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

PH. BONTÉ, J.M. MOUCHEL, A.J. THOMAS, M.F. LE CLOAREC, J.P. DUMOULIN, S. SOGON, L. TESSIER

Abstract


This study was undert a ken 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 (7Be, 234Thxs) and artificial (131I) radionuclides, as well as the longer-lived natural 210P bxs and its descendant the 210Po. 137Cs 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 131I, 7Be and 234Thxs. The best estimate of the sediment mass present in the river is (24-41)·103 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).


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