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.Letter ID: 0217

Commenter: Olon Plunk Organization: Xcel Energy, Inc.

Letter ID: 0217 Comment ID: 21549 Letter Lines: 64 to 73

Comment Code: 1330 1.1.3.1 Support use of GS in general

Comment: As we studied the feasibility of developi ng an IGCC project with CCS, we learned a great deal about geologic storage of CO 2. We learned that ge ologic storage is currently one of the most important m eans by which power companies can reduce CO2 emissions while still producing reliab le and affordable energy from coal.

International studies as well as studies here in this country attest to the importance of geologic storage of CO2 in any strategy to reduce increasing carbon concentrations in the atmosphe re. The development of a rational regulatory framework to address geologic stor age of CO2 is essential in order to balance the nation's need for energy secu rity, through increased use of domestic energy resources, with concerns over global climate change.

Individual Response: EPA acknowledges the comment. Pleas e see the remainder of EPA’s responses to this comment for further deta ils. For additional information on EPA’s final regulatory approach, please see the final rule and preamble as well as the rulemaking docket.

Letter ID: 0217 Comment ID: 21550 Letter Lines: 75 to 85

Comment Code: 1250 1.1.1.4 Flexibility is needed (p erformance-based standards)

Comment: We also learned, however, that there ar e number of critical policy, legal, and

technical issues that must be resolv ed before geologic storage can be implemented on a full-scale commercial basi s. The proposed rule is a significant early step in helping to resolve thes e issues. The rule, however, must be designed to facilitate, not unduly hi nder, progress towards commercial deployment of geologic storage. Large-sc ale underground injecti on and storage of CO2 remains an emerging technology and it is important that a ny rule adopted by EPA be flexible enough to adapt to adva nces in science and technology moving forward. We offer these comments in suppor t of a flexible and appropriately tailored rule that will facilitate geol ogic storage technol ogy and projects while protecting underground sour ces of drinking water.

Individual Response: EPA clarifies that the Class VI GS requirements are designed to ensure that, where deployed, GS is conducted in a ma nner that protects USDWs from endangerment. Encouraging investment in CCS projects and facilit ating deployment are out side the scope of the GS rulemaking.

To address the comment that the rule should accommodate future technological developments

1related to GS, EPA adds that it is using an ad aptive approach to regulating GS under the UIC Program. Even after finalization of the GS regulat ion, EPA plans, every six years, to collect and review project data on GS research projects to determine whether the appropriate amount and types of information and appropriate documenta tion are being collected, and to determine if modifications to the requirements are appropriate or necessary.

Letter ID: 0217 Comment ID: 21551 Letter Lines: 87 to 100

Comment Code: 1480 1.2.2.2.2 CO2 is not a pollutant

Comment Code: 1680 1.3.8 Don't overstate ris k, don't stigmatize CO2

Comment: CO2 Characterization

CO2 is produced continually by natural pr ocess throughout the natural world and poses no significant hazard to human health when properly managed. EPA is correct to note in the proposed rule that a potential barrier to CCS and geologic storage involves how the captur ed CO2 is treated within existing regulatory frameworks. EPA is incorrect, however, to suggest or presume that captured CO2 is a waste, or that ge ologic storage via a Class VI well constitutes disposal. In seve ral instances, the proposed ru le refers to CO2 as a potential hazardous waste and suggests the need for regulatory requirements for

Class VI wells that exceed those deemed necessary for Class I hazardous waste injection wells. Creating such a regulato ry framework that implies an undue level of risk associated with geol ogic storage is misleading and could stigmatize proposed projects in the eyes of both the public and investors.

Individual Response: EPA clarifies that the GS rule does not address the status of CO2 as either a pollutant or a commodity. In addition, whether or not a fluid could be sold on the market as a commodity does not override EPA's authority under the SDWA to protect USDWs from endangerment. EPA stresses that the focus of the GS rule is to ensure that, where GS is deployed, it is conducted in a manner that protects USDWs.

Under the SDWA, the injection of any "fluid" is subject to the requirements of the UIC program. "Fluid" is defined under 40 CFR 144.3 as any mate rial or substance which flows or moves, whether in a semisolid, liquid, slud ge, gas or other form or state, and it includes liquids, gases, and semisolids (i.e., slurries) in jected into the subsurface.

Under §146.82(a)(7), owners or operators of GS wells must characterize their CO2 stream as part of their permit application. If the injectate is de termined to meet the definition of a hazardous

waste under RCRA, and as defined in 40 CFR Part 261, the stringent UIC Class I hazardous waste requirements for injection of hazardous waste apply. Thus, injection of corrosive CO2 into Class VI wells is prohibited. In addition, owners or operators must peri odically monitor their injectate throughout injecti on operations under §146.90(a).

