Sulfate reduction in ground water of southeastern Montana

Ground water in southeastern Montana was investigated to determine if sulfide production was bacterially mediated. Sulfate—reducing bacteria were detected in 25 of 26 ground—water samples in numbers ranging from 2.0 x 10¹ to greater than 2.4 x 10⁴ bacteria per 100 milliliters. Stable sulfur isotope fractionation studies indicate a biological role in sulfate reduction. However, sulfate—reducing activity as determined by use of a radioactive sulfur isotope was observed in only 1 of 16 samples. Bacterial dissimilatory sulfate reduction is postulated to be responsible for a major part of the sulfide produced in these ground waters. These bacteria are most likely active in the adsorbed state, possibly in subsurface microzones where environmental conditions are conducive to sulfate reduction.