Temporal and spatial variations of anthropogenic radionuclides in Japan Sea waters

90Sr, 137Cs and 239+240Pu data covering three decades were examined for temporal and spatial variations of their concentrations and inventories in waters of the Japan Sea/East Sea. The time trend of radionuclide concentrations in surface water showed a gradual decrease over the whole period and the effective half-lives of 90Sr, 137Cs and 239+240Pu were estimated to be about 15, 20 and 15 years, respectively. The averaged concentrations of 90Sr, 137Cs and 239+240Pu for the year 2000 were estimated to be 1.6, 2.8 Bq m−3 and 6.6 mBq m−3, respectively. 90Sr and 137Cs concentrations in the south of the Japan Sea were higher than the concentrations in the north, whereas 239+240Pu showed a reverse pattern. The radionuclide concentrations decreased with time in the surface layer and increased in the subsurface layer for 90Sr and 137Cs, or increased below the subsurface layer for 239+240Pu. The averaged inventories of 90Sr, 137Cs and 239+240Pu in Japan Sea waters in the 1990s were estimated to be 1.8 , 3.1 kBq m−2 and 63 Bq m−2, respectively, and were 30–40% higher than in the Pacific Ocean. The area of high inventory intruded from the north of the Japan Sea into the south, and the low inventory region appeared in the central part of the Sea around the Yamato Rise. Distinctive changes and distributions of radionuclides found in this study are explained by the geographic and oceanographic characteristics of the Japan Sea.