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Approximately 70 % of the area highly 137 Cs-contaminated by the Fukushima Daiichi Nuclear Power Plant accident is forested. Decontamination works in most of these forests have not progressed, and the forestry industry remains stagnant. Although the long-term dynamics of 137 Cs in the forest ecosystem will be controlled by the amount of 137 Cs absorbed by roots in the future, temporal changes in 137 Cs of tree roots have rarely been reported. In the present study, we monitored the depth distribution of 137 Cs in the soil and absorptive very fine (VF) roots of 0.5 mm or less in a Japanese cedar forest from 2011 to 2023. As a result, the 137 Cs inventory in the mineral soil increased over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots tended to decrease since 2020, although there was a large variation. Temporal decrease in the exchangeable 137 Cs fraction with fixation and temporal increase in VF root biomass with their growth were not clearly observed, the 137 Cs concentration in the VF roots at 0-2 cm decreased with the decrease in 137 Cs concentration in the litter layers. Although the 137 Cs concentration in the VF roots below 2 cm tended to increase with increasing 137 Cs concentration in the soil at the same depth, the downward migration of 137 Cs within the soil can reduce the amount of 137 Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, 137 Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil. This natural migration process can be regarded as a "self-cleaning" of the forest ecosystem, the green and sustainable remediation using such self-cleaning should be actively adopted for the future forest management., 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 © 2024 Elsevier B.V. All rights reserved.)

After the Fukushima Daiichi nuclear power plant accident, the terrestrial environment became severely contaminated with radiocesium. Consequently, the river and lake water in the Fukushima area exhibited high radiocesium levels, which declined subsequently. The partition coefficient of 137 Cs between the suspended sediment (SS) and dissolved phases, K d , was introduced to better understand the dynamic behavior of 137 Cs in different systems. However, the K d values in river water, ranging from 2 × 10 4 to 7 × 10 6  L kg -1 , showed large spatiotemporal variability. Therefore, the factors controlling the 137 Cs partition coefficient in natural water systems should be identified. Herein, we introduce a chemical model to explain the variability in 137 Cs K d in natural water systems. The chemical model includes the complexation of Cs + with mineral and organic binding sites in SS, metal exchange reactions, and the presence of colloidal species. The application of the chemical model to natural water systems revealed that Cs + is strongly associated with binding sites in SS, and a major chemical interaction between 137 Cs and the binding sites in SS is the isotope exchange reaction between stable Cs and 137 Cs, rather than metal exchange reactions with other metal ions such as potassium ions. To explain the effect of the SS concentration on K d , the presence of colloidal 137 Cs passing through a filter is significant as the dominant dissolved species of 137 Cs in river water. These results suggest that a better understanding of stable Cs dissolved in natural water is important for discerning the geochemical and ecological behaviors of 137 Cs in natural water., 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 © 2024 Elsevier Ltd. All rights reserved.)

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.

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 ( 137 Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the 137 Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The 137 Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating 137 Cs migration. However, the rate of decrease subsequently slowed due to the 137 Cs supply from the river. The 137 Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate 137 Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in 137 Cs concentrations will be smaller. The 137 Cs concentration in the Koise River will continue to decrease; thus, the difference in the 137 Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of 137 Cs in the entire lake were 3.72 × 10 12 Bq in December 2012 and 4.18 × 10 12 Bq in August 2016. The accumulated amount of 137 Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate 137 Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of 137 Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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.)

An international review of radioecological data derived after the accident at the Fukushima Daiichi nuclear power plant was an important component of activities in working group 4 of the IAEA Models and data for radiological impact assessment, phase II (MODARIA II) programme. Japanese and international scientists reviewed radioecological data in the terrestrial and aquatic environments in Japan reported both before and after the accident. The environmental transfer processes considered included: (a) interception and retention radionuclides by plants, (b) loss of radionuclides from plant and systemic transport of radionuclides in plants (translocation), (c) behaviour of radiocaesium in soil, (d) uptake of radionuclides from soil by agricultural crops and wild plants, (e) transfer of radionuclides from feedstuffs to domestic and wild animals, (f) behaviour of radiocaesium in forest trees and forest systems, (g) behaviour of radiocaesium in freshwater systems, coastal areas and in the ocean, (h) transport of radiocaesium from catchments through rivers, streams and lakes to the ocean, (i) uptake of radiocaesium by aquatic organisms, and (j) modification of radionuclide concentrations in food products during food processing and culinary preparation. These data were compared with relevant global data within IAEA TECDOC-1927 'Environmental transfer of radionuclides in Japan following the accident at the Fukushima Daiichi Nuclear Power Plant'. This paper summarises the outcomes of the data collation and analysis within MODARIA II work group 4 and compares the Japan-specific data with existing radioecological knowledge acquired from past and contemporary radioecological studies. The key radioecological lessons learned are outlined and discussed., (© 2022 Society for Radiological Protection. Published on behalf of SRP by IOP Publishing Limited. 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 vertical distribution of 137 Cs 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 137 Cs 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 137 Cs has been conducted since 2011. As a result, the 137 Cs 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 137 Cs inventory in the litter layer has been exponentially decreasing, the annual 137 Cs 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 137 Cs. Comparing these 137 Cs fluxes with the apparent amounts of downward migration of 137 Cs estimated from the change in the vertical distribution of 137 Cs, the contribution rate of the infiltrated water to downward migration of 137 Cs 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 137 Cs, 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., 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.)

