Rates of microbial hydrogen oxidation and sulfate reduction in Opalinus Clay rock

Abstract Hydrogen gas (H 2 ) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H 2 is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm −3 day −1 for H 2 , and 0.14 and 0.20 μmol cm −3 day −1 for sulfate. Based on the stoichiometry of the reaction, there is an excess of H 2 consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H 2 is lost via diffusion. These rate estimates are critical to evaluate whether biological H 2 consumption can negate H 2 production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.