Basin-scale modelling of soil-to-river 137Cs transfer in the Rhône river system

Cesium-137 ( 137 Cs) is an anthropogenic radionuclide whose persistence in soils and sediments represents a sustained source of environmental contamination. The Rhône River basin has been affected by 137 Cs contamination from atmospheric nuclear weapons testing and the Chornobyl accident. In this study we simulate spatially the 137 Cs dynamics in the Rhône River system, including the main stem and several major tributaries. The approach builds on an existing SWAT (Soil and Water Assessment Tool) configuration of the Rhône basin and extends a radionuclide-specific mass-balance formulation previously developed at the sub-basin scale. Time-evolving soil inventories of 137 Cs from 1986 to 2016 are reconstructed using historical deposition maps. These reconstructions incorporate SWAT-derived erosion and soil turnover into a vertical migration model, providing spatially explicit initial conditions for simulations starting in 2016. Model outputs for the 2016-2020 period are evaluated against extensive observations of particulate and dissolved 137 Cs collected throughout the Rhône network. Simulated soil 137 Cs concentrations reproduce large-scale spatial patterns inherited from historical fallout and agree well with observations across most land-use classes. The modelled source term decreases annually by 2.2 %, of which 97.9 % is driven by radioactive decay. Soil erosion mobilizes 0.8 TBq yr −1 , but most of this activity is trapped within the river system. Consequently, only 0.13 ± 0.03 TBq yr −1 (a small fraction of the total inventory) is exported at the basin outlet. This modelling framework provides a coherent basis for quantifying long-term radionuclide fluxes in large river systems affected by diffuse radioactive contamination.