ИСТИНА

Redistribution of Fukushima-derived radiocesium through river system poses potential risks for residents in the downstream area of river basin. Niida river basin (265 km2 ) on north eastern part of Fukushima prefecture is indeed a case in point. The river basin has highly contaminated area on its upstream part and urban area on its downstream part. The upstream area of the basin had been subject to decontamination works, such as scraping surface soil and reversal tillage and so on, since 2012. Quantitative evaluation of 137Cs wash-off associated with sediment dynamics is required for mitigating radiological risk for the residents and for understanding the influences of decontamination works. This study presents results of the long-term observation of radiocesium wash-off within the catchment and water sampling during flooding events triggered by a typhoon. Based on the results, we try to unveil processes of 137Cs wash-off of catchment scale