CO2 Releases from Deep Storage Formations into Drinking Water Aquifers - Assessment of Impacts on Drinking Water Quality

Geological storage of supercritical CO2 is envisioned as a means of mitigating the release of greenhouse gases into the atmosphere. However, the potential exists for CO2 to migrate from the deep geologic formations to overlying aquifers that serve as sources of drinking water, which could lead to geochemical alterations that have detrimental effects on drinking water quality. For example, elevated CO2 levels in drinking water aquifers can enhance the solubility and decrease the sorbed fraction of trace metals and radionuclides to an extent that concentrations may reach undesirable levels at the local scale. Therefore, an assessment of these effects is necessary to determine the risks associated with geologic sequestration of CO2. In this study, the effects of CO2 intrusion into a sandstone aquifer (with and without calcite cement present) on the water chemistry and on the mobility of trace metals and radionuclides were investigated. The aquifer was assumed to be unpolluted such that sorption, not solubility, was likely to be the predominant process controlling heavy metal and radionuclide mobility. Four elements with very different geochemical behaviors were selected for the study - lead, copper, arsenic, and uranium - and sorption was assumed to occur on ferric oxyhydroxides coating the sandstone matrix. Two-dimensional simulations were conducted using the coupled reactive-transport code MULTIFLO to determine the changes in aquifer water chemistry - spatially and temporally - as a function of CO2 flux from a leaking CO2 sequestration aquifer. Lead, copper, arsenic, and uranium Kd values as a function of pH and pCO2 were derived using equilibrium thermodynamic calculations and used to assess the impact of CO2 leakage on heavy metal and radionuclide mobility based on the MULTIFLO results. This work was funded by the Southwest Research Institute Internal Research and Development Project 20- R9826.