[HCG34-P02] Radiocesium Inter-Annual Variations and Spatial Dynamics in Watershed in Iitate, Fukushima, Japan
キーワード：セシウム、土壌侵食、経年変化、空間的変動
A large amount of radiocesium, 137Cs and 134Cs, were released and deposited on the soil surface due to the accident at Fukushima Daiichi Nuclear Power Plant, FDNPP, due to the Great East Japan Earthquake that occurred on March 11, 2011. Among them, 137Cs is easily absorbed by clay minerals and soil organic matter, and its physical half-life is 30.2 years. 137Cs are redistributed to waterways, rivers, lakes and coastal areas by rainfall-runoff process, so long-term influences on residents’ lives and agricultural, forestry and fishery products in the area surrounding the accident are concerned. In order to evaluate the influence of secondary contamination of 137Cs, it is necessary to grasp the temporal and spatial dynamics in the watershed including areas with high 137Cs deposits.
In this study, fixed point observations was conducted on two rivers, Hiso river and Mano river, in Iitate village, Fukushima Prefecture from 2013 to 2018. In addition, multiple observation points were set up in watersheds, and simultaneous water sampling was done at multiple points. We measured SS concentration and 137Cs concentration from water samples, grasped the long-term change in 137Cs runoff, and identified the catchment area where 137Cs runoff was noticeable.
The 137Cs concentration of SS dropped drastically with extreme large flood due to Typhoon Etau in 2015, and the concentration remained low until 2018. The same result was obtained in 2018. In Hiso river, 137Cs concentration of SS in 2018 increased slightly compared to 2017. This is thought to be the effect of the decontamination work in the Nagadoro district.
As a result of simultaneous multipoint observation, spatial distribution of 137Cs concentration in the watershed was changing as merging the branch streams into the main stream. In addition, it was revealed that sediment with high 137Cs concentration flowed in from Nagadoro district where decontamination works was incomplete.
In this study, fixed point observations was conducted on two rivers, Hiso river and Mano river, in Iitate village, Fukushima Prefecture from 2013 to 2018. In addition, multiple observation points were set up in watersheds, and simultaneous water sampling was done at multiple points. We measured SS concentration and 137Cs concentration from water samples, grasped the long-term change in 137Cs runoff, and identified the catchment area where 137Cs runoff was noticeable.
The 137Cs concentration of SS dropped drastically with extreme large flood due to Typhoon Etau in 2015, and the concentration remained low until 2018. The same result was obtained in 2018. In Hiso river, 137Cs concentration of SS in 2018 increased slightly compared to 2017. This is thought to be the effect of the decontamination work in the Nagadoro district.
As a result of simultaneous multipoint observation, spatial distribution of 137Cs concentration in the watershed was changing as merging the branch streams into the main stream. In addition, it was revealed that sediment with high 137Cs concentration flowed in from Nagadoro district where decontamination works was incomplete.