Applied Geochemistry 30 (2013) 136–147

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Applied Geochemistry

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CO2 leakage impacts on shallow groundwater: Field-scale reactive-transportsimulations informed by observations at a natural analog site

Elizabeth H. Keating a,⇑, J. Alexandra Hakala b, Hari Viswanathan a, J. William Carey a, Rajesh Pawar a,George D. Guthrie b, Julianna Fessenden-Rahn a

a Los Alamos National Laboratory, United Statesb National Energy and Technology Laboratory, United States

a r t i c l e i n f o

Article history:Available online 23 August 2012

0883-2927/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apgeochem.2012.08.007

⇑ Corresponding author. Address: MS T003, EarthDivision, Los Alamos National Laboratory, Los AlamoTel.: +1 505 665 6714; fax: +1 505 665 8737.

E-mail address: ekeating@lanl.gov (E.H. Keating).

a b s t r a c t

It is challenging to predict the degree to which shallow groundwater might be affected by leaks from aCO2 sequestration reservoir, particularly over long time scales and large spatial scales. In this study obser-vations at a CO2 enriched shallow aquifer natural analog were used to develop a predictive model whichis then used to simulate leakage scenarios. This natural analog provides the opportunity to make directfield observations of groundwater chemistry in the presence of elevated CO2, to collect aquifer samplesand expose them to CO2 under controlled conditions in the laboratory, and to test the ability of multi-phase reactive transport models to reproduce measured geochemical trends at the field-scale. The fieldobservations suggest that brackish water entrained with the upwelling CO2 are a more significant sourceof trace metals than in situ mobilization of metals due to exposure to CO2. The study focuses on a singletrace metal of concern at this site: U. Experimental results indicate that cation exchange/adsorption anddissolution/precipitation of calcite containing trace amounts of U are important reactions controlling U ingroundwater at this site, and that the amount of U associated with calcite is fairly well constrained. Sim-ulations incorporating these results into a 3-D multi-phase reactive transport model are able to repro-duce the measured ranges and trends between pH, pCO2, Ca, total C, U and Cl� at the field site.Although the true fluxes at the natural analog site are unknown, the cumulative CO2 flux inferred fromthese simulations are approximately equivalent to 37.8E�3 MT, approximately corresponding to a.001% leak rate for injection at a large (750 MW) power plant. The leakage scenario simulations suggestthat if the leak only persists for a short time the volume of aquifer contaminated by CO2-induced mobi-lization of U will be relatively small, yet persistent over 100 a.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

This paper considers the potential for geologic sequestration ofCO2 to lead to upward leakage of CO2, resulting in impacts to shal-low groundwater resources in overlying aquifers. There are manyquestions to be answered regarding these impacts, such as: howlarge an area would be impacted? Over what time scales? Wouldthe impacts be reversible? Would the changes have health effectson individuals who depend on the resource for drinking water? Re-cently a number of studies have emerged in the literature to ad-dress these issues, most focused on either modeling (Wang andJaffe, 2004; Zheng et al., 2009; Carroll et al., 2009; Wilkin and DiG-iulio, 2010) or laboratory measurements (Smyth et al., 2009; Careyet al., 2009; Lu et al., 2010; Little and Jackson, 2010). It remains tobe seen, however, how applicable these types of studies are to

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explaining field-scale phenomenon. Direct observations of CO2/shallow aquifer interactions have been made in controlled releaseexperiments (Kharaka et al., 2010; Apps et al., 2010) and naturalanalog studies (Evans et al., 2002; Aiuppa et al., 2005; Keatinget al., 2009; Flaathen et al., 2009). Attempting to reconcile observa-tions made at these sites with laboratory and simulation studies isone focus of this paper.

Field studies at a site in northern New Mexico, USA (Keatinget al., 2009) have provided insight into the impacts of CO2 on ashallow aquifer. At this site, natural CO2 upwelling from depth re-sults in very high dissolved CO2 levels in a number of shallowdrinking water wells and, in fact, causes one well to geyser almostpure CO2 gas daily. Groundwater total dissolved solids ranges from301 to 5891 mg/L and is enriched locally in Na, Cl�, HCO�3 and inminor elements such as U, As, Pb and F�. In this paper, data col-lected at this site are used to build a 3-D reactive-transport modelwhich captures the essential geochemical reactions that controlCO2/aquifer interactions at the site and which may determine tracemetal concentrations. Model parameters are, in part, determinedby laboratory measurements on aquifer samples from the site

E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147 137

(Hakala et al., 2009; Carey et al., 2009). In the first step, the modelis partially validated by comparison of model outputs to measuredwater chemistry data at numerous wells. This comparison buildsconfidence that the model is capable of representing, at least tothe first order, impacts of CO2 on groundwater chemistry. In thenext step, the model is applied to simulating a variety of CO2 leak-age scenarios, largely outside the range of conditions which exist atthis natural analog site but which might exist above an actual CO2

sequestration reservoir. These scenarios provide insight into howan aquifer with geochemical characteristics such as the one studiedhere might respond to leaks. The focus is U, a trace element withclear connections to human health risks and which happens tobe common in aquifer sediments and groundwater at the naturalanalog site at Chimayó. Published laboratory studies have shownthat U can be released from aquifer samples when exposed toCO2. While U-bearing minerals are not common in most US drink-ing water aquifers likely to overly CO2 sequestration sites, the geo-chemical issues governing U mobility are very similar to thoseaffecting more common trace metals and so lessons learned at thissite should be transferable to others.

2. Background

Risk assessment of geologic sequestration requires quantitativepredictions of the impact of CO2 and co-contaminants (e.g. H2S,brine) on shallow aquifers. It is likely that no single prediction willbe accurate, for many reasons that will be discussed below. Never-theless, an ensemble of predictions that reflects predictive uncer-tainty can bound the problem, provide sensitivity information,and thus be very useful to risk assessment.

