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appropriately addressed.

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Responses on each topic area are compiled and are also available in the rulemaking docket.
Executive Summary: Recent results of the SACROC  

Groundwater Geochemistry Study

Romanak, K.D., Smyth, R.C., Yang, C., Hovorka, S.D., Nicot, J.P., Lu, J.

Gulf Coast Carbon Center, Bureau of Economic Geology, The University of Texas at Austin

Executive Summary

The purpose of this document is to report a summary of recent results and research of the

SACROC groundwater quality study and their relation to geologic sequestration (GS) of CO2. The

research was conducted by scientists at the Gulf Coast Carbon Center (GCCC) as part of the

NETL Southwest Carbon Partnership Phase II (SWP) research. The results of this study were not

available before the publication of the July 25, 2008, proposed rule,  “Federal Requirements

Under the Underground Injection Control (UIC) Program for Carbon Dioxide (CO2) Geologic

Sequestration (GS)  Wells'' (73 FR 43492). The work is currently being written for peer review

publication in Applied Geochemistry to be submitted by the end of 2009, and is useful for

consideration of amendments to the Safe Water Drinking Act. Additional details can be found in

Smyth et. al., 2009; Romanak et. al., 2008 and 2009; and in the document, Modeling shallow

groundwater geochemistry and carbon isotopes: test of methodology for CO2 storage evaluation

at an EOR site, west Texas, USA, published at http://www.beg.utexas.edu/gccc/.  

Our examination of groundwater geochemistry at a site where CO2 has been injected for

decades shows that monitoring of natural parameters (pH, alkalinity, etc.) in shallow aquifers

may be impractical and/or ineffective for evaluation of CO2 storage permanence.  At the very

least, detailed analysis of background variability and processes controlling groundwater

geochemistry would be required on a case by case basis to confirm correct interpretations. Our

analysis shows that to detect possible CO2 leakage into an underground source of drinking

water (USDW) it is important to characterize the hydrogeologic context.  Sampling at a few

wells without understanding the hydrogeologic context could lead to erroneous

interpretations. Errors could be either at sites where CO2 leakage has occurred but is not

detectable or at sites where non‐injection induced changes are mistakenly assumed to indicate

leakage. We have found that background variability in aquifers may mask the effects of even

moderate anthropogenic CO2 input at GS sites rendering it impossible to discriminate changes

in natural biogenic CO2 from anthropogenic inputs.  

Introduction

Monitoring groundwater resources above geologic carbon sequestration reservoirs is proposed

for ensuring that potable water supplies remain protected and also for establishing that CO2 is

adequately sequestered with respect to the biosphere. Research regarding the impact of CO2

leaks on potable aquifers is focused on, 1) assessing geochemical outcomes of increased CO2

and their effects on drinking water quality, 2) determining the potential for widespread usage

of shallow aquifer geochemistry for CO2 storage evaluation and, 3) identifying the parameters

best suited for monitoring groundwater resources.  
The success of using shallow groundwater geochemistry as a tool for CO2 storage evaluation

depends upon the ability to identify one or more geochemical parameters sensitive enough to

provide a recognizable geochemical shift upon addition of exogenous CO2 to the aquifer. This

requires a shift that is larger than normal background variability and is therefore detectable as

an indicator of change. Widespread application of these methods in a variety of GS locations

also necessitates that the type and magnitude of geochemical shift is similar or at least

predictable in any geochemical environment even though processes and mineralogies differ

widely among aquifers.    

Indeed the position of many researchers is that a predictable consequence of input of CO2 into

a dilute aquifer is an increase in dissolved inorganic carbon (DIC) as alkalinity, H2CO3 or HCO3

‐

, a

decrease in pH, and mobilization of cations including toxic metals from mineral dissolution and

desorption reactions (Assayag et. al., 2009; Apps, et. al., 2009; Carroll et. al., 2009; Wang and

Jaffe, 2004).  A popular suggestion is that these parameters are therefore suitable for

monitoring groundwater resources.  The results of the SACROC ground water study, which is

the only field study to date of potable water at a large‐scale industrial CO2 storage analogue,

give strong evidence against such assumptions. At SACROC, natural and man‐induced processes

that are common in many shallow aquifers have created; 1) a geochemical system that is more

complex and reacts differently to CO2 than the simple system put forth by previous researchers,

and 2) a background variability that would mask even a relatively large‐scale exogenous CO2

input.  The implications are that GS monitoring strategies using natural groundwater

constituents must, at the very least, be site‐specific and may not be useful at all owing to the

variability and complexity of groundwater systems.

