Post-deposition early-phase migration and retention behavior of radiocesium in a litter-mineral soil system in a Japanese deciduous forest affected by the Fukushima nuclear accident

Long-term behaviour of Cs-137, Cs-133 and K in beech trees of French forests

In the long-term after atmospheric deposit onto a forest ecosystem, Cs-137 becomes incorporated into the biogeochemical cycle of stable elements and progressively reaches a quasi-equilibrium state. This study aimed at determining to what extent Cs-137 activity distribution in tree vegetation could be predicted from that of stable caesium (Cs-133) and potassium (K), which are known to be stable chemical analogues and competitors for Cs-137 intake in tree organs. Field campaigns that focused on beech trees (Fagus sylvatica L.) were conducted in 2021 in three French forest stands with contrasted characteristics regarding either the contribution of global vs. Chornobyl fallouts, soil or climatic conditions. Decades after Cs-137 fallouts, it was found that more than 80% of the total radioactive inventory in the system remained confined in the top 20 cm mineral layers, while organic layers and beech vegetation (including roots) contributed each to less than 1.5%. The enhanced downward migration of Cs-137 in cambisol than podzol forest sites was presumably due to migration of clay particles and bioturbation. The distribution of Cs-137 and Cs-133 inventories in beech trees was very similar among sites but differed from that of K due a higher accumulation of Cs isotopes in roots (40-50% vs. < 25% for K). The aggregated transfer factor (Tag) of Cs-137 calculated for aerial beech organs were all lower than those reported in literature more than 20 years ago, this suggesting a decrease of bioavailability in soil due to ageing processes. Regarding their variability, Tags were generally lower by a factor 5 at the cambisol site, which was fairly well explained by a much higher value of RIP (radiocesium immobilisation potential). Cs-137 concentrations in trees organs normalized by the soil exchangeable fractions were linearly correlated to those of Cs-133 and the best fit was found for the linear regression model without intercept indicating that no more contribution of the foliar uptake could be observed on long term. Provided that the vertical distribution of caesium concentrations and fine root density are properly measured or estimated, Cs-133 was shown to be a much better proxy than K to estimate the root transfer of Cs-137.

Influential factors of long-term and seasonal 137Cs change in agricultural and forested rivers: Temperature, water quality and an intense Typhoon Event

In this study, the effect of temperature, water quality, and the impact of an intense typhoon event on change in 137Cs 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 137Cs concentration fluctuated seasonally with water temperature in all rivers, and the increase in dissolved 137Cs 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 137Cs 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 137Cs concentrations in forested rivers is not only water temperature effect on ion exchange, but rather the input of 137Cs and K+ (competing with 137Cs for exchange sites on mineral particles) into the water phase via litter leaching. Suspended solids concentrations in agricultural rivers correlated negatively with 137Cs concentrations in suspended solids, suggesting an increased proportion of coarse particles or the input of soils with low 137Cs concentration from decontaminated agricultural land. At some sites, 137Cs 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 137Cs.

Radioactivity of anthropogenic and natural radionuclides in marine sediments of the Chaun Bay, East Siberian Sea

Natural radioactive isotopes serve as a useful proxy of geological and geochemical processes in marine environment, while radiocesium serves as an indicator of man-made contamination. Monitoring of natural and anthropogenic radioactivity under conditions of the climate changes in the Arctic region is of high importance in investigations of this natural system. For the first time, we report the data on spatial distribution of natural (232Th, 226Ra, 40K) and anthropogenic (137Cs) radionuclide activities in the marine sediments from Chaun Bay (East Siberian Sea). The measured activity concentrations varied in the range 23.7-77.9 (mean 39.2) Bq kg-1 for 232Th, 16.5-39.3 (mean 26.6) Bq kg-1 for 226Ra, 535-991 (mean 726) Bq kg-1 for 40K, and 0.5-4.7 (mean 2.0) Bq kg-1 for 137Cs. The radiocesium level in the sediments showed no local sources of anthropogenic pollution in the Chaun Bay, while the average activity concentration of 40K was 1.8 times higher than worldwide.

Effect of soil organic matter on the fate of 137Cs vertical distribution in forest soils

Understanding the fate of the vertical distribution of radiocesium (¹³⁷Cs) in Japanese forest soils is key to assessing the radioecological consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The ¹³⁷Cs behavior in mineral soil is known to be primarily governed by interaction with clay minerals; however, some observations suggest the role of soil organic matter (SOM) in enhancing the mobility of ¹³⁷Cs. Here we hypothesized that soil organic carbon (SOC) concentration profile determines the ultimate vertical pattern of ¹³⁷Cs distribution in Japanese forest soils. In testing this hypothesis, we obtained soil samples that were collected before the FDNPP accident at four forest sites with varying SOC concentration profiles and quantified the detailed vertical profile of ¹³⁷Cs inventory in the soils roughly half a century after global fallout in the early 1960 s. Results showed that the higher the SOC concentration in the soil profile, the deeper the ¹³⁷Cs downward penetration. On the basis of the data for surface soils (0-10 cm), the ¹³⁷Cs retention ratio for each of the 2-cm thick layers was evaluated as the ratio of ¹³⁷Cs inventory in the target soil layer to the total ¹³⁷Cs inventory in and below the soil layer. A negative correlation was found between the ratio and SOC concentration of the layer across all soils and depths. This indicates that the ultimate fate of ¹³⁷Cs vertical distribution can be predicted as a function of SOC concentration for Japanese forest soils, and provides further evidence for SOM effects on the mobility and bioavailability of ¹³⁷Cs in soils.

