239+240Pu and 137Cs in sediments from Tokyo Bay:: Distribution and inventory

Four sediment cores were collected from Tokyo Bay and analyzed for their Pu239+240 and Cs-137 concentrations. Shallow nearshore sediments from Tokyo Bay have Pu239+240 inventories which average 214+/-14 MBq/km(2). They are five times greater than the supply expected from the atmospheric global fallout at the same latitude of 42 MBq/km(2). The measured mean Cs-137 inventory of 433+/-93 MBq/km(2) is approximately one fifth the value expected from global fallout. Furthermore, the Pu239+240/Cs-137 activity ratios, with a mean ratio of 0.50+/-0.14, are significantly greater than the ratio expected from the global fallout of 0.021. Excess Pu239+240 inventories can be considered to have been introduced into Tokyo Bay as weathering products by soil erosion, transported via rivers and winds, and to be much more efficiently scavenged from seawater by particles resuspended at the sediment-water interface.