Lawrence Berkeley National Laboratory

Storage of CO2 in deep geologic formations is under intensive investigation. The potential for leakage from CO2 storage reservoirs into potable aquifers and its impact on the groundwater quality is a potential concern. CO2 dissolution in groundwater decreases pH. The increased acidity can affect the solubility of minerals containing such hazardous inorganic chemical constituents as Pb, As, Hg, Sb, Se, Cu, Ni, Co, Zn, Cd, Mo, and U, and cause their concentration to increase in the potable water. Dissolution of some chemical constituents could be further enhanced through carbonate complexation, possibly exceeding EPA specified health based limits (HBLs) for drinking water. Because many geochemical and hydrogeological factors can affect the dissolution and precipitation behavior of these constituents, their mobilization can be best studied through reactive geochemical transport modeling. We have investigated mobilization of Pb and As in an aquifer in response to CO2 intrusion using the TOUGHREACT simulator. A large number of simulations have been performed to address sensitivities to system variables including (1) rock mineral composition, (2) adsorption, (3) cation exchange, (4) co-precipitation, (5) reaction kinetics, (6) hydrological and CO2 flow rate boundary conditions. Our simulations show that lead (Pb) concentration can increase above health-based limits even for relatively small amounts of CO2 intrusion, but only when the aquifer is poorly buffered and if model assumptions are conservative. If adsorption is taken into account, the resulting lead concentration is considerably lower. A simple model for co-precipitation with carbonate minerals results in removal of significant amounts of lead from the aqueous phase. A relatively small calcite inventory of about 2% (representative of a Gulf Coast sandstone) provides a relatively good buffer helping to maintain an elevated pH, and minimize adverse effects of CO2 intrusion on lead concentrations. For arsenic, similar simulation results are generally obtained.