The effects of the physical properties of highly compacted smectitic clay (bentonite) on the culturability of indigenous microorganisms

The Canadian approach for long-term management of used nuclear fuel waste is Adaptive Phased Management (APM) which includes development of a Deep Geological Repository (DGR) and placement of nuclear fuel waste in corrosion-resistant copper containers excavated at a depth of 500–1000 m in a suitable host rock formation. The containers would be surrounded by compacted bentonite-based buffers and backfills. This study examined the conditions required to suppress microbial activity in compacted bentonite, such that microbially influenced corrosion (MIC) of copper waste containers in a future DGR would be insignificant. Experiments (of 40–90 days duration) were carried out to determine the effects of dry density and porewater salinity on swelling pressure, water activity (aw) and the culturable and viable microbial community indigenous to MX-80 Wyoming bentonite. A low aw (< 0.96) and a swelling pressure > 2 MPa appear to suppress microbial aerobic culturability below background levels (2.1 × 102 Colony-Forming Units/g) in as-purchased bentonite. To actually impose conditions of aw < 0.96 and swelling pressure > 2 MPa in compacted bentonite in a DGR, dry density needs to be maintained at 1.6 g/cm3 or higher for porewater salinities at ≤ 50 g/L. High porewater salinity (> 100 g/L) also keeps aw < 0.96 and aerobic culturability below background values. Under such conditions, cells likely survive as dormant cells or inactive spores (as suggested by phospholipid fatty acid analysis), which reduces the possibility of significant MIC. Observations in natural clay-rich environments support these findings. The geological timescale of microbial survival in dormant form is at present unknown. Interfacial locations in a DGR could form environments where (temporarily at least) the physical conditions necessary to suppress microbial activity would not always be met. The extent and potential importance of such interfaces with respect to revived microbial activity (and MIC) in a DGR require further study.