Distribution of trace metals in suspended matter of the Scheldt estuary

Abstract

The distribution of trace metals (Ni, Co, Cr, Zn, Cu, Cd, Pb, Mn) in suspended matter and sediments collected in the Scheldt estuary is investigated as a function of grain size, Al and particulate organic carbon (POC) content. Normalized trace-metal content with respect to Al allows characterization of the origin of the solids and evaluation of their degree of contamination. The suspended matter exhibits strong changes in chemical composition in the tidal zone, indicating mixing of heavily contaminated particles of continental origin with less contaminated marine particles. Flood events strongly influence the trace-metal content of the suspended matter by inducing resuspension processes and intensive transfer of solid material from the river system to the sea. The evolution of the composition of the particles in the mixing zone of the estuary indicates that the trace elements do not behave conservatively and that they are also affected by the changing physico-chemical conditions (salinity, pH, redox) observed in the Scheldt. In order to gain a better understanding of the origin and behaviour of the particulate trace elements in the estuary, a mixing model of suspended matter based on the conservative property of Al is proposed, which enables quantification of the proportion of marine vs. continental solids and to detect non-conservative behaviour of trace elements.