO2 GS. 

Commenter Name: Diane VanDe Hei Letter ID: 0213

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22927

Other EPA Comment Codes and Descriptions: 5560 2.10.3 Enhancing the public participation

process (seeking comment) 

Comment: In addition, AMWA recommends that local drinking water utilities be included in 

the AoR review process, as drinking water or potential drinking water sources 

may become threatened due to a change in plume direction or behavior. Public 

involvement also needs to be a mandatory part of the AoR reevaluation process. 

156
Response: EPA agrees that coordination between GS project owners or operators, program

Directors, drinking water utilities that may be affected by GS activities, and the public is important.

EPA encourages GS project operators and permitting authorities to use the Internet and other

available tools to explain potential GS projects, describe the technology, and post information on the

latest developments, including schedules for hearings, briefings, and other opportunities for

involvement. EPA has added a requirement at §124.10(c)(1)(xi) that state and local oil and gas

regulatory agencies and state agencies regulating mineral exploration and recovery, the PWSS

program director, and all agencies that oversee injection wells in the state must be notified as part of

the public notice process for a Class VI permit.

EPA encourages permitting authorities to notify and involve as many stakeholders as possible

including drinking water organizations and local water utilities. §124.10(c)(1)(x)(A) requires

notification "to any unit of local government having jurisdiction over the area where the facility is

proposed to be located; and (B) to each State agency having any authority under State law with

respect to the construction or operation of such facility." 

EPA anticipates that drinking water associations and local water utilities will express interest in and

be notified of draft permits and public hearings as required under §124.10(c)(1)(ix) and 40 CFR Part

25.5 (where EPA is providing public information) which requires the permitting authority to notify

all interested parties. 

EPA notes that public notice is required if the Director determines that a major permit modification

is necessary as a result of an AoR reevaluation or other indication that the plume is moving in a

manner other than predicted. See §144.39(a)(5).

Commenter Name: Diane VanDe Hei Letter ID: 0213

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22944

Other EPA Comment Codes and Descriptions: 5560 2.10.3 Enhancing the public participation

process (seeking comment) 

Comment: plume direction or behavior. Public involvement also needs to be a mandatory 

part of the AoR reevaluation process. 

Response: EPA notes that, public notice is required if the Director determines that a major permit

modification is necessary as a result of an AoR reevaluation. See §144.39(a)(5).

Commenter Name: Olon Plunk Letter ID: 0217

Commenter Organization: Xcel Energy, Inc. Comment ID: 21560

Comment: corrective action in a timely manner. Finally, the rule should explicitly allow 

for future expansion of the AoR if necessary, provided the objective to protect 

USDWs continues to be met. 

Response: EPA acknowledges that the AoR for a GS project may change over time, and for this

reason, the rule requires owner or operator to periodically review the AoR. EPA believes that

periodic reevaluation of the AoR as required at §146.84(e) is appropriate to address the unique

conditions posed by GS, including the anticipated injection volumes and associated size of CO2

plumes, as well as to address potential uncertainty in AoR modeling. 

157
EPA clarifies that the purpose of the requirement to perform an initial AoR delineation at

§46.84(c)(1) is to predict as closely as possible, the ultimate extent of the CO2 plume and pressure

front. The reevaluation should be considered adjustments to address changes that could not have

been anticipated before injection commences.

Commenter Name: M. Granger Morgan Letter ID: 0233

Commenter Organization: Carnegie Mellon University on behalf of the CCS

Reg Project

Comment ID:

21908

Other EPA Comment Codes and Descriptions: 3630 2.2.4.1 Comments in favor of allowing

phased corrective action 

Comment: Specific comments on the proposed rule 

17. Iterative Area of Review: We support the proposal to reevaluate iteratively 

the area of review over the project life cycle and to allow phased corrective 

action (sec. 146.84). 

Response: EPA acknowledges the commenter’s support of AoR reevaluations and phased corrective

action. EPA believes that the AoR reevaluation process at §146.84(e) provides additional

opportunities to verify that the models have accurately predicted movement of the CO2 plume and to

make any needed adjustments to corrective action or monitoring at the site, and has retained this

approach in the final rule.

EPA agrees that phased corrective action may provide many benefits to a project including, but not

limited to, spreading the burden of cost, avoiding delays in project start-up, allowing for use of

future, improved corrective action techniques and addressing unanticipated changes in the movement

of the CO2 plume or pressure front. EPA believes that the phased corrective action approach is

appropriate for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 25257

Other EPA Comment Codes and Descriptions: 4610 2.6.1 Testing and monitoring plan

requirement (seeking comment) 

Comment: Perhaps inadvertently, the proposed rule does not explicitly require updating of 

several of the required plans. In most but not all cases, there is a rather 

ambiguous requirement to "maintain" a given plan. Explicit requirements to make 

updates as warranted appear to be missing from provisions relating to: 

• the testing and monitoring plan required by §146.90 (the ambiguous word 

"maintain" is used and there appears to be no requirement that estimated 

plugging costs be kept current); 

Response: The final rule includes provisions that owners or operators review and update the AoR

and corrective action plan, the testing and monitoring plan, and the emergency and remedial

response plan throughout the life of the project, as specified in §146.84(c), §146.90(j), and

§146.94(d). Owners or operators must review each plan and identify necessary amendments to the

plan or demonstrate to the Director that no amendment is needed. These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director.

158
Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24838

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 5700 2.12 Comments on the proposed duration of Class VI permits (seeking comment) 

Comment: Rule Provision: 144.36 

EPA Proposal: 6. Section 144.36 is amended by revising the first two sentences 

in paragraph (a) to read as follows: 

§ 144.36 Duration of permits. 

(a) Permits for Class I and V wells shall be effective for a fixed term not to 

exceed 10 years. UIC Permits for Class II, III and VI wells shall be issued for 

a period up to the operating life of the facility. * * * 

* * * * * 

GWPC Recommendations: Support the proposed language. 

Rationale: The proposed provision is considered acceptable as long as the 

iterative reevaluation process established for AoRs also applies to other 

aspects of the project including modeling, characterization, operational 

decisions, monitoring, financial assurance, etc. It is recognized that 

Directors may also depend on 144.39 regarding modification and revocation of 

Class VI permits. 

Response: EPA acknowledges the commenter’s support and notes that the final rule retains this

requirement. The final rule maintains the lifetime permit provision and clarifies that the permit

extends through the post-injection site care period (144.36(a)). It also adds a requirement that the

Director review each Class VI permit at least once every 5 years to determine whether it should be

modified, revoked and reissued, terminated or a minor modification made as provided in §§144.39,

144.40, or 144.41. The Director’s review will be informed by the owners’ or operators’ iterative

reevaluation of the project. Specifically, the final rule includes a requirement that the AoR be

reevaluated at least every five years, and also requires owners or operators to review and update the

AoR and corrective action plan, the testing and monitoring plan, and the emergency and remedial

response plan throughout the life of the project (146.84(e), 146.90(j), and 146.94(d)). Owners or

operators must review each plan and identify necessary amendments or demonstrate to the Director

that no amendment is needed. In no case can reviews occur less often than once every five years. The

iterative process and subsequent plan reevaluations ensure that time and resources are committed to

the most critical aspects of the permit in preventing endangerment to USDWs. EPA believes that the

iterative process of developing and executing the various plans required of GS well operators in the

final rule (e.g., for AoR and corrective action, testing and monitoring, injection well plugging, postinjection site care, and emergency and remedial response) is tailored to the unique aspects of—and is

more appropriate for–GS than establishing a finite permit duration. 

EPA emphasizes that the final rule requires owners or operators to revise their cost estimates for

financial assurance within 60 days of amendments to the AoR and corrective action plan, the

injection well plugging plan, the post-injection site care and site closure plan, or the emergency and

remedial response plan (40 CFR 146.85(c)(2). 

159
EPA emphasizes that, the Director has the authority to revise the permit if project operations change

or if the Director believes based on monitoring or operational data that the project may endanger

USDWs. EPA has also added text to 144.39(a)(5) that permit modifications would be required if the

Director determines that a modification is necessary based on an AoR reevaluation, amendments to

any of the project plans, or a review of the testing and monitoring results. The Director may also

choose to require minor permit modifications under 40 CFR 144.41, depending on the extent of the

plan amendment. All permit modifications, except minor modifications under 40 CFR 144.41,

require an opportunity for public comment on the portion of the permit that is being reopened

pursuant to requirements under 40 CFR Part 124.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24883

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: Rule Provision: 146.84(b)(2) 

EPA Proposal: (2) A description of: 

(i) The minimum fixed frequency, not to exceed 10 years, the owner or operator 

proposes to reevaluate the area of review; 

(ii) The monitoring and operational conditions that would warrant a reevaluation 

of the area of review prior to the next scheduled reevaluation as determined by 

the minimum fixed frequency established in paragraph (b)(2)(i) of this section. 

(iii) How monitoring and operational data (e.g., injection rate and pressure) 

will be used to inform an area of review reevaluation; and 

GWPC Recommendations: Delete (i) and (ii) 

Rationale: Suggest deletion of (i) as this is handled by "manner and at a 

frequency acceptable to the Director in (b) 

Suggest deletion of (ii) for same reason as comment on (i) 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: John McManus Letter ID: 0294

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Commenter Organization: American Electric Power Comment ID: 25317

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 4700 2.7 Proposed Recordkeeping and Reporting Requirements (40 CFR 146.91) 

Comment: In addition, we recommend the following new §146.95: 

"146.95 Plan Maintenance and Update 

"(a) Owners or operators must submit an annual statement, signed by an 

appropriate company official, confirming that the company has: 

"(i) reviewed the monitoring and operational data that are relevant to a 

decision on whether to reevaluate the area of review and the monitoring and 

operational data that are relevant to a decision on whether to update a plan 

identified in §146.82(p) or (u) through (x) or the quality assurance plan for 

all testing and monitoring requirements; and 

"(ii) determined whether any updates were warranted by material change in the 

monitoring and operational data or in the evaluation of the monitoring and 

operational data by the owner or operator. 

"(b) Owners or operators must submit either the updated plan or a summary of 

the modifications for each plan for which an update was determined to be 

warranted pursuant to subsection (a) of this section. The Director may require 

submission of copies of any updated plans and/or additional information 

regarding whether or not updates of any particular plans are warranted. 

"(c) The Director may require the revision of any required plan whenever the 

Director determines that such a revision is necessary to comply with the 

requirements of this subchapter." 

Response: EPA agrees with the commenter that review of the GS project plans is appropriate. The

final rule includes provisions that owners or operators review and update the AoR and corrective

action plan, the testing and monitoring plan, and the emergency and remedial response plan

throughout the life of the project (these provisions are added at §146.84(c), §146.90(j), and

§146.94(d)). Owners or operators must review each plan and identify necessary amendments to the

plan or demonstrate to the Director that no amendment is needed. These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director.

Comment Code: 3530 2.2.3.1 AoR reevaluation frequency (seeking comment) 

Individual Comments

Commenter Name: Karl R. Moor Letter ID: 0109

Commenter Organization: Southern Company, on behalf of Edison Electric

Institute (EEI)

Comment ID:

22006

Comment: For example, the Agency has proposed a 10-year default period for operators to 

161
reevaluate periodically the proper scope of the Area of Review. If such a 

review were required when triggered by appropriate circumstances, the Area of 

Review would be reviewed only when necessary, but as often as necessary. In 

this way, allowing site- specific performance characteristics to guide 

regulatory requirements means that those projects that pose the greater risk for 

potential harm to sources of drinking water reevaluate their Areas of Review as 

often as needed, and those projects that do not pose such risks are not burdened 

by unnecessary reevaluations. At a minimum, a default period for reevaluations 

of the proper scope of the Area of Review should be determined by site-specific 

characteristics at the time of permitting. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. 

However, EPA believes that setting a minimum reevaluation frequency is appropriate. In the final

rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10

years to at least every 5 years in the final GS rule. EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS. 

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24748

Other EPA Comment Codes and Descriptions: 1250 1.1.1.4 Flexibility is needed (performancebased standards) 

Comment: 5. EPA should provide more flexibiilty in operation. For example, allow an EOR 

operator to demonstrate that CO2 is sequestered, and instead of a 10-year 

default period for reevaluation of the area of review, allow the timing for such 

reevaluation to be determined based on site-specific characteristics and sitespecific monitoring of the CO2 plume and pressure front. 

Response: EPA disagrees that the GS regulation should include performance-based standards for

AoR calculation. However, EPA believes that consideration of site-specific conditions is appropriate

for the AoR calculation and reevaluation process. EPA believes that the flexibility incorporated into

the AoR and corrective action plans and the provision for AoR reevaluation as required at

§146.84(e) provide for an ongoing review of the site and a dialogue between the Director and the

owner or operator to ensure that the most appropriate modeling and corrective action technologies

are used to protect USDWs from endangerment.

However, EPA believes that setting a minimum reevaluation frequency is appropriate. In the final

rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10

years to at least every 5 years in the final GS rule. EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS.

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Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24772

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

Comment: (2) A description of: (i) The minimum fixed frequency, not to exceed 10 years, 

the owner or operator proposes to reevaluate the area of review; (ii) The 

monitoring and operational conditions that would warrant a reevaluation of the 

area of review prior to the next scheduled reevaluation as determined by the 

minimum fixed frequency established in paragraph (b)(2)(i) of this section.

{Suggest that there be no minimum fixed frequency for reevaluation of the AoR, 

but that such review be based on site-specific circumstances. } 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data.

Commenter Name: Dawn A. Thorne Letter ID:

0134

Commenter Organization: University of Utah Energy and Geoscience Institute on

behalf of the Southwest Regional Partnership

Comment

ID: 21655

Other EPA Comment Codes and Descriptions: 3560 2.2.3.2 Conditions that merit reevaluation of

the AoR (seeking comment) 

Comment: - Section 146.84(b)(2)(i) "A description of The minimum fixed frequency, 

not to exceed 10 years, the owner or operator proposes to reevaluate the area of 

review; " 

Section 146.84(b)(2)(ii) "The monitoring and operational conditions that would 

warrant a reevaluation of the area of review prior to the next scheduled 

reevaluation as determined by the minimum fixed frequency established in 

paragraph (b)(2)(i) of this section " 

Response/Discussion: The SWP recommends adopting a risk/performance-based 

approach to AoR re¬evaluation. Since each site is unique, it may be best for the 

operator to work directly with the EPA Director to establish an evaluation 

schedule that would occur as often as needed, but not more than needed. The SWP 

agrees that re-evaluations should be based on updating, revising, and 

calibrating computational models with site-specific data. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

163
operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. 

However, EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Dwight Lockwood, PE, QEP Letter ID: 0141

Commenter Organization: Global Energy, Inc. Comment ID: 23108

Comment: It seems that the geology and capacity of the horizon can dictate the frequency 

but perhaps five years will prove reasonable in many cases. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from 10 years to 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22571

Other EPA Comment Codes and Descriptions: 4650 2.6.5 CO2 plume and pressure front tracking

(seeking comment) 

Comment: MMV should be conducted hand-in-hand with modeling over the entire period that 

MMV is occurring, with each new round of measurements being incorporated into a 

new iteration of the appropriate site models. An iterative measurement-modeling 

approach will allow for the best understanding of the shape and size of the AoR. 

It will also help identify areas of uncertainty in the model that may be reduced 

with changes in the MMV plan. Changes identified in the size of the AoR and for 

risk of leakage will help ensure that MMV is carried out at the right frequency 

and in the right locations. The timing of the MMV operations should be flexible 

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and adjustable based on the performance of the site and the site models. It is 

likely that the site data from early in the operation will probably be more 

valuable than data from later in the operation for assessing the performance of 

the site and identifying and reducing uncertainties. MMV operations that are 

not continuous should be conducted more frequently at the start of the 

operational phase and then be allowed to become less frequent if the site is 

performing safely and in concert with the model predictions. The initial, more 

frequent monitoring should be specified in the permit as an experimental phase 

of the injection project. The continuation of injection should be contingent 

upon successful containment of the injected CO2 during the experimental phase. 

Response: EPA agrees with commenters on the importance of monitoring at GS sites and

emphasizes that monitoring of injection activities will be an important component of every GS

project. Monitoring associated with injection wells and the movement of the CO2 plume is necessary

to verify that the injectate is confined in the target formations and does not endanger USDWs. 

EPA agrees that monitoring data should be used to review model assumptions, verify computational

model projections, and, when necessary, provide input for model refinements. As data are collected

at an injection site, model parameters can be adjusted to match observations (i.e., for model

calibration), which in turn improves the predictive capability of the model. As a GS project proceeds,

both monitoring and modeling will evolve, with monitoring results supporting effective AoR

reevaluations and leading to improved model predictions.

As described above, EPA does not agree that each GS site should have to be permitted as a Class V

experimental technology well before being authorized to operate as a full-scale Class VI well. EPA

does not consider it appropriate to permit CO2 injection wells that are testing the injectivity or

appropriateness of an individual formation (e.g., as a prelude to a commercial-scale operation) as

Class V experimental technology wells. Such wells should be permitted as Class VI wells. 

EPA clarifies that some monitoring frequencies are specified in §146.90; others will be established

in the monitoring and testing plan submitted with the permit application under §146.90. The Director

may require changes in the frequencies when the plan is updated, as is required at least every 5 years

(or when there are changes to the facility or at the request of the Director).

EPA is developing guidance on testing and monitoring to assist owners or operators and Directors in

evaluating requirements under §146.90, including proposed monitoring plans. The guidance

describes available technologies and recommended approaches for implementing a testing and

monitoring plan, including pre-operational geophysical logging and testing of injection wells, MITs,

injection well testing during operation, groundwater quality monitoring, tracking the CO2 plume and

pressure front, and soil gas and surface air monitoring which may be required by the UIC Director

for specific GS projects. The guidance can be found on EPA’s website at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22582

Comment: Preamble Questions: EPA seeks comment on requiring the reevaluation of the site 

AoR on a periodic basis, under what conditions the AoR should be reevaluated, 

and the appropriateness of a 10-year minimum fixed frequency for AoR 

reevaluation. EPA Notice 72. 

165
NGWA Response: Yes, reevaluation is needed and at a higher frequency than 10 

years, particularly at the beginning. 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. 

In addition, the Director has discretion to require more frequent AoR reevaluation if he or she has

reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22583

Other EPA Comment Codes and Descriptions: 3630 2.2.4.1 Comments in favor of allowing

phased corrective action 

Comment: Preamble Questions: EPA proposes to allow a phased approach to corrective action 

but says this may not be appropriate in all situations; therefore EPA is 

proposing that the Director have the discretion to decide to allow this 

approach, based on the understanding of relevant geologic and site conditions. 

EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. EPA Notice 

74. 

NGWA Response: Yes, phased corrective action is a good idea. It will allow 

targeting of those wells most in need of corrective action and optimize 

financial resources as the pressure/CO2 plume may deviate from the initial AoR. 

That's why it is important to review AoR more often than 10 years. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Also, as described above, EPA changed the minimum frequency for the AoR reevaluation at

§146.84(e) from 10 years to 5 years in the final GS rule.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22593

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: 146.84(b)(2)(i) 

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Proposed revision: (i) The owner or operator will reevaluate the AOR after two 

yearsof operational testing (prior to issuance of the operating permit), and 

after 5 years and then no less frequently than every 5 years for the life of the 

project; 

Rationale: NGWA believes that a frequency of every 10 years is too long to 

identify early discrepancies between the site performance and the site model. 

Seeing early difference will allow for earlier model updating and provide 

additional reduction in leakage risk. 

