Evaluation of sediment and Cs redistribution in the Oginosawa River catchment near the Fukushima Dai-ichi Nuclear Power Plant using integrated watershed modeling

福島第一原子力発電所近くの荻ノ沢川流域を対象に統合型流域モデルを用いた土砂およびCsの再分布の評価

佐久間 一幸   ; Malins, A.  ; 舟木 泰智 ; 操上 広志  ; 新里 忠史  ; 中西 貴宏   ; 森 康二*; 多田 和広*; 小林 嵩丸*; 北村 哲浩   ; 細見 正明*

Sakuma, Kazuyuki; Malins, A.; Funaki, Hironori; Kurikami, Hiroshi; Niizato, Tadafumi; Nakanishi, Takahiro; Mori, Koji*; Tada, Kazuhiro*; Kobayashi, Takamaru*; Kitamura, Akihiro; Hosomi, Masaaki*

福島第一原子力発電所の南西15kmに位置する荻ノ沢川を対象に、水循環流域シミュレーターGETFLOWSを用いて、流域内の土砂とCsの再分布を評価した。河道へのCsの供給は主に河川近傍と森林のガリで発生し、河川から離れた森林域における寄与は小さいことが示唆された。森林内の表層土壌中のCsは、主に物理減衰と下方浸透、系外にわずかに流出することで減少していた。将来的に河川近傍から河川へのCsの供給量が小さくなることが示唆された。

The Oginosawa River catchment lies 15 km south-west of the Fukushima Dai-ichi nuclear plant. The General-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) code was used to study sediment and Cs redistribution within the catchment. Cesium-137 input to watercourses came predominantly from land adjacent to river channels and forest gullies. Forested areas far from the channels only made a minor contribution to Cs input to watercourses, total erosion of between 0.001-0.1 mm from May 2011 to December 2015. The 2.3-6.9% y decrease in the amount of Cs in forest topsoil over the study period can be explained by radioactive decay (approximately 2.3% y), along with a migration downwards into subsoil and a small amount of export. The amount of Cs available for release from land adjacent to rivers is expected to be lower in future than compared to this study period, as the simulations indicate a high depletion of inventory from these areas.