Microbial Characterization Of Groundwater From Boreholes CR9 And CR18 At Crl (2007-2009) - Implications For A Possible Future Repository For Radioactive Non-fuel Waste

Abstract

A microbiological characterization study was carried out on groundwater samples taken from various depths in boreholes CR9 and CR18 at Chalk River Laboratories (CRL), over a three-year period (2007 -; 2009), as part of the technical feasibility study for siting a proposed Geologic Waste Management Facility (GWMF) at the CRL site. The characterization included a variety of methods to determine total, viable and culturable cell counts, as well as identifications using molecular methods. Results indicated a total population of 10⁴ to 10⁵ cells/mL of which < 1% could be cultured. However, a large percentage of this population was in fact viable and appeared to survive in a viable-but-not-culturable (VBNC) state. The microbial results combined with geochemical observations suggested that the oligotrophic biogeochemical system in CR9 may have exhausted the process of nitrate reduction but, due to a lack of electron donors (dissolved organic carbon, DOC) and acceptors (e.g., Fe(III)), is not able to lower Eh values sufficiently to allow the occurrence of sulphate reduction and methanogenesis as major processes. The results from a biofilm experiment in CR18 showed that most cells appeared to be suspended rather than attached. From a future GWMF perspective, the presence of a population of largely VBNC cells implies that, given an increased source of electron donors (DOC) and acceptors (e.g., metals) leached from the waste, microbial activity could increase significantly. This can have both positive effects, such as a lower redox potential and lower radionuclide solubility, and negative effects, such as increased radionuclide mobility, and ¹⁴C-containing gas production. It is expected that ultimately the biogeochemical system would return to its original oligotrophic conditions but the rate at which this would occur is, at present, uncertain because both waste leach rates and in situ microbial metabolic rates are unknown. The current results, limited to samples from CR9 and CR18, 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.