Trace metals in suspended matter from the Rhine/Meuse estuary

Abstract

The contents of trace metals in suspended matter of the river Rhine have decreased during the last few decades but still exceed background contents. Contamination factors for suspended matter in the river Rhine at Lobith are found to be: Cd (7), Cu (5), Zn (9), Pb (3.5), Ni (2.5) and Cr (3). In the Rhine/Meuse estuary geochemical trace-metal behaviour is influenced by industrial discharges. Budget calculations show that the additional enrichment due to industry is a few percent (0 to 3%) for Cr, Cu, Hg, Ni, Pb and Zn, but is much higher for As (11.5%) and Cd (48.6%). Suspended-matter samples collected in the estuary along a salinity gradient were analysed for major elements and trace metals. The results show a decrease in the suspended-matter contents of Cd, Co, Cu, Cr, Ni, Pb and Zn as a function of salinity. The Sr content shows an increase with salinity. By using Cr as a conservative tracer, the differences in trace-metal contents between freshwater and seawater can be attributed to the physical mixing of relatively contaminated Rhine/Meuse suspended matter with relatively uncontaminated North Sea suspended matter. The results show an enrichment in Mn, Co, Ni and Pb in the samples collected in May 1988 in comparison to the samples collected in March 1989. This may be due to the enhanced oxidation of reduced Mn at higher temperatures. As a result of the formation of Mn-oxyhydroxides the trace metals Ni, Co and Pb are scavenged from the solution.