### 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.

### The importance of considering rock-water interactio n when studying potential

impacts of geological CO 2 

### sequestration on drinking water resources

Changbing Yang, Jean-Philippe Nicot, Rebecca C. Smy th, Katherine D. Romanak, and

Susan D. Hovorka

Summary

It is of fundamental importance that water-rock rea ctions be taken into account in the

interpretation of ground water chemistry. Not consi dering the rock contributions to the

rock-water-CO 2 

### reactive system can lead to incorrect extrapolatio ns and overemphasize

the risk to water.

Conclusions of modeling exercises must also be seri ously weighted by the soundness of

the model assumptions. For example, we think that t he role of sulfides in terms of risk to

water as a consequence of CO 2 

### leakage in the report of Birkholzer et al. (2008) has been

overemphasized, as these species are not the minera ls commonly observed in fresh-water

aquifers.

Introduction

Researchers from the Bureau of Economic Geology (BE G) at the University of Texas at

Austin are conducting experimental and modeling stu dies relevant to geologic

sequestration of CO 2 

### (CO2GS) with specific regard to potential impacts to shallow

groundwater resources. In addition, BEG researchers have recently completed a fieldbased groundwater sampling study at the SACROC oilf ield in west Texas and are

continuing research on CO2GS at the Cranfield oilfi eld in Mississippi. Other researchers

in the field of geologic carbon sequestration have recently published work on potential

impacts to water resources as a result of CO 2 

### injection directly into deep brine

formations, or by accidental release of injectate C O 2 

### into drinking water resources either

through laboratory experiments or groundwater model ing studies.

Themes we want to emphasize as being critical to st udies of potential impacts to drinking

water resources from CO 2 

### injection are:

• CO 2 

### occurs naturally in shallow aquifers. CO 2 

### concentrations in a single aquifer

can vary temporally and spatially as a result of ag ricultural and industrial

practices. For example, Scanlon et al. (2005, 2007) have documented enhanced

recharge from land use changes. Enhanced recharge o f groundwater through the

vadose zone can potentially change CO 2 

### concentrations in the saturated zone.

• The capacity for carbonate minerals (carbonate rock s and carbonate cement in

clastic rocks) to buffering changes in pH with infl ux of CO 2 

### is under represented

in the literature (i.e. Carroll et al., 2009). An e xception to this statement is

Keating et al. (2009) who documented nicely the buf fering capacity of carbonate

cement in a high-CO 2 

### concentration natural analog system in New Mexico.

• Realistic rock-water reactions are critical to accu rate prediction of impacts to

groundwater as a result of CO 2 

### injection (numerous articles referenced herein).

For example, Romanak et al. (in preparation) have a lso documented the

importance of carbonates but in a more complex fash ion through the incongruent

### dissolution of dolomite as a process dominating the hydrochemistry of the

Dockum aquifer that overlies the SACROC oilfield in west Texas.

The following text is a review of recently publishe d studies on potential impacts to

groundwater from GS with some examples from BEG’s o n-going and soon to be

published work. We start by citing published work o n the importance of rock-water

interactions in both deep and shallow aquifers for interpreting results of groundwater

modeling. We then focus on one specific study (Birk holzer et al., 2008) that has been

completed by researchers at the Lawrence Berkeley N ational Laboratories (LBNL).

CO 2 

### -water-rock interactions discussed in literature

Rock-water reactions are ubiquitous in groundwater systems and cannot be excluded

from models used to predict impacts to groundwater from introduction of CO 2 

### . Work of

Kaszuba et al. (2003) concludes that because of the importance of rock-water reactions in

potential geologic sequestration systems, much more than simple aqueous dissolution of

CO 2 

### in water must be factored into any realistic model of potential impacts to deep

formation fluids or freshwater aquifers from introd uction of anthropogenic CO 2



### .

Earlier work by Zhu and Burden (2001) states that t he complexity of aquifer fluid

chemistry is defined by the degree of interaction w ith aquifer mineralogy, which includes

solid matrix, grain-coatings and colloids. They are critical of groundwater modeling

efforts that only consider pore-fluid compositions and ignore or greatly simplify aquifer

mineralogy. Conclusions from Zhu and Burden (2001) are that contaminant fate and

transport model predictions depend upon mineral com position of the aquifer. In fact, they

state that the amount of calcite in the aquifer mat rix determines the distance of

contaminant plume migration.

