Investigation of Microorganisms in Bentonite Deposits

Microbiological characteristics of bentonite deposits were investigated as a natural analogue of microbial behavior in the buffer material for geological disposal of radioactive waste. Distributions of microorganisms in bentonite were examined at four sites in two different bentonite deposits in Japan. The sites included pond bottom, wetland, and wet mine gallery environments where bentonite layers have been left undisturbed for 2 to 30 years. Excavation was performed without using drilling water and the center parts of the cores were used for microbial examination. Plate counts with R2A medium of aerobic and anaerobic bacteria at the drilling mouth ranged from 10⁵ to 10⁷/g DW (dry weight) and from 10³ to 10⁶/g DW, respectively. The CFDA-AM (Carboxyfluorescein diacetate acetoxymethyl ester) cell counts ranged from 10⁶ to 10⁹/g DW. Bacterial numbers in the bentonite layers declined with distance from the drilling mouth; both aerobes and anaerobes were less than 10²/g DW and CFDA-AM cell counts less than 10⁶/g DW for core samples taken from approximately 1 m depth, except at the pond bottom. These results suggest that microbial activity in natural bentonite is lower than in typical soils and aquatic sediments and does not spread easily.

Keywords:

• bentonite
• montmorillonite
• bacteria
• radioactive waste
• buffer material

The present research was funded by the Japan Nuclear Cycle Development Institute (JNC), Naka-gun, Ibaraki, Japan. The authors thank A. Honda (JNC), C. Oda (JNC), R. P. Jensen (JNC), and Y. Otsuka (a former member of JNC) for their great support and valuable advice. We also would like to thank T. Senju (Ishikawajima Inspection and Instrumentation Co., Ltd., Japan) for assistance in microbiological examination, and K. Aoki (Horonobe Underground Research Center, Japan), T. Kanno and K. Ishii (Ishikawajima-Harima Heavy Industries Co., Ltd., Japan) for their generous advice.

Notes

^a Distances from the drilling mouth at site B reflect correction for core extention. At site A, depths from the pond bottom (top of the bentonite layer) are noted in parentheses, reflecting the deviation of the drilling axis from the vertical.

^b No data.

^a Pond water. All analyses except temperature were performed on a sample collected on Feb. 22, 2002.

^b Springwater.

^c Groundwater stream in the gallery.

^d Water dripping from the gallery ceiling.

^e No data.