Dissolution of a mineral phase in potable aquifers due to CO2 releases from deep formations;: effect of dissolution kinetics

Injection of supercritical CO2 into deep saline aquifers is a technique for sequestration of large amounts of CO2 that is currently being investigated as a means to ameliorate the release of greenhouse gases into the atmosphere. Because complete characterization of these geological formations is not possible, the likelihood that some fraction of the injected CO2 Will leak into overlying aquifers needs to be considered. If the leaking CO2 were to reach shallow groundwater aquifers, it could lead to geochemical alterations with detrimental effects on the water quality of these potable aquifers. Identification and assessment of these effects is necessary to analyze risks associated with geologic sequestration of CO2 adequately. In order to assess if there is a potential of solubilizing trace metals, metalloids and/or selected radionuclides by CO2 releases from deep formations into potable aquifers, a series of simulations were conducted. Numerical simulations were conducted for a series of CO2 release scenarios and different aquifer geochemical properties. The effect of CO2 induced pH changes as well as trace metal solubilization was assessed using a geochemical transport model. Results show that elevated CO2 levels in freshwater aquifers can enhance the dissolution of trace metals so that concentrations may reach undesirable levels at the local scale. Transport models demonstrate the importance of assessing the areal extent of this CO2 release, as well as the need to gain thorough understanding of the key kinetic processes related to CO2 solubilization and the dissolution of a trace metal containing mineral phase. (C) 2004 Elsevier Ltd. All rights reserved.