Wigand et al. (2008) go on to state that not only d oes introduction of supercritical CO 2 

### into geologic systems affect brine composition thro ugh aqueous dissolution, it also

affects the intensity of rock-water interactions. T he phenomenon of increased rock-water

interaction with increase in dissolved CO 2 

### in brine is becoming common knowledge

through DOE/NETL- and EPA-sponsored research on geo logic sequestration (i.e.,

Keating et al. 2009).

Review of LBNL shallow groundwater modeling

LBNL (Birkholzer et al., 2008) evaluated mobilizati on of a set of trace elements (called

hazardous constituents in the report 1 

### ), such as arsenic, barium, cadmium, mercury, lead,

uranium and zinc from a set of mineral species that may potentially be mobilized in the

event of CO 2 

### leak from the deep storage reservoirs using a comp rehensive computational

model. The mineral assemblages in the aquifer rocks were selected indirectly upon

evaluation of 38,000 groundwater samples from aquif ers throughout the United States,

retrieved from the Unites States National Water Inf ormation System (NWIS). The report

concluded that some trace elements may be mobilized and exceed the maximum

1 We disagree with the term “hazardous constituents” used in the Birkholzer et al., (2008) report becau se

the trace elements occur in groundwater naturally e ven if their concentrations may increase owing to C O 2

intrusion.

### contamination levels (MCL) resulting from enhanced dissolution of the mineral

assemblages caused by CO 2 

### intrusion into shallow groundwater aquifers. Howev er, the

model results rely on a fundamental assumption with large uncertainties in model

parameters. We disagree with the assumption for con structing the EQ3/6 models and the

methodology they used for investigating mobilizatio n of trace elements from the aquifer

rocks. We believe that this report illustrates the trappings of solely using groundwater

compositions to estimate impact of CO 2 

### on the system.

The fundamental assumption in the Birkholzer et al. (2008) report for constructing the

EQ3/6 models is that mobilization of trace elements is mainly dominated by enhanced

dissolution of a set of specific mineral assemblage s (for instances, sulfides in Table 3.6,

p37 in the report). This set of specific mineral as semblages was selected from

mineralogical analysis of coal seam aquifers, which formed in very strong reducing

environments. Based on such an assumption, the auth ors calculated saturation indices of

the selected mineral assemblages over chemical anal yses of 38000 groundwater samples

and determined whether a specific mineral phase wou ld be present in a rock in

equilibrium with the water sample. In a second step , assuming this phase is present, the

authors computed the amount of trace elements that could be released under addition of

CO 2 

### . This assumption is unreasonable and not well supp orted at all in the literature.

There are many other factors that could affect mobi lization of the tracer elements, such as

aqueous complexation (by ligands), ad/desorption fr om metal oxide and clay surfaces,

and solid solutions in common minerals (carbonates, feldspars, and clay minerals). For

instance, behavior and contribution of arsenic in n atural water are related to MeOx (either

simple desorption in oxidizing conditions or reduci ng conditions) (for example, Smedley

and Kinniburgh, 2002, Scanlon et al. 2009).

BEG researchers did a set of batch experiments by r eacting rock samples taken from

major aquifers in Texas with water and CO 2 

### gas at a pressure of 1 atm (Lu et al., 2009,

Smith etal. 2009). They found that concentrations o f some trace elements may be

elevated after CO 2 

### intrusion, such as arsenic (Figure 1). However, no evidences of XRD

and SEM analysis indicated that arsenopyrite existe d in the rock samples. Lead, which

according to the Birkholzer (2008) could be the mos t problematic element after CO 2 

### intrusion, however, is non-detectable in the 10 bat ch experiments. Of the 114 analyses of

potable groundwater in the vicinity of the SACROC o ilfield in west Texas, 100 samples

(88% of the total samples) had Pb concentrations be low a detection limit of 0.0002

milligrams per liter (mg/L) (Smyth et al., in prepa ration). Our experiences indicate that

Pb in drinking water resources commonly comes from corrosion of submersible water

pumps and accompanying conduit pipes and not from t he aquifer rocks.

In conclusion, the methodology employed in the Birk holzer (2008) report on potential

impacts of CO 2 

### addition to shallow groundwater resources by using only numerical

modeling techniques is questionable and may be misl eading if the controlling minerals/or

controlling phases assumed in the numerical models are not the right ones. In addition,

large uncertainties in the model parameters, such a s Eh values and solubility constants

may lead to wrong conclusions. Regulations of CCS b ased on such conclusions drawn

### from the model results presented in the Birkholzer (2008) report could be biased and may

be impractical when applied to actual sites.

