BIOGEOCHEMICAL BEHAVIOR OF CD, CU, PB AND ZN IN THE SCHELDT ESTUARY DURING THE PERIOD 1981-1983

Cu, Zn, Cd and Pb have been analysed in suspended matter and water sam ples from the Scheldt estuary, collected during five cruises between M ay 1982 and May 1983. In order to evaluate the overall metal behaviour in the estuary, continuous longitudinal profiles of the total, the pa rticulate and the dissolved metal concentrations are discussed in rela tion to various specific physico-chemical parameters. By means of corr elation and multi-regression techniques, several estuarine processes c ould be identified which dominate and/or control the behaviour of the individual trace metals in three distinct areas of the Scheldt estuary . The behaviour of the four selected metals was found to show some com mon features, but is besides this also characterised by many individua l and specific properties. The total metal concentrations seem to co-v ary well with the turbidity in the maximum turbidity zone between sali nities 2 and 10 psu. In the downstream area a dilution profile is obse rved for the total metal concentrations of Cu and Cd as well as for th e dissolved concentrations of all metals. Dissolved metal production r ates are high for Cu and Cd, but much lower for Zn and Pb, confirming the different longitudinal dissolved metal profiles. For Cu and Cd the se dissolved metal profiles are characterised by a broad mobilisation area, covering the whole middle estuary. Pb on the other hand, shows a more confined mobilisation area, and for Zn no such area was found. D uring winter the dissolved metal production rates seem to be controlle d, predominantly, by desorption processes. During summer dissolved met al production is correlated mainly to the dissolved oxygen content, su ggesting the domination of redox processes solely or in combination wi th desorption processes. In some cases, co-existent with nearly zero o xygen concentrations, removal of dissolved metals was observed, possib ly reflecting precipitation reactions. Longitudinal particulate metal profiles all show a negative deviation versus the dilution profile. Cu and Cd provide the higher deviations, characterised by a narrow enric hment zone preceding a broad mobilisation area. Finally, all particula te metal profiles appeared to correlate well with POC, when expressed in volumetric units.