Tracking leakage from a natural CO 2 reservoir (Montmiral, France) through the chemistry and isotope signatures of shallow groundwater

Natural accumulations and releases of CO 2 provide the opportunity to study the CO 2 trapping and migration mechanisms, the potential impacts of CO 2 leaks, and the monitoring tools to assess the impacts of geological storage of anthropogenic CO 2 . Previous studies on the deep CO 2 reservoir of Montmiral (France), focusing on soil gases, groundwater, as well as deep fluids, did not detect any signs of leaks and despite high CO 2 fluxes suspiciously high δ-super-13C values have not been stated. In order to further investigate whether some CO 2 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 (δ-super-13C of DIC, -super-87Sr/-super-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 δ-super-13C values are compatible with modeled carbonate dissolution under open or closed conditions with respect to CO 2 . A -super-13C‐enriched subset of samples shows clear signs of incongruent dissolution of Mg‐Sr‐calcite or dolomite, corroborated by -super-87Sr/-super-86Sr ratios, so that mixing with isotopically heavy deep CO 2 is not required to explain the observed chemical and isotope data. The absence of any sign of CO 2 leakage into shallow groundwater would support the fact that the reservoir and caprock have been trapping the CO 2 efficiently over millions of years.