Response: As described above, EPA agrees that reevaluations of the AoR every ten years may not

be sufficient to ensure protection of USDWs from endangerment, and has changed the minimum

frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in the final

GS rule. 

In addition, the Director has discretion to require more frequent AoR reevaluation if he or she has

reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22597

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: 146.84(f) 

Proposed revision: (f) At the minimum fixed frequency, not to exceed two years 

if under operational testing (prior to issuance of the operating permit), or 5 

years (if operating permit has been issued), as specified in the area of review 

and corrective action plan or when monitoring and operational conditions 

warrant, owners or operators must: 

Rationale: This makes this consistent with NGWA's proposed revision to Section 

146.84(b)(2)(i) 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. In addition, the Director

has discretion to require more frequent AoR reevaluation if he or she has reason to believe, based on

site-specific information, that a five-year reevaluation schedule is not sufficiently protective of

USDWs.

Commenter Name: Richard Driggans Letter ID: 0145

Commenter Organization: Tennessee Valley Authority Comment ID: 23152

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Comment: 3. Area of Review (AoR) - The proposed rule requested comment on requiring reevaluation of the site AoR on a periodic basis and the appropriateness of a 10 

year minimum fixed frequency for AoR re-evaluation. TVA supports a reevaluation frequency not to exceed 10 years. Periodic AoR re-evaluation will be 

needed given the enormous volumes of CO2 that would be injected, the potentially 

high mobility of CO2 in the subsurface, the lengthy timeframes likely for CO2 GS 

operations, and the uncertainty of the original model-predicted AoR limits, 

especially where multiple injections might occur below the same aquifer (noncontiguous but merging CO2 pressure fronts). 

In practice, much of what is required in an AoR evaluation (e.g., well field 

monitoring, computational fluid dynamics model calibration/verification, and 

updated prediction of future limits of the CO2 plume) will be performed on an 

on-going basis to support well-field operation and monitoring. If after 10 

years, updated modeling shows no significant change in AoR limits, the AoR reevaluation should merely require presenting the latest modeling results and 

possibly confirming that no new penetrations have been identified and therefore 

no new corrective actions are necessary. 

Response: EPA believes that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. 

In addition, the Director has discretion to require more frequent AoR reevaluation if he or she has

reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs. 

EPA adds that if site monitoring data agrees with the existing AoR delineation, a model recalibration

may not be necessary, and the AoR reevaluation may consist of a demonstration that the current

AoR delineation is adequate based on site monitoring data. 

Commenter Name: Brianne Metzger Letter ID: 0151

Commenter Organization: Spectra Energy Corp Comment ID: 21820

Comment: We also support the Agency’s position that any reevaluation of the AoR occur at 

a minimum fixed frequency, not to exceed 10 years. As well, we agree with 

providing the Director discretion to require more frequent reevaluation, as 

appropriate, if the monitoring or modeling suggests that the CO2 plume is not 

behaving as originally predicted and could result in endangerment to 

underground sources of drinking water (USDWs). Based on our experience with 

AGR, we believe that a frequency of between 3-5 years may be more reasonable 

and appropriate and would support such a determination; but in any event, we 

agree that such reevaluation should not exceed 10 years. 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

168
and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at §146.84(e) from ten years to at least every five

years in the final GS rule. EPA believes that this revised frequency addresses commenters' concerns

about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology,

risk-based targeting of monitoring activities, and the unique nature of CO2 GS.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22112

Comment: 12. EPA seeks comment on requiring reevaluation of the area of review on a periodic 

basis, under what conditions the area of review should be reevaluated and the 

appropriateness of a 10 year minimum fixed frequency. MDEQ agrees that 

reevaluations are appropriate. However, MDEQ suggests that during the first 10 

year period a 2-year frequency should be used to calibrate the models to actual 

conditions. Additionally, after any seismic event a reevaluation should be 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Steven Pirner Letter ID: 0153

Commenter Organization: South Dakota Department of Environment and

Natural Resources

Comment ID:

22145

Comment: 146.84 Area of review and corrective action - (b) (2) (i) the need to reevaluate the area of review a maximum of every 10 years - great clause, we have 

potential for problems in our Class II program where operators could potentially 

over inject, especially if the life of permit expires 

Response: EPA acknowledges the commenters’ support for reevaluation of the AoR. Based on

evaluation of the public comments on this topic, EPA has changed the minimum frequency for the

AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in the final GS rule. EPA

believes that this revised frequency addresses commenters' concerns about the inherent uncertainties

in modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of

monitoring activities, and the unique nature of CO2 GS. 

In response to the comment that operators could potentially over-inject, EPA clarifies that the

purpose of the requirement to perform an initial AoR delineation at §46.84(c)(1) is to predict, as

closely as possible, the ultimate extent of the CO2 plume and pressure front. The reevaluation

should be considered as adjustments to address changes that could not have been anticipated before

injection commences. 

169
Commenter Name: James Childress Letter ID: 0158

Commenter Organization: Gasification Technologies Council (GTC) Comment ID: 23780

Comment: EPA seeks comment on requiring the reevaluation of the site AoR on a periodic 

basis, under what conditions the AoR should be reevaluated, and the 

appropriateness of a 10-year minimum fixed frequency for AoR reevaluation. EPA 

Notice 72. 

10 years is a reasonable period of time to reevaluate the site AoR during 

injection. 

Response: Based on evaluation of the public comments on this topic, EPA has changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years.

EPA believes that this revised frequency addresses concerns raised by commenters representing

water associations, an energy company, associations, industries, states, and environmental groups

about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology,

risk-based targeting of monitoring activities, and the unique nature of CO2 GS. 

The 5-year reevaluation frequency, particularly during early stages of GS deployment, will also

support EPA's adaptive approach to the GS rulemaking. EPA is using an adaptive approach to

regulating GS. The Agency plans, every six years, to collect and review data on GS projects to

determine whether the appropriate amount and types of information and appropriate documentation

are being collected, and to determine if modifications to the requirements are appropriate or

necessary.

Commenter Name: Michael J. Miller Letter ID: 0168

Commenter Organization: Electric Power Research Institute (EPRI) Comment ID: 22500

Comment: Also preamble, page 43507 "EPA seeks comment on requiring the reevaluation of 

the site AoR on a periodic basis, under what conditions the AoR should be 

reevaluated, and the appropriateness of a 10 year minimum fixed frequency for 

AoR reevaluation." 

The proposed rule uses an arbitrary 10-year minimum fixed frequency for AoR 

reevaluation and does not present the technical criteria EPA used in determining 

this frequency. EPRI believes that it would be more logical to base the 

frequency of AoR reevaluation on performance measures including the observed 

speed of the plume and its anticipated arrival time at the edge of the computed 

AoR. Or, if plume speed cannot be observed directly (e.g., no CO2 is observed in 

wells completed in the monitoring zone) nor indirectly (e.g., using remote 

monitoring techniques like seismic), then the model predictions for CO2 plume 

speed and arrival at the AoR boundary should be used to set the reevaluation 

frequency. 

Response: EPA disagrees that the requirement at §146.84(e) for a periodic AoR reevaluation is

arbitrary. EPA proposed the ten-year frequency based on input from stakeholders and experts at the

technical workshops on "Geological Considerations and AoR Studies" and "Modeling and Reservoir

Simulation for Geologic Carbon Storage." EPA also requested comment on the ten-year frequency.

In the preamble to the proposed rule, EPA identified benefits to periodically reevaluating the AoR,

including allowing refinement to the AoR delineation based on real-world conditions and monitoring

results, increasing confidence in the modeled predictions, and targeting monitoring in those areas

170
subject to the greatest potential risk.

Based on the comments received (including those by water associations, an energy company,

associations, industries, states, and environmental groups), EPA has changed the minimum

frequency for the AoR reevaluation from 10 years to at least every 5 years. EPA believes that this

revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS. 

EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure protection of

USDWs from endangerment and address the uncertainties inherent in predictive modeling. Periodic

AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify, based on

monitoring and operational data, that the CO2 plume is moving within the subsurface as predicted.

EPA believes that less frequent reevaluations may not be sufficient to appropriately model plume

and pressure front movement, establish conditions for corrective action, and ensure protection of

USDWs from endangerment.

EPA adds that the GS rule at §146.84(e) requires the AoR reevaluation, along with the iterative

process of developing and executing the various plans required of GS well operators (e.g., for AoR

and corrective action, testing and monitoring, injection well plugging, post-injection site care, and

emergency and remedial response) instead of period permit renewals (e.g., of 10 years for Class I

wells). EPA believes the iterative process and subsequent reevaluations of the AoR and corrective

action, testing and monitoring, and emergency and remedial response plans are preferable to a

periodic permit reapplication and review of the entire permit because they focus time and resources

to the most critical aspects of the permit. Additionally, AoRs reevaluated after at least every 5 years

of injection will provide data on GS projects to inform EPA's adaptive rulemaking approach.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24166

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f) 

API Comments: (f) At the minimum fixed frequency, not to exceed 10 years, as 

specified in the approved area of review and corrective action plan, or When 

monitoring and operational conditions warrant, owners or operators must: 

Rationale: Rather than a fixed time period, API recommends following a 

performance based approach that would be based on significant changes in 

injection parameters or anomalous monitoring results. 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

171
EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Based on the comments received (including those by water associations, an energy company,

associations, industries, states, and environmental groups), EPA has changed the minimum

frequency for the AoR reevaluation from ten years to at least every five years. EPA believes that this

revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS. 

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24226

Comment: Rule Provisions: 146.84 

EPA Preamble Question: 14. EPA seeks comment on requiring the reevaluation of 

the site AoR on a periodic basis, under what conditions the AoR should be 

reevaluated, and the appropriateness of a 10- year minimum fixed frequency for 

AoR reevaluation. EPA Notice 43507. 

API Response: Rather than a fixed time period, API recommends following a 

performance based approach that would be based on significant changes in 

injection parameters or anomalous monitoring results. 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23486

Comment: 9. Area of Review and Corrective Action 

USWAG appreciates the importance and complexity of proper AoR evaluation. See 

id. The Proposal would require Owners or Operators to invest a significant 

amount of time and resources in the initial delineation of the AoR. Provided 

that the Program's requirements are satisfied in the initial determination of 

the AoR, USWAG believes that a requirement for AoR reevaluations every ten years 

or at any other fixed period is unnecessary; this will result in the arbitrary 

imposition of costs that may be wholly unwarranted. A more appropriate approach 

would be performance-based, with reevaluation required upon occurrence of some 

triggering event. One example of a triggering event could be a significant 

172
deviation from previously modeled scenarios, provided that the level of 

deviation that would require reevaluation is clearly defined by the regulating 

agency. As with many other decisions related to implementation of the Class VI 

CO2 GS program, determinations such as these are best made on a local level by 

regulators having the most familiarity with the characteristics and constraints 

of a given geologic setting for a CO2 GS project. 

Response: EPA disagrees that the requirement at §146.84(e) for a periodic AoR reevaluation is

arbitrary. EPA proposed the ten-year frequency based on input from stakeholders and experts at the

technical workshops on "Geological Considerations and AoR Studies" and "Modeling and Reservoir

Simulation for Geologic Carbon Storage." EPA also requested comment on the ten-year frequency.

In the preamble to the proposed rule, EPA identified benefits to periodically reevaluating the AoR,

including allowing refinement to the AoR delineation based on real-world conditions and monitoring

results, increasing confidence in the modeled predictions, and targeting monitoring in those areas

subject to the greatest potential risk.

Based on the comments received (including those by water associations, an energy company,

associations, industries, states, and environmental groups), EPA has changed the minimum

frequency for the AoR reevaluation from 10 years to at least every 5 years. EPA believes that this

revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS. 

EPA adds that the GS rule at §146.84(e) requires the AoR reevaluation, along with the iterative

process of developing and executing the various plans required of GS well operators (e.g., for AoR

and corrective action, testing and monitoring, injection well plugging, post-injection site care, and

emergency and remedial response) instead of period permit renewals (e.g., of 10 years for Class I

wells). EPA believes the iterative process and subsequent reevaluations of the AoR and corrective

action, testing and monitoring, and emergency and remedial response plans are preferable to a

periodic permit reapplication and review of the entire permit because they focus time and resources

to the most critical aspects of the permit. Additionally, AoRs reevaluated after at least every 5 years

of injection will provide data on GS projects to inform EPA's adaptive rulemaking approach.

Commenter Name: Pamela Faggert Letter ID: 0175

Commenter Organization: Dominion Comment ID: 23523

Comment: 3. Area of Review 

Unlike existing UIC classes of wells, the Area of Review (AoR), the area within 

which all pre-existing wells must be identified and studied to determine whether 

they have been properly completed or plugged, will be determined on a sitespecific basis using computational fluid flow models. Whereas Dominion supports 

the site-specific determination for AoR, we urge EPA to eliminate the arbitrary 

10-year maximum between periodic reevaluation of the AoR. Tying reevaluation to 

the triggering of appropriate circumstances will provide equivalent protection 

of USDW while also ensuring that those projects posing greater relative risks 

receive greater scrutiny through more stringent reevaluation requirements while 

those projects posing fewer relative risks are relieved of costly regulatory 

burdens. 

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The Agency should avoid requiring an arbitrary 10 year maximum between 

periodic reevaluations of the AoR and instead should rely on site 

specific circumstances to determine periodic review schedules. 

Response: EPA disagrees that the requirement at §146.84(e) for a periodic AoR reevaluation is

arbitrary. EPA proposed the ten-year frequency based on input from stakeholders and experts at the

technical workshops on "Geological Considerations and AoR Studies" and "Modeling and Reservoir

Simulation for Geologic Carbon Storage." EPA also requested comment on the ten-year frequency.

In the preamble to the proposed rule, EPA identified benefits to periodically reevaluating the AoR,

including allowing refinement to the AoR delineation based on real-world conditions and monitoring

results, increasing confidence in the modeled predictions, and targeting monitoring in those areas

subject to the greatest potential risk.

Based on the comments received (including those by water associations, an energy company,

associations, industries, states, and environmental groups), EPA has changed the minimum

frequency for the AoR reevaluation from 10 years to at least every 5 years. EPA believes that this

revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS. 

EPA adds that the GS rule at §146.84(e) requires the AoR reevaluation, along with the iterative

process of developing and executing the various plans required of GS well operators (e.g., for AoR

and corrective action, testing and monitoring, injection well plugging, post-injection site care, and

emergency and remedial response) instead of period permit renewals (e.g., of 10 years for Class I

wells). EPA believes the iterative process and subsequent reevaluations of the AoR and corrective

action, testing and monitoring, and emergency and remedial response plans are preferable to a

periodic permit reapplication and review of the entire permit because they focus time and resources

to the most critical aspects of the permit. Additionally, AoRs reevaluated after at least every 5 years

of injection will provide data on GS projects to inform EPA's adaptive rulemaking approach.

Commenter Name: Quinlan J. Shea Letter ID: 0177

Commenter Organization: Edison Electric Institute Comment ID: 23552

Comment: EEI is concerned that a broad, unconstrained requirement for periodic 

reevaluation of the area of review (AOR) that would apply in the same manner to 

all potential project sites is unwarranted as a matter of presumption. The 

schedule for AOR review should be a matter addressed through permitting and a 

plan for development and operation. 

Response: EPA is retaining the approach of setting a minimum fixed frequency for reevaluating the

AoR but has, based on commenters' input, changed the frequency from 10 years to at least every 5

years, as specified in §146.84(e). EPA agrees with commenters that site-specific information is an

important factor in determining an appropriate AoR reevaluation frequency, and adds that the owner

or operator's AoR and corrective action plan must describe the minimum fixed frequency for

reevaluating the AoR and the site-specific conditions on which this frequency is based.

Commenter Name: Quinlan J. Shea Letter ID: 0177

Commenter Organization: Edison Electric Institute Comment ID: 23586

Comment: C. Periodic Reevaluation of AOR and Corrective Action Requirements 

174
The second general area of concern addressed in these comments relates to EPA‘s 

proposed provisions concerning the AOR for CO2 storage projects. In a departure 

from AOR procedures currently in place under the UIC program, the proposed 

regulations would require operators to reevaluate the proper scope of the AOR 

periodically at a minimum fixed frequency to be set by the program directorcurrently proposed as no less than once every 10 years, with discretion to 

require more frequent reevaluation. The apparent assumption underlying this 

proposed requirement is that the location of injected CO2, and thus the location 

and extent of potential impact, could change--or perhaps more importantly, could 

change from what is expected at the outset of a project--over time. 

As EEI commented in our May 15, 2008, pre-rulemaking comments, a broad, 

unconstrained requirement for periodic reevaluation of the AOR that would apply 

in the same manner to all potential project sites is unwarranted as a matter of 

presumption. Although the proposed rules provide for some discretion in terms 

of the review period, they nonetheless would require periodic reevaluation for 

every project at least once every 10 years. Several factors argue against such 

an approach: 

- Consideration of specific circumstances should be required: While EEI is not 

necessarily opposed to an appropriately drafted provision requiring periodic 

reevaluation of the AOR under certain circumstances, any requirement for such 

review should not be a universally applicable requirement potentially applied 

devoid of any context. Rather, AOR reevaluation provisions should be triggered 

by reasonable parameters, taking into consideration site-specific issues, as 

discussed in the general principles set forth above. AOR issues are inherently 

site-specific; different projects--and even different geographical regions of 

the country--likely warrant different approaches. A default no-less-than-onceevery-10-years provision does not allow for adequate consideration of specific 

circumstances. Tying reevaluation to the triggering of appropriate 

circumstances will provide equivalent protection for USDW while also ensuring 

that those projects posing greater relative risks receive greater scrutiny 

through more stringent reevaluation requirements and those projects posing fewer 

relative risks are relieved of unnecessary regulatory burdens. At a minimum, 

any default period for reevaluation should be determined based on site-specific 

characteristics at the time of project permitting. 

- Unknown trade-offs militate against a universal requirement: It is impossible 

to know what the relevant trade-offs are as to the initial AOR versus potential 

later assessments of the AOR. For example, in certain instances it is 

foreseeable that a project owner may seek a relatively more constrained AOR for 

an initial portion of the project life, and propose periodic AOR reevaluations 

as a function of project experience. In other instances, a broader initial AOR 

may be appropriate, with a correspondingly reduced need for periodic 

reevaluations. Further, operators will generally model the entire life of the 

project and will likely include additional acreage in the AOR in any event. 

Response: EPA is retaining the approach of setting a minimum fixed frequency for reevaluating the

AoR but has, based on commenters' input, changed the frequency from 10 years to at least every 5

175
years, as specified in §146.84(e). 

EPA agrees that site-specific factors, including monitoring and operational data, should be

considered in the reevaluation schedule, and notes that the AoR and corrective action plan must

specify the site-specific monitoring and operational conditions that warrant reevaluation, as specified

in §146.84(b)(2)(ii). In addition, the Director has discretion to require more frequent AoR

reevaluation if he or she has reason to believe, based on site-specific information, that at least every

5 year reevaluation schedule is not sufficiently protective of USDWs.

Commenter Name: Frederick R. Eames Letter ID: 0178

Commenter Organization: CCS Alliance Comment ID: 23855

Comment: EPA also proposes requiring the reevaluation of the site AoR on a periodic 

basis, suggesting a 10-year minimum fixed frequency for AoR reevaluation. The 

CCS Alliance agrees that a 10-year minimum is reasonable for reevaluating the 

AoR delineation, given that long-term CO2 storage may produce an AoR differing 

from other types of UIC injection plumes and should therefore be reevaluated to 

assure project safety and stability. 

Response: Based on evaluation of the public comments on this topic, EPA has changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years.