Impact evaluation of typhoons and remediation works on spatiotemporal evolution of air dose rate in two riverside parks in Fukushima, Japan after the Dai-ichi nuclear power plant accident

This study elucidated the impacts of typhoon events and remediation works on the spatiotemporal evolution of the air dose rate in riverside areas frequented by residents. Spatial distribution of the air dose rate and radiocesium concentration in the sediments were measured in two riverside parks located near each other in Fukushima Prefecture, Japan, for 2015-2020. The air dose rates measured by walk surveys were interpolated using ordinary kriging to generate air dose rate maps, to facilitate a comparison between the results at different points in time during the measurement campaigns. After the typhoons that occurred during 2015-2018, the air dose rate near the riverside in one park decreased, but not in the other, because the erosion and sediment deposition patterns differed between them. This could be due to the presence of a dam upstream, which serves a flood mitigation function. However, the extreme event of typhoon Hagibis in 2019 dropped the air dose rates near the riversides in both parks. In contrast to the typhoon events which affected the riverside areas, remediation works such as decontamination undertaken during 2015-2019 reduced the air dose rates around the garden and lawn areas which are frequently used as recreational sites. Modeling the temporal evolutions in the air dose rates for the entire area of the riverside parks revealed that 35% of the reduction was caused by physical decay of radiocesium on average, 14% by vertical migration of radiocesium in the soil through precipitation, and 51% by the three typhoons and remediation works during 2015-2019. The contribution of 20% from the strongest typhoon Hagibis highlights the fact that floods resulting from large typhoons are effective in causing natural attenuation of air dose rates in riverside parks.

Effect of radioactive cesium-rich microparticles on radioactive cesium concentration and distribution coefficient in rivers flowing through the watersheds with different contaminated condition in Fukushima

Radioactive cesium-rich microparticles (CsMPs) derived from the Fukushima Daiichi Nnuclear Power Plant accident were detected from soils and river water around Fukushima Prefecture, Japan. Because CsMPs are insoluble and rich in radioactive cesium (RCs), they may cause the overestimation of solid-water distribution coefficient (K_d) for RCs in the water. Previous studies showed the proportion of RCs derived from CsMPs on RCs concentration in soils collected from areas with different contaminated levels. Because the proportion of RCs concentration derived CsMPs to the RCs concentration of soils in the less contaminated areas is higher than that in the highly contaminated areas, the effect of CsMPs on particulate RCs concentration in river water may be larger in the less contaminated areas. However, the difference in the effects of CsMPs on the particulate RCs concentration and K_d in river water flowing through watersheds with different contaminated levels has not been clarified. In this study, we investigated the effect of CsMPs on the particulate RCs concentration and K_d in two rivers, Takase River and Kami-Oguni River, flowing through the watersheds with different RCs contaminated levels in Fukushima Prefecture. CsMPs might enter rivers due to soil erosion because they were detected only in some samples collected from both rivers during flood events. CsMPs accounted for more than half of particulate RCs concentration in some water samples collected in the flood condition. In particular, the proportion of CsMPs in particulate RCs for the Kami-Oguni River was greater than that for the Takase River. However, when evaluating for the entire water sampling in the flood condition, a proportion of RCs concentration derived from CsMPs in the average RCs concentrations per unit mass of SS in both river waters collected in the flood condition was not large. CsMPs might temporarily increase the particulate RCs concentration and K_d in the flood event, but CsMPs did not significantly affect them when evaluated throughout the event.

A new approach to extracting biofilm from environmental plastics using ultrasound-assisted syringe treatment for isotopic analyses

Plastics are one of the ubiquitous and artificial types of substrates for microbial colonization and biofilm development in the aquatic environment. Characterizing plastic-associated biofilms is key to the better understanding of organic material and mineral cycling in the "Plastisphere"-the thin layer of microbial life on plastics. In this study, we propose a new method to extract biofilms from environmental plastics, in order to evaluate the properties of biofilm-derived organic matter through stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope signatures and their interactions with radionuclides especially radiocesium (¹³⁷Cs). The extraction method is simple and cost-effective, requiring only an ultrasonic bath, disposable plastic syringes, and a freeze drier. After ultrasound-assisted separation from the plastics, biofilm samples were successfully collected via a sequence of syringe treatments, with less contamination from plastics and other mineral particles. Effective removal of small microplastics from the experimental suspension was satisfactorily achieved using the method with syringe treatments. Biofilm-derived organic matter samples (14.5-65.4 mg) from four river mouths in Japan showed ¹³⁷Cs activity concentrations of <75 to 820 Bq·kg^-1 biofilm (dw), providing evidence that environmental plastics, mediated by developed biofilms, serve as a carrier for ¹³⁷Cs in the coastal riverine environment. Significant differences in the δ¹³C and δ¹⁵N signatures were also obtained for the biofilms, indicating the different sources, pathways, and development processes of biofilms on plastics. We demonstrate here a straightforward method for extracting biofilms from environmental plastics; the results obtained with this method could provide useful insights into the plastic-associated nutrient cycling in the environment.

