The influence of salinity on the solubility of Zn and Cd sulphides in the Scheldt estuary

In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme MINEQL + .Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S = 15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S = 15 than when oxidation occurs at S = 2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the, 1970s, the anoxic part of the estuary reached S = 15-20, but since the early 1980s it has dropped to S = 2-10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary

Distribution and behaviour of trace metals in the Scheldt estuary in the period 1991-1994

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Transport and transformation of trace metals in the Scheldt estuary

Water samples collected along the Scheldt estuary during several cruises were analysed for fundamental physico-chemical parameters and for dissolved and particulate trace metals (Cd, Co, Cr, Cu, Mn, Ni, Ph and Zn). Mean suspended matter concentration and upstream dissolved concentrations of Ni, Cr and Mn are increasing with river discharge, suggesting a resuspension of fluid mud and release of dissolved elements from interstitial waters. Dissolved and particulate Ni and Cr behave conservatively during estuarine mixing, while Mn is removed from the dissolved phase and Cd, Cu and Zn are on the contrary mobilized. The distribution coefficients, KD's, show at low salinities the following sequence Cd > Cu > Zn > Ni. The KD's of Cd, Cu and Zn are well correlated with dissolved oxygen content suggesting a redissolution of sulphide produced in the anoxic part of the estuary. The analysis of suspended matter in the upper Scheldt confirms the high contamination of this estuary for Cd, Cr, Cu, Pb and Zn. The long residence time of the water in the estuary, due to its small river discharge and large tidal influence and the important variations of the river input make it difficult to estimate metal fluxes to the North Sea, without the use of an elaborated coupled hydrodynamical/biogeochemical model

Geochemical composition of sediments in the Scheldt

Superficial sediments of the Scheldt estuary were collected with a Van Veen grab at 57 stations between Temse and Vlissingen. They were analysed for major elements (Si, Al, Fe, Ca, Mg, Na, K, Cinorg and Corg) and trace metals (Cd, Pb, Cu, Zn, Cr, Ni, Co, Mn and Li). Factor analysis indicates that 44% of the variance can be explained by one factor which exhibits a high saturation for trace metals, organic matter, Al and Fe, all variables typical of fine mud. The high scores of this first factor are almost exclusively present in the upper estuary except for one area in front of Terneuzen. The second factor, which explains 23% of the variance, is typical of the carbonates and the third one (19% of the variance) is representative of the clay minerals. These two factors are more evenly distributed over the estuary. As usual, a strong influence of granulometry on the distribution of trace elements in the sediments was observed. Intercomparison of their composition within the Scheldt or with those of other aquatic systems requires thus a normalization procedure. This problem has been studied in detail by analysing various size fractions (63–16, 16–8, 8–4, <4 m) obtained by elutriation of the sample or by using a parameter characteristic of the fine fraction such as the concentration of a typical element (Al, Fe, Li, Corg). The normalization of trace metals allowed us to evaluate an enrichment factor of the trace elements in the estuarine deposits due to mans activities. In addition, it demonstrates the decrease of the anthropogenic impact on the composition of sediments by comparing the composition of sediments collected in 1976 and in 1994.Université libre de Bruxelle