Anthropogenic radionuclides in seawater in the East Sea/Japan Sea: Results of the first-stage Japanese–Korean–Russian expedition

Measurements of anthropogenic radionuclides in seawater samples near dumping sites of the East Sea/Japan Sea were conducted, as the first-stage Japanese–Korean–Russian Joint Expedition, in March–April 1994, to assess radioactive contamination of the East Sea/Japan Sea after the dumping of radioactive wastes by the former Soviet Union and Russian Federation. Concentrations of 137Cs, 90Sr and 239,240Pu in surface waters of the East Sea/Japan Sea near dumping sites were in the ranges of 2.8–3.6,1.6–2.0, and 8–25 Bq kg-1, respectively. Surface 137Cs and 90Sr concentrations were the same order of magnitude as those observed in the North Pacific, whereas surface 239,240Pu concentration was significantly higher than those observed in the North Pacific. Vertical profiles of 137Cs and 90Sr, with surface maxima and decreasing with depth, showed higher depth gradients than those observed in the North Pacific, reflecting deep convection in the north central East Sea/Japan Sea. The higher 239,240Pu concentration in the surface waters of the north central East Sea/Japan Sea may reflect rapid recycling of deep Pu. The results revealed that most of the recent radioactivity observed in the north central East Sea/Japan Sea was of global fallout origin from atmospheric nuclear testing and partly the Chernobyl fallout. In this survey, there was no clear evidence of an increase in radionuclide concentrations due to the dumping of radioactive wastes by the former Soviet Union and Russian Federation.