New predictions of 137Cs dynamics in forests after the Fukushima nuclear accident

Long-term changes in 90Sr pools of Scots pine biomass in the Chornobyl Red Forest

The trenches of the waste burial site in the Chornobyl Red Forest represent a big reservoir of radionuclides for the artificial plantation of Scots pine established in that area, but the long term dynamics of tree biomass contamination, especially with 90Sr, remains unclear. The present study was conducted between 2005 and 2018 on two groups of trees of the same age. The IN group is represented by trees growing on the trench containing highly radioactive contaminated fertile soil and organic matter, while the OUT group is located outside the trench. Within a little more than one decade, the total aboveground biomass doubled in the trees of the group OUT and increased more than four times in the group IN. In the group OUT, the concentrations of 90Sr have decreased in all biomass compartments compared to 2005, while in the group IN, the concentrations demonstrated a trend to increase. Regression analysis shows that both decrease in the compartment concentrations in the group OUT (slope coefficient 0.55) and increase in the group IN (1.58) were significant. As a result of the changes in the biomass inventories and 90Sr concentrations, in absence of changes in plantation density, the contamination of total aboveground biomass by 90Sr in the group OUT would have increased slightly in 2018 (from approximately 18 GBq ha-1 to 23 GBq ha-1) compared to 2005, while in the group IN it would have increased almost 6-fold, reaching approximately 560 GBq ha-1, or about (19 ± 9) % of the total 90Sr inventory in the trench area. Trenches of the Red Forest were shown to act as long-lasting hot spots of 90Sr bioavailability for forest trees.

Estimation of spatio-temporal distribution of 137Cs concentrations in litter layer of forest ecosystems in Fukushima using FoRothCs model

The nuclear power plant accident in Fukushima had led to pollution of forest ecosystems with ¹³⁷Cs in 2011. In this study, we simulated the spatiotemporal distribution of ¹³⁷Cs concentrations of litter layer in the contaminated forest ecosystems in two decades from 2011, which is one of the key environmental components of ¹³⁷Cs migration in the environment due to the high bioavailability of ¹³⁷Cs in the litter. Our simulations showed that ¹³⁷Cs deposition is the most important factor in the degree of contamination of the litter layer but vegetation type (evergreen coniferous/deciduous broadleaf) and mean annual temperature are also important for changes over time. Deciduous broadleaf trees had higher initial concentrations in the litter layer due to the direct initial deposition on the forest floor. However, the concentrations remained higher than those in evergreen conifers after 10 years due to redistribution of ¹³⁷Cs by vegetation. Moreover, areas with lower average annual temperatures and lower litter decomposition activity retained higher ¹³⁷Cs concentrations in the litter layer. The results of the spatiotemporal distribution estimation of the radioecological model suggest that, in addition to ¹³⁷Cs deposition, elevation and vegetation distribution should also be considered in the long-term management of contaminated watersheds, which can be informative in identifying hotspots of ¹³⁷Cs contamination on a long-term scale.

Thirty-year simulation of environmental fate of 137Cs in the Abukuma River basin considering the characteristics of 137Cs behavior in land uses

