Pyrite mineralization related to microbial activity in low-permeability sediments

The geochemical and physical factors affecting the microbial ecology of fine-grained sediments were determined for an unconfined aquifer in south-central Washington. The distribution of anaerobic bacteria was related to variations in concentration of sediment organic matter and to gradients of electron acceptors oxygen, nitrate, and sulfate within pore waters, and bioavailable Fe (III) in solids. Sulfate-, iron-reducing, and fermentative bacteria were detected in clay-rich sediments that contained secondary pyrite in microsites associated with sediment organic matter. Low rates of microbial metabolism of organic matter linked to sulfate and Fe (III) respiration were inferred to result in the precipitation of secondary pyrite. The limiting factors in pyrite mineralization of these sediments included spatial heterogeneity of bacteria, organic matter, and bioavailable Fe (III) and diffusion control of soluble electron acceptors in low-permeability (< 10{sup -7} cm/s) sediments.