Mobilisation of Heavy Metals in Contaminated Sediments in the River Meuse, The Netherlands

Due to seasonal variation in bottom-water temperature and degradation of organic matter, the depths of the redox boundaries fluctuate in sediments of the river Meuse. This is reflected by a non-steady state behaviour of heavy metals in the surface sediments. Levels of acid-volatile sulphides suggest that dissolved concentrations of heavy metals in the anoxic pore waters are determined by their respective sulphide phases. However, complexation with dissolved organic ligands may significantly increase dissolved concentrations of heavy metals. In most sediments studied, a distinct peak in dissolved concentrations of heavy metals is measured immediately below the sediment-water interface. This concentration peak may be attributed to degradation of organic matter and oxidation of sulphides. Dissolved concentration gradients indicate that upward diffusion of heavy metals from the sediment can contribute to concentrations in the surface water, although significant effects may be confined to specific locations. In addition, it is shown that release of heavy metals as dissolved species to the surface water is negligible compared to particulate-bound fluxes of heavy metals to the sediment.