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 L^STP 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 H_2(aq) addition, the aquifer physico-chemistry has been modified: low increase in pH, high decrease in redox potential and O_2(aq). These results confirm the feasibility of detecting and monitoring H₂ in shallow aquifers in very low concentration conditions and highlight the potential impacts. This is of first importance for establishing the surveillance and security aspects related to with H₂ storage.