(137)Cs trapped by biomass within 20 km of the Fukushima Daiichi Nuclear Power Plant

The impact of sampling scale: A comparison of methods for estimating external contaminant exposure in free-ranging wildlife

The impacts of contaminants on wildlife are dose dependent, and thus being able to track or predict exposure following contamination events is important for monitoring ecosystem health. However, the ability to track exposure in free-ranging wildlife is often severely limited. Consequently, researchers have predominantly relied on simple methods for estimating contaminant exposures in wildlife with little regard for spatial contaminant heterogeneity or an animal's use of diverse habitats. We evaluated the influence sampling scale (i.e., how finely contaminant distribution and organism's spatial use of the landscape is mapped) has on (1) realism and (2) conservativeness of exposure estimates. To do this, we monitored the actual exposure of wild boar (Sus scrofa) in Fukushima, Japan to radioactive contamination using GPS-coupled contaminant monitors placed on individual animals. We compared empirical exposures to estimates generated by combining varying amounts of information about an individual boar's location and/or movement, with the distribution of contamination on the landscape. We found that the most realistic exposure estimates were produced when finer-scale contaminant distribution surveys (e.g., airborne surveys) were combined with more accurate estimates of an individual's space use (e.g., home ranges or core areas). Importantly, estimates of exposure based on single point surveys at a trap site (a simple method commonly used in the literature), did not correlate with actual exposure rates, suggesting dose-effects studies using this method may result in spurious conclusions. These results suggest that researchers seeking realistic estimates of exposure, such as in dose-effect studies, should ensure they have adequately accounted for fine-scale contaminant distribution patterns and areas of higher use by study organisms. However, conservative estimates of exposure (i.e., intentionally over-predicting exposure as is done in initial tiers of ecological risk analyses) were not as scale sensitive and could be achieved with a single known location and coarse contaminant distribution maps.

Vertical distribution and transport of radiocesium via branchflow and stemflow through the canopy of cedar and oak stands in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident

Aiming to fill a need for data regarding radiocesium transport via both branchflow and stemflow through forests impacted by radioactive fallout, this study examined the vertical variation of radiocesium flux from branchflow and stemflow through the canopies of young Japanese cedar (Cryptomeria japonica (L. f.) D. Don) and Japanese oak (Quercus serrata Murray) trees in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident. In forested areas approximately 40 km northwest of the location of the Fukushima Dai-ichi accident, the ¹³⁷Cs concentration varied significantly among sampling periods and between the two forests, with the oak stand exhibiting higher ¹³⁷Cs concentrations and depositional fluxes than the cedar stand. Expressed per unit trunk basal area, the depositional flux of ¹³⁷Cs generated from the cedar and oak stands was 375 and 2810 Bq m^-2 year^-1, respectively. Of this total, 71% and 48% originated from the cedar and oak canopy, respectively, while the remainder originated from the trunk. Accordingly, the origin of radiocesium was more balanced for the oak stand with almost half of the flux coming from the canopy (48%) and the other half from the trunk (52%). Only about a quarter (29%) of the radiocesium flux originated from the trunk in Japanese cedar. Results from this work provide needed data that can enable a more thorough conceptualization of radiocesium cycling in forests. Coupling these empirical results with a physically-based model would likely lead to better forest management and proactive strategies for rehabilitating radioactively-contaminated forests and reducing the exposure risk of radiation dose rate for those that utilize forest products.

Absence of Detectable Radionuclides in Breast Milk in Sendai, Japan in 2012 Even by High-Sensitivity Determination: Estimated Dose among Infants after the Fukushima Nuclear Disaster

