Dissolved and particulate trace metal geochemistry in the scheldt estuary, S. W. Netherlands (water column and sediments)

Abstract

The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.

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

“…For veri®cation purposes, data sets were used from October and November 1978 (Duinker et al, 1983;Nolting et al, 1999b), 1987(Zwolsman and Van Eck, 1993Zwolsman et al, 1997) and 1991±1995 (Paucot, 1997;Paucot and Wollast, 1997).…”

“…In 1978, in the Scheldt estuary anoxia extended as far as S 15: Between 1978 and 1987 the anoxic zone receded upstream to S , 5 (Duinker et al, 1983;Zwolsman and Van Eck, 1993;Zwolsman et al, 1997;Nolting et al, 1999b) (Fig. 4).…”

“…4. Field data from 1978 (Duinker et al, 1983;Nolting et al, 1999b) and 1987(Zwolsman and Van Eck, 1993Zwolsman et al, 1997). Note the different scales between the studies.…”

Contrasting behaviour of trace metals in the Scheldt estuary in 1978 compared to recent years

“…These changes affect the distribution of trace metals over the dissolved and particulate phases with respect to source waters (Duinker et al, 1982a;Turner et al, 1993;Zwolsman and Van Eck, 1993;Turner, 1996;Paucot and Wollast, 1997).…”

Role of metal species in flocculation rate during estuarine mixing

“…Redox processes is of major concern for mobilization of trace metals in estuaries where an oxygen gradient exists in addition to the salinity gradient (Gerringa et al, 2001;Abdel-Ghani et al, 2009). A combination of high organic matter content and a long residence time, may result in development of anoxic conditions that may be either restricted to the sediments (Westerlund et al, 1986;Kerner and Wallmann 1992) or can occur in the water column (Zwolsman and Van Eck, 1993;Zwolsman et al, 1997;Shiller and Mao, 1999). Sulfides of elements (e.g.…”