The actual response of an aquifer to a CO2 leak will depend onboth hydrologic processes (multi-phase flow in heterogeneousmedia) and geochemical processes (water/CO2/rock interactions).The details of the response are likely to vary both in time andspace. Decades of research in contaminant transport modelinghas led to the conclusion that spatial variability in aquifer proper-ties, which introduces a largely irreducible uncertainty is best ad-dressed using probabilistic approaches. These methodologies aremature and easily accessible in a number of software packages. Re-search in geochemical modeling, in contrast, has been largely dom-inated by deterministic approaches. These have focused onfundamental thermodynamics of water/mineral interactions, ionexchange, and sorption/desorption reactions. Much of the researchthat supports the selection of appropriate reactions to include in amodel focuses on a single grain (TEM, etc.) or a very small aquifersample interrogated by laboratory experiments. These experimen-tal approaches offer tremendous insight into fundamental pro-cesses, yet the translation of these insights into useful field-scalesimulations is plagued by issues of non-ideal thermodynamics ofmineral phases in real aquifer sediments, geochemical variability,and the very significant observation that apparent reaction kineticrates at field-scales are usually different than those measured inlaboratory experiments (Langmuir, 1997, p. 56; Zhu, 2005).

The most commonly cited risks posed to groundwater are (1)increasing trace metal concentrations, due to CO2 induced pHdepression within the aquifer and (2) increased concentrations ofco-contaminants originating within the sequestration reservoir,including H2S, organics, and brine (which may include trace met-als) (Lemieux, 2011). Several studies of in situ (within the aquifer)trace metal mobilization due to pH depression have appeared inthe literature recently. All laboratory studies (Carey et al., 2009;Smyth et al., 2009; Little and Jackson, 2010; Lu et al., 2010) havefound some degree of measurable impact of CO2 on trace metalconcentrations. Little and Jackson (2010) ran experiments for400 days and found steadily increasing concentrations of Li, Co,

Ba and U and decreases in Mo and As. More complex behaviorwas observed by Smyth et al. (2009), Carey et al. (2009), and Luet al. (2010). Lu et al. (2010) exposed aquifer samples to CO2 fortwo weeks and found two types of responses: ‘‘Type I’’, cationswhose concentrations increased rapidly then increased slowly orremained at a constant value (B, Ba, Ca, Co, K, Mg, Mn, Sr and Zn)and ‘‘Type II’’ cations whose concentrations increased rapidly fol-lowed by a gradual decrease (Al, As, Cs, Cu, Fe, Mo, Ni, Rb, U andV). Carey et al. (2009) exposed aquifer samples to CO2 for 2 weeksand noticed Type I behavior for Ba, Type II behavior for Ca, U andAl, and virtually no change in As.

One hypothesis for the ‘‘Type II’’ behavior is the formation ofnew mineral phases that scavenge trace metals from solution. Thisprocess appears to be important in natural analog sites such asVesuvius, Italy (Aiuppa et al., 2005). The process of metal scaveng-ing can be catalyzed by pH buffering reactions such as silicate orcalcite dissolution. It is unclear, however, what time scales areappropriate for these reactions. Natural analog studies (Aiuppaet al., 2005; Evans et al., 2002; Keating et al., 2009) do not lendthemselves to studying reaction rates since these systems havetypically have been exposed to CO2 for very long periods of time.Controlled release studies, such as Kharaka et al. (2010) and Span-gler et al. (2010) have the potential to test whether or not Type IIbehavior will be important in CO2 leakage scenarios, and, if so, onwhat time scales it might occur. Unfortunately, these types of testsare expensive to conduct and will typically only be conducted forshort time periods, perhaps too short to detect Type II behavior.For example, type I behavior dominated over the 1-month con-trolled CO2 release experiment in Bozeman, Montana (Kharakaet al., 2010) (modest upward trends in As, Cu, Zn, Cd, Pb, Al and Se).

Modeling field-scale trace metal transport is a daunting task be-cause of complex relationships between the fluid and solid phasesand the many model parameters that are difficult to measure in thefield or reliably extrapolate from laboratory experiments. A rangeof approaches has been taken, including decoupling flow fromtransport (Aiuppa et al., 2005; Apps et al., 2010; Keating et al.,2009) and conducting 1-D (Keating et al., 2011) or fully 3-D reac-tive transport modeling (Wang and Jaffe, 2004; Zheng et al.,2009, 2012). The Wang and Jaffe (2004) study, which was entirelytheoretical, emphasized the sensitivity to mineral dissolutionkinetics. The Zheng et al. (2009) study was also largely theoretical,and focused on Pb and As. Their model included very detailed sur-face complexation reactions and mineral dissolution kinetic ratelaws. One interesting conclusion of this model was that adsorp-tion/desorption reactions were much more important than precip-itation/dissolution reactions. No comparisons of simulation outputto in situ water chemistry measurements were provided. Perhapsthe most integrated modeling/laboratory studies/field measure-ment analysis published to date is the trace metal study of Aiuppaet al. (2005) at the CO2-rich natural analog site, Mt. Vesuvius, Italy.It is remarkable that despite extensive laboratory analysis and asophisticated approach to modeling complex geochemistry, onlymodest success was achieved in quantitative agreement betweensimulated and measured trends in trace metal concentrations formany elements, and fairly poor success was obtained for others.The limitations of this very rigorous and site-specific study pro-vides a useful perspective for what can and cannot be achievedby extrapolation of theory and laboratory results to the field-scale.

3. Site overview

The small community of Chimayó is located near the easternmargin of the Espanola Basin, a sub-basin of the Rio Grande Riftin New Mexico, USA. The primary aquifer in the basin is a thick se-quence of Santa Fe Group alluvial sediments. In some locations,

138 E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147

layers of Pennsylvanian meta-sedimentary rocks and carbonateslie beneath the Santa Fe Group. Due to tectonic extension, north–south trending faults are ubiquitous in the basin. Figs. 1 and 2show a geologic map and an idealized cross-section is shown forthe site (Koning et al., 2002; Koning, 2003). Groundwater generallyflows east to west from the recharge zone (Sangre de Cristo Moun-tains) to the regional discharge zone (the Rio Grande). The aquiferis generally unconfined, but due to occasional clay-rich layers, usu-ally laterally discontinuous, confined or semi-confined conditionsoccur locally. Aquifer sediments are quartz and feldspar-rich sand-stones and gravel beds. Secondary calcite is common. Uranium isalso a common element in aquifer sediments (generally in tracequantities but some parts of the basin have U prospects). Manywaters in the basin exceed the EPA standard for U (30 lg/L,1.26e�7 M).

