Distribution of environmental radionuclides in sediment cores of the Jinheung catchment

Sediment core samples taken from the Jinheung catchment located in the middle of the Korean Peninsula were used to know environmental radionuclide distribution. The grain sizes of the sediment cores were found at depth of about 17 cm suggesting that it might have occurred during a dry period of 1969. The radionuclides, Cs-137, Np-237, Pu-239, Pu-240, U-234, U-238, Th-228, Th-230, Th-232, were analyzed by sector type ICP-MS and gamma-spectrometry. The Cs and Pu distribution changed with the depth, in which the maximum ranged from 14 to 22 cm. This was due to the high activity of the results of nuclear bomb tests in the air from 1960s and showed different distribution pattern on the soil surface. The average activity ratio of Pu-240/Pu-239 and Np-237/Pu-239 was 0.173 and 0.45, respectively. These values were similar to the north hemisphere global fallout ratio of Pu-240/Pu-239 (0.18) and Np-237/Pu-239 (0.45). The Np-237/Pu-239 ratio showed a higher value than the global fallout ratio above 14 cm depth. The U, Th and their daughter radionuclides kept secular equilibrium in the sediment core because the average activity ratios were nearly 1.