Biogeochemical dynamics of iron and sulfur in sediments from hydro-electric dams submitted to wetting and drying cycles

Abstract

Water level changes due to the decommissioning of hydro-electric dams can result in sediment exposure to air. Oxidation of sediments can decrease the pH as a result of iron sulfides changing into iron oxides. The present study was designed to simulate drying and wetting cycles of shallow lake sediments from two lakes (Stump and Black Donald Lakes in Ontario), in order to assess the mineralogical changes of Fe-rich minerals. Our results indicate that the total reactive iron fraction of the sediment increased after the wetting and drying cycles. This increase was caused by the weathering of pyrite and Fe-silicates and their subsequent transformation into more Fe-reactive mineral species. The pH of the surface sediments also decreased following the addition of simulated rainwater and the oxidation of iron sulfides in the sediments. This preliminary study shows that the decommissioning of hydro-electric dams will have an effect on the biogeochemical cycles of iron and sulfur.