The aim of this study was to estimate radionuclide levels in breast milk and the transferred dose to their infants in Sendai (100 km from Fukushima), Japan after the 2011 Fukushima nuclear disaster. Radionuclide concentrations were analyzed in 101 specimens of breast milk collected in 2012. Median values for minimum detectable activities were 0.39, 0.34, 1.1, 1.89, and 17.1 Bq/kg for ¹³⁷Cs, ¹³⁴Cs, ¹³¹I, ^110mAg, and ⁴⁰K, respectively. Only radionuclides from ⁴⁰K were detected. To estimate potential exposure and radiocesium dose, we assumed that the samples contained each minimum detectable activity level. The mean minimum detectable activity concentrations (standard deviation) of ¹³⁷Cs and ¹³⁴Cs were 0.42 (0.15) and 0.37 (0.14) Bq/kg, respectively. Means of estimated dietary intakes of ¹³⁷Cs and ¹³⁴Cs among infants were 0.35 (0.12) and 0.31 (0.11) Bq/day, respectively. The committed effective doses of radiocesium in infants aged 3 and 12 months via breastmilk were estimated at 5.6 (2.1) and 3.3 (1.2) μSv/year, respectively. Dietary intakes of ¹³⁷Cs and ¹³⁴Cs in breastfeeding mothers were back-calculated at 1.9 (0.71) and 1.7 (0.65) Bq/day, respectively. The study verified no discernible exposure to radionuclides among infants. The most conservative estimates were below the Japanese internal exposure limit of 1 mSv/year.

Use of thermal treatment with CaCl2 and CaO to remove 137Cs in the soil collected from the area near the Fukushima Daiichi Nuclear Power Plant

A massive amount of soils and inflammable materials of plants etc. contaminated by radiocesium are generated from decontamination work in the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident affected area. In present study, the removal experiments of ¹³⁷Cs in a soil collected from the FDNPP accident affected area were carried out in a lab-scale electrical heating horizontal furnace through thermal treatment with CaCl₂ addition over a temperature of 900 - 1300 °C. The results indicated that the average radioactive concentration of ¹³⁷Cs in the soil was 52.8 Bq/g. The removal ratio of ¹³⁷Cs in the soil treated at 1300 °C was 96.3 % when 20 % CaCl₂ was added. The addition of CaCl₂ and CaO mixture exhibited a synergistic effect on the removal of ¹³⁷Cs, relative to the addition of CaCl₂ alone. Accordingly, the addition of CaCl₂ or its mixture with CaO during thermal treatment is suggested to remove ¹³⁷Cs in the soil collected from the FDNPP accident affected area. Additionally, segregation of the soil sample to fine and coarse fraction and then treated individually are also recommended.

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).

Six-year monitoring study of radiocesium transfer in forest environments following the Fukushima nuclear power plant accident

The study investigated temporal changes in the ¹³⁷Cs concentrations in vegetal and hydrological samples collected from various forests in Yamakiya District, Kawamata Town of Fukushima prefecture over six years following the Fukushima Dai-ichi nuclear power plant accident. Cesium-137 was detected in all forest environmental samples. However, the concentration in most samples decreased exponentially with time. The ¹³⁷Cs concentrations in throughfall samples exhibited a double-exponential decreasing trend with time. Temporal changes in the ¹³⁷Cs concentration in vegetal samples and stemflow were approximated by using a single-exponential equation. A comparison of the decline coefficient for the latter observation period (>2 y since the accident) revealed that the declining trend of ¹³⁷Cs concentrations varied between foliage and the outer barks of the Japanese cedar and Japanese konara oak trees. The ¹³⁷Cs concentration in cedar needles decreased exponentially while that in konara oak leaves was constant over the last six years. Conversely, the declining trend of ¹³⁷Cs concentration in the outer bark of konara oak exceeded that of cedar. The results suggested that self-decontamination processes and internal recycling of ¹³⁷Cs varied among tree species and different tree parts. The results indicated that the leaching of ¹³⁷Cs in the throughfall in Japanese cedar was dependent on the ¹³⁷Cs concentration in needles. However, a comparison of ¹³⁷Cs concentrations in vegetal and hydrological samples from each sampling year showed that the leaching rate decreased with time. Conversely, the ¹³⁷Cs concentrations in the stemflow were independent of the concentrations in the outer bark. The declining trend of ¹³⁷Cs concentrations in litterfall (λ: 0.31-0.33 y^-1) was similar to that of the mean of new/old needles (λ: 0.26-0.33 y^-1) for cedar stands. With respect to the hydrological components, the ¹³⁷Cs concentration in the stemflow (λ: 0.32-0.33 y^-1) decreased at a slightly slower rate than that in the throughfall (λ: 0.36-0.54 y^-1) for the cedar forest. The decline coefficients of ¹³⁷Cs concentration in the aforementioned types of hydrological components slightly exceeded that for the vegetal samples. The results suggest that monitoring of ¹³⁷Cs concentrations in hydrological components and vegetal samples can aid in further understanding the leaching mechanisms of ¹³⁷Cs from trees to rainwater.

