Anthropogenic radionuclides in seawater of the Japan Sea - The results of recent observations and the temporal change of concentrations

Between 1996 and 2002, a wide-area research project on anthropogenic radionuclides was carried out in an area covering the Japanese and Russian Exclusive Economic Zones of the Japan Sea, through a collaboration of Japanese and Russian institutes. The aim was to investigate the migration behavior of anthropogenic radionuclides (Sr-90, (137)cs, and (239+240)pu) in the sea. Four expeditions conducted in the Japan Sea between 2001 and 2002 found that the observed concentrations and distributions of radionuclides were similar to those found in previous investigations. Inventories estimated from the concentration data indicate that larger amounts of these radionuclides accumulate in the Japan Sea seawater (by a factor of 1.5-2.1) than are supplied by global fallout in the same latitude belt. Further, we found that the Sr-90 and (CS)-C-137 concentrations in the intermediate layer show temporal variations with time scales of I to several years. The results of cross-analysis using the data of Cs-137 and dissolved oxygen suggest that the distribution and variation of radionuclide concentrations in the intermediate layer may reflect water mass movement in the upper part of the Japan Sea.