Inverse Modeling of Water-Rock-CO 2 Batch Experiments: Potential Impacts on Groundwater Resources at Carbon Sequestration Sites Changbing Yang,* ,† Zhenxue Dai, ‡ Katherine D. Romanak, † Susan D. Hovorka, † and Ramó n H. Treviñ o † † Bureau of Economic Geology, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, United States ‡ Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States * S Supporting Information ABSTRACT: This study developed a multicomponent geo- chemical model to interpret responses of water chemistry to introduction of CO 2 into six water-rock batches with sedimentary samples collected from representative potable aquifers in the Gulf Coast area. The model simulated CO 2 dissolution in groundwater, aqueous complexation, mineral reactions (dissolution/precipitation), and surface complexation on clay mineral surfaces. An inverse method was used to estimate mineral surface area, the key parameter for describing kinetic mineral reactions. Modeling results suggested that reductions in groundwater pH were more significant in the carbonate-poor aquifers than in the carbonate-rich aquifers, resulting in potential groundwater acidification. Modeled concentrations of major ions showed overall increasing trends, depending on mineralogy of the sediments, especially carbonate content. The geochemical model confirmed that mobilization of trace metals was caused likely by mineral dissolution and surface complexation on clay mineral surfaces. Although dissolved inorganic carbon and pH may be used as indicative parameters in potable aquifers, selection of geochemical parameters for CO 2 leakage detection is site-specific and a stepwise procedure may be followed. A combined study of the geochemical models with the laboratory batch experiments improves our understanding of the mechanisms that dominate responses of water chemistry to CO 2 leakage and also provides a frame of reference for designing monitoring strategy in potable aquifers. ■ INTRODUCTION One of the critical issues related to geological CO 2 sequestration (GCS) is the potential impact of CO 2 leakage from the storage formations on underground sources of drinking water (USDW) in overlying aquifers. Although CO 2 itself is not hazardous to the water quality of USDW, increased CO 2 concentrations in USDW caused by CO 2 leakage could lead to a decrease in groundwater pH, potentially enhancing mineral dissolution, adsorption/desorption, and cation ex- change reactions, subsequently resulting in mobilization of major ions and trace metals from aquifer sediments into groundwater, 1-8 and thus potentially degrade groundwater quality. A variety of approaches, including laboratory experi- ments, 4,6,9,10 field tests, 2,8,11,12 numerical modeling, 5,13-17 and natural and industrial analogs, 18-20 have been used to (1) assess risks of CO 2 leakage from deep reservoirs into USDWs and (2) determine plausible geochemical parameters that might be used to detect CO 2 leakage in potable aquifers. 13 Few laboratory experiments of water-rock-CO 2 interaction and field tests were reported in the literature to investigate how CO 2 was introduced, in contact with shallow-aquifer sediments and groundwater. The results show that groundwater pH was decreased, and some major and trace metals may be mobilized after introducing CO 2 into the reactors. 1,2,4,6,8,11,21 Limitations of laboratory batch experiments were also obvious, such as, high water-rock ratios (usually 4:1), a small amount of sedimentary samples, and low CO 2 pressure (∼1 atmospheric) which was applied to the water-rock system. 2,8 To date, much of our understanding of impacts of the leaked CO 2 on drinking groundwater quality is based on modeling investigations because numerical modeling studies can help identify the physical and geochemical processes that control the effects of CO 2 introduced into an aquifer and provide information for risk assessments related to GCS. Wang and Jaffe 14 simulated dissolution of galena in shallow aquifers as a result of CO 2 leakage and concluded that a decrease in pH would result in large concentrations of Pb in shallow groundwater. Zheng et al. 13 simulated reactive transport of CO 2 into a shallow aquifer for 100 yr and concluded that CO 2 leakage could mobilize As and Pb in shallow aquifers through mineral dissolution and surface complex desorption. However, most modeling studies are subject to considerable uncertainty inherent in model Received: September 18, 2013 Revised: January 30, 2014 Accepted: February 4, 2014 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es4041368 | Environ. Sci. Technol. XXXX, XXX, XXX-XXX