Research Objectives and Approach

To test methodologies for CO2 storage evaluation using shallow aquifers, we targeted a potable

water source (Dockum aquifer) above a long‐duration (>35 years) large‐scale (~250 km2

)

engineered CO2 injection site in west Texas.  The SACROC oil field in Scurry County, TX, currently

operated by Kinder Morgan has injected CO2 for enhanced oil recovery (EOR) since 1972 with

approximately 50% recovery and recycling of injected CO2. This site provides the opportunity to

investigate (1) if CO2 from over 35 years of injection can be identified in the shallow aquifer and

(2) if groundwater chemistry can be effectively used as a monitoring tool at sequestration sites.  

To accomplish our objectives, we compared the groundwater geochemistry of samples

collected in the area of CO2 injection (inside SACROC) to that of groundwater samples collected

from areas that have not experienced CO2 injection (outside SACROC). In addition, we utilize

regional historical data from the Texas Water Development Board (TWDB) collected either prior

to or spatially removed from any influence of engineered CO2 injection from the SACROC or

nearby Sharon Ridge Field to establish regional background variability in geochemistry.  

Geochemical perturbations lying outside the normal range of groundwater variability could

signal anthropogenic CO2 input. In addition, we use stable carbon isotopes to source‐identify

dissolved CO2 to distinguish natural microbial inputs from anthropogenic ones.   
Methods

Approximately 113 fresh to slightly saline groundwater samples within Triassic‐age Dockum and

8 samples from Permian‐age evaporite deposits were collected from 60 private water wells (50‐ 

to 500‐ft‐deep) and 1 spring during five water sampling trips, June 2006 ‐ November 2008.

Samples were analyzed in the field for alkalinity, dissolved oxygen, conductivity, pH, and

temperature and in the laboratory for major cations and anions, trace elements, δ13C, 18O, and

2

H isotopes, dissolved organic and inorganic carbon, and headspace gases. Ten samples of

brines co‐produced by Kinder Morgan from the 6,000‐ to 7,000‐ft injection/production zone

were also analyzed for these parameters. Additional geochemical data on co‐produced brines

were provided by Kinder Morgan and historical records for Dockum and Permian water

geochemistry collected by the Texas Water Development Board between 1936 and 2008 from

eight Texas counties were also added to the data set.  Carbon isotopes of CO2 being injected for

enhanced oil recovery, soil organic matter, and soil carbonate from Dockum cores were

collected and analyzed for δ13C. Two samples of Dockum core from depth intervals of 450‐460

ft and 370‐380 ft were examined for mineralogy using SEM with aid of an energy dispersive X‐

ray system which can identify chemical composition of point area.

Results and Discussion

Consideration of regional geology and stratigraphy of the area, geochemical trends of collected

data, and past and present farming and pre‐CO2‐EOR practices, was used to identify

background geochemical influences on Dockum geochemistry. The major influences are; 1)

mixing of Dockum groundwater with underlying Permian evaporite waters along a well‐

documented Permian dissolution zone (i.e., Dutton, 1989), 2) leaching into the Dockum of

historically‐produced brine formerly stored in unlined surface pits, 3) temporal geochemical

variations related to Dockum aquifer pumping and local irrigation practices, 4) groundwater

interaction with spatially and compositionally variable stratigraphic units, 5) cation exchange,

and 6) dedolomitization. Potential sources of CO2 in Dockum groundwater were identified as; 1)

CO2 injected into the production zone, 2) CO2 produced from microbial activities, and 3)

degassing from rock‐water reactions.  