Evaluation of Environmental Contamination and Estimated Radiation Exposure Dose Rates among Residents Immediately after Returning Home to Tomioka Town, Fukushima Prefecture

On 1 April 2017, six years have passed since the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, and the Japanese government declared that some residents who lived in Tomioka Town, Fukushima Prefecture could return to their homes. We evaluated environmental contamination and radiation exposure dose rates due to artificial radionuclides in the livelihood zone of residents (living space such as housing sites), including a restricted area located within a 10-km radius from the FDNPS, immediately after residents had returned home in Tomioka town. In areas where the evacuation orders had been lifted, the median air dose rates were 0.20 μSv/h indoors and 0.26 μSv/h outdoors, and the radiation exposure dose rate was 1.6 mSv/y. By contrast, in the “difficult-to-return zone,” the median air dose rate was 2.3 μSv/h (20 mSv/y) outdoors. Moreover, the dose-forming artificial radionuclides (radiocesium) in the surface soil were 0.018 μSv/h (0.17 mSv/y) in the evacuation order-lifted areas and 0.73 μSv/h (6.4 mSv/y) in the difficult-to-return zone. These findings indicate that current concentrations of artificial radionuclides in soil samples have been decreasing in the evacuation order-lifted areas of Tomioka town; however, a significant external exposure risk still exists in the difficult-to-return zone. The case of Tomioka town is expected to be the first reconstruction model including the difficult-to-return zone.

Impact of clearcutting on radiocesium export from a Japanese forested catchment following the Fukushima nuclear accident

Changes in 137Cs export over time following clearcutting were investigated in a Japanese forested catchment affected by the Fukushima nuclear accident. A total of 13% of the catchment area was clear-cut 2 years after the accident. Annual suspended solids (SS) export at the catchment outlet increased 1.4 to 2.0 times after clearcutting; however, 137Cs export increased slightly (up to 1.1 times), corresponding to 0.21% to 0.30% of the 137Cs inventory in the catchment. The smaller change in 137Cs export than in SS export was due to a rapid decrease in the activity concentration following clearcutting. This decrease was likely caused by both natural attenuation and SS derived from sources with a low activity concentration in the clear-cut area. Monitoring of the sediment transport from hillslopes in small-scale experimental plots showed that the 137Cs yield in the skid trail was 3.6 to 21 times greater than those in clear-cut and unlogged forest floors. This significant 137Cs transport was due to greater soil erosion (by up to two orders of magnitude) along the skid trail, despite the lower activity concentration than those in the other plots. This indicates that while skid trails were involved in the rapid decrease of the activity concentration of SS, they were a potential source of the increased export of 137Cs and SS. Net 137Cs export increased by clearcutting (the export excluding the decrease accompanied by natural attenuation) was estimated to account for only 0.092% of the inventory in the catchment for 2.5 years. These results imply that the impact of clearcutting on 137Cs export was temporary in this catchment.

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 migration in the litter layer of different forest types in Fukushima, Japan

Cesium-137 (¹³⁷Cs) migration in the litter layer consists of various processes, such as input via throughfall, output via litter decomposition, and input from deeper layers via soil organism activity. We conducted litter bag experiments over 2 years (December 2014-November 2016) to quantify the inputs and outputs of ¹³⁷Cs in the litter layer in a Japanese cedar plantation (Cryptomeria japonica) and a mixed broadleaf forest dominated by Quercus serrata located 40 km northwest of the Fukushima Dai-ichi Nuclear Power Plant. The experiments included four conditions, combining contaminated and non-contaminated litter and deeper layer material, and the inputs and outputs were estimated from the combination of ¹³⁷Cs increases and decreases in the litter layer under each condition. The ¹³⁷Cs dynamics differed between the two forests. In the C. japonica forest, some ¹³⁷Cs input via throughfall remained in the litter layer, and downward ¹³⁷Cs flux passed through the litter layer was 0.42 (/year).Upward flux of ¹³⁷Cs from the deeper layer was very restricted, < 0.017 (/year). In the broadleaf forest, migration of ¹³⁷Cs in throughfall into deeper layers was restricted, downward ¹³⁷Cs flux was less than 0.003 (/year).Upward input of ¹³⁷Cs from the deeper layer was prominent, 0.037 (/year). ¹³⁷Cs output via litter decomposition was observed in both forests. The flux in the C. japonica forest was slower than that in the broadleaf forest, 0.12 and 0.15 (/year), respectively.

