The Scheldt estuary revisited: occurrence and behaviour of particulate polychlorinated biphenyls and polycyclic aromatic hydrocarbons

Temporal and spatial variations in particulate PolyChlorinated Biphenyl (PCB) and Polycyclic Aromatic Hydrocarbons (PAH) concentrations were studied in the Scheldt estuary during a one-year period (May 1995-June 1996). This sampling campaign was arecurrence of a campaign held almost a decade ago. Both PCB and PAH concentrations in the Scheldt river were substantially lower in the 90's, but the Scheldt is still one of the most severely polluted rivers in the North Sea area. The annual input into the estuary was approximately 40 kg for total PCBs (sum of 8 congeners) and 3300 kg for total PAHs (19 compounds). The fraction transported to the North Sea is relatively small: 10-20 %.In the most recent sampling campaign the behaviour of particulate PCBs and PAHs found earlier was confirmed: as a result of the mixing of riverine with marine particulates the concentrations of the micropollutants generally decrease with increasing salinities, The quicker restoration of dissolved oxygen in the upper estuary gave rise to a more intense degradation of organic matter, and, probably as a result of cometabolism, degradation of low molecuiar weight PAH. Typical calculated half lives were 260 days for pyrene and 390 for fluoranthene

Potential chemical time bombs in the Schelde estuary

The Schelde estuary is heavily polluted with many different (micro)pollutants. This results in high concentrations in the abiotic and biotic compartments of the Schelde estuary and in various effects. the present day suboxic and anoxic conditions in the upper estuary are probably responsible for the high distribution coefficients for some trace metals in the lower Schelde estuary (Western Schelde). a future reduction in discharges with a high biological oxygen demand will therefore not only result in higher oxygen concentrations, but probably also in lower distribution coefficients for trace metals in the Western Schelde. Simulations show that lower trace metal distribution coefficients will probably result in higher concentrations of dissolved (bioavailable) trace metals, even with substantially reduced discharge rates, due to the desorption of trace metals from resuspended bottom sediments

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

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 af dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phyto- plankton 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. Reabsorption 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 absorption 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

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

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