GENERAL DESCRIPTION OF THE SCHELDT ESTUARY

A general description of the Scheldt estuary. including the hydrology, the sediment transport, the productivity and the biodegradation with respect to their influence on the trace metal behaviour in the Scheldt estuary, is given. The river basin can be divided in several sections according to their morphological, hydrodynamical and sedimentary prop erties. The zone from km 78 to 55, which corresponds roughly with the salinity zone from 2 to 10 psu, is the zone of high turbidity, high se dimentation and of oxygen depletion, especially in the summer period. That area is called the geochemical filter because the solid/dissolved distribution of the trace metals is controlled by redox. adsorption/d esorption, complexation and precipitation/coprecipitation processes. T he sedimentation rate in that area is estimated at 280 Mkg y(-1). In- the downstream estuary the phytoplankton activity increases due to the restoration of oxygen and to the much lower turbidity values. That ar ea is called the biological filter because incorporation of trace meta ls by the plankton communities lowers the trace metal concentrations d uring the productivity period, while transformation of metal species, especially observed with mercury, occurs during that period too.