Geochemisch gedrag van zware metalen in het Schelde-estuarium = Geochemistry of trace metals in the Scheldt estuary
Zwolsman, J.J.G. (1999). Geochemisch gedrag van zware metalen in het Schelde-estuarium = Geochemistry of trace metals in the Scheldt estuary. Geologica Ultraiectina, 171. PhD Thesis. Universiteit Utrecht: Utrecht. ISBN 90-5744-030-X. 183 pp.
Part of: Geologica Ultraiectina. Universiteit Utrecht: Utrecht, more
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Document type: Dissertation
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| Keywords |
Chemistry > Geochemistry
Trace elements > Trace metals
Water bodies > Coastal waters > Coastal landforms > Coastal inlets > Estuaries
Marine/Coastal; Brackish water |
| Abstract |
The distribution of trace metals has been studied in abiotic compartments of the Scheldt estuary (water column and sediments). Seasonal surveys, carried out in 1987-1988, indicate that the geochemistry of dissolved trace metals (Cd, Cu, Zn) is determined by the redox status of the upper estuary, and by primary production in the lower estuary .During the warmer period (spring till autumn), the incoming river water is anoxic, leading to very low dissolved metal concentrations due to precipitation of metal su1phides. During winter, the river water is usually not completely devoid of oxygen (although highly undersaturated), precluding metal sulphide precipitation. TherefQre, the dissolved metal concentrations in the incoming river water are much higher during winter.In the upper estuary, during winter, dissolved trace metals are removed from solution, e.g. by adsorption onto Fe and MnS)X-yhydroxides, formed in the water column. Dissolved Mn and Fe are introduced into the water column by inflow of reducing pore-waters. After their release from the sediments, dissolved Fe is immediately oxidised, but part of the dissolved Mn escapes from oxidation and is transported to the lower estuary .The fluvial suspended matter (rich in trace metals) is mixed with metal-poor m3rine material, transported to the upper estuary by the estuarine circulation. A major part of the suspended matter input is deposited in the high-turbidity zone, around the city of Antwerp. Sediment resuspension is important in the high-turbidity zone, because of continuous dredging and tidal disturbance. Due to the suboxic condition of the water column, trace metals are precipitated as sulphides in the sediments close to the sediment-water interface. This means that post-depositional mobility of trace metals in the sediments is probably insignificant.In the eastern (brackish) part of the lower estuary, the incoming dissolved Mn is oxidised, because of the increasing oxygen concentration in the water column. Coprecipitation of Ni and Co with the Mn oxides formed is observed, both in the suspended matter and in the salt marshes. Trace metal sulphides in the suspended matter, imported from the upper estuary, are oxidised in the brackish water zone, leading to a dissolved Zn maximum at relatively low salinity (6-9), whereas the maxima of Cd and Cu occur more seawards in the salinity range 9- 21. The pore-waters in the brackish water zone cause an additional (but yet unknown) flux of dissolved trace metals to the water column. Another flux of dissolved trace metals to the brackish water zone comes from the salt marshes, which only partially retain their particulate metal inputs (e.g. 50% of the particulate Cd flux to the marshes is mobilised). The suspended matter, transported from the upper to the lower estuary, is progressively diluted with marine material, leading to continuously decreasing trace metal contents in the suspended matter (and also in the sediments) with increasing salinity. Trace metal profiles in the intertidal sediments of the brackish water zone appear to be significantly influenced by early diagenetic reactions, which is related to the well-resolved spacing of the redox zones in these sediments. Post- depositional mobility of trace metals is even more important in the salt marshes of the lower estuary , because of the thick oxic top layer present in salt marsh sediments (20 to 40 cm). In the western (marine) part of the lower estuary, conservative behaviour of dissolved trace metals is generally observed, except during phytoplankton blooms. The reason for this is that the mobilisation of trace metals from the suspended matter has been terminated (trace metal sulphides have become completely oxidised), whilst sediment-water exchanges are also small because the marine sediments are typically medium to coarse sands, i.e. low in trace metals. In more silty sediments (intertidal flats), trace metals are mobilised to some extent, but they are trapped during |
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