Upwelling of deep, CO2-rich water occurs in the Jemez Moun-tains to the west and in a small area of geothermal springs �25 km to the north (Ojo Caliente). The appearance of high CO2 lev-els in Chimayó groundwater was first documented by Cumming(1997). Stable isotope data analyses (Cumming, 1997; Keatinget al., 2009) have been inconclusive in determining the exactsource of the CO2. A relatively shallow well (�50 m) in the commu-nity geysers almost pure CO2 daily. High-CO2 wells appear to beclustered along ‘‘Robert’s fault’’ (see Fig. 1), which is the inferredconduit for upwelling of deep CO2. A geochemical study of tracemetals in Chimayó groundwaters (Keating et al., 2009) identifiedseveral types of waters: background (no elevated CO2 levels), andtwo types of elevated CO2 waters: brackish and non-brackish. Near

Fig. 1. Site map (Koning et al., 2002; Koning, 2003). Symbols show location of water sa

the southern end of the fault (near the geysering well) the CO2-en-riched waters are brackish. Near the northern end of the fault, CO2-enriched waters are non-brackish. Examination of trace metal(loid)data suggested that high levels of U, Pb and As are transported intothe aquifer with upwelling brackish water rather than being mobi-lized in situ by CO2 influx. This was somewhat unexpected since, inthe case of U, the formation of U–carbonate complexes should re-sult in desorption of U. Apparently, at this site, the entrainment ofmetal-bearing brine with CO2 in layers beneath the aquifer is amore important process than CO2-driven geochemical reactionswithin the aquifer. This hypothesis will be further explored in thispaper, using 3-D reactive transport modeling.

4. Model development

4.1. Multi-phase groundwater flow

The 3-D flow model of the site builds on the conceptual modeland hydrologic data presented by Cumming (1997). The extent ofthe model is indicated in Fig. 1, and was chosen so as to includeall the wells for which water chemistry data are available, includ-ing both CO2-impacted and background wells. An orthogonal meshwas generated for the problem, of dimension 5 � 3.5 � 0.5 km,shown in Fig. 3. East–west resolution varies from 564.8 m at theboundaries to �70 m near the fault. North–south resolution andvertical resolution is uniform (117 m and 30 m, respectively). Mul-ti-phase flow and reactive-transport calculations were performed

mples (Keating et al., 2011; Cumming, 1997). Black rectangle shows model extent.

Fig. 2. Geologic cross-section from Koning et al. (2002) and Koning (2003).

Fig. 3. Location of Robert’s Fault, Santa Cruz River, and sampled wells in relation to the computational mesh. Figure has no vertical exaggeration. Green horizontal planeindicates location of confining bed separating shallow and deep aquifers. Vertical pink bars indicate locations of cross-sections AA0 and BB0 . Simulated CO2 injection occurs atintersection of the vertical projection of the fault (red line) with the base of the model (blue arrow). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Table 1Hydrologic properties. All boundaries and initial conditions: 15 �C.

Hydrologic property Permeability(log10 m2)

Shallow aquifer �12.5Confining unit �16.0Carbonate layer �14.5Fault zone �12.0

Boundary conditionsEastern edge Specified water flux Shallow aquifer 4 kg/s

Deep carbonate 0.8 kg/sWestern edge Specified pressure head Shallow aquifer 1800 m

Deep carbonate 1810 mSanta Cruz River Specified water flux 1 kg/sFault base Specified CO2 flux 0.3 kg/s

Specified water flux 15 kg/sRelative permeability Linear: k = KS

E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147 139

using the simulator FEHM (Finite-Element-Heat-and-Mass-trans-fer) code (Zyvoloski, 1997). The complex stratigraphy shown inFigs. 1 and 2 was simplified to a two-layer system: a shallow rela-tively permeable, sedimentary aquifer underlain by a lower perme-ability carbonate aquifer. The two units are separated by a thinconfining layer, which is broken by a north–south trending, high-permeability conduit representing Robert’s Fault (Fig. 1). Inflowto the two horizontal layers is at the eastern edge (roughly corre-sponding to the contact between the Santa Fe group and the Pre-Cambrian); discharge occurs at the western edge. Downward flowoccurs along the Santa Cruz River, representing dilute stream re-charge. Inflows and permeabilities were specified to achieveapproximately the correct gradient in the shallow aquifer ofapproximately 0.01 m/m, in accordance with estimates providedby Cumming (1997, p. 19), and to produce slightly higher pressuresin the lower units. These parameters are shown in Table 1. Steady-state flow simulations were conducted, followed by transient flowwith CO2 flux applied at the base of the fault.

4.2. Coupled flow/reactive transport

4.2.1. Identification of rock/water reactionsThe specification of reaction networks to include in the simula-

tions is guided by previous batch and 1-D geochemical modeling(Keating et al., 2009, 2011) and laboratory experiments (Hakalaet al., 2009; Carey et al., 2009). The aqueous geochemistry of U

has been the focus of many studies (Hsi and Langmuir, 1985; Lang-muir, 1997; Dong et al., 2005; Fox et al., 2006; Rossberg et al.,2009); many of these have focused on sorption reactions. Uraniumsorbs to mineral surfaces such as ferrihydrite and quartz; however,formation of uranyl–carbonate complexes and U–Ca–CO3 com-plexes can inhibit sorption (Dong et al., 2005; Fox et al., 2006). Be-cause of this, it is reasonable to expect that the introduction of CO2

could cause formation of aqueous U complexes and U desorption.

140 E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147

Additionally, there is also some indication from experiments per-formed by Carey et al. (2009) and from preliminary modeling workby Keating et al. (2009, 2011) on this site that U concentrationsmay be controlled by precipitation/dissolution of calcite containingsmall amounts of U in solid solution.

