This comment letter response contains two parts:   

(1) The original submittal from the commenter.

(2) A data base printout of EPA’s response to each point raised by the commenter.

Notes about the response letters:

• The response letters were generated by a data base used to track the content of all public

comment submittals and ensure that the Agency responds to each. Each comment letter

was assigned a Letter ID that corresponds to the last four digits of the docket number of the

original comment (as posted on www.regulations.gov).

• Each response letter includes: comment text; an EPA‐assigned comment code; and an

individual response. These are described and explained below.  

• The data base contains Comment text from the letters submitted by commenters.  

Many hard copy letters were scanned into electronic format for inclusion in the data

base. As a result, there may be some typographical errors attributable to optical

character recognition issues.  However, all original submittals were reviewed in the

process of drafting responses to ensure that the comment content was understood and

appropriately addressed.

• The content of each comment letter was divided into topic areas and assigned a unique

Comment ID.  Additionally, one or more Comment Codes (i.e., unique topic area ID) was

assigned to each discrete point raised by a commenter. This information was added to

the data base. To inform final rulemaking decisions, all comments and information

received on a given topic were reviewed by technical experts in each rule area (e.g., on

construction requirements).  

• Following decisions on the final rule requirements, Individual responses were drafted

by topic area to ensure completeness and consistency of the responses with the final

rule and other responses to similar comments.  Each discrete point made by a

commenter was given a unique response.

• Upon completion of comment responses, all information was entered into the data base.

Each letter response is a compilation of each comment and its individual response,

compiled in the order of the original comment letter.  As a result of this process, there is

repetition in the response text.  The Agency believes this approach ensures completeness

and attention to each comment letter that was submitted.   

• All comments in a topic area were reviewed, evaluated, and responded to concurrently.  

Responses on each topic area are compiled and are also available in the rulemaking docket.
Attention: Docket ID No. EPA-HQ-OW-2008-0390 

 Comments on the EPA’s Proposed Federal Requirements under the Underground 

Injection Control (UIC) Program for Carbon Dioxide (CO2) Geologic Sequestration 

(GS) wells; Notice of Data Availability. 

The Notice of Data Availability (NODA) references useful new studies that can help to inform 

EPA’s proposed rule-making for geologic storage of CO2. However, Anadarko Petroleum 

Corporation (APC) sees nothing in the new information that suggests that EPA should depart 

from the adaptive approach to regulation that EPA has discussed as the basic objective for the 

geologic sequestration (GS) rule. Adherence to an adaptive approach for regulations will 

facilitate new information being implemented in a responsible way. The proven success of 30+ 

years of CO2 enhanced oil recovery, 70+ years of natural gas storage, and 150 years of oil and 

gas production, demonstrates the industry knows how to safely and effectively drill wells, inject 

and store fluids, and monitor for containment. In addition, CO2 injection for purposes of 

enhanced oil or gas recovery (“EOR/EGR”) has been successfully regulated as Class II injection 

for decades, and we agree with EPA that there is nothing to justify any disruption of the existing 

regulatory regime for the thousands of injection wells already in use in these operations. 

Injection Depth for GS Projects

APC is supportive of EPA’s effort to introduce flexibility on the depth of injection relative to the 

location of USDWs. APC holds that CO2 can be injected above a USDW while protecting the 

USDW and maintaining access to the USDW for future use. Anadarko’s Salt Creek EOR project 

in Wyoming, which has a USDW below the injection zone, is an excellent example of how 

industry can operate CO2 injection wells in an EOR context without endangering or limiting 

access to USDWs that are located below the injection zone. Accessing USDWs below a CCS 

site does not present significant technological hurdles. Industry has the technology to drill 

through over-pressured gas reservoirs which are analogous to CCS sites. There are some 

excellent examples of off-shore technologies for drilling through salt formations and shallow gas 

pockets that involve the same type of pressure issues that would be encountered in drilling 

through a CCS site. 

The EPA’s stated basis for requiring sequestration above a USDW, that “placing distance 

between the injection formation and USDWs decreases risks to USDWs” (NODA p.44809) is 

too simplistic. Distance will undoubtedly help, but distance does not deserve to be singled out 

like this; it will not make up for otherwise poor site characteristics nor does the lack of depth 

defeat a good site. The nature and characteristics of the confining material is paramount and 

must be considered along with distance in determining the sufficiency of containment at any 

particular site. Site selection and characterization must focus on the trapping and containment 

mechanisms themselves without giving undue weight to distance, which plays a role but is not 

always determinative. 

APC supports the concept of allowing injection above a USDW; however, we are concerned 

about the waiver process as currently designed. First, the EPA has not demonstrated how 

injection above a USDW presents an additional risk that would warrant a significant increase in 

the permitting requirements. If the proposed regulations are adopted, then a properly sited 

project will not pose any greater danger sited above a USDW as below one. Characterization of 

the confining zones will still be necessary whether they prevent against upward movement or 

downward movement. The regulations as proposed have been written broadly enough to require 
the site characterization to encompass both above and below a proposed injection zone, where 

appropriate. 