The Fukushima Daiichi Nuclear Power Plant accident caused a radioactive contamination of deposited radionuclides, including ¹³⁷Cs, on the land surface. Cesium-137 deposited on the land surface was strongly adsorbed on soil particles and was then washed off through soil erosion. Trends of temporal variation of ¹³⁷Cs wash-off varied greatly depending on land use. Therefore, it is important to reflect the characteristics of ¹³⁷Cs migration processes in each land use to clarify the long-term fate of ¹³⁷Cs. In this study, a 30-year simulation of environmental fate of ¹³⁷Cs was conducted using a distributed radiocesium prediction model, taking into account the characteristics of the ¹³⁷Cs behavior in each land use. Overall, in the Abukuma River basin, the ¹³⁷Cs transported into the ocean for 30 years was estimated to correspond to 4.6 % of the initial deposition in the basin, and the effective half-life of ¹³⁷Cs deposited in the basin was estimated to be 3.7 years shorter (by 11.6 %) than its physical half-life. These results suggested that ¹³⁷Cs deposited from the accident could still remain for decades. Based on the analysis of the ¹³⁷Cs behavior in land use, in 2011, the contribution of ¹³⁷Cs export to the ocean from urban lands was estimated to correspond to 70 % of the total ¹³⁷Cs export. Meanwhile, from 2012 to 2040, the contribution of ¹³⁷Cs export from agricultural lands was estimated to correspond to 75 % of the total ¹³⁷Cs export. The reduction ratios excluding radioactive decay of ¹³⁷Cs remained in areas with and without human activities for 30 years after the accident, defined as the ratios of the total outflow to the initial deposition, were estimated to be 11.5 %-17.7 % and 0.4 %-1.4 %, respectively. These results suggested that human activities enhance the reduction of ¹³⁷Cs remaining in land in the past and future.

Radiocesium mobility in different parts of the two major tree species in Fukushima

Radiocesium (¹³⁷Cs) released in the Fukushima Dai-ichi Nuclear Power Plant accident is still cycling in the forest ecosystem. We examined the mobility of ¹³⁷Cs in the external parts-leaves/needles, branches, and bark-of the two major tree species in Fukushima, Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata). This variable mobility will likely lead to spatial heterogeneity of ¹³⁷Cs and difficulty in predicting its dynamics for decades. We conducted leaching experiments on these samples by using ultrapure water and ammonium acetate. In Japanese cedar, the ¹³⁷Cs percentage leached from current-year needles was 26-45% (ultrapure water) and 27-60% (ammonium acetate)-similar to those from old needles and branches. In konara oak, the ¹³⁷Cs percentage leached from leaves was 47-72% (ultrapure water) and 70-100% (ammonium acetate)-comparable to those from current-year and old branches. Relatively poor ¹³⁷Cs mobility was observed in the outer bark of Japanese cedar and in organic layer samples from both species. Comparison of the results from corresponding parts revealed greater ¹³⁷Cs mobility in konara oak than in Japanese cedar. We suggest that more active cycling of ¹³⁷Cs occurs in konara oak.

How dynamic transfer models can complement an equilibrium-based approach: Case studies of radiocesium transfer to forest trees following accidental atmospheric release

Accidental release of radionuclides caused by nuclear accidents like those in Fukushima and Chernobyl can result in pulses of radioactivity entering the forest environment. Due to intense recycling in the forest, equilibrium between radioactivity concentrations in trees and in soil may not be reached during the period of severe short-term radionuclide transport following the accident. Another question arises as to whether the equilibrium hypothesis using empirical concentration ratios (CRs) can be applied to the long-term period. Using two atmospheric ¹³⁷Cs fallout scenarios in the Fukushima and Chernobyl sites, this study investigated whether the CR approach could provide conservative predictions of ¹³⁷Cs levels in trees following ¹³⁷Cs fallout events by comparing predictions from the CR approach using data gathered for trees by the IAEA to those from dynamic transfer models and actual measured data. The inter-comparisons also aimed to investigate whether the CR approach could account for the variability of ¹³⁷Cs levels across different tree organs. The results showed that caution may be necessary when using the CR approach, which relies on the IAEA dataset, to estimate ¹³⁷Cs accumulation in forest trees in the short - and long term following atmospheric ¹³⁷Cs fallout events. A calculation by TRIPS 2.0 demonstrated the importance of considering the distribution within tree organs for in-depth analysis of radiological impact of forest trees. Our findings suggest that it may be preferable to use CR values based on site-specific data rather than generic data collected from various sites. This is particularly relevant when studying the sites where the bioavailability of ¹³⁷Cs for trees and thus possible exposures are higher. This study also showed that dynamic modeling approaches could offer an alternative means of estimating CR values of the entire tree or specific tree organs in situations where empirically derived values are not available.

