Plutonium isotopes in the lower reaches of the River Rhône over the period 1945–2000: fluxes towards the Mediterranean Sea and sedimentary inventories

Plutonium isotopes in the Rhône River originate from both the weathering of the catchment basin contaminated by global atmospheric fallout, and the liquid effluents released from the Marcoule reprocessing plant since 1961. Due to a new treatment process applied to the liquid effluents, a decrease of two orders of magnitude in the industrial plutonium discharged into the River Rhône has been registered from 1991. Today, 238Pu industrial inputs to the River Rhône are still about 10 times higher than those derived from global fallout, while 239+240Pu inputs from industrial and global fallout sources are of similar importance, i.e. 1 GBq y−1. Our results indicate that the river sedimentary compartment act either as a sink or a delayed-source term of plutonium for the freshwaters depending on the hydraulic regime and flood events. This compartment may then represent an important industrial delayed-source term for the River Rhône freshwaters in the coming years as the Marcoule reprocessing plant is being dismantled. These results were obtained from samples collected from the lower course of the River Rhône over the 1987–1998 period and analysed for 238Pu and 239+240Pu activities. Both river sedimentary inventories of plutonium isotopes and effective outputs from the River Rhône towards the Gulf of Lions have been estimated for each year over the 1945–2000 period. Regarding 239+240Pu, the sedimentary inventory accumulated since 1945 is estimated to be 172±35 GBq. If mobile, this amount represents a significant delayed-source term of plutonium on the scale of the Rhône watershed.