The behavior of heavy metals in the estuarine zone is strongly dependent on the chemical characteristics imposed more particularly by the physical and biological processes occurring in this region. Many estuaries are now characterized by a drastic increase of their organic carbon load. In long residence time estuaries, this organic load can induce a permanent or seasonal anaerobic zone due to the bacterial heterotrophic activity. It is the case of the Scheldt estuary which is also characterized by the presence of high concentrations of heavy metals. In order to investigate the processes involved in the transformation of trace metals between particulate and solution during estuarine mixing, Cu, Cd, Zn, Ni and Mn were measured in solution and suspension in the estuary of the river Scheldt. Samples were obtained at six fixed positions at 1-hour intervals during complete tidal cycles in selected overlapping salinity regimes. The resultant plots of the concentrations in solution vs. salinity suggest conservative mixing of nickel. However, manganese, cadmium and zinc plots have pronounced maxima, which occur at different locations and salinities. Dissolved manganese is produced in the low pH, low dissolved oxygen area (around 10 ‰ S). Cadmium and zinc maxima occur at higher salinities (21 an 17 ‰ S respectively). Possible explanations include the effect of pH, redox potential, formation of soluble complexes during mixing of fresh water and seawater, biological uptake or release.
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