Effects of low-dose radiation on human blood components after in vitro exposure to gamma radiation from 137Cs radioactivity

This current study was designed to determine the effects of in vitro exposure to radioactive cesium-137 on human blood components. Whole blood samples were given a radiation dose of 0.02, 0.05, 0.1, 0.2, and 0.3 mGy of gamma radiation using a ¹³⁷Cs radioactive standard source. The whole blood samples that were exposed to 0 mGy served as sham-controls. The spectrofluoroscopic technique was used to determine the autofluorescence spectrum of protein in plasma or red blood cells by using excitation wavelength and range of emission wavelengths at 280 nm and 300-550 nm, respectively. The hemolysis of red blood cells was evaluated by determination of the release of hemoglobin from the red blood cells to the supernatant. Complete blood counts were also determined in whole blood. The results showed that there was no change in the ratio of fluorescence emission intensity at 340 nm of wavelength of protein extract from irradiated whole blood or red blood cells compared to the corresponding non-irradiated control. The hemolysis value did not change in irradiated whole blood when compared to the corresponding non-irradiated group. In addition, complete blood count values in irradiated groups did not differ from non-irradiated group. These current results suggested that there were no harmful effects of the low-dose gamma radiation from radioactive ¹³⁷Cs on blood components when human whole blood was exposed to gamma radiation in an in vitro condition.

Effect of composite amendments on physicochemical properties of copper tailings repaired by herbaceous plants

Phytoremediation is considered to be the most environmentally friendly green restoration technology for dealing with mine waste. Adding amendments can improve the substrate environment for plant growth and enhance remediation efficiency. Herbaceous plants have become the preferred species for vegetation restoration in abandoned mines because of their fast greening and simple management. After 8 weeks of pot experiments in the early stage, it was shown that the plant height and fresh weight of the plants treated with 5% conditioner and 0.5% straw (C₂S₂) were significantly higher than those of other treatments. Considering that, in this paper, to explore the effect of composite amendments on physicochemical properties of copper tailings repaired by herbaceous plants, the untreated copper tailings were employed as the control group, whereas copper tailings repaired by ryegrass (Lolium perenne L.), vetiver grass (Chrysopogon zizanioides L.), and tall fescue (Festuca arundinacea) with or without conditioners and straw combination into the compound amendments were taken separately as the test group. After 6 months of planting, the pH, electrical conductivity, water content, available potassium, organic matter, total nitrogen, and available phosphorus in the main physical and chemical properties of copper tailings in each experimental area were analyzed. The results showed that the electrical conductivity, organic matter, and total nitrogen content of copper tailings were improved to a certain extent by planting plants without treatment. Meanwhile, compared with the control group, all indexes of planting plants showed an upward trend after adding composite amendments. Among them, pH, water content, and available potassium content of copper tailings were enhanced more obviously. Furthermore, as discovered from the gray correlation analysis results, vetiver grass planted with composite amendments has the best comprehensive effect of improving the physicochemical properties of copper tailings, followed by tall fescue and ryegrass.

