Tidal asymmetry: the use of artificial radionuclides in sediments (the Seine estuary, France)
Résumé
The Seine estuary is the outlet of the catchment area of the Paris Basin, where fine sediments and a number of anthropogenic elements and substances end in. Among them, artificial radionuclides can be used as tracers of sediment sources and mixing processes. They may originate from upstream (atmospheric fallout from Chernobyl accident in 1986 and from nuclear weapons testing in the 1960s, licensed discharges from nuclear facilities...) or from downstream (La Hague reprocessing plant -Central Channel). In this macrotidal estuary, trapping and upstream migration of sediment is in process, due to the tide asymmetry; it is named "tidal pumping". It has been previously documented using 60Co, a short-lived radionuclide, originating from the La Hague reprocessing plant (north of Cotentin peninsula). The average upstream velocity of 60Co-labelled sediment particles has been estimated to be in the order of 10 km per year. The plutonium 239, 240 and the americium 241 have much longer decay period and could, therefore, give the opportunity to better understand the dynamics of the "tidal pumping" on a longer term.