Impact of Water Saturation on Microbial Hydrogen Consumption in Porous Media

Hydrogen (H₂) production from steel corrosion challenges the long-term stability of deep geological repositories for radioactive waste storage. Subsurface microbial communities are known to consume H₂, but previous work has primarily focused on fully saturated porous environments. However, gas production will prevent full water saturation of the repository tunnels, where microbial H₂ consumption is expected. Here, we investigate H₂ consumption rates at four saturation levels in sand-bentonite, a candidate tunnel backfill material for the Swiss repository concept. The rates ranged from 0.14 μmol·d^-1·cm^-3_{sand-bentonite} (at 70% saturation) to 2.23 μmol·d^-1·cm^-3_{sand-bentonite} (at 90% saturation), exceeding those observed in fully saturated environments. Throughout the 90 days of the experiment, no oxidation was detected below 70% saturation. A sharp rise in H₂ consumption rate between 70% and 80% saturation reflected the reduced water potential leading to saturation of small pores. This study is the first to examine microbial H₂ consumption in a natural, partially saturated porous medium, an essential step toward accurately modeling H₂ sinks as a function of repository saturation. Beyond nuclear waste disposal, our findings may also inform strategies for subsurface H₂ storage.