MICROBIAL CHARACTERIZATION OF GROUNDWATER FROM BOREHOLES CR9 AND CR18 AT CRL - IMPLICATIONS FOR A POSSIBLE FUTURE REPOSITORY FOR RADIOACTIVE NON-FUEL WASTE

A microbiological characterization study was carried out on groundwater samples from various depths in boreholes CR9 and CR18 at Chalk River Laboratories (CRL) as part of the technical feasibility study for siting a proposed Geologic Waste Management Facility (GWMF) for radioactive non-fuel waste at CRL. Several methods were used to determine total, viable and culturable cell counts, and identify species. Samples contained a total population of 10⁴ to 10⁵ cells/mL of which < 1% could be cultured. However, a large percentage of the total population was viable and may be surviving in a viable-but-not-culturable (VBNC) state. The combined microbial and geochemical results suggest an oligotrophic biogeochemical system in CR9 that has exhausted the process of nitrate reduction but, due to lack of electron donors (ED; i.e., dissolved organic carbon, DOC) and acceptors (EA; e.g., Fe(III)) is not able to lower Eh values sufficiently to allow sulphate reduction and methanogenesis to become major processes. Biofilm sampling in CR18 showed that most cells appeared suspended rather than attached. The presence of a population of largely VBNC cells implies that, given an increased source of ED (DOC) and EA (e.g., metals) leached from the waste, microbial activity could increase significantly in a GWMF. This can have both positive effects (e.g., lower Eh and radionuclide (RN) solubility) and negative effects (e.g., increased RN mobility, ¹⁴C-containing gas production). Ultimately the biogeochemical system is expected to return to its original oligotrophic condition but the rate at which this would occur is uncertain because waste leach rates and in situ microbial metabolic rates are unknown. The current results do not preclude the feasibility of a GWMF at the CRL site, but suggest that microbial effects need to be considered in the safety assessment of a GWMF.