To further explore possible mechanisms controlling U at thissite, experimental work was performed on outcrop samples col-lected in the Chimayó area, representing both clay-rich and sandyzones within the Santa Fe group (Carey et al., 2009; Hakala et al.,2009). Samples were categorized by lithology (Lithosomes A andB, Fig. 2). First, in order to assess the potential maximum U contentthat could be leached from the aquifer solids, aqua regia digestionswere performed (per extraction schemes described in Shang andZelazny, 2008) on five samples. Sequential extractions were alsoperformed using a technique that targets exchangeable ions andions associated with carbonates (Shang and Zelazny, 2008), as wellas the modified BCR technique, which targets three potentialphases containing trace elements: exchangeable/carbonate, reduc-ible, and oxidizable fractions (Rauret et al., 1999). The goal behindthe different extractions was to determine whether U associatedwith different phases in the heterogeneous Chimayo sedimentscould be distinguished.

For the aqua regia digestions, 0.5 g of dry sample was reactedfor 5 h on a hot stir plate with an aqua regia solution that consistedof 3 mL HCl (trace metal grade) and 1 mL HNO3 (trace metal grade),diluted to 50 mL volume with distilled, de-ionized water. For themodified BCR extractions, in the first step (to probe for U presentin exchangeable/carbonate fractions of the sediments) 1 g of<100 mesh samples were reacted with 40 mL of 0.11 M acetic acid(pH 2.5) in centrifuge vials, and rotated on an end-over-end shakerfor 16 h. The reacted sample was centrifuged, and the supernatantpreserved for chemical analysis by ICP–OES and ICP–MS. This pro-cess was repeated for each sample until the pH of the post-extrac-tion supernatant reached �2.5; the sample subsequently wasrinsed with distilled, deionized water. During the second step (toprobe for U associated with reducibles), 40 mL of 0.05 M hydroxyl-amine HCl solution was added to the residual solids and rotated onan end-over-end shaker for 16 h. The Chimayo samples only re-quired one volume of the hydroxylamine HCl reduction step (Ehelectrode measurements verified that the redox potential for theextraction supernatants were similar to the procedural blank).The samples were centrifuged, and the supernatant preserved foranalysis. Solid residues subsequently were washed with distilled,deionized water prior to the final step. In the final step (to probefor U associated with oxidizables), solid residues were reacted with8 mL of 30% H2O2 for 1 h at room temperature, then at 85 �C for 1 h.One sample (sample A2 in Table 2) showed continued signs of reac-tion at the elevated temperature, and was allowed to react untilvisible signs of reaction had stopped (�3 h). All samples then werereacted with 1.0 M ammonium acetate (pH 2) and rotated on an

Table 2U concentrations (g/kg) and quantitative XRD results for the outcrop samples.

Sample Chimayo aquifer sediment Color inFig. 2

Aquaregia

Modified BCR metho

Exchangeables/carbonates

A1 Lithosome A Mixed (Gray + Red)(Roadcut)

Yellow 1.14E�02

A2 Lithosome A, Gray Isolate(Roadcut)

Yellow 1.08E�01 1.68E�03

A3 Lithosome A, Red Isolate(Roadcut)

Yellow 1.12E�02 7.13E�04

AB Lithosome B (Roadcut) Green 7.18E�03 2.76E�04B1 Lower Lithosome B (Streamcut) Light Blue 9.40E�03 2.64E�04B2 Lithosome B (Streamcut) Green 5.52E�03 1.07E�04

end-over-end shaker for 16 h; the samples were centrifuged, andthe supernatant preserved for analysis.

In the extraction designed to distinguish exchangeable- versuscarbonate-bound U, in the first step 2 g of <100 mesh samplewas reacted with 80 mL of distilled, deionized water and rotatedon an end-over-end shaker for 15 min to isolate U from thewater-soluble salt fraction. The sample bottles were subsequentlycentrifuged, and the supernatant was preserved for chemical anal-ysis. In the second step (to probe for U associated with exchange-ables), the residual solid was reacted with 80 mL of 1 Mammonium acetate (pH 7) and rotated on an end-over-end shakerfor 16 h. After centrifugation, the supernatant was preserved foranalysis and the sample was washed with distilled, de-ionizedwater. In the final step (to probe for U associated with carbonates),the residuals were reacted with 80 mL of 0.11 M acetic acid and ro-tated on an end-over-end shaker for 16 h. After centrifugation, thesupernatant was preserved for analysis.

Results of the digestions and sequential extractions are shown inTable 2. The aqua regia digestions revealed that all samples containU, with group A samples containing an order of magnitude more Uthan group B. Uranium contents ranged from 1.1E�1 to 9.4E�3 gU/kg sample. The sequential extractions using the modified BCRmethod show that less than % 1 of the U can be readily extracted,and, for all but one sample, the exchangeable/carbonate fractioncontains most of the U. The second sets of extractions, focused ondistinguishing between carbonates and exchangebles, showed thatall samples had fairly significant U extracted from carbonates andfor one sample the extractable U came entirely from carbonate.

Additionally, in order to specifically evaluate possible changesin U concentrations due to reactions between CO2-charged ground-water and the aquifer samples, batch reaction experiments wereperformed. Bottles containing size-fractionated splits (40 mesh)of the aquifer samples were suspended in a synthetic ‘‘back-ground’’ groundwater; this water was generated in the laboratoryusing analytical-grade salts and distilled, de-ionized water basedon the chemistry reported for Well 15 in Keating et al. (2009). Inorder to evaluate the contribution of CO2 towards releasing U inthese experiments, the fluid chemistry of Well 15 was chosen asit represents the chemistry of regional groundwaters that are unaf-fected by fluid intrusion or CO2. The target water-to-rock ratio forall experiments was 35:1, with an initial solids mass of either 23 gor 100 g. Hydrated CO2 was bubbled into the experimental solu-tions through a glass frit under 1 atmosphere pressure. Two setsof experiments were conducted for 2–3 weeks and consisted of a3–4 day equilibration period without CO2, followed by spargingof the system with 1 atm CO2. In one of the experiments, the CO2

was stopped during the final 4 days to examine potential reversiblechanges in geochemical behavior. Liquid aliquots were collectedand analyzed at discrete time points during the course of theexperiments. Fluid chemistry was monitored for pH, major cations

d Distinguishing exchangeable vs.carbonate

Quantitative XRD

Reducibles Oxidizables Water-soluble

Exchangeable Carbonate Weight percentcalcite

5.24E�04 4.96E�03 6.96E�05 3.78E�05 5.10E�04 4.7

1.14E�04 1.21E�04 4.37E�05 5.20E�04 1.94E�04 1.8

7.75E�05 8.07E�05 0.00E+0 9.80E�07 8.86E�05 0.41.20E�04 1.19E�04 0.00E+0 2.70E�04 6.71E�05 1.64.75E�05 5.55E�05 0.00E+0 9.83E�05 2.33E�05 0.6

E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147 141

and anions, and trace elements using standard analytical labora-tory techniques (e.g., ICP–MS and ICP–OES).