Meta-analysis of radiocesium contamination data in Japanese forest trees over the period 2011-2013

The fate and dispersion of radiocesium in forests affected by the Fukushima atmospheric fallouts have been efficiently characterized by Japanese scientists thanks to monitoring surveys of radioactive contents in contaminated soil, water, and vegetation samples at numerous sites. In this paper, we carry out a meta-analysis of the field surveys conducted over the period 2011-2013 in evergreen coniferous and deciduous broadleaf forests of Fukushima or neighboring prefectures. The review focuses on contamination data acquired in tree vegetation - about 1500 spatio-temporal measurements of concentrations, inventories and depuration fluxes - with a particular interest for organs that were directly exposed to the atmospheric fallouts and subjected to depuration mechanisms (foliage, branches and outer bark). To reduce the spatial variability between the sites, radioactive data were normalized by the total deposit estimated at each site. Our analysis highlights the overall consistency of field observations despite the variety of experimental protocols, disparate sampling periods, differences in the forest stand characteristics and variability of the atmospheric deposition conditions. Assuming that the sites conformed to the same dynamics (within the range of residual variability), we then derive, discuss, and compare the mean representative evolutions of radiocesium contamination in the two categories of forest. Thanks to a simple mass balance approach, we finally demonstrate that: (i) about 90% of the radiocesium deposit was intercepted by evergreen coniferous vegetation, (ii) 80% of the deposit was gradually transferred to the forest floor in 3years, according to a well characterized depuration kinetics, and (iii) about 4% was readily absorbed by the foliage and translocated to internal organs (inner bark, stem wood and roots) at a rate of about 10^-4d^-1.

Temporal changes in the radiocesium distribution in forests over the five years after the Fukushima Daiichi Nuclear Power Plant accident

To elucidate the temporal changes in the radiocesium distribution in forests contaminated by the Fukushima Daiichi Nuclear Power Plant accident, we monitored the ¹³⁷Cs concentration and inventory within forests from 2011 to 2015 across nine plots containing variable tree species and different contamination levels. The ¹³⁷Cs concentrations in needles and branches decreased exponentially at all coniferous plots, with effective ecological half-lives of 0.45–1.55 yr for needles and 0.83–1.69 yr for branches. By contrast, the ¹³⁷Cs concentration in deciduous konara oak leaves did not change over the five years. The concentration of ¹³⁷Cs in oak wood increased by 37–75%, whereas that in Japanese red pine decreased by 63% over the five years. In Japanese cedar and hinoki cypress, the ¹³⁷Cs concentration in wood showed an increasing trend in half of the plots. The changes in ¹³⁷Cs in the organic and mineral soil layers were not strongly related to the tree species or contamination level. Our multi-site, multi-species monitoring results revealed that the pattern of temporal changes in radiocesium in the 9 forest plots was similar overall; however, changes in ¹³⁷Cs in needles/leaves and wood differed among tree species.

Radiocaesium partitioning in Japanese cedar forests following the "early" phase of Fukushima fallout redistribution

Our study focused on radiocaesium (¹³⁷Cs) partitioning in forests, three vegetation periods after the Fukushima Daiichi nuclear power plant accident. ¹³⁷Cs distribution in forest components (organic and mineral soil layers as well as tree compartments: stem, bark, needles, branches and roots) was measured for two Japanese cedar stand ages (17 and 33 years old). The results showed that around 85% of the initial deposit was found in the forest floor and topsoil. For the youngest stand almost 70% of the deposit is present in the forest floor, whereas for the oldest stand 50% is present in the 0-3 cm mineral soil layer. For trees, old and perennial organs (including dead and living needles and branches, litter fall and outer bark) directly exposed to the fallout remained the most contaminated. The crown concentrated 61-69% of the total tree contamination. Surprisingly the dead organs concentrated 25 ± 9% (young cedars) to 36 ± 20% (mature cedar) of the trees' residual activity, highlighting the importance of that specific compartment in the early post-accident phase for Japanese cedar forests. Although the stem (including bark) represents the highest biomass pool, it only concentrates 3.3% and 4.6% of the initial ¹³⁷Cs deposit for mature and young cedars, respectively.

