Investigation of environmental migration and behavior by trace of Nagasaki atomic bomb recorded in sediment 2

Sediment in a reservoir accumulates substances in its layers which records the environmental change in the past. Yamazaki et al. found the trace of Nagasaki atomic bombardment recorded in the sediments of 'A core' sampled at Nishiyama reservoir, using the information of chronological order estimated from the stratigraphic distribution of 137Cs and geological features in the sediments. The determination of origin of the substances can provide useful information to determine the chronological order in the sediments accurately. The ratio of 240Pu/239Pu and the morphology of particles give the information of their origin. To make a further research on the traces of the atomic bombardment, another sediment core sample (B core) collected from Nishiyama reservoir was analyzed in this study. From the depth profile of B core, it was observed that 137Cs and 239+240Pu concentrations had two peaks. The 240Pu/239Pu ratio in B core sediments was lower than that of 1960's nuclear tests, suggesting the Pu originated from Nagasaki atomic bomb. The concentrations of Cu and Pb had similar profiles to 137Cs concentration. The deeper layer samples, in which maximum 137Cs concentrations were observed, contained particles of Cu metal and its alloy scattered by the bomb blast. The stratigraphic distribution of 137Cs and 239+240Pu and the ratio of 240Pu/239Pu indicated that the sediments below 200 cm contained the trace of Nagasaki atomic bombardment. Sand layer found at the depth of 150-200 cm was considered to be a result of the flood in 1953. At the depth of 120-150 cm, the trace of Nagasaki atomic bombardment come from the surface soil around the reservoir after the floods. Above 120 cm, 137Cs and Cu originated from nuclear tests and industrial activities, respectively. (author)