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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.)

Following the Fukushima Daiichi Nuclear Power Plant accident in 2011, most of the released 137Cs remained in the litter and surface soil of the adjacent forest floor. However, 137Cs absorption by large soil invertebrates near this site has not been estimated. The aim of this study was to understand the role of soil macroinvertebrates in 137Cs uptake from forest litter into forest ecosystems. Breeding experiments were conducted using scarab beetle larvae (Protaetia orientalis). Dissection experiments revealed that 85% of the total 137Cs was concentrated in the digestive tract of larvae, while a low proportion was absorbed into the skin and muscle tissues. The 137Cs absorption rate, indicating the transfer of 137Cs from consumed litter to larval tissue, was low (0.39%). 137Cs concentrations decreased to one-fourth from larva to imago, possibly due to excretion from the digestive tract and during eclosion. In the elimination experiment, biological half-lives were 0.26-0.64 and 0.11-0.47 days and 3.35-48.30 and 4.01-17.70 days for the digestive tract and muscle/skin tissues in the fast and slow components, respectively, corresponding to 137Cs discharge from the gastrointestinal tract and physiological clearance. In the sequential extraction experiment, litter digestion by flower chafer larvae significantly reduced the bioavailable fraction of 137Cs including water-soluble, exchangeable, oxidized, and organic forms, from 23.2% in litter to 17.7% in feces. Residual 137Cs was not reduced by digestion, probably because it was fixed in soil clay. Our study on breeding experiments of the Scarabaeidae beetle confirmed the low bioavailability of 137Cs in the litter in Fukushima. However, litter feeders may play an important role in transferring 137Cs to higher trophic levels in the forest ecosystem by extracting the bioavailable fraction of the vast stock of 137Cs on the forest floor., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Jo 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.)

Understanding the seasonality of 137 Cs concentrations in aquatic animals is crucial for reviving local inland fisheries. The seasonality of 137 Cs concentrations in animals is expected to vary, even if focal species consume similarly contaminated foods because the 137 Cs excretion rate is species-specific, and 137 Cs uptake by foraging autochthonous food resources also vary among seasons. Here, we conducted a seasonal monitoring survey of dissolved 137 Cs concentrations as an indicator of the contamination level of food resources and measured 137 Cs concentrations in two carnivorous aquatic animals (Palaemon paucidens and Rhinogobius sp.) that share a trophic niche in a stream connected to a dam reservoir. The dissolved 137 Cs concentration had clear seasonality-high in summer and low in winter. The 137 Cs concentrations in the animals revealed a different seasonal pattern-it peaked in October in P. paucidens and peaked in February in Rhinogobius. Overall, the 137 Cs concentration was relatively higher in P. paucidens than in Rhinogobius, suggesting that P. paucidens has a lower excretion rate than Rhinogobius. Consequently, the seasonality of the 137 Cs concentration in P. paucidens showed temporal changes similar to those of the dissolved 137 Cs concentration, which were likely affected by 137 Cs uptake through foraging, whereas that in Rhinogobius was controlled by 137 Cs excretion. This study shows that the seasonality of 137 Cs concentration can differ between sympatric animals that share a trophic niche. Accumulating knowledge and comparing the seasonality of 137 Cs concentrations in fisheries species based on the balance between uptake and excretion will be valuable to determine the appropriate seasons to obtain less-contaminated products., 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 © 2022 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.)

