Distribution of dissolved and particulate radiocesium concentrations along rivers and the relations between radiocesium concentration and deposition after the nuclear power plant accident in Fukushima

•Particulate 137Cs concentration in river water was greater than the dissolved 137Cs.•High dissolved 137Cs was observed where greater 137Cs deposition occurred.•Dissolved 137Cs was strongly correlated with the amount of 137Cs deposition.•Particulate 137Cs was not highly correlated with the amount of 137Cs deposition.•137Cs in SS [Bq/g] was determined by 137Cs deposition regardless of the river scale.

This study involved measurement of concentrations of dissolved and particulate radiocesium (134Cs and 137Cs) in river water, and determination of the quantitative relations between the amount of deposited 137Cs and 137Cs concentrations in river waters after the Fukushima Daiichi nuclear power plant accident. First, the current concentrations of dissolved and particulate 134Cs·137Cs were determined in a river watershed from 20 sampling locations in four contaminated rivers (Abukuma, Kuchibuto, Shakado, and Ota).Distribution characteristics of different 137Cs forms varied with rivers. Moreover, a higher dissolved 137Cs concentration was observed at the sampling location where the 137Cs deposition occurred much more heavily. In contrast, particulate 137Cs concentration along the river was quite irregular, because fluctuations in suspended solids concentrations occur easily from disturbance and heavy precipitation. A similar tendency with dissolved 137Cs distribution was observed for the 137Cs concentration per unit weight of suspended solids.Regression analysis between deposited 137Cs and dissolved/particulate 137Cs concentrations was performed for the four rivers. The results showed a strong correlation between deposited 137Cs and dissolved 137Cs, and a relatively weak correlation between deposited 137Cs and particulate 137Cs concentration for each river. However, if the particulate 137Cs concentration was converted to 137Cs concentration per unit weight of suspended solid, the values showed a strong correlation with deposited 137Cs.