Potential chemical time bombs in the schelde estuary

Abstract

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...

Spatial distribution and correlation characteristics of heavy metals in the seawater, suspended particulate matter and sediments in Zhanjiang Bay, China

“…According to the correlation analysis among the partition coefficients of heavy metals and the environmental parameters, we could deduce that high primary production could cause Fe and Pb to be more easily partitioned into the particulate phase and could cause Cu and Cd to be more easily partitioned into the dissolved phase. Similar conclusions were also obtained in other studies [ 3 , 8 , 54 ]. Similar with Fe and Pb to some extent, high concentrations of Chl a , DO and high pH were favorable for the partitioning of Mn and Zn to into the particulate phase.…”

Temporal and spatial trends in heavy metal concentrations in the marine mussel Mytilus edulis from the Western Scheldt estuary (The Netherlands)

“…In relation to chemical speciation, studies have shown that the distribution of heavy metals is strongly influenced by salinity, dissolved organic carbon and dissolved oxygen in the WS (i.e. Van Eck & De Rooij, 1993;Paucot & Wollast, 1997;Zwolsman et al, 1997;Gerringa et al, 1998). In spring when the freshwater inflow in the estuary is largely anoxic, dissolved metal concentrations tend to be very low especially in the upper estuary and metal partitioning in those conditions favours adsorption to suspended particles and sediments.…”

The subtidal macrobenthos in the mesohaline part of the Schelde Estuary (Belgium): influenced by man?

“…The Schelde Estuary is one of the larger north-west European estuaries with a complete salinity gradient, including a large freshwater tidal area. Especially the part between the Dutch^Belgian border and the Rupel is severely impacted and heavily polluted by domestic, industrial and agricultural waste loads and the concentrations of micropollutants are high (Van Eck & De Rooij, 1993;Soetaert & Herman, 1995a,b;Zwolsman, 1999). From the 1980s on water quality has started to improve slowly, and this improvement continues in the 1990s (Van Damme et al, 1995;Van Eck et al, 1998).…”