• Investigations of rivers in Fukushima Prefecture have revealed that most cesium exists in the dissolved state in clear water with low flow rates. Since 2012, the concentration of radioactive cesium in these regions has been lower than the detection limit of radiation detectors (about 1 Bq/L).^*1
• When the flow rate of a river increases, the concentration of floating suspended materials increases. Radioactive cesium is strongly adsorbed on such suspended matter (particulate state). For that reason, when the water level is high, the concentration of cesium in the particulate state increases, while that in dissolved states does not change so much.
• Investigations of the Ukedo River, Fukushima Prefecture have revealed that the concentration of radioactive cesium is lower than the maximum value for drinking water (10 Bq/L) even when the water level is high.

• 

  Fig.1　Relationship between flow rate (water level) and concentration of floating suspended materials in the downstream Ukedo River (near Ukedo Bridge) （2014-2015）

• 

  Fig.2　Relationship between flow rate (water level) and concentration of ¹³⁷Cs in the downstream Ukedo River (near Ukedo Bridge) （2014-2015）

Reference：*1　Report on the Project for the Nuclear Regulatory Agency, “Data Accumulation for Distribution of Radioactive Materials Released from Fukushima Daiichi Nuclear Power Station, TEPCO and Development of Migration Model” (FY 2014).

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Reference

1. 原子力規制委員会(2015): 平成26年度東京電力株式会社福島第一原子力発電所事故に伴う放射性物質の分布データの集約及び移行モデルの開発事業成果報告書. https://radioactivity.nra.go.jp/ja/docs/reps/rad-dist/aggregate-results-report/2014 (2024.11.18アクセス)