Second, for the waiver system to work effectively, the final decision making authority for a 

waiver should be with the agency holding primacy for the proposed Class VI UIC program. Any 

requirement that would provide for two separate agencies of state government to have the same 

authority over a single permitting process is inefficient and unnecessarily duplicative creating a 

risk of inconsistent permitting decisions. EPA’s current proposal would appear to give the 

Public Water Supply Supervision (PWSS) Program Director essentially veto power over the 

waiver. APC feels this is not appropriate due to the fact that the PWSS Program Director (and 

the Director’s staff) is not as knowledgeable regarding underground injection as the UIC 

Program Director (and the Director’s staff). In addition, APC questions what authority the EPA 

has to dictate to those states with primacy which agencies must approve a permit. Instead, APC 

recommends that the PWSS, like all other interested agencies, be included in the comment period 

making sure that they have the opportunity to raise any concerns they may have with the project. 

Comments on Research, Data Analysis, and Findings 

Class VI well construction, operational, and monitoring requirements 

EPA has noted in the NODA that the SECARB project “employs, demonstrates, and validates 

the EPA’s proposed Class VI well construction, operational, and monitoring requirements” 

(NODA, p. 44806). However, APC notes that no one project “validates” a given construction, 

operation, or monitoring technique in a general sense. The well involved in this project, by 

requirement of the Mississippi Department of Environmental Quality, meets Class I hazardous 

waste injection well design standards which were applied in an abundance of caution. 

Construction to these Class IH standards was not proposed voluntarily by the SECARB project 

team. The construction of this well and the resulting protectiveness only demonstrate that the 

design level is sufficient to be protective of USDWs not that it is the only design level sufficient 

to be protective. A rich database of successful well construction is also offered by the CO2-EOR 

and underground gas storage experience. APC continues to urge EPA to reconsider its proposed 

well construction and operation requirements which while protective, are much more prescriptive 

and rigorous than necessary for commercial-scale implementation of CCS and do not reflect 

industry’s 35-plus years of success in injecting CO2 under the Class II UIC program. The Class 

II EOR wells constructed for these projects - and their operating practices - have an excellent 

record of mechanical integrity and of protecting USDWs. Some of the proposed requirements, 

such as cementing the long string casing to surface, are not only unnecessarily burdensome, but 

might also interfere with operations and could introduce unintended risks. 

Monitoring: Tracers & Soil and Air Monitoring 

There are many monitoring tools that could be shown to have some utility, but it does not follow 

that every tool – or any particular tool – should be used for any given geological storage project, 

especially as the scaling-up to commercial projects would be cost-prohibitive for some of the 

tested monitoring techniques. The testing of particular monitoring technologies does not show 

the necessity of these techniques or the inadequacy of alternatives that were not tested. 

The Pump Canyon Project represents just one test of traces; the outcome should not be 

generalized. Use of tracers should be a site specific decision. More specifically, the 

perfluorocarbon (PFT) tracers used in the Escatawpa pilot project are of concern, as they are 
greenhouse gases much more potent than CO2. In addition, PFTs have a high-risk of 

contamination which makes them very impractical when utilizing beyond a pilot-scale project. 

Tracers may be useful for CO2 storage in some circumstances, but potential unintended 

consequences should be considered both when selecting whether tracers are an appropriate tool, 

and if so, what kind of tracer is most appropriate for the project. 

Soil and air monitoring, used at both the Escatawpa and Aneth Field pilot projects, are 

unnecessary under most foreseeable circumstances. Natural CO2 concentration in soil can have 

high spatial and temporal variation resulting from complex environmental interactions, 

particularly in wet, warm environments. In the air, CO2 has a high detection limit; a leak would 

have to be significant and sustained to detect it with air monitoring technologies. Leakage 

detection would be particularly difficult in industrial areas or near population centers, where CO2 

concentration in the air is already high and variable. Though having knowledge of CO2 flux in 

the soil and air at a given site location may prove useful for site characterization, soil and air 

monitoring will not be effective in most places. The general strategy of monitoring programs 

will be to detect leakage long before it reaches the surface, rendering limited utility to these 

techniques. The benefit of these near surface strategies is unlikely to outweigh the cost in most 

situations. 

Risk to Groundwater from Trace Element Mobilization 

APC urges EPA to not take the results of the LBNL study out of context or as more conclusive 

than they are. The NODA even states that the model results were based on “several assumptions 

and parameter values with a large degree of uncertainty” (NODA, p.44807). It must also be 

noted that the LBNL study goal was not, contrary to EPA’s assertion, to model a “representative 

groundwater” (NODA p. 44807). 

The probability of trace element contaminating groundwater requires the multiplied probabilities 

of CO2 movement out of the intended storage zone, finding a migration path to a USDW, passing 

through or ending up in a zone with minerals that contain heavy metal assemblages, 

environmental conditions favorable to dissolution, and an undetected, unremediated leak that 

mobilizes these elements into a USDW as opposed to immobilizing them through mineral 

precipitation or adsorption onto clays/organics. The probability of USDW contamination, based 

on the probabilities of each of these individual events, will be different for every site. The risk of 

trace metal contamination of groundwater will typically be small, knowable, and manageable at 

well selected and managed geologic storage sites as noted in the text “the majority of results did 

not show mobilization at levels exceeding the MCL or AL” (NODA, p. 44807). 

Also, actual field results – as opposed to the modeling exercise - have shown the risk of 

contamination is low. For example, at SACROC oilfield in Scurry County, TX, CO2 has been 

injected for EOR since 1972, and according to recent publications they have not seen evidence of 

certain elements mobilized from injection zones at SACROC into overlying USDWs (reports 

referenced in detail in both the API’s NODA response as well as the CSC’s NODA response). 

