Metrological assessment of on-site geochemical monitoring methods within an aquifer applied to the detection of H2 leakages from deep underground storages

International audience ; To manage potential risks due to H2 leaks into the near-surface geosphere from H2 underground storages (e.g. salt caverns, aquifer), reliable monitoring methods along with a precise knowledge of the geochemical environmental impacts are necessary. Thus, the evolution of some prominent parameters in soil and aquifers can be determined: gas concentrations, redox potential, ionic balance and trace elements. As part of the ROSTOCK'H project, Ineris simulated H2 leakage by injection of dissolved H2 into a shallow aquifer (~20 m deep) in an experimental site within the Paris basin. This experiment aimed to testing advanced monitoring techniques and studying hydrogeochemical impacts at shallow depths. The aquifer water has calcium-bicarbonate facies and a neutral pH. Eight piezometers were aligned over 80 m according to the aquifer main flow (west-east). Hydrogeochemical monitoring devices were set up. One of the piezometers was equipped with a completion connected to a Raman probe and a specific Mid-IR cell for continuous measurement of aqueous gases. At the experiment outset, 5 m3 of water were extracted from the aquifer to be saturated with H2 under atmospheric conditions, before being reinjected through the injection well. About 100 LSTP of dissolved H2 (concentration of 1,8 mg/L) was injected in the aquifer. The H2 injection was preceded by the injection of underground water containing tracers (He(aq), uranine and LiCl) in order to warn the H2 plume arrival in the piezometers located downstream of the injection well. The concentrations of aqueous gases (He, H2, N2, O2, CO2, H2S and CH4) were measured in a control piezometer (20 m upstream) and in six piezometers up to 60 m downstream. Thus, the maximum H2 contents were detected up to 20 m downstream of the injection well: 0.6 mg/L at 5 m, 0.17 mg/L at 7 m then 1.8 µg/L of H2 at 10 and 20 m during the first week. Following the H2(aq) addition, the aquifer physico-chemistry has been modified: low increase in pH, high decrease in redox .