Erosion investigation and sediment quality measurements for a comprehensive risk assessment of contaminated aquatic sediments.

In this paper, an assessment strategy is introduced which allows one to evaluate the ecological hazard of contaminated sediments in connection with the risk of in-stream erosion. Special techniques for sediment sampling, non-intrusive density profiling, and depth related measurement of erosion are presented, which, in combination with ecological aspects, lead to a comprehensive risk assessment of fluvial sediments. The strategy was applied to a lock-regulated reach of the River Neckar in Germany. The spatial pattern of contamination in the river reservoir was found to be remarkably heterogeneous. At some sites, very high heavy metal concentrations were detected at the sediment surface. A sudden increase in contamination with depth at other sites could be attributed to an erosional unconformity. The critical shear stress of erosion for old contaminated sediments is higher than for recently deposited material. Nevertheless, during major flood events, bottom shear stress in the river exceeds the critical shear stresses of erosion of all sediments. Accordingly, there is a substantial risk that old contaminated sediment can be mobilised from the reservoir and transported downstream.