Impacts of thermo-hydro-mechanical experiments on the microbial activity in compacted bentonite at the Kamaishi Mine, Northeast Japan

Abstract

Microbial activity has been investigated for the bentonite buffer and surrounding host rock (granodiorite) at the Kamaishi Mine in Iwate, Northeast Japan. For the host rock, the total number of bacteria and viable microorganisms was enumerated for deep groundwater in granodiorite which was sampled from six hydraulically separated zones. The six groundwater samples were taken from two horizontal boreholes drilled from drift KD-90. Using acridine orange direct staining, the total number of bacteria was 8.6 × 10 2 to 1.2 × 10 6 cells/ml. From the most probable number (MPN) technique, heterotrophs numbered in the range of 5.3 × 10 2 to 3.9 × 10 4 CFU/ml. Nitrate-reducing bacteria dominated, in the range of 4.9 × 10 2 to 5.4 × 10 4 MPN/ml, while the presence of sulphate-reducing bacteria and denitrifying bacteria was also confirmed. The coupled thermo-hydro-mechanical (T-H-M) experiments named “engineered barrier experiments” were carried out to examine the in situ performance of buffer material. At the end of the heating and cooling phases, bentonite samples were taken for microbial analysis to determine if the naturally present microbial population in the buffer material survived the conditions (i.e. compaction, heat and desiccation) in a simulated vault environment. Heterotrophic aerobic bacteria were enumerated by the pour plate method using R2A media. The results confirmed the existence of heterotrophs, which disappeared in bentonite samples with low water content (< 12%). These results suggest that microbial activity is severely limited near waste container in the vault for some time after disposal, due to desiccation as a result of the heat output of the waste container. Such knowledge will be useful in assessing the potential effects of microbial activity on deep geological disposal of high level radioactive waste.