Late phase radiocesium dynamics in Fukushima forests post deposition

The long term dynamics of radiocesium in typical forest ecosystems was studied in the radioactive contaminated areas in Fukushima Prefecture. Six observations sites located in Yamakiya Village (Kawamata Town; since 2014), Tsushima Village (Namie Town, since 2015), and Tomioka Town (since 2017) were monitored. The forests consisted of artificial plantations of Japanese cedar (Cryptomeria japonica) at Yamakiya Village, Tsushima Village, and Tomioka Town. Tsushima Village also had a natural mixed forest dominated by Japanese red pine (Pinus densiflora), and Tomioka Town had a young and a mature artificial plantation of Japanese cypress (Chamaecyparis obtuse). Concentrations of ¹³⁷Cs were monitored in the samples collected from the main aboveground biomass compartments, fresh litterfall, forest litter, and soil. Concentrations of exchangeable forms of ¹³⁷Cs and stable K were measured in soil samples. During the observation period, the litter radiocesium inventories at all sites decreased significantly to approximately 1% or less of the total ground deposition. Approximately 80% of the total radiocesium inventory is localized in the upper 5-cm layer of soil and there is little downward migration of radiocesium. At the sites with the longest monitoring series (Yamakiya and Tsushima), the radiocesium expectation depths and expectation mass depths were relatively constant at 2-3 cm and 5-6 kg m^-2, respectively. Aboveground biomass compartments showed similar decreasing trends in radiocesium aggregated transfer factors, T_ag, in the compartments that were exposed to atmospheric fallout in March 2011 (old foliage, small branches, and outer bark). The mean T_ag in cedar stand compartments currently are in the range of 10^-3-10^-2 m² kg^-1 dw. However, the mean T_ag and their dynamic trend significantly differed in the wood compartments of the cedar stands, which may indicate root uptake differences of orders of magnitude between observation sites. The difference in radiocesium concentration in wood between the sites becomes less pronounced when normalized by the ratio of exchangeable ¹³⁷Cs/K in the soils.

Strategic roadmap to assess forest vulnerability under air pollution and climate change

Although it is an integral part of global change, most of the research addressing the effects of climate change on forests have overlooked the role of environmental pollution. Similarly, most studies investigating the effects of air pollutants on forests have generally neglected the impacts of climate change. We review the current knowledge on combined air pollution and climate change effects on global forest ecosystems and identify several key research priorities as a roadmap for the future. Specifically, we recommend (1) the establishment of much denser array of monitoring sites, particularly in the South Hemisphere; (2) further integration of ground and satellite monitoring; (3) generation of flux-based standards and critical levels taking into account the sensitivity of dominant forest tree species; (4) long-term monitoring of N, S, P cycles and base cations deposition together at global scale; (5) intensification of experimental studies, addressing the combined effects of different abiotic factors on forests by assuring a better representation of taxonomic and functional diversity across the ~73,000 tree species on Earth; (6) more experimental focus on phenomics and genomics; (7) improved knowledge on key processes regulating the dynamics of radionuclides in forest systems; and (8) development of models integrating air pollution and climate change data from long-term monitoring programs.

Relationship of 137Cs with Fungal Spore Tracers in the Ambient Aerosols from Fukushima after the 2011 Nuclear Accident, East Japan

Even after 7 years of the nuclear accident that occurred in 2011 at the Fukushima Dai-ichi nuclear power plant (F1NPP), high levels of 137Cs have been detected in ambient aerosols from some polluted areas of Fukushima. Higher levels of radionuclides were often observed in the atmosphere during and after rain events. We presume that biological processes such as fungal activity associated with higher relative humidity may be involved with a possible emission of radioactivity to the atmosphere, which was originally emitted from the F1NPP accident and was deposited over the ground, forest, soil, etc. Here, we report, for the first time, relationships of 137Cs and organic tracers of fungal spores (i.e., arabitol, mannitol and trehalose) in the aerosol samples collected from Fukushima, Japan. Although we found twice-higher concentrations of 137Cs at nighttime than at daytime, fungal spore tracers did not show a consistent trend to 137Cs, that is, organic tracers at nighttime were similar with those at daytime or were even higher in daytime. This study has not clearly demonstrated that fungal spores are the important source of high levels of 137Cs at nighttime. The current unclear relationship is probably associated with the sampling strategy (four consecutive days with a sampling on/off program for day/nighttime samples) taken in this campaign, which may have caused a complicated meteorological situation.

