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.
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