Unexpected oxygen calls for rethinking redox conditions in deep geological repositories

Oxidation-reduction (redox) conditions are among the key factors affecting the long-term safety of nuclear waste disposal. In Finland and Sweden, where high level nuclear waste will be deposited at a depth of 400-450 m in crystalline bedrock, underground repositories are estimated to become anoxic within 200 years after closure (King et al 2010). This scenario follows the general understanding of biological and abiotic consumption of photosynthetic oxygen in the subsurface but does not consider possible in-situ oxygen production. However, non-photosynthetic oxygen is known to be produced by microbiological dismutation reactions and radiolysis of water even under the dark conditions prevailing in the deep subsurface (e.g., Ershov & Gordeev, 2008; Ruff et al. 2023). Such continuous supply of oxygen in deep groundwater environments can greatly influence the long-term safety of nuclear waste disposal, as corrosion reactions, microbial activity, and the mobility of uranium and other redox sensitive elements are strongly affected by the availability of oxygen.

 In our study, we investigated dissolved oxygen concentrations and isotopic composition, microbial communities and their metabolic pathways, as well as reaction thermodynamics to reveal dark oxygen production and potential for oxic niches in deep (> 125 m) bedrock groundwaters in Finland. Our results show that oxygen is commonly found in the deep subsurface, its isotopic composition differs from that of atmospheric air, and deep microbial communities contain genes for protection against oxygen radicals and oxygen-dependent metabolic pathways. Reactions involving oxygen are thermodynamically favored, which would suggest rapid consumption of oxygen. However, their progression is constrained by the availability of other reactants or the accumulation of reaction products, and therefore only a subset of oxygen-driven reactions is energetically feasible in the deep subsurface, potentially allowing the development of oxic niches. Considering these results, redox conditions in the deep subsurface should be reevaluated.

References

Ershov, B. G. & Gordeev, A. V., 2008. A model for radiolysis of water and aqueous solutions of H₂, H₂O₂ and O₂. Radiation Physics and Chemistry 77, 928-935.

King, F., Lilja, C., Pedersen, K., Pitkänen, P., Vähänen, M., 2010. An update of the state-of-the-art report on the corrosion of copper under expected conditions in a deep geologic repository. SKB-TR-10-67, Swedish Nuclear Fuel and Waste Management Co., 176 p.

Ruff, S. E., Humez, P., Hrade de Angelis, I., Diao, M., Nightingale, M., Cho, S., Connors, L., Kuloyo, O. O., Seltzer, A., Bowman, S., Wankel, S. D., McClain, C. N., Mayer, B., Strous, M., 2023. Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems. Nature Communications 14, 3194.