EPA believes that this revised frequency addresses concerns raised by commenters representing

water associations, an energy company, associations, industries, states, and environmental groups

about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology,

risk-based targeting of monitoring activities, and the unique nature of CO2 GS. 

The 5-year reevaluation frequency, particularly during early stages of GS deployment, will also

support EPA's adaptive approach to the GS rulemaking. EPA is using an adaptive approach to

regulating GS. The Agency plans, every six years, to collect and review data on GS projects to

determine whether the appropriate amount and types of information and appropriate documentation

are being collected, and to determine if modifications to the requirements are appropriate or

necessary.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23653

Comment: In its comments, AWWA makes several recommendations for the following specific 

items to be included in the final regulation: 

- Reevaluation of the area of review annually for the first ten years, and then 

every five years for the remaining life of the project; 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

176
has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23691

Comment: Category: Area of Review (AoR) 

EPA Request for Comment: EPA seeks comment on requiring the reevaluation of the 

site AoR on a periodic basis, under what conditions the AoR should be 

reevaluated, and the appropriateness of a 10-year minimum fixed frequency for 

AoR reevaluation. EPA Notice 43507. 

EPA proposes to allow a phased approach to corrective action but says this may 

not be appropriate in all situations; therefore EPA is proposing that the 

Director have the discretion to decide to allow this approach, based on the 

understanding of relevant geologic and site conditions. EPA invites public 

comment on the merits and frequency of reevaluation of the AoR as well as the 

phased corrective action approach for GS wells. EPA Notice 43508. 

AWWA Response: AWWA recommends that the area of review be reevaluated more 

frequently than suggested by EPA in the proposed regulation. AWWA recommends 

that the AoR be reevaluated annually for the first ten years of the project, and 

then every five years for the remaining operating period of the project. 

However, criteria should be established to trigger a shorter review period 

(based on a risk management approach) any time during the five-year time 

periods. For example, reevaluation of the AoR could be triggered by a shift in 

plume direction or sudden change in plume size or pressure. The local drinking 

water utilities, commissions, and planning boards need to be included in the AoR 

review process in the event that one of their drinking water sources becomes 

threatened due to a change in plume direction or behavior. Public involvement 

also needs to be a mandatory part of the AoR reevaluation process 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23733

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Part 146.84(d) Based upon our comments in the previous section of this document, 

177
AWWA recommends revising this part to read: 

At the minimum fixed frequency, not to exceed one (1) year for the first ten 

(10) years of the operation of the carbon sequestration project and not to 

exceed five (5) years after the first ten year time period, as specified in the 

approved area of review and corrective action plan, or when monitoring and 

operational conditions warrant, owners or operators must: 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25184

Other EPA Comment Codes and Descriptions: 3560 2.2.3.2 Conditions that merit reevaluation of

the AoR (seeking comment) 

Comment: M. EPA seeks comment on requiring the reevaluation of the site AoR on a periodic 

basis, under what conditions the AoR should be reevaluated, and the 

appropriateness of a 10 year minimum fixed frequency for AoR reevaluation. 

NYSDEC-DMN Response: AoR reevaluation frequency could be based on the amount of 

CO2 injected relative to the total proposed injection volume of CO2. For 

example, reevaluation of AoR could be required for every 20% injected but in no 

event less than every five years or at any other time at the discretion of the 

Director. Such a frequency would allow an operator to gather sufficient 

operating history and monitoring data for subsequent reevaluation. 

Response: EPA disagrees with the commenter's suggestion to tie the AoR reevaluation frequency to

the volume or percentage of CO2 injected. EPA believes that it is important to set an AoR

reevaluation frequency (i.e., at least every five years) to ensure that the project is reviewed and any

necessary updates to the project plans or the permit are made on a regular schedule, regardless of the

pace at which the injection is completed. 

EPA believes that the commenter's proposed approach has the potential to lead to AoR reevaluation

schedules that are not sufficiently frequent to ensure protection of USDWs from endangerment, if for

some reason the variables (rate or volume) are such that reevaluation based on percentages are not

triggered within 5 years. In addition, EPA believes this approach may complicate the administrative

process of determining and tracking the reevaluation schedule. 

For this reason, EPA has, at §146.84(e), adopted the 5 year interval (an adjustment from the ten year

interval in the proposed rule) recommended by many commenters. EPA adds that the Director has

178
discretion to require reevaluation more frequently, such as when monitoring data differ significantly

from modeled predictions, or when there are appreciable operational changes (e.g., injection rates). 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24336

Comment: 3. AOR Reevaluation 

The proposed rule requires periodic reevaluation of the AOR determination on a 

frequency of at least once every ten years. We recommend an alternative 

approach. 

Reevaluation of the AOR is a significant exercise that ought to be triggered 

only when information suggests that there is actually a need for reevaluation. 

At the same time, it is important to recognize that injection well operations 

provide an ongoing stream of information - injection pressure information in 

particular - that might suggest the need for reevaluation of the AOR. 

Consequently, it does not make sense to require reevaluation of the AOR on any 

fixed schedule; instead, it makes sense to consider the need to reevaluate the 

AOR on a regular basis, and to reevaluate the AOR if - but only if - there is 

evidence of a need to do so (i.e., if information becomes available that is 

materially inconsistent with assumptions underlying an existing AOR 

determination). 

The proposed rule would already require a semi-annual report of injection 

operations. 73 Fed. Reg. at 43540. We recommend that these periodic reports be 

annual, consistent with Class II requirements, and that these reports include an 

assessment of the need for reevaluation of the AOR. Specifically, we support the 

Ground Water Protection Council’s CO2 Committee recommendation 7.6 that 

operators: 

Report annually, or at a frequency acceptable to the Director, the 

characteristics and cumulative volumes (annual and total) of injected 

fluids, injection pressures, injection rates, annular pressures for each 

injection well, the results of any mechanical integrity tests conducted 

during this time period, available site integrity monitoring results, 

the extent of the injected CO2 and the pressure front based on current 

data. 

Update site operations as appropriate and site modeling results 

annually, or at a frequency acceptable to the Director, to reflect 

current data. Compare the updated results to the previous results. 

Identify and explain to the Director any discrepancies between the 

updated results and the previous results. 

This approach will provide a reasonably frequent consideration of the need for 

reevaluation of the AOR, ensuring that the AOR is reevaluated promptly when 

necessary but that there is no need to reevaluate the AOR when monitoring 

results remain consistent with modeling results such that a requirement to 

reevaluate the AOR would be unnecessary. 

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Response: EPA agrees that site-specific factors, including monitoring and operating data, should be

considered in the reevaluation schedule, and notes that the AoR and corrective action plan must

specify the site-specific monitoring and operational conditions that warrant reevaluation, as specified

in §146.84(b)(2)(ii). In addition, the Director has discretion to require more frequent AoR

reevaluation if he or she has reason to believe, based on site-specific information, that a five-year

reevaluation schedule is not sufficiently protective of USDWs. However, EPA believes that setting a

minimum reevaluation frequency is appropriate. In the final rule, the Agency changed the minimum

frequency for the AoR reevaluation at §146.84(e) from ten years to at least every five years in the

final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. In addition, the Director

has discretion to require more frequent AoR reevaluation if he or she has reason to believe, based on

site-specific information, that a five-year reevaluation schedule is not sufficiently protective of

USDWs.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24518

Comment: Rule Provisions or Federal Register Cite: 146.84 

EPA Preamble Question: 14. EPA seeks comment on requiring the reevaluation of 

the site AoR on a periodic basis, under what conditions the AoR should be 

reevaluated, and the appropriateness of a 10-year minimum fixed frequency for 

AoR reevaluation. EPA Notice 43507. 

CSC Response: Rather than a fixed time period, CSC recommends following a 

performance based approach that would require reevaluation of the AOR as, but 

only as, it is necessary, as determined by consideration of whether there are 

significant unexpected changes in injection parameters or monitoring results. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from ten years to at least every five years in the final GS rule. EPA believes that this revised

frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2

movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and

the unique nature of CO2 GS. In addition, the Director has discretion to require more frequent AoR

reevaluation if he or she has reason to believe, based on site-specific information, that a five-year

reevaluation schedule is not sufficiently protective of USDWs.

Commenter Name: Andy Horn Letter ID: 0196

Commenter Organization: Colorado Ground Water Association (CGWA) Comment ID: 23934

Comment: Issue 12. The timeframe of Area of Review Reevaluation should be revised to 

require more frequent reevaluation during the beginning of a project 

180
implementation and then less frequent reevaluation as the GS reservoir is shown 

to maintain integrity. 

The AoR should schedule be reevaluated annually during the first five years, 

biannually during the five to 10-year period and then on a 5-year schedule 

afterwards until closure. Following closure, reevaluation should be performed on 

a 10-year frequency until the operator is released from liability. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every five years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

To the comment regarding post closure reevaluations, EPA presumes the commenter is referring to

the post-injection site care period. (EPA clarifies that site closure occurs after a demonstration that

the GS project no longer poses a risk to USDWs.) 

Commenter Name: Paul Pike Letter ID: 0198

Commenter Organization: Ameren Corporation Comment ID: 23947

Comment: AREA OF REVIEW AND CORRECTIVE ACTION 

Ameren will be required to invest a significant amount of time and resources in 

the initial delineation of the Area of Review ("AoR"), but we believe that 

requiring AoR reevaluations every ten years or at any other fixed period is 

unnecessary. A performance-based approach would be more appropriate, with 

reevaluation required upon occurrence of some triggering event; one example of a 

triggering event could be a significant deviation from previously modeled 

scenarios, provided that the level of deviation that would require reevaluation 

is agreed to be the permitee and the regulating agency in the permitting 

process. 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

181
Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24581

Comment: Rule Provisions: 146.84 

EPA Rule Issue: 14. EPA seeks comment on requiring the reevaluation of the site 

AoR on a periodic basis, under what conditions the AoR should be reevaluated, 

and the appropriateness of a 10- year minimum fixed frequency for AoR 

reevaluation. EPA Notice 43507. 

Anadarko Petroleum Corporation (APC) Response: Ten years is acceptable as long 

as reevaluation at this frequency is coupled with the ongoing duty to monitor 

for changes that would trigger an earlier reevaluation of the AoR. 

Response: Based on evaluation of the public comments on this topic, EPA has changed the

minimum frequency for the AoR reevaluation at §146.84(e) from ten years to at least every five

years. EPA believes that this revised frequency addresses concerns raised by commenters

representing water associations, an energy company, associations, industries, states, and

environmental groups about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. 

The five-year reevaluation frequency, particularly during early stages of GS deployment, will also

support EPA's adaptive approach to the GS rulemaking. EPA is using an adaptive approach to

regulating GS. The Agency plans, every six years, to collect and review data on GS projects to

determine whether the appropriate amount and types of information and appropriate documentation

are being collected, and to determine if modifications to the requirements are appropriate or

necessary.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24664

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f) 

Anadarko Petroleum Corporation (APC) Comments: (f) At the minimum fixed 

frequency, not to exceed 10 years, as specified in the APPROVED area of review 

and corrective action plan, or when monitoring and operational conditions 

warrant, owners or operators must: 

Rationale: Clarification. 

Response: EPA did not make the suggested change, but agrees that AoR reevaluations must take

place as specified in approved AoR and corrective action plan. EPA clarifies that owners or

operators must develop, implement, and update a Director-approved AoR and corrective action plan

as required under §146.84(b). EPA is developing guidance for owners or operators on how to

prepare the required GS project plans. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Thomas Curtis Letter ID: 0204

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Commenter Organization: American Water Works Association (AWWA) Comment ID: 22184

Comment: - Reevaluation of the Area of Review should occur annually for the first ten 

years, and then every five years for the remaining life of the project. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Diane VanDe Hei Letter ID: 0206

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22211

Comment: geosequestration project. In addition, an annual reevaluation of the project’s 

area of review (AoR) should be conducted for at least the first 10 years of a 

geosequestration project and then every five years for the remaining operating 

period of the project. CCS is a developing technology and additional data and 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22433

Comment: Area of Review Reevaluation. . .. . . . . . . . . . . . . . . . Page 6 

Comparisons of monitoring data with model results should be made and reported 

annually, especially in the initial years of operation, to ensure that the site 

is behaving as expected or predicted. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

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GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Christopher Murray Letter ID: 0207

Commenter Organization: Pacific Northwest National Laboratory Comment ID: 22444

Comment: AREA OF REVIEW REEVALUATION 

PROPOSED REGULATION 

p. 43507, c. 3 When monitoring data differ significantly from modeled 

redictions or when there are appreciable operational changes (e.g., rates) 

reevaluation may be mandated prior to the minimum fixed frequency. At no time 

would area of review reevaluations occur less frequently than every 10 years. 

DISCUSSION 

The conclusion reached by EPA that the AoR should be reevaluated over time based 

on comparisons of incoming monitoring and site characterization data with model 

predictions is very important. However, the definition of "significantly" needs 

to be clarified in the regulation, otherwise interpretation will be a constant 

point of confusion. 

Based on PNNL's experience in modeling GS projects, comparison of monitoring 

data with model prediction, especially in the initial years of operation, would 

be extremely valuable in assessing our understanding of the reservoir behavior. 

Models of CO2 injection into deep formations are sensitive to estimates of 

permeability in the formation. Permeability can vary over orders of magnitude in 

a formation, and this can have a large impact on estimates of pressure response, 

and hence the AoR. As the size of the injected CO2 plume grows with time, it will 

encounter areas of the formation that may have different permeability than 

was measured at the injection well, and the AoR may vary over time. 

PNNL RECOMMENDATION 

We recommend that comparisons of monitoring data with model results be made and 

reported annually, to ensure that the site is behaving as expected or predicted. 

If at any time the monitoring data or comparisons of observed and predicted 

pressure response suggest anomalous behavior, this could suggest a need for a 

reevaluation of the AoR (and AoM; see previous comments), at the discretion of the 

Director. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

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commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

EPA also agrees with the commenter that inconsistencies between monitoring data and predictions

about CO2 plume and pressure front movement would warrant a reevaluation of the AoR. For this

reason, the AoR and corrective action plan must specify the site-specific monitoring and operational

conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii).

EPA is developing guidance on preparing and updating AoR and corrective action plans that will

clarify the requirements for reevaluations. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Michael Stroben Letter ID: 0209

Commenter Organization: Duke Energy Comment ID: 22684

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: REEVALUATION OF AREA OF REVIEW ("AoR") 

Current UIC regulations require the owner/operator of a proposed injection site 

to determine the AoR based on the area necessary to include the calculated cone 

of pressure influence. The amount of CO2 that will be injected at a GS site over 

the life of a project will be tens if not hundreds of millions of tons. This 

quantity of CO2 along with uncertain dispersion characteristics makes it next to 

impossible to determine the correct AoR utilizing a fixed radius or simple 

mathematical method. 

Acceptable methods for determining the AoR should include but not be limited to 

multiphase fluid flow models. It is anticipated that modeling will improve over 

time as experience with CO2 sequestration increases. Demonstration projects 

should not be constrained by allowing only the use of current models. Any model 

based on accepted science that has been validated should be acceptable. Models 

should be calibrated using actual site data once it becomes available. 

The AoR for a GS site for CO2 will be dynamic and in all probability irregular 

in shape. Based on the quantity of CO2 to be sequestered at a site, the AoR 

could be hundreds of square miles. A significant amount of time and money would 

be needed to complete an up-front evaluation of the life of a project AoR. A 

large portion of this effort could be wasted since the injection plumb may not 

travel as anticipated. As a result of this uncertainty, Duke Energy recommends a 

phased evaluation for the AoR. The AoR should be reviewed periodically (every 

ten years or as agreed upon by the Director). This would allow any necessary 

corrective actions to be focused where needed. This approach would allow 

projects to proceed more expeditiously, reduce start-up costs and because of the 

periodic review, provide greater protection to USDW. 

Response: EPA clarifies that the AoR must be delineated as part of the permit application, as

185
required at §§146.82(a)(2) and 146.82(c)(1). The Agency believes that AoR delineation using

computational modeling that accounts for the multiphase flow of CO2 and native fluids (i.e.,

naturally occurring salty water), including density-driven flow (i.e., buoyancy), is necessary to

ensure protection of USDWs from endangerment. The computational models used for this purpose

are required to predict the fate of injected CO2 and changes in formation pressure. EPA believes that

the requirements at 146.84(c)(1) to delineate the AoR using computational modeling are appropriate

and encompass the significant processes that could affect the location of the CO2 plume and

associated pressure front. 

EPA agrees that modeling technology is likely to improve over time, and EPA believes that future

AoR reevaluations, as required at 146.84(e), should incorporate the best modeling available. Periodic

reevaluations also ensure that a deliberative effort is made to verify, based on monitoring and

operational data, that the CO2 plume is moving within the subsurface as predicted. However, EPA

clarifies that the AoR delineation process will not be phased. Owners or operators must delineate the

entire AoR of the project prior to receiving an operating permit. This evaluation must include the

entire region surrounding the geologic sequestration project where USDWs may be endangered by

the injection activity.

EPA agrees that phased corrective action may provide many benefits to a project including, but not

limited to, spreading the burden of cost, avoiding delays in project start-up, allowing for use of

future, improved corrective action techniques and addressing unanticipated changes in the movement

of the CO2 plume or pressure front. 

Commenter Name: Richard Guinn Letter ID: 0210

Commenter Organization: Eastman Chemical Company Comment ID: 22730

Comment: EPA seeks comment on requiring the reevaluation of the site AoR on a periodic 

basis, under what conditions the AoR should be reevaluated, and the 

appropriateness of a 10-year minimum fixed frequency for AoR reevaluation. EPA 

Notice 72. 

10 years is a reasonable period of time to reevaluate the site AoR during 

injection. 

Response: Based on evaluation of the public comments on this topic, EPA has changed the

minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years.

EPA believes that this revised frequency addresses concerns raised by commenters representing

water associations, an energy company, associations, industries, states, and environmental groups

about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology,

risk-based targeting of monitoring activities, and the unique nature of CO2 GS. 

The 5-year reevaluation frequency, particularly during early stages of GS deployment, will also

support EPA's adaptive approach to the GS rulemaking. EPA is using an adaptive approach to

regulating GS. The Agency plans, every six years, to collect and review data on GS projects to

determine whether the appropriate amount and types of information and appropriate documentation

are being collected, and to determine if modifications to the requirements are appropriate or

necessary.

Commenter Name: John King Letter ID: 0211

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Commenter Organization: Shell Exploration and Production Company

(SEPCo)

Comment ID:

22768

Other EPA Comment Codes and Descriptions: 3400 2.2.2 Corrective Action 

Comment: 2.Corrective Action 

Corrective action is a familiar UIC program requirement. Given the potential 

scale of GS operations, EPA has proposed requirements for a corrective action 

plan contemplating periodic reassessment of the AOR and a phased implementation 

of corrective action. In general, this approach is prudent and practical, 

providing what will ultimately be a rigorous approach to corrective action while 

reducing regulatory burdens by permitting a phased implementation of corrective 

action requirements. However, SEPCo does not believe that a minimum frequency 

for reevaluation of the AOR should be required. 

Recommendation: 

The need for reevaluation of the AOR should be required -- if at all -- when 

there is site specific evidence that there is actually a need to reevaluate the 

AOR. Where monitoring results remain consistent with modeling results, a 

requirement to reevaluate the AOR should not be required. 

Response: EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations or planning reevaluations only when a need

is demonstrated may not be sufficient to appropriately model plume and pressure front movement,

establish conditions for corrective action, and ensure protection of USDWs from endangerment. EPA

agrees that site-specific factors should be considered in the reevaluation schedule, and notes that the

AoR and corrective action plan must specify the site-specific monitoring and operational conditions

that warrant reevaluation, as specified in §146.84(b)(2)(ii).