Spatial and vertical distribution of 137Cs activity concentrations in lake sediments of Turawa Lake (Poland)

The main objective of this research was to study the spatial and vertical distribution of ¹³⁷Cs activity concentrations in the bottom sediments of Turawa Lake 32 years after the Chernobyl fallout to investigate possible factors responsible for the post-fallout migration and accumulation of ¹³⁷Cs in the selected reservoir. The results demonstrated a strong relationship between the increasing ¹³⁷Cs and ⁴⁰K activity concentrations and the decreasing grain size of sediments. Significant amounts of ¹³⁷Cs were detected in the bottom sediments deposited in the deeper parts of the reservoir (especially near the dam). Therefore, this research showed that Turawa Lake can be an important trap for sediments polluted with ¹³⁷Cs. Moreover, disturbed vertical distribution of ¹³⁷Cs activity concentrations in the sediment columns collected from the littoral zone of this lake was observed, which is probably related to the bottom erosion intensified by wind-wave action, bioturbations, and water-level fluctuations. In the profundal zone, the vertical distribution of ¹³⁷Cs activity concentrations was undisturbed, which indicates stable sedimentation conditions in this part of Turawa Lake.

Ten-year trends in vertical distribution of radiocesium in Fukushima forest soils, Japan

To elucidate interannual changes in the vertical distribution of ¹³⁷Cs in forest ecosystems contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, we investigated ¹³⁷Cs inventories in forest soils (both organic and mineral soil horizons) at 10 sampling plots with different ¹³⁷Cs deposition levels and dominant species for up to 10 years after the accident. We examined the temporal variation of the ¹³⁷Cs inventories by depth with exponential regression models (assuming that the transition and partitioning of ¹³⁷Cs are still active) and exponential offset regression models (assuming a shift to a stable ¹³⁷Cs distribution, defined as the "quasi-equilibrium steady-state" in the Chernobyl accident). In the organic horizon, the ¹³⁷Cs inventories were exponentially decreasing, and it might take more time to converge in the quasi-equilibrium steady-state at most plots. In the mineral soil horizon, most of ¹³⁷Cs was found in the surface layer of the mineral soil horizon (0-5 cm). In this layer, the inventories first increased and then become relatively constant, and the exponential offset model was selected at most plots, suggesting entry into the quasi-equilibrium steady-state over the observation period. Although we also observed exponentially increasing trends in a lower layer (5-10 cm) of the mineral soil horizon, there was no clear increasing or decreasing trend of ¹³⁷Cs inventory in the deeper mineral soil layers (10-15 and 15-20 cm). Our calculation of the relaxation depth and migration center revealed that downward migration of ¹³⁷Cs is not significant in terms of the overall ¹³⁷Cs distribution in the mineral soil horizon over 10 years.

Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident 137Cs fallout

In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with ¹³⁷Cs. However, ¹³⁷Cs transfer processes determining tree contamination (particularly for stem wood, a prominent commercial resource) remain insufficiently understood. We propose a model for simulating dynamic behavior of ¹³⁷Cs in a forest tree-litter-soil system and applied it to contaminated forests of cedar plantation and natural oak stand in Fukushima to elucidate relative impact of distinct ¹³⁷Cs transfer processes determining the tree contamination. The transfer of ¹³⁷Cs to the trees occurred mostly (>99%) through surface uptake of ¹³⁷Cs trapped by needles and bark during the fallout. Root uptake of soil ¹³⁷Cs was several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, internal contamination of the trees proceeded through an enduring recycling (translocation) of ¹³⁷Cs absorbed on the tree surface. A significant surface uptake of ¹³⁷Cs through bark was suggested, contributing to 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining uptake occurred at needles) of the total uptake by the trees, although that pathway still needs to be evaluated by experimental evidence. It was suggested that the activity concentration of ¹³⁷Cs in stem wood of the trees at these sites are currently (as of 2021) decreasing by ~3% per year, mainly through radioactive decay of ¹³⁷Cs and partly through dilution effect from tree growth. Although further refinement of the model is recommended, for example by including tree species specific ¹³⁷Cs transportation in stem, these findings provide vital information for planning of forestry reactivation in Fukushima; e.g., removal of forest floor organic layer will not reduce the tree contamination for a long term because of the ¹³⁷Cs absorption via the tree surface substantially greater than root uptake of ¹³⁷Cs deposited to the floor.

Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of 137Cs derived from the FDNPP accident in a cedar forest soil

The vertical distribution of ¹³⁷Cs in forest soil is important for predicting air dose rates and future cycling in forest ecosystems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the ¹³⁷Cs flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of ¹³⁷Cs has been conducted since 2011. As a result, the ¹³⁷Cs concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the ¹³⁷Cs inventory in the litter layer has been exponentially decreasing, the annual ¹³⁷Cs fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14-0.17% of the deposition density of ¹³⁷Cs. Comparing these ¹³⁷Cs fluxes with the apparent amounts of downward migration of ¹³⁷Cs estimated from the change in the vertical distribution of ¹³⁷Cs, the contribution rate of the infiltrated water to downward migration of ¹³⁷Cs from litter to soil was calculated to be 8.5-17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6-0.8% on a measured basis and estimated to be 3.0 ± 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of ¹³⁷Cs, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots.

An application research for near-surface repository of strontium-90 sorption kinetic model on mudrocks

In this study,90Sr was used as the test radionuclide to characterize the sorption kinetics and effects of initial 90Sr activity and remaining 90Sr in solid concentration were simulated for a near-surface repository. The study focused on the sorption characteristics of radionuclides in unsaturated groundwater environment (or vadose zone) is the important information for investigating the near-surface disposal of intermediate and low-level radioactive waste (ILLW). Moreover, the 90Sr sorption experiments reached equilibrium within 56 h, which fit to the first order sorption kinetic model, and the remaining 90Sr in mudrock samples showed obvious sorption equilibrium hysteresis, which fit to the second order sorption kinetic model. Before reaching the maximum sorption capacity, the sorption rate constant increases with 90Sr increasing; the distribution coefficient (Kd) of 56 h decreases with the remaining 90Sr decreasing. In addition, it showed that the slow sorption process dominated before the sorption reaches equilibrium. In fact, a reliable safety assessment methodology for on-going near-surface repository required a lot of the radionuclides parameters with local environment including the radionuclides sorption/desorption rate constant and maximum sorption capacity.

Decontamination of radioactive cesium-contaminated soil/concrete with washing and washing supernatant- critical review

We reviewed washing of radioactive Cs-contaminated concrete and soil based on the fate of Cs in concrete and soil, including sorption materials for treatment of supernatant solution. In non-aged cement materials (the calcium silicate hydration (C-S-H) phase), it was possible to decontaminate Cs using ion exchange with monovalent cations, such as NH₄⁺. The clay components in the soil and aggregates were important factors in optimization of the efficiency and mechanism for Cs decontamination with washing solution. The parameters (reagent component, pH, and temperature) of the washing solution should be determined considering soil mineral type (here, weathered biotite (WB) with vermiculite), since monovalent cations such as NH₄⁺ and K⁺ can inhibit Cs decontamination due to collapse of the hydrated and expanded interlayer regions with cation exchange. In this case, hydrothermal treatment or H₂O₂ dosing was necessary to expand the collapsed interlayer region for Cs removal by washing with cation exchange or organic acids. Acid and a chelating agent significantly enhanced Cs-release with dissolution of the adsorbent layer containing iron and aluminum oxides. The important characteristics of important and emerging sorption materials for treatment of the radioactive Cs-contaminated supernatant after washing treatment are discussed. Sorbents for treatment of washing supernatant are divided in to two main categories. Clay minerals, metal hexacyanoferrates, and ammonium molybdophosphates are discussed in the inorganic class of materials. Hypercrosslinked polymers, supramolecular sorbents, carbon nanotubes, and graphene oxide are covered in the carbon-based sorbents for Cs removal from water.

Contamination of Slovak Bilberry (Vaccinium Myrtillus L.) with Radiocaesium 137Cs in Selected Slovak Locations

The aim of this study was to determine the activity of post-Chernobyl 137Cs in Slovak bilberry (Vaccinium myrtillus L.) from selected locations of Slovakia depending upon: the season, location and the soil-plant transfer factor (TF). The 137Cs activity was determined in samples of forest soil and bilberry bushes (stems, leaves and fruits) growing on the soil in three locations: Arboretum Mlyňany (1), Hodruša-Hámre (2) and Javorníky (3). Altogether we collected 35 samples; namely 15 samples from Arboretum Mlyňany, 10 samples from Hodruša-Hámre and 10 from Javorníky. The samples of soil were examined also for the activity of 40K. The samples were collected in the spring, summer and autumn in the years 2019 and 2020 and were analysed by a gamma-ray spectrometry method. The highest levels of 137Cs activity concentrations were determined in Hodruša-Hámre in September where they reached 161 ± 13 Bq.kg−1 in the soil, 3.95 ± 1.07 Bq.kg−1 in the stems and 14.7 ± 4.26 Bq.kg−1 in the leaves. The lowest levels were determined in the Arboretum Mlyňany in October where the 137Cs activity in the soil reached 13.1 ± 1 15 Bq.kg−1, while in the stems, leaves and fruits it was lower than the minimum detectable level. In the latter location, we also determined the highest activity of 40K, the radioactive isotope physiologically and metabolically similar to 137Cs. The mass activity of 137Cs in all samples of bilberries were lower than the minimum detectable activity (MDA). The comparison of the results obtained in this study with the limit for total mass activity of radiocaesium (134Cs + 137Cs) in the food (600 Bq.kg−1) allowed us to conclude that the fruits, stems and leaves of Slovak bil-berries present neither health nor environmental risk.

