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Following the Fukushima Daiichi Nuclear Power Plant disaster in March 2011, the Japanese government initiated an unprecedented decontamination programme to remediate 137 Cs-contaminated soils and allow population return. This programme involved the removal of topsoil under farmland and residential land, and its replacement with "fresh soil" composed of granitic saprolite. However, decontamination was limited to these two land uses, without remediating forests, which cover 70 % of the surface area in the affected region. In this unprecedented context, the specific impact of this unique decontamination programme on 137 Cs transfers in river systems remains to be quantified at the catchment scale. In this study, based on the analysis of a sediment core collected in June 2021 in the Mano Dam reservoir draining a decontaminated catchment, the effects of soil decontamination on particle-bound 137 Cs dynamics and sediment source contributions in response to a succession of extreme precipitation events were retrospectively assessed. The sequence of sediment layer deposition and its chronology were reconstructed through the analysis of several diagnostic properties (organic matter, elemental geochemistry, visible colourimetry, granulometry) and contextual information. During abandonment (2011-2016), cropland contribution decreased (31 %). Concurrently, 137 Cs activity and deposition flux decreased (19 and 29%year -1 , respectively). Following decontamination (2017), sediment transfer increased (270 %) in response to increased contributions from decontaminated cropland and "fresh soil" (625 % and 180 % respectively). Meanwhile, forest contributions remained stable. In contrast, 137 Cs activity dropped (65 %), although 137 Cs deposition flux remained constant. Forests acted as a stable source of 137 Cs. Accordingly, 137 Cs deposition flux after decontamination (2016-2021) was similar to that observed during the 5-years period of land abandonment (2011-2016), as a result of the regrowth of spontaneous vegetation over farmland, protecting soil against erosion. Future research should further investigate the impact of longer land abandonment that prevailed in some regions decontaminated lately on the 137 Cs fluxes in the rivers., Competing Interests: Declaration of competing interest Olivier Evrard reports financial support was provided by French National Research Agency. Olivier Evrard reports financial support was provided by French Alternative Energies and Atomic Energy Commission (CEA). Olivier Evrard reports financial support was provided by French National Center for Scientific Research (CNRS). Thomas Chalaux-Clergue reports financial support was provided by French Alternative Energies and Atomic Energy Commission. Thomas Chalaux-Clergue reports financial support was provided by Japan Society for the Promotion of Science (JSPS). Olivier Evrard reports financial support was provided by Environmental Radioactivity Research Network Center (ERAN). If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Collection and cooking of wild vegetables have provided seasonal enjoyments for Japanese local people as provisioning and cultural ecosystem services. However, the Fukushima Daiichi Nuclear Power Plant accident in March 2011 caused extensive radiocesium contamination of wild vegetables. Restrictions on commercial shipments of wild vegetables have been in place for the last 10 years. Some species, including buds of Aralia elata, are currently showing radiocesium concentrations both above and below the Japanese reference level for food (100 Bq/kg), implying that there are factors decreasing and increasing the 137Cs concentration. Here, we evaluated easy-to-measure environmental variables (dose rate at the soil surface, organic soil layer thickness, slope steepness, and presence/absence of decontamination practices) and the 137Cs concentrations of 40 A. elata buds at 38 locations in Fukushima Prefecture to provide helpful information on avoiding collecting highly contaminated buds. The 137Cs concentrations in A. elata buds ranged from 1 to 6,280 Bq/kg fresh weight and increased significantly with increases in the dose rate at the soil surface (0.10-6.50 μSv/h). Meanwhile, the 137Cs concentration in A. elata buds were not reduced by decontamination practices. These findings suggest that measuring the latest dose rate at the soil surface at the base of A. elata plants is a helpful way to avoid collecting buds with higher 137Cs concentrations and aid in the management of species in polluted regions., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Sakai et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

