Potential plutonium remobilisation linked to marine sediment resuspension: First estimates based on flume experiments

In coastal environments, resuspension is a key issue regarding contaminant remobilisation from the sediments. A first attempt to quantify both sedimentary bed erosion and related plutonium isotope remobilisation from eroded particles was carried out through flume experiments. Erosion fluxes under controlled hydrodynamic stresses and their resulting plutonium remobilisation were measured for cohesive sediments sampled at several locations (8 to 97 m depth) near the Rhone river mouth (North-western Mediterranean Sea). Surface sediments were characterised in terms of plutonium content (238Pu and 239,240Pu) and particle size distribution. Laboratory resuspension experiments were performed under realistic hydrodynamic conditions close to those prevailing in coastal zones. Critical shear stresses of erosion ranged between 0.068 and 0.087 N m− 2 whereas erosion rate varied within a factor of 3 (57-176 g m− 2 h− 1). After 1 hour of resuspension, the plutonium activity increased in the seawater particulate phase by factors ranging from 2 to 14, depending on the site at which the sediment was sampled. Plutonium fluxes ranged between 2 and 56 mBq m− 2 h− 1 for 239,240Pu depending on location. The highest fluxes were measured for sediments from the deepest site, where the highest Pu activities and mud content were found. During our resuspension experiments, no significant increase was found in dissolved Pu activities. These laboratory experiments have provided a unique opportunity to investigate the behaviour of plutonium at the sediment-water interface in a coastal environment. They emphasised the importance of sediment resuspension in plutonium remobilisation and its possible dispersion on continental shelves.