Final Report: Risk-Based Decision Making for Assessing Potential Impacts of Geologic CO2 Sequestration on Drinking-Water Sources

EPA Grant Number: R834387
Title: Risk-Based Decision Making for Assessing Potential Impacts of Geologic CO2 Sequestration on Drinking-Water Sources
Investigators: McCray, John , Maxwell, Reed , Sitchler, Alexis
Institution: Colorado School of Mines
EPA Project Officer: Klieforth, Barbara I
Project Period: February 1, 2010 through January 31, 2013 (Extended to January 31, 2014)
Project Amount: $899,987
RFA: Integrated Design, Modeling, and Monitoring of Geologic Sequestration of Anthropogenic Carbon Dioxide to Safeguard Sources of Drinking Water (2009) RFA Text |  Recipients Lists
Research Category: Drinking Water , Water

Objective:

The research was comprised of a combination of numerical modeling and carefully designed experiments to achieve the following objectives:

  1. Understand how subsurface properties are related to the susceptibility of an aquifer to degradation from CO2.
  2. Develop a framework that can be used to assess the risk associated with CO2 leakage into a potential drinking water source from the aquifer properties.

Summary/Accomplishments (Outputs/Outcomes):

The findings of our research are best summarized in the numerous published papers that resulted from this work (see publications). Below, we provide a brief summary of each paper that has been published, is in review, or is being submitted, along with an explanation of how the research adds to our understanding of, or solutions for, environmental problems or is otherwise of benefit to protection of the environment and human health, including relevant practical applications.

The summaries below start with experimental research that investigated the integrity of caprocks (i.e., that protect overlying aquifers from the carbon dioxide storage zone), then describe experimental and data-mining research intended to understand the impacts of potential leakage of CO2 or brine into an aquifer.  Finally, we present research that developed a state-of-the-art risk assessment framework associated with leakage of metals (from either brine or CO2) into aquifers, downstream transport, and human exposure to these contaminants via water wells.

Marcon and Kaszuba (2013, 2014a) conducted hydrothermal experiments to investigate the integrity of caprocks after exposure to supercritical CO2 at realistic pressures and temperatures. Carbon dioxide-water-rock interactions are important within a broad range of anthropogenic and geologic systems including geologic carbon sequestration, enhanced oil and gas recovery, enhanced geothermal systems, hydrocarbon production through hydraulic fracturing, and CO2-charged thermal springs. Understanding water-rock interactions and overall chemistry of CO2-charged systems is crucial to help mitigate potential environmental concerns such as leakage of inorganic compounds into overlying potable aquifers as a result of injecting CO2 or overall integrity of a caprock seal.  Mobilizing trace metals with injection of supercritical CO2 into deep saline aquifers is a concern for geologic carbon sequestration because of potential subsequent leakage into overlying drinking water aquifers.  This study investigated the release of regulated metals from two zones within a carbonate reservoir: at the boundary between the caprock and the reservoir boundary and deeper within the carbonate reservoir. The experiments emulated saline formations (I = 3.3 m) in the Paradox Basin, Utah; an idealized Desert Creek limestone was used for the reservoir and the Gothic Shale as the caprock.  Experimental results show that CO2 injection decreased the pH by 1 to 2 units; concomitant mineral dissolution produces elevated Ba, Cu, Fe, Pb, and Zn concentrations in the brine. Concentrations subsequently decrease to approximately steady state values after 120-330 hours as a result of secondary mineralization of clays and metal sulfides (i.e., Fe, As, and Co sulfides). At the end of the experiments, both iron, an element of secondary concern, and lead exceeded the U.S. Environmental Protection Agency (EPA) maximum contaminant limits (MCLs), whereas Ba, Cu, and Zn concentrations remained below the limits. Transition elements Cd, Cr, Cu, and Zn as well as Pb, behaved in a similar manner, increasing in concentration with injection but continually decreased after about 830 hours until termination of the experiments. Nickel, not a regulated element, also was readily mobilized, and is associated with human health concerns at elevated concentrations.  If a CO2-saturated brine migrates from a carbonate reservoir and mixes with a potable aquifer, the experimental results suggest that Ba, Cu, and Zn will not be contaminants of concern; however, Fe, Ni, and Pb may require careful attention. Experimentally observed trends of decreasing trace metal concentrations suggest that these metals (Fe, Ni, and Pb) could become less of a concern during the life of a carbon repository. Finally, these experiments indicate that the caprock plays an active role in trace metal evolution within the system. The caprock provides a large source of metals, but secondary mineralization and adsorption may remove metals of concern from solution.  These experiments lend considerable insight into potential contaminants that may enter an aquifer upward leakage for a specific carbon-sequestration location, would help practitioners design monitoring and preventative remediation plans to help protect drinking water, and could help evaluate the source of potential contamination.  However, such experiments should be conducted for a particular site (injection formation and caprock). 

