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This article discusses tritium concentrations in monthly precipitation in part of the Difficult-to-Return Zone in Namie Town during 2012–2021. The tritium concentrations, which were measured with a low background liquid scintillation counter after carrying out an enrichment procedure, fluctuated seasonally from 0.10 ± 0.02 to 0.85 ± 0.02 Bq L−1. This range of concentrations is concluded to not be unusual based on comparisons with the concentrations at other sites and estimates of the past range of the concentrations. Moreover, no significant variations in observed tritium concentrations were observed due to decommissioning work at the Fukushima Dai-ichi Nuclear Power Plant. These results contribute to understanding the background level of tritium concentration in precipitation before the oceanic discharge of treated water from the Fukushima plant. In addition, this article evaluates the amount of tritium supplied to the ocean by terrestrial rainwater pouring into the Pacific Ocean via Ukedo River, which flows through Namie Town; this information will contribute to the discussion on the impact of the oceanic discharge of treated water. [ABSTRACT FROM AUTHOR]

To understand the behavior of atmospheric 129I that originated from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, activity concentrations of 129I in samples of atmospheric particulate matter (PM), comprising coarse (>1.1 μm) and fine (<1.1 μm) fractions (separated using a single stage impactor), were measured on a nearly monthly basis from October 2012 to October 2014 at a site in an area evacuated after the FDNPP accident. Total atmospheric 129I activity concentrations in PM at the site ranged between 0.15 and 2.17 nBq m−3. Specific activity concentration of 129I in total atmospheric PM ranged between 40.8 and 336 mBq kg−1, with a mean and standard deviation of 142 and 77.6 mBq kg−1, respectively. The specific activity in PM tended to be higher than not only the background values reported from soil collected before the FDNPP accident but also than those reported from the contaminated soil after the accident (range: 1.6–57 mBq kg−1; mean and standard deviation: 10.6 and 12.3 mBq kg−1, respectively). Total 129I/127I atomic ratios in PM ranged from 2.0 × 10−8 to 59.8 × 10−8, with a mean and standard deviation of 15.0 × 10−8 and 14.4 × 10−8, respectively. These ratios were generally lower than those of the contaminated soil collected after the FDNPP accident (range: 4.9 × 10−8–443 × 10−8; mean and standard deviation: 74.2 × 10−8 and 85.4 × 10−8, respectively). The 129I concentration and 129I/127I atomic ratio in atmospheric PM showed different characteristics from that of contaminated soils, suggesting that the presence of other atmospheric PMs plays a more important role as the host for 129I. [ABSTRACT FROM AUTHOR]

After the Fukushima Dai-ichi nuclear power plant accident in March 2011, the fluvial discharge of 137Cs from watersheds to rivers was analyzed between 2011 and 2021. The concentrations of dissolved and particulate 137Cs were measured in river water samples collected from two rivers (the Hiso and Wariki rivers, mainly draining farmlands and forests, respectively) draining approximately 4–7 km2 watersheds in a montane area (the areal deposition of 134+137Cs; 1–3 MBq·m−2 in March 2011) of Iitate Village, Fukushima. Over the 10 yr analysis, the particulate 137Cs concentrations in the Hiso and Wariki rivers decreased by 70 and 50 times, respectively, and that of the dissolved form decreased by 150 and 130 times, respectively. Moreover, the apparent K d (distribution coefficient) of 137Cs for water samples from both rivers have increased gradually over these periods. In 2011, the 137Cs discharge rates through the Hiso and Wariki rivers were 0.63% and 0.46% per year of the total amount of 137Cs deposited in their catchments, respectively; however, by 2021, these rates had decreased to 0.09% and 0.03% per year. The cumulative 137Cs discharge rates over 10 yr in the farmland- and forest-dominated rivers were 1.95% and 1.33%, respectively. These rates of the farmland-dominated river were ∼1.4-fold greater than those of the forest-dominated river. Moreover, ∼90% of the of the discharge occurred in particulate form while the remaining ∼10% was in the dissolved form. Thus, 137Cs deposited within these watersheds due to the accident was gradually discharged by the rivers over the 10-yr period; however, the majority remains stored in soils and litters etc. of the catchment area. These results indicate that 137Cs outflow from land-to-ocean will be limited in the future, as the river export of 137Cs is expected to continue decreasing. [Display omitted] • 137Cs levels during 2011–2021 were monitored in contaminated watershed rivers (4–7 km2) of a mountainous area in Fukushima. • In 2011, 137Cs discharges in farmland and forest dominant rivers amounted to 0.63 and 0.46% of total 137Cs inventories. • <2% of the total 137Cs deposited has flowed out the rivers in past decade. [ABSTRACT FROM AUTHOR]