Trace metals in the eastern part of the North Sea: 2. Flows of Cd, Cu, Hg, Pb and Zn through the coastal area

The residual flow lines of the Belgian coast are more or less parallel to the coast except in the area where the northeasterly flowing watermass coming from the Channel encounters the southwesterly-oriented Scheldt outflow. The result is a residual hydrodynamical front with, at the northern side, a less saline estuarine watermass, and at the southern side a more saline marine watermass. From the steady-state salinity pattern, diffusion coefficients perpendicular to the residual flow were deduced; they ranged from 21 to 108 m².s-1.The steady-state profiles of dissolved metals versus salinity indicate a dilution effect in the coastal-estuarine watermass and almost constant concentration in the marine watermass. Offshore fluxes of Cd, Cu, Hg, Pb and Zn based on diffusive and mixing processes are calculated. They can explain the concentration increases in the coastal flow tube (315 km³. y-1), but are insignificant beyond that flow tube. The ratios of the Scheldt output to the offshore flux vary from 38 to 85%, depending on the kind of metal. The ratio of the Scheldt output to the metal flow parallel to the coast is smaller than 1%.The steady-state concentration profiles for particulate metals versus salinity are fairly constant in the coastal-estuarine and marine watermasses, but decrease very abruptly from the first to the second watermass. Assuming a conservative behaviour of the particulate metals, offshore fluxes and the resulting concentration increases in the coastal flow tube were deduced. These calculated concentration increases agree fairly well with the observed values. The contribution of the Scheldt estuary to the flows parallel to the coast ranges from 1.6 to 3.3%