Radiocesium contamination in living and dead foliar parts of Japanese cedar during 2011-2015

Radiocesium (¹³⁷Cs) activity concentrations, mainly derived from the Fukushima accident of March 2011, were measured in green foliar parts without separation by age (bulk green foliar parts; GL) and litterfall (LF) of Japanese cedar (Cryptomeria japonica) from 2011 to 2015. In all samples, ¹³⁷Cs concentrations decreased exponentially over time, but were always higher in LF (7.36-0.58 Bq g-DW^-1) than in GL (2.10-0.06 Bq g-DW^-1). The difference in the decreasing rate between GL and LF would reflect a difference in the dominant factor of the decrease between living and dead tissues (i.e., internal translocation and weathering, respectively). Over this same timeframe, potassium (K) concentrations in both GL and LF experienced repetitive periodical changes within a certain range (0.38-3.0 mg g-DW^-1 for LF and 2.08-4.77 mg g-DW^-1 for GL, respectively). Thus, there was no specific correlation between ¹³⁷Cs and K concentrations in LF and GL. However, analyses of the age classified green foliar parts (GL-S) and dead foliar parts still retained on trees (DL) could indicate another view. The annual changes in residual rates of both ¹³⁷Cs and K concentrations in GL-S demonstrated very similar two-phase reductions (i.e., a faster reduction in each expansion year than in the following years) and an obvious linear correlation between each other. Radiocesium concentration in DL were always higher than in any part of GL-S sampled at the same timing, but K concentrations showed the reverse relation. It is probable that ¹³⁷Cs is basically translocated from older parts to the developing parts (as long as the former are alive) via a seasonal nutritional flow of K; however, a part of ¹³⁷Cs translocation would cease considerably earlier than the cessation of K translocation.

Characteristics of initial deposition and behavior of radiocesium in forest ecosystems of different locations and species affected by the Fukushima Daiichi Nuclear Power Plant accident

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, information about stand-level spatial patterns of radiocesium initially deposited in the surrounding forests was essential for predicting the future dynamics of radiocesium and suggesting a management plan for contaminated forests. In the first summer (approximately 6 months after the accident), we separately estimated the amounts of radiocesium ((134)Cs and (137)Cs; Bq m(-2)) in the major components (trees, organic layers, and soils) in forests of three sites with different contamination levels. For a Japanese cedar (Cryptomeria japonica) forest studied at each of the three sites, the radiocesium concentration greatly differed among the components, with the needle and organic layer having the highest concentrations. For these cedar forests, the proportion of the (137)Cs stock in the aboveground tree biomass varied from 22% to 44% of the total (137)Cs stock; it was 44% in highly contaminated sites (7.0 × 10(5) Bq m(-2)) but reduced to 22% in less contaminated sites (1.1 × 10(4) Bq m(-2)). In the intermediate contaminated site (5.0-5.8 × 10(4) Bq m(-2)), 34% of radiocesium was observed in the aboveground tree biomass of the Japanese cedar stand. However, this proportion was considerably smaller (18-19%) in the nearby mixed forests of the Japanese red pine (Pinus densiflora) and deciduous broad-leaved trees. Non-negligible amounts of (134)Cs and (137)Cs were detected in both the sapwood and heartwood of all the studied tree species. This finding suggested that the uptake or translocation of radiocesium had already started within 6 months after the accident. The belowground compartments were mostly present in the organic layer and the uppermost (0-5 cm deep) mineral soil layer at all the study sites. We discussed the initial transfer process of radiocesium deposited in the forest and inferred that the type of initial deposition (i.e., dry versus wet radiocesium deposition), the amount of rainfall after the accident, and the leaf biomass by the tree species may influence differences in the spatial pattern of radiocesium by study plots. The results of the present study and further studies of the spatial pattern of radiocesium are important for modeling future radiocesium distribution in contaminated forest ecosystems.