Results from these experiments are shown in Fig. 4. The first setof batch reaction experiments compared relatively quartz-rich(Lithosome B) and clay-rich (Lower Lithosome B) samples. Duringthe pre-equilibration stage without CO2, both samples react withthe synthetic groundwater to produce 1 and 4 lg/L U in thequartz-rich and clay-rich samples, respectively (Fig. 4a). Spargingof CO2 through the samples produces an immediate increase in Uconcentration in the Lower Lithosome B which reaches almost14 lg/L U. This is associated with a drop in pH from 7.8 to 6.3(not shown). In contrast, Lithosome A changes little with a slightincrease to 2 lg/L U (pH change from 7.9 to 6.2). Loss of CO2 inthe system results in an apparently reversible loss of U from LowerLithosome B.

By contrast, the behavior of Ca in both samples is similar andshows a sharp increase from about 210 to about 400 mg/L follow-ing the addition of CO2. Loss of CO2 results in a sharp decrease in Caconcentration that is interpretted to result from the precipitationof calcite. The correlation between the behavior of U and Ca ob-served in the Lower Lithosome B samples suggests that calcite dis-solution and precipitation govern the relatively greater mobility ofU in the Lower Lithosome B samples. The low concentration of U

a

b

Fig. 4. Experimental data for laboratory batch experiments. (a) Clay-rich sample (Lower

found in Lithosome B does not respond to CO2 and is apparentlynot directly connected with calcite. This association of mobile Uwith carbonate is reflected in the sequestration extraction data inwhich greater U in sediments is associated with higher carbonateconcentration. The second set of experiments (Fig. 4b) showedqualitatively similar behavior.

Due to the complex nature of these sediments, it is hypothe-sized that a variety of geochemical processes may occur that con-trol the concentration of U in solution. Measured changes in Caduring the experiments show that [Ca] trends with [U] (Fig. 4). Cal-culation of calcite saturation indices, using Geochemists Work-bench Professional v. 7.0 and the thermo.dat database, indicatesthat the fluid is always either in equilibrium with calcite (logQ/Kof 0 ± 0.5) or undersaturated. Both conditions are compatible withcalcite dissolution. Based on these results, U release to solution viacalcite dissolution is a realistic mechanism for the initial release ofU in these systems, and calcite precipitation is a probable mecha-nism of U scavenging after the end of CO2 sparging.

In order to estimate the concentration of U that could be re-leased to solution through dissolution of U-bearing calcite, threeindependent calculations were made from experimental results.The first two calculations are derived from CO2 batch experiments:the molar ratio of U to Ca increase due to CO2 exposure was �2E�6

U (µg/l)

Lithosome B) and the quartz-rich sample (Lithosome B) and (b) Sample A1 (Table 2).

Table 3Aqueous complexation reactions.

Log10(Keq)

H2CO3 () HCO�3 þ Hþ �6.3

H2CO3 () CO2�3 þ 2Hþ �16.6

Hþ () OH� �14.0

3H2CO3 þ Ca2þ þ UO2 () CaUO2ðCO3Þ�23 þ 6Hþ �24.2

3H2CO3 þ 2Ca2þ þ UO2 () Ca2UO2ðCO3Þ3 þ 6Hþ �20.0

2H2CO3þ UO2 () UO2ðCO3Þ�22 þ 4Hþ �16.4

3H2CO3 þ UO2 () UO2ðCO3Þ�43 þ 6Hþ �28.4

142 E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147

in the A1 experiment (Fig. 4b) and was �7E�6 in experiments onsimilar Chimayó samples (Fig. 4a). The third estimate is derivedfrom the carbonate extraction step for sample B2, corrected forthe total weight percent of calcite, resulting in a molar ratio of�9E�6. These three estimates are remarkably consistent, andcould be thought of as natural variability within the sediments.The lower end of this range was used for the base-case simulationsto follow. The calcite dissolution/precipitation reaction assumed is

H2CO3 þ Ca2þ þ AUO2 () Cað1� AÞUO2ðAÞCO3ðsÞ

where A = 2.E�6. Sensitivity to uncertainty in this assumed value isexplored later in the paper. It is necessary to adjust log10 Keq = 8.33by 0.71 (8.2) to match the slight supersaturation evident in allgroundwater samples in Chimayó (Keating et al., 2009). To accountfor possible sorption/desorption reactions due to formation of aque-ous uranyl carbonates, a number of aqueous complexation reactionswere included, listed in Table 3.

A common approach to simulating U sorption in laboratoryexperiments is by surface complexation models (Waite et al.,1994). To gain computational efficiency a more simple methodwas used: the linear isotherm (Kd) approach. As a pragmatic meth-od to approximate pH-dependent sorption/desorption without thecomputational burden of surface complexation reactions differentaqueous U complexes were allowed to sorb at different Kd values.To reproduce the trends measured by Fox et al. (2006) who foundthat the presence of Ca2UO2(CO3)3 reduces sorption onto ferrihy-drite and quartz from 83% to 57%, Ca2UO2(CO3)3 was assigned aKd value that results in approximately 50% sorption (Kd = 0.04).Much larger values of Kd were assigned to the other complexesand to the free U ion, thus introducing the possibility of pH-depen-dent sorption/desorption. A more detailed study of appropriate Kd

values for these sediments would account for the effects of clayminerals, which were not considered by Fox et al. (2006). Heretheir affect is not accounted for.

