Transport and transformation of trace metals in the scheldt estuary

Water samples collected along the Scheldt estuary during several cruises were analysed for fundamental physico-chemical parameters and for dissolved and particulate trace metals (Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn). Mean suspended matter concentration and upstream dissolved concentrations of Ni, Cr and Mn are increasing with river discharge, suggesting a resuspension of fluid mud and release of dissolved elements from interstitial waters. Dissolved and particulate Ni and Cr behave conservatively during estuarine mixing, while Mn is removed from the dissolved phase and Cd, Cu and Zn are on the contrary mobilized. The distribution coefficients, KDʹs, show at low salinities the following sequence Cd > Cu > Zn > Ni. The KDʹs of Cd, Cu and Zn are well correlated with dissolved oxygen content suggesting a redissolution of sulphide produced in the anoxic part of the estuary. The analysis of suspended matter in the upper Scheldt confirms the high contamination of this estuary for Cd, Cr, Cu, Pb and Zn. The long residence time of the water in the estuary, due to its small river discharge and large tidal influence and the important variations of the river input make it difficult to estimate metal fluxes to the North Sea, without the use of an elaborated coupled hydrodynamical/biogeochemical model.