TEMPORAL CHANGES IN RADIOCESIUM DEPOSITION ON THE FUKUSHIMA FLOODPLAIN

There has been significant concern about increases in the exposure dose in living areas due to the accumulation of radiocesium discharged from contaminated mountainous forests in Fukushima. In this study, we investigated the history of radiocesium deposition on several floodplains in Fukushima following the nuclear power plant accident. Radiocesium concentrations in river suspended particles and the air dose rates on the floodplains were observed continuously. The annual sediment accumulation on the floodplains was 5.5-200 kg m(-2), and the observed radiocesium concentration decreased with the decrease in the radiocesium concentration of suspended particles. The air dose rates on the floodplains were gradually decreasing with time. In 2015, with heavy flood discharge, a sediment accumulation of 180-200 kg m(-2) and a sharp decrease in the air dose rate were observed at the Takase River, which does not have a reservoir. Conversely, the sediment accumulation at the Ukedo River was significantly reduced due to deposition in an upstream reservoir.