4.3. Initial and boundary conditions

To specify initial geochemical conditions for the shallow aquifer(and lateral recharge to the aquifer from the eastern inflow bound-ary) data collected in ‘background’ (low pCO2) wells in Chimayówas used. Recharge occurring along the Santa Cruz River was alsospecified using field data. The lower carbonate layer was dividedinto two zones. As described in Keating et al. (2009) one of thestriking features of the geochemical trends in the shallow aquiferat Chimayó is that some waters are impacted by CO2 and others(near the southern end of the fault) are impacted by a mixture ofCO2 and brackish, Na–Cl water. Apparently the upwelling CO2 en-trains Na–Cl water in some locations and not others. To reproducethis feature, Na–Cl-type water was placed in the southern portionof the underlying carbonate. The inflow boundary to the lower car-bonate aquifer was set to be equivalent to the initial condition. Thelower carbonate aquifer was assumed to be 100% calcite; the upperaquifer was assumed to be 0.4% calcite, in accordance with the XRD

results. All layers in the model were assumed to be single porosity,porous flow media.

The simulations were conducted in pairs. The first simulationwas used to establish geochemical equilibrium with the reactionslisted in Table 3. This produced equilibrium with calcite through-out the model. The dominant aqueous U complex was found tobe Ca2UO2(CO3)3, hence approximately 50% of total U was sorbedto mineral surfaces. This simulation also established a small zoneof elevated TDS near the fault, due to upward vertical gradientsand upwelling of deep water. No CO2 was introduced. The secondsimulation perturbed this equilibrium by introducing a flux ofCO2 along the northern portion of the base of Robert’s Fault anda flux of CO2 and brackish water along the southern portion. AsCO2 enters a cell in the model, instantaneous dissolution in wateris assumed to occur. The maximum allowable dissolved concentra-tion is controlled by the pressure and temperature of that cell, asdefined by CO2 solubility tables published by Duan and Sun(2003). If excess CO2 exists, it flows as a free phase.

5. Comparison of simulations with measured data

The first step is to verify that the numerical model is capable ofreproducing the general trends observed at Chimayó. The focus iscarbonate and U geochemistry and the possible effects of brackishwater; therefore, comparisons with Ca, pH, pCO2 and Cl are empha-sized. There are two possible perspectives from which to view thefield data. The first is that these data represent a single snapshot intime of an aquifer as a plume of CO2 (locally, with brackish water)moves through it. In this perspective, the variability of groundwa-ter chemistry is largely due to spatial variability in the characteris-tics of the CO2 plume itself, and the spatial relationship betweenthe plume and individual wells. Since the length of time that theCO2 plume has been in the aquifer is not known, it is difficult to se-lect the appropriate time in the simulation to perform this ‘timesnapshot’ comparison. Another difficulty for this type of compari-son is that it requires detailed information about the spatial char-acteristics of the CO2 plume, for which there is virtually noinformation. An alternative perspective is that the data representwaters exposed to the same plume, but for different lengths oftime. Using this perspective, points can be selected in the modeldomain corresponding to the sampled wells and the time-varyingchanges in geochemistry compared to the observed spatial vari-ability in water chemistry at the site.

Several model parameters were varied in a qualitative attemptto reproduce geochemical trends measured at the site. These in-cluded the injection rate of CO2 at the base of the fault, the durationof the CO2 injection, the initial concentration of U in the brackishwater in the deep aquifer, and the concentration of U in the leakat the base of the fault. Although no formal sensitivity analysiswas conducted, it was apparent that results were particularlysensitive to the total cumulative amount of CO2 injected and theconcentration of U in the leak at the base of the fault. In Figs. 5and 6 results are presented for a case of injecting CO2 at a rate of0.3 kg/s for 4 a (37.8E�3 MT total CO2). Initial and boundary condi-tions for this model are shown in Table 4. The water chemistry datafrom the site are compared to time-varying water chemistry in themodel domain. The open symbols on these plots are nodes in themodel corresponding to the wells (black spherical symbols inFig. 3). The pH/pCO2 and Ca/pCO2 comparisons (Figs 5a and b) showreasonably good agreement at early times. The simulated values be-gin to depart from measured values at later times, corresponding tothe recovery phase (after CO2 injection) when the dissolved CO2

plume spreads and becomes more dilute. Importantly, the rangeof measured pH and pCO2 is fairly well reproduced. Larger injectionrates (or larger injection durations) resulted in overpredicting

pCO2

pH

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

6.0

6.5

7.0

7.5

8.0

8.5

pCO2

Ca

-3 -2 -1 0 10

0.005

0.01

0.015

Year:Year: 2 5 10 20 30

2 5 10 20 30

Fig. 5. Simulated (open symbols) and measured (red symbols) pH, pCO2 (log10 atm), and Ca (moles/l). Simulation results are colored according to the time of the modeloutput. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Cl

U

0.000 0.005 0.010 0.015 0.0200

2E-07

4E-07

6E-07

8E-07

1E-06

Cl

Tota

l C

0.000 0.005 0.010 0.015 0.020 0.0250.00

0.05

0.10

0.15

0.20

Year: 2 5 10 20 30Year: 2 5 10 20 30

Fig. 6. Simulated (open symbols) and measured (closed red symbols) total dissolved carbon, chloride, and uranium (all moles/l). Simulation results are colored according tothe time of the model output. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 4Initial and boundary conditions (units are m M).

Initial conditions Ca CT Cl pH Ub

Aquifer 1.52 5.516 0.62 8.5 2.e�4Deep (northern) 1.52 5.516 0.62 8.5 2.e�4Deep (southern) 1.52 5.516 20 8.5 7.e�4Fault base inflow 1.52 100 a 8.5 a

River 1.52 0.1 .62 6.0 2.e�6

a Same as the deep aquifer (north or south).b Total U (aqueous + sorbed).

E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147 143

maximum pCO2. Reproducing the measured slope of the pH/pCO2/Ca trends indicates that the simulations have the right degree of pHbuffering and that calcite buffering is an adequate proxy for the(undoubtedly) more complex set of reactions occurring at the site.