1.E-01 1.E+00 1.E+01 1.E+02

-12 -8 -4 0 4 8 12 16

Time (days)As ppb Batch 1

Batch 2

Batch 3

Batch 4



### Figure 1. Evolution of arsenic concentration in fou r batch experiments (time 0

represents introduction of CO 2 

### into the batch reactor)

References

Birkholzer, J., 2008, Research project on CO2 geolo gical storage and groundwater

resources: water quality effects caused by CO2 intr usion into shallow groundwater:

Technical Report LBNL-1251E, 352p.

Carroll, S., Hao, Y., and Aines, R., 2009, Geochemi cal detection of carbon dioxide in

dilute aquifers: Geochemical Transactions 10:4; doi : 10.1186/1467-4866-10-4.

Kaszuba, J.P., Janecky, D. R., Snow, M.G., 2003, Ca rbon dioxide reaction processes in a

model brine aquifer at 200 C and 200 bars: implicat ions for geologic sequestration of

carbon: Applied Geochemistry, v. 18, p. 1065-1080.

Keating, HE, Julianna Fessenden, Nancy Kanjorski, D aniel J. Koning, and Rajesh Pawar,

2009, The impact of CO2 on shallow groundwater chem istry: observations at a natural

analog site and implications for carbon sequestrati on, under review.

### Lu, J., Partin JW, Hovorka, SD, Wong, Corinne Wong (2009) Potential risks to freshwater resources as a result of leakage from CO 2 

### geological storage—a batch-reaction

experiment, Environmental Geology, under review.

Scanlon BR, Reedy RC, Stonestrom DA, Prudic DE, Den nehy KF, 2005, Impact of land

use and land cover change on groundwater recharge a nd quality in the southwestern

USA. Global Change Biol 11: 1577-1593

Scanlon BR, Reedy RC, Tachovsky JA, 2007, Semiarid unsaturated zone chloride

profiles: Archives of past land use change impacts on water resources in the southern

High Plains, United States. Water Resour Res 43: Ar t. No. W06423

Scanlon, B. R., Nicot, J. -P., Reedy, R. C., Kurtzm an, D., Mukherjee, A., and Nordstrom,

D. K., 2009, Elevated naturally occurring arsenic i n a semiarid oxidizing system:

Southern High Plains aquifer, Texas, USA: Applied G eochemistry,

doi:10.1016/j.apgeochem.2009.08.0004.

Shiraki, M., and Dunn, T.L., 2000, Experimental stu dy on water-rock interactions during

CO2 flooding in the Tensleep Formation, Wyoming, U. S.A.:Applied Geochemistry, v.

15, p. 265-279.

Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and

distribution of arsenic in natural waters. Appl. Ge ochem. 17, 517–568.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Pa rtin, C. Wong, C. Yang, 2009,

Assessing risk to fresh water resources from long t erm CO2 injection – laboratory and

field studies, Energy Procedia, p1957-1964.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Pa rtin, C. Wong, and C. Yang, 2009,

Hydrogeology of Dockum groundwater near the SACROC CO2 EOR oilfield, Scurry

County, Texas, in preparation.

Wigand, M., Carey, J.W., Schutt, H., Spangenberg, E ., and Erzinger, J., 2008,

Geochemical effects of CO2 sequestration in sandsto nes under simulated in situ

conditions of deep saline aquifers: Applied Geochem istry, v. 23, p. 2735-2745.

Zhu, C. and Burden, D. S., 2001, Mineralogical comp ositions of aquifer matrix as

necessary initial conditions in reactive contaminan t transport models: Contaminant

Hydrology, v. 51, p. 145-161.Letter ID: 0371

Commenter: Changbing Yang

Organization: University of Texas at Austin, Bureau of Economic Geology (BEG)

Letter ID: 0371 Comment ID: 27948 Letter Lines: 18 to 253

Comment Code: 2400 2.4. Other Research Offered by Commenters (seeking comment)

Comment: It is of fundamental importance that water

‐ rock reactions be taken into account

in the interpretation of ground water chemistry. Not considering the rock

contributions to the rockwater

‐ CO2 reactive system can lead to incorrect

extrapolations and overemphasize the risk to water.

