Geochemical monitoring for H2 leakage: Impact of dissolved H2 injection into biogeochemistry of a shallow chalky aquifer

Ensuring the secure underground storage of hydrogen (H₂) is crucial for future energy systems. Shallow aquifers, as gas retention areas for leaks from deeper reservoirs, are key monitoring zones. The ANR Hystoren project aims to understand the impact of H₂ leakage into the biogeochemistry of a shallow carbonate aquifer. A controlled leakage simulation was conducted at the experimental Saint-Emilion site (France) injecting water in equilibrium with H2 and tracer gases (Krypton and Helium). Direct and indirect parameters such as concentrations of dissolved H2, CO2, CH4, He, Kr, isotopic C and H compositions, water physico-chemical parameters and microbial communities were tracked on monitoring wells over a scale of several meters for a week. Here we will focus on the geochemical results to identify the most sensitive parameters indicative of H2 leakage. On-site H₂ concentrations, measured with Unisens© probes, allowed direct observation of the H2 plume across µM to mM scales. Laboratory analyses confirmed but showed lower concentrations, indicating reduced sensitivity method to monitor dissolved H₂. The evolution of krypton concentrations (ex-situ measurements) correlate closely with H₂ concentrations. The integration of these methodologies highlighted the complementarity of in-situ and ex-situ approaches and their importance in detecting early signs of leakage.