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

Biofilm-mediated interactions between plastics and radiocesium in coastal environments

A ubiquitous distribution of plastic debris has been reported in aquatic and terrestrial environments; however, the interactions between plastics and radionuclides and the radioactivity of environmental plastics remain largely unknown. Here, we characterize biofilms developing on the surface of plastic debris to explore the role of plastic-associated biofilms as an interaction medium between plastics and radiocesium (137Cs) in the environment. Biofilm samples were extracted from plastics (1-50 mm in size) collected from two contrasting coastal areas in Japan. The radioactivity of plastics was estimated based on the 137Cs activity concentration of the biofilms and compared seasonally with surrounding environmental samples (i.e., sediment and sand). 137Cs traces were detected in biofilms with activity concentrations of 21-1300 Bq·kg-1 biofilm (dry weight), corresponding to 0.04-4.5 Bq·kg-1 plastic (dry weight). Our results reveal the interaction between 137Cs and plastics and provide evidence that organic and mineral components in biofilms are essential in 137Cs retention in environmental plastics. Given the ubiquitous distribution of plastic debris in the environment, more attention should be directed to bioaccumulation and the radioecological impacts of plastic-associated radionuclides on ecosystems.

Interactions between micaceous minerals weathering and cesium adsorption

The environmental behavior of radioactive cesium (RCs) in contaminated areas is generally governed by soil and sediment components and natural weathering conditions. In this study, desorption tests and spectroscopic approaches were used to explore the interaction between the weathering of micaceous minerals (i.e., biotite and phlogopite) and the adsorption of Cs⁺ and the critical role of weathering in the environmental behavior of RCs. Results showed that the reaction sequence between weathering and Cs⁺ adsorption significantly affected the surface species of Cs⁺ and the structure of biotite and phlogopite. Regardless of whether it occurred before, after, or during Cs⁺ adsorption, weathering generated more high-affinity adsorption sites, namely, interlayer sites (ITs) and frayed edge sites (FESs), to different extents, and then facilitated the uptake of Cs⁺ at FESs and ITs on micaceous minerals in a poorly exchangeable state. Cs⁺ stabilized the micaceous mineral structure once it was absorbed within collapsed interlayers by hindering cation exchange and preventing further destruction during weathering. As important weathering factors, high temperature and Ca²⁺ content promoted the binding of Cs⁺ in the interlayers of biotite and phlogopite by enhancing interlayer cation exchange. These findings are beneficial for a better understanding of the environmental behaviors of RCs in the hydrosphere and pedosphere.

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.

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.

Ten years of investigations of Fukushima radionuclides in the environment: A review on process studies in environmental compartments

In March 2011, severe nuclear accident happened at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the gigantic earthquake and following huge tsunami wave. A lot of investigations to assess environmental and radiological impacts of released radionuclides have been conducted by domestic and international organizations. Environmental radioactivity research related to the FDNPP accident has spread widely over different scientific fields due to specific features of the accident, and specifically its impact on the marine environment. The present paper summarizes major lessons learned from the environmental investigations of the FDNPP accident. Environmental radioactivity studies have typical interdisciplinary character; especially physics and chemistry are fundamental as a base of process studies in the environment. In this sight, we review chemical aspects regarding FDNPP-derived radiocesium transfer within and between compartments (atmosphere, ocean and land). We also discuss future trends in investigations of behavior of anthropogenic radionuclides in the environment, important not only for a better understanding of impacts of the FDNPP accident on the environment, but also for improving our general knowledge of the total environment in the Anthropocene era and its protection for the future.

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.

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.

Accumulation of 137Cs in aggregated organomineral assemblage in pasture soils 8 years after the accident at the Fukushima Daiichi nuclear power plant

Despite the presence of minerals that allow Cs fixation in soils, ¹³⁷Cs remains available to crops for several years after its deposition, particularly in pasture soils. Larger amounts of organic matter derived from herbage residues are accumulated in pasture soils than in tilled farmland soils. As the above-ground part of herbage crops initially received airborne ¹³⁷Cs during the accident at Fukushima Daiich nuclear power plant (FDNPP), the organic matter originated from the contaminated herbage should play an important role in the fate of ¹³⁷Cs in soils. To evaluate the role of organic matter on ¹³⁷Cs distribution between potentially mobile and immobile fractions, we compared the distribution of ¹³⁷Cs and stable ¹³³Cs, which are differently associated with organic matter, by sequential extraction and density fractionation. Soil samples were collected 8 years after the accident from Andosols in pasture fields located about 160 km southwest of FDNPP. More than 90% of ¹³⁷Cs was not extracted even after oxidative digestion of organic matter, suggesting that most ¹³⁷Cs was strongly associated with soil minerals. Density fractionation results showed that the ¹³⁷Cs/¹³³Cs ratio was highest in the density fraction of 1.6-1.8 g cm^-3, in which organic matter -including fragmented and decomposed plant detritus -was associated with minerals. Mineral-free organic matter, mostly composed of fresh plant detritus (<1.6 g cm^-3), had a higher ¹³⁷Cs/¹³³Cs ratio than that of crops harvested in the same year of soil sampling. Thus, the transfer of ¹³⁷Cs from soil to plants decreased with cultivation cycles. Our results demonstrate that plant-available ¹³⁷Cs in pasture soil decreased with aging time, not only through increased ¹³⁷Cs fixation in mineral-dominated fractions but also through its physical sequestration in aggregates.

