Remobilization of histroically contaminated sediments during high discharges in the River Rhine

Abstract

During the post-war period industrial activity increased resulting in higher emission rates, which caused an overall degradation of biological conditions in the Rhine over the period 1960-1970 (Wolff, W.J., 1978). Thereafter, the concentrations of most heavy metals in the water considerably decreased. But, high discharge events, associated with climate change can cause resuspension of old contaminated sediments. Data analysis of the River Rhine at Lobith shows a decreasing contaminant concentration with increasing discharge in the lower regimes. This may be caused by dilution. During higher discharges, however, contaminant concentrations appear to increase slightly. Distinct relations are hard to establish due to a significant decrease in the amount of data points with increasing discharge. In addition, higher discharges cause a dilution effect and suspension of bigger grains, these have a lower sorption capacity for contaminants. These processes cover up actual effects. Therefore, a model is used to examine high discharge events. Using Delft3D-WAQ, two numerical experiments are performed, one with a constant emission pattern and the other with a variable emission pattern with emission peaks between 1968 and 1975. Model results demonstrate that the scenario with the variable emission pattern shows a deterioration of suspended particulate matter quality with increasing discharge, indicating a mobilization of historically contaminated sediment layers.