Dynamics and Controls of Microbial Hydrogen Consumption in Porous Media

While nuclear energy contributes significantly to the global energy supply, the long-term management of radioactive waste remains challenging. Deep geological repositories, in which the waste is buried within a low-permeability host rock, are considered the safest storage option. However, hydrogen gas (H2) generated by steel container corrosion may accumulate and create an overpressure that threatens host rock integrity. Access tunnels will be filled with gas-permeable material (such as a sand-bentonite mixture) that mechanically lowers pressure by gas diffusion, but microbial H2 consumption is key to enhancing safety and cost-effectiveness. Hydrogenotrophy is well established in deep subsurface sulfate-reducing bacteria, whereas the activity of other hydrogenotrophs under H2-replete conditions remains poorly constrained. This study examines microbial H2 consumption under repository-relevant conditions in partially saturated sand-bentonite exposed to elevated H2 concentrations. Over 3 months, continuous gas composition and pressure monitoring showed rapid H2 depletion driven by its microbial oxidation coupled to sulfate reduction, methanogenesis, and iron reduction. Water saturation and electron acceptor availability strongly controlled H2 consumption rates. These results clarify the constraints on microbial H2 oxidation in deep geological repositories and inform safety assessments related to gas-induced overpressure.