Concentration of radiocaesium 137Cs in the bottom sediments of Lake Obuchi in Rokkasho Village, Japan

In the vicinity of Lake Obuchi in Rokkasho Village, Japan, are present a nuclear fuel reprocessing plant, a low-level radioactive waste disposal center, and a uranium enrichment plant. Once these facilities are in operation, it is presumed that radionuclides will be released into the environment. Some of the released radionuclides would be accumulated in the lake by tidal activity and atmospheric fallout. In order to evaluate the impact of the released radioactive substances, it is necessary to evaluate the level of radionuclides in the lake sediments before the above mentioned facilities go into operation. This investigation has focused on the concentration of radiocaesium 137Cs in the bottom sediments of the lake. The following results were obtained from the relationships between the concentrations of 137Cs in the sediments and the shape of the water basin, particle size of sediments, the content of organic matter in the sediments. The Lake Obuchi can be divided broadly into two parts, based on a particle size of the sediment. The first part is the mud area (the Futamata River Mouth, the west basin of the lake and in the main lake basin), where most of sediment was composed of silt-size fraction (<0,075 mm). The second part is the sand area (the estuary of lake, the Obuchi River and in the northeast region), where most of sediment was composed of medium-size fraction (0.125 - 0.5 mm). 137Cs concentrations in the mud area were found to be high (approximately 10 Bq kg-1 dry), but the sand area were found to be low (< 1 Bq kg-1 dry). It has been shown that the 137Cs concentration of the bottom sediment in Lake Obuchi increased with decreasing the grain-size and the increasing amount of organic matter present. The cumulative level of 137Cs in the bottom sediment is influenced by grain-size and physical-chemical property of the sediment. (author)