Présentations Colloques

One concern regarding CO2 geological storage is the potential impact a leak of gas, or reservoir-associated brines, may have on groundwater quality in overlying potable aquifers. Sites where geological CO2 is naturally leaking into near-surface aquifers provide opportunity to study large, long-term processes in different geological settings. We present results from two locations in Italy- i) the Latera caldera has silicate mineralogy with gas leakage (CO2, trace H2S, CH4)+ ii) the San Vittorino site has carbonate mineralogy with leaking gas (CO2, trace H2S, CH4) and groundwater. **At both sites shallow boreholes were augered parallel to the hydraulic gradient. Samples were analysed on-site for physical-chemical parameters and in the lab for major and trace elements. **A significant difference between the response of the two aquifers was observed. The low buffering capacity of the silicates at Latera result in pH values as low as 3.5 compared to minimum pH values of only 6 in the San Vittorino carbonates. Redox was found to be equally important in controlling mineral stability and associated ion mobility. H2S is particularly important due to its oxidation and the formation of sulfate ion complexes, yielding species and processes not unlike those in acid mine drainage. Different elements were anomalous in the leakage areas of the two sites, such as very high aluminum values in Latera silicates or elevated magnesium in San Vittorinio carbonates. Down gradient attenuation processes lower the concentrations of most elements, although this is more evident at the Latera site due to the longer transect. **This work highlights the importance of aquifer mineralogy and trace gas composition on the potential impact of leakage from a CCS storage reservoir on groundwater quality. The utility of studying natural leaking sites is also illustrated.