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Diane VanDe Hei Letter ID: 0213

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22943

Other EPA Comment Codes and Descriptions: 3560 2.2.3.2 Conditions that merit reevaluation of

the AoR (seeking comment) 

Comment: - 73 FR 43507 -EPA seeks comment on requiring the reevaluation of the site AoR 

on a periodic basis, under what conditions the AoR should be reevaluated, and 

the appropriateness of a 10 year minimum fixed frequency for AoR reevaluation. 

Response: As carbon sequestration is an emerging technology that has not yet 

been tested on a large scale in the U.S., AMWA believes at this time that a tenyear minimum frequency for AoR evaluation is not sufficient. AMWA recommends 

instead that the AoR be reevaluated annually for the first ten years of the 

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project and then every five years for the remaining operating period of the 

project. 

AMWA also recommends that the final rule include criteria to trigger a shorter 

review period at earlier intervals should monitoring detect a sudden change in 

CO2 plume movement, size or pressure. In addition, AMWA recommends that local 

drinking water utilities be included in the AoR review process, as existing or 

potential drinking water sources could become threatened due to a change in 

plume direction or behavior. Public involvement also needs to be a mandatory 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. 

EPA agrees that other aspects about a GS project may trigger the need for an AoR reevaluation, and

adds that the AoR and corrective action plan must also specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). EPA agrees that

site-specific information is an important factor in determining when AoR reevaluations are

appropriate, and expects that the owner or operator's and Director's combined knowledge of the local

geologic conditions will be a critical element in developing plans that protect USDWs from

endangerment. In addition, the Director has discretion to require more frequent AoR reevaluation if

he or she has reason to believe, based on site-specific information, that a five-year reevaluation

schedule is not sufficiently protective of USDWs.

Regarding the suggestion that local drinking water utilities be included in the AoR review process,

EPA added text to §144.39(a)(5) that permit modifications would be required if the Director

determines that a modification is necessary based on an AoR reevaluation, amendments to any of the

project plans, or a review of the testing and monitoring results. EPA encourages permitting

authorities to notify and involve as many stakeholders as possible about permit modifications,

including drinking water organizations and local water utilities. 

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24021

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3400 2.2.2 Corrective Action 

Comment: Proposed Rule: (b) The owner or operator of a Class VI well must prepare, 

maintain, and comply with a plan to delineate the area of review for a proposed 

geologic sequestration project, periodically reevaluate the delineation, and 

perform corrective action that meets the requirements of this section and is 

acceptable to the Director. As a part of the permit application for approval by 

the Director, the owner or operator must submit an area of review and corrective 

action plan that includes the following information: 

Schlumberger Suggested Revision: (b) The owner or operator of a Class VI well 

188
must prepare, maintain, and comply with a plan to delineate the area of review 

for a proposed geologic sequestration project, reevaluate the delineation, and 

perform corrective action that meets the requirements of this section in a 

manner and at a frequency acceptable to the Director. As a part of the permit 

application, the owner or operator must submit an area of review and corrective 

action plan that includes the following information: 

Schlumberger Rationale: - 

Response: EPA disagrees with the proposed changes and retains the provisions as proposed. For

additional information on EPA's approach to AoR and corrective action, please see the final rule and

preamble.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24022

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: (i) The minimum fixed frequency, not to exceed 10 years, the 

owner or operator proposes to reevaluate the area of review; 

Schlumberger Suggested Revision: Revision: (i) The owner or operator will 

reevaluate the AOR after two years, 5 years and then no less frequently than 

every 5 years for period of injection. This schedule may be relaxed at the 

discretion of the director in the post injection period if appropriate data 

exist to support the change. 

Schlumberger Rationale: We feel that a frequency of every 10 years is too long 

to identify early discrepancies between the site performance and the site model. 

Seeing early difference will allow for earlier model updating and provide 

additional reduction in leakage risk and a better understanding of the site. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24029

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: (f) At the minimum fixed frequency, not to exceed 10 years, as 

specified in the area of review and corrective action plan, or when monitoring 

189
and operational conditions warrant, owners or operators must: 

Schlumberger Suggested Revision: (f) At the same frequency as the reevaluation 

of the AOR as specified in 146.84(c ) (2) (i), or when monitoring and 

operational conditions warrant, owners or operators must: 

Schlumberger Rationale: - 

Response: EPA disagrees with the proposed changes and retains the provisions as proposed. EPA

believes that the commenter’s suggested edit has substantively the same meaning as the proposed

text. For additional information on EPA's approach to AoR and corrective action, please see the final

rule and preamble.

Commenter Name: A. James Mayer Letter ID: 0230

Commenter Organization: A. J. Mayer International on behalf of the American

Energy Security Initiative

Comment ID:

21888

Comment: 5. EPA should provide more flexibility in operational requirements including 

allowing an EOR/EGR/ECBM operator to demonstrate that CO2 is sequestered rather 

than the prescriptive 10-year default period for reevaluation of the area of 

review. 

Response: EPA clarifies that the AoR reevaluation required at §146.84(e) is not intended to serve as

a replacement for monitoring of the site or demonstrating containment. While the two are linked—

monitoring and operational data will inform the AoR reevaluation and changes to the calculated AoR

may necessitate changes to the testing and monitoring plan—EPA believes that periodic monitoring,

including ground water sampling and plume and pressure front tracking, are necessary to ensure that

CO2 is not endangering USDWs. 

EPA is retaining the approach of setting a minimum fixed frequency for reevaluating the AoR but

has, based on commenters' input, changed the frequency from 10 years to at least every 5 years, as

specified in §146.84(e). EPA agrees with commenters that site-specific information is an important

factor in determining an appropriate AoR reevaluation frequency, and adds that the owner or

operator's AoR and corrective action plan must describe the minimum fixed frequency for

reevaluating the AoR and the site-specific conditions on which this frequency is based. 

Commenter Name: Steven B. Corneli Letter ID: 0234

Commenter Organization: NRG Energy, Inc. Comment ID: 23163

Comment: 2. AoR Reevaluation 

The proposed rule requires a re-evaluation of the AoR determination on a 

frequency not to exceed once every ten years. NRG agrees with the Carbon 

Sequestration Council’s recommended alternative approach to re-evaluate the AoR 

only when site specific operation and monitoring data suggest that there is a 

need for a new evaluation of the AoR. Periodic AoR re-evaluation will most 

likely be necessary given the enormous volumes of CO2 that would be injected 

into the subsurface, the lengthy timeframes likely for GS operations, and the 

potential uncertainty of the original model prediction. We believe that it is 

impractical and cost prohibitive to require re-evaluation of the AoR on a fixed 

190
schedule. Re-evaluation of the AoR should only be conducted when monitoring and 

testing data are determined to be inconsistent with the modeling results. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. However, EPA believes that setting a minimum reevaluation frequency is appropriate. In

the final rule, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e)

from 10 years to at least every 5 years in the final GS rule. EPA believes that this revised frequency

addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the

emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique

nature of CO2 GS. 

EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure protection of

USDWs from endangerment and address the uncertainties inherent in predictive modeling. Periodic

AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify, based on

monitoring and operational data, that the CO2 plume is moving within the subsurface as predicted.

EPA believes that less frequent reevaluations may not be sufficient to appropriately model plume

and pressure front movement, establish conditions for corrective action, and ensure protection of

USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data.

Commenter Name: Abbie Krebsbach Letter ID: 0235

Commenter Organization: Montana-Dakota Utilities Co. Comment ID: 23184

Comment: Periodic Reevaluation of Area of Review (AOR) 

The proposed regulations would require operators to reevaluate the proper scope 

of the AOR periodically at a minimum fixed frequency to be set by the program 

director, currently proposed as no less than once every 10 years, with the 

discretion to require more frequent reevaluation. AOR reevaluation provisions 

should be triggered by reasonable parameters, taking into consideration sitespecific issues. AOR issues are inherently site-specific; different projects, 

and even different geographical regions of the country, likely warrant different 

approaches. 

Montana-Dakota believes that tying reevaluation to the triggering of appropriate 

circumstances will provide equivalent protection for USDW while also ensuring 

that those projects posing greater relative risks receive greater scrutiny 

through more stringent reevaluation requirements, and those projects posing 

fewer relative risks are relieved of unnecessary regulatory burdens. At a 

minimum, any default period for reevaluation should be determined based on sitespecific characteristics at the time of project permitting. 

Response: EPA agrees that site-specific information is an important factor in determining when

191
AoR reevaluations are appropriate, per §146.84(e), and expects that the owner or operator's and

Director's combined knowledge of the local geologic conditions will be a critical element in

developing plans that protect USDWs from endangerment. 

EPA also believes that periodic AoR reevaluation at a set frequency is necessary to ensure protection

of USDWs from endangerment and address the uncertainties inherent in predictive modeling.

Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify, based on

monitoring and operational data, that the CO2 plume is moving within the subsurface as predicted. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23234

Comment: 14. EPA seeks comment on requiring the reevaluation of the site AoR on a 

periodic basis, under what conditions the AoR should be reevaluated, and the 

appropriateness of a 10-year minimum fixed frequency for AoR reevaluation. EPA 

Notice 43507. 

It is appropriate to reevaluate the AOR on a periodic basis. The conditions and 

frequency under which the AOR is reevaluated should be based upon monitoring. 

Response: EPA agrees that site-specific factors, including monitoring data, should be considered in

the reevaluation schedule, and notes that the AoR and corrective action plan must specify the sitespecific monitoring and operational conditions that warrant reevaluation, as specified in

§146.84(b)(2)(ii). In addition, the Director has discretion to require more frequent AoR reevaluation

if he or she has reason to believe, based on site-specific information, that a five-year reevaluation

schedule is not sufficiently protective of USDWs. However, EPA believes that setting a minimum

reevaluation frequency is appropriate. In the final rule, the Agency changed the minimum frequency

for the AoR reevaluation at §146.84(e) from 10 years to 5 years in the final GS rule. EPA believes

that this revised frequency addresses commenters' concerns about the inherent uncertainties in

modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring

activities, and the unique nature of CO2 GS.

Commenter Name: Kristin Henry Letter ID: 0242

Commenter Organization: Sierra Environmental Law Program, Sierra Club Comment ID: 23329

Other EPA Comment Codes and Descriptions: 4750 2.7.5 Comments on the merit of annual

reports (seeking comment) 

Comment: An update or redefinition of the AOR at a minimum fixed frequency of ten years 

is not sufficient. The operator should be required to submit an annual 

assessment under stating whether they deem a revision of the AOR necessary, 

explaining their reasoning based on site-specific operational and other data. 

The Director should examine and approve or reject this submission, requiring a 

192
revision of the AOR as needed. The same updating procedure should apply to all 

other plans: monitoring, closure, etc. We believe that this arrangement 

encourages honest and truthful reporting of data and site performance without 

placing excessive burdens on redefining the AOR if such revision is not needed. 

It also leaves a clear accountability trail for both operator and Director in 

case of disputes. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. In addition, the Director has discretion to require more frequent AoR reevaluation if he or she

has reason to believe, based on site-specific information, that a five-year reevaluation schedule is not

sufficiently protective of USDWs.

EPA also agrees that periodic reviews of the other plans based on the AoR reevaluation is

appropriate. The final GS rule includes provisions that owners or operators review and update the

AoR and corrective action plan, the testing and monitoring plan, and the emergency and remedial

response plan throughout the life of the project, as specified in §146.84(c), §146.90(j), and

§146.94(d). Owners or operators must review each plan and identify necessary amendments to the

plan or demonstrate to the Director that no amendment is needed. These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director.

EPA agrees that the AoR reevaluations offer a good opportunity to promote a dialogue between the

owner or operator and the permitting authority. The final GS rule, at §§146.84(c), 146.90(j), and

146.94(d), requires owners or operators to review and update the AoR and corrective action plan, the

testing and monitoring plan, and the emergency and remedial response plan throughout the life of

the project. Owners or operators must review each plan and identify necessary amendments to the

plan or demonstrate to the Director that no amendment is needed. These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director. In no case can reviews

occur less often than once every five years. For additional information, please see the final rule and

the preamble for a discussion of EPA's final approach to recordkeeping and reporting.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22308

Comment: Even though proposed §146.84(f) requires "reevaluation" of the AOR at a 

"minimum" fixed frequency of ten years or "when operational and monitoring 

conditions warrant" (the "and" should be an "or"), §146.84 runs the risk that 

revisions will not be made on a timely basis. Default periods have a way of 

becoming the standard way of doing business. We recommend imposing a continual 

obligation on operators to assess whether the AOR should be revised, a 

requirement for an annual report stating whether circumstances during the 

preceding year warranted a revision, and a requirement that revisions be done 

when required by the Director as well as when the operator determines that 

193
conditions warrant. To accomplish this, it will be useful to break down the 

concept of "reevaluation" into two ideas - "assessment" of the need to revise 

the AOR and the actual process of "revision." There are several portions of 

§146.84 that will need to be changed in order to incorporate these suggestions. 

We believe that these suggestions will incentivize honest and truthful reporting 

of data and site performance while avoiding the excessive burden of "reevaluating" or revising the Area of Review if such work is not needed. Moreover, 

a continuous obligation to assess whether revisions are needed, when coupled 

with the annual reporting requirement just suggested, will create a clear 

accountability trail for both operator and Director in case of disputes. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient, and has changed the minimum frequency for the AoR reevaluation from 10 years to at

least every 5 years, as specified in §146.84(e). EPA believes that this revised frequency addresses

commenters' concerns that more frequent reevaluations are necessary to address the inherent

uncertainties in modeling CO2 movement, the emerging nature of GS technology, risk-based

targeting of monitoring activities, and the unique nature of CO2 GS. To the comment that a

"minimum frequency" tends to become a standard frequency, which may not be adequate for some

sites, EPA clarifies that the Director has discretion to require more frequent AoR reevaluations based

on site specific information to ensure that USDWs are protected from endangerment.

EPA recognizes that there are many unknowns about how GS projects will unfold including: the

practicalities of permitting, operating, monitoring, and closing a GS project. To address this, the

Agency is using an adaptive approach to regulating GS. The Agency plans, every six years, to collect

and review data on GS projects to determine whether the appropriate amount and types of

information and appropriate documentation are being collected, and to determine if modifications to

the requirements are appropriate or necessary.

EPA disagrees that an annual reevaluation of the AoR is appropriate, because this timeframe would

not, in many cases, allow for the generation and collection of sufficient data on which evaluate

modeled predictions. However, EPA adds that the Director has discretion to require more frequent

reevaluations based on site specific information such as when monitoring data differ significantly

from modeled predictions, or when there are appreciable operational changes (e.g., injection rates). 

EPA adds that the final GS rule includes provisions that owners or operators review and update the

AoR and corrective action plan throughout the life of the project, as specified in §146.84(c). Owners

or operators must review the plan and identify necessary amendments to the plan or demonstrate to

the Director that no amendment is needed. The review must be performed within one year of an AoR

reevaluation, following any significant changes to the facility (e.g., the addition of monitoring or

injection wells), or when required by the Director. EPA believes that these new provisions could

address the commenter’s suggestions.

Commenter Name: Walter L. Baker Letter ID: 0253

Commenter Organization: Utah Division of Water Quality Comment ID: 22349

Comment: RE-EVALUATION OF THE AREA OF REVIEW 

Given that there is not extensive experience in tracking the movement of large 

volumes of injected CO2 and associated formation fluids, we support EPA's intent 

to require periodic re-evaluation of the Area of Review. We feel strongly that 

194
monitoring results should inform the Director in determining an appropriate 

frequency for re-evaluation and that this should be an iterative process 

resulting in the continuing refinement of the site-specific predictive model. 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient, and has changed the minimum frequency for the AoR reevaluation from 10 years to 5

years, as specified in §146.84(e). EPA believes that this revised frequency addresses commenters'

concerns that more frequent reevaluations are necessary to address the inherent uncertainties in

modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring

activities, and the unique nature of CO2 GS. 

EPA agrees that site-specific information, including monitoring data, is an important factor in

determining when AoR reevaluations are appropriate, and adds that the final rule also requires

owners or operators to identify in the AoR and corrective action plan site-specific triggers for

reevaluation before five years. EPA anticipates that anomalous monitoring data would be one such

trigger. 

EPA recognizes that there are many unknowns about how GS projects will unfold including: the

practicalities of permitting, operating, monitoring, and closing a GS project. To address this, the

Agency is using an adaptive approach to regulating GS. The Agency plans, every six years, to collect

and review data on GS projects to determine whether the appropriate amount and types of

information and appropriate documentation are being collected, and to determine if modifications to

the requirements are appropriate or necessary.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24890

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal: (f) At the minimum fixed frequency, not to exceed 10 years, as 

specified in the area of review and corrective action plan, or when monitoring 

and operational conditions warrant, owners or operators must: 

(1) Reevaluate the area of review in the same manner specified in paragraph (c) 

(1) of this section; 

(2) Identify all wells in the reevaluated area of review that require corrective 

action in the same manner specified in paragraph (c)(2) of this section; 

GWPC Recommendations: (f) As specified in the approved area of review and 

corrective action plan, or when monitoring and operational conditions warrant, 

owners or operators must 

(1) Reevaluate the area of review in the same manner specified in paragraph (c) 

(1) of this section; 

(2) Identify all wells in the reevaluated area of review that require corrective 

action in the same manner specified in paragraph (c)(2) of this section; 

Rationale: Delete "At the minimum fixed frequency, not to exceed 10 years". 

195
Insert "approved" before "area of review" in (f). 

Response: EPA disagrees with the suggested edit, and believes that periodic AoR reevaluation at a

set frequency is necessary to ensure protection of USDWs from endangerment and address the

uncertainties inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a

deliberate effort is made to verify, based on monitoring and operational data, that the CO2 plume is

moving within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Letter ID: 0263

Commenter Organization: Interstate Oil & Gas Compact Commission Comment ID: 22800

Comment: III A. 2 Proposed Area ofReview and Corrective Action Requirements 

O. EPA seeks comment on requiring the reevaluation of the site AoR on a periodic 

basis, under what conditions the AoR should be reevaluated, and the 

appropriateness of a 10 year minimum fixed frequency for AoR reevaluation. 

IOGCC Response: Note: This comment is based on the traditional AoR requirement. 

Most states do an annual project review with the operator not only to evaluate 

the project's performance but to determine if additional measures are needed. 

Setting a frequency should be performance based and should be left to the 

Director's discretion, but in no event less than every two years. Only if the 

review determines that remodeling, further study or additional downhole monitoring is necessary, should such work be done. Additionally, such 

additional work should be done as needed, not at a set time interval. 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA adds that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs.

Commenter Name: Steven Chester Letter ID: 0264

Commenter Organization: Michigan Department of Environmental Quality Comment ID:

196
(MDEQ) 22835

Comment: EPA (III. A. 2.) proposes periodic re-evaluation of the Area of Review (AoR), 

with a 10year minimum fixed frequency. 