Marcon and Kaszuba (2014b) conducted a second set of hydrothermal experiments to react a different idealized injection reservoir material (Fe-rich dolomite), caprock material (illite clay), and caprock + reservoir rocks in a water at 1600°C and 25MPa for ~55 days.  Steady state was achieved in 17 days before CO2 was injected and monitoring was conducted for 38 days to examine changes from injection of CO2. Major and minor water chemistry, total dissolved CO2, and pH were monitored throughout the duration of the experiments. In all three experiments, Ca, Fe, Mg, Mn, SiO­2, and SO­4 increase with injection, but slowly decline through termination of the experiments. The aqueous data supported by geochemical equilibrium modeling and SEM results suggests initial dissolution of minerals followed by re-precipitation as carbonates, sulfides, sulfates, and clays. Trace metals in these experiments do not exceed the U.S. EPA’s primary or secondary MCL, but geochemical patterns denote valuable information for metal release, co-precipitation, and sorption of metals in a sequestration scenario.  Arsenic declines in concentration after the addition of CO2. Strontium does not decrease in concentration in any of the three experiments. The concentration of barium decreased after injection when the caprock proxy is included, but continued to increase in concentration in the deeper storage reservoir, without the caprock present. All three experiments display a similar pH decrease of 1.5 units after the addition of CO2. Formation of secondary clays, sulfates, sulfides, and metal oxides provides direct evidence for the removal of heavy metals of concern from formation waters within a storage reservoir in a sequestration scenario. The presence of clay mineral helps reduce metal mobilization by 1-2 orders of magnitude. Experiments that contain a caprock proxy display an increase in surface area suggesting a change in the clay structures after reactions with CO2- saturated waters, whereas the carbonate reservoir experiments display an overall decrease in surface area suggesting greater dissolution than secondary mineralization.  This study demonstrates the complexity of subsurface material dissolution that can release regulated metals along with metal precipitation that can temporarily or permanently immobilize released metals, that the dominant process depends on the type of rock, and that geochemical modeling is needed to elucidate the transport mechanisms.

Bearup, et al. (2012) conducted a data-mining and geochemical modeling study to better understand groundwater time scales wherein kinetic metal desorption and mineral dissolution are important mechanisms with a goal of improving realistic modeling of metal release. In this study, release rate constants were compiled and the Damköhler number was applied to calculate residence times where kinetic formulations are relevant. Metal desorption rate constants were compiled for arsenic, barium, cadmium, copper, lead, mercury, nickel, and zinc, and span 6 orders of magnitude, while mineral dissolution rate constants compiled for calcite, kaolinite, smectite, anorthite, albite, K-feldspar, muscovite, quartz, goethite, and galena ranged over 13 orders of magnitude. This Damköhler analysis demonstrated that metal-desorption kinetics are potentially influential at residence times up to about 2 years, depending on the metal and groundwater conditions. Kinetic mineral dissolution should be considered for nearly all residence times relevant to groundwater modeling, provided the rate, solubility, and availability of the mineral generates a non-negligible concentration. Geochemical models of competitive desorption and dissolution for an illustrative metal demonstrate total metal concentrations may be sensitive to dissolution rate variations despite the predominance of release from desorption. Ultimately, this analysis provides constraints on relevant processes for incorporation into transport models, yet demonstrates that site-specific information would be needed for accurate geochemical modeling. 