The LBNL report’s authors realize the limitations to their models and EPA should also. Despite 

having set up the simulation so that trace element mobilization would likely result, the authors 

conclude that “. . . model uncertainties, kinetic factors, and post-withdrawal water treatment 

methods all suggest diligence rather than alarm.” 
Basin Scale Impacts of CO2 Injection 

EPA states that the LBNL study of Basin-Scale impacts “illustrates the importance of basin-scale 

evaluation of reservoir pressures and far-field pressures resulting from CO2 injection” (NODA, 

p. 44808). APC would caution EPA from basing regulations or permit requirements on the 

outcomes of the study due to the constraints of the model and assumptions. In addition the state 

of understanding the modeling of basin-scale CO2 sequestration impacts is in its early stages and 

thus is not appropriate for use in rulemaking at this time. Conclusions based on the modeled 

brine flow out of the basin and far field pressure effects should be considered tentative at best 

given the significant (and potentially incorrect) assumptions made in the model. This is 

particularly true given that the modeled brine out-flow and far field pressure effects are very 

small, and would not likely have been observed if the model had included lateral heterogeneities 

in permeability and porosity and higher compressibility. 

Under the proposed rule, basin-wide model results are not applicable to Area of Review 

decisions for individual site selection. The multi-stakeholder group recommendations dated May 

14, 2009, provide an approach for handling multi-project basins where pressures fronts may 

intersect and influence. Adaptive rule-making could allow for adjustments to encompass multiproject scenarios in the same basin if needed in the future. However, it raises the question of 

who would be responsible for doing basin-wide analysis and at what point should it be 

conducted. 

CO2 Modeling 

EPA states that “modeling the injection and sequestration of CO2 poses unique challenges” 

(NODA p. 44808). While on its face true, the statement exaggerates the challenges. Industry is 

constantly being faced with new situations that pose “unique challenges”. For example in the 

Canadian oil sands, industry routinely simulates heavy oil thermal recovery operations which 

require steam injection into shallow reservoirs at high temperature, causing thermal alteration of 

the oil, reactivity of mineral species, and overpressure. These operations are more complicated 

and more chemically reactive than CO2 storage operations. Industry has extensive experience in 

these types of simulations, and though not a perfect analog for CO2 storage, it shows that we can 

model highly complex and dynamic subsurface processes. 

The fact is that the state of modeling for CO2 storage is more than adequate to proceed with EPA 

rule promulgation. Additional research is of course encouraged, but there are no major technical 

breakthroughs in the realm of modeling that need to occur before beginning commercial-scale 

CO2 storage operations in the United States. Adaptive regulations will facilitate new information 

being implemented in a responsible way. 

APC takes exception to EPA’s assertion that there is a “. . . need to couple multiphase flow, 

reactive transport, and geomechanical processes” (NODA p.44808). Such a model is certainly 

possible but will likely be intractable, expensive, and it will still be at the mercy of the quality of 

the input. It is more important to correctly capture the range of processes and possible outcomes 

at a given location than to use a particular software package or type of modeling. Robust yet less 

complex models capturing the most important physical and chemical processes can accomplish 

the modeling goals necessary for safe storage. 
Letter ID: 0380  

Commenter: Rebecca Thingelstad  

Organization: Anadarko Petroleum Corporation (APC)  

Letter ID: 0380 Comment ID: 28090 Letter Lines: 22 to 36  

Comment Code: 1000 1. General Comments on the Notice of Data Availability  

Comment: The Notice of Data Availability (NODA) references useful new studies that can  

help to inform EPA’s proposed rule‐making for geologic storage of CO2. However,  

Anadarko Petroleum Corporation (APC) sees nothing in the new information that  

suggests that EPA should depart from the adaptive approach to regulation that  

EPA has discussed as the basic objective for the geologic sequestration (GS)  

rule. Adherence to an adaptive approach for regulations will facilitate new  

information being implemented in a responsible way. The proven success of 30+  

years of CO2 enhanced oil recovery, 70+ years of natural gas storage, and 150  

years of oil and gas production, demonstrates the industry knows how to safely  

and effectively drill wells, inject and store fluids, and monitor for  

containment. In addition, CO2 injection for purposes of enhanced oil or gas  

recovery ("EOR/EGR") has been successfully regulated as Class II injection for  

decades, and we agree with EPA that there is nothing to justify any disruption  

of the existing regulatory regime for the thousands of injection wells already  

in use in these operations.  

Individual Response: EPA is following an adaptive approach to the GS rulemaking to incorporate new

research, data, and information about GS. EPA intends to continue to collect data on GS and to

incorporate such data as it implements its adaptive approach to GS rulemaking to incorporate new

research, data, and information about GS. The Agency believes that an adaptive approach provides Class

VI owners or operators the flexibility to use and apply newly developed technologies for GS as they

become available in the future. For additional information on the adaptive approach, see the responses

to comments on the proposed rule in the rulemaking docket.

EPA recognizes that the oil and gas industry has decades of experience injecting CO2 for the purpose of

ER. The GS rulemaking builds upon existing experience and knowledge about safe injection practices in

ER and other injection industries, with tailored requirements that address the unique nature of CO2

injection for GS.  

The final rule does not preclude CO2 injection for ER under Class II permits. However, the rule requires

re‐permitting of Class II wells (as Class VI) if the primary purpose of the injection transitions from ER to
long‐term storage, and establishes specific, risk‐based criteria to be considered by the Director in

making this determination (§144.19). For more information on the transition from Class II to Class VI,

see the responses to comments on the proposed rule in the rulemaking docket.  

