Spatial and temporal distribution of trace metals in sediments from the Scheldt estuary, south-west Netherlands

The spatial and temporal distribution of trace metals has been studied in sediments of the Scheldt estuary, in order to identify trends in space and time. Sediment trace metal data, obtained by 11 laboratories, were considered for the period 1959-90. It was found that trace metal data obtained by complete sample digestion (e.g. HClO4-HF) were incomparable to those obtained by strong acid leaching (e.g. HNO3-HCl). However, various strong acid leaches were found to yield comparable results for sediment trace metals, except for chromium. Hence, trace metal data obtained by strong acid digestion could be merged into a comprehensive database (except for chromium), which was analysed by an ANOVA procedure. Sediment trace metal contents were normalized against a common standard, based on their grain size fraction and organic matter content. Spatial variations were investigated for four areas encompassing the entire Scheldt estuary within the periods 1978-85 and 1986-90. For one specific area, mid-estuary, trace metal data were studied from 1959 to 1987. For each area and period considered, the sediment trace metal content was calculated at the average percentage < 16 mu m and organic matter content of the whole data set. The results showed that: (1) The upper Scheldt estuary is strongly polluted by trace metals, especially cadmium; (2) Viewed in historic perspective, pollution of the Scheldt estuary by trace metals was maximal at the end of the 1950s but decreased considerably through the 1970s and again in the 1980s. Monitoring data of suspended matter composition and trace metal profiles in a radiodated sediment core tend to confirm this pollution history; (3) The trace metal content of the sediments decreases in the downstream direction due to mixing of metal-rich fluvial sediments and metal-poor marine sediments; (4) Although physical mixing is the most important process that controls the trace metal content of the sediments, evidence is presented for mobilization of cadmium with increasing salinity, whilst this is suggested for arsenic. (C) 1996 Academic Press Limited