Characteristic mineral composition of sediment transporting radiocaesium from upstream to downstream within a Fukushima river catchment

福島県内の河川水系における上流から下流までの放射性セシウムの移動媒体としての堆積物の鉱物学的特徴

萩原 大樹  ; 小西 博巳*; 舟木 泰智 ; 中西 貴宏   ; 飯島 和毅  

Hagiwara, Hiroki; Konishi, Hiromi*; Funaki, Hironori; Nakanishi, Takahiro; Iijima, Kazuki

Deposited radiocaesium is transported in river systems by soil particles such as clay minerals and redistributed to downstream areas. It is therefore important to understand both the distribution of mineral species contained in the soil particles and their behavior within river systems. This paper describes the sediment mineralogy and radiocaesium concentration from an upstream reservoir down to the estuary within the Tomioka river catchment, situated in the east of the Fukushima Prefecture. Radiocaesium concentrations in soil particles increased with decreasing grain size. Results of XRD analyses showed that the principal minerals contained in the reservoir sediment were quartz, feldspar, hornblende, augite and various clay minerals. The relative abundance of clay minerals increased with decreasing grain size. By means of TEM observation with heat treatment, it was clarified that the clay minerals included vermiculite and halloysite. It was also clarified that the clay mineral composition was similar throughout the river system with the exception of one estuarine sample, which included smectite. The mineral compositions indicate that the transport of radiocaesium within soil particles in the Tomioka river system is dominated by clay minerals which originate from Abukuma granite.