Mid-long term change of particulate/dissolved 137Cs concentration in river water and the impact of Typhoon Hagibis in 2019

In Fukushima Prefecture, radiocesium as the particulate and dissolved form has been discharging from the mountains and forests since the Fukushima Dai-ichi nuclear power plant (FDNPP) accident in 2011. In particular, in October 2019, the watershed around the FDNPP was subjected to extensive flooding due to Typhoon Hagibis, resulting in significant changes in the hydrological environment. In this study, we investigated the characteristics of changes in particulate/dissolved ¹³⁷Cs concentrations in the main 3 rivers in the north region of FDNPP 3–9 years after the nuclear accident and the impact of the typhoon on ¹³⁷Cs dynamics in river water.

Monthly observations of river waters in baseflow conditions showed a decrease in dissolved ¹³⁷Cs concentration with an environmental half-life of 2–10 years, and seasonal fluctuation such as increasing in summer and decreasing in winter. The annual amplitude of the dissolved ¹³⁷Cs concentration in water released from dams was smaller and the peak of the concentration was observed later than that in river sites where the influence of dams is small. The ¹³⁷Cs concentrations in the suspended solids did not show any significant seasonal variation, and the environmental half-life of 1–8 years was relatively faster than the dissolved forms observed at the same site. Immediately after Typhoon Hagibis in 2019, the dissolved ¹³⁷Cs concentration decreased significantly compared to the previous years, especially at two dam lake discharge sites. At the two sites, the dissolved ¹³⁷Cs concentration did not recover to the level predicted by the pre-typhoon data even one year after the typhoon event, but no significant decrease in ¹³⁷Cs concentration in suspended solids was observed. These differences in the environmental behavior of ¹³⁷Cs in different forms suggest that there are limitations in predicting particulate and dissolved ¹³⁷Cs concentrations with a fixed parameter such as partition coefficient.