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Horizontal and vertical distributions of $$^{137}$$Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

福島第一原子力発電所周辺の河川近傍の河川近傍の海底土に含まれる放射性セシウム(137)の空間分布について

鶴田 忠彦; 原田 久也*; 御園生 敏治; 松岡 稔幸; 程塚 保行*

Tsuruta, Tadahiko; Harada, Hisaya*; Misono, Toshiharu; Matsuoka, Toshiyuki; Hodotsuka, Yasuyuki*

福島県の沿岸域(沖合約5km程度まで)について、海底地形、海底土の堆積状況及び海底土の放射性Csの分布状況について調査を行った。放射性Csが農集する地点は凹状の窪地を有する特異な地点に限られ、その他の地点は、陸域と比較して放射性Cs量として2から3オーダー程度少ないことが明らかになった。

The seafloor topography was divided into flat and terrace seafloors based on their topographical features and seabed sediments were distributed in an area that was half a degree of the entire investigation area. The $$^{137}$$Cs inventory was several tens of kBq/m$$^{2}$$ and the grain sizes (the D50 values) were nearly constant (fine sand) on the flat seafloor. On the terrace seafloor, the $$^{137}$$Cs inventory was larger than that on the flat seafloor, and the grain size varied from silt to coarse sand. The grain size distributions appear to be influenced by the mean shear stress at the seafloor bottom, and a significant factor in the mean shear stress is thought to be the seafloor topography. Distributions of remarkably large $$^{137}$$Cs inventories, more than several thousands of kBq/m$$^{2}$$, are thought to be confined to a small area. Vertical changes in the $$^{137}$$Cs inventories suggested that the $$^{137}$$Cs inventories have significantly decreased in large areas of the shallow sea.

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