Wunsch, et al. (2013b) conducted bench-scale experiments to simulate CO2 leakage into carbonate aquifer rocks, obtained from the USGS Core Laboratory, pressurized to conditions realistic for drinking water aquifers. CO2 leakage from underground CO2 sequestration and storage poses potential risks to degradation of water quality in shallow aquifers. Increased CO2 concentrations can result in decreased pH and lead to subsequent metal release from mineral dissolution or desorption from mineral surfaces. Dissolution of carbonate minerals present in aquifer sediments or rocks will buffer pH and are generally thought to reduce the potential risk of metal release in the event of a CO2 leak. As a result, much of the research on geochemical impacts of CO2 leakage has focused on siliciclastic aquifers with little to no carbonate minerals present. However, carbonate minerals contain trace amounts of metals in their crystal structure that will be released into solution with dissolution and may pose a risk to drinking water quality. Here, we perform laboratory water-rock experiments to analyze the potential for metal release due to carbonate mineral dissolution in limestone aquifers. Rock samples from three limestone aquifers were dissolved in batch reactors with varying partial-pressures of CO2 (from 0.01 to 1 bar) in the headspace. As CO2 dissolved into the fluid and decreased the pH, the carbonate minerals dissolved and released metals into solution. The concentrations of calcium, magnesium, strontium, barium, thallium, uranium, and cobalt increased but remained below any regulatory limits. The concentrations of arsenic and nickel increased and exceeded primary drinking water standards set by the U.S. EPA and the State of California, respectively. Potential sources of metals in the rocks were determined through detailed sample characterization using sequential extractions, laser ablation inductively coupled mass spectrometry, and high resolution mineralogical mapping with QEMSCAN. We found that calcite dissolution released more metals to solution than pyrite dissolution or metal desorption from mineral surfaces in these experiments. Geochemical models based on the experimental data were used to evaluate the relative importance of calcite versus pyrite dissolution over a 30-year time frame. Under both oxic and sub-oxic conditions, calcite dissolution is the dominant source of metals to solution immediately after exposure to CO2. Pyrite dissolution becomes the dominant source at later times as the fluid reaches equilibrium with respect to calcite. For all model scenarios, the cumulative contribution of metals to solution was dominated by calcite dissolution..

Wunsch, et al. (2013c) followed up on the study described above, but with a focus on dolomite aquifer materials, also obtained from the USGS Core Laboratory, and details a thorough investigation on the role of mineral composition and mechanisms on trace element release in the presence of CO2. Detailed characterization of samples from dolomite formations demonstrated stronger associations of metal releases with dissolution of carbonate mineral phases relative to sulfide minerals or surface sorption sites. Aqueous concentrations of Sr2+, Co2+, Mn2+, Ni2+, Tl+, and Zn2+ increased when these dolomite rocks were exposed to elevated concentrations of CO2. The aqueous concentrations of these metals correlate to aqueous concentrations of Ca2+ throughout the experiments. All of the experimental evidence points to carbonate minerals as the dominant source of metals from these dolomite rocks to solution under experimental CO2 leakage conditions. Aqueous concentrations of Ca2+ and Mg2+ predicted from numerical simulation of kinetic dolomite dissolution match those observed in the experiments when the surface area is 3 to 5 orders of magnitude lower than the surface area of the samples measured by gas adsorption, suggesting the reactive surface area cannot be properly measured using this technique.  These two studies suggest that the pH buffering benefit of carbonate mineral dissolution in the event of a CO2 leak may be offset by the potentially negative effect of trace metal release from the crystal structure. The studies highlight the need for detailed sample characterization at individual sites to identify sources of metals when assessing the potential risk of CO2 leakage into shallow aquifers.

Kirsch, et al. (2014) evaluated metal release from sandstone aquifer materials using experiments similar to those used by Wunsch, et al. (above) for the carbonate rocks, and also conducted geochemical simulations to analyze the resulting data. This work was partially funded by this EPA STAR grant. This study targeted the geochemical response of siliclastic aquifer rocks after contact with CO2 at pressures representative of drinking water aquifers. Specifically, the study used three sandstones of the Mesaverde Group in northwestern Colorado. The goal was to test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO2-impacted aquifer.  Two batch experiments were conducted. Samples were reacted for 27 days with water and CO2 at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO2 was introduced into the system, but did not exceed primary MCLs set by the U.S. EPA. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation was supported by a geochemical model, which could simulate the observed changes in fluid composition through CO2-induced calcite and dolomite dissolution.

