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Dissolved radiocesium (mainly 134 Cs and 137 Cs) is thought to be leached mainly from suspended sediment in downstream rivers, while organic matter, such as leaf litter, contributes to catchments in forested headwater streams. It is also known that dissolved 137 Cs in headwater streams exhibit seasonal variation with water temperature. Some mechanisms have been proposed as the causes: ionic competition of potassium ion (K + ) and ammonium ion (NH 4 + ), leaching associated with the decomposition of organic matter, and thermodynamic adsorption-desorption processes. We investigated the relationship between K + and the dissolved organic carbon (DOC) concentrations and seasonal changes in dissolved 137 Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in 137 Cs concentrations in coarse organic matter, suspended sediments, and dissolved forms at four sites (one decontaminated site and three undecontaminated sites) in the Yamakiya area since 2011. The distribution coefficients (Kdcss and Kdorg) of dissolved 137 Cs concentrations relative to suspended sediment and coarse organic matter 137 Cs concentrations were calculated, and differences in temporal changes due to decontamination were investigated. In addition, we examined the relationship between water temperature and DOC, K + , NH 4 + and 137 Cs concentrations in the headwater catchments. The suspended sediment 137 Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved 137 Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH 4 + has been detected in headwater streams in our catchments. In the SET and ISH watersheds, where the distance from the groundwater spring is short, a correlation was found between DOC concentration and dissolved 137 Cs concentration. In contrast, in the IBO watershed, where the distance from the groundwater spring is long, temperature dependence and a good correlation between K + and 137 Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved 137 Cs in forested headwater streams at short distances from the spring, but the influence of competing ions is expected to increase gradually as the water flows downstream., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yuichi Onda reports financial support and travel were provided by University of Tsukuba. Yuichi Onda reports a relationship with University of Tsukuba that includes: employment., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Radionuclides released and deposited because of the 2011 Fukushima Dai-ichi Nuclear Power Plant accident caused an increase in air dose rates in Fukushima Prefecture forests. Although an increase in air dose rates during rainfall was previously reported, the air dose rates in the Fukushima forests decreased during rainfall. This study aimed to develop a method to estimate rainfall-related changes in air dose rates, even in the absence of soil moisture data, in Namie-Town and Kawauchi-Village, Futaba-gun, Fukushima Prefecture. Moreover, we examined the relationship between preceding rainfall (Rw) and soil moisture content. The air dose rate was estimated by calculating the Rw in Namie-Town from May to July 2020. We found that the air dose rates decreased with increasing soil moisture content. The soil moisture content was estimated from Rw by combining short-term and long-term effective rainfall using half-live values of 2 h and 7 d and considering the hysteresis of water absorption and drainage processes. Furthermore, the soil moisture content and air dose rate estimations showed a good agreement with coefficient of determination (R 2 ) scores >0.70 and >0.65, respectively. The same method was tested to estimate the air dose rates in Kawauchi-Village from May to July 2019. At the Kawauchi site, variation in estimated value is relatively large due to the presence of water repellency in dry conditions, and the amount of 137 Cs inventory was low, so estimating air dose from rainfall remained a challenge. In conclusion, rainfall data were successfully used to estimate soil moisture and air dose rates in areas with high 137 Cs inventories. This leads to the possibility of removing the influence of rainfall on measured air dose rate data and could contribute to the improvement of methods currently used to estimate the external air dose rates for humans, animals, and terrestrial forest plants., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Yuichi Onda reports financial support was provided by University of Tsukuba., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The March 2011 Fukushima Dai-ichi Nuclear Power Plant accident in Japan released 520 PBq of radionuclides compared to a total release of 5300 PBq from the Chornobyl Nuclear Power Plant accident. Both nuclear accidents resulted in deposition of radiocesium throughout the northern hemisphere, and a plethora of studies have been performed regarding radiocesium ( 137 Cs) behavior. However, few studies have assessed the impact of precipitation on 137 Cs deposition in forests. Wide-scale environmental measurements from 2011 and 2016 were used to determine the differences in 137 Cs deposition because of precipitation following the Fukushima accident. In areas where wet deposition processes were dominant, dense forests generally had lower ambient dose rates and levels of contamination on forest floors than other stands with fewer stems per hectare in 2011. Similar tendencies were not observed in areas that were primarily subject to dry deposition nor were any trends observed in 2016. 137 Cs was retained in dense forest canopies for an extended period regardless of the deposition mode. Additionally, it was found that the initial retention of radionuclides by forest canopies is in general higher for areas with predominantly dry deposition. Incorporation of radiocesium into wood tissues was the same for both wet and dry deposition.