Letter ID: 0380 Comment ID: 28091 Letter Lines: 38 to 52  

Comment Code: 3210 3.2.1. Comments Generally in Support of the Approach  

Comment Code: 3420 3.4.2. Request for Information on Areas Where Injection Depth and USDWs Are a

Concern (seeking comment)  

Comment: Injection Depth for GS Projects  

APC is supportive of EPA’s effort to introduce flexibility on the depth of  

injection relative to the location of USDWs. APC holds that CO2 can be injected  

above a USDW while protecting the USDW and maintaining access to the USDW for  

future use. Anadarko’s Salt Creek EOR project in Wyoming, which has a USDW below  

the injection zone, is an excellent example of how industry can operate CO2  

injection wells in an EOR context without endangering or limiting access to  

USDWs that are located below the injection zone. Accessing USDWs below a CCS  

site does not present significant technological hurdles. Industry has the  

technology to drill through over‐pressured gas reservoirs which are analogous to  

CCS sites. There are some excellent examples of off‐shore technologies for  

drilling through salt formations and shallow gas pockets that involve the same  

type of pressure issues that would be encountered in drilling through a CCS  

site.  

Individual Response: EPA acknowledges the commenter's support of the waiver approach and site‐

specific flexibility in regulatory development and permitting as described in the NODA and Request for

Comment. The Agency acknowledges that there are some situations where injection into non‐USDW

formations above the lowermost USDW can be performed in a manner that protects USDWs. The final

GS rule at §146.95 establishes a process by which owners and operators can seek a waiver of the Class

VI injection depth requirements. The Agency believes the waiver approach: 1) responds to concerns

about local and regional geologic storage capacity limitations imposed by the proposed injection depth

requirements; 2) allows for a more site‐specific assessment of injection depth for GS projects; 3)

accommodates injection into different formation types; 4) allows for injection of CO2 for GS into non‐

USDWs above and/or between USDWs when appropriate and where it can be demonstrated that

USDWs will be protected; and 5) responds to concerns about the use of aquifer exemptions for GS.

Finally, EPA's approach to addressing injection depth variability through a waiver process responds to

concerns about future drinking water resource availability and the need to ensure that high quality

water remains available in sufficient quantities to supply drinking water needs.
EPA acknowledges that technologies and past experience exist to inject through over‐pressured

formations while maintaining zonal isolation (e.g., oil and gas industry experience in drilling through

shallow pockets of gas or other zones that have higher pressures). EPA has reviewed information on

these types of projects and has determined that, if properly constructed, wells can be drilled through

pressurized zones without causing fluid movement. However, to provide additional USDW protection

and ensure that any planned drilling is conducted appropriately, the final GS rule requires the Director

to consider planned or permitted water, hydrocarbon, or mineral resource exploitation potential of the

proposed injection formation and other formations both above and below the injection zone to

determine if there are any plans to drill through the formation to access resources in or beneath the

proposed injection zone/formation as specified in §146.95(b)(1)(vii). EPA believes that these additional

considerations are necessary when permitting such wells to ensure protection of all USDWs and that the

additional requirements that apply to owners or operators of Class VI wells granted a waiver of the

injection depth requirements at §146.95 will provide this protection.  

Letter ID: 0380 Comment ID: 28092 Letter Lines: 65 to 74  

Comment Code: 3200 3.2. Comments on Alternative Injection Depth Waiver Approach  

Comment: APC supports the concept of allowing injection above a USDW; however, we are  

concerned about the waiver process as currently designed. First, the EPA has not  

demonstrated how injection above a USDW presents an additional risk that would  

warrant a significant increase in the permitting requirements. If the proposed  

regulations are adopted, then a properly sited project will not pose any greater  

danger sited above a USDW as below one. Characterization of the confining zones  

will still be necessary whether they prevent against upward movement or downward  

movement. The regulations as proposed have been written broadly enough to  

require the site characterization to encompass both above and below a proposed  

injection zone, where appropriate.  

Individual Response: EPA believes that that injection for the purpose of GS into non‐USDW formations

above the lowermost USDW warrants tailored requirements to address the unique challenges and risks

relative to injection below the lowermost USDW. For example, there is a risk of downward migration of

fluids if there is no lower confining layer or inadequate confinement. In addition, there is potential that

wells will be drilled through the injection zone in the future to reach USDWs. EPA believes that the

additional requirements that apply to owners or operators of Class VI wells granted a waiver of the

injection depth requirements at §146.95 will provide additional considerations for the owner or

operator and Director when permitting such wells to ensure protection of all USDWs.  
Letter ID: 0380 Comment ID: 28093 Letter Lines: 76 to 90  

Comment Code: 3350 3.3.5. Requirement to Submit Waiver Application to UIC and PWSS Program

Directors  

Comment: Second, for the waiver system to work effectively, the final decision making  

authority for a waiver should be with the agency holding primacy for the  

proposed Class VI UIC program. Any requirement that would provide for two  

separate agencies of state government to have the same authority over a single  

permitting process is inefficient and unnecessarily duplicative creating a risk  

of inconsistent permitting decisions. EPA’s current proposal would appear to  

give the Public Water Supply Supervision (PWSS) Program Director essentially  

veto power over the waiver. APC feels this is not appropriate due to the fact  

that the PWSS Program Director (and the Director’s staff) is not as  

knowledgeable regarding underground injection as the UIC Program Director (and  

the Director’s staff). In addition, APC questions what authority the EPA has to  

dictate to those states with primacy which agencies must approve a permit.  

Instead, APC recommends that the PWSS, like all other interested agencies, be  

included in the comment period making sure that they have the opportunity to  

raise any concerns they may have with the project.  

