Tracking leakage from a natural CO2 reservoir (Montmiral, France) through the chemistry and isotope signatures of shallow groundwater

 Natural accumulations and releases of CO2 provide the opportunity to study the CO2 trapping and migration mechanisms, the potential impacts of CO2 leaks, and the monitoring tools to assess the impacts of geological storage of anthropogenic CO2. Previous studies on the deep CO2 reservoir of Montmiral (France), focusing on soil gases, groundwater, as well as deep fluids, did not detect any signs of leaks and despite high CO2 fluxes suspiciously high δ13C values have not been stated. In order to further investigate whether some CO2 has leaked from the reservoir toward the surface, we focus here on the major and trace element geochemistry of the shallow aquifers overlying the reservoir with a special focus on the carbonate system, using isotope tracers potentially sensitive to leaks (δ13C of DIC, 87Sr/86Sr and stable isotopes of water). A forward modeling of the potential evolution of groundwater in case of leaks was performed, combining equilibrium calculations of the carbonate system and an ad hoc carbon isotope model. Most observed δ13C values are compatible with modeled carbonate dissolution under open or closed conditions with respect to CO2. A 13C-enriched subset of samples shows clear signs of incongruent dissolution of Mg-Sr-calcite or dolomite, corroborated by 87Sr/86Sr ratios, so that mixing with isotopically heavy deep CO2 is not required to explain the observed chemical and isotope data. The absence of any sign of CO2 leakage into shallow groundwater would support the fact that the reservoir and caprock have been trapping the CO2 efficiently over millions of years.