This study investigated the transport of 137 Cs within a forest ecosystem by examining temporal changes in the inventory and determining the major pathways of transfer following significant atmospheric deposition. A forested area of eastern Japan was monitored for four years immediately after the Fukushima nuclear power plant accident in March 2011 that released a large amount of radionuclides. The long physical half-life of 137 Cs means that contamination can persist for decades, so it is vital to understand the mechanisms underlying the 137 Cs dynamics in ecosystems. We sampled litterfall, throughfall, and soil, mainly from a cedar stand, over a four-year period, and analyzed the 137 Cs concentrations of each sample to determine the transfer rate and total inventory. After validating our methodology through a comparison with results from an earlier study, we determined the temporal changes in the 137 Cs distribution and in the major transfer pathway. Results showed that most 137 Cs intercepted by canopies was transferred rapidly over the first nine months, and that the major pathway was not litterfall but throughfall. The ecological half-life of the 137 Cs stocked in the canopy was calculated for both the early and later stages of contamination. Although the former is consistent with previous results, the latter ecological half-life is somewhat longer, probably because of dependence on the meteorological and tree physiological conditions at the site. This study presents valuable new data on the post-Fukushima 137 Cs contamination, enhancing our understanding of the associated dynamics in forest ecosystems., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Changes in 137Cs export over time following clearcutting were investigated in a Japanese forested catchment affected by the Fukushima nuclear accident. A total of 13% of the catchment area was clear-cut 2 years after the accident. Annual suspended solids (SS) export at the catchment outlet increased 1.4 to 2.0 times after clearcutting; however, 137Cs export increased slightly (up to 1.1 times), corresponding to 0.21% to 0.30% of the 137Cs inventory in the catchment. The smaller change in 137Cs export than in SS export was due to a rapid decrease in the activity concentration following clearcutting. This decrease was likely caused by both natural attenuation and SS derived from sources with a low activity concentration in the clear-cut area. Monitoring of the sediment transport from hillslopes in small-scale experimental plots showed that the 137Cs yield in the skid trail was 3.6 to 21 times greater than those in clear-cut and unlogged forest floors. This significant 137Cs transport was due to greater soil erosion (by up to two orders of magnitude) along the skid trail, despite the lower activity concentration than those in the other plots. This indicates that while skid trails were involved in the rapid decrease of the activity concentration of SS, they were a potential source of the increased export of 137Cs and SS. Net 137Cs export increased by clearcutting (the export excluding the decrease accompanied by natural attenuation) was estimated to account for only 0.092% of the inventory in the catchment for 2.5 years. These results imply that the impact of clearcutting on 137Cs export was temporary in this catchment., Competing Interests: The authors have declared that no competing interests exist.

To evaluate the mobility and bioavailability of 137 Cs in soils, we compared the extraction of 137 Cs with stable Cs and ammonium solutions from 137 Cs-contaminated minerals and soils. The extraction yields of 137 Cs with stable Cs were significantly lower than those with ammonium for minerals with frayed edge sites, but such differences were not observed for minerals without frayed edge sites. The amount of 137 Cs extracted with stable Cs from soils was lower than, or equal to, that extracted with ammonium. The above results suggest that stable Cs extracted the 137 Cs from easily accessible sites. Plant available 137 Cs was assessed using Kochia (Bassia scoparia) cultivated in pots of contaminated soils, and compared with soil parameters including extractable 137 Cs and K, and radiocesium intercept potential. The 137 Cs/K ratio extracted with stable Cs solution was found to be a potential index for evaluation of the easily mobile and bioavailable fraction of 137 Cs in soil.

Predicting the environmental fate of 137 Cs in forest ecosystems along with the concentrations of 137 Cs in tree parts are important for the managements of radioactively contaminated forests. In this study, we calibrate the Forest RothC and Cs model (FoRothCs), a forest ecosystem 137 Cs dynamics model, using observational data obtained over six years from four forest sites with different levels of 137 Cs contamination from Fukushima Prefecture. To this end, we applied an approximate Bayesian computation (ABC) technique based on the observed 137 Cs concentrations (Bq kg -1 ) of five compartments (leaf, branch, stem, litter, and soil) in a Japanese cedar plantation. The environmental decay (increment) constants of the five compartments were used as the summary statistics (i.e., the metric for model performance) to infer the five parameters related to 137 Cs transfer processes in FoRothCs. The ABC technique successfully reconciled the model outputs with the observed trends in 137 Cs concentrations at all sites during the study period. Furthermore, the estimated parameters are in agreement with the literature values (e.g., the root uptake rates of 137 Cs). Our study demonstrates that model calibration with ABC based on the trends in 137 Cs concentrations of multi compartments is useful for reducing the prediction uncertainty of 137 Cs dynamics in forest ecosystems., (Copyright © 2018 The Authors. Published by Elsevier Ltd.. All rights reserved.)

A study is presented on the applicability of the distribution coefficient (K d ) absorption/desorption model to simulate dissolved 137 Cs concentrations in Fukushima river water. The upstream Ota River basin was simulated using GEneral-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) for the period 1 January 2014 to 31 December 2015. Good agreement was obtained between the simulations and observations on water and suspended sediment fluxes, and on particulate bound 137 Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved 137 Cs in the river water were much harder to reproduce with the simulations. By tuning the K d values for large particles, it was possible to reproduce the mean dissolved 137 Cs concentrations during base flow periods (observation: 0.32 Bq/L, simulation: 0.36 Bq/L). However neither the seasonal variability in the base flow dissolved 137 Cs concentrations (0.14-0.53 Bq/L), nor the peaks in concentration that occurred during storms (0.18-0.88 Bq/L, mean: 0.55 Bq/L), could be reproduced with realistic simulation parameters. These discrepancies may be explained by microbial action and leaching from organic matter in forest litter providing an additional input of dissolved 137 Cs to rivers, particularly over summer, and limitations of the K d absorption/desorption model. It is recommended that future studies investigate these issues in order to improve simulations of dissolved 137 Cs concentrations in Fukushima rivers., (Copyright © 2018 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.)

