Influence of an aggregate extraction in remobilizing contaminants from sediments in surface waters of the Bay of Seine (France)

In coastal areas where monitoring data show that fine sediments contain contaminants, remobilization of these materials during aggregate extractions, which create a turbidity plume, might influence water quality. The eastern bay of Seine (France) is under the influence of the Seine plume, one of the most contaminated rivers in Europe. This study aimed at investigating remobilization of metals and organics from sediments in surface water during and after an aggregate dredging, through their behavior between total and dissolved phases. In this study area located 20 kilometers off Le Havre, sediments where sampled to identify potential contaminants. Unfiltered and filtered (< 0,2 µm) surface seawater samples were collected at different stages of the aggregate extraction : before (T0), in the turbidity plume at 20 minutes (T1), 1h (T2) and 2h (T3), and 1 h after the end (T4). Substances looked for were metals (Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, V, Zn) and several organic contaminants including DEHP, PCBs, and PAHs. Results showed a significant increase in Cr, Cu, Hg, Ni, Pb, V and Zn in total samples at T1 and to a lesser extent at T3, and a tendency to return to initial concentrations at T4. DEHP showed a different behavior with decreasing concentrations from T1 to T3, before tending to initial level at T4. Dissolved concentrations remained relatively stable over the whole experiment. This work therefore shows effective remobilization of contaminants which remain mostly adsorbed on particles, and a tendency to returning to initial levels during clearing away of the turbidity plume. A schematic modeling (with a physical 3D model) of the turbidity plume confirms its limited extent and duration. Moreover, these results represent new data on substances that are of high concern in the bay of Seine, an area subject to multiple anthropogenic pressures.