Individual Response: EPA disagrees that consultation with the PWSS Director is duplicative: the

communication with the PWSS Director is consultative and does not constitute a final decision making.

The final GS rule includes a requirement that the UIC Program Director consult with the PWSS Directors

of all states, Territories, and Tribes having jurisdiction within the AoR of a well for which a waiver is

sought as specified in §146.95(b)(2). EPA believes this process is necessary and appropriate to ensure

that water system concerns are considered when evaluating a waiver application.  

Letter ID: 0380 Comment ID: 28094 Letter Lines: 92 to 117  

Comment Code: 2110 2.1.1. SECARB (Escatawpa, MS)  

Comment: Comments on Research, Data Analysis, and Findings  

Class VI well construction, operational, and monitoring requirements  

EPA has noted in the NODA that the SECARB project "employs, demonstrates, and  

validates the EPA’s proposed Class VI well construction, operational, and  

monitoring requirements" (NODA, p. 44806). However, APC notes that no one  

project "validates" a given construction, operation, or monitoring technique in  
a general sense. The well involved in this project, by requirement of the  

Mississippi Department of Environmental Quality, meets Class I hazardous waste  

injection well design standards which were applied in an abundance of caution.  

Construction to these Class IH standards was not proposed voluntarily by the  

SECARB project team. The construction of this well and the resulting  

protectiveness only demonstrate that the design level is sufficient to be  

protective of USDWs not that it is the only design level sufficient to be  

protective. A rich database of successful well construction is also offered by  

the CO2‐EOR and underground gas storage experience. APC continues to urge EPA to  

reconsider its proposed well construction and operation requirements which while  

protective, are much more prescriptive and rigorous than necessary for  

commercial‐scale implementation of CCS and do not reflect industry’s 35‐plus  

years of success in injecting CO2 under the Class II UIC program. The Class II  

EOR wells constructed for these projects ‐ and their operating practices ‐ have  

an excellent record of mechanical integrity and of protecting USDWs. Some of the  

proposed requirements, such as cementing the long string casing to surface, are  

not only unnecessarily burdensome, but might also interfere with operations and  

could introduce unintended risks.  

Individual Response: EPA acknowledges that a single project does not prove that a given design is

effective in all cases or is the only design that would be effective, and disagrees that the proposed rule

requirements are more prescriptive than necessary. The results of the SECARB project, however, do

show that the proposed requirements are effective in the studied case and can provide for safe injection

of carbon dioxide that protects USDWs. For more information on the construction requirements for GS

wells, see the responses to comments on the proposed rule in the rulemaking docket.

EPA believes that GS is sufficiently different from Class II ER operations to warrant the tailored

construction requirements for GS wells at §146.86. For example, the volume of CO2 anticipated to be

injected in Class VI wells is significantly greater than for Class II wells. Reservoir pressure conditions will

be different, with Class VI injection operations involving large volumes of CO2 and likely having higher

pressures within their storage formations than Class II ER projects. The Class VI regulations also address

the unique physical characteristics of CO2 including its buoyancy and potential corrosivity. See the

response to comments document for the proposed rule for additional information on EPA’s final

approach to construction of GS wells.

EPA intends to use an adaptive approach to the GS regulation. 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.  
Letter ID: 0380 Comment ID: 28095 Letter Lines: 119 to 136  

Comment Code: 2130 2.1.3. SWP (Pump Canyon, NM)  

Comment: Monitoring: Tracers & Soil and Air Monitoring  

There are many monitoring tools that could be shown to have some utility, but it  

does not follow that every tool ‐ or any particular tool ‐ should be used for  

any given geological storage project, especially as the scaling‐up to commercial  

projects would be cost‐prohibitive for some of the tested monitoring techniques.  

The testing of particular monitoring technologies does not show the necessity of  

these techniques or the inadequacy of alternatives that were not tested.  

The Pump Canyon Project represents just one test of traces; the outcome should  

not be generalized. Use of tracers should be a site specific decision. More  

specifically, the perfluorocarbon (PFT) tracers used in the Escatawpa pilot  

project are of concern, as they are greenhouse gases much more potent than CO2.  

In addition, PFTs have a high‐risk of contamination which makes them very  

impractical when utilizing beyond a pilot‐scale project. Tracers may be useful  

for CO2 storage in some circumstances, but potential unintended consequences  

should be considered both when selecting whether tracers are an appropriate  

tool, and if so, what kind of tracer is most appropriate for the project.  

Individual Response: EPA acknowledges that the pilot sites studied involved extensive monitoring that

may not necessarily be appropriate for all GS sites. However, a robust monitoring program is critical to

ensuring protection of USDWs at GS projects from endangerment, and EPA clarifies that the use of any

monitoring technique in the pilot projects does not amount to an endorsement by EPA or an intent by

EPA to require that particular method. By testing multiple technologies, the pilot sites provide valuable

data that demonstrate whether or not these technologies are viable and informed the final GS rule

about the appropriate use of monitoring technologies.

EPA acknowledges that the appropriate monitoring methods and technologies will be determined on a

site‐specific basis. Data from the projects presented supports the need for a site‐specific monitoring

plan that will allow use of a range of monitoring technologies suitable for each unique GS site.

EPA recognizes that certain monitoring technologies, such as tracers are subject to variability and may

not be appropriate or necessary in all settings. The GS rule at §146.90 gives the Director discretion to

determine what methods are appropriate. All monitoring must be performed according to the Director‐

approved testing and monitoring plan. See the response to comments document for the proposed rule

for additional information on EPA’s final approach to monitoring at GS sites.