Machine learning analysis of 137Cs contamination of terrestrial plants after the Fukushima accident using the random forest algorithm

Radioactive contamination of terrestrial plants was extensively investigated and quantitatively modeled after the Fukushima nuclear power plant accident. This phenomenon, which is important for ecosystem functioning and protection of human health, is influenced by multiple factors, including plant species, time after the accident, and climate. Machine learning algorithms such as random forests (RF) have a record of strong performance on large multi-dimensional data sets, but, to our knowledge, combined data on post-Fukushima plant contamination with radionuclides were not yet subjected to a machine learning analysis. Here we performed such analysis on two large published data sets: (1) ¹³⁷Cs activity concentrations in four common Japanese forest tree species. (2) Plant/soil ¹³⁷Cs concentration ratios in multiple perennial plant species. The goal was to show the usefulness of machine learning for identifying and quantifying the main trends of ¹³⁷Cs contamination in terrestrial plants. Each data set was split randomly into training and testing parts, RF was fitted and tuned on the training parts, and its performance was assessed on the testing parts by three metrics: coefficient of determination (R²), root mean squared error, and mean absolute error. Synthetic noise variables and the Boruta algorithm were used in a customized procedure to identify the most important predictor variables, which consistently outperformed random noise. Good agreement between observations and RF predictions (e.g. R²∼0.9 on testing data) was obtained on both data sets. The effects of the most important predictors (e.g. time after the accident, ¹³⁷Cs land contamination level, and plant species) and interactions between them were quantified by partial dependence plots. These results of machine learning analyses of large data collections can help to complement previous modeling efforts, and to clarify the patterns of ¹³⁷Cs contamination of plants after the Fukushima accident.

Effects of species and geo-information on the 137Cs concentrations in edible wild mushrooms and plants collected by residents after the Fukushima nuclear accident

After the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), much of the wild and edible mushrooms and plants in the surrounding areas were contaminated with radiocesium (137Cs). To elucidate their concentration characteristics, we analyzed 137Cs radioactivity data in edible forest products brought in for food inspection by the residents of Kawauchi Village, 12-30 km away from the FDNPP, from 2012 to 2019. A Bayesian model to estimate 137Cs concentration was constructed. Parameters of the normalized concentration of species (NCsp) for mushrooms were similar to those of the same species obtained in a previous study. Although NCsp values were highly varied among species, mycorrhizal mushrooms tended to have high NCsp values, followed by saprotrophic mushrooms, and wild edible plants values were low. Also, half of mycorrhizal mushroom species (8 of 16) showed an increasing trend in concentration with time; however, saprotrophic mushrooms and wild plants generally demonstrated a decreasing trend (22 of 24). The model considering the sub-village location information decreased the error of individual samples by 40% compared to the model not considering any location information, indicating that the detailed geo-information improved estimation accuracy. Our results indicate that the radioactivity data from samples collected by local residents can be used to accurately assess internal exposure to radiation due to self-consumption of contaminated wild mushrooms and plants.

Dynamics of radiocaesium within forests in Fukushima-results and analysis of a model inter-comparison

Forests cover approximately 70% of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident in 2011. Following this severe contamination event, radiocaesium (¹³⁷Cs) is anticipated to circulate within these forest ecosystems for several decades. Since the accident, a number of models have been constructed to evaluate the past and future dynamics of ¹³⁷Cs in these forests. To explore the performance and uncertainties of these models we conducted a model inter-comparison exercise using Fukushima data. The main scenario addressed an evergreen needleleaf forest (cedar/cypress), which is the most common and commercially important forest type in Japan. We also tested the models with two forest management scenarios (decontamination by removal of soil surface litter and forest regeneration) and, furthermore, a deciduous broadleaf forest (konara oak) scenario as a preliminary modelling study of this type of forest. After appropriate calibration, the models reproduced the observed data reliably and the ranges of calculated trajectories were narrow in the early phase after the fallout. Successful model performances in the early phase were probably attributable to the availability of comprehensive data characterizing radiocaesium partitioning in the early phase. However, the envelope of the calculated model end points enlarged in long-term simulations over 50 years after the fallout. It is essential to continue repetitive verification/validation processes using decadal data for various forest types to improve the models and to update the forecasting capacity of the models.

