Recent inputs and budgets of 90Sr, 137Cs, 239,240Pu and 241Am in the northwest Mediterranean Sea

Abstract

Monthly atmospheric depositional fluxes of 137Cs, 239,240Pu and 241Am, coupled with measurements of these radionuclides in seawater of the northwest Mediterranean Sea, are presented. Precipitation rates are the dominant factor in controlling the temporal deposition of anthropogenic radionuclides onto the northwest Mediterranean; however, special events such as Saharan dust input bring a significant amount of radionuclides to this area. The average annual deliveries of 137Cs, 238Pu, 239,240Pu and 241Am through the atmosphere into the northwest Mediterranean were 990, 0.9, 22 and 7.9 mBq m−2, respectively. The corresponding amounts delivered annually into the northwest Mediterranean were 0.02 TBq for 239,240Pu, 0.007 for 241Am, and 0.84 for 137Cs. The present atmospheric inputs represent about 0.08% for 239,240Pu, 0.58% for 241Am, and 0.02% for 137Cs of their water-column inventories. While the present surface 239,240Pu concentrations in the west Mediterranean have significantly decreased since 1976, those recorded in deeper layers (below 1000 m) have increased. The observed differences between the 241Am and 239,240Pu profiles have been caused by the different scavenging efficiencies of the two radionuclides, and also by 241Am in-growth from 241Pu. The activity ratios of 241Am/239,240Pu and 239,240Pu/137Cs through the water column appear to be lower than the global fallout ratio, whereas 137Cs/90Sr activity ratios reflect the global fallout ratio. Higher concentrations of 90Sr and 137Cs were found in the intermediate layer (200–600 m), where higher salinity and temperature levels prevail. This indicates the intrusion of Levantine Intermediate Water from the Eastern Basin into the northwest Mediterranean Sea. The estimated radionuclide inventories in the water column for 2001 were around 3700 Bq m−2 for 137Cs, 2200 for 90Sr, 52 for 239,240Pu, and 7.0 for 241Am.