The benthic sediments in Lake Coeur d'Alene, northern Idaho, have been contaminated by metals (primarily Zn, Pb, and Cu) from decades of upstream mining activities. As part of ongoing research on the biogeochemical cycling of metals in this area, a diffusive reactive-transport model has been developed to simulate metal transport in the lake sediments. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network with multiple terminal electron acceptors under redox disequilibrium conditions. Here, the model is applied to evaluate the competing effects of heavy-metal mobilization through biotic reductive dissolution of Fe(III) (hydr)oxides, and immobilization as biogenic sulfide minerals. Results indicate that the relative rates of Fe and sulfate reduction could play an important role in metal transport through the environment, and that the formation of (bi)sulfide complexes could significantly enhance metal solubility, aswell as desorption from Fe hydroxides. © 2007 Taylor & Francis Group, London.