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.