- Staff believes that the frequency of re-evaluation should be based on 

the results of the initial modeling and subsequent monitoring and that a fixed 

10-year minimum should not be mandated. 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every five

years is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Michael Moore Letter ID: 0265

Commenter Organization: North American Carbon Capture & Storage

Association (NACCSA)

Comment ID:

22852

Comment: - Area of Review Reevaluation: NACCSA questions the inclusion of a 

requirement that the area of review by reevaluated at least every ten years. 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

Commenter Name: Michael Moore Letter ID: 0265

Commenter Organization: North American Carbon Capture & Storage

Association (NACCSA)

Comment ID:

22864

Comment: 3. Area of Review Reevaluation 

EPA's proposal would require that the owner or operator periodically reevaluate 

the AoR during the injection operation. Reevaluations would occur at a minimum 

fixed frequency, not to exceed 10 years, as agreed upon by the Director. We 

question the need for this requirement to be mandatory in light of the rule's 

extensive monitoring requirements. In the event that on going monitoring indicated 

a problem, appropriate action could be taken at that time without separately 

requiring a complete AoR reevaluation according to a pre-defined schedule. 

We recommend that EPA provide the AoR reevaluation is at the discretion of the 

197
Administration if the Administration determines that site-specific data, 

reservoir characteristics and/or monitoring or modeling data dictate that such 

a reevaluation would be warranted. 

Response: As described above, EPA believes that periodic AoR reevaluation at a set frequency of at

least every 5 years is necessary to ensure protection of USDWs from endangerment and address the

uncertainties inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a

deliberate effort is made to verify, based on monitoring and operational data, that the CO2 plume is

moving within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

In response to the suggestion that site-specific factors should dictate the reevaluation schedule, EPA

notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: John McManus Letter ID: 0294

Commenter Organization: American Electric Power Comment ID: 25316

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: 8. Reevaluation of the Area of Review and Revision and Maintenance of Plans 

Although the proposed §146.84(f) would require "reevaluation" of the aor at a 

"minimum" fixed frequency of ten years or "when operational and monitoring 

conditions warrant," the specific language of §146.84 runs the risk that 

revisions will not be made on a timely basis. We recommend imposing a continual 

obligation on operators to assess whether the aor itself, the aor and corrective 

action plan and all other plans required by the regulations should be revised, 

requiring an annual certification stating that the owner or operator has 

reviewed the circumstances (including operating and monitoring data) during the 

preceding year to determine whether these circumstances warranted a revision of 

the aor and any plans, and requiring that revisions be done when required by the 

director as well as when the operator determines that conditions warrant. To 

accomplish this, it is useful to break down the concept of "reevaluation" into 

two ideas - "assessment" of the need to revise the aor and all plans and the 

actual process of "revision." we believe that these suggestions will result in 

increased accuracy and reliability in the site performance data while avoiding 

work that is not warranted by the site data and site performance. Moreover, a 

continuous obligation to assess whether revisions are needed, when coupled with 

an annual certification requirement (see proposed §146.91(d) below) will create 

a clear accountability trail for both operator and director in case of disputes. 

Given this annual certification process, we believe it is unnecessary to require 

198
a reevaluation on a fixed basis every 10 years in every case. 

An annual certification requirement can also be used to enhance the ability of 

both Director and operators to keep track of which plans require updating and 

whether or not plans that ought to be updated have in fact been revised. Such a 

shared understanding is essential if there is to be the "ongoing dialogue" 

between regulators and operators that EPA anticipates. We considered the 

inclusion of an annual report as described in the preamble at 73 Fed. Reg. 

43518, but concluded that the benefits of that particular report would be 

outweighed by the burden. The annual certification we are recommending would be 

uch simpler. We recommend: 

That §146.84(f) be revised to read as follows: 

"(f) (1) Notwithstanding the requirement in paragraph (2)(i) of this subsection 

to perform a reevaluation of the area of review at the frequency set forth in 

the area of review and corrective action plan, the owner or operator must also 

conduct the following whenever warranted by material change in the monitoring 

and operational data or in the evaluation of the monitoring and operational 

data by the owner or operator: 

"(i) reevaluate the area of review by performing all of the actions specified 

in paragraphs (c)(1) through (3) of this section to delineate the area of 

review and identify all wells that require corrective action; 

"(ii) perform corrective action on wells requiring corrective action in the 

reevaluated area of review in the same manner specified in paragraph (d) of 

this section; and 

"(iii) submit an amended area of review and corrective action plan or 

demonstrate to the Director through monitoring data and modeling results that 

no change to the area of review and corrective action plan is needed. 

"(2) Except as provided in paragraph (3) of this subsection, the owner or 

operator shall also conduct a reevaluation by performing all of the actions 

specified in subparagraphs (1)(i) through (iii) of this subsection: 

"(i) Within the time periods specified in the area of review and corrective 

action plan for conducting such a reevaluation of the area of review; or 

"(ii) If an area of review evaluation or reevaluation has not been performed 

pursuant to §146.84(c) and (d) for the geologic sequestration project during 

the preceding ten (10) years, then within six months after the passage of that 

10-year period. 

"(3) The Director shall waive the requirement for a reevaluation pursuant to 

either §146.84(f)(2)(i) or (ii) if the owner or operator demonstrates to the 

Director through monitoring and operational data and modeling results, such as 

those reviewed annually pursuant to section 146.95, that such a reevaluation is 

not warranted by material change." 

199
Response: EPA disagrees with the proposed changes and believes that periodic AoR reevaluation at

a set frequency of at least every 5 years is necessary to ensure protection of USDWs from

endangerment and address the uncertainties inherent in predictive modeling. Periodic AoR

reevaluations at §146.84(e) ensure that a deliberate effort is made to verify, based on monitoring and

operational data, that the CO2 plume is moving within the subsurface as predicted. EPA believes that

less frequent reevaluations may not be sufficient to appropriately model plume and pressure front

movement, establish conditions for corrective action, and ensure protection of USDWs from

endangerment.

EPA agrees that site-specific information will influence the need for AoR reevaluations, and clarifies

that if site monitoring data agrees with the existing AoR delineation, a model recalibration may not

be necessary, and the AoR reevaluation may consist of a demonstration that the current AoR

delineation is adequate based on site monitoring data. 

Commenter Name: Scott Anderson Letter ID: 0520

Commenter Organization: Environmental Defense Fund Comment ID: 21430

Other EPA Comment Codes and Descriptions: 3300 2.2.1 Delineating the AoR/Modeling CO2

movement 

Comment: We think the AOR's area of reviews need to be calculated as intended and updated 

as necessary. We think ten year updates is far too infrequent and we think that 

there are some technical problems with the way the AOR is defined in the rule. 

We believe the EPA's intention is to use elevated pressure as the main guide to 

AOR delineation, but in our judgment the definition just does not say that. 

Response: EPA agrees that reevaluations of the AoR every ten years may not be sufficient to

appropriately model plume and pressure front movement, establish conditions for corrective action,

and ensure protection of USDWs from endangerment. For these reasons, the Agency changed the

minimum frequency for the AoR reevaluation at 146.84(e) from 10 years to at least every 5 years in

the final GS rule. EPA believes that this revised frequency addresses commenters' concerns about the

inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, riskbased targeting of monitoring activities, and the unique nature of CO2 GS. 

EPA agrees that the AoR definition should account for the impact of the CO2 plume on USDWs.

The Agency has carefully considered all comments on the AoR definition at 146.84(a) and modified

it to read as follows: The region surrounding the geologic sequestration project where USDWs may

be endangered by the injection activity. The area of review is delineated using computational

modeling that accounts for the physical and chemical properties of all phases of the injected CO2

stream and is based on available site characterization, monitoring, and operational data. 

Comment Code: 3560 2.2.3.2 Conditions that merit reevaluation of the AoR (seeking comment) 

Individual Comments

Commenter Name: Dawn A. Thorne Letter ID:

0134

200
Commenter Organization: University of Utah Energy and Geoscience Institute on

behalf of the Southwest Regional Partnership

Comment

ID: 21655

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: - Section 146.84(b)(2)(i) "A description of The minimum fixed frequency, 

not to exceed 10 years, the owner or operator proposes to reevaluate the area of 

review; " 

Section 146.84(b)(2)(ii) "The monitoring and operational conditions that would 

warrant a reevaluation of the area of review prior to the next scheduled 

reevaluation as determined by the minimum fixed frequency established in 

paragraph (b)(2)(i) of this section " 

Response/Discussion: The SWP recommends adopting a risk/performance-based 

approach to AoR re¬evaluation. Since each site is unique, it may be best for the 

operator to work directly with the EPA Director to establish an evaluation 

schedule that would occur as often as needed, but not more than needed. The SWP 

agrees that re-evaluations should be based on updating, revising, and 

calibrating computational models with site-specific data. 

Response: EPA agrees that site-specific factors should be considered in the reevaluation schedule,

and notes that the AoR and corrective action plan must specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). In addition, the

Director has discretion to require more frequent AoR reevaluation if he or she has reason to believe,

based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective

of USDWs. 

However, EPA believes that periodic AoR reevaluation at a set frequency is necessary to ensure

protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22591

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 4710 2.7.1 Proposed submissions required for permit applications (seeking comment) 

Comment: Proposed Rule: 146.82(c) 

Revision or Comment: Proposed Revision: 2(c ) Re-evaluation of Area of Review. 

After completion of the operational testing period, the Area of Review shall be 

re-evaluated, using a calibrated, multi-phase fluid flow model. The data 

collection during operational testing must be adequate for calibration of the 

model. If a calibrated model cannot be prepared using data collected during 

201
operational testing, the owner or operator shall apply for a new period of 

operational testing of up to two years in order to collect the required data. 

No rationale provided 

Response: EPA agrees that AoR reevaluations should be based on the same type of modeling that is

used to perform the initial delineation, but disagrees that the suggested change is necessary. EPA

clarifies that all AoR modeling (including that which supports reevaluations) should be based on

accepted, verified, and validated scientific methods. Under requirements at §146.84(b), the owner or

operator must demonstrate in the proposed AoR and corrective action plan, to the satisfaction of the

Director that the modeled AoR (and all updates) will be based on detailed geologic data collected on

the site. EPA anticipates that any model that does this in a comprehensive and satisfactory manner

would be approved by the Director.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22594

Other EPA Comment Codes and Descriptions: 5600 2.11 Comments related to UIC Program

primacy for Class VI wells 

Comment: Proposed Rule: 146.84(c) 

Comment: There should be somewhere in the rules a requirement that "mutual 

interference effects" should trigger reevaluation and re-delineation of the AORs 

of all projects that are interferring with each other. In essence, when a new 

project is submitted for permitting, there should be a system-wide, "regional" 

entity charged with ensuring that AORs for each project are appropriate, and 

perhaps that a "hybrid" or "combo AOR" be defined for the set of mutuallyinterferring projects. 

No rationale provided 

Response: EPA anticipates that the impacts of multiple operators injecting into hydraulically

connected formations will be accounted for in the initial AoR calculation. EPA adds that the owner

or operator must demonstrate, in the proposed AoR and corrective action plan, to the satisfaction of

the Director that the modeled AoR will be based on detailed geologic data collected on the site and

includes consideration of existing conditions such as the presence of multiple operators. EPA adds

that if any new GS projects are permitted within the AoR of a GS project between AoR evaluations,

the AoR reevaluation would need to address such projects. EPA also expects that the permitting of

the newer GS project would have considered the presence of an existing GS project.

EPA understands that the CO2 plumes for large GS projects may cross State, Tribal, Territorial, EPA

Regional, or national boundaries. The final GS rule, at §146.82(a)(2), requires the owner or operator

to identify all State, Tribal, and Territorial boundaries within the AoR. Based on this information, the

Director must provide written notification to all States, Tribes, and Territories to inform them of the

permit application, as specified in §146.82(b) and ensure that they are involved in any necessary

processes (e.g., development of the emergency and remedial response plan, as specified in §146.94).

The Agency encourages transparency in the permitting process and anticipates that State-State/StateTribal communication on GS permitting will facilitate information sharing and encourage safe,

protective projects. 

202
Because some GS projects will require coordination across State, Tribal and Territorial jurisdictions,

the Agency will work with the appropriate entities to facilitate communication among States, Tribes,

and Territories interested in obtaining primacy during the implementation phase of the GS

regulation. Such communication may involve other organizations such as the Sierra Club, NDWAC,

associations, and other experts. However, the responsibility for addressing such subjects ultimately

rests with the States, Tribes, and Territories with primacy. In addition, EPA is developing guidances

for states that describe the process of applying for and receiving primary enforcement responsibility

(primacy) for GS Class VI wells under Section 1422 of SDWA, as well as the technical and

administrative issues on developing GS regulations that are the equivalent of minimum Federal

criteria and requirements. The guidance will be available at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22113

Comment: conditions. Additionally, after any seismic event a reevaluation should be 

automatically triggered regardless of the year. 

Response: EPA thanks the commenter for their input on the types of conditions that could warrant

reevaluation of the AoR at §146.84(e). EPA agrees that site-specific information is an important

factor in determining when AoR reevaluations are appropriate, and expects that the owner or

operator's and Director's combined knowledge of the local geologic conditions will be a critical

element in developing plans that protect USDWs from endangerment. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in evaluating requirements and proposed AoR and corrective action plans

under §146.84. The guidance describes EPA's recommended approaches for the data to be considered

in an initial AoR delineation, how to apply computational modeling to delineate the AoR,

performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The

guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA emphasizes that, while a seismic event would certainly warrant an AoR reevaluation, it is

anticipated that the extensive geologic characterization of a proposed GS site (including its seismic

history) collected pursuant to §146.82(a)(3)(v) would reduce the likelihood of such an event. EPA is

developing guidance on site characterization for GS projects to assist owners or operators and

Directors in evaluating permit requirements and applications under §146.82. The guidance describes

how best to characterize the regional geology and hydrogeology of a proposed GS site as well as

how best to perform the necessary geochemical, geomechanical, and geophysical analyses of the

injection and confining zone.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25184

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: M. EPA seeks comment on requiring the reevaluation of the site AoR on a periodic 

basis, under what conditions the AoR should be reevaluated, and the 

203
appropriateness of a 10 year minimum fixed frequency for AoR reevaluation. 

NYSDEC-DMN Response: AoR reevaluation frequency could be based on the amount of 

CO2 injected relative to the total proposed injection volume of CO2. For 

example, reevaluation of AoR could be required for every 20% injected but in no 

event less than every five years or at any other time at the discretion of the 

Director. Such a frequency would allow an operator to gather sufficient 

operating history and monitoring data for subsequent reevaluation. 

Response: EPA disagrees with the commenter's suggestion to tie the AoR reevaluation frequency to

the volume or percentage of CO2 injected. EPA believes that it is important to set an AoR

reevaluation frequency (i.e., at least every five years) to ensure that the project is reviewed and any

necessary updates to the project plans or the permit are made on a regular schedule, regardless of the

pace at which the injection is completed. 

EPA believes that the commenter's proposed approach has the potential to lead to AoR reevaluation

schedules that are not sufficiently frequent to ensure protection of USDWs from endangerment, if for

some reason the variables (rate or volume) are such that reevaluation based on percentages are not

triggered within 5 years. In addition, EPA believes this approach may complicate the administrative

process of determining and tracking the reevaluation schedule. 

For this reason, EPA has, at §146.84(e), adopted the 5 year interval (an adjustment from the ten year

interval in the proposed rule) recommended by many commenters. EPA adds that the Director has

discretion to require reevaluation more frequently, such as when monitoring data differ significantly

from modeled predictions, or when there are appreciable operational changes (e.g., injection rates). 

Commenter Name: John King Letter ID: 0211

Commenter Organization: Shell Exploration and Production Company

(SEPCo)

Comment ID:

22748

Comment: - Area of Review Reevaluation: 

- The need for reevaluation of the AOR should be required – if at all – when 

there is site-specific evidence that there is actually a need to reevaluate the 

AOR. Where monitoring results remain consistent with modeling results, a 

requirement to reevaluate the AOR should not be required. 

Response: EPA thanks the commenter for their input on the types of conditions that could warrant

reevaluation of the AoR at §146.84(e). EPA agrees that site-specific information, including

monitoring data, is an important factor in determining when AoR reevaluations are appropriate, and

expects that the owner or operator's and Director's combined knowledge of the local geologic

conditions will be a critical element in developing plans that protect USDWs from endangerment. 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners or

operators and Directors in how best to evaluate requirements and proposed AoR and corrective

action plans under §146.84. The guidance describes EPA's recommended approaches for the data to

be considered in an initial AoR delineation, how to apply computational modeling to delineate the

AoR, performing corrective action at GS sites, and, when appropriate, completing AoR

reevaluations. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

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Commenter Name: Diane VanDe Hei Letter ID: 0213

Commenter Organization: Association of Metropolitan Water Agencies

(AMWA)

Comment ID:

22943

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: - 73 FR 43507 -EPA seeks comment on requiring the reevaluation of the site AoR 

on a periodic basis, under what conditions the AoR should be reevaluated, and 

the appropriateness of a 10 year minimum fixed frequency for AoR reevaluation. 

Response: As carbon sequestration is an emerging technology that has not yet 

been tested on a large scale in the U.S., AMWA believes at this time that a tenyear minimum frequency for AoR evaluation is not sufficient. AMWA recommends 

instead that the AoR be reevaluated annually for the first ten years of the 

project and then every five years for the remaining operating period of the 

project. 

AMWA also recommends that the final rule include criteria to trigger a shorter 

review period at earlier intervals should monitoring detect a sudden change in 

CO2 plume movement, size or pressure. In addition, AMWA recommends that local 

drinking water utilities be included in the AoR review process, as existing or 

potential drinking water sources could become threatened due to a change in 

plume direction or behavior. Public involvement also needs to be a mandatory 

Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment. For these reasons, the

Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at

least every 5 years in the final GS rule. EPA believes that this revised frequency addresses

commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging

nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2

GS. 

EPA agrees that other aspects about a GS project may trigger the need for an AoR reevaluation, and

adds that the AoR and corrective action plan must also specify the site-specific monitoring and

operational conditions that warrant reevaluation, as specified in §146.84(b)(2)(ii). EPA agrees that

site-specific information is an important factor in determining when AoR reevaluations are

appropriate, and expects that the owner or operator's and Director's combined knowledge of the local

geologic conditions will be a critical element in developing plans that protect USDWs from

endangerment. In addition, the Director has discretion to require more frequent AoR reevaluation if

he or she has reason to believe, based on site-specific information, that a five-year reevaluation

schedule is not sufficiently protective of USDWs.

Regarding the suggestion that local drinking water utilities be included in the AoR review process,

EPA added text to §144.39(a)(5) that permit modifications would be required if the Director

determines that a modification is necessary based on an AoR reevaluation, amendments to any of the

project plans, or a review of the testing and monitoring results. EPA encourages permitting

authorities to notify and involve as many stakeholders as possible about permit modifications,

including drinking water organizations and local water utilities. 

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Comment Code: 3630 2.2.4.1 Comments in favor of allowing phased corrective action 

Individual Comments

Commenter Name: Doug Louis Letter ID: 0121

Commenter Organization: Kansas Corporation Commission Comment ID: 22079

Comment: Phased corrective action for GS wells that lie in front of the path of a CO2 

plume would allow companies to concentrate current capital expenditures on wells 

that represent the highest risk. Wells that might be affected in the distant 

future would be deemed a lower risk thus given a lower priority. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation. EPA clarifies that

wells that might not be affected for some time are not necessarily considered to pose a lower risk;

rather they are considered to not pose a risk for the first several years of injection.

Commenter Name: Dawn A. Thorne Letter ID:

0134

Commenter Organization: University of Utah Energy and Geoscience Institute on

behalf of the Southwest Regional Partnership

Comment

ID: 21656

Comment: - Section 146.84(3)(d) "Owners or operators of Class VI wells must perform 

corrective action on all wells in the area of review that are determined to need 

corrective action using methods necessary to prevent the movement of fluid into 

or between USDWs including use of corrosion resistant materials, where 

appropriate." 

