[H-CG21] Nuclear Energy and Geoscience

[HCG21-P04] Verification study on borehole investigation technologies: (5) Impact of drilling mud fluid on microbial analysis

*Toru Nagaoka¹, Shin-ichi Hirano¹, Tokihiro Kurita¹ (1.Central Research Institute of Electric Power Industry)

Keywords:Deep underground, Microbial community, Borehole investigation, Drilling mud fluid, Contamination

Microorganisms live in the deep underground environment, resulting in the impacts on nuclear waste geological disposal barrier system, such as migration behavior of radionuclides in groundwater and corrosion of metallic overpack¹⁾. In order to assess microbial impacts on the disposal system, it is important to reveal the microbial abundance and the species in the deep underground environment as geological disposal site. Therefore, we sampled rock cores, groundwater and drilling mud fluid at the approximately same depth (G.L. -695m) from the NUMO-CRIEPI joint borehole investigation at the Yokosuka area of our institute and analyzed microorganisms in the samples by commonly used method, i.e., total cell count by fluorescence staining, viable cell count using agar medium, quantification of functional genes, amplicon sequence analysis targeting 16S rRNA genes. As a result of evaluating the impact of drilling mud on microbial analysis, it was difficult to completely prevent cross-contamination of rock cores and groundwater by drilling mud fluid as expected. In particular, viable cell count was significantly affected by highly alkaline drilling mud fluid, whereas microbial community analysis could be succeeded. These results suggest that it is necessary to apply novel analysis techniques for microbial activities in the samples collected from borehole investigations, and finally quantitative assessment methods for microbial impacts on nuclear waste disposal barrier systems need to be developed.

References
1) Lloyd and Cherkouk (2021): The microbiology of nuclear waste disposal, Elsevier inc.