Navarre-Sitchler, et al. (2014) conducted two intermediate scale-tank experiments to investigate the role of competitive desorption of metals from clays by Ca2+ released to solution through calcite dissolution, which has been proposed by some recent field studies (i.e., Trautz, et al., 2012, ES&T) and the impact of heterogeneous distribution of clays on metal release.  We chose Ni for this study because of its potentially high toxicity. Results from this study show a slight increase in mass flux of metal from sorbed surface sites on kaolinite in the presence of high CO2 waters (PCO2 ~ 1 atm) relative to flushing the tank with deionized water. In the heterogeneously packed tank the increase in mass release was two-fold (1.3x10-3 compared to 2.6x10-3 mg Ni sec-1). The difference in the homogeneous tank also was two-fold higher compared to deionized (DI) water, but the release rates were higher in this tank (2.0x10-3 mg Ni sec-1 for DI water and 4.0x10-3 mg Ni sec-1 for high CO2 water). The Ni release rate increased substantially when Ca2+ was introduced into the flushing solution. In both tanks after 14 days of high Ca2+ input water the release rate stabilized at 1.5x10-1 mg Ni sec-1. The observed difference in release rate with heterogeneity at low release rates in high CO2 water, but not at the high release rates when Ca2+ was increased, suggests that equilibrium was reached along the flow path in the tank. If the experiment had been run for a longer time, we would expect an eventual difference in release rate between the two tank configurations. Overall, release rates from the homogeneous packed tank was higher than the heterogeneously packed tank illustrating the role of physical heterogeneity in the subsurface on geochemical changes in water quality due to CO2 intrusion. Data analysis and modeling for this intermediate-scale experimental effort will continue after the period of performance of this EPA STAR grant. 

Wunsch, et al. 2013a conducted a data-mining and simple mixing-model study to better understand impact of brine leakage into underground sources of drinking water (USDWs), given that some researchers have presented results that suggest brine leakage may be more likely than CO2 leakage because of the larger radius of influence of pressurized brine in the injection formation. This study thus aimed to characterize the geochemical composition of deep brines, with a focus on constituents that pose a human health risk and are regulated by the U.S. EPA.  The study conducted a statistical analysis of the NATCARB brine database (more than 23,000 data points across the U.S.) to better understand the potential for exceeding drinking water regulatory limits, and also for influencing agricultural productivity.  The statistical analysis, combined with simple mixing model calculations, show total dissolved solids and concentrations of chloride, boron, arsenic, sulfate, nitrate, iron, and manganese may exceed regulatory or plant tolerance levels. Twelve agricultural crops evaluated for decreased productivity in the event of brine leakage would experience some yield reduction due to increased TDS at brine-USDW ratios of < 0.1, and a 50% yield reduction at < 0.2 brine-USDW ratio. A brine-USDW ratio as low as 0.004 may result in yield reduction in the most sensitive crops. The U.S. EPA TDS secondary standard is exceeded at a brine fraction of approximately 0.002. To our knowledge, this is the first study to consider agricultural impacts of brine leakage, even though agricultural withdrawals of groundwater in the United States are almost three times higher than public and domestic withdrawals.