Leaching characteristics of 137Cs for forest floor affected by the Fukushima nuclear accident: A litterbag experiment

In forest ecosystems, forest litter is considered an active medium for radiocesium (¹³⁷Cs). To understand discharge mechanisms of highly bioavailable dissolved ¹³⁷Cs from forests to river systems, we investigated the characteristics of ¹³⁷Cs leaching from forest litter as observed from litterbag experiments. Leaching experiments with conifer needle and deciduous broadleaf litters were then conducted. After soaking conifer needles and broadleaf litters for 20 min, 140 min, and 1 day, the mean values of the ¹³⁷Cs leaching ratios were 0.13-2.0% and 0.81-6.6%, respectively, indicating that ¹³⁷Cs leaching ratios are different between forest litter types. To elucidate the factors affecting ¹³⁷Cs leaching from forest litter, a multi-regression analysis of ¹³⁷Cs leaching ratios was conducted against antecedent mean precipitation and temperature before sampling the litterbag and accumulated temperature during the litterbag experiments. The ¹³⁷Cs leaching ratios showed a negative correlation to the antecedent mean precipitation for both litters and the accumulated temperature for broadleaf litters, whereas it exhibited a positive correlation with the antecedent mean temperature for both litters and the accumulated temperature for conifer needle litters. It was proposed that the fraction of ¹³⁷Cs in labile sites in forest litter increased/decreased due to litter decomposition by antecedent/accumulated temperature, and that this fraction can be washed off by the antecedent precipitation. The different effects of accumulated temperature on ¹³⁷Cs leaching from conifer needles and broadleaf litters could be due to their different decomposition rates. Our results contribute further the understanding of the mechanisms associated with dissolved ¹³⁷Cs discharge from forested catchments.

Meta-analysis of radiocesium contamination data in Japanese cedar and cypress forests over the period 2011-2017

Since Fukushima accident, dozens of field studies have been conducted in order to quantify and understand the behaviour of atmospheric radiocesium (¹³⁷Cs) fallouts in contaminated forests of Fukushima and neighbouring prefectures. In this paper, we carry out a detailed review of data acquired over 2011-2017 in Japanese cedar and cypress plantations, focusing on aerial tree organs, soil layers and tree-to-soil depuration fluxes. To enable comparison and reinforce the consistency between sites, radiological measurements were normalized by the deposit and interpolated onto the same spatio-temporal frame. Despite some (poorly explained) residual variability, we derived a "mean" pattern by log-averaging data among sites. These "mean" results were analysed with the help of a simple mass-balance approach and discussed in the light of post-Fukushima literature. We demonstrated that the activity levels and dynamics in all compartments were consistent and generally well reproduced by the mass balance approach, for values of the interception fraction between 0.7 and 0.85. The analysis indicated that about 5% of the initial deposit remained in the aerial vegetation after 6 years, more than two thirds of intercepted ¹³⁷Cs being transferred to the soil due to throughfall. The simulations indicated that foliar uptake might have contributed between 40% and 100% to the activity transferred to stem wood. The activity concentration in canopy organs rapidly decreased in the first few months then more slowly, according to an effective half-life of about 1.6 years. The activity level in the organic layer peaked in summer 2011 then decreased according to an effective half-life of 2.2 years. After a rapid increase in 2011, the contamination of mineral horizons continued to increase more slowly, 85% of ¹³⁷Cs incoming through the organic layer being retained in the 0-5 cm layer according to a mean residence time longer than in the upper layer (7 against 1.5 years).

Differences in leaching characteristics of dissolved radiocaesium and potassium from the litter layer of Japanese cedar and broadleaf forests in Fukushima, Japan

Cesium is an element that belongs to the same group as K, and is known to show similar behaviour to that of K in plants. In this study, we conducted a serial leaching test for 120 h to compare the leaching characteristics of dissolved ¹³⁷Cs and K in forest litter, obtained from Japanese cedar and deciduous broadleaf forests located 40 km from the site of the 2011 Fukushima Dai-ichi nuclear power plant accident. The litter was collected in 2018 and was divided into three groups according to the decomposition level. The cumulative leachable fraction of ¹³⁷Cs at 120 h ranged from 0.3% to 3.3%, suggesting that most of the ¹³⁷Cs in the litter was hardly leachable in water. The leachable fraction of ¹³⁷Cs generally decreased with the decomposition level of the litter, implying that the easily leachable ¹³⁷Cs eluted into the water during the first stage of decomposition. Meanwhile, the cumulative leachable fraction of K at 120 h was approximately 10 times greater than that of ¹³⁷Cs and ranged from 22.7% to 54.8%. The leaching speeds of ¹³⁷Cs and K decreased suddenly with elapsed time regardless of the tree species, decomposition degree, or element. Our findings contribute to the long-term understanding of the ¹³⁷Cs cycle in forest ecosystems.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified ¹³⁷Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated ¹³⁷Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the ¹³⁷Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of ¹³⁷Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1-2 years after the accident) for Japanese forest ecosystems.

Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of ¹³⁷Cs from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of ¹³⁷Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of ¹³⁷Cs in litter and soil layers for 6 y (2011-2017) following the previous study (2011-2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The ¹³⁷Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80-95% of the deposited ¹³⁷Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of ¹³⁷Cs inventory in the litter layer to the total ¹³⁷Cs inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22-0.44 y^-1. The slower migration was observed in the YC site, probably because of higher initial interception of ¹³⁷Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the ¹³⁷Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the ¹³⁷Cs concentration in the litter within 2-3 y of the accident. The ¹³⁷Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of ¹³⁷Cs concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y^-1. In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of ¹³⁷Cs and its factors regarding both forest and soil characteristics.

Applicability of Kd for modelling dissolved 137Cs concentrations in Fukushima river water: Case study of the upstream Ota River

A study is presented on the applicability of the distribution coefficient (K_d) absorption/desorption model to simulate dissolved ¹³⁷Cs 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 ¹³⁷Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs concentrations in Fukushima rivers.

Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with ¹³⁷Cs. It is important to characterize the behavior of ¹³⁷Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, ¹³⁷Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and ¹³⁷Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of ¹³⁷Cs could be suppressed rapidly in Japanese forests. At all sites, ¹³⁷Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous ¹³⁷Cs supply from the organic layer. The diffusion coefficients of ¹³⁷Cs in the mineral soil were estimated to be 0.042-0.55 cm² y^-1, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited ¹³⁷Cs will migrate to the mineral soil but only less than 10% of the total ¹³⁷Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the ¹³⁷Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

Web-building spider Nephila clavata (Nephilidae: Arachnida) can represent 137Cs contamination of arthropod communities and bioavailable 137Cs in forest soils at Fukushima, Japan

Large areas of Fukushima's forests were contaminated with radiocesium (¹³⁷Cs) after the Fukushima Dai-ichi Nuclear Power Plant accident. Most of the contaminated forests have not been decontaminated, and bioavailable ¹³⁷Cs is likely to circulate within the forest environment's food web. Nephila clavata (Nephilidae: Arachnida) is a top predator in the forest arthropod community, and this web-building spider potentially consumes many arthropod species presented in the grazing and detrital food chains. We tested whether ¹³⁷Cs in the spider could serve as a proxy for ¹³⁷Cs contamination of these arthropod communities. We also examined whether N. clavata could serve as a proxy for soil bioavailable ¹³⁷Cs. Nephila clavata was similarly or more contaminated with ¹³⁷Cs compared with lower-trophic-level arthropods such as herbivores and other predators at the same trophic level. Thus, the ¹³⁷Cs activity of N. clavata could represent the extent to which the arthropod community was contaminated with ¹³⁷Cs. Data from nine ¹³⁷Cs-contaminated sites in Fukushima showed a significant positive correlation between soil bioavailable ¹³⁷Cs and N. clavata's ¹³⁷Cs activity05 but the coefficient of determination was only moderate (R² = 0.43), suggesting that N. clavata is only a weak proxy of soil bioavailable ¹³⁷Cs. Our results also showed that the bioavailable fraction of ¹³⁷Cs in Fukushima was strongly correlated with the total inventory and that the K and Na contents of the soil determined the soil-to-spider transfer factor for ¹³⁷Cs and the ¹³⁷Cs activity in N. clavata, respectively. These results improve our understanding of ¹³⁷Cs transfer from the soil to arthropod species.

Six-year trends in exchangeable radiocesium in Fukushima forest soils

We investigated the changes in ¹³⁷Cs exchangeable fraction in the soils of contaminated forest ecosystems until 6 years after the Fukushima radioactive atmospheric deposits. For this investigation, we performed chemical extractions of ¹³⁷Cs from both organic and mineral soil layer sampled from two forest stands, Japanese cedar (Cryptomeria japonica) and broadleaf deciduous, mainly konara oak (Quercus serrata), which are located 26-27 km inland from the Fukushima Dai-ichi Nuclear Power Plant. The exchangeable ¹³⁷Cs as a proportion of the total ¹³⁷Cs inventory was only 10% in the organic layer and 6.1% in the mineral soil layer about 5 months after the accident at the Japanese cedar site. We observed an exponential decrease in the proportion of exchangeable ¹³⁷Cs in both organic and mineral soil layer samples over the 6-years observation period at both sites. The proportion significantly decreased within 2-4 years after the accident, becoming almost constant (2-4%). These results support the interpretation that contaminated forests have entered a steady-state phase of ¹³⁷Cs cycling, although we need more integration for an improved vision of the future of Fukushima forests.

