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Geochemistry of major elements and trace metals in suspended matter of the Scheldt estuary, southwest Netherlands
Zwolsman, J.J.G.; van Eck, G.Th.M. (1999). Geochemistry of major elements and trace metals in suspended matter of the Scheldt estuary, southwest Netherlands. Mar. Chem. 66(1-2): 91-111. dx.doi.org/10.1016/S0304-4203(99)00026-2
In: Marine Chemistry. Elsevier: Amsterdam. ISSN 0304-4203, more
Peer reviewed article  

Available in  Authors 
    VLIZ: Open Repository 117672 [ OMA ]

Keywords
    Estuaries; Geochemical cycle; Suspended inorganic matter; Trace metals; ANE, Netherlands, Westerschelde [Marine Regions]; Marine; Brackish water

Authors  Top 
  • Zwolsman, J.J.G., more
  • van Eck, G.Th.M., more

Abstract
    The geochemistry of suspended matter from the Scheldt estuary has been studied in eight surveys in 1987–1988. Samples were analyzed for major elements (Al, Ca, Fe, K, Mg, Na, Si, Ti, POC, N, P, S) and trace metals (Ag, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Ni, Pb, Sn, Sr, V, Zn). Physical mixing of fluvial and marine particulates leads to a continuous decrease in the trace metal content of the suspended matter with increasing salinity. Principal component analysis shows that the effect of desorption processes (e.g., of Cd, Cu, and Zn) on the suspended matter composition is relatively minor as compared to that of particle mixing. A particulate S maximum is present in the upper estuary, reflecting resuspension of reduced sediments. Pore water infusion into the (suboxic) upper estuary is a major source of Fe and Mn to the suspended matter. Due to differences in oxidation kinetics, precipitation of dissolved Mn occurs later (in the lower estuary) than that of Fe (in the upper estuary). Coprecipitation with Mn (hydr)oxides is observed for Ni and Co, but not for the other metals studied. Phytoplankton activity leads to a seasonal shift in the suspended matter composition in the lower estuary. During the spring bloom, the contents of trace metals and lithogenic elements are decreased, in favour of biogenic elements (POC, N, P). This observation is attributed to dilution of mineral particles by phytoplankton which, apparently, has lower trace metal levels. However, the Ba content of the suspended matter is increased during the bloom, which is ascribed to biological formation of barite. Another effect of the spring bloom is depletion of dissolved Cd and Zn (but not of Cu), leading to an increase in their distribution coefficients in the lower estuary. Phytoplankton may both directly (through biological uptake) and indirectly (by increasing the pH) be involved in the seasonal shift of Cd and Zn from the dissolved to the particulate phase. Comparison of recent data on suspended matter composition with historical data shows that the trace metal burden of the Scheldt river has decreased considerably between 1980 and 1995. The decrease in trace metal levels of the fluvial suspended matter amounts to 88% for Cd, 85% for Hg, 74% for As, 59% for Cu, and 50–54% for Cr, Ni, Pb and Zn over the time span considered. However, the Mn content of the fluvial suspended matter has doubled from the early 1970s until the mid 1990s, reflecting the gradual increase in the dissolved oxygen concentration of the river water over the last 20 years.

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