The findings at SACROC are pertinent to a wide variety of hydrogeologic environments where

GS sites may be implemented and therefore must be considered when developing a near‐

surface monitoring program. They illustrate the importance of understanding hydrochemical

context and background variability to implementing monitoring protocol. These points are

further illustrated below:

1. Geochemical Context ‐ The geochemical driving forces of the hydrochemical system must

be defined before the consequences of CO2 input can be predicted. In the Dockum aquifer,

as well as in many other aquifers, mixing of concentrated waters (in this case Na‐Cl brines

with cation exchange and Ca‐SO4 Permian evaporite waters) drives dedolomitization.

Dissolution of dolomite and precipitation of calcite (dedolomitization) fueled by calcium

ions is widely documented and common in many hydrologic systems (Pacheco and Szocs,

2006; Back and Baedecker, 1989; Back et. al., 1979; Plummer and Back, 1980; Hanshaw

and Back, 1985) and creates a significantly different carbonate environment than that of
simple calcite dissolution. Whereby calcite dissolution is driven by CO2, dedolomitization is

driven by calcium ions which create undersaturation of the system with respect to

dolomite and oversaturation with respect to calcite. Input of CO2 into these very different

systems has been modeled to produce very different outcomes. In the simple system, DIC

increases with increases in the partial pressures of CO2 (PCO2) as previous researchers

have predicted. However, in the case of dedolomitization, evolution of groundwater

proceeds towards lower HCO3‐ concentrations. Increases in PCO2 simply shift this overall

trend towards higher HCO3‐ but retain the same overall evolutionary trajectories. This

complex geochemical reaction would yield inaccurate conclusions if dedolomitization had

not been identified as a dominant process.   

2. Background variability ‐ Because CO2 exists naturally in an aquifer, stable carbon isotopes

were used to determine if exogenous CO2 in the Dockum aquifer could be distinguished

from natural CO2. Stable carbon isotope ratios of various carbon sources in the system, the

PHREEQC numerical code, and isotope mass balance equations were used to; 1) model

isotopic trends during dedolomitization under open and closed systems and, 2) identify the

most likely source of CO2 in the Dockum aquifer. We modeled carbon isotopic and DIC

trends during dedolomitization in open‐ and closed‐systems with and without injectate

and microbial CO2 input. Whereas isotopic trends for calcite dissolution are

straightforward, isotopic trends during dedolomitization are complex. When coupled with

high background geochemical variability such as in the Dockum aquifer system, stable

carbon isotopes do not yield definitive results. Hence stable carbon isotopes do not appear

to be the best tool for positive identification of injectate CO2 in the Dockum aquifer – and

possibly in other complex aquifer systems as well.

Conclusions

Based on a field study and modeling of a long‐running, large‐scale engineered CO2‐EOR site, we

conclude that shallow aquifer geochemistry has limited applicability for monitoring

permanence of CO2 storage at GS sites. Many natural and man‐induced factors create

hydrochemical variability that may mask the effects of CO2 input in common hydrogeologic

environments. Detailed costly studies of each geochemical system would be necessary to

assure that interpretations are correct. Assumptions about CO2 leakage formed by observation

of geochemical parameters without knowledge of controlling geochemical processes would

lead to incorrect conclusions.  

References

Apps, J. A., Zhang, Y., Zheng, L., Xu, T., Birkholzer, J.T., 2009, Identification of thermodynamic

controls defining the concentrations of hazardous elements in potable ground waters and the

potential impact of increasing carbon dioxide partial pressure. Energy Procedia 1(1): 1917‐1924.

Assayag, N., Matter, J., Ader, M., Goldberg, D., and Agrinier, P., 2009, Water‐rock interactions

during a CO2 injection field‐test: Implications on host rock dissolution and alteration effects.

Chemical Geology 265(1‐2): 227‐235.
Back, W., Hanshaw, B.B., Pyle, T.E. Plummer, L.N., and Weidie A.E., 1979, Geochemical

significance of groundwater discharge and carbonate solution to the formation of Caleta Xel Ha,

Quintana Roo, Mexico. Water Resources Research, 15(6): 1521–1535.

   

Back, W. and Baedecker, M. J., 1989, Chemical hydrogeology in natural and contaminated

environments. Journal of Hydrology, 106(1‐2): 1‐28.