Untangling radiocesium dynamics of forest-stream ecosystems: A review of Fukushima studies in the decade after the accident

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

Sorption and desorption experiments using stable cesium: considerations for radiocesium retention by fresh plant residues in Fukushima forest soils

We conducted sorption experiments with stable cesium (133Cs) solution in different organic matter samples, aiming to understand the sorption of radiocesium (134Cs and 137Cs) in the initial throughfall by fresh plant residues (e.g., needles, wood, and bark from Japanese cedar trees) in the Oi horizon in forests in Fukushima. Among the organic matter samples, bark and wattle tannin sorbed relatively large amounts of Cs, whereas wood and cellulose powder sorbed small amounts. In contrast, samples containing clay minerals showed much higher Cs sorption. We also conducted desorption experiments, and suggested that Cs on the organic matter samples were relatively mobile.

137Cs and 40K activities and total K distribution in the sclerotia of the Wolfiporia cocos fungus from China

The activity concentration of ¹³⁷Cs and ⁴⁰K and total K content in the sclerotia of the Chinese medicinal fungus Wolfiporia cocos collected mainly from Yunnan province of China during the period 2013-2015 were investigated. W. cocos in Yunnan is collected from the wild and is cultivated in field conditions and the wood substrate used is derived from the local pine (Pinus yunnanensis Franch.) logs from neighborhood forests. The outer part of sclerotia was found to be richer than the inner one in both ¹³⁷Cs and ⁴⁰K with median values of 7.3 and 3.2 Bq kg^-1 dry weight (dw) for ¹³⁷Cs, 220 and 140 Bq kg^-1 dw for ⁴⁰K. The median K concentrations were 6800 mg kg^-1 dw in the outer and 3700 mg kg^-1 dw in the inter parts. No statistically significant correlation was found for activity concentrations between the inner and outer parts, both for ¹³⁷Cs and ⁴⁰K (p > 0.05). Using the median activities of ¹³⁷Cs, the nominal values of effective dose (mSv) for exposed adults annually consuming 50 g of sclerotia, were estimated at 0.0035 mSv and 0.084 mSv (outer part), and 0.0020 mSv and 0.040 mSv (inner part) per capita, respectively. Sclerotia of W. cocos seemed to be a relatively good source of K.

Chemical sequential extraction of O horizon samples from Fukushima forests: Assessment for degradability and radiocesium retention capacity of organic matters

To investigate how radiocesium (¹³⁷Cs) is retained in the O horizon via interactions with organic matter, we collected O horizon samples in Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata) forest sites in Fukushima during the 8 years following the Fukushima Dai-ichi Nuclear Power Plant accident. To assess degradability and ¹³⁷Cs retention capacity of organic matter, we conducted chemical sequential extraction with organic solvent and sulfuric acid, collecting the following fractions: organic solvent extractives (Fraction 1), acid-soluble carbohydrates (Fraction 3), and acid-insoluble residue (Fraction 4). In all samples, across sampling years and sites, ¹³⁷Cs content in Fractions 1, 3, and 4, as a proportion of the total ¹³⁷Cs content, was 0.0-23.6%, 18.4-42.9%, and 44.8-76.0%, respectively. Generally, ¹³⁷Cs is considered to be electrostatically bound to organic matter and relatively mobile, making it easily extractable by sulfuric acid treatment. However, we observed a relatively high proportion of ¹³⁷Cs in Fraction 4, suggesting strong retention of ¹³⁷Cs and their immobility in the O horizon. Complex organic matter such as lignin or tannin may contribute this retention. We also noted that some part of ¹³⁷Cs may be also retained by clay minerals in the O horizon. Although organic matter in Fractions 1 and 3 is considered to decompose faster than that in Fraction 4, over the observation period the ¹³⁷Cs proportion and net rate of decrease in ¹³⁷Cs content (in total and in each fraction) remained nearly constant. This result implies that decomposition of organic matter and the consequent release of bound ¹³⁷Cs may be partly compensated by additional input of ¹³⁷Cs from the canopy and ¹³⁷Cs recycling by soil microorganisms. Our study highlights the potential role of organic matter in the O horizon as a temporary reservoir of ¹³⁷Cs and a driver of the ¹³⁷Cs cycle in forest ecosystems.

