The impact of the Scheldt input on the trace metal distribution in the Belgian coastal area (results of 1981–1983 and 1995–1996)

Abstract

Offshore fluxes of Cu, Zn, Cd, Pb and Hg were calculated based on residual flow patterns and salinity gradients along the Belgian coast. The residual flow lines along the Belgian coast are more or less parallel to the coast except in the area where the north-easterly flowing watermass coming from the Channel encounters the south-westerly-oriented Scheldt outflow, forming a residual hydrodynamical front. From the steady-state salinity pattern, diffusion coefficients perpendicular to the residual flow were deduced; they ranged from 21 to 108 m² s⁻¹.

Offshore fluxes of dissolved and particulate trace metals based on diffusive and mixing processes are calculated. The steady state profiles of dissolved metals show a dilution effect in the coastal waters, reaching an almost constant concentration in the marine watermass in the 1981–1983 dataset. The ratios of the Scheldt input of trace metals to the total dissolved offshore flux vary from 38 to 55% (1981–1983), depending on the kind of metal, and from 55 to 91% (1995–1996). The ratio of the Scheldt input to the dissolved metal flow parallel to the coast, is in both periods (1981–1983 and 1995–1996), smaller than 1%. The steady-state concentration profiles of particular 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 particular metals, offshore fluxes and the resulting concentration increases agree fairly well with the observed values. The ratios of the Scheldt input to the particulate trace metal offshore flux vary between 30 to 46% (1981–1983) and 13 to 37% (1995–1996). The contribution of the Scheldt estuary to the flows parallel to the coast ranges from 1.6 to 2.9% (1981–1983) and from 0.6 to 1.6% (1995).