   

Back, W., Hanshaw, B.B., Plummer, L.N., Rahn, P.H., Rightmire, C.T., and Rubin, M., 1983,

Process and rate of dedolomitization: mass transfer and 14C dating in a regional carbonate

aquifer. Geological Society of America Bulletin, v. 94, p. 1415‐1429.

   

Carroll, S., Hao, Y, and Aines, R., 2008, Geochemical detection of CO2 in dilute aquifers.

Geochemical Transactions, 10: 4.  

Dutton, A.R., 1989, Hydrogeochemical processes involved in salt‐dissolution zones, Texas

panhandle, U.S.A.: Hydrological Processes, v. 3, p. 75‐89.

Hanshaw, B. B. and Back, W., 1985, Deciphering hydrological systems by means of geochemical

processes. Hydrological Sciences Journal, 30(2): 257‐271.

Pacheco, F. A. L. and Szocs, T., 2006, Dedolomitization reactions driven by anthropogenic

activity on loessy sediments, SW Hungary. Applied Geochemistry 21(4): 614‐631.

Plummer, L. N. and Back, W., 1980, The mass balance approach; application to interpreting the

chemical evolution of hydrologic systems. Am J Sci 280(2): 130‐142.

Romanak, K. D., Smyth, R. C., Yang, C., and Hovorka, S. D., 2009, Modeling shallow groundwater

geochemistry and carbon isotopes: test of methodology for CO2 storage evaluation at an EOR

site, West Texas, USA (abs.): Eos, v. 90, no. 22, Abstract V13C‐03.

Romanak, K. D., and Smyth, R. C., 2008, Geochemical effects of CO2‐enhanced oil recovery on

the shallow Dockum aquifer, west Texas (abs.), in 7th Annual Conference on Carbon Capture &

Sequestration, May 5th–8th, 2008, Pittsburgh, Pennsylvania.

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

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

field studies: Energy Procedia, v. 1, p. 1957‐1964. 

Wang, S. and Jaffe, P.R., 2004, Dissolution of a mineral phase in potable aquifers due to CO2

releases from deep formations; effect of dissolution kinetics. Energy Conversion and

Management 45(18‐19): 2833‐2848.
Letter ID: 0370  

Commenter: Katherine Romanak  

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

Center (GCCC)  

Letter ID: 0370 Comment ID: 27947 Letter Lines: 17 to 288  

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

Comment: The attached document details results of the DOE/NETL‐funded (through SWP) field  

study of drinking water resources at the SACROC oilfield in west Texas. This  

study, which has been conducted by researchers of the Bureau of Economic  

Geology, Jackson School of Geosciences at The University of Texas at Austin  

(BEG), satisfies the LBNL suggestion that field studies be conducted to assess  

impacts to drinking water resources from CO2 injection. Conclusions from this  

study are included in several comments being submitted by BEG researchers.  

In summary, BEG researchers have not detected injectate CO2 in the Dockum  

aquifer at the SACROC oilfield. However, our findings do suggest that monitoring  

of natural parameters (pH, alkalinity, cations, etc.) in shallow aquifers may  

not be effective for evaluation of CO2 storage permanence. Knowledge of  

background variability and an understanding of processes controlling groundwater  

geochemistry are necessary for making correct interpretations regarding the  

presence of sequestered CO2 in aquifers at GS sites. Regardless, there are no  

discernible impacts to drinking water resources as a result of CO2 injection at  

the SACROC oilfield.  

