Stable Hydrogen Isotope Fractionation of Hydrogen in a Field Injection Experiment: Simulation of a Gaseous H₂ Leakage

In the context of the shift toward clean, carbon-free energy, hydrogen (H 2 ) has received growing attention as an energy carrier. We monitored a simulated leakage of gaseous molecular H 2, e.g., from a pipeline. H 2 was injected into a shallow aquifer, and the resulting biogeochemical processes were monitored. For the first time, stable isotopes of hydrogen were used to track in situ H 2 transport and consumption. Isotopic composition of injected H 2 was δ 2 H = −161.1 ± 0.4‰. During the injection, initial shifts in the isotope signature of about Δ 2 H = +8‰ in well D06 (1 m from injection) and Δ 2 H = −120‰ in well D04 (2 m from injection) were observed, probably caused by a mass-dependent isotope effect associated with the pressure of the injection and the migration of the gas phase through pores and channels in the aquifer. After the injection, H 2 concentrations decreased and an equilibrium isotope exchange with water led to an isotopic depletion of H 2 (δ 2 H = −710.7 ± 2.7‰) within 28 days, presumably catalyzed by hydrogenase enzymes of microbes. The theoretical equilibrium between H 2 and water was however not reached. We hypothesize that a continuous isotopic shift in available H 2 as a result of physical transport processes resulted in a new isotope equilibrium with water, catalyzed by hydrogenases. Acetate detected in groundwater samples indicates in situ H 2 oxidation by microbial homoacetogenesis. In laboratory experiments using H 2 -amended sediments sampled from the same site, microbial H 2 oxidation was accompanied by equilibrium isotope exchange with water and homoacetogenesis and ferric iron reduction were the main microbial H 2 -consuming processes. Overall, the H 2 isotope ratio was considerably impacted by physical and microbial processes occurring in the shallow aquifer. Monitoring of the equilibrium isotope exchange between H 2 and water could be used as a proxy for ongoing microbial H 2 oxidation.