Colonization of compacted backfill materials by microorganisms

Experiments were carried out to investigate the occurrence of pore clogging in backfill by bacterial activity. Four differently prepared and treated backfill materials were used to determine the effects of the quality and preparation method of the backfill materials on the occurrence of pore clogging. The backfills were compacted in permeameters which were infused with either groundwater or sterile distilled water. A constant pressure was applied to increase the rate of saturation. Results showed different inflow rates for the four materials despite the use of the same packing method for each specimen, the same dry density for each backfill and indications of similar initial pore volumes. These differences were likely caused by the fact that the two slowest-flowing permeameters contained a mixture of Na-bentonite and illitic shale simulating a glacial lake clay. Hydraulic conductivities measured ranged from 5 x 10-11 m/s to 5 x 10-12 m/s for the backfills containing glacial lake clay and 4 x 10-12 m/s to 9 s 10-13 m/s for the backfills containing a mixture of Na-bentonite and illitic shale. Weekly samples of outflow from the permeameters were analyzed microbially. Aerobic heterotrophs were low initially but stabilized around 106 to 107 colony forming units (CFU)/mL after about one week. Anaerobic heterotrophs stabilized at around 102 to 103 CFU/mL. Sulphate-reducing bacteria (SRB) were measured by the most probable number (MPN) method. Results showed low initial numbers but they stabilized around 104 MPN/mL after one to two months. No significant numbers of aerobic or anaerobic sulphur oxidizing bacteria were found. Enumeration of methanogens indicated that they were generally present in the permeameters that contained non-autoclaved backfill. Results are partially inconclusive because of the lack of confirmation of methane gas present in the headspace of part of the MPN culture tubes. Microbial pore clogging was not evident for the two fastest-flowing permeameters over 180 d. Insufficient data were obtained for the two slowest-flowing permeameters to draw any conclusions regarding plugging. High dissolved organic carbon levels were measured in several outflow samples which suggests strongly that organic material is being leached from the clay-sized component of the backfill. This could have consequences for radionuclide transport in a vault and the character of this organic material (i.e., complexing capacity, acid-base titrations, etc.) and its transport ability through backfill should be examined further. (author)