福田, 美保  ,  山崎, 慎之介  ,  青野, 辰雄  ,  石丸, 隆  ,  神田, 穣太

The 2011 off the Pacific coast of Tohoku Earthquake and subsequent tsunami on 11 March 2011 led to the accident at TEPCO’s Fukushima Dai-ichi Nuclear Power Station (FDNPS). Large amounts of radionuclides were dispersed by the resulting hydrogen explosion, and radionuclides also leaked from the FDNPS into the terrestrial and marine environments. In the approximately 4 years that have passed since the FDNPS accident, radiocaesium activities in seawater in the Fukushima coastal area have decreased to about 10 time activities as higher as before the accident (TEPCO, 2015). Because it is sensitive for affecting river input and land erosion off the coast area, there is some controversy about the factors affecting radiocaesium. This study aimed at elucidating the factors affecting the distribution and behaviour of dissolved radiocaesium in seawater collected off the coast of Fukushima Prefecture during six months of each May and October in 2013-2015. In 2013-2015, the dissolved 137Cs activities were 1.8-220 mBq L−1 and the activities in October totally were higher than those in May. In surface-layer seawater (water depth 0-20 m), the activities at stations, which are located within 10 km of the continent, were higher than those at other stations and the trends were particularly observed in October. On the other hand, it is not unclear that the relation between the activities and distance from the continent or salinity in bottom water (water depth < 20 m). In mid-October 2013, Typhoon Wipha passed through the eastern part of the main Japanese island, and at the Haranomachi weather station (37°38′ N, 140°59 E), the total rainfall exceeded 100 mm on 16 October 2013, which was a sampling date for this study (JMA, 2013a). It seems that the result of increased fresh water input, including input of radiocaesium from rivers or land erosion caused by the heavy rain that occurred when Typhoon Wipha passed through the area in 2013. In this presentation, we will also discuss factor for radiocaesium distribution and temporal variation in 2014-2015.
International Conference on Radiological and Nuclear Chemistry

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