Non-steady state behaviour of heavy metals in contaminated freshwater sediments

The behaviour of heavy metals has been investigated in contaminated sediments of the river Meuse, The Netherlands. Due to temporal changes in temperature and degradability of organic matter, the depths of the redox boundaries fluctuate. This contributes to a non-steady state. As a result of oxidation processes, a distinct peak in heavy metal concentrations in pore water is measured at the sediment-water interface. Because the studied anoxic sediments contain low levels of sulphide, other solid phases are expected to be of importance in the binding of heavy metals. Furthermore, heterogeneity of the sediment and complexation with dissolved organic compounds may result in supersaturation of the anoxic pore waters with respect to discrete heavy metal sulphides, thus influencing heavy metal mobility. Calculations using concentration gradients of heavy metals indicate that diffusive fluxes between the sediment and the surface water contribute to concentrations in the surface water, although significant effects may be confined to specific locations.