The impact of CO2 leakage on groundwater chemistry of major and trace elements: result from a field experiment

The impact of CO2 perturbation on groundwater chemistry appears as a key point for the safety of long term CO2 geological storage. Special focus is based on the risk of remobilisation and transfer of metals species into groundwater induced by CO2 hydrogeochemical perturbation. We performed a field experiment based on single well injection of small volume (3m3) of water-CO2 saturated, equivalent of approximately 3kg of CO2 injection. This experience was performed in aquifer presented relatively low metals concentration in groundwater while rock reservoir was naturally concentrated in different metals species (Fe, Mn, U, As, Zn, Pb etc.). We found that CO2 perturbation produced important quantity of carbonate, showed by an important increase of alkalinity, correlated with a pH decrease. This increase in alkalinity is also correlated with an increase in Ca2+ and Mg2+, which suggests a dissolution of dolomite, confirmed by geochemical model simulation. On the other hand, an enrichment of several order of magnitude in metal species concentration was measured into the groundwater, especially Fe, Mn and Zn. We propose that acidification due to CO2 dissolution induced dissolution of minerals. Further, those results have showed a significant effect of carbonate speciation on the release of metals. This field experiment highlighted the complex interaction between pH, Redox, major species, metal species into CO2-water-rock interaction system.