In this study, the effect of temperature, water quality, and the impact of an intense typhoon event on change in 137 Cs concentration in the water of agricultural and forested rivers near the Fukushima Daiichi nuclear power plant (Japan) was evaluated using monthly stationary observations obtained under baseflow conditions 2.8-10.6 years after the nuclear accident in 2011. The dissolved 137 Cs concentration fluctuated seasonally with water temperature in all rivers, and the increase in dissolved 137 Cs concentration for unit increase in temperature was higher in forested rivers than in agricultural rivers. The relationship between water temperature and the apparent distribution coefficient of 137 Cs well followed the van 't Hoff equation in the two agricultural rivers, where the enthalpy of reaction was estimated as -15.6 and -19.6 kJ mol -1 . The van 't Hoff equation was not well followed for a forested river, where the suspended solids mainly comprised organic matter, suggesting that the dominant process determining dissolved 137 Cs concentrations in forested rivers is not only water temperature effect on ion exchange, but rather the input of 137 Cs and K + (competing with 137 Cs for exchange sites on mineral particles) into the water phase via litter leaching. Suspended solids concentrations in agricultural rivers correlated negatively with 137 Cs concentrations in suspended solids, suggesting an increased proportion of coarse particles or the input of soils with low 137 Cs concentration from decontaminated agricultural land. At some sites, 137 Cs concentrations in dissolved form and in suspended solids were reduced sharply in association with the passage of Typhoon Hagibis in October 2019, suggesting that Typhoon Hagibis caused large-scale surface erosion that removed the source of 137 Cs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

In the context of elevated concerns related to nuclear accidents and warfare, the lessons learnt from the Fukushima Daiichi Nuclear Power Plant accident in 2011 are important. In particular, Japanese authorities implemented an ambitious decontamination program to reduce the air dose rate in order to facilitate the return of the local inhabitants to previously evacuated areas. This approach contrasts the strategy adopted in Chernobyl, where the most contaminated areas remain off limits. Nonetheless, the effectiveness of the Japanese decontamination strategy on the dispersion of radioactive contaminant fluxes across mountainous landscapes exposed to typhoons has not been quantified. Based on the unique combination of river monitoring and modeling in a catchment representative of the most impacted area in Japan, we demonstrate that decontamination of 16% of the catchment area resulted in a decrease of 17% of sediment-bound radioactive fluxes in rivers. Decontamination operations were therefore relatively effective, although they could only be conducted in a small part of the area due to the dominance of steep forested slopes. In fact, 67% of the initial radiocesium contamination was calculated to remain stored in forested landscapes, which may contribute to future downstream radiocesium dispersion during erosive events. Given that only a limited proportion of the initial population had returned in 2019 (~30%), it raises the question as to whether decontaminating a small percentage of the contaminated area was worth the effort, the price, and the amount of waste generated?

After the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Plant accident in Japan, freshwater ecosystems near the site remained contaminated by radiocesium (RCs). Clarifying RCs concentrations in aquatic insects is crucial because fishes consume these insects that transfer RCs into freshwater ecosystems. As aquatic insects are usually measured for radioactivity in bulk samples of several tens of insects, variation in RCs concentration among individuals is not captured. In this study, we investigated the variability in 137Cs activity concentration in individual aquatic insects in detritivorous caddisfly (Stenopsyche marmorata) and carnivorous dobsonfly (Protohermes grandis) larvae from the Ota River, Fukushima. Caddisfly larvae showed sporadically higher radioactivity in 4 of the 46 caddisfly larvae, whereas no such outliers were observed in 45 dobsonfly larvae. Autoradiography and scanning electron microscopy analyses confirmed that these caddisfly larvae samples contained radiocesium-bearing microparticles (CsMPs), which are insoluble Cs-bearing silicate glass particles. CsMPs were also found in potential food sources of caddisfly larvae, such as periphyton and drifting particulate organic matter, indicating that larvae may ingest CsMPs along with food particles of similar size. Although CsMP distribution and uptake by organisms in freshwater ecosystems is relatively unknown, our study demonstrates that CsMPs can be taken up by aquatic insects., Competing Interests: The authors have declared that no competing interests exist.

Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilise in bioavailable form from sediments to water columns, which may contribute to the long-term contamination of aquatic products. This study systematically investigated the 137 Cs activities of sediment-pore water, providing a direct evidence of the remobilisation of bioavailable 137 Cs from sediments in two highly contaminated reservoirs affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. We observed that the dissolved 137 Cs activity concentration of pore water (3.0-65.8 Bq L -1 ) was one to two orders of magnitude higher than that of reservoir water. Moreover, the distribution coefficient (K d ) values for the 137 Cs of sediment-pore water (2.6-14 × 10 3  L kg -1 ) decreased with depth. The K d values were significantly and negatively correlated with the concentration of the major 137 Cs competing cation NH 4 + . Our results strongly indicate a competitive ion exchange process between 137 Cs and NH 4 + via a highly selective interaction with the frayed edge sites of phyllosilicate minerals, which is the major reason for the variability of K d values of sediment-pore water, even in the Fukushima case. Additionally, the sediment accumulation rates were relatively high, and the annual depositional rate of exchangeable 137 Cs prevailed over the annual diffusive flux of 137 Cs from the sediment to the overlying water. This finding indicates that even after 10 years since the FDNPP accident, the bioavailable 137 Cs is still continuously supplied from the catchment covered by mountainous forests, and reservoir sediments are a long-term important source of bioavailable 137 Cs in the riverine system. Our findings provide important parameter values for mid- and long-term assessments of the radiation impact of radionuclide discharges to freshwater environments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Forest-stream ecosystems are widespread and biodiverse terrestrial landscapes with physical and social connections to downstream human activities. After radiocesium is introduced into these ecosystems, various material flows cause its accumulation or dispersal. We review studies conducted in the decade after the Fukushima nuclear accident to clarify the mechanisms of radiocesium transfer within ecosystems and to downstream areas through biological, hydrological, and geomorphological processes. After its introduction, radiocesium is heavily deposited in the organic soil layer, leading to persistent circulation due to biological activities in soils. Some radiocesium in soils, litter, and organisms is transported to stream ecosystems, forming contamination spots in depositional habitats. While reservoir dams function as effective traps, radiocesium leaching from sediments is a continual phenomenon causing re-contamination downstream. Integration of data regarding radiocesium dynamics and contamination sites, as proposed here, is essential for contamination management in societies depending on nuclear power to address the climate crisis., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with 137 Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios ( 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 242 Pu/ 239 Pu) and for 137 Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, 236 U/ 238 U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Movement of 137 Cs in the bottom sediment of a shallow lake was investigated by collecting columnar sediment cores from the entire area once a year, and also from three representative points every 3 months over 3 years from the 137 Cs fallout in March 2011. The 137 Cs inventory in the sediment increased remarkably near the mouths of urban rivers; however, the distribution of the 137 Cs inventory did not change remarkably in most areas, and horizontally slow downstream movement of 137 Cs was observed., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The Fukushima nuclear accident in Japan resulted in the deposition of radiocesium over forested and rural landscapes northwest of the power plant. Although there have been several investigations into the dynamics of contaminated river sediment, less attention has been paid to the sources of deposited particulate matter in dams and reservoirs. In the Fukushima Prefecture, there are 10 significant dams and over a 1000 reservoirs for both agricultural and surface water management. These reservoirs may have trapped a significant volume of radiocesium-contaminated sediment. Therefore, characterizing the sources of contaminated particulate matter is important for the ongoing management of contamination in the region. Accordingly, the composition of particulate matter deposited in the Mano Dam reservoir, approximately 40km northwest of the power plant, was investigated with the analyses and modelling of carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N), total organic carbon (TOC) and total nitrogen (TN) concentrations. Four sediment cores, with lengths ranging 29-41cm, were sampled in the Mano Dam. Source samples from 46 forest soils, 28 cultivated soils and 25 subsoils were used to determine the source contributions of particulate matter. Carbon and nitrogen parameters were analyzed on all samples and a concentration-dependent distribution modelling approach was used to apportion source contributions. Three of the four cores sampled in the Mano Dam reservoir had distinct radiocesium peaks representative of the initial post-accident wash-off phase. Cultivated sources were responsible for 48±7% of the deposited fine particulate matter whereas forests were modelled to contribute 27±6% and subsoil sources 25±4%. Ongoing decontamination of cultivated sources in the Fukushima region should result in a decrease of contaminated matter deposition in reservoirs., (Copyright © 2017 Elsevier B.V. All rights reserved.)

