Sediment redistribution and air dose rate changes in river basins in Eastern Fukushima Prefecture

福島県東部の河川流域における土砂流送と空間線量率の変化

Malins, A.  ; 佐久間 一幸   ; 操上 広志  ; 町田 昌彦  ; 北村 哲浩   

Malins, A.; Sakuma, Kazuyuki; Kurikami, Hiroshi; Machida, Masahiko; Kitamura, Akihiro

The Fukushima Daiichi Nuclear accident resulted in the contamination of several river basins with PBqs of Cs. As radiocesium binds strongly to soils, its redistribution is primarily by soil erosion and sediment transport. Each year 0.1-1% of the Cs inventory in the basins enters into watercourses and is exported out to the Pacific Ocean. Therefore, although the total inventory in the basins does not reduce much each year due to sediment migration, a large amount of Cs in terms of absolute magnitude is redistributed by soil erosion and sediment transport. This is a particular concern for areas in the basin where eroded sediments deposit and accumulate, such as near river mouths, on floodplains in the lower basins near the coast, and in reservoirs. Moreover, the gradient of high Cs densities arising from the accident plumes over the upland areas in the west of the basin areas, compared to relatively lower Cs levels towards the coast, mean that the watercourses are generally transporting highly contaminated sediments into areas with lower contamination levels. This study combined sediment transport modelling with air dose rate simulations to understand how dose rates at areas with high soil erosion/sediment deposition rates in a river basin are being affected by radiocesium redistribution. The sediment and radiocesium transport simulations were conducted using GETFLOWS. We simulated sediment redistribution during typhoon floods, as the contamination redistribution predominantly occurs over these events during the year. The air dose rate modelling was completed with a tool designed to model Cs and Cs distributions varying both spatially and with depth in soil. The dose rate modelling took GETFLOWS results for Cs and Cs erosion and deposition as an input. We analyze the relation between the soil redistribution pattern and the air dose rate.