A new perspective on the 137Cs retention mechanism in surface soils during the early stage after the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination of the soil in multiple terrestrial ecosystems. Soil is a complex system where minerals, organic matter, and microorganisms interact with each other; therefore, an improved understanding of the interactions of ¹³⁷Cs with these soil constituents is key to accurately assessing the environmental consequences of the accident. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral–organic interaction characteristics using a density fractionation method, and then analyzed for ¹³⁷Cs content, mineral composition, and organic matter content. The results show that 20–71% of the ¹³⁷Cs was retained in association with relatively mineral-free, particulate organic matter (POM)-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the ¹³⁷Cs extractability of the soils, ¹³⁷Cs incorporation into the complex structure of POM is likely the main mechanism for ¹³⁷Cs retention in the surface soil layers. Therefore, our results suggest that a significant fraction of ¹³⁷Cs is not immediately immobilized by clay minerals and remains potentially mobile and bioavailable in surface layers of organic-rich soils.

Trend of 137Cs concentration in river water in the medium term and future following the Fukushima nuclear accident

It is a critical to examine the migration behavior of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in river systems to predict the future contamination status and propose effective countermeasures to reduce exposure. We conducted a three-year-long observation (April 2015-March 2018) of the ¹³⁷Cs concentration in two rivers which located surrounding the FDNPP. The result revealed a declining trend for the dissolved and particulate ¹³⁷Cs concentration in river water from four to seven years after the FDNPP accident. The dissolved and particulate ¹³⁷Cs concentrations for both rivers had similar temporal patterns and showed declining trends with time. However, the dissolved ¹³⁷Cs concentration had longer half-life than the particulate ¹³⁷Cs concentration and large seasonal variations related to water temperature. The environmental half-life for the dissolved ¹³⁷Cs concentration was longer than previous reported values within three years after the accident, suggesting that the declining trend for the dissolved ¹³⁷Cs concentration is gradually decreasing with time. The temperature dependency of the dissolved ¹³⁷Cs concentration became weaker year by year. From the D₁₀ equation we proposed, the dissolved ¹³⁷Cs concentration will likely remain at the same level for several decades. The results of the present study promote our understanding of both the medium- and long-term impacts of the FDNPP accident on river systems.

Spatial and temporal variation in vertical migration of dissolved 137Cs passed through the litter layer in Fukushima forests

We examined spatial variation in vertical ¹³⁷Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug-4 Oct), autumn (5 Oct-30 Nov) and winter (1 Dec-27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m² areas with 3-5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m²/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m²/month. The ratio of ¹³⁷Cs flux during the survey period to litter ¹³⁷Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The ¹³⁷Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the ¹³⁷Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20-25 times. The spatial variations in the ¹³⁷Cs flux were affected positively by those in the litter ¹³⁷Cs inventory and negatively by canopy openness.

Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of ¹³⁷Cs from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of ¹³⁷Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of ¹³⁷Cs in litter and soil layers for 6 y (2011-2017) following the previous study (2011-2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The ¹³⁷Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80-95% of the deposited ¹³⁷Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of ¹³⁷Cs inventory in the litter layer to the total ¹³⁷Cs inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22-0.44 y^-1. The slower migration was observed in the YC site, probably because of higher initial interception of ¹³⁷Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the ¹³⁷Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the ¹³⁷Cs concentration in the litter within 2-3 y of the accident. The ¹³⁷Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of ¹³⁷Cs concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y^-1. In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of ¹³⁷Cs and its factors regarding both forest and soil characteristics.

Development and assessment of a simple ecological model (TRIPS) for forests contaminated by radiocesium fallout

The management of vast forested zones contaminated by radiocesium (rCs) following the Chernobyl and Fukushima fallout is of great social and economic concern in affected areas and requires appropriate dynamic models as predictive or questioning tools. Generally, the existing radio-ecological models need less fragmented data and more ecological realism in their quantitative description of the rCs cycling processes. The model TRIPS ("Transfer of Radionuclide In Perennial vegetation Systems") developed in this study privileged an integrated approach which makes the best use of mass balance studies and available explicit experimental data for Scots pine stands. A main challenge was the differentiation and calibration of foliar absorption as well as root uptake in order to well represent the rCs biocycling. The general dynamics of rCs partitioning was simulated with a relatively good precision against an independent series of observed values. In our scenario the rCs biological cycling enters a steady-state about 15 years after the atmospheric deposits. At that time, the simulations showed an equivalent contribution of foliage and root uptake to the tree contamination. But the root uptake seems not sufficient to compensate the activity decline in the tree. The initial foliar uptake and subsequent internal transfers were confirmed to have a great possible impact on the phasing of tree contamination. An extra finding concerns the roots system acting as a buffer in the early period. The TRIPS model is particularly useful in cases where site-specific integrated datasets are available, but it could also be used with adequate caution to generic sites. This development paves the way for simplification or integration of new modules, as well as for a larger number of other applications for the Chernobyl or Fukushima forests once the appropriate data become available. According to the sensitivity analysis that involves in particular reliable estimates of net foliar uptake as well as root uptake not disconnected from rCs exchange reactions in soil.