This study investigated the spatial distribution of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident in a densely planted Japanese cedar stand. Systematic grid sampling was conducted to determine 137 Cs inventories in the layers of deposited organic material and mineral soil at two different spatial scales (hillslope [60 m 2 ] and small [1 m 2 ]). The results showed that 137 Cs inventories along the hillslope were heterogeneously distributed, with coefficients of variation for the deposited organic material and mineral soil layers of 46.4% and 48.9%, respectively. The 137 Cs inventory in each layer tended to show a lognormal distribution. The correlation between the 137 Cs inventories in deposited organic material and mineral soil in the same sampling grid was weak. The controlling mechanisms of the 137 Cs inventories in the litter and mineral soil layers differed due to differences in the underlying key processes, such as canopy-forest floor transfer due to hydrological and biological processes. No significant correlation was found between the distance from the nearest tree trunk and the 137 Cs inventory in the deposited organic layer at each sampling point. In contrast, the 137 Cs inventory in the soil tended to increase as the distance from the nearest tree trunk increased at both the hillslope and small scales. It was found that the initial spatial patterns of 137 Cs in the soil layer due to atmospheric deposition were preserved in the cedar stand. Finally, we tested the effects of soil sampling density on the reliability of mean soil 137 Cs inventory estimations in the cedar stand. The results indicated that a soil sampling area greater than 0.06 m 2  at the hillslope scale and 0.008 m 2  at the small scale enabled the mean 137 Cs inventory to be estimated with an uncertainty of less than 20% in the cedar stand., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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 137 Cs concentration varied significantly among sampling periods and between the two forests, with the oak stand exhibiting higher 137 Cs concentrations and depositional fluxes than the cedar stand. Expressed per unit trunk basal area, the depositional flux of 137 Cs 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The deposited 137 Cs is one of the long-lived radionuclides, that was released following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, has been hydrologically transported as particulates in the terrestrial environment of the Fukushima region. The impact of freeze-thaw processes and subsequent runoff affecting the 137 Cs flux and concentration in sediment discharge were revealed in bare land erosion plot following the FDNPP accident by detailed monitoring and laser scanner measurement on the soil surface. We found that surface topographic changes due to the frost-heaving during the winter-spring period, and rill formation during the summer. We also found the evident seasonal changes in 137 Cs concentration; high during the early spring and gradually decreased thereafter, then surface runoff from the plot frequently occurred during spring and autumn when rainfall was high and reached a maximum in summer. From these results, the higher 137 Cs concentration in spring was caused by a mixture of unstable surface sediment following freeze-thaw processes and then transported in the early spring, but erosion amount is not significant because of the less rainfall event. The sediment with a lower 137 Cs concentration, which was supplied from the rill erosion and its expansion, was wash-offed during the summer, contributing most of the flux from erosion in bare land in Fukushima region. In case, heavy rainfall occurs in the early spring, caution is required because high concentrations of cesium may flow down into the river., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F l/t ), the migration center (X c ) and the relaxation depth (α) using 99 soil inventory data sets. F l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high 137 Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of 137 Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of 137 Cs from organic to surface mineral soil layer after the second year. These results indicate that 137 Cs retention capacity of the organic layer can affect the apparent vertical migration of 137 Cs in the underlying mineral soil layer., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The study investigated temporal changes in the 137 Cs 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 137 Cs concentrations in throughfall samples exhibited a double-exponential decreasing trend with time. Temporal changes in the 137 Cs 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 137 Cs concentrations varied between foliage and the outer barks of the Japanese cedar and Japanese konara oak trees. The 137 Cs concentration in cedar needles decreased exponentially while that in konara oak leaves was constant