Response/Discussion: The SWP strongly supports a phased approach to corrective 

actions as indicated in the Federal Register, page 43508. This discussion 

includes the following benefits to phased corrective actions: "There are 

potential benefits to implementing phased corrective action. Phasing in the 

corrective action would benefit the owner or operator by spreading out the 

burden and costs of corrective action and not delaying initiation of the GS 

project while corrective action is performed at wells that may not be affected 

by the injection for several decades. Initial corrective action would focus on 

those penetrations that pose a potential endangerment to USDWs from injection of 

CO2 in the near term. Deferring corrective action on some of the wells at the 

outer reaches of the predicted plume can improve USDW protection by giving these 

later corrective action efforts the benefit of newer corrective action 

techniques. Additionally, this approach can prevent the unnecessary burden of 

performing corrective action in areas far from the injection zone that may never 

be impacted This approach would still assure that all wells in the AoR that need 

corrective action eventually receive it, as is the case in current UIC 

requirements." 

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Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Christine Reimer Letter ID: 0142

Commenter Organization: National Ground Water Association Comment ID: 22583

Other EPA Comment Codes and Descriptions: 3530 2.2.3.1 AoR reevaluation frequency (seeking

comment) 

Comment: Preamble Questions: EPA proposes to allow a phased approach to corrective action 

but says this may not be appropriate in all situations; therefore EPA is 

proposing that the Director have the discretion to decide to allow this 

approach, based on the understanding of relevant geologic and site conditions. 

EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. EPA Notice 

74. 

NGWA Response: Yes, phased corrective action is a good idea. It will allow 

targeting of those wells most in need of corrective action and optimize 

financial resources as the pressure/CO2 plume may deviate from the initial AoR. 

That's why it is important to review AoR more often than 10 years. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Also, as described above, EPA changed the minimum frequency for the AoR reevaluation at

§146.84(e) from 10 years to 5 years in the final GS rule.

Commenter Name: Brianne Metzger Letter ID: 0151

Commenter Organization: Spectra Energy Corp Comment ID: 21821

Comment: C. Phased Corrective Action Is An Appropriate Approach. 

Spectra Energy agrees with the Agency’s idea of allowing for phased corrective 

action. We believe this is a reasonable approach, given the potential size of 

the CO2 plume for commercial-scale operations (50 square miles or larger). 

Requiring corrective action for the entire AoR prior to injection would likely 

result in substantial delays in permitting and project implementation. We 

agree that at a minimum, as part of the AoR process, all wells that penetrate 

the confining or injection zone must be identified. One question that warrants 

clarification by EPA, however, is the ability to gain access to third party 

wells that may over time require corrective action. Most states require 

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corrective action to be performed prior to injection and the “right of access” 

is taken into consideration during the adequacy determination of the corrective 

action plan. Some states, such as Texas, require corrective action prior to 

injection unless the owner/operator, after best efforts, is unable to get 

permission to access the property of a third party. 30 TEX. ADMIN. CODE § 

305.151 (2008). Since SDWA does not provide EPA the authority to require third 

parties to provide access to an owner/operator for the purposes of conducting 

corrective action, we believe it is important for the Agency in the Final GS 

Rule to discuss and clarify how owner/operators and states will be expected to 

implement corrective action based on an iterative approach. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

EPA agrees that securing access to all wells that require corrective action is important. In general, to

meet the corrective action requirement, the owner or operator will most likely need to obtain the

right of access to wells that may require corrective action, and demonstrate they have done this in

their proposed AoR and corrective action plan. In the AoR and corrective action plan, the owner or

operator must describe how site access will be guaranteed for future corrective action, as specified in

§146.84(b)(2)(iv). The final rule retains the requirement to guarantee site access as proposed.

Commenter Name: Michael J. Miller Letter ID: 0168

Commenter Organization: Electric Power Research Institute (EPRI) Comment ID: 22501

Comment: Preamble, page 43508: "Today’s proposal would give the Director the discretion 

to allow owners or operators to perform corrective action on an iterative, 

phased basis over the operational life of a GS project." 

EPRI COMMENT #7: A phased approach for corrective action simultaneously ensures 

permanence of the stored CO2 and cost-effective management of the storage 

operation. Therefore, it will encourage GS project development. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24227

Comment: Rule Provisions: 146.84(f) 

EPA Preamble Question: 15. EPA proposes to allow a phased approach to corrective 

action but says this may not be appropriate in all situations; therefore EPA is 

proposing that the Director have the discretion to decide to allow this 

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approach, based on the understanding of relevant geologic and site conditions. 

EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. EPA Notice 

43508. 

API Response: API supports the EPA proposal. The phased approach allows the 

sound economic execution of the project over time without endangering USDWs. 

This approach prevents an operator from the unnecessary expense of remediating 

wells in an area that ultimately may not be affected by the storage project. 

Frequency of reevaluation should be based on site performance as noted in the 

prior comment. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques, and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Regarding the comment that AoR reevaluation frequency should be based on site performance, as

described above, EPA believes that periodic AoR reevaluation at a set frequency of at least every

five years is necessary to ensure protection of USDWs from endangerment and address the

uncertainties inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a

deliberate effort is made to verify, based on monitoring and operational data, that the CO2 plume is

moving within the subsurface as predicted. 

Commenter Name: James Rower Letter ID: 0173

Commenter Organization: Utility Solid Waste Activities Group (USWAG) Comment ID: 23488

Comment: Most importantly, USWAG supports a phased approach to corrective action (well 

plugging and repair) for CO2 GS wells as contemplated in the Proposal. 73 Fed. 

Reg. at 43507-08. However, a phased approach to well plugging and repair for CO2 

GS wells should be available to all Owners or Operators of CO2 GS wells rather 

than be available only at the discretion of the Director. Id. at 43508 (proposal 

"would give the Director the discretion to allow owners or operators to perform 

corrective action on an iterative, phased basis... "); see also id. at 43536-37 

(including proposed § 146.84(b), which would require Owners or Operators of 

Class VI wells to, inter alia, "perform corrective action that meets the 

requirements of this section and is acceptable to the Director"). 

As EPA recognizes, the AoR for CO2 GS sites may be extensive; requiring well 

plugging and repair to be complete for all wells in a CO2 GS AoR prior to 

commencement of the injection project would be overly burdensome, unnecessary, 

and unrealistic. This is particularly true given that the AoR may be shown, 

through periodic AoR reevaluations, to have expanded or shifted, identifying new 

wells for plugging and repair. USWAG supports an approach that would require the 

Owner or Operator of a Class VI well to perform pre-injection well plugging and 

repair on only those wells within the portion of the AoR that would be 

intersected by the CO2 plume or pressure front during the initial years of 

injection. Well plugging and repair for the rest of the AoR should be allowed to 

209
take place as the injection project progresses, ensuring that, eventually, "all 

wells in the AoR that need corrective action [well plugging and repair] 

eventually receive it." 73 Fed. Reg. at 43508. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation. EPA clarifies that, if

monitoring or modeling data indicate a change in the AoR (i.e., an expansion or shift), the owner or

operator would need to review and amend the AoR and corrective action plan and follow the

amended plan. 

EPA does not agree that a phased approach to corrective action should be available to all owners or

operators because phased corrective may not be appropriate at all GS sites. EPA believes that, given

the wide range of conditions at GS sites, the Director is in the best position to make decisions about

the appropriateness of phased corrective action based on his or her knowledge of the local geologic

and site conditions. EPA is developing guidances for states that help to describe the technical and

administrative issues a state may need to consider when developing GS regulations that are the

equivalent of minimum federal criteria and requirements; this guidance will provided recommended

approaches for Directors to exercise discretion. The guidances will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Pamela Faggert Letter ID: 0175

Commenter Organization: Dominion Comment ID: 23524

Comment: Dominion supports the requirement to conduct corrective action within the AoR 

provided the Director can authorize phased corrective action when warranted 

based on site specific performance and conditions. As with other discretionary 

authority proposed, there should be requirements establishing bounds for 

exercising such discretion to avoid arbitrary application of such authority. 

Directorial discretion should be subject to guidelines as to when phased 

corrective action should NOT be permitted. 

Response: EPA agrees that phased corrective action may provide many benefits to a project

including, but not limited to, spreading the burden of cost, avoiding delays in project start-up,

allowing for use of future, improved corrective action techniques and addressing unanticipated

changes in the movement of the CO2 plume or pressure front. EPA believes that the phased

corrective action approach is appropriate for GS projects, and has retained it at §146.84(b)(2)(iv) in

the final GS regulation.

EPA does not agree that a phased approach to corrective action should be available to all owners or

operators, because phased corrective may not be appropriate at all GS sites. EPA believes that, given

the wide range of conditions at GS sites, the Director is in the best position to make decisions about

the appropriateness of phased corrective action based on his or her knowledge of the local geologic

and site conditions. EPA is developing guidances for states that help to describe the technical and

administrative issues a state may need to consider when developing GS regulations that are the

equivalent of minimum federal criteria and requirements; this guidance will provided recommended

210
approaches for Directors to exercise discretion. The guidances will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Quinlan J. Shea Letter ID: 0177

Commenter Organization: Edison Electric Institute Comment ID: 23587

Comment: The proposed regulations also include a requirement to conduct corrective action 

within the AOR, but would give the UIC program director discretion to allow 

corrective action to be conducted in phases over the operational life of the 

project. A phased corrective action approach has merit, particularly where 

periodic reevaluations of the AOR are to be required. A phased approach will 

help ensure that all necessary corrective action is taken, but also that 

unnecessary corrective action is avoided. It may also serve to ease the initial 

financial burden on operators and provide increased certainty from an investment 

standpoint. However, the proposed rules leave the possibility of a phased 

approach to the program director‘s discretion and contain no guidelines for the 

exercise of such discretion. Any discretionary requirement should be tempered 

by suitable criteria for its exercise. Likewise, any financial assurance 

requirements related to corrective action that may be imposed should be tied to 

the corrective action approach that is to be taken in any given case. For 

example, where phased corrective action is permitted, corresponding provisions 

for phased financial assurance would provide a greater ability to price the 

activities at issue accurately. A more detailed discussion of financial 

assurance issues is below. 

Response: EPA agrees that phased corrective action should be allowed and has retained this

provision in the final rule §146.84(b)(2)(iv). However, EPA disagrees that decisions of the Director

regarding phased corrective action would be made arbitrarily. The GS rule provides for Director's

discretion because EPA believes that, given the wide range of conditions at GS sites, the Director is

in the best position to make decisions about the appropriateness of phased corrective action based on

his or her knowledge of the local geologic and site conditions.

EPA does not agree that a phased approach to corrective action should be available to all owners or

operators, because phased corrective may not be appropriate at all GS sites. EPA believes that, given

the wide range of conditions at GS sites, the Director is in the best position to make decisions about

the appropriateness of phased corrective action based on his or her knowledge of the local geologic

and site conditions. EPA is developing guidances for states that help to describe the technical and

administrative issues a state may need to consider when developing GS regulations that are the

equivalent of minimum federal criteria and requirements; this guidance will provided recommended

approaches for Directors to exercise discretion. The guidances will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Regarding financial responsibility for corrective action, EPA clarifies that the owner or operator

must demonstrate and maintain financial responsibility for all needed corrective action, regardless of

whether it is to be phased. EPA is developing guidance on financial responsibility that will discuss

approaches to how to estimate appropriate costs for all required activities. 

For additional information on EPA's approach to financial responsibility for Class VI wells, please

see the final rule and preamble.

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Commenter Name: Frederick R. Eames Letter ID: 0178

Commenter Organization: CCS Alliance Comment ID: 23856

Comment: 2. Phased Corrective Action 

The CCS Alliance phased correction action and believes such an approach is 

reasonable, given the potential size of the CO2 plume for commercial-scale GS 

operations. Requiring up-front corrective action for the entire AoR prior to 

injection could result in substantial delays in permitting and project 

implementation. At a minimum, as part of the AoR process, wells that penetrate 

the confining or injection zone must be identified. Identifying problematic 

wells up-front may increase short-term costs, but should reduce risk and 

liability over the long-term. 

Response: EPA agrees that phased corrective action may provide many benefits to a project

including, but not limited to, spreading the burden of cost, avoiding delays in project start-up,

allowing for use of future, improved corrective action techniques and addressing unanticipated

changes in the movement of the CO2 plume or pressure front. EPA believes that the phased

corrective action approach is appropriate for GS projects, and retained it at §146.84(b)(2)(iv) in the

final GS regulation.

Commenter Name: Thomas Curtis Letter ID: 0181

Commenter Organization: American Water Works Association (AWWA) Comment ID: 23692

Comment: AWWA recommends that a phased corrective action plan be developed as a result of 

the AoR reevaluation. This would prevent owner/operators from performing 

unnecessary work in the event that the plume migrates in a different direction 

than originally predicted. However, AWWA recommends that the required corrective 

actions be completed at least two years before the plume is expected to reach 

the specific location. Corrective actions should include reentering and 

replugging old or abandoned wells that have not been certified to be closed in 

accordance with the sequestration requirements. Also, in the case where old 

wells were plugged with cement that is not compatible with carbon dioxide, these 

wells should be re-entered and replugged. Finally, old wells for which no 

records exist should be re-entered and replugged to ensure that a pathway for 

carbon dioxide to escape is not provided. 

One issue that EPA needs to consider in the final regulation is the potential 

cost differential for either performing all of the necessary corrective actions 

immediately or by phasing them in over time. If the corrective actions are 

phased in, EPA needs to require that the owner/operator provide financial 

assurance that the corrective action measures will still be performed even if 

the company goes out of business and the carbon dioxide plume continues to 

migrate. 

Response: EPA believes that the phased corrective action approach is appropriate for GS projects,

and retained it at §146.84(b)(2)(iv) in the final GS regulation. EPA believes that phased corrective

action may provide many benefits to a project including, but not limited to, spreading the burden of

cost, avoiding delays in project start-up, allowing for use of future, improved corrective action

techniques and addressing unanticipated changes in the movement of the CO2 plume or pressure

front. 

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EPA agrees with the commenter that phased corrective action must account for any changes in the

AoR, as demonstrated through a reevaluation or other site-specific data. For this reason, the final

rule includes provisions that owners or operators review and update the AoR and corrective action

plan throughout the life of the project, as specified in §146.84(c). These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director. 

EPA also agrees that corrective action techniques must be appropriate for CO2 GS. All corrective

action performed under §146.84(d) must be appropriate to the GS site and the injectate/formation

fluid chemistry to ensure that the well does not allow CO2 or other fluids to move into or endanger

USDWs. 

Regarding financial responsibility for corrective action, EPA clarifies that the owner or operator

must demonstrate and maintain financial responsibility for all needed corrective action, regardless of

whether it is to be phased. EPA is developing guidance on financial responsibility that will discuss

approaches to how best to estimate appropriate costs for all required activities. The guidance will be

available on EPA's web site, at http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Bradley J. Field Letter ID: 0186

Commenter Organization: New York State Dept. of Environmental Conservation

Div. of Mineral Resources

Comment ID:

25185

Comment: N. EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. 

NYSDEC-DMN Response: Same as the two comments above for frequency of AoR 

reevaluation. With respect to the proposed phased corrective action approach for 

wells, NYSDEC-DMN agrees with the stated approach for the reasons given provided 

a permit can be suspended or revoked for failure to perform the corrective 

actions in a timely manner. However, NYSDEC-DMN believes all wells potentially 

impacted within the first five years of operation should be addressed prior to 

first injection of CO2. 

Response: EPA agrees that corrective action on wells should be completed several years in advance

of the anticipated arrival of the CO2 plume or pressure front. However, EPA does not believe that it

is appropriate to set a specific time for completing corrective action in the regulation due to

uncertainty in delineating the CO2 plume movement during certain time frames. Instead, such

decisions would be incorporated into the AoR and corrective action plan for each project, as

specified in §146.84(b)(2)(iv), which would be developed by the owner or operator and approved by

the Director based on the geology of the site and the wells in the AoR. 

All needed corrective action (whether phased or not) must be performed by the owner or operator as

described in the approved AoR and corrective action plan and on the approved schedule to ensure

that deficient wells in the AoR are addressed in a timely manner; failure to do so will either preclude

authorization to inject or, in the case of phased corrective action, be considered a violation of the

permit and subject to applicable civil, criminal, or administrative penalties under SDWA §1423. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

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Commenter Organization: Carbon Sequestration Council Comment ID: 24335

Comment: 2. Corrective Action 

Corrective action is a familiar UIC program requirement. Given the potential 

scale of GS operations, EPA has proposed requirements for a corrective action 

plan contemplating a phased implementation of corrective action. In general, 

this approach is prudent and practical, providing a rigorous approach to 

corrective action while reducing regulatory burdens by permitting a phased 

implementation of corrective action requirements. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques, and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24519

Comment: Rule Provisions or Federal Register Cite: 146.84(f) 

EPA Preamble Question: 15. EPA proposes to allow a phased approach to corrective 

action but says this may not be appropriate in all situations; therefore EPA is 

proposing that the Director have the discretion to decide to allow this 

approach, based on the understanding of relevant geologic and site conditions. 

EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. EPA Notice 

43508. 

CSC Response: CSC supports the EPA proposal. The phased approach allows the 

sound economic execution of the project over time without endangering USDWs. 

This approach prevents an operator from the unnecessary expense of remediating 

wells in an area that ultimately may not be affected by the storage project. 

Frequency of reevaluation should be based on site performance as noted in the 

prior comment. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Regarding the comment that some wells identified in the original AoR ultimately may not be

affected by the storage project, EPA clarifies that, if monitoring or modeling data indicate a change

in the AoR, the owner or operator would need to review and amend the AoR and corrective action

plan and follow the amended plan.

Regarding the comment that AoR reevaluation frequency should be based on site performance, as

described above, EPA believes that periodic AoR reevaluation at a set frequency of at least every

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five years is necessary to ensure protection of USDWs from endangerment and address the

uncertainties inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a

deliberate effort is made to verify, based on monitoring and operational data, that the CO2 plume is

moving within the subsurface as predicted. 

Commenter Name: Paul Pike Letter ID: 0198

Commenter Organization: Ameren Corporation Comment ID: 23949

Comment: A CO2 GS site may have an extensive AoR and requiring well plugging and repair 

to be complete for all wells in a CO2 GS AoR prior to commencement of the 

injection project would be unnecessary, and unrealistic. We instead support an 

approach that would require the Owner or Operator of a Class VI well to perform 

pre-injection well plugging and repair on only those wells within the portion of 

the AoR that would be intersected by the CO2 plume or pressure front during the 

initial years of injection. Well plugging and repair for the rest of the AoR 

should be allowed to take place as the injection project progresses, ensuring 

that, eventually, "all wells in the AoR that need corrective action [well 

plugging and repair] eventually receive it." 73 Fed. Reg. at 43508. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24582

Comment: Rule Provisions: 146.84(f) 

EPA Rule Issue: 15. EPA proposes to allow a phased approach to corrective action 

but says this may not be appropriate in all situations; therefore EPA is 

proposing that the Director have the discretion to decide to allow this 

approach, based on the understanding of relevant geologic and site conditions. 

EPA invites public comment on the merits and frequency of reevaluation of the 

AoR as well as the phased corrective action approach for GS wells. EPA Notice 

43508. 

Anadarko Petroleum Corporation (APC) Response: APC supports the EPA proposal. 

The phased approach allows the sound economic execution of the project over time 

without endangering USDWs. This approach prevents an operator from remediating 

wells in an area that ultimately may not be affected by the storage project. 

