Plutonium distribution and remobilization in sediments of the Rhone River mouth (North-Western Mediterranean)

The aim of the present study was to describe the distribution and remobilization of plutonium (Pu) in the sediments off the Rhone river mouth. Most of the 238Pu and 239,240Pu isotopes introduced into the Rhone River were discharged by the liquid effluents released from the Marcoule reprocessing plant, located 120 km upstream the river mouth. Due to its high affinity for particles and its long half life, 238Pu is a promising tracer to follow the dispersion of particulate matter from the Rhone River to the Mediterranean Sea. During the 3 REMORA cruises, sediment samples were specifically collected in the Rhone pro-delta area and more offshore on the whole continental shelf of the Gulf of Lions. The measurements of alpha emitters gave a first detailed spatial distribution of Pu isotope concentrations in surface sediments off the Rhone mouth. Using 137Cs concentrations and their correlations with Pu isotopes, we were able to give a first estimate of Pu inventories for the sediments of the study area. In 2001, plutonium inventories were estimated to 92 ± 7 GBq of 238Pu and 522 ± 44 GBq of 239,240Pu for an area of 500 km2 in front of the Rhone River mouth. Roughly, 50 % of these inventories are trapped in an area of 100 km2 corresponding to the extent of the Rhone pro-delta zone. In spring 2002, an ADCP, with current velocity and wave measurements, was moored off the Rhone River mouth. This unique in situ dataset highlights the major role of South-East swells in the erosion of pro-deltaic sediments and their dispersion to the South-Westward direction. Plutonium remobilization was examined using a new experimental design based on sediment resuspension processes studied within a linear recirculating flume. For Gulf of Lions sediments and for a given hydrodynamic stress, remobilization fluxes raised a maximum of 0.08 Bq.m-2.h-1 for 238Pu and 0.64 Bq.m-2.h-1 for 239,240Pu. A first plutonium budget determined for the study area indicates that at least 85 % of the Pu discharged over the 40 past years by the Marcoule reprocessing plant, are currently trapped in the sediments off the Rhone River mouth. Plutonium remobilization is potentially an important process but dispersion of contaminated sediments appeared to be limited in space. These results suggest a low exportation of particulate matter introduced by the Rhone River towards the continental shelf of the Gulf of Lions and the North-Western Mediterranean basin. (author)