Factors limiting microbial activity in volcanic tuff at Yucca Mountain

Samples of tuff aseptically collected from 10 locations in the Exploratory Shaft Facility at the site of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada Test Site were analyzed for microbiological populations, activities, and factors limiting microbial activity. Radiotracer assays (C-14-labeled organic substrate mineralization), direct microscopic counts, and plate counts were used. Radiolabeled substrates were glucose, acetate, and glutamate. Radiotracer experiments were carried out with and without moisture and inorganic nutrient amendments to determine factors limiting to microbial activities. Nearly all samples showed the presence of microorganisms with the potential to mineralize organic substrates. Addition of inorganic nutrients stimulated activities in a small number of samples. The presence of viable microbial communities within the tuff has implications for transport of contaminants.