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‐ assigned comment code; and an

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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

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• 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: 0288

Commenter: Anonymous7

Letter ID: 0288 Comment ID: 25136 Letter Lines: 17 to 53

Comment Code: 3100 2.1 Proposed Geologic Siting Requirements (40 CFR 146.83)

Comment: COMMENT ON CARBON SEQUESTRATION

Geology, like hydrology is a science of averaged parameter values. When

determining lithologic properties (e.g.. transmissivity, thickness, threedimensional position, etc.) of a geologic stratum, scientists estimate these

properties based on collection of data at a small number points selected from an

infinite number of possible points. The more deeply buried the stratum, the

greater the cost of data collection and, so, the number of data points tends to

diminish with depth. There is no way to know how representa tive the collected

data are of a geologic stratum.

As an example, imagine driving down a road at 60 mph and hitting a hole in the

highway. In 20 million years from now, th at highway likely will be buried under

hundreds of feet of sediment. If a dozen test holes are drilled from the land

surface, though the highway roadbed, the chance that that hole would be

intersected is minuscule, yet that hole in the roadbed may be the most important

feature of the highway.

Not all data are collected from drilling cores, there also are temperature data,

geophysical data, etc. An uncertainty anal ysis is performed to provide a sense

of how reliable the modeling results are, but the bottom line" is that the "weak

links" in the protectivene ss of a confining unit are still likely to be unknown.

While this lack of knowledge is not a le thal blow to modeling for, say, watersupply, these weak links can lead to se rious consequences where the purpose is

modeling the injection of supercritical CO2 fluids.

When we think of very deep strata that are, say, thousands of feet below land

surface, we take some "comfort" in thei r depth. We take this comfort, because

it's difficult to imagine that, absent a d eep mappable fault, a fluid rupturing a

very deep confining layer is likely to reach the land surface. However, where

injection strata and confining units are shallower, our level of comfort

decreases, and rightly so. It is because of the lack of sufficient data to

accurately define the nature of the su bsurface, that EPA needs to take a

protective approach to options for: 1) in jecting into formations in or above the

lowermost USDW, 2) injecting below the lowermost USDW when that USDW is

"relatively" shallow, and 3) injecti ng into a formation overlain by a brittle

confining layer, unless that c onfining layer is very deep.

12 Individual Response: EPA agrees that permitting decisions regarding GS sites must account for

a large amount of site-specific information. For this reason, the GS regul ation requires the owner

or operator to submit data about the site, incl uding information on the porosity, permeability,

mineralogy, and geochemistry of the targeted inj ection and confining zones. Operators must also

present information on the compatibility of the CO2 stream with fluids in the injection zone and

minerals in both the injection and the confining zone(s), as determined through the formation

testing program and on the compatibility of the CO2 with the materials used to construct the

well, as required at §146.82(c)(3).