Sedimentation rate and environment in Lake Shinji, Shimane Prefecture

In order to study sedimentation environments and sedimentation rates in Lake Shinji, Shimane Prefecture, the Pb-210 and Cs-137 radioactivities were measured in 15 cores taken on Oct. in 1994. The sedimentation rates in the lake varied at each location; those in the western area are about 0.25 g/cm2/y, larger than in the eastern area (about 0.1 g/cm2/y), and those in the central area are the lowest (≤0.05 g/cm2/y). This fact indicates that most of the sediments supplied by the Hii river deposited in the western area and little amount was transported to the central area. The inventories of radionuclides that indicate the amount of accumulation were larger in the western area, which is the same tendency as the sedimentation rate. They are in a positive correlation (correlation coefficient (R) is 0.78), and their relationships are different from that observed in offshore sediments of Japan sea. The fluxes of excess Pb-210 and Cs-137 are large in western area, which suggests that much amount of sediment were supplied from the Hii river. The concentrations of excess Pb-210 and Cs-137 in surface sediments are in a good correlation (R=0.76), which indicates that both nuclides in sediment grains may exist in a constant ratio. The sources of radionuclides are supposed to be terrestrial fallout and suspended particles carried from river. Although the latter source is not negligible, it is considered that the former is superior and that radionuclides moved and distributed heterogeneously in Lake Shinji. (author)