over the last six years. Conversely, the declining trend of 137 Cs concentration in the outer bark of konara oak exceeded that of cedar. The results suggested that self-decontamination processes and internal recycling of 137 Cs varied among tree species and different tree parts. The results indicated that the leaching of 137 Cs in the throughfall in Japanese cedar was dependent on the 137 Cs concentration in needles. However, a comparison of 137 Cs concentrations in vegetal and hydrological samples from each sampling year showed that the leaching rate decreased with time. Conversely, the 137 Cs concentrations in the stemflow were independent of the concentrations in the outer bark. The declining trend of 137 Cs 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 137 Cs 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 137 Cs concentration in the aforementioned types of hydrological components slightly exceeded that for the vegetal samples. The results suggest that monitoring of 137 Cs concentrations in hydrological components and vegetal samples can aid in further understanding the leaching mechanisms of 137 Cs from trees to rainwater., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Understanding solid 137 Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid 137 Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of 137 Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in 137 Cs activity concentration in sediment and solid wash-off processes of 137 Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized 137 Cs solid entrainment coefficient, defined as Sc (m 2 kg -1 ), was calculated by dividing 137 Cs activity concentration in sediment by initial 137 Cs deposition. A particle size-corrected Sc, defined as Sc correct (m 2 kg -1 ), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sc correct, ranged from 0.0072 to 0.084 m 2  kg -1 and 0.0052-0.078 m 2  kg -1 , respectively. Annual wash-off rates of solid 137 Cs were 0.0029-12% year -1 . There was no significant decreasing trend in Sc or Sc correct on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sc correct significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in 137 Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Ascertaining the initial amount of accidently released radiocesium is fundamental for determining the extent of radioactive contamination following nuclear accidents, and is of key importance to environmental transfer models. A series of the airborne monitoring surveys of radioactivity have conducted by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), and provide basic information on radioactive contamination following the accident. However, there are no clear guidelines regarding the selection of airborne monitoring survey results for estimating the initial fallout input in studies of the environmental transfer of radiocesium. This study reconstructed a fallout map of Fukushima accident-derived radiocesium based on a comparison of the radiocesium deposition densities (D l ) derived from the third and fifth airborne monitoring surveys. The D l derived from the fifth airborne monitoring survey were adjusted for variation in the measured radioactivity associated with the influence of radioactive decay, natural weathering processes, variation in the calibration procedure, and other, undefined mechanisms. The calibrated deposition density of the fifth airborne monitoring survey for each land use type (A' 5th*l ) were used to establish the initial fallout map in the East Japan area. Furthermore, the airborne monitoring surveys which were independently conducted in each prefecture area were used to complement the lack of data in the South Kanto region and the mountainous area in the North Kanto region due to snow cover during the measurement period of the fifth airborne monitoring survey. The reconstructed initial fallout map of the Fukushima accident derived 137 Cs was opened to the public via the database of the Center for Research in Isotopes of Environmental Dynamics, University of Tsukuba, Japan (www.ied.tsukuba.ac.jp/∼fukushimafallout/). Finally, the total atmospheric deposition of Fukushima Dai-ichi Nuclear Power Plant accident-derived radiocesium onto each prefecture and land uses was estimated based on the reconstructed map in this study., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011-2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014-2016 and 2011-2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10-20% faster than that induced solely by physical decay of radiocesium for the observation period 2011-2016., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011-2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014-2016 and 2011-2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10-20% faster than that induced solely by physical decay of radiocesium for the observation period 2011-2016., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Owing to