EPA disagrees that the GS regulation overstates the risk of CO2 injection or implies an undue level of risk. EPA recognizes that injecting large volumes of CO2 for GS poses unique

2challenges that, if not properly managed, could endanger drinking water s ources and have other adverse impacts. However, with proper site sele ction, the final, tailored, Class VI requirements address the large volumes of CO2 injected, its bu oyancy, potential corrosivity in the presence of water, and potential impurities in the CO2 stream. In addition, EP A expects the tailored permitting, siting, construction, operation, monitoring, and post-injection site care requirements in the GS rule to prevent leakage from injection formations and protect USDWs.

EPA does not agree that the tailored requirements will create a negative public perception of GS. The Agency believes that a demonstration to th e public that GS projects are meeting the GS rule's tailored requirements will likely improve c onfidence in the project and help address public questions and concerns.

Regarding the ability of owners or operators to secure investment in GS projects, EPA believes that financing entities will have improved conf idence in projects that incorporate multiple protective requirements.

Letter ID: 0217 Comment ID: 21552 Letter Lines: 102 to 109

Comment Code: 1630 1.3.3 RCRA/hazardous waste i ssues (seeking comment)

Comment Code: 1640 1.3.4 CERCLA issues (seeking comment)

Comment: In their comments regarding the pro posed rule, both the Edison Electric Institute (EEI) and the Utility Solid Wa ste Activity Group (USWAG) present well articulated concerns regarding the legal and regulatory issues that could arise from the inappropriate treat ment of captured CO2 with in the context of the Resource Conservation and Recovery Act, and the Comprehensive Environmental Response, Compensation and Liability Ac t. Both groups describe potential solutions to this issue; Xcel Energy endorses these commen ts and suggested

solutions.

Individual Response: EPA received many comments regardi ng the applicability of the RCRA regulations and statute to CO2 st reams that are geologically seque stered under the final Class VI UIC well requirements. As a result, EPA has deci ded to initiate a sepa rate proposed rulemaking specifically to examine the issue of RCRA appl icability to CO2 streams being geologically sequestered, including the possible option of a conditional exemption from the RCRA requirements for CO2 GS in Class VI wells. EPA will consider these comments as part of its development of that proposal. The Agency will consider issues pa rticular to RCRA in the context of that separate rulemaking, and recommends that the commenter read that proposed rule when it is published. The Agency notes that comment ers who wish to submit comments on the RCRA proposal will need to do so during the comment pe riod for that rule. Finall y, EPA also notes that

the final GS rule, which adds ne w regulations for Class VI wells under the Safe Drinking Water Act (SDWA), is not making any changes to the RCRA regulations.

EPA clarifies that CO2 itself is not listed as a hazardous s ubstance under CERCLA; however, the CO2 stream may contain a listed hazardous s ubstance (such as mercury) or may mobilize substances in the subsurface that could react with ground water to produce listed hazardous

3substances (such as sulfuric acid). Whether su ch substances may result in CERCLA liability from a sequestration facility depe nds entirely on the composition of the specific CO2 stream and

the environmental media in which it is stored (e.g., soil or ground water). CERCLA exempts from liability, under CERCLA section 107, 42 U. S.C. 9607, certain “federally permitted releases” (FPR) as defined in CERCLA, 42 U.S. C. 9601(10). Class VI permits will need to be carefully structured to ensure that they prevent potential releases from the well which are outside the scope of the Class VI permit.

For additional information, please see EPA's res ponses to specific comments by Edison Electric Institute and the Utility Solid Waste Activity Group.

Letter ID: 0217 Comment ID: 21553 Letter Lines: 111 to 143

Comment Code: 1250 1.1.1.4 Flexibility is needed (p erformance-based standards)

Comment: Default Provisions

Although underground injection of CO2 is we ll established for enhanced oil and gas recovery, it remains in it nascent st ages with respect to large-scale geologic storage. Throughout the proposed rule, EPA suggests the need for blanket prohibitions, limitations and default provisions that are overly conservative and not supported by establis hed facts. Such examples include:

- 50-year default post closure period - Mandatory 10-year reevaluation of the Area of Review (AoR) - Categorical exclusion of specific formations (e.g,. coal seams) - Prohibitions on types of well dr illing (e.g., horizontal, angled) - Mandate to always inject below the lowermost USDW - Prohibition on use of exempt aquifers - Prohibition on hydraulic fracturing - Mandatory use of tracers - Mandatory annual and electronic reporting

At this early stage, EPA should avoid establishing arbitrary limits or prohibitions that might stifle research and development, or that are so excessively conservative as to close th e door on geologic stor age projects that are otherwise safe and economically sound. The final rule must recognize that the potential risks, as well as the appro aches to mitigate or eliminate that risk, will vary greatly from project to project. The broad imposition of administrative and technical requirement s designed to be protective of the highest risk projects unnecessa rily burden low to moderate risk projects. Viable projects might not be proposed or move forward due to excessive economic burdens or regulatory uncertainty. EPA should avoi d a "one size fits a ll" approach and instead develop a framework that provides the flexibility for projects to be successfully permitted, based on each project's unique characteristics and

circumstances.