Executive Summary: Recent results of the SACROC Groundwater Geochemistry Study  

Romanak, K.D., Smyth, R.C., Yang, C., Hovorka, S.D., Nicot, J.P., Lu, J. Gulf  

Coast Carbon Center, Bureau of Economic Geology, The University of Texas at  

Austin  

   

Executive Summary   

The purpose of this document is to report a summary of recent results and  

research of the SACROC groundwater quality study and their relation to geologic  

sequestration (GS) of CO2. The research was conducted by scientists at the Gulf  
Coast Carbon Center (GCCC) as part of the NETL Southwest Carbon Partnership  

Phase II (SWP) research. The results of this study were not available before the  

publication of the July 25, 2008, proposed rule,  "Federal Requirements Under  

the Underground Injection Control (UIC) Program for Carbon Dioxide (CO2)  

Geologic Sequestration (GS)  Wells'' (73 FR 43492). The work is currently being  

written for peer review publication in Applied Geochemistry to be submitted by  

the end of 2009, and is useful for consideration of amendments to the Safe Water  

Drinking Act. Additional details can be found in Smyth et. al., 2009; Romanak  

et. al., 2008 and 2009; and in the document, Modeling shallow groundwater  

geochemistry and carbon isotopes: test of methodology for CO2storage evaluation  

at an EOR site, west Texas, USA, published at http://www.beg.utexas.edu/gccc/.  

Our examination of groundwater geochemistry at a site where CO2 has been  

injected for decades shows that monitoring of natural parameters (pH,  

alkalinity, etc.) in shallow aquifers may be impractical and/or ineffective for  

evaluation of CO2 storage permanence.  At the very least, detailed analysis of  

background variability and processes controlling groundwater geochemistry would  

be required on a case by case basis to confirm correct interpretations. Our  

analysis shows that to detect possible CO2 leakage into an underground source of  

drinking water (USDW) it is important to characterize the hydrogeologic context.  

Sampling at a few wells without understanding the hydrogeologic context could  

lead to erroneous interpretations. Errors could be either at sites where CO2  

leakage has occurred but is not detectable or at sites where non?injection  

induced changes are mistakenly assumed to indicate leakage. We have found that  

background variability in aquifers may mask the effects of even moderate  

anthropogenic CO2 input at GS sites rendering it impossible to discriminate  

changes in natural biogenic CO2 from anthropogenic inputs.  

Introduction  

Monitoring groundwater resources above geologic carbon sequestration reservoirs  

is proposed for ensuring that potable water supplies remain protected and also  

for establishing that CO2 is adequately sequestered with respect to the  

biosphere. Research regarding the impact of CO2 leaks on potable aquifers is  

focused on, 1) assessing geochemical outcomes of increased CO2 and their effects  

on drinking water quality, 2) determining the potential for widespread usage of  

shallow aquifer geochemistry for CO2 storage evaluation and, 3) identifying the  

parameters best suited for monitoring groundwater resources.  

The success of using shallow groundwater geochemistry as a tool for CO2 storage  

evaluation depends upon the ability to identify one or more geochemical  

parameters sensitive enough to provide a recognizable geochemical shift upon  
addition of exogenous CO2 to the aquifer. This requires a shift that is larger  

than normal background variability and is therefore detectable as an indicator  

of change. Widespread application of these methods in a variety of GS locations  

also necessitates that the type and magnitude of geochemical shift is similar or  

at least predictable in any geochemical environment even though processes and  

mineralogies differ widely among aquifers.  

Indeed the position of many researchers is that a predictable consequence of  

input of CO2 into a dilute aquifer is an increase in dissolved inorganic carbon  

(DIC) as alkalinity, H2CO3or HCO3?, a decrease in pH, and mobilization of  

cations including toxic metals from mineral dissolution and desorption reactions  

(Assayag et. al., 2009; Apps, et. al., 2009; Carroll et. al., 2009; Wang and  

Jaffe, 2004).  A popular suggestion is that these parameters are therefore  

suitable for monitoring groundwater resources.  The results of the SACROC ground  

water study, which is the only field study to date of potable water at a large‐  

scale industrial CO2 storage analogue, give strong evidence against such  

assumptions. At SACROC, natural and man‐induced processes that are common in  

many shallow aquifers have created; 1) a geochemical system that is more complex  

and reacts differently to CO2 than the simple system put forth by previous  

researchers, and 2) a background variability that would mask even a relatively  

large‐scale exogenous CO2 input.  The implications are that GS monitoring  

strategies using natural groundwater constituents must, at the very least, be  

site‐specific and may not be useful at all owing to the variability and  

complexity of groundwater systems.  