EPA acknowledges comments regarding the costs associated with monitoring. The Agency believes the
requirements for comprehensive monitoring are necessary to protect USDWs. A discussion of the costs

attributed to monitoring technologies (used during injection and the post‐injection site care timeframes)

is available in the Technology and Cost Document for the GS rule, which is in the federal docket for this

rulemaking.  

Letter ID: 0380 Comment ID: 28096 Letter Lines: 153 to 185  

Comment Code: 2210 2.2.1. Groundwater Quality and Mobilization of Trace Elements (seeking

comment)  

Comment: Risk to Groundwater from Trace Element Mobilization  

APC urges EPA to not take the results of the LBNL study out of context or as  

more conclusive than they are. The NODA even states that the model results were  

based on "several assumptions and parameter values with a large degree of  

uncertainty" (NODA, p.44807). It must also be noted that the LBNL study goal was  

not, contrary to EPA’s assertion, to model a 'representative groundwater" (NODA  

p. 44807).  

The probability of trace element contaminating groundwater requires the  

multiplied probabilities of CO2 movement out of the intended storage zone,  

finding a migration path to a USDW, passing through or ending up in a zone with  

minerals that contain heavy metal assemblages, environmental conditions  

favorable to dissolution, and an undetected, unremediated leak that mobilizes  

these elements into a USDW as opposed to immobilizing them through mineral  

precipitation or adsorption onto clays/organics. The probability of USDW  

contamination, based on the probabilities of each of these individual events,  

will be different for every site. The risk of trace metal contamination of  

groundwater will typically be small, knowable, and manageable at well selected  

and managed geologic storage sites as noted in the text "the majority of results  

did not show mobilization at levels exceeding the MCL or AL" (NODA, p. 44807).  

Also, actual field results ‐ as opposed to the modeling exercise ‐ have shown  

the risk of contamination is low. For example, at SACROC oilfield in Scurry  

County, TX, CO2 has been injected for EOR since 1972, and according to recent  

publications they have not seen evidence of certain elements mobilized from  

injection zones at SACROC into overlying USDWs (reports referenced in detail in  

both the API’s NODA response as well as the CSC’s NODA response). The LBNL  

report’s authors realize the limitations to their models and EPA should also.  

Despite having set up the simulation so that trace element mobilization would  
likely result, the authors conclude that ". . . model uncertainties, kinetic  

factors, and post‐withdrawal water treatment methods all suggest diligence  

rather than alarm."  

Individual Response: EPA clarifies that the LBNL study was conducted to study the potential

vulnerability of a wide range of aquifer types in the unlikely case of CO2 intrusion to provide a

foundation for further studies on ground water quality concerns, such as site‐specific studies including

laboratory and field experiments and associated modeling with the objective of reducing uncertainty of

assumptions and parameters.  

EPA notes that all of the cited elements (and others not cited here) are generally present in natural

ground waters regardless of location, albeit typically at very small (i.e., trace) concentrations. However,

it is true that analyses for many potentially hazardous trace elements are not made routinely, and that

the analytical methods used have often been insufficiently sensitive to quantify the concentrations

actually present. Regarding the presence or absence of minerals hosting the potentially hazardous trace

elements, very few studies have been conducted on the mineralogy of potable water aquifer host rocks

in which the modes of occurrence of trace elements have been characterized. Birkholzer et al. (2008)

evaluated concentrations in soils and sediments from USGS’ National Geochemistry Database andlooked

at evidence from crustal abundances and abundances in domestic coals. They concluded that many

trace elements occur ubiqui¬tously in soils, sediments, and aquifer rocks throughout the world.

EPA agrees (and mentioned in the NODA) that the risk of CO2 leaking from deep storage is low for

properly managed GS projects, but the leakage probability is not necessarily zero (i.e., pre‐injection site

characterization may fail to detect leakage pathways or monitoring during and after injection may miss

small‐scale signals of local leaks). Therefore, the potential consequences of unlikely leakage events need

to be understood and predictive capabilities need to be developed as a basis for risk and vulnerability

assessment.  

EPA agrees that risk assessment studies, as well as additional geochemical investigations on ground

water vulnerability, should be site specific. For this reason, all GS project owner or operators must

submit detailed, site‐specific information about a proposed GS site, including information on the

porosity, permeability, mineralogy, and geochemistry of the targeted injection and confining zones as

well as all USDWs within the AoR. Operators must also present information on the compatibility of the

CO2 stream with fluids in the injection zone and minerals in the injection and confining zones

(§146.82(c)(3)). The data can help the Director determine the potential for geochemical reactions

between the injectate and the host geologic formations. For additional information on EPA’s approach

to site characterization for GS wells, see the responses to comments on the proposed rule in the

rulemaking docket.

EPA notes the cited paper by Smyth et al. (2009) suggests that no contamination of an overlying aquifer

has occurred at the SACROC oil field. However, the paper does not establish (1) if this is because no CO2

leakage occurred, (2) if leakage occurred but the leaking CO2 has not yet reached the aquifer, or (3) if
CO2 has migrated into the aquifer but has not caused noticeable geochemical response. The latter is

unlikely, as other studies clearly suggested mobilization of metals in response to increased CO2 levels in

the water. For example, the same cited paper by Smyth et al. (2009) reports on laboratory experiments

with exposure of diverse aquifer rocks to CO2, showing geochemical response within hours to days after

introduction of CO2. Kharaka et al. (2009) report on a field experiment with injection of CO2 into

shallow groundwater, also showing a rapid and systematic geochemical response in the aquifer, with

concentration increases observed for several trace metals. These studies support the LBNL findings that

CO2 intrusion into USDW, however unlikely, can lead to mobilization of trace elements.  

