Nuclear accident-derived ¹²⁹I in several river water, eastern Fukushima, Japan, 2016-2020

Radionuclides, including ¹²⁹I, were released into the atmosphere by the Fukushima nuclear power plant accident. We measured the dissolved ¹²⁹I concentration in 11 rivers in eastern Fukushima from 2016 to 2020 to clarify the ¹²⁹I concentration level in river water under base-flow conditions. During the study period, the maximum ¹²⁹I concentration in the river water was 1.6 × 10^-6 Bq L^-1 in the Ukedo River in 2016, and the minimum was 1.2 × 10^-7 Bq L^-1 in the Uda River in 2017, with a difference of over an order of magnitude between the two. Water samples from the Ukedo, Niida-U, and Nagadoro Rivers in eastern Fukushima were strongly affected ¹²⁹I from the Fukushima accident. In addition, we found a significant correlation between concentration of ¹²⁹I in river water and mean ¹²⁹I contamination density in the catchment. The time-series variation of ¹²⁹I in river water from 2016 to 2020 was small and showed no year-to-year decrease. The ¹²⁹I concentration in each river hardly decreased compared to the ¹³⁷Cs decay-corrected concentration during the observation period, and it was expected that a certain concentration of dissolved ¹²⁹I would continue to flow out over a long period of time.