Research Objectives and Approach  

To test methodologies for CO2 storage evaluation using shallow aquifers, we  

targeted a potable water source (Dockum aquifer) above a long‐duration (>35  

years) large‐scale (~250 km2) engineered CO2 injection site in west Texas.  The  

SACROC oil field in Scurry County, TX, currently operated by Kinder Morgan has  

injected CO2 for enhanced oil recovery (EOR) since 1972 with approximately 50%  

recovery and recycling of injected CO2. This site provides the opportunity to  

investigate (1) if CO2 from over 35 years of injection can be identified in the  

shallow aquifer and (2) if groundwater chemistry can be effectively used as a  

monitoring tool at sequestration sites.  

To accomplish our objectives, we compared the groundwater geochemistry of  

samples collected in the area of CO2 injection (inside SACROC) to that of  

groundwater samples collected from areas that have not experienced CO2 injection  

(outside SACROC). In addition, we utilize regional historical data from the  

Texas Water Development Board (TWDB) collected either prior to or spatially  
removed from any influence of engineered CO2 injection from the SACROC or nearby  

Sharon Ridge Field to establish regional background variability in geochemistry.  

Geochemical perturbations lying outside the normal range of groundwater  

variability could signal anthropogenic CO2 input. In addition, we use stable  

carbon isotopes to source‐identify dissolved CO2 to distinguish natural  

microbial inputs from anthropogenic ones.  

Methods  

Approximately 113 fresh to slightly saline groundwater samples within Triassic‐  

age Dockum and 8 samples from Permian‐age evaporite deposits were collected from  

60 private water wells (50‐ to 500‐ft?deep) and 1 spring during five water  

sampling trips, June 2006 ‐ November 2008. Samples were analyzed in the field  

for alkalinity, dissolved oxygen, conductivity, pH, and temperature and in the  

laboratory for major cations and anions, trace elements, delta^13carbon,^18 O,  

and 2^Hydrogen H isotopes, dissolved organic and inorganic carbon, and headspace  

gases. Ten samples of brines co‐produced by Kinder Morgan from the 6,000‐ to  

7,000‐ft injection/production zone were also analyzed for these parameters.  

Additional geochemical data on co‐produced brines were provided by Kinder Morgan  

and historical records for Dockum and Permian water geochemistry collected by  

the Texas Water Development Board between 1936 and 2008 from eight Texas  

counties were also added to the data set.  Carbon isotopes of CO2 being injected  

for enhanced oil recovery, soil organic matter, and soil carbonate from Dockum  

cores were collected and analyzed for delta^13carbon Two samples of Dockum core  

from depth intervals of 450?460 ft and 370‐380 ft were examined for mineralogy  

using SEM with aid of an energy dispersive X‐ ray system which can identify  

chemical composition of point area.  

Results and Discussion  

Consideration of regional geology and stratigraphy of the area, geochemical  

trends of collected data, and past and present farming and pre‐CO2‐EOR  

practices, was used to identify background geochemical influences on Dockum  

geochemistry. The major influences are; 1) mixing of Dockum groundwater with  

underlying Permian evaporite waters along a well? documented Permian dissolution  

zone (i.e., Dutton, 1989), 2) leaching into the Dockum of historically?produced  

brine formerly stored in unlined surface pits, 3) temporal geochemical  

variations related to Dockum aquifer pumping and local irrigation practices, 4)  

groundwater interaction with spatially and compositionally variable  

stratigraphic units, 5) cation exchange, and 6) dedolomitization. Potential  

sources of CO2 in Dockum groundwater were identified as; 1) CO2 injected into  

the production zone, 2) CO2 produced from microbial activities, and 3) degassing  
from rock‐water reactions.  

