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.
Comments on the Proposed Rule 

Federal Requirements under the Underground Injection Control Program for Carbon 

Dioxide Geologic Sequestration Wells, 40 CFR Parts 144 and 146, Environmental 

Protection Agency (Docket ID No. EPA–HQ–OW–2008–0390) 

This proposed rule was published in the Federal Register, Vol. 73, No. 144, pp. 43492-43541. 

These comments were prepared by Edward Mehnert, Illinois State Geological Survey, 

Champaign, IL. Contact info: mehnert@isgs.illinois.edu or 217/244-2765. 

Technical Comments

p. 43497, IIC: USEPA needs to develop an effective way to handle cross-state boundary issues. 

Cross-state boundary issues could be a problem when dealing with pressure fronts in a setting 

like the Mt. Simon Sandstone, which is present beneath several Midwestern states. In addition, 

many stationary CO2 sources are located along state boundaries (i.e., power plants along major 

rivers). 

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. 

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? The Twin Cities 

Metropolitian Groundwater Flow model 

(http://www.pca.state.mn.us/water/groundwater/metromodel.html) is an example of this concept. 

p.43514, IIIA6 Monitoring Requirements: The complete list of monitoring procedures may not 

be necessary in all settings, such as deep saline aquifers (>5,000 ft below ground surface). I 

hope that these rules will remain flexible such that they can benefit from the research currently 

being conducted by the USDOE-funded partnerships. Their Phase III efforts should provide 

valuable information on the feasibility and value of various monitoring methods. 
Letter ID: 0221 

Commenter: Edward Mehnert 

Organization: Illinois State Geological Survey 

Letter ID: 0221 Comment ID: 21636 Letter Lines: 28 to 33 

Comment Code: 1310 1.1.2.4 Comments on other "out of scope" issues, e.g., pore space 

ownership 

Comment: Technical Comments p. 43497, IIC: USEPA needs to develop an effective way to 

handle cross-state boundary issues. Cross-state boundary issues could be a 

problem when dealing with pressure fronts in a setting like the Mt. Simon 

Sandstone, which is present beneath several Midwestern states. In addition, 

many stationary CO2 sources are located along state boundaries (i.e., power 

plants along major rivers). 

Individual Response: EPA acknowledges that CO2 plumes may cross state or even international 

boundaries and encourages cooperation and coordination among permitting authorities. The final 

Class VI 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 

and ensure that they are involved in any necessary processes; this new requirement was added to 

the final rule at §146.82(b). The Agency encourages transparency in the permitting process and 

anticipates that State-State/State-Tribal communication on GS permitting will facilitate 

information sharing and encourage safe, protective projects. 

Letter ID: 0221 Comment ID: 21637 Letter Lines: 35 to 41 

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

Comment Code: 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 ( 

well and the "freshwater" section of the aquifer may be more than 100 miles. 

Individual 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 

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

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

Letter ID: 0221 Comment ID: 21638 Letter Lines: 43 to 52 

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

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? 

The Twin Cities Metropolitian Groundwater Flow model 

(http://www.pca.state.mn.us/water/groundwater/metromodel.html) is an example of 

this concept. 

Individual 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. 

Letter ID: 0221 Comment ID: 21639 Letter Lines: 54 to 59 

Comment Code: 4600 2.6 Proposed Plume and Pressure Front Monitoring Requirements (40 

CFR 146.90) 

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Comment: p.43514, IIIA6 Monitoring Requirements: The complete list of monitoring 

procedures may not be necessary in all settings, such as deep saline aquifers 

(>5,000 ft below ground surface). I hope that these rules will remain flexible 

such that they can benefit from the research currently being conducted by the 

USDOE-funded partnerships. Their Phase III efforts should provide valuable 

information on the feasibility and value of various monitoring methods. 

Individual Response: EPA agrees with commenters and believes that flexibility is important in 

developing and implementing a monitoring plan that is appropriately tailored for a specific GS 

site. The purpose of the requirement at §146.90 to develop site-specific testing and monitoring 

plans is to ensure that the monitoring performed at a site is appropriate for site operational 

conditions and the geologic setting. EPA believes that this is the best approach for addressing 

potential risks at GS sites. These flexible provisions in the monitoring requirements for Class VI 

wells enable the owner or operator to tailor monitoring programs to the specific needs of each 

project in consultation with the Director. 

The final regulation provides for flexibility in: 1) methodologies used to collect these data, 2) the 

spatial resolution at which the data should be collected, and 3) how often the data should be 

collected. The number and placement of monitoring wells will be based on site-specific 

information such as the calculated AoR, injection rate and volume, site geology, baseline data, 

and the presence of artificial penetrations within the AoR. In some instances, there may be a 

limited number of options for complying with EPA's minimum requirements. For example, 

monitoring for geochemical changes will require use of monitoring wells and collection of 

groundwater samples for chemical analysis. Many options currently exist for complying with 

other requirements, such as tracking the position of the plume and pressure front within the 

injection zone. 

EPA believes, however, that implementing a testing and monitoring plan that includes all of the 

required elements under §146.90 is essential to ensuring USDW protection. Please see the 

preamble to the final rule for a complete justification of the elements of a GS testing and 

monitoring regime. 

Flexibility is also provided regarding how to monitor for the specific data required by the final 

rule. The testing and monitoring plan is developed by the owner or operator in consultation with 

the Director and is designed to ensure that a detailed, Director-approved document describing 

how monitoring will be conducted during the lifetime of the project exists. 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. 

EPA is using an adaptive approach to regulating GS. EPA will collect and review project data on 

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early GS projects to determine whether the appropriate amount and types of information and 

appropriate documentation are required and being collected. The Agency will evaluate the 

information and determine if modifications to the requirements are necessary.