137Cs in river systems can be taken up by aquatic animals. In this paper, total 137Cs concentration in the Ukedo River system and 137Cs presence in dissolved, acid-soluble and -insoluble fractions in river water samples were determined. Total 137Cs concentration had the maximum value of 2.08 Bq kg-1 below the Ogaki Dam in May 2012. In the base flow condition, > 87% of 137Cs existed in the dissolved state, and the acid-soluble state and the insoluble state were <13% and the undetectable level, respectively. After the heavy rain event, the particulate fraction (acid-soluble state + insoluble state) was elevated to 65%, which corresponds to twice as much as that in the base flow condition. Particulate 137Cs, especially the acid-soluble Cs, can play a role when taken into the food chain in aquatic ecosystems.
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