Experimental quantification of radiocesium recycling in a coniferous tree after aerial contamination: Field loss dynamics, translocation and final partitioning

After foliar interception of radioactive atmospheric fallout by forest trees, the short-term recycling dynamics of radiocesium from the tree to the soil as well as within the tree is a primary area of uncertainty in the modeling of the overall cycle. The partitioning of radiocesium transfers in a spruce tree exposed to aerial deposits was investigated during one growth season to reveal the dynamics and significance of underlying processes. The rate of radiocesium loss resulting from foliage leaching (wash-off) was shown to have a functional dependence on the frequency of rainy episodes in a first early stage (weathering 60% of initial contamination during 70 days) and on the amount of precipitation in a second stage (weathering 10% of initial deposits during the following 80 days). A classical single exponential decay model with offset and continuous time as predictor lead to a removal half-life t1/2 of intercepted radiocesium of 25 days. During the growth season, the similar pattern of the internal (134)Cs content in new shoots and initially contaminated foliage confirmed that radiocesium was readily absorbed from needle surfaces and efficiently translocated to growing organs. In the crown, a pool of non-leachable (134)Cs (15-30%) was associated with the abiotic layer covering the twigs and needle surfaces. At the end of the growth season, 30% of the initial deposits were relocated to different tree parts, including organs like stemwood (5%) and roots (6%) not directly exposed to deposition. At the scale of the tree, 84% of the residual activity was assimilated by living tissues which corresponds to a foliar absorption rate coefficient of 0.25 year(-1) for modeling purposes. According to the significant amount of radiocesium which can be incorporated in tree through foliar uptake, our results support the hypothesis that further internal transfers could supply the tree internal cycle of radiocesium extensively, and possibly mask the contribution of root uptake for a long time.

Carbon, cesium and iodine isotopes in Japanese cedar leaves from Iwaki, Fukushima

Japanese cedar leaves from Iwaki, Fukushima were analyzed for carbon, cesium and iodine isotopic compositions before and after the 2011 nuclear accident. The Δ14C values reflect ambient atmospheric 14C concentrations during the year the leaves were sampled/defoliated, and also previous year(s). The elevated 129I and 134,137Cs concentrations are attributed to direct exposure to the radioactive fallout for the pre-fallout-expended leaves and to internal translocation from older parts of the tree for post-fallout-expended leaves. 134Cs/137Cs and 129I/137Cs activity ratios suggest insignificant isotopic and elemental fractionation during translocation. However, fractionation between radioiodine and radiocesium is significant during transportation from the source.

Radiocesium transfer from hillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review

The devastating tsunami triggered by the Great East Japan Earthquake on March 11, 2011 inundated the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) resulting in a loss of cooling and a series of explosions releasing the largest quantity of radioactive material into the atmosphere since the Chernobyl nuclear accident. Although 80% of the radionuclides from this accidental release were transported over the Pacific Ocean, 20% were deposited over Japanese coastal catchments that are subject to frequent typhoons. Among the radioisotopes released during the FDNPP accident, radiocesium ((134)Cs and (137)Cs) is considered the most serious current and future health risk for the local population. The goal of this review is to synthesize research relevant to the transfer of FDNPP derived radiocesium from hillslopes to the Pacific Ocean. After radiocesium fallout deposition on vegetation and soils, the contamination may remain stored in forest canopies, in vegetative litter on the ground, or in the soil. Once radiocesium contacts soil, it is quickly and almost irreversibly bound to fine soil particles. The kinetic energy of raindrops instigates the displacement of soil particles, and their bound radiocesium, which may be mobilized and transported with overland flow. Soil erosion is one of the main processes transferring particle-bound radiocesium from hillslopes through rivers and streams, and ultimately to the Pacific Ocean. Accordingly this review will summarize results regarding the fundamental processes and dynamics that govern radiocesium transfer from hillslopes to the Pacific Ocean published in the literature within the first four years after the FDNPP accident. The majority of radiocesium is reported to be transported in the particulate fraction, attached to fine particles. The contribution of the dissolved fraction to radiocesium migration is only relevant in base flows and is hypothesized to decline over time. Owing to the hydro-meteorological context of the Fukushima region, the most significant transfer of particulate-bound radiocesium occurs during major rainfall and runoff events (e.g. typhoons and spring snowmelt). There may be radiocesium storage within catchments in forests, floodplains and even within hillslopes that may be remobilized and contaminate downstream areas, even areas that did not receive fallout or may have been decontaminated. Overall this review demonstrates that characterizing the different mechanisms and factors driving radiocesium transfer is important. In particular, the review determined that quantifying the remaining catchment radiocesium inventory allows for a relative comparison of radiocesium transfer research from hillslope to catchment scales. Further, owing to the variety of mechanisms and factors, a transdisciplinary approach is required involving geomorphologists, hydrologists, soil and forestry scientists, and mathematical modellers to comprehensively quantify radiocesium transfers and dynamics. Characterizing radiocesium transfers from hillslopes to the Pacific Ocean is necessary for ongoing decontamination and management interventions with the objective of reducing the gamma radiation exposure to the local population.