The comparison with total C and Cl (Fig. 6a) is also good. Thesimulation correctly predicts a maximum total C concentration of�0.15 M/L. This value is very sensitive to the total amount of CO2

injected, and so matching these measured data provides some con-straint on the CO2 fluxes at the site. The two groups of waters, highCO2/Cl and high CO2/low Cl, present at the site are both reproducedby the simulations. The match becomes fairly good beginningaround 2 a after the start of CO2 injection. The simulated trendsin U and Cl (Fig. 6b) also show good agreement for all but veryearly times in the simulation. These results were very sensitiveto the presumed [U] and [Cl] concentrations in the deep aquifer

and in the upward leaking water, and to assumptions regardingU sorption. If U sorption is allowed in the deeper layers and withinthe fault itself, decades were required for the [U] and [Cl] concen-trations to match the field data. By this time, simulated pCO2 levelswould be much too high in the aquifer. This result suggests eitherthat CO2 and brackish water upwell on different temporal scales orthat U only weakly sorbs as it is transported up into the shallowaquifer. The agreement achieved between measured and simulatedU, Cl, pH, pCO2, and total C provides confidence that the conceptualmodel of calcite buffering, release and scavenging of U by calciteprecipitation/dissolution, and entrainment of U with high Clwaters transported up along the fault is a reasonable one, and sim-ulations of these processes are practical at the field scale.

The spatial evolution of the CO2 plume along a vertical E–Wcross-section is shown in Fig. 7. At all times, the CO2 is entirelyin the dissolved phase (no free phase) and does not reach the uppersurface of the model (the water table). This is notable consideringthat aquifer permeability was assumed to be isotropic. The ten-dency for lateral migration is due to the combined effects of the in-creased density of CO2-enriched water and ambient lateral flux ofgroundwater. The maximum mass fraction is �4%. Immediatelyafter cessation of injection (Fig. 7), the plume has reached the shal-low aquifer and is beginning to spread, mostly in the downstream(western) direction. The maximum [Ct] has already declined. By100 a, there is significant downstream spread. The plume still hasnot quite reached the top of the aquifer.

Fig. 7. Dissolved CO2 (moles/L) at 1 years, 4 years, and 100 years along cross-section BB0 . Horizontal line indicates confining bed.

Fig. 8. Cumulative changes in U concentrations, along cross-section BB0 .

Fig. 9. Cumulative amount of calcite dissolved (moles) in the shallow aquifer alongcross-section BB0 (note: dark blue area along base is below the shallow aquifer – nodata here). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Fig. 10. Cumulative changes in U concentrations, along cross-section AA0 .

144 E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147

The zone where U concentrations are changing in response tothe plume is smaller, as shown in Fig. 8. By inspection of the aque-ous complex concentration and sorption results, it is evident thatthe Ca2UO2(CO3)3 complex dominates everywhere. The fraction oftotal U sorbed does not change significantly in time or space (totalvariation over all times everywhere in the model: 46.1–47.2%). Thisresult is consistent with the hypothesis proposed in Keating et al.(2011) that the dominance of the Ca2UO2(CO3)3 complex in thisCa-rich system at all pH values measured at the site will mutethe importance of desorption reactions as the aquifer responds toCO2. In fact, the changes in [U] shown in Fig. 8 are mostly due toentrainment of high [U] water from the deeper aquifer. In this por-tion of the aquifer, the [U] of inflowing water is greater than [U]in situ and so clearly no in situ U desorption could occur. As shownin Fig. 9, the zone of calcite dissolution is fairly small compared tothe zone of enhanced total C (Fig. 7). The situation is very differentnear the southern end of the fault (Fig. 10). Here changes are aboutan order of magnitude higher. This is entirely due to the entrain-ment of brackish water containing elevated [U]. In summary, sim-ulated changes in U concentrations in the shallow aquifer aredominated by entrainment of brackish water containing elevated[U] and continued upwelling of brackish water from the base ofthe fault. Some release of U is caused by dissolution of calcite,

but this effect is subtle compared to the entrainment. No signifi-cant desorption of [U] occurs, due to the stability of the Ca2UO2(-CO3)3 complex at all pH values during the simulation.

6. CCS Leakage scenarios

As mentioned above, the Chimayó site would never be a candi-date for CO2 sequestration due to the obvious presence of leakagepathways and the lack of a suitable reservoir at depth for storage.However, it shares physical and geochemical characteristics withother sedimentary aquifers in semi-arid environments and as suchcan serve to represent a class of aquifers that might be overliepromising CO2 sequestration reservoirs. The conceptualization isof an aquifer with physical and geochemical characteristics suchas the Chimayó site, overlying a sequestration reservoir which isleaking upwards to the aquifer through a fault.

In order to calculate a leak rate relevant to risk assessment at anindustrial CO2 geologic sequestration site, a rate is considered

a b

Fig. 11. Volume of plume of water (m3) exceeding the EPA drinking water standard for uranium and pH, showing sensitivity to (a) injection duration (colors) and injectionrate (line type), and (b) U content in calcite (colors) and injection duration (line type). (For interpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147 145

which is of the highest magnitude that would still meet the guide-lines of 99% CO2 retention over 1 ka proposed by IPCC (2005). Theleak is assumed to be evenly distributed along a fault similar indimensions to Robert’s Fault (an approximately 3.5 km � 70 m fea-ture). Using an approximate ratio of 2:5 MT CO2 injected per MWof power produced at the plant (Spangler et al., 2010), for a500 MW plant, this would correspond to a leak rate of 0.2 kg/s.For a large plant, 2.1 GW, this would correspond to 0.84 kg/s. Thesenumbers are very rough estimates, since many aspects of an actualleak are highly uncertain, particularly the footprint of the leakingfeature. The simulations described above introduce CO2 at the baseof the fault at a rate of 0.3 kg/s, which would roughly correspond tothe maximum allowable leak rate from injecting produced CO2

from a �750 MW plant, given the assumptions listed above. Insome sense, therefore, the simulation results (and correspondingobserved water chemistry measurements at Chimayó) presentedabove are relevant to scenarios of interest for an industrial-scalesequestration project, if the leak were short-lived. The simulatedplume in the non-brackish water impacted portion of the aquifer(Fig. 8) would correspond to a ‘pure CO2’ leak; the other plumes(Fig. 10) would be relevant to a mixture of displaced brine andCO2. Reasons to expect that a leak could be short-lived includeself-sealing of the leaking pathway, due to degassing of CO2 andprecipitation of carbonates, or mitigation strategies such as pres-sure management.

