Benthic exchange of sedimentary metals (Cd, Cu, Fe, Mn, Ni and Zn) in the Deûle River (Northern France)

Abstract

Environmental context Exchange processes at the water–sediment interface can release metals to riverine waters, having negative effects on organisms in the water column. We investigate the geochemical processes and metal exchange between the surface sediment and the overlying water under metal contamination conditions. Results suggest that the sediment can be a significant source of metal pollution in aquatic systems, particularly during anoxic events. Abstract Experiments were performed on the Deûle River (Northern France), which is strongly polluted by smelting plants, in the aim to investigate the influence of diagenetic processes and benthic macro-faunal activity on trace metal (Cd, Cu, Ni and Zn) and major metal (Fe, Mn) exchanges occurring at the water–sediment interface. Diffusive metal fluxes were determined from pore water metal concentration gradients measured in sediment cores. Benthic metal fluxes were evaluated using incubation chambers under dark conditions, and by further examining key variables (O2, CO2, redox potential and pH) affecting metal release and sequestration processes. As a whole, it was demonstrated that benthic fluxes were strongly dependent upon medium oxygenation and generation of colloidal iron oxides and hydroxides at the overlying water–sediment interface, raising the possibility of trace-metal adsorption and (co)precipitation.