Letter ID: 0380 Comment ID: 28097 Letter Lines: 187 to 210  

Comment Code: 2220 2.2.2. Basin‐Scale Hydrologic Impacts of CO2 Storage (seeking comment)  

Comment: Basin Scale Impacts of CO2 Injection  

EPA states that the LBNL study of Basin‐Scale impacts "illustrates the  

importance of basin‐scale evaluation of reservoir pressures and far‐field  

pressures resulting from CO2 injection" (NODA, p. 44808). APC would caution EPA  

from basing regulations or permit requirements on the outcomes of the study due  

to the constraints of the model and assumptions. In addition the state of  

understanding the modeling of basin‐scale CO2 sequestration impacts is in its  

early stages and thus is not appropriate for use in rulemaking at this time.  

Conclusions based on the modeled brine flow out of the basin and far field  

pressure effects should be considered tentative at best given the significant  

(and potentially incorrect) assumptions made in the model. This is particularly  

true given that the modeled brine out‐flow and far field pressure effects are  

very small, and would not likely have been observed if the model had included  

lateral heterogeneities in permeability and porosity and higher compressibility.  

Under the proposed rule, basin‐wide model results are not applicable to Area of  

Review decisions for individual site selection. The multi‐stakeholder group  

recommendations dated May 14, 2009, provide an approach for handling multi‐  

project basins where pressures fronts may intersect and influence. Adaptive  

rule‐making could allow for adjustments to encompass multiproject scenarios in  

the same basin if needed in the future. However, it raises the question of who  

would be responsible for doing basin‐wide analysis and at what point should it  

be conducted.  

Individual Response: The results of the modeling study did not cause EPA to change any aspects of the

site characterization or AoR delineation requirements for GS projects. The final GS rule focuses on
permitting of individual GS wells and therefore does not include requirements for basin‐scale

evaluations. However, EPA agrees that proactive management of basin scale concerns to maintain

pressures within acceptable ranges is important, and encourages owners or operators and permitting

authorities to communicate planned GS activities to maximize coordination and minimize pressure

interference. If a permit applicant (or the permitting authority) is aware of pending GS projects near a

proposed well, the AoR delineation performed pursuant to §146.84 should account for the impacts of

multiple owners and operators injecting within a single basin. 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. The proximity of injection sites to each other and the relative

timing of each injection operation might indicate a possible increase in risk for the containment of the

total CO2 and/or the effects of hydraulic perturbations and brine displacement. For additional

information regarding EPA’s requirements for AoR delineation for GS projects, see the responses to

comments on the proposed rule in the rulemaking docket.

Regarding the model assumptions, EPA acknowledges that basin‐scale predictions are uncertain due to

the limited information generally available for large subsurface volumes. While the example modeling

study cited in the NODA and Request for Comment, and its underlying assumptions, are based on the

best information available, significant prediction uncertainty remains and results should be viewed as

illustrative, as acknowledged in Birkholzer and Zhou (2009). EPA clarifies, however, that the model

choice for pore compressibility is discussed in detail in Zhou et al. (2009), and is believed to be well

constrained. The pore compressibility used in the model was obtained through a pumping test in central

Illinois, at a depth of 2000 m just north of the assumed injection area. Additional pumping tests available

in shallower parts of the storage reservoir corroborate the chosen value. These values used in the model

are within the range of pore compressibility reported for well‐consolidated sandstones at in situ

effective stresses representative of Mt. Simon. The model assumption made about lateral homogeneity

in permeability and porosity is important for determining brine flow and far‐field pressure, but it is not

clear without better knowledge about lateral variability whether inclusion of heterogeneity would

increase or decrease far‐field effects.  

Letter ID: 0380 Comment ID: 28098 Letter Lines: 212 to 241  

Comment Code: 2310 2.3.1. Review of Modeling Studies (seeking comment)  

Comment: CO2 Modeling  

EPA states that "modeling the injection and sequestration of CO2 poses unique  

challenges" (NODA p. 44808). While on its face true, the statement exaggerates  

the challenges. Industry is constantly being faced with new situations that pose  

'unique challenges". For example in the Canadian oil sands, industry routinely  

simulates heavy oil thermal recovery operations which require steam injection  
into shallow reservoirs at high temperature, causing thermal alteration of the  

oil, reactivity of mineral species, and overpressure. These operations are more  

complicated and more chemically reactive than CO2 storage operations. Industry  

has extensive experience in these types of simulations, and though not a perfect  

analog for CO2 storage, it shows that we can model highly complex and dynamic  

subsurface processes.  

The fact is that the state of modeling for CO2 storage is more than adequate to  

proceed with EPA rule promulgation. Additional research is of course encouraged,  

but there are no major technical breakthroughs in the realm of modeling that  

need to occur before beginning commercial‐scale CO2 storage operations in the  

United States. Adaptive regulations will facilitate new information being  

implemented in a responsible way.  