4Long-Term Liability

Individual Response: The Agency believes that the Class VI requirements balance site-specific flexibility with USDW protection. Rather than setting performance or technology-based standards for GS projects, EPA is adopting a flex ible approach, where operators of GS projects would develop and implement site-specific plans for delineating the AoR, testing and monitoring the site, performing corrective actio n, and post-injection site care . EPA believes that the plans— which would be reviewed and approved by the Director and periodical ly updated throughout the life of the project—promote an ongoing dialogue between the operato r and the Director that will encourage use of the most appropriate t echnologies for a particular project.

EPA clarifies that the Class VI GS requirements are designed to ensure that, where deployed, GS is conducted in a manner that protects USDWs fr om endangerment. Encouraging investment in CCS projects and facilitating deployment are outside the scope of the GS rulemaking.

Letter ID: 0217 Comment ID: 21554 Letter Lines: 145 to 155

Comment Code: 5460 2.9.2 Comments on transfer of lia bility for long-term care

Comment: Owners and operators must have a means to bound and quantify th e potential longterm liabilities associated with a geologic storage project, otherwise, it could be impossible to secure the necessary in surance and financing. As a matter of national policy, the success of geologic st orage is essential to our future energy security and climate mitigation st rategies. These projects provide a public benefit. As such, the regulatory framework should provide a mechanism whereby the long-term liability for a properly sited, permitted, operated and closed project can be capped or quantif ied, and then later transferred or shared. Unless the potential open-ended a nd long-term liability imposed by the proposed rule is resolved, it is question able whether geologic storage can be implemented on a large-scale basis.

Individual Response: EPA acknowledges the commenter’s con cern regarding the applicability of multiple statutes to GS projects and the need for a single clear liability regime for owners or operators. EPA is working to create a nationally c onsistent financial responsibility regime within its authority to ensure that fi nancial resources are available to protect USDWs throughout the life of the GS project. However, the authority under the SDWA does not extend to release owners or operators from long-term responsib ility for the potential economic costs of impacts unrelated to USDWs. EPA does not have authority to transfer liability from one entity to another and SDWA does not provide for the creation of a third party fi nancial instrument where EPA is the trustee. In the UIC program, liability under SDWA §1423 remain s with the owner or operator for all well classes until all regulatory requirements have been met. EPA understa nds that Congress is considering legislation that may address the liabi lity issue. EPA clarifies that the GS regulation does not promote or encourage the use of GS; rath er it is designed to ensure consistency in permitting underground injection of CO2 at GS operations across the United States and provide requirements to prevent endangerment of USDWs in anticipation of the eventual use of GS to

5reduce CO2 emissions.

EPA disagrees that caps should be placed on fi nancial responsibility requirements in the proposed rule. A cap may result in the endangerm ent of a USDW if the appropriate amount of money provided under the financial in strument is less then what is needed to protect the USDW.

EPA believes that the instruments chosen for fina ncial responsibility must cover the complete

costs of these activities.

Letter ID: 0217 Comment ID: 21555 Letter Lines: 157 to 164

Comment Code: 5430 2.9.1 Comments on proposed financia l responsibility requirements (40 CFR 146.85)

Comment Code: 5432 2.9.1.1 Need to adjust financial responsibility mechanisms (seeking comment)

Comment Code: 5434 2.9.1.2 Need for additional instruments (seeking comment)

Comment: Financial Assurance

Xcel Energy agrees that some method of financial assurance is appropriate for geologic storage projects. However, it is important to retain the concepts of both self-assurance and phased-assurance. The amount of financial assurance required should be limited to what is know n, or can be estimated with reasonable accuracy, today. Financial assurance shoul d be periodically adjusted as new

information becomes available.

Individual Response: EPA will allow owners or operators to use one or more or multiple financial responsibility instruments for well plug ging and site closure. However, EPA will not allow for a separate financial re sponsibility demonstration for well plugging and PISC; this must be done at the time of permit app lication. This will ensure that resources are available to prevent the endangerment of USDWs in the event that the owner or operator experiences financial difficulties at any time during the life of the GS project.