the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident a vast amount of radiocesium was released polluting the land. Afterwards, a variety of decontamination practices has been done, reducing the ambient dose rates. In this study we evaluated the effectiveness of eight forest decontamination practices by means of monitoring the radiocesium ( 137 Cs) concentration in soil and leaf samples, and the daily discharge rates in ten plots during 27 months (May 2013-July 2015). A forest plantation located 16 km southwest to the FDNPP and within the exclusion area was selected. Radiocesium concentrations were analysed using a germanium gamma ray detector. The differences in radiocesium activities between the different plots were statistically significant (p < 0.05) and four homogeneous groups were distinguished. Tree thinning and litter removal greatly reduced the radioactivity and the two plots devoted to these practices presented the highest discharge rates of 137 Cs (Th + LR; 350-380 Bq/m 2 day), followed by the two Th plots (163-174 Bq/m 2 day). The clearcutting with LR and the LR plots (104 and 92 Bq/m 2 day) also had higher rates than those rates in the control plots (51 Bq/m 2 day). We only observed low rates in the two plots with matting (19-25 Bq/m 2 day). The temporal variability was explained by (i) the different rainfall depths registered during the measurement intervals (accumulated precipitation from 14 to 361 mm); and (ii) the fluctuations of the total surface coverage. The decrease trend in radiocesium concentration was high in 2013, moderate in 2014 and low in 2015 owing to the vegetation recovery after the countermeasures, thus reducing the possibility of the second pollution of the neighbouring areas. The average proportions of contribution of 137 Cs discharge by soil and leaf fraction were 96.6% and 3.4%., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Cesium-137 ( 137 Cs) 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 137 Cs 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 137 Cs increases and decreases in the litter layer under each condition. The 137 Cs dynamics differed between the two forests. In the C. japonica forest, some 137 Cs input via throughfall remained in the litter layer, and downward 137 Cs flux passed through the litter layer was 0.42 (/year).Upward flux of 137 Cs from the deeper layer was very restricted, < 0.017 (/year). In the broadleaf forest, migration of 137 Cs in throughfall into deeper layers was restricted, downward 137 Cs flux was less than 0.003 (/year).Upward input of 137 Cs from the deeper layer was prominent, 0.037 (/year). 137 Cs 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., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

An experiment has been conducted to evaluate the additional dose reduction by clear felling contaminated forestry in Fukushima Prefecture, Japan, and using the timber to cover the areas with wood chips. A portable gamma spectrometry system, comprising a backpack containing a 3 × 3″ NaI(Tl) detector with digital spectrometer and GPS receiver, has been used to map dose rate and radionuclide activity concentrations before, after and at stages during this experiment. The data show the effect of the different stages of the experiment on dose rate at different locations around the site. The spectrometric data have allowed the assessment of the contributions of natural and anthropogenic radionuclides to the dose rate at different parts of the site before and after the experiment. This has clearly demonstrated the value of radiometric methods in evaluating remediation, and the effect of other environmental processes. The value of spectrometric methods which directly measure radionuclide concentrations has also been shown, especially through the identification of the contribution of natural and anthropogenic activity to the measured dose rate. The experiment has shown that clearing trees and applying wood chips can reduce dose rates by 10-15% beyond that achieved by just clearing the forest litter and natural redistribution of radiocaesium., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Radiocaesium wash-off associated with soil erosion in different land use was monitored using USLE plots in Kawamata, Fukushima Prefecture, Japan after the Fukushima Dai-ichi Nuclear Power Plant accident. Parameters and factors relating to soil erosion and (137)Cs concentration in the eroded soil were evaluated based on the field monitoring and presented. The erosion of fine soil, which is defined as the fraction of soil overflowed along with discharged water from a sediment-trap tank, constituted a large proportion of the discharged radiocaesium. This indicated that the quantitative monitoring of fine soil erosion is greatly important for the accurate evaluation of radiocaesium wash-off. An exponential relationship was found between vegetation cover and the amount of eroded soil. Moreover, the radiocaesium concentrations in