APC takes exception to EPA’s assertion that there is a ". . . need to couple  

multiphase flow, reactive transport, and geomechanical processes" (NODA  

p.44808). Such a model is certainly possible but will likely be intractable,  

expensive, and it will still be at the mercy of the quality of the input. It is  

more important to correctly capture the range of processes and possible outcomes  

at a given location than to use a particular software package or type of  

modeling. Robust yet less complex models capturing the most important physical  

and chemical processes can accomplish the modeling goals necessary for safe  

storage.  

Individual Response: EPA believes that modeling of CO2 injection for GS does pose several unique

challenges, based on review of relevant technical literature, and participation in several technical

symposia and workshops related to computational modeling in this area. While this type of multiphase,

non‐isothermal, multi‐component computational modeling may be familiar in certain advanced

industries, it is certainly novel to the UIC community, and is more sophisticated than modeling typically

required in U.S. EPA regulations. However, EPA agrees with the commenter that advanced

computational modeling of GS sites is feasible, and trusts that experts in industry will meet the

challenges posed by GS modeling. For this reason, EPA has required computational modeling for

delineation of the AoR in a Class VI permit application.  

EPA also agrees with the commenter that coupling of multiphase flow, reactive transport, and

geomechanical processes may not be necessary in all cases for modeling to delineate the AoR in a Class

VI permit application. EPA clarifies that today’s federal regulation requires that models used for AoR

delineation be able to account for multiphase flow and the buoyancy of CO2. However, the Director has

the discretion to require more advanced models be used for AoR delineation, including additional

coupling of reactive transport and geomechanical processes. EPA agrees with the commenter that these

types of fully coupled models are possible, and believes that in certain cases they will be necessary for

accurate delineation of the AoR. For many projects, however, simpler models, as advocated by the
commenter, may be suitable for AoR delineation. EPA believes that the Director of individual projects is

in the best position to determine the specific technical requirements of the model(s) used.  

Letter ID: 0380 Comment ID: 28571 Letter Lines: 54 to 63  

Comment Code: 3100 3.1. Comments on Concerns About Injection Depth for Class VI Wells  

Comment: The EPA’s stated basis for requiring sequestration above a USDW, that "placing  

distance between the injection formation and USDWs decreases risks to USDWs"  

(NODA p.44809) is too simplistic. Distance will undoubtedly help, but distance  

does not deserve to be singled out like this; it will not make up for otherwise  

poor site characteristics nor does the lack of depth defeat a good site. The  

nature and characteristics of the confining material is paramount and must be  

considered along with distance in determining the sufficiency of containment at  

any particular site. Site selection and characterization must focus on the  

trapping and containment mechanisms themselves without giving undue weight to  

distance, which plays a role but is not always determinative.  

Individual Response: EPA believes that suitable confining layers (e.g., low‐permeability, low‐porosity

formations identified through proper site characterization process) along with proper well construction,

operation, monitoring, and post‐injection site care are key to protecting USDWs from endangerment.  

Additionally, the Agency believes that distance between the injection zone and USDWs can impede

vertical fluid movement, allow for pressure dissipation, and provide additional sites for monitoring,

mitigation, and remediation if conditions warrant.  

However, EPA acknowledges that there are situations where injection into non‐USDW formations above

the lowermost USDW can be performed in a manner that protects USDWs, as long as appropriate

protections are in place. The final GS rule at §146.95 establishes a process by which owners and

operators can seek a waiver of the Class VI injection depth requirements.  

Letter ID: 0380 Comment ID: 28658 Letter Lines: 138 to 151  

Comment Code: 2110 2.1.1. SECARB (Escatawpa, MS)  

Comment Code: 2120 2.1.2. SWP (Aneth Field, UT)  

Comment: Soil and air monitoring, used at both the Escatawpa and Aneth Field pilot  

projects, are unnecessary under most foreseeable circumstances. Natural CO2  

concentration in soil can have high spatial and temporal variation resulting  
from complex environmental interactions, particularly in wet, warm environments.  

In the air, CO2 has a high detection limit; a leak would have to be significant  

and sustained to detect it with air monitoring technologies. Leakage detection  

would be particularly difficult in industrial areas or near population centers,  

where CO2 concentration in the air is already high and variable. Though having  

knowledge of CO2 flux in the soil and air at a given site location may prove  

useful for site characterization, soil and air monitoring will not be effective  

in most places. The general strategy of monitoring programs will be to detect  

leakage long before it reaches the surface, rendering limited utility to these  

techniques. The benefit of these near surface strategies is unlikely to outweigh  

the cost in most situations.  

Individual Response: EPA acknowledges that the pilot sites studied involved extensive monitoring that

may not necessarily be appropriate for all GS sites. However, EPA believes that a robust monitoring

program is critical to ensuring protection of USDWs at GS projects from endangerment, but clarifies that

the use of any monitoring technique in the pilot projects does not amount to an endorsement by EPA or

an intent by EPA to require that particular method. By testing multiple technologies, the pilot sites

provide valuable data that demonstrate whether or not these technologies are viable and informed the

final GS rule about the appropriate use of monitoring technologies.

EPA acknowledges that the appropriate monitoring methods and technologies will be determined on a

site‐specific basis. Data from the projects presented supports the need for a site‐specific monitoring

plan that will allow use of a range of monitoring technologies suitable for each unique GS site.

EPA recognizes that certain monitoring technologies, particularly surface air and soil gas monitoring are

subject to variability and may not be appropriate or necessary in all settings. The GS rule at §146.90

gives the Director discretion to determine what methods are appropriate. All monitoring must be

performed according to the Director‐approved testing and monitoring plan. See the response to

comments document for the proposed rule for additional information on EPA’s final approach to

monitoring at GS sites.