Allowing a phased approach for corrective action is also imperative for economic 

success of many projects. The upfront capital cost for sequestration will be 

extensive, and requiring corrective action upfront may be economically 

unfeasible. For instance Anadarko has spent over $10 million addressing historic 

wells at one of our EOR operations; this site is significantly smaller than 

those that may be involved in large sequestration projects. Therefore, allowing 

corrective actions in a phased manner will provide the flexibility needed to 

215
address operational and economic issues. Beyond the potential for significant 

economic expenditure, the availability of work-over rigs may also provide a 

difficult time-delay for starting sequestration projects. 

Frequency of reevaluation should be based on site performance as noted in the 

prior comment. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques, and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Regarding the comment that AoR reevaluation frequency should be based on site performance, as

described above, EPA believes that periodic AoR reevaluation at a set frequency of at least every

five years is necessary to ensure protection of USDWs from endangerment and address the

uncertainties inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a

deliberate effort is made to verify, based on monitoring and operational data, that the CO2 plume is

moving within the subsurface as predicted. 

Commenter Name: John Corra Letter ID: 0205

Commenter Organization: Wyoming Department of Environmental Quality

(DEQ)

Comment ID:

22203

Comment: 6. Various issues where we agree with US EPA that the Director should have the 

discretion to determine, or require: 

d. Allowing for a phased approach to corrective action. 

Response: EPA agrees that there are potential benefits to phased corrective action, including

distributing the burden and costs of corrective action over time and not delaying initiation of the GS

project. EPA believes that the phased corrective action approach is appropriate for GS projects, and

retained it at §146.84(b)(2)(iv) in the final GS regulation. However, EPA recognizes that a phased

approach to corrective action may not be appropriate in all situations, and that the Director is in the

best position to make decisions about the appropriateness of phased corrective action based on his or

her knowledge of the local geologic and site conditions. For this reason, the GS rule affords the

Director discretion in determining whether phased corrective action is appropriate at a given GS site.

Commenter Name: Olon Plunk Letter ID: 0217

Commenter Organization: Xcel Energy, Inc. Comment ID: 21559

Comment: deliberate and proactive preventive measures. In addition, Xcel Energy advocates 

a phased approach to well plugging and repair, allowing expansion of the scope 

of wells within the AoR over time on an iterative basis as dictated by actual 

project progress and/or new information. As noted in the proposed rule, the AoR 

can also include pre-existing wells owned by other owners or operators. Geologic 

storage project owners and operators should not be held responsible to close, 

repair, or replace these pre-existing wells or take other corrective action 

where the pre-existing well owner or responsible party has an existing 

216
obligation to do so under the law. Current or past owners or operators of preexisting wells should not be rewarded for ignoring their responsibilities under 

the law, and should be penalized for failure to implement already required 

corrective action in a timely manner. Finally, the rule should explicitly allow 

Response: EPA clarifies that the purpose of the requirement to perform corrective action at

§146.84(d) is to ensure that all wells in the AoR that have the potential to cause movement of CO2

or other fluids into or between USDWs receive appropriate corrective action. All needed corrective

action must be performed by the owner or operator as described in the approved AoR and corrective

action plan, regardless of ownership. Failure to do so would either preclude authorization to inject

or, in the case of phased corrective action, be considered a violation of the permit and subject to

applicable civil, criminal, or administrative penalties. 

EPA clarifies that, if monitoring or modeling data indicate a change in the AoR, the owner or

operator would need to review and amend the AoR and corrective action plan and follow the

amended plan.

Commenter Name: Tiffany Rau Letter ID: 0224

Commenter Organization: Hydrogen Energy International Comment ID: 21677

Comment: necessary. HEI agrees with the proposal to allow a phased approach to conduct 

corrective action in an Area of Review that has pre-existing well penetrations. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Brian Williams Letter ID: 0225

Commenter Organization: BP Alternative Energy North America, Inc. (BP) Comment ID: 21722

Comment: necessary. BP agrees with the proposal to allow a phased approach to conduct 

corrective action in an Area of Review that has pre-existing well penetrations. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: M. Granger Morgan Letter ID: 0233

Commenter Organization: Carnegie Mellon University on behalf of the CCS

Reg Project

Comment ID:

21908

Other EPA Comment Codes and Descriptions: 3500 2.2.3 Area of Review Reevaluation (seeking

comment) 

Comment: Specific comments on the proposed rule 

217
17. Iterative Area of Review: We support the proposal to reevaluate iteratively 

the area of review over the project life cycle and to allow phased corrective 

action (sec. 146.84). 

Response: EPA acknowledges the commenter’s support of AoR reevaluations and phased corrective

action. EPA believes that the AoR reevaluation process at §146.84(e) provides additional

opportunities to verify that the models have accurately predicted movement of the CO2 plume and to

make any needed adjustments to corrective action or monitoring at the site, and has retained this

approach in the final rule.

EPA agrees that phased corrective action may provide many benefits to a project including, but not

limited to, spreading the burden of cost, avoiding delays in project start-up, allowing for use of

future, improved corrective action techniques and addressing unanticipated changes in the movement

of the CO2 plume or pressure front. EPA believes that the phased corrective action approach is

appropriate for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Steven B. Corneli Letter ID: 0234

Commenter Organization: NRG Energy, Inc. Comment ID: 23164

Comment: 3. Corrective Action 

EPA proposes to allow a phased approach to corrective action for GS wells. NRG 

agrees that this approach is prudent and practical given the enormous volumes of 

CO2 that would be injected for a geologic sequestration project. Implementation 

of a phased corrective action to address risk associated with artificial 

penetrations such as plugged well borings would reduce project costs and also 

reduce the regulatory burdens of the permitting process. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

Commenter Name: Abbie Krebsbach Letter ID: 0235

Commenter Organization: Montana-Dakota Utilities Co. Comment ID: 23185

Comment: A phased corrective action approach as proposed in the rule has merit 

particularly where periodic reevaluations of the AOR are to be required. A 

phased approach will help ensure that all necessary corrective action is taken, 

but also that unnecessary corrective action is avoided. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques, and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation. 

218
Regarding the comment that some wells may not need corrective action, EPA clarifies that, if

monitoring or modeling data indicate a change in the AoR, the owner or operator would need to

review and amend the AoR and corrective action plan and follow the amended plan. EPA clarifies

that all wells in the AoR of a GS project that need corrective action must receive it at some point.

Commenter Name: Mark Bohrer Letter ID: 0241

Commenter Organization: North Dakota Industrial Commission Comment ID: 23235

Comment: 15. EPA proposes to allow a phased approach to corrective action but says this 

may not be appropriate in all situations; therefore EPA is proposing that the 

Director have the discretion to decide to allow this approach, based on the 

understanding of relevant geologic and site conditions. EPA invites public 

comment on the merits and frequency of reevaluation of the AoR as well as the 

phased corrective action approach for GS wells. EPA Notice 43508. 

A phased approach is appropriate. Corrective action for some aspects of the 

project may not be necessary at the outset and a phased approach would allow an 

operator to optimize capital expenditures to make a project more feasible. 

Wholesale corrective action at the outset of a project may ultimately result in 

corrective action in areas unaffected by the project. In addition, the 

procurement of rigs and other equipment and the time necessary to conduct 

corrective actions in areas that might not be affected for a significant period 

of time could unnecessarily delay a sequestration project. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation. EPA clarifies that all

wells in the AoR of a GS project that need corrective action must receive it at some point. 

Regarding the comment that some wells identified in the original AoR ultimately may not be

affected by the storage project, EPA clarifies that, if monitoring or modeling data indicate a change

in the AoR, the owner or operator would need to review and amend the AoR and corrective action

plan and follow the amended plan.

Commenter Name: Walter L. Baker Letter ID: 0253

Commenter Organization: Utah Division of Water Quality Comment ID: 22350

Comment: ALLOWING FOR DIRECTOR DISCRETION IN PHASED APPROACH TO

CORRECTIVE ACTION 

We support EPA's intention to allow Director's discretion in allowing a phased 

approach to corrective action for wells that penetrate the injection zone within 

the Area of Review. As stated above, this allows for a risk-based, sitespecific approach in setting permit requirements for the protection of USDWs and 

other natural resources thereby increasing the number of sites that could 

potentially be used for CCGS. 

219
Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation.

EPA recognizes that a phased approach to corrective action may not be appropriate in all situations,

and that the Director is in the best position to make decisions about the appropriateness of phased

corrective action based on his or her knowledge of the local geologic and site conditions. For this

reason, the GS rule affords the Director discretion in determining whether phased corrective action is

appropriate at a given GS site.

Commenter Name: Kevin Frederick Letter ID: 0525

Commenter Organization: Wyoming Department of Environmental Quality Comment ID: 21401

Comment: And finally discretion to allow for a phased approach to corrective action. 

Response: EPA agrees that there are potential benefits to phased corrective action, including

distributing the burden and costs of corrective action over time and not delaying initiation of the GS

project. EPA believes that the phased corrective action approach is appropriate for GS projects, and

retained it at §146.84(b)(2)(iv) in the final GS regulation. However, EPA recognizes that a phased

approach to corrective action may not be appropriate in all situations, and that the Director is in the

best position to make decisions about the appropriateness of phased corrective action based on his or

her knowledge of the local geologic and site conditions. For this reason, the GS rule affords the

Director discretion in determining whether phased corrective action is appropriate at a given GS site.

Commenter Name: Doug Louis Letter ID: 0530

Commenter Organization: Kansas Corporation Commission Comment ID: 21342

Comment: Phased corrective action for GS wells that lie in front of the path of a CO2 

plume would allow companies to concentrate current capital expenditures on wells 

that represent the highest risk. Wells that might be affected in the distant 

future would be deemed a lower risk, thus given a lower priority. 

Response: EPA acknowledges the commenter’s support of phased corrective action and agrees that

phased corrective action may provide many benefits to a project including, but not limited to,

spreading the burden of cost, avoiding delays in project start-up, allowing for use of future,

improved corrective action techniques and addressing unanticipated changes in the movement of the

CO2 plume or pressure front. EPA believes that the phased corrective action approach is appropriate

for GS projects, and retained it at §146.84(b)(2)(iv) in the final GS regulation. EPA clarifies that

wells that might not be affected for some time are not necessarily considered to pose a lower risk;

rather they are considered to not pose a risk for the first several years of injection.

Comment Code: 3660 2.2.4.2 Comments opposed to allowing phased corrective action 

Individual Comments

220
Commenter Name: John Hanger Letter ID: 0130

Commenter Organization: Pennsylvania Dept. of Env. Protection Comment ID: 21506

Comment: -The rule proposes phased corrective action (plugging of existing penetrations) 

after injection begins, but the corrective action is to occur in advance of the 

CO2 plume. The rule should require all corrective action to occur before 

injection begins, or, if phased corrective action is to be approved, then the 

operator must post a 100 percent performance bond with the permitting agency 

before the permit is issued. The performance bond is to be in the amount 

necessary for the permitting agency to immediately complete the corrective 

action should the owner/operator fail to do so in a timely manner. 

Response: EPA disagrees that phased corrective action is inappropriate for GS projects, and retains

this provision in the final GS regulation. EPA acknowledges the concern regarding the uncertainty in

AoR modeling, and notes that addressing this is the intent of the AoR reevaluation process. EPA

adds that the final rule includes a requirement that the AoR be reevaluated every 5 years or when

monitoring or operational conditions warrant a reevaluation as identified at §146.84(b)(2)(ii). 

Allowing phased corrective action is at the discretion of the Director. Any state, in developing a

primacy package that is concerned that phased corrective action cannot be safely implemented based

on the geologic or other conditions can prohibit Director's discretion to allow phased corrective

action. 

EPA clarifies that owners or operators must also demonstrate and maintain financial responsibility

for all corrective action, as specified in §146.85, whether it is performed prior to injection or phased.

Commenter Name: Steven Chester Letter ID: 0264

Commenter Organization: Michigan Department of Environmental Quality

(MDEQ)

Comment ID:

22836

Comment: EPA (I II . A. 2.) proposes phased corrective action for wells in the AoR. 

- Staff believes phased corrective action should not be allowed, since 

there is much uncertainty in the modeled results--specifically in the times 

at which the plume will reach various wells--due to typically sparse or nonexistent geologic data in the AoR. 

Response: EPA disagrees that phased corrective action is inappropriate for GS projects, and retains

this provision in the final GS regulation. EPA acknowledges the concern regarding the uncertainty in

AoR modeling, and notes that addressing this is the intent of the AoR reevaluation process. EPA

adds that the final rule includes a requirement that the AoR be reevaluated every five years or when

monitoring or operational conditions warrant a reevaluation as identified at §146.84(b)(2)(ii). 

Allowing phased corrective action is at the discretion of the Director. Any state, in developing a

primacy package that is concerned that phased corrective action cannot be safely implemented based

on the geologic or other conditions, can prohibit Director's discretion to allow phased corrective

action. 

Comment Code: 3700 2.2.5 Comments on the requirement for an AoR and corrective action plan 

221
Individual Comments

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24771

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: (b) The owner or operator of a Class VI well must prepare, maintain, and comply 

with a plan to delineate the AOR for a proposed geologic sequestration project, 

periodically reevaluate the delineation, and perform corrective action that 

meets the requirements of this section AT A FREQUENCY AND IN A MANNER THAT 

and is acceptable to the Director. As a part of the permit application 

for approval by the Director, the owner or operator must submit an area of 

review and corrective action plan that includes the following information: 

Response: EPA disagrees with the suggested change, and is retaining the approach of setting a

minimum fixed frequency for reevaluating the AoR but has, based on commenters' input, changed

the frequency from 10 years to at least every 5 years, as specified in §146.84(e). EPA agrees with

commenters that site-specific information is an important factor in determining an appropriate AoR

reevaluation frequency, and adds that the owner or operator's AoR and corrective action plan must

describe the minimum fixed frequency for reevaluating the AoR and the site-specific conditions on

which this frequency is based. EPA adds that the Director will evaluate the proposed plan in

connection with a review of a vast amount of geologic and proposed operating data in determining

whether to approve the plan. EPA is developing guidance on recommended approaches for preparing

all of the required GS project plans, which will address considerations for determining an appropriate

AoR reevaluation schedule. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24772

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3530 2.2.3.1 AoR reevaluation frequency (seeking comment) 

Comment: (2) A description of: (i) The minimum fixed frequency, not to exceed 10 years, 

the owner or operator proposes to reevaluate the area of review; (ii) The 

monitoring and operational conditions that would warrant a reevaluation of the 

area of review prior to the next scheduled reevaluation as determined by the 

minimum fixed frequency established in paragraph (b)(2)(i) of this section.

{Suggest that there be no minimum fixed frequency for reevaluation of the AoR, 

but that such review be based on site-specific circumstances. } 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

222
EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data.

Commenter Name: Leslie Savage Letter ID: 0122

Commenter Organization: Railroad Commission of Texas (RRC) Comment ID: 24773

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3400 2.2.2 Corrective Action 

Comment: (II)(iii) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

(iv) How corrective action will be conducted to meet the requirements of 

paragraph (d) of this section, including what corrective action will be 

performed prior to injection and what, if any, portions of the area of review 

will have corrective action addressed on a phased basis and how the phasing will 

be determined; how corrective action will be adjusted if there are changes in 

the area of review; and how THE site WILL BE accessed will be guaranteed for 

future corrective action. 

{Word Choice. Secure rather than guarantee. } 

Response: The purpose of the corrective action requirement at §146.84(d) is to ensure that all wells

in the AoR that have the potential to cause the movement of CO2 or other fluids into or between

USDWs receive appropriate corrective action. In general, to meet this requirement, the owner or

operator will mostly likely need to obtain the right of access to wells that may require corrective

action, and demonstrate they have done this in their proposed AoR and corrective action plan. For

this reason, the owner or operator must describe in the AoR and corrective action plan how site

access will be guaranteed for future corrective action, as specified in §146.84(b)(2)(iv). The final

rule retains the requirement as proposed.

Commenter Name: Bonnie Lovelace Letter ID: 0152

Commenter Organization: Montana Department of Environmental Quality

(MDEQ)

Comment ID:

22118

Other EPA Comment Codes and Descriptions: 4610 2.6.1 Testing and monitoring plan

requirement (seeking comment); 4710 2.7.1 Proposed submissions required for permit applications

(seeking comment) 

Comment: 17. EPA seeks comment on the merits of requiring plans for monitoring, area of 

review and post-injection site care as part of a permit application. MDEQ agrees 

that this is necessary and appropriate. A"cradle-to-grave" approach is wise in 

this instance and allows for updating the permit application materials as new 

technologies are forthcoming. 

Response: EPA acknowledges the commenter’s support of the GS project plans. EPA believes that

the requirement for the plans associated with GS projects will promote an ongoing dialogue between

the owner or operator and the Director to ensure that USDWs are protected from endangerment.

EPA is developing guidances for owners or operators describing recommended approaches to

prepare the project plans. The guidances will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

223
On the topic of monitoring and plan revision: EPA agrees with commenters on the importance of

monitoring at GS sites and emphasizes that monitoring of injection activities will be an important

component of every GS project. EPA also believes that periodic updates to the plans are appropriate.

EPA clarifies that the final GS regulation includes a requirement that the AoR be reevaluated at least

every 5 years, and it specifies at §146.90(j) that the operator revise the testing and monitoring plan to

incorporate the results of the most recent AoR reevaluation and recent operational and monitoring

data.

EPA agrees that incorporating the plans into the permit rather than establishing specific requirements

for corrective action, monitoring, injection well plugging, and post-injection site care in the

regulation provide the appropriate flexibility to address the variety of site-specific conditions that

may be encountered at GS sites. This approach is retained in the final rule. Please see the final rule

and the discussion of the required GS project plans in the preamble for additional information.

EPA also believes that it is appropriate to require owners and operators to submit all plans, including

the injection well plugging and post-injection site care and site closure plans, at the time of permit

application. EPA believes that the Director needs a satisfactory level of assurance that the owner or

operator has planned for these events before injection commences and that the owner or operator has

ensured that appropriate financial resources are available to fund all required activities, as required at

§146.85. The final rule retains requirements for submitting these plans with the permit application at

§146.82(a). EPA believes that the requirement for this and other plans associated with GS projects

will promote an ongoing dialogue between the owner or operator and the Director to ensure USDWs

are protected from endangerment. In addition, these plans will support a demonstration that owners

and operators have adequate financial resources to meet well plugging and post-injection site care

and site closure requirements.

Commenter Name: Michael Mudd Letter ID: 0167

Commenter Organization: FutureGen Alliance (FGIA) Comment ID: 22473

Other EPA Comment Codes and Descriptions: 4610 2.6.1 Testing and monitoring plan

requirement (seeking comment) 

Comment: A-7 Record Keeping and Reporting Requirements III-A-7 We support the requirement 

for 1) a monitoring and testing plan 2) an AoR and Correction Action Plan and 

3) a Post-Injection Site Care and Site Closure plan. 

Response: EPA acknowledges the commenter’s support of the GS project plans. EPA believes that

the requirement for the plans associated with GS projects will promote an ongoing dialogue between

the owner or operator and the Director to ensure that USDWs are protected from endangerment.