Conclusions of modeling exercises must also be seriously weighted by the

soundness of the model assumptions. For example, we think that the role of

sulfides in terms of risk to water as a consequence of CO2 leakage in the report

of Birkholzer et al. (2008) has been overemphasized, as these species are not

the minerals commonly observed in fresh

‐ water aquifers.

The importance of considering rock

‐ water interaction when studying potential

impacts of geological CO2 sequestration on drinking water resources

Changbing Yang, Jean

‐ Philippe Nicot, Rebecca C. Smyth, Katherine D. Romanak, and

Susan D. Hovorka

Summary

It is of fundamental importance that water

‐ rock reactions be taken into account

in the interpretation of ground water chemistry. Not considering the rock

contributions to the rock

‐ water

‐ CO2 reactive system can lead to incorrect

extrapolations and overemphasize the risk to water.

Conclusions of modeling exercises must also be seriously weighted by the

soundness of the model assumptions. For example, we think that the role of

sulfides in terms of risk to water as a consequence of CO2 leakage in the report

of Birkholzer et al. (2008) has been overemphasized, as these species are not

the minerals commonly observed in fresh

‐ water aquifers.Introduction

Researchers from the Bureau of Economic Geology (BEG) at the University of Texas

at Austin are conducting experimental and modeling studies relevant to geologic

sequestration of CO2 (CO2GS) with specific regard to potential impacts to

shallow groundwater resources. In addition, BEG researchers have recently

completed a field

‐ based groundwater sampling study at the SACROC oilfield in

west Texas and are continuing research on CO2GS at the Cranfield oilfield in

Mississippi. Other researchers in the field of geologic carbon sequestration

have recently published work on potential impacts to water resources as a result

of CO2 injection directly into deep brine formations, or by accidental release

of injectate CO2 into drinking water resources either through laboratory

experiments or groundwater modeling studies.

Themes we want to emphasize as being critical to studies of potential impacts to

drinking water resources from CO2 injection are:

‐ CO2 occurs naturally in shallow aquifers. CO2 concentrations in a single

aquifer can vary temporally and spatially as a result of agricultural and

industrial practices. For example, Scanlon et al. (2005, 2007) have documented

enhanced recharge from land use changes. Enhanced recharge of groundwater

through the vadose zone can potentially change CO2 concentrations in the

saturated zone.

‐ The capacity for carbonate minerals (carbonate rocks and carbonate cement in

clastic rocks) to buffering changes in pH with influx of CO2 is under

represented in the literature (i.e. Carroll et al., 2009). An exception to this

statement is Keating et al. (2009) who documented nicely the buffering capacity

of carbonate cement in a high

‐ CO2 concentration natural analog system in New

Mexico.

‐ Realistic rock

‐ water reactions are critical to accurate prediction of impacts

to groundwater as a result of CO2 injection (numerous articles referenced

herein). For example, Romanak et al. (in preparation) have also documented the

importance of carbonates but in a more complex fashion through the incongruent

dissolution of dolomite as a process dominating the hydrochemistry of the Dockum

aquifer that overlies the SACROC oilfield in west Texas.

The following text is a review of recently published studies on potential

impacts to groundwater from GS with some examples from BEG’s on

‐ going and soon

to be published work. We start by citing published work on the importance of

rock

‐ water interactions in both deep and shallow aquifers for interpretingresults of groundwater modeling. We then focus on one specific study (Birkholzer

et al., 2008) that has been completed by researchers at the Lawrence Berkeley

National Laboratories (LBNL).

CO2

‐ water

‐ rock interactions discussed in literature

Rock

‐ water reactions are ubiquitous in groundwater systems and cannot be

excluded from models used to predict impacts to groundwater from introduction of

CO2. Work of Kaszuba et al. (2003) concludes that because of the importance of

rock

‐ water reactions in potential geologic sequestration systems, much more than

simple aqueous dissolution of CO2 in water must be factored into any realistic

model of potential impacts to deep formation fluids or freshwater aquifers from

introduction of anthropogenic CO2.

Earlier work by Zhu and Burden (2001) states that the complexity of aquifer

fluid chemistry is defined by the degree of interaction with aquifer mineralogy,

which includes solid matrix, grain

‐ coatings and colloids. They are critical of

groundwater modeling efforts that only consider pore

‐ fluid compositions and

ignore or greatly simplify aquifer mineralogy. Conclusions from Zhu and Burden

(2001) are that contaminant fate and transport model predictions depend upon

mineral composition of the aquifer. In fact, they state that the amount of

calcite in the aquifer matrix determines the distance of contaminant plume

migration.

