Geochemistry of major elements and trace metals in suspended matter of theScheldt estuary, southwest Netherlands

The geochemistry of suspended matter from the Scheldt estuary has been stud ied in eight surveys in 1987-1988. Samples were analyzed for major elements (Al, Ca, Fe, K, Mg, Na, Si, Iri, POC, N, P, S) and trace metals (Ag, Ba, B e, Cd, Co, Cr, Cu, Li, Mn, Ni, Pb, Sn, Sr, V, Zn). Physical mixing of fluvi al and marine particulates leads to a continuous decrease in the trace meta l content of the suspended matter with increasing salinity. Principal compo nent analysis shows that the effect of desorption processes (e.g., of Cd, C u, and Zn) on the suspended matter composition is relatively minor as compa red to that of particle mixing. A particulate S maximum is present in the u pper estuary, reflecting resuspension of reduced sediments. Pore water infu sion 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 (hy the lower estuary) than that of Fe (in the upper estuary). Coprecipitation with Mn (hydr)oxides is observed for Ni an d 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 eleme nts are decreased, in favour of biogenic elements (POC, N, P). This observa tion is attributed to dilution of mineral particles by phytoplankton which, apparently, has lower trace metal levels. However, the Pa content of the s uspended matter is increased during the bloom, which is ascribed to biologi cal 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 distr ibution coefficients in the lower estuary. Phytoplankton may both directly (through biological uptake) and indirectly (by increasing the pH) be involv ed in the seasonal shift of Cd and Zn from the dissolved to the particulate phase. Comparison of recent data on suspended matter composition with hist orical data shows that the trace metal burden of the Scheldt river has decr eased considerably between 1980 and 1995. The decrease in trace metal level s of the fluvial suspended matter amounts to 88% for Cd, 85% for Hg, 74% fo r As, 59% for Cu, and 50-54% for Cr, Ni, Pb and Zn over the time span consi dered. However, the Mn content of the fluvial suspended matter has doubled from the early 1970s until the mid 1990s, reflecting the gradual increase i n the dissolved oxygen concentration of the river water over the last 20 ye ars. (C) 1999 Elsevier Science B.V. All rights reserved.