This review article covers the processes affecting trace metals in deposited sediments. The sediment-water system can be divided in three parts: the oxic layer, the anoxic layer and the oxic-anoxic interface. Available data show that trace metals like Cu, Zn and Cd occur as sulphides in marine and estuarine anoxic sediments. Calculations show that organic complexation is unlikely and the dominant species are sulphide and bisulphide complexes. Chromium and arsenic are probably present as adsorbed species on the sediments; their concentrations in the pore waters, therefore depend on the concentrations in the sediments. The oxic-anoxic interface plays the major role in the potential flux of trace metals from the sediments, however this interface is not well studied at present. Changes from an anoxic to an oxic environment as occurs during dredging and land disposal of contaminanted sediments may cause a remobilization of some trace metals.

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The Schelde estuary is heavily polluted with many different (micro)pollutants. This results in high concentrations in the abiotic and biotic compartments of the Schelde estuary and in various effects. The present day suboxic and anoxic conditions in the upper estuary are probably responsible for the high distribution coefficients for some trace metals in the lower Schelde estuary (Western Schelde). A future reduction in discharges with a high biological oxygen demand will therefore not only result in higher oxygen concentrations, but probably also in lower distribution coefficients for trace metals in the Western Schelde. Simulations show that lower trace metal distribution coefficients will probably result in higher concentrations of dissolved (bioavailable) trace metals, even with substantially reduced discharge rates, due to the desorption of trace metals from resuspended bottom sediments. KEY WORDS Schelde estuary Sediment pollution modelling Trace metals Organic pollutants SCHELDE ESTUARY: THE NEED FOR POLLUTION MODELLINGThe Schelde estuary, which drains a densely populated and industrialized area in northern France, west Belgium and the southwest Netherlands, is faced with two major problems: (1) extensive dredging and dumping of (contaminated) sediments to keep the port of Antwerp accessible; and (2) large domestic and industrial waste water discharges, a significant proportion of which is discharged without treatment.The Schelde estuary (Figure 1; Wollast, 1988;Zwolsman and Van Eck, 1993) is a macrotidal coastal plain estuary with a tidal range of 4-5 m and an average water depth of 10 m. The river Schelde and its tributaries drain about 21600 km2 and the river discharge ranges from 20 m3 s-' in summer to 400 m3 s-' during winter, averaging 104 m3 s-' over the period 1949-86. The length of the estuarine mixing zone is 60-120 km and the residence time of the water in the estuary ranges from one to three months. The water column generally shows a well mixed circulation pattern, except during extreme river discharges in winter. The estuary can be divided into two zones with different hydrographical characteristics.The upper estuary or Zeeschelde, located in northwest Belgium, is heavily polluted by organic wastes, trace metals and organic contaminants. One of the most striking features of this area is the presence of an anoxic water column during most of the year . Previous studies in the Schelde estuary have suggested that this anoxic zone plays an important part in the behaviour of trace metals (Salomons and Eysink, 1981;Duinker, et al., 1982). The bottom sediments consist of sands, sandy muds and clayey muds. The origin of the sediments is predominantly fluvial, but the marine component of the sediments increases in a seaward direction. The finest sediments are found in the so-called high turbidity zone (HTZ) between the city of Antwerp and the Dutch-Belgian border (Wartel, 1977). The HTZ is an important sedimentation zone.The lower estuary or Western Schelde, situated between the Dutch-Belgian bo...

A comprehensive, integrated system of 50 models was developed to evaluate policies that include mixes of building new facilities and changing operating rules to improve water supply, as well as adjusting prices and regulations to reduce demands. Analysis performed with the system resulted in a new national water management policy, saving hundreds of millions of dollars in investment expenditures and reducing agricultural damage by about $15 million per year, while decreasing thermal and algae pollution. The methodology was adopted by the Dutch government and has been used to train water resource planners from many nations.