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 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 fieldbased 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 interpreting results 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 report1), 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 

 

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

assemblages caused by CO2 intrusion into shallow 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. 

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). Our experiences 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. 

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. 

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 four batch experiments (time 0 

represents introduction of CO2 into the batch reactor) 

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 freshwater resources as a result of leakage from CO2 geological storage—a batch-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. 
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 interpreting  
results 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 into  
shallow 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). Our  
experiences 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‐a  
atch‐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 of
modeling. 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.