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

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