Investigations of the fate of xenobiotic chemicals in laboratory systems that accommodate the microbial ecology of sediments are described. These systems permit examination of biochemical activities in the sediment bed with particular emphasis at the sediment-water interface. Sediment may contain thousands of microcommunities, each containing the same genotypic array of metabolic potential. Each community, however, will demonstrate, depending on the surrounding conditions, a certain phenotypic response that reflects a small portion of its total metabolic potential.