Groundwater hydrogeochemistry in injection experiments simulating CO₂ leakage from geological storage reservoir

Geologic carbon sequestration has the potential to reduce greenhouse gas concentrations in the atmosphere. However, one barrier to large scale implementation is concern for water quality degradation from leakage of high CO₂ fluids into drinking water aquifers. The hydrogeochemical response to simulated CO₂ leakage was studied to estimate major and trace element release and to develop criteria for water quality monitoring and risk assessment. In this study, approximately 3100L aquifer water enhanced with 1 atmosphere pressure CO₂ gas was injected into a fracture zone located at 362-366m below the ground surface in a sandstone/siltstone/mudstone interbedded aquifer in the Newark Basin. This was followed by a 3-6 week long incubation and then continuous monitoring of the hydrogeochemistry in the pumped-back water samples. Relative to background conditions, the recovered aquifer water displayed a decrease of pH, increase of alkalinity, Ca, Mg and Si concentrations, decrease of sulfate and Mo concentrations, and increased concentrations of trace elements including Fe, Mn, Cr, Co, Ni, Cu, Zn, Rb, Sr, Ba and U. These changes in aquifer water geochemistry can be explained by (a) dissolution of silicate and carbonate minerals and (b) trace element release that appear to be dependent on pH and pCO₂ and affected by the altered redox conditions in the aquifer. Rapid and simultaneous changes of pH, specific conductance, major and trace metal release in aquifer water could be used as indicators of CO₂ leakage from geologic sequestration sites. Hydrogeochemical parameters including pH, total dissolved solids and trace elements, particularly Fe, Mn, and Zn, need to be monitored in compliance with the U.S. Environmental Protection Agency (EPA) drinking water regulations.