Continuous nitrogen fertilization retards the vertical migration of Fukushima nuclear accident-derived cesium-137 in apple orchard soil

We monitored the levels of cesium-137 (¹³⁷Cs) in the soils of five orchards for six years following the Fukushima Daiichi Nuclear Power Plant accident on 11 March 2011 and found that the vertical distribution of accident-derived ¹³⁷Cs varied significantly among the orchards with varying land-use and fertilizer management. Based on these results, this study evaluated how nitrogen (N) fertilizer management may have affected the vertical migration of ¹³⁷Cs in the orchard soils. We selected an experimental orchard producing 'Jonathan' apples, where a long-term N-fertilizer trial has continued since 1973, with an N-fertilized plot (N plot; N added at 20 g m^-2 y^-1) and a non-fertilized plot (0 N plot). Five years after the accident, the vertical migration of accident-derived ¹³⁷Cs was significantly lower in the N plot (2.3 cm) than in the 0 N plot (4.3 cm), suggesting greater ¹³⁷Cs retention in the surface of the N plot. Application of a cesium bromide (¹³³CsBr) tracer suggested that the retarded vertical migration of ¹³⁷Cs in the N plot may be related to significantly lower amounts of exchangeable ¹³³Cs and significantly higher proportions of non-exchangeable ¹³³Cs in the upper 2 cm. We did not find any evidence of the aboveground plants contributing to more ¹³³Cs retention in the N plot. However, greater ¹³⁷Cs retention in the surface (0-2 cm) of the N plot may be due to more dissolved organic carbon (DOC), and high DOC may have contributed to deeper ¹³⁷Cs migration in both the plots. Our results suggest that continuous N fertilizer application significantly retarded the migration of ¹³⁷Cs by approximately one-half and resulted in less ¹³⁷Cs reaching the mobile exchangeable form in the deeper root-zone layers.

Baseline radioecological data for the soil and selected bioindicator organisms in the temperate forest of Plitvice Lakes National Park, Croatia

The aim of this study was to provide baseline radioecological data for the temperate forest ecosystem in Plitvice Lakes National Park. Emphasis was placed on the determination of naturally occurring radionuclides since there is an acknowledged lack of data for these radionuclides in non-accident conditions in wildlife, even for bioindicator organisms. Activity concentrations of ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²³²Th, ⁴⁰K, ¹³⁴Cs, and ¹³⁷Cs were measured by gamma spectrometry in soil and bioindicators: earthworms, conifer needles, mosses, and lichens. From the measured activity concentrations, concentration ratios were calculated to quantify the transfer of these radionuclides from soil to bioindicators. Our results show that soil activity concentrations are biased toward results from other studies conducted within the Dinaric mountain region. However, in moss and lichen samples, we measured higher activity concentrations of ²²⁶Ra and lower activity concentrations of ⁴⁰K and ¹³⁷Cs in comparison to similar studies. Also, we estimated lower concentration ratios for all radionuclides from soil to these organisms, except for ²¹⁰Pb, in comparison to generic values. The transfer of ²³⁸U was generally low for all of the bioindicator organisms. For conifer needles, a correlation was found between activity concentrations of ²²⁶Ra and ¹³⁷Cs in soil and related concentration ratios. Correlation was also found between the activity concentration of ⁴⁰K in soil and transfer of ⁴⁰K and ¹³⁷Cs to mosses and lichens. A comparison with literature data highlighted the lack of ²²⁶Ra related concentration ratios for conifer trees and especially for earthworms. Therefore, the results of this study could supplement the sparse data currently available on radionuclide background data in similar ecosystems and related soil-to-wildlife transfer of radionuclides. Dose rate assessments, performed by the ERICA Tool, estimated that 96% of the overall exposure of wildlife in the Park area is due to the background dose rates, while 0.06 μGy h^-1 on average can be attributed as an incremental dose rate from ¹³⁴Cs and ¹³⁷Cs.

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.

