Abstract

In industrial areas, watercourses constitute potential hazards due to the underlying heavily contaminated sediments. Physical disturbances at the water-sediment interface, induced by natural or anthropogenic events, can result in resuspension of contaminated sediments and remobilisation of pollutants into the water column. Metallic pollutants are largely particle-associated and their fate in aquatic systems partly depends on particle dynamics. Environmental radionuclides are widely used as specific tracers able to identify and model particulate transfer processes in various aquatic media. In this study, short-lived radionuclide 7Be is applied to quantify dynamic processes of particulate material at monthly time scale in a highly urbanized and canalized shallow river: the Scarpe River, North of France. Radioactive method based on the use of 7Be radioisotope (53 days half-life) focuses on short-term sedimentation and resuspension rate. From April 2005 to August 2006, 7Be activities were measured in superficial bed sediment cores and in suspended particles monthly collected at three sites. 7Be Activities generally decreased with depth in the top 0-5 cm of sediment cores, ranging from an undetectable level to 248 Bq.kg^-1. Inventories calculated from the activities of 7Be vary from an undetectable level to 0.152 Bq.cm^-²: observed spatial and temporal variations could reflect discrepancies in sediment dynamics. Partitioning the 7Be stock into “new” and “residual” components showed that an alternation of periods of sedimentation and resuspension occurred at the water-sediment interface. Net maximum deposition (0.39 g.cm^-2) measured for one month with the 7Be method was in the same order of magnitude as the average annual sedimentation rate estimated from 137Cs and 210Pb methods. Inversely, short resuspension periods were evidenced with a maximum erosion rate reaching 0.44 g.cm^-².month^-1.

Keywords: sediment dynamic, deposition, resuspension, Beryllium-7, Scarpe River.