Radiocesium has been deposited after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and affix to the soils inland. The soils flowed into Lake Teganuma through Ohori River. Field observation was carried out to quantify radiocesium, associated with sediments, transported around Lake Teganuma and Ohori River from October 2011 to March 2014. We quantified the sediment transported to Ohori River and found that the total radiocesium stock in Ohori River was 2,190 MBq which was mostly attached to the fine-grained sediment of silt in 2012. The radiocesium, transported as a suspended sediment in the Ohori River watershed, was approximately 10 percent of the total fallout of 2 TBq. The total flux of radiocesium in Ohori River were 3,800 MBq in 2012 and reduced to 6.65 MBq in 2013. These results denote that most of the radiocesium-polluted sediments were transported downstream in 2011 and sharply decreased after 2012. When a similar study was conducted in Abukuma River, only 1% of the total fallout was calculated indicating transport was 10 times that of the Abukuma River. This is attributed to be urbanization of Teganuma basin in comparison to the Abukuma basin, thus more cesium were transported in a shorter time period.
2015 Japan Society on Water Environment