This presentation explores the role of microorganisms in the engineered barrier system of a deep geological repository (DGR) for used nuclear fuel, with a focus on bentonite clay and surrounding subsurface environments. Microbial activity, particularly the production of sulfide by sulfate-reducing bacteria (SRB), poses a potential risk to corrosion of copper that will surround used fuel containers. To assess this, a combination of cultivation and molecular analyses were used to characterize microbial communities in a range of bentonite clays and DGR-relevant environments. Results show that dry compacted bentonites at high densities and low water activity limit microbial growth, supporting the role of these factors as effective barriers for microorganisms. Field studies at the Grimsel Underground Research Laboratory (MaCoTe experiment) confirmed microbial community stability over a decade in saturated modules under repository-like conditions. Laboratory enrichments recovered various taxa, including aerobic heterotrophs and SRB, but only under permissive conditions. Subsurface rock samples yielded no detectable microbial signal, whereas groundwater contained low abundance microbial communities including members of CPR and DPANN lineages. Together, these findings demonstrate the effectiveness of bentonite as a microbiological barrier, while highlighting the importance of water availability, density, and swelling pressure in controlling microbial survival. This research helps inform microbial risk assessment and long-term safety evaluations for DGR systems in Canada and beyond.

Learning Objectives:

1. Describe characteristics of the subsurface and how they impact microbial communities

2. Contrast microorganisms found in bentonite clay with those found in the subsurface

3. List characteristics of a deep geological repository and explain how these will prevent microbial growth