Wigand et al. (2008) go on to state that not only does introduction of

supercritical CO2 into geologic systems affect brine composition through aqueous

dissolution, it also affects the intensity of rock

‐ water interactions. The

phenomenon of increased rock

‐ water interaction with increase in dissolved CO2 in

brine is becoming common knowledge through DOE/NETL

‐ and EPA

‐ sponsored research

on geologic sequestration (i.e., Keating et al. 2009).

Review of LBNL shallow groundwater modeling LBNL (Birkholzer et al., 2008)

evaluated mobilization of a set of trace elements (called hazardous constituents

in the report [Footnote1]), such as arsenic, barium, cadmium, mercury, lead,

uranium and zinc from a set of mineral species that may potentially be mobilized

in the event of CO2 leak from the deep storage reservoirs using a comprehensive

computational model. The mineral assemblages in the aquifer rocks were selected

indirectly upon evaluation of 38,000 groundwater samples from aquifers

throughout the United States, retrieved from the Unites States National Water

Information System (NWIS). The report concluded that some trace elements may be

mobilized and exceed the maximum contamination levels (MCL) resulting from

enhanced dissolution of the mineral assemblages caused by CO2 intrusion intoshallow groundwater aquifers. However, the model results rely on a fundamental

assumption with large uncertainties in model parameters. We disagree with the

assumption for constructing the EQ3/6 models and the methodology they used for

investigating mobilization of trace elements from the aquifer rocks. We believe

that this report illustrates the trappings of solely using groundwater

compositions to estimate impact of CO2 on the system.

[Footnote 1]: We disagree with the term "hazardous constituents" used in the

Birkholzer et al., (2008) report because the trace elements occur in groundwater

naturally even if their concentrations may increase owing to CO2 intrusion.

The fundamental assumption in the Birkholzer et al. (2008) report for

constructing the EQ3/6 models is that mobilization of trace elements is mainly

dominated by enhanced dissolution of a set of specific mineral assemblages (for

instances, sulfides in Table 3.6, p37 in the report). This set of specific

mineral assemblages was selected from mineralogical analysis of coal seam

aquifers, which formed in very strong reducing environments. Based on such an

assumption, the authors calculated saturation indices of the selected mineral

assemblages over chemical analyses of 38000 groundwater samples and determined

whether a specific mineral phase would be present in a rock in equilibrium with

the water sample. In a second step, assuming this phase is present, the authors

computed the amount of trace elements that could be released under addition of

CO2. This assumption is unreasonable and not well supported at all in the

literature. There are many other factors that could affect mobilization of the

tracer elements, such as aqueous complexation (by ligands), ad/desorption from

metal oxide and clay surfaces, and solid solutions in common minerals

(carbonates, feldspars, and clay minerals). For instance, behavior and

contribution of arsenic in natural water are related to MeOx (either simple

desorption in oxidizing conditions or reducing conditions) (for example, Smedley

and Kinniburgh, 2002, Scanlon et al. 2009).

BEG researchers did a set of batch experiments by reacting rock samples taken

from major aquifers in Texas with water and CO2 gas at a pressure of 1 atm (Lu

et al., 2009, Smith etal. 2009). They found that concentrations of some trace

elements may be elevated after CO2 intrusion, such as arsenic (Figure 1).

However, no evidences of XRD and SEM analysis indicated that arsenopyrite

existed in the rock samples. Lead, which according to the Birkholzer (2008)

could be the most problematic element after CO2 intrusion, however, is non ‐

detectable in the 10 batch experiments. Of the 114 analyses of potable

groundwater in the vicinity of the SACROC oilfield in west Texas, 100 samples

(88% of the total samples) had Pb concentrations below a detection limit of

0.0002 milligrams per liter (mg/L) (Smyth et al., in preparation). Ourexperiences indicate that Pb in drinking water resources commonly comes from

corrosion of submersible water pumps and accompanying conduit pipes and not from

the aquifer rocks.

[PLEASE SEE P. 4 of EPA

‐ HQ

‐ OW

‐ 2008

‐ 0390

‐ 0371.PDF FOR IMAGE OF FIGURE 1]

Figure 1: Evolution of arsenic concentration in

four batch experiments (time 0 represents introduction of CO2 into the batch

reactor)

In conclusion, the methodology employed in the Birkholzer (2008) report on

potential impacts of CO2 addition to shallow groundwater resources by using only

numerical modeling techniques is questionable and may be misleading if the

controlling minerals/or controlling phases assumed in the numerical models are

not the right ones. In addition, large uncertainties in the model parameters,

such as Eh values and solubility constants may lead to wrong conclusions.

