Soil-to-river Cesium-137 transfer in a catchment coupling the SWAT model and a mass balance equation

Nuclear accidents and atmospheric tests have released large quantities of radionuclides into the environment. Among them, Cesium-137 (¹³⁷Cs) is of major concern due to its long-term persistence. To support post-accidental management, predictive tools are needed to assess its environmental transfers. This study presents a novel approach to simulate the transfer of ¹³⁷Cs from soils to the river outlet in a watershed impacted by radioactive atmospheric fallout. The Soil and Water Assessment Tool (SWAT) was coupled with a trace metal transfer equation to simulate daily concentrations of ¹³⁷Cs in suspended sediments in the Ardèche watershed (2138 km², France). The total ¹³⁷Cs stock in soils was estimated at 7.7 TBq based on a soil sampling campaign. Modelled ¹³⁷Cs concentrations range from 0 to 43.0 Bq kg^-1, whereas measured values range from 3.5 to 20.1 Bq kg^-1. The discrepancies are mainly due to limitations in SSC observations used for model calibration, as well as the particle-trap sampling method, which tends to underestimate the actual concentrations. Moreover, results indicate that 83 % of the annual ¹³⁷Cs transport occurs in the particulate phase with 75 % of the total annual ¹³⁷Cs flux at the outlet exported during the top 10 % highest flow days. On average, 263.0 GBq y^-1 of ¹³⁷Cs are eroded from the watershed soils, the river exports 1.62 GBq y^-1 of ¹³⁷Cs at the outlet and stores 256.0 GBq y^-1 within the river itself and floodplain. This corresponds to an annual export of only 0.02 % of the total ¹³⁷Cs soil stock.