Buffering of suspended sediment transport in lowland river during low water stages: quantification in river Seine using environmental radionuclides

International audience; This study was undertaken to test the application of environmental radioactive tracers for estimating sediment mass and sediment residence time in rivers. A continuous sampling of the Seine river suspended matter (SM) using sediment traps was made during two months, between Paris and the estuary, along a 120 km long river section. The hydrological regime corresponded to the low water stage, where the SM transport is reduced. The measured tracers in the SM include short-lived natural ($^7$Be, $^{234}$Th$_{xs}$) and artificial ($^{131}$I) radionuclides, as well as the longer-lived natural $^{210}$Pb$_{xs}$ and its descendant the $^{210}$Po. $^{137}$Cs was used to check grain-size effects. A simple steady state model allowed us to estimate the total sediment mass, i.e. the SM, plus the resuspendable matter (RM), and the sediment residence time. Despite their different half-lives (8 to 53 days) and their different geochemical properties, consistent results were obtained with $^{131}$I, $^7$Be and $^{234}$Th$_{xs}$. The best estimate of the sediment mass present in the river is (24-41)·10$^3$ tons; it is essentially composed of the RM which is 10-17 times more abundant than the SM. In these hydrological conditions, the sediment residence time is quite long (1.6-2.8 months).