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### 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 CO 2

. 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 (CO 2 ) 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 input at GS sites rendering it impossible to discriminate changes

in natural biogenic CO 2 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 CO 2 is

adequately sequestered with respect to the biosphere. Research regarding the impact of CO 2

leaks on potable aquifers is focused on, 1) assessing geochemical outcomes of increased CO 2

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

of shallow aquifer geochemistry for CO 2 storage evaluation and, 3) identifying the parameters

best suited for monitoring groundwater resources.The success of using shallow groundwater geochemistry as a tool for CO 2 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 CO 2 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 CO 2 into

a dilute aquifer is an increase in dissolved inorganic carbon (DIC) as alkalinity, H 2 CO 3 or HCO 3 ‐

, 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 CO 2 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 CO 2 than the simple system put forth by previous researchers,

and 2) a background variability that would mask even a relatively large

‐ scale exogenous CO 2

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 CO 2 storage evaluation using shallow aquifers, we targeted a potable

water source (Dockum aquifer) above a long

‐ duration (>35 years) large

‐ scale (

~ 250 km 2 )

engineered CO 2 injection site in west Texas. The SACROC oil field in Scurry County, TX, currently

operated by Kinder Morgan has injected CO 2 for enhanced oil recovery (EOR) since 1972 with

approximately 50% recovery and recycling of injected CO 2

. This site provides the opportunity to

investigate (1) if CO 2 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 CO 2 injection (inside SACROC) to that of groundwater samples collected

from areas that have not experienced CO 2 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 CO 2 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 CO 2 input. In addition, we use stable carbon isotopes to source

‐ identify

dissolved CO 2 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, δ 13 C, 18 O, 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 δ 13 C. 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

‐ CO 2 ‐ 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 CO 2 in Dockum groundwater were identified as; 1)

CO 2 injected into the production zone, 2) CO 2 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 CO 2 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

‐ SO 4 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 ofsimple calcite dissolution. Whereby calcite dissolution is driven by CO 2, dedolomitization is

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

dolomite and oversaturation with respect to calcite. Input of CO 2 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 CO 2 (PCO 2 ) as previous researchers

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

proceeds towards lower HCO 3 ‐ concentrations. Increases in PCO 2 simply shift this overall

trend towards higher HCO 3 ‐ 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 CO 2 exists naturally in an aquifer, stable carbon isotopes

were used to determine if exogenous CO 2 in the Dockum aquifer could be distinguished

from natural CO 2

. 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 CO 2 in the Dockum aquifer. We modeled carbon isotopic and DIC

trends during dedolomitization in open ‐ and closed

‐ systems with and without injectate

and microbial CO 2 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 CO 2 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 CO 2 ‐ EOR site, we

conclude that shallow aquifer geochemistry has limited applicability for monitoring

permanence of CO 2 storage at GS sites. Many natural and man

‐ induced factors create

hydrochemical variability that may mask the effects of CO 2 input in common hydrogeologic

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

assure that interpretations are correct. Assumptions about CO 2 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 CO 2 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 CO 2 storage evaluation at an EOR

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

‐ 03. s

Romanak, K. D., and Smyth, R. C., 2008, Geochemical effects of CO 2 ‐ 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 CO 2 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 CO 2

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 GulfCoast 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 uponaddition 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 spatiallyremoved 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) degassingfrom 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 positiveidentification 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.

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.