This study involved measurement of concentrations of dissolved and particulate radiocesium ((134)Cs and (137)Cs) in river water, and determination of the quantitative relations between the amount of deposited (137)Cs and (137)Cs concentrations in river waters after the Fukushima Daiichi nuclear power plant accident. First, the current concentrations of dissolved and particulate (134)Cs·(137)Cs were determined in a river watershed from 20 sampling locations in four contaminated rivers (Abukuma, Kuchibuto, Shakado, and Ota). Distribution characteristics of different (137)Cs forms varied with rivers. Moreover, a higher dissolved (137)Cs concentration was observed at the sampling location where the (137)Cs deposition occurred much more heavily. In contrast, particulate (137)Cs concentration along the river was quite irregular, because fluctuations in suspended solids concentrations occur easily from disturbance and heavy precipitation. A similar tendency with dissolved (137)Cs distribution was observed for the (137)Cs concentration per unit weight of suspended solids. Regression analysis between deposited (137)Cs and dissolved/particulate (137)Cs concentrations was performed for the four rivers. The results showed a strong correlation between deposited (137)Cs and dissolved (137)Cs, and a relatively weak correlation between deposited (137)Cs and particulate (137)Cs concentration for each river. However, if the particulate (137)Cs concentration was converted to (137)Cs concentration per unit weight of suspended solid, the values showed a strong correlation with deposited (137)Cs., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Measurements in forest catchments in Fukushima Prefecture indicate that cesium‐137 (137Cs) leaching from forest litter affects dissolved 137Cs levels in river water. Existing simulations have been unsuccessful in reproducing this phenomenon due to mechanistic limitations in the models connecting the forest ecosystem and the river system. This paper introduces a new coupled watershed‐geochemical model to address this. We connected a forest ecosystem compartment model to the 3D watershed model General‐purpose Terrestrial Fluid‐Flow Simulator to describe 137Cs transfer between forests and the river system. The compartment model included nonleachable and leachable 137Cs sites in the organic layer. The latter sites model how leachable 137Cs stocks increase with ambient temperature owing to decomposing organic matter. The model was tested by simulating dissolved and particulate 137Cs discharges from a forested catchment upstream of the Ohta River, Fukushima Prefecture. The results for dissolved 137Cs concentrations in river water for 1 January 2014, to 31 December 2015, correlated well with catchment measurements. The simulations reproduced dissolved 137Cs concentration peaks that occurred during three typhoon events and the seasonal variations under baseflow conditions. The results support the theory that leaching from the organic layer in forests is a primary factor affecting river water dissolved 137Cs concentrations. The wider contribution of this study is it introduces a coupled watershed‐geochemical model, applicable not only for evaluating the dynamics of 137Cs, but also those of other substances that adsorb onto sediments and soil particles, and leach from forest organic matter. Plain Language Summary: Cesium‐137 (137Cs) derived from the Fukushima Daiichi Nuclear Power Plant is a critical radioisotope remaining from the fallout owing to its high activity, long half‐life (30.1 y), and potential impacts on humans and ecosystems. Approximately 70% of all land in Fukushima Prefecture is covered with forests, where 137Cs is predominantly present. The behavior of dissolved 137Cs in the environment must be investigated because this form of 137Cs is readily absorbed by living beings. Researchers have reported that dissolved 137Cs concentrations in river water under baseflow conditions vary seasonally and rise during storms, indicating that leaching from organic matter in forests is a dissolved 137Cs source. We developed a new simulation model for the dynamics of 137Cs between the forest ecosystem and the river system. The simulations reproduced the dissolved 137Cs concentration peaks that occurred during previous typhoons and the seasonal variations that occurred under baseflow conditions. Therefore, the model's results support the theory that leaching from the organic layer in forests is a primary factor affecting dissolved 137Cs concentrations in river water. The model introduced here can not only be used for simulating 137Cs, but also has wider applicability for simulating the transport mechanisms of other contaminants through forest watersheds. Key Points: A coupled watershed‐geochemical model is introduced for simulating dissolved contaminant transport from forests into river waterThe model reproduced dissolved 137Cs concentration trends in Ohta River, Fukushima Prefecture, in base‐ and stormflow periodsThe results supported the theory that the organic layer is an important contamination source for river water [ABSTRACT FROM AUTHOR]