Siirila, et al. (2012a) present a quantitative risk assessment framework to predict potential human health risk from CO2 leakage into drinking water aquifers.  This work was partially funded by this EPA STAR grant.  Aquifer contamination may occur due to CO2 or brine leakage into groundwater via transport through faults and fractures, through faulty well bores, or through leaky confining materials. Contaminants of concern include aqueous salts and dissolved solids, gaseous or aqueous-phase organic contaminants, and acidic gas or aqueous-phase fluids that can liberate metals from aquifer minerals. This framework incorporates the potential mobilization of metals due to a decrease in pH; transport of these metals down gradient to municipal receptors; distributions of contaminated groundwater to multiple households; and exposure and health risk to individuals using this water for household purposes. The framework is stochastic, incorporates detailed variations in hydrogeological and geostatistical parameters and discriminates between uncertain and variable parameters using a two-stage, or nested, Monte Carlo approach. This approach is demonstrated using example simulations with hypothetical, yet realistic, aquifer characteristics and leakage scenarios. These example simulations show a greater risk for arsenic than for lead for both cancer and non-cancer endpoints, an unexpected finding. Higher background groundwater gradients also yield higher risk. The overall risk and the associated uncertainty are sensitive to the extent of aquifer stratification and the degree of local-scale dispersion. These results highlight the importance of hydrologic modeling in risk assessment, and that the influence of hydrogeologic and aquifer transport factors may be equally important to chemical toxicity when calculating risk. A linear relationship between carcinogenic and noncarcinogenic risk was found for arsenic and suggests action levels for carcinogenic risk will be exceeded in exposure situations before noncarcinogenic action levels, a reflection of the ratio of cancer and non-cancer toxicity values. The paper results had implications for ranking aquifer vulnerability due to geologic configuration, aquifer mineralogy, and leakage scenarios.

Siirila and Maxwell (2012b) presented a new perspective on human health risk assessment through development of a time-dependent methodology that accounted for the effect of varying exposure durations.  The Time Dependent Risk Assessment (TDRA) stochastically considers how joint uncertainty and inter-individual variability (JUV) associated with human health risk change as a function of time. In contrast to traditional, time independent assessments of risk, this new formulation relays information on when the risk occurs, how long the duration of risk is, and how risk changes with time. Because the true exposure duration (ED) often is uncertain in a risk assessment, we also investigate how varying the magnitude of fixed size durations (ranging between 5 and 70 years) of this parameter affects the distribution of risk in both the time independent and dependent methodologies. To illustrate this new formulation and to investigate these mechanisms for sensitivity, an example of arsenic contaminated groundwater is used in conjunction with two scenarios of different environmental concentration signals resulting from rate dependencies in geochemical reactions. Cancer risk is computed and compared using environmental concentration ensembles modeled with sorption as 1) a linear equilibrium assumption (LEA) and 2) first order kinetics (Kin). Results show that the information attained in the new time dependent methodology reveals how the uncertainty in other time-dependent processes in the risk assessment may influence the uncertainty in risk. The study also showed that individual susceptibility also affects how risk changes in time, information that would otherwise be lost in the traditional (time independent) methodology. These results are especially pertinent for forecasting changes in risk through time, and for risk managers who wish to evaluate them.

Siirila and Maxwell (2012c) evaluated effective reaction rates of kinetically driven solutes in large-scale, statistically anisotropic media and the associated human health risk implications.  The interplay between regions of high and low hydraulic conductivity, degree of aquifer stratification, and rate-dependent geochemical reactions in heterogeneous flow fields is investigated, focusing on impacts of kinetic sorption and local dispersion on plume retardation and channeling. Human health risk is used as an endpoint for comparison via a nested Monte Carlo scheme, explicitly considering joint uncertainty and variability. Kinetic sorption is simulated with finely resolved, large-scale domains to identify hydrogeologic conditions where reactions are either rate limited (nonreactive), in equilibrium (linear equilibrium assumption is appropriate), or are sensitive to time-dependent kinetic reactions. By utilizing stochastic ensembles, effective equilibrium conditions are examined, in addition to parameter interplay. In particular, the effects of preferential flow pathways and solute mixing at the field-scale (marcrodispersion) and subgrid (local dispersion, LD) are examined for varying degrees of stratification and regional groundwater velocities (v). Results show effective reaction rates of kinetic ensembles with the inclusion of LD yield disequilibrium transport, even for averaged (or global) Damköholer numbers associated with equilibrium transport. Solute behavior includes an additive tailing effect, a retarded peak time, and results in an increased cancer risk. The inclusion of LD for nonreactive solutes in highly anisotropic media results in either induced solute retardation or acceleration, a new finding given that LD has previously been shown to affect only the concentration variance. The distribution, magnitude, and associated uncertainty of cancer risk are controlled by the up scaling of these small-scale processes, but are strongly dependent on groundwater velocity and the nature of the contaminant source.

