Fig.1　¹³⁷Cs concentrations in riverwater down the Kido dam in 2014

• When water level is low, the concentration of ¹³⁷Cs (particulate state and dissolved state) in river water was lower than 0.1Bq/L.
• When water level is high, the concentration of ¹³⁷Cs in dissolved states increases to about 1 Bq/L, that is lower than maximum permissible value of drinking water (10 Bq/L).
• The concentration of radioactive cesium in the dissolved state is lower than 0.1 Bq/L even during high flow periods. The concentration of radioactive cesium tends to decrease with decreasing turbidity. Therefore most of radioactive cesium in river water is in the particulate state, and it is presumed that the concentration of radioactive cesium will decrease with time (see also; Q Is there a continuous input of cesium contamination to rivers from forests)

According to the Ministry of Environment, radioactive materials were lower than 0.1 Bq/L in any other rivers in Fukushima Prefecture.
Reference:　”Results for Monitoring of Radioactive Materials in Public Water in Fukushima Prefecture” Homepage of the Ministry of Environment

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Reference

1. 環境省, “福島県内の公共用水域における放射性物質モニタリングの測定結果（速報）（1月-2月分）”, 2017-03-30.