EPA agrees that it is importan t to periodically update cost estimates that support financial responsibility determinations. EP A clarifies that the GS rule, at §146.85(c), requires the owner or operator to provide annual updates to the cost estimate to account for any amendments to the AoR and corrective action plan, the injection well plugging plan, the PISC and site closure plan, and the emergency and remedial response plan. Th e owner or operator must also update the cost estimate annually for inflation. EPA adds that the Director can also require the owner or operator to provide an adjustment of the cost estimate if the Director determines that the original demonstration is no longer adequa te, as specified at §146.85(e).

EPA will retain the use of self-insurance as an instrument for financial responsibility for Class VI wells. Owner or operators may self insure th rough a financial test prov ided certain conditions are met such as demonstration of profitability and stability. Furtherm ore, EPA is developing guidance that describes the type s of financial instruments, including self insurance, which owners or operators can use to meet financia l responsibility requirements for GS wells. The

6financial responsibility guidance will include considerations fo r Directors regarding how to evaluate the appropriateness of a specific fina ncial responsibility instrument for a specific project. This guidance is av ailable on EPA's website at http://water.epa.gov/type/groundwat er/uic/wells_sequest ration.cfm. EPA notes that Directors may disallow certain instruments.

Letter ID: 0217 Comment ID: 21556 Letter Lines: 166 to 176

Comment Code: 5600 2.11 Comments related to UIC Program primacy for Class VI wells

Comment Code: 5610 2.11.1 Allowing partial primacy for Class VI wells (seeking comment)

Comment: State Primacy

Xcel Energy supports the concept of st ate primacy. States should have the ability to obtain delegated authority for Class VI wells only, as opposed to being required to develop delegable programs for all cl asses of wells. As noted by EEI and USWAG, permitting decisions are best made at the state level where knowledge of local conditions and public co ncerns is the greatest. In addition, it is essential that the final rule establish a "lead state" concept for projects where the AoR may cross state boundaries. Xcel Energy supports assigning the project permitting lead role to the state where the Class VI

wellhead is located.

Individual Response: EPA agrees that states should be al lowed to obtain primacy for Class VI wells independently of other well classes, and wi ll allow independent primacy for Class VI wells under §145.1(i) of the final GS rule. In the past , EPA has not accepted UIC primacy applications

for individual well classes unde r Section 1422 of SDWA. In other words, if a State wanted primacy for Class I wells, the State would also ne ed to accept responsibilit y for Class III, IV, and V wells. This policy has been in place since the initiation of the UIC Program in the early 1980s and was intended to encourage states to take full responsibility fo r UIC programs, avoid duplication of efforts, and provide for administrative efficiencies.

However, given the potential for CCS (encom passing capture, transportation, and geologic sequestration) to be an important part of a na tional priority to address climate change, EPA believes that providing states the flexibility to obtain primacy for Class VI wells independently of other well classes can help foster effective management of CCS. EPA believes that allowing independent Class VI primacy may encourage states to take res ponsibility for implementation of Class VI regulations and provide for a more co mprehensive approach to managing CCS projects by promoting the integration of GS activities under SDWA in to a broader framework for managing CCS. Furthermore, GS operations involve ancillary activities (e.g., pipeline operations, pore space ownership, land use rights, a nd surface access) for which states can call upon other authorities that exist at the state leve l (but outside UIC author ity) to provide a more comprehensive CCS management approach.

EPA also believes that allowing states to apply only for Class VI primacy will shorten the primacy approval process for states. EPA's willingness to accept independent primacy

7applications for Class VI wells represents a shift in its longstandi ng policy of discouraging "partial primacy." It is very important to note that this shift in policy allowing for Class VI primacy does not apply to any other well Cl ass under Section 1422. EPA will continue to discourage applications for pa rtial primacy for all other well types under Section 1422.

EPA clarifies that states should be able to obtain primacy for Cla ss VI wells independent of other well classes provided they devel op regulations that are at leas t as stringent as the minimum Federal requirements (e.g., permitting, inspec tion, operation, monitoring, and recordkeeping requirements) in the final GS rule. EPA believes that existing programs can adapt their expertise to the new requirements for Class VI wells and w ill be able to develop a primacy application for

Class VI wells under section 1422 of SDWA and en sure that their GS pr ograms integrate into national climate change priorities an d protect USDWs from endangerment.

