137Cs inventory in sediment near the Rhone mouth: role played by different sources

The low tidal range of the Mediterranean Sea and the high sediment load of the Rhone induce the formation of an important submarine delta. The ¹³⁷Cs inventory in sediment on a 480 km² area near the Rhone mouth reached 19.6 TBq in 1990. The spatial distribution of both sediment accumulation rates and ¹³⁷Cs concentrations in this area confirm the existence of a spatially well-delimited zone, i.e. the prodelta, where both parameters are the highest. Thus, 40% of the total amount of ¹³⁷Cs stored in this 480 km² area was found within the prodelta (30 km²) in the close vicinity of the river mouth. This attests to the efficiency of the ¹³⁷Cs trapping by the prodelta sediments. A study on the part the different ¹³⁷Cs sources in the Mediterranean Sea play in contributing to this inventory has been carried out. These sources are (i) direct deposition from both global fallout and the Chernobyl accident, (ii) indirect inputs arising from the erosion of surface soils of the Rhone catchment area that were also contaminated by the previous fallout, (iii) liquid effluents from the nuclear industry into the Rhone waters. Assuming that direct deposition is evenly distributed over the study area (480 km²) and that the particulate ¹³⁷Cs input from the Rhone is entirely trapped in this same area, these sources account for about 50% of the inventory. These hypotheses are unlikely in a coastal area subject to various physical disturbances but they are conservative enough to assess the lower limit of the nuclear liquid releases contribution to this inventory to be 50%.