Estimation of the rate of 137Cs root uptake into stemwood of Japanese cedar using an isotopic approach

Japanese cedar (Cryptomeria japonica) is the main timber species in Japan. The prediction of the temporal changes in the ¹³⁷Cs concentration in the stemwood of Japanese cedar after the Fukushima nuclear accident is essential for optimizing forest management in contaminated areas. However, it is difficult to estimate the respective contributions of root and foliar uptake to ¹³⁷Cs accumulation in stemwood from simple field measurements, especially in trees that contain the residue of initially-deposited ¹³⁷Cs. In this study, we devised a method for estimating the rate of ¹³⁷Cs root uptake into stemwood using the ¹³³Cs content in stemwood and the ¹³⁷Cs/¹³³Cs ratio in the exchangeable fraction of soil. As a trial, the method was applied to a cedar stand in Fukushima Prefecture, using available monitoring data from prior studies over 5 years from August 2011 to August 2016. The mean annual rate of ¹³⁷Cs root uptake into stemwood over this period was estimated as 53 ± 20 Bq m^-2 yr^-1. We note that our method likely provided a maximum estimate, because it is based on the assumptions that ¹³³Cs in wood is exclusively supplied by root uptake, and that Cs isotopes are taken up by roots in the top 5 cm of mineral soil. Moreover, the mean annual increase of the ¹³⁷Cs inventory in stemwood during the study period was measured as 108 Bq m^-2 yr^-1, although this value was associated with considerable uncertainty (95% confidence interval from -109 to 324 Bq m^-2 yr^-1). As a result, the maximum estimated rate of ¹³⁷Cs root uptake into stemwood accounted for around half of the measured rate of ¹³⁷Cs accumulation in stemwood. Our results show that the Cs isotopic approach has potential to distinguish the main pathway of stemwood contamination (i.e., root vs. foliar uptake) following radioactive fallout.

Assessment of potentially reusable edible wild plant and mushroom gathering sites in eastern Fukushima based on external radiation dose

Edible wild plant/mushroom gathering, an essential food acquisition and outdoor recreation activity in rural areas, has declined in the area near the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in eastern Japan. The present study first evaluated the spatial distribution of potential gathering sites of various edible wild plant/mushroom species before the accident by administering a face-to-face questionnaire survey to local gatherers as well as utilizing the group analytical hierarchy process (AHP) and geographic information systems (GIS). Then, the damage to and future reusability of previous gathering sites were estimated from the perspective of the external radiation dose by overlaying maps of potential gathering sites and the time-series air dose rate (ADR) up to 2050 incorporating different gathering frequency scenarios. The study area is located in Kawauchi village in the eastern Fukushima prefecture, at 12-30 km southwest of FDNPP. The spatial distributions of gathering sites before the accident differed widely among species at the local scale because of their different environmental preferences. In contrast, the temporal variation in the reusability of the gathering sites was notably small among different species. The external radiation dose in the potential gathering sites declined sharply in the early post-accident years and gradually in the later years through the physical decay of radiocesium, i.e., ¹³⁴Cs and ¹³⁷Cs, with different half-lives. Moreover, the gathering frequency and heterogeneous distribution of radiocesium substantially affected the temporal variation in reusability for the gathering sites. These results indicate that an early resumption of gathering would be possible by reducing the gathering frequency and avoiding higher ADR areas, whereas it will take a much longer period to resume gathering in areas with higher ADR. Further research is required that considers both internal and external radiation doses in the geospatial context for the restoration and safer use of edible wild plants/mushrooms.

