Interface-associated microbial growth in anoxic pressure vessels containing compacted gapfill bentonite

Deep geological repositories (DGR) for belowground storage of used nuclear fuel rely on highly compacted bentonite to limit growth of microorganisms, including those that can contribute to microbiologically influenced corrosion, like sulfate-reducing bacteria (SRB). This study examined how a range of bentonite dry densities (1.25-1.60 g cm-3) affected microbial community abundance and composition in bentonite incubated in pressure vessels for 1, 4, or 12 months under anoxic conditions.

Culturable heterotroph abundance increased in some lower-density (i.e., 1.25 and 1.40 g cm-3) samples after 1 month, consistent with previous experiments. Unlike previous studies, outer layer bentonite samples had higher abundances of culturable SRB than the starting material. SRB-associated sequences were more prevalent in swabs from pressure vessel surfaces than in bentonite, suggesting interface-associated SRB growth.

Although culturable SRB increased for several outer layer pressure vessel samples, abundances remained relatively low and did not increase beyond the values measured at the 1-month timepoint. Overall, a minimum dry density of 1.45 g cm-3 limited microbial growth in inner layer bentonite samples.