Trace Metals in the Westerschelde Estuary: A Case-Study of a Polluted, Partially Anoxic Estuary

Oceans and lakes are characterized by large residence times and can thus be considered as being in an equilibrium or quasi-equilibrium state. In contrast, estuaries, which constitute the interface between the fresh water and marine ecosystems, are biogeochemically and physically very dynamic. Strong gradients in physical and chemical variables such as temperature, pH, salinity, dissolved oxygen, nutrients, amount and composition of particulates result from the mixing of fresh water and saline end members. These gradients provoke increased biogeochemical processes, generally resulting in a geochemical filter (the area of high turbidity, showing enhanced adsorption-desorption processes) and a biological filter (the area of high plankton activity showing, besides a high uptake of nutrients, a high accumulation of trace metals). Both filters disturb the conservative mixing pattern of the trace metals in an estuary.
In addition, four aspects make the Scheldt estuary very unusual and different from other estuaries:

• the Scheldt is a tide-governed estuary which means larger residence times;
• the upper estuary receives large inputs of biodegradable organic matter which leads to oxygen depletion in winter and to anoxic conditions in summer;
• inputs of toxic pollutants occur in the upper estuary by the industrial park of Antwerp;
• the anoxic zone, the zone of pollutant input and the zone of the turbidity maximum coincide geographically.
  The Scheldt estuary is thus an ideal field laboratory for the study of biogeochemical processes which do not occur elsewhere, or only partially, to a very low extent or in a specific compartment, i.e. the sediments. More specifically, redox processes involving trace metals, the formation of non-labile metal complexes, and the methylation of mercury occur as a result of anoxic conditions and the high load of organic matter. Several models describing indetail the transport, the sedimentation and the speciation of the trace metals in this well-mixed estuary (all major estuaries in EC countries adjacent to the North Sea or the Atlantic Ocean are well-mixed) are presented in this book. A comparison of the metal concentrations over a 15-year period shows a net reduction of the pollution level.

Résumé
In addition, four aspects make the Scheldt estuary very unusual and different from other estuaries: the Scheldt is a tide-governed estuary which means larger residence times;

Contenu
General description of the Scheldt estuary.- Biogeochemical behaviour of Cd, Cu, Pb and Zn in the Scheldt estuary during the period 19811983.- The biogeochemical behaviour of Cd, Cu, Pb and Zn in the Scheldt estuary: results of the 1995 surveys.- Behaviour and speciation of mercury in the Scheldt estuary (water, sediments and benthic organisms).- Effect of organic complexation on the behaviour of dissolvd Cd, Cu and Zn in the Scheldt estuary.- The impact of the Scheldt input on the trace metal distribution in the Belgian coastal area (results of 19811983 and 19951996).- A box-model of metal flows through the Scheldt estuary (19811983 and 19921995).- Water quality modelling of the western Scheldt estuary.- A time-dependent flow model for heavy metals in the Scheldt estuary.- Evolution of trace metal concentrations in the Scheldt estuary (19781995). A comparison with estuarine and ocean levels.