the discharged soil were greatly affected by the land use. These results indicate that radiocaesium wash-off related to vegetation cover and land use is crucially important in modelling radiocaesium migration., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in extensive radioactive contamination of the environment via deposited radionuclides such as radiocesium and (131)I. Evaluating the extent and level of environmental contamination is critical to protecting citizens in affected areas and to planning decontamination efforts. However, a standardized soil sampling protocol is needed in such emergencies to facilitate the collection of large, tractable samples for measuring gamma-emitting radionuclides. In this study, we developed an emergency soil sampling protocol based on preliminary sampling from the FDNPP accident-affected area. We also present the results of a preliminary experiment aimed to evaluate the influence of various procedures (e.g., mixing, number of samples) on measured radioactivity. Results show that sample mixing strongly affects measured radioactivity in soil samples. Furthermore, for homogenization, shaking the plastic sample container at least 150 times or disaggregating soil by hand-rolling in a disposable plastic bag is required. Finally, we determined that five soil samples within a 3 m × 3-m area are the minimum number required for reducing measurement uncertainty in the emergency soil sampling protocol proposed here., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

To explore the behavior of radionuclides released after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011, and the distribution of radiocesium in paddy fields, we monitored radiocesium (Cs) and suspended sediment (SS) discharge from paddy fields. We proposed a rating scale for measuring the effectiveness of surface soil removal. Our experimental plots in paddy fields were located ∼40 km from the FDNPP. Two plots were established: one in a paddy field where surface soil was not removed (the "normally cultivated paddy field") and the second in a paddy field where the top 5-10 cm of soil was removed before cultivation (the "surface-removed paddy field"). The amounts of Cs and SS discharge from the paddy fields were continuously measured from June to August 2011. The Cs soil inventory measured 3 months after the FDNPP accident was approximately 200 kBq m(-2). However, after removing the surface soil, the concentration of Cs-137 decreased to 5 kBq m(-2). SS discharged from the normally cultivated and surface-removed paddy fields after puddling (mixing of soil and water before planting rice) was 11.0 kg and 3.1 kg, respectively, and Cs-137 discharge was 630,000 Bq (1240 Bq m(-2)) and 24,800 Bq (47.8 Bq m(-2)), respectively. The total amount of SS discharge after irrigation (natural rainfall-runoff) was 5.5 kg for the normally cultivated field and 70 kg for the surface-removed field, and the total amounts of Cs-137 discharge were 51,900 Bq (102 Bq m(-2)) and 165,000 Bq (317 Bq m(-2)), respectively. During the irrigation period, discharge from the surface-removed plot showed a twofold greater inflow than that from the normally cultivated plot. Thus, Cs inflow may originate from the upper canal. The topsoil removal process eliminated at least approximately 95% of the Cs-137, but upstream water contaminated with Cs-137 flowed into the paddy field. Therefore, to accurately determine the Cs discharge, it is important to examine Cs inflow from the upper channel. Furthermore, puddling and irrigation processes inhibit the discharge of radiocesium downstream. This indicates that water control in paddy fields is an important process in the prevention of river pollution and radionuclide transfer.

The Fukushima Dai-ichi nuclear accident led to massive atmospheric deposition of radioactive substances onto the land surfaces. The spatial distribution of deposits has been estimated by Japanese authorities for gamma-emitting radionuclides through either airborne monitoring surveys (since April 2011) or in situ gamma-ray spectrometry of bare soil areas (since summer 2011). We demonstrate that significant differences exist between the two surveys for radiocaesium isotopes and that these differences can be related to dry deposits through the use of physically based relationships involving aerosol deposition velocities. The methodology, which has been applied to cesium-134 and cesium-137 deposits within 80-km of the nuclear site, provides reasonable spatial estimations of dry and wet deposits that are discussed and compared to atmospheric numerical simulations from the Japanese Atomic Energy Agency and the French Institute of Radioprotection and Nuclear Safety. As a complementary approach to numerical simulations, this field-based analysis has the possibility to contribute information that can be applied to the understanding and assessment of dose impacts to human populations and the environment around Fukushima.

