RADIOISOTOPES

According to the research conducted by the Government of Japan and Chiba Prefectural Government, the sediments of Lake Tega and its tributaries are still highly contaminated by radiocaesium (over 1,000 Bq/kg) derived from the Fukushima Daiichi Nuclear Power Plant. To investigate the possible sources that would transport and redistribute radiocaesium to the Lake Tega system, which is urbanized compared to Fukushima Prefecture, we have selected three retention basins, corresponding to upstream of Lake Tega, located in Kashiwa City, Chiba Prefecture. Temporal changes in concentrations of radiocaesium in water and sediment samples were determined during 2013–2014. The concentrations of radiocaesium in both samples tended to decrease during the sampling period, although some of the sediment samples were still high radiocaesium concentrations exceeding 10 kBq/kg. These observations suggest that the radiocaesium in the retention basins, which play an important role in supplying radiocaesium to the downstream area, is highly contaminated compared to the downstream area. Increments of the concentrations of radiocaesium in particulate form were observed from the water samples collected immediately after the passage of large typhoon Wipha. This result implies that heavy rainfall events may be playing an important role in the migration of existing radiocaesium from the catchment areas to the Lake Tega system.