Initial decrease in the ambient dose equivalent rate after the Fukushima accident and its difference from Chernobyl

In 2011, after the Fukushima Dai-ichi Nuclear Power Plant accident, the initial decrease in the ambient dose equivalent rate (dH*(10) dt-1), an alternative quantity to the effective dose, was studied using monitoring data obtained from March 16, 2011. The dH*(10) dt-1 was normalized by the 137Cs activity per unit area (norm-dH*(10) dt-1) to analyze the data across monitoring sites with different deposition levels. The norm-dH*(10) dt-1 showed a rapid decrease during the first 60 days, followed by slow decrease and was modeled using two exponential functions. The norm-dH*(10) dt-1 obtained in areas dominated by paved surfaces and buildings showed a faster decrease than the unpaved-dominant field, and this decrease was facilitated in residential areas compared with the evacuation zone. The decrease in norm-dH*(10) dt-1 was compared with simulation results using parameters obtained in Europe after the Chernobyl Nuclear Power Plant accident that represent a decrease due to radionuclide migration (e.g., soil penetration and horizontal wash-off). The simulation results showed a faster decrease than our results, implying that there was less radiocesium migration in Fukushima than in Europe. The results also suggested that the regional variation in the decrease rate led to uncertainty regarding the external dose estimation.

Change in the chemical form of 137Cs with age in needles of Japanese cedar

Radiocesium (¹³⁷Cs) derived from the accident of Fukushima Dai-ichi Nuclear Power Plant remains in forests. Although a large proportion of the ¹³⁷Cs in forests has been transferred to soils, the rates of transfer to soils depend on the chemical form of ¹³⁷Cs, which determines the mobility of ¹³⁷Cs in plant tissues and subsequently during decomposition of leaf litter. In order to understand the dynamics of ¹³⁷Cs in Sugi (Japanese cedar, Cryptomeria japonica) forests, we identified the chemical forms, such as water soluble, ion-exchangeable, and residual of ¹³⁷Cs, ¹³³Cs, K, and Rb in needle-bearing Sugi branches of different ages across several years. Compared with the results for K and Rb, Cs (¹³³Cs + ¹³⁷Cs) tended to change from a water-soluble form to an immobilized form with aging of needle-bearing branch segments. In addition, it was observed that a larger proportion of the immobilized Cs were accumulated in the green outer portions of the stems through aging.

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

Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest. In this paper, we updated model predictions of temporal changes in the ¹³⁷Cs dynamics using the latest observation data and newly provided maps of the predicted ¹³⁷Cs activity concentration for wood, which is the most commercially important part of the tree body. Overall, the previous prediction and latest observation data were in very good agreement. However, further validation revealed that the migration from the soil surface organic layer to the mineral soil was overestimated for evergreen needleleaf forests. The new prediction of the ¹³⁷Cs inventory showed that although the ¹³⁷Cs distribution within forests differed among forest types in the first 5 years, the difference diminished in the later phase. Besides, the prediction of the wood ¹³⁷Cs activity concentrations reproduced the different trends of the ¹³⁷Cs activity concentrations for cedar, oak, and pine trees. Our simulation suggests that the changes of the wood ¹³⁷Cs activity concentration over time will slow down after 5–10 years. Although the model uncertainty should be considered and monitoring and model updating must continue, the study provides helpful information on the ¹³⁷Cs dynamics within forest ecosystems and the changes in wood contamination.

Numerical study of transport pathways of 137Cs from forests to freshwater fish living in mountain streams in Fukushima, Japan

The accident at the Fukushima Dai-ichi Nuclear Power Plant in 2011 released a large quantity of radiocesium into the surrounding environment. Radiocesium concentrations in some freshwater fish caught in rivers in Fukushima Prefecture in October 2018 were still higher than the Japanese limit of 100 Bq kg^-1 for general foodstuffs. To assess the uptake of ¹³⁷Cs by freshwater fish living in mountain streams in Fukushima Prefecture, we developed a compartment model for the migration of ¹³⁷Cs on the catchment scale from forests to river water. We modelled a generic forest catchment with Fukushima-like parameters to ascertain the importance of three export pathways of ¹³⁷Cs from forests to river water for the uptake of ¹³⁷Cs by freshwater fish. The pathways were direct litter fall into rivers, lateral inflow from the forest litter layer, and lateral transfer from the underlying forest soil. Simulation cases modelling only a single export pathway did not reproduce the actual trend of ¹³⁷Cs concentrations in river water and freshwater fish in Fukushima Prefecture. Simulations allowing a combined effect of the three pathways reproduced the trends well. In the latter simulations, the decreasing trend of ¹³⁷Cs in river water and freshwater fish was due to a combination of the decreasing trend in the forest leaves/needles and litter compartments, and the increasing trend in soil. The modelled ¹³⁷Cs concentrations within the forest compartments were predicted to reach an equilibrium state at around ten years after the fallout due to the equilibration of ¹³⁷Cs cycling in forests. The model suggests that long term ¹³⁷Cs concentrations in freshwater fish in mountain streams will be controlled by the transfer of ¹³⁷Cs to river water from forest organic soils.

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.