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 CO ₂ adequately. In order to assess if there is a potential of solubilizing trace metals, metalloids and/or selected radionuclides by CO ₂ 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.