Export of radioactive cesium from agricultural fields under simulated rainfall in Fukushima

In this study, we investigated the impact of rainfall on runoff, soil erosion and consequently on the discharge of radioactive cesium in agricultural fields in Fukushima prefecture using a rainfall simulator. Simulated heavy rainfalls (50 mm h(-1)) generated significant runoff and soil erosion. The average concentration of radioactive cesium (the sum of (134)Cs and (137)Cs) in the runoff sediments was ∼3500 Bq kg(-1) dry soil, more than double the concentrations measured in the field soils which should be considered in studies using the (137)Cs loss to estimate long-term soil erosion. However, the estimated mass of cesium discharged through one runoff event was less than 2% of the cesium inventory in the field. This suggested that cesium discharge via soil erosion is not a significant factor in reducing the radioactivity of contaminated soils in Fukushima prefecture. However, the eroded sediment carrying radioactive cesium will deposit into the river systems and potentially pose a radioactivity risk for aquatic living organisms.

Analysis of Japanese radionuclide monitoring data of food before and after the Fukushima nuclear accident

In an unprecedented food monitoring campaign for radionuclides, the Japanese government took action to secure food safety after the Fukushima nuclear accident (Mar. 11, 2011). In this work we analyze a part of the immense data set, in particular radiocesium contaminations in food from the first year after the accident. Activity concentrations in vegetables peaked immediately after the campaign had commenced, but they decreased quickly, so that by early summer 2011 only a few samples exceeded the regulatory limits. Later, accumulating mushrooms and dried produce led to several exceedances of the limits again. Monitoring of meat started with significant delay, especially outside Fukushima prefecture. After a buildup period, contamination levels of meat peaked by July 2011 (beef). Levels then decreased quickly, but peaked again in September 2011, which was primarily due to boar meat (a known accumulator of radiocesium). Tap water was less contaminated; any restrictions for tap water were canceled by April 1, 2011. Pre-Fukushima (137)Cs and (90)Sr levels (resulting from atmospheric nuclear explosions) in food were typically lower than 0.5 Bq/kg, whereby meat was typically higher in (137)Cs and vegetarian produce was usually higher in (90)Sr. The correlation of background radiostrontium and radiocesium indicated that the regulatory assumption after the Fukushima accident of a maximum activity of (90)Sr being 10% of the respective (137)Cs concentrations may soon be at risk, as the (90)Sr/(137)Cs ratio increases with time. This should be taken into account for the current Japanese food policy as the current regulation will soon underestimate the (90)Sr content of Japanese foods.

Uptake and translocation of radiocesium in cedar leaves following the Fukushima nuclear accident

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

Changes in radiocesium contamination from Fukushima in foliar parts of 10 common tree species in Japan between 2011 and 2013