The findings at SACROC are pertinent to a wide variety of hydrogeologic  

environments where GS sites may be implemented and therefore must be considered  

when developing a near? surface monitoring program. They illustrate the  

importance of understanding hydrochemical context and background variability to  

implementing monitoring protocol. These points are further illustrated below:  

1. Geochemical Context ‐ The geochemical driving forces of the hydrochemical  

system must be defined before the consequences of CO2 input can be predicted. In  

the Dockum aquifer, as well as in many other aquifers, mixing of concentrated  

waters (in this case Na‐Cl brines with cation exchange and Ca‐SO4 Permian  

evaporite waters) drives dedolomitization. Dissolution of dolomite and  

precipitation of calcite (dedolomitization) fueled by calcium ions is widely  

documented and common in many hydrologic systems (Pacheco and Szocs, 2006; Back  

and Baedecker, 1989; Back et. al., 1979; Plummer and Back, 1980; Hanshaw and  

Back, 1985) and creates a significantly different carbonate environment than  

that of simple calcite dissolution. Whereby calcite dissolution is driven by  

CO2, dedolomitization is driven by calcium ions which create undersaturation of  

the system with respect to dolomite and oversaturation with respect to calcite.  

Input of CO2 into these very different systems has been modeled to produce very  

different outcomes. In the simple system, DIC increases with increases in the  

partial pressures of CO2 (PCO2) as previous researchers have predicted. However,  

in the case of dedolomitization, evolution of groundwater proceeds towards lower  

HCO3‐ concentrations. Increases in PCO2 simply shift this overall trend towards  

higher HCO3‐ but retain the same overall evolutionary trajectories. This complex  

geochemical reaction would yield inaccurate conclusions if dedolomitization had  

not been identified as a dominant process.  

2. Background variability ‐ Because CO2 exists naturally in an aquifer, stable  

carbon isotopes were used to determine if exogenous CO2 in the Dockum aquifer  

could be distinguished from natural CO2. Stable carbon isotope ratios of various  

carbon sources in the system, the PHREEQC numerical code, and isotope mass  

balance equations were used to; 1) model isotopic trends during dedolomitization  

under open and closed systems and, 2) identify the most likely source of CO2 in  

the Dockum aquifer. We modeled carbon isotopic and DIC trends during  

dedolomitization in open? and closed?systems with and without injectate and  

microbial CO2 input. Whereas isotopic trends for calcite dissolution are  

straightforward, isotopic trends during dedolomitization are complex. When  

coupled with high background geochemical variability such as in the Dockum  

aquifer system, stable carbon isotopes do not yield definitive results. Hence  

stable carbon isotopes do not appear to be the best tool for positive  
identification of injectate CO2 in the Dockum aquifer ‐ and possibly in other  

complex aquifer systems as well.  

Conclusions  

Based on a field study and modeling of a long‐running, large?scale engineered  

CO2‐EOR site, we conclude that shallow aquifer geochemistry has limited  

applicability for monitoring permanence of CO2 storage at GS sites. Many natural  

and man‐induced factors create hydrochemical variability that may mask the  

effects of CO2 input in common hydrogeologic environments. Detailed costly  

studies of each geochemical system would be necessary to assure that  

interpretations are correct. Assumptions about CO2 leakage formed by observation  

of geochemical parameters without knowledge of controlling geochemical processes  

would lead to incorrect conclusions.  

References  

Apps, J. A., Zhang, Y., Zheng, L., Xu, T., Birkholzer, J.T., 2009,  

Identification of thermodynamic controls defining the concentrations of  

hazardous elements in potable ground waters and the potential impact of  

increasing carbon dioxide partial pressure. Energy Procedia 1(1): 1917‐1924.  

Assayag, N., Matter, J., Ader, M., Goldberg, D., and Agrinier, P., 2009,  

Water?rock interactions during a CO2 injection field‐test: Implications on host  

rock dissolution and alteration effects. Chemical Geology 265(1?2): 227‐235.  

Back, W., Hanshaw, B.B., Pyle, T.E. Plummer, L.N., and Weidie A.E., 1979,  

Geochemical significance of groundwater discharge and carbonate solution to the  

formation of Caleta Xel Ha, Quintana Roo, Mexico. Water Resources Research,  

15(6): 1521–1535.  

Back, W. and Baedecker, M. J., 1989, Chemical hydrogeology in natural and  

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Individual Response: EPA has reviewed the paper on “Recent results of the SACROC Groundwater

Geochemistry Study.” EPA has been monitoring the progress at SACROC, among other GS projects. 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 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.