Regulations of CCS based on such conclusions drawn from the model results

presented in the Birkholzer (2008) report could be biased and may be impractical

when applied to actual sites.

References

Birkholzer, J., 2008, Research project on CO2 geological storage and groundwater

resources: water quality effects caused by CO2 intrusion into shallow

groundwater: Technical Report LBNL

‐ 1251E, 352p.

Carroll, S., Hao, Y., and Aines, R., 2009, Geochemical detection of carbon

dioxide in dilute aquifers: Geochemical Transactions 10:4; doi:

10.1186/1467

‐ 4866

‐ 10

‐ 4.

Kaszuba, J.P., Janecky, D. R., Snow, M.G., 2003, Carbon dioxide reaction

processes in a model brine aquifer at 200 C and 200 bars: implications for

geologic sequestration of carbon: Applied Geochemistry, v. 18, p. 1065

‐ 1080.

Keating, HE, Julianna Fessenden, Nancy Kanjorski, Daniel J. Koning, and Rajesh

Pawar, 2009, The impact of CO2 on shallow groundwater chemistry: observations at

a natural analog site and implications for carbon sequestration, under review.

Lu, J., Partin JW, Hovorka, SD, Wong, Corinne Wong (2009) Potential risks to

fresh

‐ water resources as a result of leakage from CO2 geological storage

‐ aatch

‐ reaction experiment, Environmental Geology, under review.

Scanlon BR, Reedy RC, Stonestrom DA, Prudic DE, Dennehy KF, 2005, Impact of land

use and land cover change on groundwater recharge and quality in the

southwestern USA. Global Change Biol 11: 1577

‐ 1593

Scanlon BR, Reedy RC, Tachovsky JA, 2007, Semiarid unsaturated zone chloride

profiles: Archives of past land use change impacts on water resources in the

southern High Plains, United States. Water Resour Res 43: Art. No. W06423

Scanlon, B. R., Nicot, J.

‐ P., Reedy, R. C., Kurtzman, D., Mukherjee, A., and

Nordstrom, D. K., 2009, Elevated naturally occurring arsenic in a semiarid

oxidizing system: Southern High Plains aquifer, Texas, USA: Applied

Geochemistry, doi:10.1016/j.apgeochem.2009.08.0004.

Shiraki, M., and Dunn, T.L., 2000, Experimental study on water

‐ rock interactions

during CO2 flooding in the Tensleep Formation, Wyoming, U.S.A.:Applied

Geochemistry, v. 15, p. 265

‐ 279.

Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and

distribution of arsenic in natural waters. Appl. Geochem. 17, 517

‐ 568.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Partin, C. Wong, C. Yang,

2009, Assessing risk to fresh water resources from long term CO2 injection

‐

laboratory and field studies, Energy Procedia, p1957

‐ 1964.

Smyth, R., S. Hovorka, J. Lu, K. Romanak, J. Partin, C. Wong, and C. Yang,

2009, Hydrogeology of Dockum groundwater near the SACROC CO2 EOR oilfield,

Scurry County, Texas, in preparation.

Wigand, M., Carey, J.W., Schutt, H., Spangenberg, E., and Erzinger, J., 2008,

Geochemical effects of CO2 sequestration in sandstones under simulated in situ

conditions of deep saline aquifers: Applied Geochemistry, v. 23, p. 2735

‐ 2745.

Zhu, C. and Burden, D. S., 2001, Mineralogical compositions of aquifer matrix as

necessary initial conditions in reactive contaminant transport models:

Contaminant Hydrology, v. 51, p. 145

‐ 161.

Individual Response: EPA has reviewed the paper on “The importance of considering rock

‐ water

interaction when studying potential impacts of geological CO2 sequestration on drinking water

resources.” EPA acknowledges the submittal and will continue to review studies related to the

evaluation of field results and the limitations of sampling and analysis, as well as the limitations ofmodeling. EPA adds that is has been monitoring the progress at SACROC, among other GS projects. EPA

intends to continue to collect data on GS and to incorporate such data and information as it implements

its adaptive approach to the GS rulemaking to incorporate new research, data, and information about

GS. For more information on the adaptive approach, see the responses to comments on the proposed

rule in the rulemaking docket.