Sediment dynamic equilibrium, a key for assessing a coastal anthropogenic disturbance using geochemical tracers: Application to the eastern part of the Bay of Seine

Geochemical studies are becoming more and more frequent in the context of the increasing pressure of human activities on marine coastal ecosystems and represent an appropriate tool to assess anthropogenic disturbances. Moreover, it is difficult to find discriminant markers. The eastern part of the Bay of Seine (English Channel) is highly impacted by the presence of harbour activities, fishing and sediment extraction. Dredged sediment from the Grand Port Maritime du Havre (GPMH) are deposited at the subtidal Octeville site, in the north-eastern part of the mouth of the Seine estuary (mixed sediment area). To understand natural and anthropogenic sedimentary mechanisms in this area, a geochemical and sedimentological study was conducted at the beginning of 2016. A dense sampling campaign including 179 stations was carried out between Cap de La Heve and Cap d'Antifer. For comparison, sampling was carried out in the harbour (13 samples in basins strongly or very weakly dredged), in the dredged grab itself and in the Seine estuary sediment (one station in the brackish zone and two stations in the river freshwater zone). Elemental compositions were determined by X-Ray Fluorescence spectrometry and infrared spectrometry. Using PCA (Principal Component Analysis), seven constituents were selected (Si, As, S, Pb, Rb, organic Br and TOC) to determine the area influenced by dumped spoil and the sediment transport directions. Sediment areas in dynamic equilibrium display a TOC gradient perpendicular to the coastline, linked to granulometric variations due to a combination of the swell and tidal currents. In the study area, dredged sediment are finer grained and have undergone changes due to the influence of diagenetic processes characteristic of the harbour environment. As a result, these sediment are enriched in sulphides, Pb, TOC and Rb, which allows us to highlight the in-situ deposited sediment spoil. Dumped sediment and the area subject to their influence are clearly identified since they locally disrupt the natural dynamic equilibrium state.