EPA clarifies that issuance a nd oversight of a GS permit, including any associated AoR boundary related coordination, is the responsibility of the primacy (Sta te, Tribal or Territorial) or regional DI program in the state where the inje ction well is located. EP A understands that the CO2 plumes for large GS projects may cross Stat e, Tribal, Territorial, EP A Regional, or national boundaries. The final GS rule, at 146.82(a)(2), re quires the owner or opera tor to identify all

State, Tribal, and Territorial bounda ries 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 146.82(b) and ensure that they are involved in any necessary processes (e.g., development of the emergency and remedial re sponse plan (146.94)). 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: 0217 Comment ID: 21557 Letter Lines: 178 to 191

Comment Code: 3120 2.1.2 Need to identify additional confining and containment zones (seeking comment)

Comment: Secondary Containment

Requiring secondary containment as a matte r of standard practice is unnecessary and will likely significantly decrease th e number of geologic storage sites available. Such a requirement exceeds the standard for Class I hazardous waste injection wells, creating the logical, yet erroneous inference that CO2 injection in a Class VI well poses a great er risk than Class I injection of a hazardous waste. Given the proposed rule's basic premise that the design of a Class VI well and the designated receiving formation must be protective of USDWs in order to be permitted in the firs t instance, the need for secondary containment is redundant and unnecessary. Secondary containment should remain only as an option to be proposed by the project applicant in instances where legitimate concerns over the primary cont ainment capabilities of the receiving formation preclude permitti ng in the normal manner.

8Individual Response: EPA clarifies that secondary confini ng zones are not required for all GS sites. Rather, secondary confini ng zones are required when the Di rector determines that these zones are needed to maximize the protection of USDWs from endangerment. EPA retains the requirement to identify additional containment/confining zones, if required by the Director, as proposed (e.g., §146.83(b)). EPA believes that, in some situations, the presence of additional confining and containment zones above the pr imary (i.e., lowermost) confining zone can improve USDW protection. The Agency acknowledge s that the additional confining formations may also, in site-specific circumstances, fu rther impede vertical fluid movement, provide additional sites for monitoring, mitigation, and reme diation, and allow for pressure dissipation. EPA disagrees that the requirement would reduce the number of sites available for GS or that the requirement implies that CO2 poses a greater thre at than Class I hazardous wells. The intent of this requirement is to ensure that a proposed site provides adequate protection of USDWs from endangerment; if this protection cannot be demonstrated, the Dir ector would not issue a permit

for GS at the site.

Letter ID: 0217 Comment ID: 21558 Letter Lines: 193 to 199

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

Comment: Corrective Actions/AoR

Xcel Energy is concerned th at the term "corre ctive action" as used broadly in the proposed rule infers to the public th at a problem has occurred. We concur

with USWAG's comment that it would be be tter to refer specifically to "well plugging and repair" to avoid stigmatizi ng projects for what are in fact deliberate and proactive preventive measur es. In addition, Xcel Energy advocates

Individual Response: EPA notes that the term "corrective ac tion" at 146.84(d) has been used to describe the process of addre ssing improperly plugged wells in th e 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).

Letter ID: 0217 Comment ID: 21559 Letter Lines: 199 to 210

Comment Code: 3630 2.2.4.1 Comments in favor of allowing phased corrective action

Comment: deliberate and proactive preventive measur es. In addition, Xcel Energy advocates a phased approach to well plugging and repa ir, 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,

9repair, or replace these pre-existing we lls or take other corrective action where the pre-existing well owner or responsible party has an existing obligation to do so under the law. Current or past owners or op erators of preexisting wells should not be rewarded for ignoring their responsibilities under the law, and should be penalized for fa ilure to implement already required corrective action in a timely manner. Finall y, the rule should explicitly allow

Individual Response: EPA clarifies that the purpose of th e 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 be tween USDWs receive appr opriate 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 subjec t to applicable civil, criminal , or administrative penalties.

EPA clarifies that, if monitoring or modeling data indicate a cha nge in the AoR, the owner or operator would need to review and amend the AoR and corrective action plan and follow the amended plan.

Letter ID: 0217 Comment ID: 21560 Letter Lines: 210 to 212

Comment Code: 3500 2.2.3 Area of Review Reevaluation (seeking comment)

Comment: corrective action in a timely manner. Finall y, the rule should explicitly allow for future expansion of the AoR if nece ssary, provided the objective to protect

USDWs continues to be met.

Individual Response: EPA acknowledges that the AoR for a GS project may change over time, and for this reason, the rule requ ires owner or operator to pe riodically 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 inje ction volumes and associated size of CO2 plumes, as well as to addre ss potential uncertainty in AoR modeling.

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 consid ered adjustments to address changes that could not have been anticipated before injection commences.

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