Atchley, et al. (2013a) conducted a human health risk assessment of CO2 leakage into overlying aquifers using a stochastic, geochemical reactive transport approach, which built on the work described above by including more complex geochemical reactions in the model formulation rather than simple linear equlibrium partitioning type reactions. Increased human health risk associated with groundwater contamination from potential CO2 leakage into a potable aquifer is predicted by conducting a joint uncertainty and variability (JUV) risk assessment. The approach explicitly incorporates heterogeneous flow and geochemical reactive transport in an efficient manner and is used to evaluate how differences in representation of subsurface physical heterogeneity and geochemical reactions change the calculated risk for the same hypothetical aquifer scenario where a CO2 leak induces increased lead (Pb2+) concentrations through dissolution of galena (PbS). A nested Monte Carlo approach was used to take Pb2+ concentrations at a well from an ensemble of numerical reactive transport simulations (uncertainty) and sample within a population of potentially exposed individuals (variability) to calculate risk as a function of both uncertainty and variability. Pb2+ concentrations at the well were determined with numerical reactive transport simulation ensembles using a streamline technique in a heterogeneous 3D aquifer. Variances of log hydraulic conductivity (s2lnK) were used as a quantitative measure of the degree of heterogeneity (s2lnK).  Three ensembles with s2lnK values of 1, 3.61, and 16 were simulated. Under the conditions simulated, calculated risk is shown to be a function of the strength of subsurface heterogeneity s2lnK and the choice between calculating Pb2+ concentrations in groundwater using equilibrium with galena and kinetic mineral reaction rates. Calculated risk increased with an increase in s2lnK of 1 to 3.61, but decreased when s2lnK was increased from 3.61 to 16 for all but the highest percentiles of uncertainty. Using a Pb2+ concentration in equilibrium with galena under CO2 leakage conditions (PCO2 = 30 bar) resulted in lower estimated risk than the simulations where Pb2+ concentrations were calculated using kinetic mass transfer reaction rates for galena dissolution and precipitation. This study highlights the importance of understanding both hydrologic modeling and detailed geochemical calculations when numerical simulations are used to perform quantitative risk calculations.

Atchley, et al. (2013b) used streamlines to simulate stochastic reactive transport in heterogeneous aquifers and investigate kinetic metal release and transport in CO2 impacted drinking water aquifers. Using streamlines as the computational domain greatly reduces the model simulation time for risk assessment calculations that include both heterogeneities in subusurface properties as well as detailed geochemical reactions, to enable simulations to be conducted on traditional desktop computers.  The Lagrangian streamline approach stochastically represents uncertainty in spatial hydraulic conductivity distribution and is coupled to kinetic reactive transport in a heterogeneous 3-D domain. This methodology is designed to efficiently account for uncertainties inherent in subsurface reactive transport while also including hydrogeochemical processes. A hypothetical CO2 leak from a geological carbon storage site into an overlying aquifer is used to simulate reactive transport where contamination may occur. Uncertainty in subsurface hydraulic conductivity is accounted for using correlated, Gaussian random fields in a Monte Carlo approach. In this approach, 100 realizations of each ensemble were simulated with variances of the natural log of hydraulic conductivity (s2lnK) of 1, 3.61, and 16. Peak ensemble lead concentrations were found at s2lnK of 3.61, the middle of the variances simulated. The value for s2lnK within an aquifer was found to influence chemical residence time, which in turn determined the equilibrium state of the plume along the flow path and at the pumping well thus driving geochemical conditions. However, macrodispersion due to heterogeneous flow paths caused lower contaminant concentrations at the pumping well due to dilution with uncontaminated water. Furthermore, a strong link between s2lnK and the probability of well capture was found, suggesting that proper characterization of the s2lnK within an aquifer will help to quantify the impact of uncertainty on risks of groundwater contamination.

Atchley, et al. (2014) investigated the role of coupled physical and geochemical heterogeneity in hydro-geochemical transport by simulating 3-D transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: (1) spatially homogeneous reactive mineral surface area, (2) reactive surface area positively correlated to hydraulic heterogeneity, and (3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (t). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical disequilibrium between fluids and the mineral galena that resulted in increased aqueous lead concentrations (Pb2+). Precipitation of galena is reactivated as calcite dissolution slowly buffers the aquifer pH change and a new geochemical equilibrium is achieved. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface areas in coordination with physical heterogeneity on the effective reaction rate (Krxn) and Pb2+ concentrations before equilibrium is re-established.  Dissimilarities between ensemble Pb2+ concentration and Krxn are attributed to how geochemical heterogeneity affects the time (teq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn and Pb2+ concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in teq and Leq.  This paper helps explain how hypothesized relationships between hydraulic conductivity and reactive geochemical zones may influence contaminant transport.