The Fukushima accident dispersed significant amounts of radioactive cesium (Cs) in the landscape. Our research investigated, from June 2011 to November 2013, the mobility of leachable Cs in forests canopies. In particular, (137)Cs and (134)Cs activity concentrations were measured in rainfall, throughfall, and stemflow in broad-leaf and cedar forests in an area located 40 km from the power plant. Leachable (137)Cs loss was modeled by a double exponential (DE) model. This model could not reproduce the variation in activity concentration observed. In order to refine the DE model, the main physical measurable parameters (rainfall intensity, wind velocity, and snowfall occurrence) were assessed, and rainfall was identified as the dominant factor controlling observed variation. A corrective factor was then developed to incorporate rainfall intensity in an improved DE model. With the original DE model, we estimated total (137)Cs loss by leaching from canopies to be 72 ± 4%, 67 ± 4%, and 48 ± 2% of the total plume deposition under mature cedar, young cedar, and broad-leaf forests, respectively. In contrast, with the improved DE model, the total (137)Cs loss by leaching was estimated to be 34 ± 2%, 34 ± 2%, and 16 ± 1% of the total plume deposition under mature cedar, young cedar, and broad-leaf forests, respectively. The improved DE model corresponds better to observed data in literature. Understanding (137)Cs and (134)Cs forest dynamics is important for forecasting future contamination of forest soils around the FDNPP. It also provides a basis for understanding forest transfers in future potential nuclear disasters., (Copyright © 2014 Elsevier B.V. All rights reserved.)

The deposition of Fukushima-derived radiocesium via falling litter in a coniferous forest 180 km downwind immediately following the nuclear power plant accident was investigated. The litterfall contribution to the transfer of radiocesium from the forest canopy to the forest floor was determined, and this pathway was compared with hydrological pathways. The results demonstrated that during the observation period, a total of approximately 5.5 kBq m(-2) of Fukushima-derived radiocesium was deposited on the forest floor through throughfall (53%), stemflow (2.3%) and litterfall (45%) routes. The data revealed that the contributions of hydrological pathways became less important as time passed. However, the litterfall route, which transferred approximately 31% (2.5±0.6 kBq m(-2)) of the local fallout within the observation period, continued depositing radiocesium onto the forest floor., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Soil, vegetation and other ecological compartments are expected to be highly contaminated by the deposited radionuclides after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident triggered by a magnitude 9.0 earthquake and the resulting tsunami on 11 March 2011. However, there is no field measurement data on the depth distributions of radiocaesium and (131)I concentrations in soil profile. In this study, the depth distribution of the deposited radionuclides in the cultivated soil profile was investigated in one of the most contaminated area after FDNPP accident. The result of this study demonstrated that greater than 86% of total radiocaesium and 79% of total (131)I were absorbed in the upper 2.0 cm in the soil profile. The relaxation mass depth (h(0)) derived from the depth distribution of radiocaesium and (131)I in the soil profile at the study site were 9.1 kg m(-2) and 10.4 kg m(-2), respectively. The h(0) of (137)Cs in the studied soil profile was greater than those for the cultivated soils nearby the Chernobyl NPP. The positive relationship was found between clay content of topsoil and the h(0) of (137)Cs. However, further analysis is required to clarify the effect of clay content on the initial penetration depth of deposited (137)Cs in soil profile., (Copyright © 2011 Elsevier Ltd. All rights reserved.)