RELEASE OF METALS FROM POLLUTED SEDIMENTS IN A SHALLOW LAKE - QUANTIFYING RESUSPENSION

The contribution of erosion of bed sediment to the load of metals leaving Lake Ketelmeer, a shallow lake in the Netherlands fed by the IJssel branch of the River Rhine, is reported. Transport of suspended matter and associated trace metals was measured using both centrifuges and sediment traps at several locations in the lake. Mass balances of suspended matter and heavy metals were calculated using data from these field measurements. Metal/scandium-ratios were used to identify the source of the suspended matter in the lake. Since the bed sediment is more polluted than the suspended sediment entering the lake, higher metal/scandium-ratios were found for bottom sediment in the lake compared with those for suspended matter entering the lake from the River IJssel. Using the metal/scadium-ratio in suspended matter from the lake, it was calculated that bottom sediments made up 43 % of the suspended matter leaving the lake. This implies an erosion flux of bottom sediment of 16 g m-2 d-1. For cadmium, mercury, chromium and zinc, this erosion process accounts for more than 50 % of the pollutant load leaving Lake Ketelmeer and entering Lake IJsselmeer.