Calculations for ambient dose equivalent rates in nine forests in eastern Japan from 134Cs and 137Cs radioactivity measurements

Understanding the relationship between the distribution of radioactive ¹³⁴Cs and ¹³⁷Cs in forests and ambient dose equivalent rates (H˙^∗(10)) in the air is important for researching forests in eastern Japan affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study used a large number of measurements from forest samples, including ¹³⁴Cs and ¹³⁷Cs radioactivity concentrations, densities and moisture contents, to perform Monte Carlo radiation transport simulations for H˙^∗(10) between 2011 and 2017. Calculated H˙^∗(10) at 0.1 and 1 m above the ground had mean residual errors of 19% and 16%, respectively, from measurements taken with handheld NaI(Tl) scintillator survey meters. Setting aside the contributions from natural background radiation, ¹³⁴Cs and ¹³⁷Cs in the organic layer and the top 5 cm of forest soil generally made the largest contributions to calculated H˙^∗(10). The contributions from ¹³⁴Cs and ¹³⁷Cs in the forest canopy were calculated to be largest in the first two years following the accident. Uncertainties were evaluated in the simulation results due to the measurement uncertainties in the model inputs by assuming Gaussian measurement errors. The mean uncertainty (relative standard deviation) of the simulated H˙^∗(10) at 1 m height was 11%. The main contributors to the total uncertainty in the simulation results were the accuracies to which the ¹³⁴Cs and ¹³⁷Cs radioactivities of the organic layer and top 5 cm of soil, and the vertical distribution of ¹³⁴Cs and ¹³⁷Cs within the 5 cm soil layers, were known. Radioactive cesium located in the top 5 cm of soil was the main contributor to H˙^∗(10) at 1 m by 2016 or 2017 in the calculation results for all sites. Studies on the ¹³⁷Cs distribution within forest soil will therefore help explain radiation levels henceforth in forests affected by the FDNPP accident. The merits of this study are that it modelled multiple forests for a long time period, with the important model inputs being informed by field measurements, and it quantified how the measurement uncertainties in these inputs affected the calculation results.

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F_l/t), the migration center (X_c) and the relaxation depth (α) using 99 soil inventory data sets. F_l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F_l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X_c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X_c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high ¹³⁷Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of ¹³⁷Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of ¹³⁷Cs from organic to surface mineral soil layer after the second year. These results indicate that ¹³⁷Cs retention capacity of the organic layer can affect the apparent vertical migration of ¹³⁷Cs in the underlying mineral soil layer.

Radiocesium concentrations in mushrooms collected in Kawauchi Village five to eight years after the Fukushima Daiichi Nuclear Power Plant accident

Following the Fukushima Daiichi Nuclear Power Plant accident in March 2011, radionuclides such as iodine-131, cesium-134 and cesium-137 were released into environment. In this study, we collected wild mushrooms from the Kawauchi Village of Fukushima Prefecture, located less than 30 km southwest of the Fukushima nuclear power plant, to evaluate their radiocesium (134Cs+137Cs) concentrations and the risk of internal radiation exposure in local residents. 342 mushroom samples were collected from 2016 to 2019. All samples were analysed for radiocesium content by a high-purity germanium detector. Among 342 mushroom samples, 260 mushroom samples (76%) were detected the radiocesium exceeding the regulatory limit of radiocesium (100 Bq/kg for general foods in Japan). The median of committed effective dose from ingestion of wild mushrooms was in the range of 0.015-0.053 mSv in 2016, 0.0025-0.0087 mSv in 2017, 0.029-0.110 mSv in 2018 and 0.011-0.036 mSv in 2019 based on the assumption that Japanese citizens consumed wild mushrooms for 1 year. Thus, our study showed that although radiocesium is still detected in mushrooms collected in Kawauchi village even after 5 to 9 years later, the committed effective dose due to consuming mushrooms was lower than 1 mSv per year. Long-term comprehensive follow-up should monitor radiocesium concentrations in wild mushrooms to support the recovery of the community after the nuclear disaster.