Journal Articles on this Report : 13 Displayed | Download in RIS Format

Other project views: All 70 publications 14 publications in selected types All 14 journal articles
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Journal Article Atchley AL, Maxwell RM, Navarre-Sitchler AK. Human health risk assessment of CO2 leakage into overlying aquifers using a stochastic, geochemical reactive transport approach. Environmental Science & Technology 2013;47(11):5954-5962. R834387 (2012)
R834387 (Final)
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  • Abstract: ES&T-Abstract
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  • Journal Article Atchley AL, Navarre-Sitchler AK, Maxwell RM. The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates.Journal of Contaminant Hydrology 2014;165:53-64. R834387 (Final)
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  • Journal Article Atchley AL, Maxwell RM, Navarre-Sitchler AK. Using streamlines to simulate stochastic reactive transport in heterogeneous aquifers: kinetic metal release and transport in CO2 impacted drinking water aquifers. Advances in Water Resources 2013;52:93-106. R834387 (2012)
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  • Journal Article Bearup LA, Navarre-Sitchler AK, Maxwell RM, McCray JE. Kinetic metal release from competing processes in aquifers. Environmental Science & Technology 2012;46(12):6539-6547. R834387 (2011)
    R834387 (2012)
    R834387 (Final)
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  • Journal Article Kirsch K, Navarre-Sitchler AK, Wunsch A, McCray JE. Metal release from sandstones under experimentally and numerically simulated CO2 leakage conditions. Environmental Science & Technology 2014;48(3):1436-1442. R834387 (Final)
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  • Journal Article Marcon V, Kaszuba J. Trace metal mobilization in an experimental carbon sequestration scenario. Procedia Earth and Planetary Science 2013;7:554-557. R834387 (Final)
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  • Journal Article Marcon V, Kaszuba JP. Carbon dioxide-brine-rock interactions in a carbonate reservoir capped by shale: experimental insights regarding the evolution of trace metals. Geochimica et Cosmochimica Acta 2015;168:22-42. R834387 (Final)
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  • Journal Article Siirila ER, Maxwell RM. Evaluating effective reaction rates of kinetically driven solutes in large-scale, statistically anisotropic media: human health risk implications. Water Resources Research 2012;48(4):W04527. R834387 (Final)
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  • Journal Article Siirila ER, Maxwell RM. A new perspective on human health risk assessment: development of a time dependent methodology and the effect of varying exposure durations. Science of the Total Environment 2012;431:221-232. R834387 (Final)
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  • Journal Article Siirila ER, Navarre-Sitchler AK, Maxwell RM, McCray JE. A quantative methodology to assess the risks to human health from CO2 leakage into groundwater. Advances in Water Resources 2012;36:146-164. R834387 (2011)
    R834387 (2012)
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  • Journal Article Wunsch A, Navarre-Sitchler AK, Moore J, Ricko A, McCray JE. Metal release from dolomites at high partial-pressures of CO2. Applied Geochemistry 2013;38:33-47. R834387 (2012)
    R834387 (Final)
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  • Journal Article Wunsch A, Navarre-Sitchler AK, McCray JE. Geochemical implications of brine leakage into freshwater aquifers. Groundwater 2013;51(6):855-865. R834387 (2012)
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  • Journal Article Wunsch A, Navarre-Sitchler AK, Moore J, McCray JE. Metal release from limestones at high partial-pressures of CO2. Chemical Geology 2014;363:40-55. R834387 (2012)
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  • Supplemental Keywords:

    Carbon sequestration, leakage, risk assessment, metals, brine, sandstones, carbonates, dolomites, limestones

    Progress and Final Reports:

    Original Abstract
  • 2010
  • 2011 Progress Report
  • 2012 Progress Report