Radiocesium distribution in aggregate-size fractions of cropland and forest soils affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination in soils in a range of terrestrial ecosystems. It is well documented that the interaction of ¹³⁷Cs with soil constituents, particularly clay minerals, in surface soil layers exerts strong control on the behavior of this radionuclide in the environment; however, there is little understanding of how soil aggregation-the binding of soil particles together into aggregates-can affect the mobility and bioavailability of ¹³⁷Cs in soils. To explore this, soil samples were collected at seven sites under different land-use conditions in Fukushima and were separated into four aggregate-size fractions: clay-sized (<2 μm); silt-sized (2-20 μm); sand-sized (20-212 μm); and macroaggregates (212-2000 μm). The fractions were then analyzed for ¹³⁷Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and 69%-83% of ¹³⁷Cs was associated with macroaggregates and sand-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and approximately 80% of ¹³⁷Cs was in the clay- and silt-sized fractions. Across all sites, the ¹³⁷Cs extractability was higher in the sand-sized aggregate fractions than in the clay-sized fractions. Mineralogical analysis showed that, in most soils, clay minerals (vermiculite and kaolinite) were present even in the larger-sized aggregate fractions. These results demonstrate that larger-sized aggregates are a significant reservoir of potentially mobile and bioavailable ¹³⁷Cs in organic-rich (forest and orchard) soils. Our study suggests that soil aggregation reduces the mobility of particle-associated ¹³⁷Cs through erosion and resuspension and also enhances the bioavailability of ¹³⁷Cs in soils.

Applicability of Kd for modelling dissolved 137Cs concentrations in Fukushima river water: Case study of the upstream Ota River

A study is presented on the applicability of the distribution coefficient (K_d) absorption/desorption model to simulate dissolved ¹³⁷Cs 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 ¹³⁷Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs concentrations in Fukushima rivers.

Stemflow-induced spatial heterogeneity of radiocesium concentrations and stocks in the soil of a broadleaved deciduous forest

The transport of radiocesium from the canopy and quantification of the spatial distribution of radiocesium in the soil of konara oak forests are important to better understand the variability of ¹³⁷Cs stocks in the soil between proximal and distal stem areas as well as fine-scale variations around the tree trunk. Moreover, a better understanding of fine-scale spatial variabilities of ¹³⁷Cs concentrations and stocks will provide insights for optimizing soil sampling strategies to provide a more robust estimation of contamination at the stand scale. This study aims to elucidate the transport of ¹³⁷Cs by stemflow in a radioactively contaminated konara oak forest in Tsukuba, Japan by describing and quantifying the fine-scale spatial distribution of ¹³⁷Cs in the soil and preferential flowpaths of stemflow on the tree stem by a dye tracing experiment. ¹³⁷Cs concentrations and stocks were higher in the soils of the proximal stem area than distal stem area when they corresponded with the preferential flowpaths of stemflow. There was a significant relationship between canopy projection area of individual trees and average soil ¹³⁷Cs concentrations and stocks, even though canopies of the trees overlapped. Our results demonstrate that the spatiality of ¹³⁷Cs concentrations and stocks in the soil of the proximal stem area are governed (at least partially) by the preferential flowpaths of stemflow along the tree trunk. In addition, higher ¹³⁷Cs concentrations and stocks in the near-trunk soils of trees with larger crown areas might be caused by an enhanced ability to capture dry deposition.

Coupling the advection-dispersion equation with fully kinetic reversible/irreversible sorption terms to model radiocesium soil profiles in Fukushima Prefecture

Radiocesium is an important environmental contaminant in fallout from nuclear reactor accidents and atomic weapons testing. A modified Diffusion-Sorption-Fixation (mDSF) model, based on the advection-dispersion equation, is proposed to describe the vertical migration of radiocesium in soils following fallout. The model introduces kinetics for the reversible binding of radiocesium. We test the model by comparing its results to depth profiles measured in Fukushima Prefecture, Japan, since 2011. The results from the mDSF model are a better fit to the measurement data (as quantified by R²) than results from a simple diffusion model and the original DSF model. The introduction of reversible sorption kinetics means that the exponential-shape depth distribution can be reproduced immediately following fallout. The initial relaxation mass depth of the distribution is determined by the diffusion length, which depends on the distribution coefficient, sorption rate and dispersion coefficient. The mDSF model captures the long tails of the radiocesium distribution at large depths, which are caused by different rates for kinetic sorption and desorption. The mDSF model indicates that depth distributions displaying a peak in activity below the surface are possible for soils with high organic matter content at the surface. The mDSF equations thus offers a physical basis for various types of radiocesium depth profiles observed in contaminated environments.

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.