Dispersion of radiocesium-contaminated bottom sediment caused by heavy rainfall in Joso City, Japan

A large-scale heavy rainfall disaster occurred in Joso City, Japan, in September 2015, and one third of the city area (40 km2) was flooded by the Kinu River. Artificial radionuclides such as 134Cs and 137Cs were known to have accumulated in the river bottom sediment after their release in the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. It was thought that these radionuclides might have been dispersed by the rainfall disaster. A car-borne survey of absorbed dose rate in air had been made by the authors in Joso City in August 2015. Then, the present study made a second car-borne survey in October 2015, to evaluate changes in the rate after the rainfall disaster. The absorbed dose rate in air and the standard deviation (range) measured in the flooded areas of Joso City after the disaster were 68 ± 9 nGy h-1 (39–98 nGy h-1), which was 10% higher than the rate before it. Additionally, higher dose rates (> 60 nGy h-1) were observed for the flooded areas after the disaster; furthermore, up to 886 Bq kg-1 of activity concentration from 134Cs and 137Cs was observed in these flooded areas, and this was 11 times higher than the activity concentration before the disaster. These results suggested the dispersion of artificial radionuclides accumulated in the bottom sediment of the Kinu River after the Fukushima Daiichi Nuclear Power Plant accident occurred by the heavy rainfall disaster.