EPA is developing guidance for owners or operators describing recommended approaches to prepare

the project plans. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

EPA agrees that developing, implementing, and periodically reviewing the required GS project plans

is an important part of assuring that a GS project is operating properly and is crucial for the early

detection of potential endangerment to USDWs. EPA also believes that periodic updates to the plans

are appropriate. EPA clarifies that the final GS regulation includes a requirement that the AoR be

reevaluated at least every 5 years, and it specifies at §146.90(j) that the operator revise the testing

and monitoring plan to incorporate the results of the most recent AoR reevaluation and recent

operational and monitoring data. EPA also agrees that the site-specific plans required for GS projects

224
will maximize protection of USDWs from endangerment.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24159

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(b) 

API Comments: (b) The owner or operator of a Class VI well must prepare, 

maintain, and comply with a plan to delineate the area of review for a proposed 

geologic sequestration project, periodically reevaluate the delineation, and 

perform corrective action that meets the requirements of this section and is

IN A MANNER AND AT A FREQUENCY acceptable to the Director. As a part of the 

permit application, for approval by the Director the owner or operator must 

submit an area of review and corrective action plan that includes the following 

information: 

Rationale: Clarification. 

Response: EPA disagrees with the suggested change, and is retaining the approach of setting a

minimum fixed frequency for reevaluating the AoR but has, based on commenters' input, changed

the frequency from ten years to at least every five years, as specified in §146.84(e). EPA agrees with

commenters that site-specific information is an important factor in determining an appropriate AoR

reevaluation frequency, and adds that the owner or operator's AoR and corrective action plan must

describe the minimum fixed frequency for reevaluating the AoR and the site-specific conditions on

which this frequency is based. EPA adds that the Director will evaluate the proposed plan in

connection with a review of a vast amount of geologic and proposed operating data in determining

whether to approve the plan. EPA is developing guidance describing recommended approaches to

preparing all of the required GS project plans, which will address considerations for determining an

appropriate AoR reevaluation schedule. The guidance will be available on EPA's web site at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24160

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Comment: Rule Provision: 146.84(b)(2) 

API Comments: (2) A description of: 

(i) The minimum fixed frequency, not to exceed 10 years, the owner or operator 

proposes to reevaluate the area of review;

(ii) The monitoring and operational conditions that would warrant a 

reevaluation of the area of review prior to the next scheduled reevaluation as 

determined by the minimum fixed frequency established in paragraph (b)(2)(i) of 

this section.

(iii)(I) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

225
No rationale provided 

Rule Provision: 146.84(b)(2) 

API Comments: (iv II) How corrective action will be conducted to meet the 

requirements of paragraph (d) of this section, including what corrective action 

will be performed prior to injection and what, if any, portions of the area of 

review will have corrective action addressed on a phased basis and how the 

phasing will be determined; how corrective action will be adjusted if there are 

changes in the area of review; and how site access will be guaranteed

ARRANGED for future corrective action. 

Rationale: Future access cannot be guaranteed for an area in advance that may 

not even be within the boundaries of the AOR/ project. 

Response: EPA disagrees with the suggested changes. Regarding reevaluation of the AoR, EPA

believes that periodic AoR reevaluation at a set frequency of at least every five years is necessary to

ensure protection of USDWs from endangerment and address the uncertainties inherent in predictive

modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate effort is made to verify,

based on monitoring and operational data, that the CO2 plume is moving within the subsurface as

predicted. EPA believes that less frequent reevaluations may not be sufficient to appropriately model

plume and pressure front movement, establish conditions for corrective action, and ensure protection

of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

To the comment on site access, EPA clarifies that the purpose of the corrective action requirement at

§146.84(d) is to ensure that all wells in the AoR that have the potential to cause the movement of

CO2 or other fluids into or between USDWs receive appropriate corrective action. In general, to

meet this requirement, the owner or operator will most likely need to obtain the right of access to

wells that may require corrective action, and demonstrate they have done this in their proposed AoR

and corrective action plan. The final rule retains the requirement as proposed.

Commenter Name: Kyle Isakower Letter ID: 0170

Commenter Organization: American Petroleum Institute Comment ID: 24167

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f)(4) 

API Comments: (4) Submit an amended area of review and corrective action plan or 

demonstrate to the Director through monitoring data and modeling results that no 

amendment CHANGE to the area of review and corrective action plan is needed. 

Rationale: Clarification 

Response: EPA disagrees with the suggested changes and believes the proposed text is clear as

written. For additional information on EPA's approach to AoR and corrective action, please see the

226
final rule and preamble.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24425

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(b) 

Proposed CCS Group Recommendations: (b) The owner or operator of a Class VI well 

must prepare, maintain, and comply with a plan to delineate the area of review 

for a proposed geologic sequestration project, periodically reevaluate the 

delineation, and perform corrective action that meets the requirements of this 

section and is IN A MANNER AND AT A FREQUENCY acceptable to the Director. As a 

part of the permit application, for approval by the Director the owner or 

operator must submit an area of review and corrective action plan that includes 

the following information: 

Basis for CCS Group Comments: Clarification. 

Response: EPA disagrees with the suggested change, and is retaining the approach of setting a

minimum fixed frequency for reevaluating the AoR but has, based on commenters' input, changed

the frequency from ten years to at least every five years, as specified in §146.84(e). EPA agrees with

commenters that site-specific information is an important factor in determining an appropriate AoR

reevaluation frequency, and adds that the owner or operator's AoR and corrective action plan must

describe the minimum fixed frequency for reevaluating the AoR and the site-specific conditions on

which this frequency is based. EPA adds that the Director will evaluate the proposed plan in

connection with a review of a vast amount of geologic and proposed operating data in determining

whether to approve the plan. EPA is developing guidance describing recommended approaches to

preparing all of the required GS project plans, which will address considerations for determining an

appropriate AoR reevaluation schedule. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24426

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Comment: Rule Provision: 146.84(b)(2)(i) & (ii) 

Proposed CCS Group Recommendations: (i) The minimum fixed frequency, not to 

exceed 10 years, the owner or operator proposes to reevaluate the area of 

review; (ii) The monitoring and operational conditions that would warrant a 

reevaluation of the area of review prior to the next scheduled reevaluation as 

determined by the minimum fixed frequency established in paragraph (b)(2)(i) of 

this section.

Basis for CCS Group Comments: These provisions in (i) and (ii) are rendered 

unnecessary by the revisions to section 146.84(b). 

227
Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every five

years is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24427

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Comment: Rule Provision: 146.84(b)(2)(iii) 

Proposed CCS Group Recommendations: iii (I) How monitoring and operational 

data (e.g., injection rate and pressure) will be used to inform an area of 

review reevaluation; and 

Basis for CCS Group Comments: Agree with EPA’s rationale. 

Response: EPA acknowledges the comment and the final rule retains this provision.

Commenter Name: Robert F. Van Voorhees Letter ID: 0190

Commenter Organization: Carbon Sequestration Council Comment ID: 24437

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f)(4) 

Proposed CCS Group Recommendations: (4) Submit an amended area of review and 

corrective action plan or demonstrate to the Director through monitoring data 

and modeling results that no amendment CHANGE to the area of review and 

corrective action plan is needed. 

Basis for CCS Group Comments: Clarification 

Response: EPA disagrees with the suggested change and believes the proposed text is clear as

written. For additional information on EPA's approach to AoR and corrective action, please see the

final rule and preamble.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24655

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(b) 

228
Anadarko Petroleum Corporation (APC) Comments: (b) The owner or operator of a 

Class VI well must prepare, maintain, and comply with a plan to delineate the 

area of review for a proposed geologic sequestration project, periodically

reevaluate the delineation, and perform corrective action that meets the 

requirements of this section and is IN A MANNER AND AT A FREQUENCY 

acceptable to the Director. As a part of the permit application, for approval 

by the Director the owner or operator must submit an area of review and 

corrective action plan that includes the following information: 

Rationale: Clarification. 

Response: EPA disagrees with the suggested changes and believes the proposed text is clear as

written. EPA believes that review and approval of the AoR and corrective action plan, including the

reevaluation frequency of at least every five years, by the Director (and in the context of all

information submitted by the owner or operator with the permit application) is important to ensuring

USDW protection.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24656

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Comment: Rule Provision: 146.84(b)(2) 

Anadarko Petroleum Corporation (APC) Comments: (i) The minimum fixed 

frequency, not to exceed 10 years, the owner or operator proposes to reevaluate 

the area of review; 

(ii) The monitoring and operational conditions that would warrant a reevaluation 

of the area of review prior to the next scheduled reevaluation as determined by 

the minimum fixed frequency established in paragraph (b)(2)(i) of this 

section.

(iii)(i) How monitoring and operational data (e.g., injection rate and 

pressure) will be used to inform an area of review reevaluation; and 

No rationale provided 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every five

years is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

229
Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24657

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3400 2.2.2 Corrective Action 

Comment: Rule Provision: 146.84(b)(2)(iv) 

Anadarko Petroleum Corporation (APC) Comments: (iv II) How corrective action 

will be conducted to meet the requirements of paragraph (d) of this section, 

including what corrective action will be performed prior to injection and what, 

if any, portions of the area of review will have corrective action addressed on 

a phased basis and how the phasing will be determined; how corrective action 

will be adjusted if there are changes in the area of review; and how site access 

will be guaranteed ARRANGED for future corrective action. 

Rationale: Future access cannot be guaranteed for an area in advance that may 

not even be within the boundaries of the AOR/ project. 

Response: The purpose of the corrective action requirement at §146.84(d) is to ensure that all wells

in the AoR that have the potential to cause the movement of CO2 or other fluids into or between

USDWs receive appropriate corrective action. In general, to meet this requirement, the owner or

operator will most likely need to obtain the right of access to wells that may require corrective

action, and demonstrate they have done this in their proposed AoR and corrective action plan. The

final rule retains the requirement as proposed.

Commenter Name: Craig R. Walters Letter ID: 0200

Commenter Organization: Anadarko Petroleum Corporation Comment ID: 24665

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Rule Provision: 146.84(f)(4) 

Anadarko Petroleum Corporation (APC) Comments: (4) Submit an amended area of 

review and corrective action plan or demonstrate to the Director through 

monitoring data and modeling results that no amendment CHANGE to the area of 

review and corrective action plan is needed. 

Rationale: Clarification. 

Response: EPA disagrees with the suggested change and believes the proposed text is clear as

written. For additional information on EPA's approach to AoR and corrective action, please see the

final rule and preamble.

Commenter Name: Andrew Duguid Letter ID: 0226

Commenter Organization: Schlumberger Carbon Services Comment ID: 24030

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: Proposed Rule: (4) Submit an amended area of review and corrective action plan 

or demonstrate to the Director through monitoring data and modeling results that 

230
no amendment to the area of review and corrective action plan is needed. 

Schlumberger Suggested Revision: (4) Submit an amended area of review and 

corrective action plan or demonstrate to the Director through monitoring data 

and modeling results that no change to the area of review and corrective action 

plan is needed. 

Schlumberger Rationale: - 

Response: EPA disagrees with the suggested change and believes the proposed text is clear as

written. For additional information on EPA's approach to AoR and corrective action, please see the

final rule and preamble.

Commenter Name: Kurt Waltzer Letter ID: 0250

Commenter Organization: Clean Air Task Force Comment ID: 22309

Other EPA Comment Codes and Descriptions: 4750 2.7.5 Comments on the merit of annual

reports (seeking comment) 

Comment: The rule should require updating when warranted of all of the proposed "plans" 

and should require an annual report that summarizes whether not updates have 

been warranted, and whether or not updates have been undertaken 

We applaud EPA for proposing to require the creation, use, and updating of 

various "plans" during the operating life of a facility. By creating such an 

iterative process, and basing it on monitoring and other operational data that 

become available through time, EPA can assure that facilities are managed in a 

way that adapts to changing circumstances. This approach also has the potential 

to enable what the Preamble calls an "ongoing dialogue" between operators and 

regulators, especially if the rule is enhanced by the annual reporting 

requirement suggested below for inclusion in §146.91. 

Perhaps inadvertently, the proposed rule does not explicitly require updating of 

several of the required plans. In most but not all cases, there is a rather 

ambiguous requirement to "maintain" a given plan. Explicit requirements to make 

updates as warranted appear to be missing from provisions relating to: 

Response: EPA acknowledges the commenters' support of the AoR and corrective action plan, and

retains the requirement in the final regulation §146.84(b). EPA believes that the iterative process by

which monitoring and operational data are used to validate the original AoR calculation, along with

the combined knowledge and ongoing dialogue between the owner or operator and the Director on

the local geologic conditions will be critical in developing and implementing plans that protect

USDWs from endangerment.

EPA adds that the final rule includes provisions that owners or operators review and update the AoR

and corrective action plan, the testing and monitoring plan, and the emergency and remedial

response plan throughout the life of the project, as specified in §146.84(c), §146.90(j), and

§146.94(d). Owners or operators must review each plan and identify necessary amendments to the

plan or demonstrate to the Director that no amendment is needed. These reviews must be performed

within one year of an AoR reevaluation, following any significant changes to the facility (e.g., the

addition of monitoring or injection wells), or when required by the Director.

231
EPA clarifies that the final rule does not require an annual report.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24882

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal: (b) The owner or operator of a Class VI well must prepare, 

maintain, and comply with a plan to delineate the area of review for a proposed 

geologic sequestration project, periodically reevaluate the delineation, and 

perform corrective action that meets the requirements of this section and is 

acceptable to the Director. As a part of the permit application for approval by 

the Director, the owner or operator must submit an area of review and corrective 

action plan that includes the following information: 

(1) The method for delineating the area of review that meets the requirements of 

paragraph (c) of this section, including the model to be used, assumptions that 

will be made, and the site characterization data on which the model will be 

based; 

GWPC Recommendations: (b) The owner or operator of a Class VI well must prepare, 

maintain, and comply with a plan to delineate the area of review for a proposed 

geologic sequestration project, reevaluate the delineation, and perform 

corrective action that meets the requirements of this section in a manner and at 

a frequency acceptable to the Director. As a part of the permit application, the 

owner or operator must submit an area of review and corrective action plan that 

includes the following information: 

Rationale: Suggest deletion of "periodically." See Workgroup recommendation 7.6 

from 

Recommendations for the Regulatory Management of Geologic Sequestration of CO2 

Under the UIC Program 

Replace with "in a manner and at a frequency" 

Delete "for approval by the Director,". Director’s approval is implicit in a 

permit application. 

Response: EPA disagrees with the suggested change, and is retaining the approach of setting a

minimum fixed frequency for reevaluating the AoR but has, based on commenters' input, changed

the frequency from 10 years to at least every 5 years, as specified in §146.84(e). EPA agrees with

commenters that site-specific information is an important factor in determining an appropriate AoR

reevaluation frequency, and adds that the owner or operator's AoR and corrective action plan must

describe the minimum fixed frequency for reevaluating the AoR and the site-specific conditions on

which this frequency is based. EPA adds that the Director will evaluate the proposed plan in

connection with a review of a vast amount of geologic and proposed operating data in determining

whether to approve the plan. EPA is developing guidance describing approaches to preparing all of

the required GS project plans, which will address considerations for determining an appropriate AoR

232
reevaluation schedule. The guidance will be available on EPA's web site, at

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm.

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24883

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language; 3500 2.2.3 Area of Review Reevaluation (seeking comment) 

Comment: Rule Provision: 146.84(b)(2) 

EPA Proposal: (2) A description of: 

(i) The minimum fixed frequency, not to exceed 10 years, the owner or operator 

proposes to reevaluate the area of review; 

(ii) The monitoring and operational conditions that would warrant a reevaluation 

of the area of review prior to the next scheduled reevaluation as determined by 

the minimum fixed frequency established in paragraph (b)(2)(i) of this section. 

(iii) How monitoring and operational data (e.g., injection rate and pressure) 

will be used to inform an area of review reevaluation; and 

GWPC Recommendations: Delete (i) and (ii) 

Rationale: Suggest deletion of (i) as this is handled by "manner and at a 

frequency acceptable to the Director in (b) 

Suggest deletion of (ii) for same reason as comment on (i) 

Response: EPA believes that periodic AoR reevaluation at a set frequency of at least every 5 years

is necessary to ensure protection of USDWs from endangerment and address the uncertainties

inherent in predictive modeling. Periodic AoR reevaluations at §146.84(e) ensure that a deliberate

effort is made to verify, based on monitoring and operational data, that the CO2 plume is moving

within the subsurface as predicted. EPA believes that less frequent reevaluations may not be

sufficient to appropriately model plume and pressure front movement, establish conditions for

corrective action, and ensure protection of USDWs from endangerment.

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: Michel Paque Letter ID: 0259

Commenter Organization: Ground Water Protection Council Comment ID: 24891

Other EPA Comment Codes and Descriptions: 1240 1.1.1.3 Suggested changes/edits to rule

language 

Comment: EPA Proposal:(3) Perform corrective action on wells requiring corrective action 

in the reevaluated area of review in the same manner specified in paragraph (c) 

(3) of this section; and 

233
(4) Submit an amended area of review and corrective action plan or demonstrate 

to the Director through monitoring data and modeling results that no amendment 

to the area of review and corrective action plan is needed. 

GWPC Recommendations: (3) Perform corrective action on wells requiring 

corrective action in the reevaluated area of review in the same manner specified 

in paragraph (c)(3) of this section; and 

(4) Submit an amended area of review and corrective action plan or demonstrate 

to the Director through monitoring data and modeling results that no change to 

the area of review and corrective action plan is needed. 

Rationale: Suggest "change" instead of "amendment" in (4) because the term 

amendment is typically construed to mean an official permit modification 

process. 

Response: EPA disagrees with the suggested change regarding the term “amendment.” EPA

believes that, in some cases an amendment to the AoR and corrective action plan may warrant a

permit modification. EPA added text to §144.39(a)(5) that permit modifications, requiring public

notice and comment, are required if the Director determines that a modification is necessary based

on an AoR reevaluation, amendments to any of the project plans, or a review of the testing and

monitoring results. EPA made this change because the Agency recognizes that the AoR reevaluation

process and any subsequent modifications to the plans may generate significant levels of public

interest. 

EPA clarifies that if site monitoring data agrees with the existing AoR delineation, a model

recalibration may not be necessary, and the AoR reevaluation may consist of a demonstration that

the current AoR delineation is adequate based on site monitoring data. 

Commenter Name: George Peridas Letter ID: 0534

Commenter Organization: Natural Resources Defense Council (NRDC) Comment ID: 21462

Other EPA Comment Codes and Descriptions: 4600 2.6 Proposed Plume and Pressure Front

Monitoring Requirements (40 CFR 146.90) 

Comment: The requirements for extensive monitoring is another example. This architectural 

design nature of having to continue to update the proposed plans, operational 

parameters, closure plans, etcetera as modeling data comes in and that is 

monitoring comes in. I think it is a very sound one which should be maintained 

Response: EPA acknowledges the commenter's support of the AoR and corrective action plan, and

retains the requirement in the final regulation §146.84(b). EPA believes that the iterative process by

which monitoring and operational data are used to validate the original AoR calculation, along with

the combined knowledge and ongoing dialogue between the owner or operator and the Director on

the local geologic conditions will be critical in developing and implementing plans that protect

USDWs from endangerment.

EPA agrees that monitoring data should be used to review model assumptions, verify computational

model projections, and, when necessary, provide input for model refinements. As data are collected

at an injection site, model parameters can be adjusted to match observations (i.e., for model

calibration or), which in turn improves the predictive capability of the model. As a GS project

234
proceeds, both monitoring and modeling will evolve, with monitoring results supporting effective

AoR reevaluations and leading to improved model predictions.

EPA believes that developing and following testing and monitoring plans is an important part of

assuring that a GS project is operating properly and is crucial for the early detection of potential

endangerment to USDWs. EPA also believes that periodic updates to the plan are appropriate. EPA

clarifies that the final GS regulation includes a requirement that the AoR be reevaluated at least

every 5 years, and it specifies at §146.90(j) that the operator revise the testing and monitoring plan to

incorporate the results of the most recent AoR reevaluation and recent operational and monitoring

data.

235