To simplify the problem, in the leakage scenarios the possibleintroduction of U in brine displaced by injection of CO2 was ne-glected. Therefore, the only possible source of U in these simula-tions is in situ release from calcite in the shallow aquifer. Thisreaction is reversible, as indicated in the batch experiments(Fig. 4b). The proxy for drinking water impacts due to CO2 leakageis the total volume of aquifer with pH < 6.5 and with [U] above theprimary drinking water standard. The focus is on the first 100 aafter the leak. A number of simulations were developed, assessingthe impact of potentially important factors including leakage rate,leakage duration, sorption coefficient for U, and content of U in car-bonate minerals. The first set of nine simulations considered com-binations of leak duration (2, 4 and 6 a) and leak rate (0.3, 0.6 and0.15 kg/s). The second set of simulations measured the sensitivityof the U plume volume to the U content of calcite (2.E�6, 7E�6,2E�5 M U per M CaCO3). This is roughly the variation in molar ra-tios that could be deduced from the sequential extractions.

The results are shown in Fig. 11. The U profiles are always aboutone order of magnitude smaller than the pH plume. This is becauseof the limited amount of calcite dissolution occurring, and the verysmall amounts of U released by dissolution. The total amount ofcalcite in the system (initially 4%) is only very slightly reducedby the end of 100a. Some of the simulated U plumes continue togrow after the cessation of the leak. This suggests that U releasedue to calcite dissolution can be maintained for long periods of

146 E.H. Keating et al. / Applied Geochemistry 30 (2013) 136–147

time, long after the cessation of the leak, but that the impact of thisrelease is relatively minor. Even though the simulated U ‘plumes’are relatively small (about 300 m3), even nine decades after thecessation of the CO2 leak the plumes have not significantly de-clined in size. Small declines in the pH and U plume volumes occurshortly after the cessation of the CO2 leak. This could be caused byeither metal scavenging (calcite precipitation) or by spreading/dilution of the plume. All three parameters considered appear tobe of approximately similar importance.

In a final set of simulations (not shown), the sorption parame-ters for U complexes were varied over three orders of magnitude,from 0.004 to 4. The simulated CO2-induced [U] plumes were com-pletely insensitive to this parameter. This is due to the fact that theCa2UO2(CO3)3 complex dominates at all geochemical conditionsrepresented by these simulations; and so differential sorption be-tween aqueous forms of U has no effect.

7. Discussion and conclusions

Studying the hydrology and geochemistry of the CO2-impactedaquifer at Chimayó offers clues as to how a drinking water aquifermight respond to a leak at a CO2 sequestration reservoir. The mod-eling results presented here utilize insights gained from analyzingfield data and conducting laboratory experiments. One importantinsight is the importance of processes occurring in strata betweenthe leaking reservoir and the aquifer, including entrainment ofbrackish water. Another is the importance of calcite, which notonly buffers pH changes but also can be both a source of trace met-als and a mechanism for trace metal scavenging. These interactionscan be complex when considered in the context of three-dimen-sional, transient reactive transport.

Uranium sorption/desorption did not occur in response to CO2

inflow in these simulations. This is due to the fact that a singleaqueous complex, Ca2UO2(CO3)3, dominated at all pH values inthe aquifer. This is in contrast to laboratory and theoretical studiesby Fox et al. (2006) and Dong et al. (2005) who hypothesized thatdiffering complexes would dominate at different pH values andsorption would be reduced in pH ranges where U–Ca complexesdominate. This may be a unique aspect to the particular geochem-ical environment at Chimayó, which has relatively high alkalinityand Ca concentrations. However, in a certain sense the release(or scavenging) of U by calcite precipitation/dissolution shouldproduce similar results as pH-dependent sorption/desorption.Regardless of the mechanism, the simulations of the Chimayó siteand other leakage scenarios show that the volume of the aquiferwhere U exceeds the EPA drinking water standard is relativelysmall (at most, approximately 300 m3) even for fairly high leakagerates. The pH < 6.5 plume, on the other hand, is relatively large andcontinues to expand long after the leak ceases. In no cases consid-ered do the pH < 6.5 plumes begin to shrink in size before the endof the 100-a simulations.

One effect of U sorption is that transport to the aquifer fromdeeper layers will be delayed relative to non-sorbing speciesincluding dissolved Cl and CO2. Depending on the sorptive capacityof the deeper strata and leakage pathways (e.g. wellbores, faultgouge), trace metal transport from below may be very, very slow.Thus, if a leak can be detected and mitigated (perhaps throughpressure management) the problem could be solved before thedeeper trace metals are transported to the shallow aquifer. Becauseof complex three-dimensional effects and the competing effects ofU release and scavenging by calcite, the predicted size of the ‘U-plume’ does not grow monotonically with time. Locally, both in-creases and decreases of U concentrations could be expected at dif-ferent times after the leak. Trace metal concentration monitoring,therefore, should not be relied on for leak detection.

The simulations presented here represent only a small range ofconditions that should be explored for a full risk assessment at aCO2 sequestration site. Other factors, such as the geometry and sizeof the leakage pathway, the types of geochemical reactions (includ-ing surface complexation) that could affect trace metal release, andthe heterogeneity in the aquifer should be considered. Nonethe-less, the simulations presented show the potential complexity ofan aquifer response, particularly with respect to trace metals.These simulations also show that a CO2 leak could have verylong-lasting effects on groundwater pH. Further study of the impli-cations of these effects would be necessary to determine if mitiga-tion would be necessary, and, if so, to design effective mitigationstrategies.

Acknowledgments

This research was funded by the Department of Energy ZERT IIproject. We appreciate the thoughtful comments of two anony-mous reviewers.

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