To understand radiocesium transfer in the forest insect food web, we investigated the activity concentrations of radiocesium in forest insects in the Fukushima and Ibaraki Prefectures approximately 1.5-2.5 years after the Fukushima Dai-ichi Nuclear Power Plant. We analyzed 34 species of insects sampled from 4 orders and 4 feeding functional groups (herbivore, carnivore, omnivore, and detritivore) from three sites in each prefecture. 137Cs activity concentrations were lowest in herbivorous species and were especially high in detritivorous and omnivorous species that feed on forest litter and fungi. Radiocesium activity concentrations in any given species reflected the degree of contamination of that species' primary food sources since radiocesium activity concentrations were found to be the lowest in leaves and grass and the highest in litter, bark, and fungi. This study confirmed that litter and other highly contaminated forest components such as fungi, decaying wood, bryophytes, and lichens serve as sources of 137Cs transfer into the forest insect community., Competing Interests: The authors have declared that no competing interests exist.

Cryptomeria japonica trees in the area surrounding Fukushima, Japan, intercepted (137)Cs present in atmospheric deposits soon after the Fukushima nuclear accident in March 2011. To study the uptake and translocation of (137)Cs in C. japonica leaves, we analyzed activity concentrations of (137)Cs and the concentration ratios of (137)Cs to (133)Cs ((137)Cs/(133)Cs) in old and new leaves of C. japonica collected from a forest on Mount Tsukuba between 9 and 15 months after the accident. Both isotopes were also analyzed in throughfall, bulk precipitation and soil extracts. Water of atmospheric and soil origin were used as proxies for deciphering the absorption from leaf surfaces and root systems, respectively. Results indicate that 20-40% of foliar (137)Cs existed inside the leaf, while 60-80% adhered to the leaf surface. The (137)Cs/(133)Cs ratios inside leaves that had sprouted before the accident were considerably higher than that of the soil extract and lower than that of throughfall and bulk precipitation. Additionally, more than 80% of (137)Cs in throughfall and bulk precipitation was present in the dissolved form, which is available for foliar uptake, indicating that a portion of the (137)Cs inside old leaves was presumably absorbed from the leaf surface. New leaves that sprouted after the accident had similar (137)Cs/(133)Cs ratios to that of the old leaves, suggesting that internal (137)Cs was translocated from old to new leaves. For 17 species of woody plants other than C. japonica, new leaves that sprouted after the accident also contained (137)Cs, and their (137)Cs/(133)Cs ratios were equal to or higher than that of the soil extract. These results suggested that foliar uptake and further translocation of (137)Cs is an important vector of contamination in various tree species during or just after radioactive fallout., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Chemical transport models played key roles in understanding the atmospheric behaviors and deposition patterns of radioactive materials emitted from the Fukushima Daiichi nuclear power plant after the nuclear accident that accompanied the great Tohoku earthquake and tsunami on 11 March 2011. However, model results could not be sufficiently evaluated because of limited observational data. We assess the model performance to simulate the deposition patterns of radiocesium ((137)Cs) by making use of airborne monitoring survey data for the first time. We conducted ten sensitivity simulations to evaluate the atmospheric model uncertainties associated with key model settings including emission data and wet deposition modules. We found that simulation using emissions estimated with a regional-scale (∼ 500 km) model better reproduced the observed (137)Cs deposition pattern in eastern Japan than simulation using emissions estimated with local-scale (∼ 50 km) or global-scale models. In addition, simulation using a process-based wet deposition module reproduced the observations well, whereas simulation using scavenging coefficients showed large uncertainties associated with empirical parameters. The best-available simulation reproduced the observed (137)Cs deposition rates in high-deposition areas (≥ 10 kBq m(-2)) within 1 order of magnitude and showed that deposition of radiocesium over land occurred predominantly during 15-16, 20-23, and 30-31 March 2011.