Yearly changes in radiocesium ((137)Cs) contamination, primarily due to the Fukushima accident of March 2011, were observed in the foliar parts of 10 common woody species in Japan (Chamaecyparis obtusa, Cedrus deodara, Pinus densiflora, Cryptomeria japonica, Phyllostachys pubescens, Cinnamomum camphora, Metasequoia glyptostroboides, Prunus × yedoensis, Acer buergerianum, and Aesculus hippocastanum). The samples were obtained from Abiko (approximately 200 km SSW of the Fukushima Dai-ichi Nuclear Power Plant) during each growing season between 2011 and 2013, and the foliar parts were examined based on their year of expansion and location in each trees. The radiocesium concentrations generally decreased with time; however, the concentrations and rates of decrease varied among species, age of foliar parts, and locations. The radiocesium concentrations in the 2012 current-year foliar parts were 29%-220% of those from 2011, while those from 2013 fell to between 14% and 42% of the 2011 values. The net decontamination in the foliage was higher in evergreen species than in deciduous species. The radiocesium concentrations in the upper foliar parts were higher than those in the lower parts particularly in C. japonica. In addition, the radiocesium concentrations were higher in the current-year foliar parts than in the 1-year-old foliar parts, particularly in 2013. Thus, the influence of the direct deposition of the fallout was reduced with time, and the translocation ability of radiocesium from old to new tissues became more influential. Similar to the behavior of potassium in trees, Cs redistribution probably occurred primarily due to internal nutrient translocation mechanisms.

Removal efficiency of radioactive cesium and iodine ions by a flow-type apparatus designed for electrochemically reduced water production

The Fukushima Daiichi Nuclear Power Plant accident on March 11, 2011 attracted people’s attention, with anxiety over possible radiation hazards. Immediate and long-term concerns are around protection from external and internal exposure by the liberated radionuclides. In particular, residents living in the affected regions are most concerned about ingesting contaminated foodstuffs, including drinking water. Efficient removal of radionuclides from rainwater and drinking water has been reported using several pot-type filtration devices. A currently used flow-type test apparatus is expected to simultaneously provide radionuclide elimination prior to ingestion and protection from internal exposure by accidental ingestion of radionuclides through the use of a micro-carbon carboxymethyl cartridge unit and an electrochemically reduced water production unit, respectively. However, the removability of radionuclides from contaminated tap water has not been tested to date. Thus, the current research was undertaken to assess the capability of the apparatus to remove radionuclides from artificially contaminated tap water. The results presented here demonstrate that the apparatus can reduce radioactivity levels to below the detection limit in applied tap water containing either 300 Bq/kg of ¹³⁷Cs or 150 Bq/kg of ¹²⁵I. The apparatus had a removal efficiency of over 90% for all concentration ranges of radio–cesium and –iodine tested. The results showing efficient radionuclide removability, together with previous studies on molecular hydrogen and platinum nanoparticles as reactive oxygen species scavengers, strongly suggest that the test apparatus has the potential to offer maximum safety against radionuclide-contaminated foodstuffs, including drinking water.

Radiation dose rates now and in the future for residents neighboring restricted areas of the Fukushima Daiichi Nuclear Power Plant

Radiation dose rates were evaluated in three areas neighboring a restricted area within a 20- to 50-km radius of the Fukushima Daiichi Nuclear Power Plant in August-September 2012 and projected to 2022 and 2062. Study participants wore personal dosimeters measuring external dose equivalents, almost entirely from deposited radionuclides (groundshine). External dose rate equivalents owing to the accident averaged 1.03, 2.75, and 1.66 mSv/y in the village of Kawauchi, the Tamano area of Soma, and the Haramachi area of Minamisoma, respectively. Internal dose rates estimated from dietary intake of radiocesium averaged 0.0058, 0.019, and 0.0088 mSv/y in Kawauchi, Tamano, and Haramachi, respectively. Dose rates from inhalation of resuspended radiocesium were lower than 0.001 mSv/y. In 2012, the average annual doses from radiocesium were close to the average background radiation exposure (2 mSv/y) in Japan. Accounting only for the physical decay of radiocesium, mean annual dose rates in 2022 were estimated as 0.31, 0.87, and 0.53 mSv/y in Kawauchi, Tamano, and Haramachi, respectively. The simple and conservative estimates are comparable with variations in the background dose, and unlikely to exceed the ordinary permissible dose rate (1 mSv/y) for the majority of the Fukushima population. Health risk assessment indicates that post-2012 doses will increase lifetime solid cancer, leukemia, and breast cancer incidences by 1.06%, 0.03% and 0.28% respectively, in Tamano. This assessment was derived from short-term observation with uncertainties and did not evaluate the first-year dose and radioiodine exposure. Nevertheless, this estimate provides perspective on the long-term radiation exposure levels in the three regions.