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment., 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 The Authors. Published by Elsevier B.V. All rights reserved.)

Forests are important sources of dissolved radiocesium ( 137 Cs) discharge downstream. To improve understanding of dissolved 137 Cs discharge processes during rainstorms, we investigated the relationship between rainfall-runoff hydrological processes and the discharge of 137 Cs leached from litter. Leaching tests were conducted with broadleaf litter collected in the area where saturated overland flow was generated during rainstorms in a broadleaf-tree-dominated forest. According to the leaching test results, the 137 Cs leaching rate was higher in the early stage of the test and decreased afterward. There was no significant difference in the overall results between the agitation and non-agitation cases. The 137 Cs leaching rate from litter after the 24-h test was up to 33.7%. A large proportion of the original 137 Cs activity was present even after the tests, as leaching from litter during rainstorms in the headwater area could be an additional source of dissolved 137 Cs in the stream water. If mixing of 137 Cs originating from groundwater, soil water, and rainfall with the hydrological processes is assumed, differences between the observed and estimated 137 Cs in the surface runoff water became larger under high flow conditions. This analysis indicates additional 137 Cs loading on surface runoff water during rainstorms, where saturated surface area can expand as the surface runoff rate increases. Contact area between surface runoff and litter accumulated on the forest floor should increase and accelerate 137 Cs leaching from the litter. Therefore, 137 Cs leaching in the saturated surface area that is temporarily formed during rainstorms can play a principal role in dissolved 137 Cs discharge during rainfall-runoff events. Contaminated litter in the temporally saturated region of forested headwaters is an important factor contributing to elevated levels of dissolved 137 Cs during rainstorms in the Fukushima area., 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.)

Radiocesium released from the Fukushima Daiichi nuclear power plant (FDNPP) and deposited in the terrestrial environment has been transported to the sea through rivers. To study the long-term effect of riverine transport on the remediation process near the FDNPP, a monitoring project was initiated by the University of Tsukuba. It was commissioned by the Ministry of Education, Culture, Sports, Science, and Technology, and the Nuclear Regulatory Commission in June 2011, and was taken over by the Fukushima Prefectural Centre for Environmental Creation from April 2015. The activity concentration and monthly flux of radiocesium in a suspended form were measured in the project. This provides valuable measurement data to evaluate the impact of the accidentally released radiocesium on residents and the marine environment. It can also be used as verification data in the development and testing of numerical models to predict future impacts.

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, immediate soil and vegetation sampling were conducted according to the action plan of nuclear emergency monitoring; however, analysing the monitoring dataset was difficult because the sampling protocols were not standardised. In this study, the sampling protocols applied just after the FDNPP accident were reviewed, and the monitoring data were analysed. The detailed protocols and results can provide a sound basis for guidelines of soil and vegetation sampling for nuclear emergency monitoring. The activity concentrations of 137 Cs and 131 I in weed samples measured immediately after the FDNPP accident were related to the air dose rate at 1 m. Consequently, vegetation sampling is recommended when the additional dose rate (above background) is higher than 0.1 μSv/h. To enhance the efficiency of a protective response in the case of a nuclear accident, predetermined sampling points for soil and vegetation sampling should be considered in the preparedness plan for nuclear emergencies. Furthermore, sampling and analytical measurement capacities (time, people, cost) during the early phase after nuclear emergencies need to be considered in the preparedness and action plan, and sampling and measurement exercises are highly recommended., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Cesium is an element that belongs to the same group as K, and is known to show similar behaviour to that of K in plants. In this study, we conducted a serial leaching test for 120 h to compare the leaching characteristics of dissolved 137 Cs and K in forest litter, obtained from Japanese cedar and deciduous broadleaf forests located 40 km from the site of the 2011 Fukushima Dai-ichi nuclear power plant accident. The litter was collected in 2018 and was divided into three groups according to the decomposition level. The cumulative leachable fraction of 137 Cs at 120 h ranged from 0.3% to 3.3%, suggesting that most of the 137 Cs in the litter was hardly leachable in water. The leachable fraction of 137 Cs generally decreased with the decomposition level of the litter, implying that the easily leachable 137 Cs eluted into the water during the first stage of decomposition. Meanwhile, the cumulative leachable fraction of K at 120 h was approximately 10 times greater than that of 137 Cs and ranged from 22.7% to 54.8%. The leaching speeds of 137 Cs and K decreased suddenly with elapsed time regardless of the tree species, decomposition degree, or element. Our findings contribute to the long-term understanding of the 137 Cs cycle in forest ecosystems., (Copyright © 2020. Published by Elsevier Ltd.)

Data on reduction of radioactivity in plants are highly important for making decision on emergency response and remediation of contaminated areas. Dynamics of the 131 I and 137 Cs concentrations in the weed leaves sampled in the areas affected by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in Japan was assessed for the period March-December 2011. The effective half-lives for 131 I and 137 Cs of 3.8-4.2 days and 7.1-13.3 days in the weed leaves were calculated for the first two months after the deposition. The approach for assessment of the aggregated transfer factors based on the ambient dose equivalent rate was suggested and validated. The geometric means of the soil to the weed leaves aggregated transfer factors were estimated for June-December 2011. Soil to crop 137 Cs- concentration ratios (buckwheat, brown rice and soybean) were estimated for 2011-2016. Soil to crop concentration ratios were found to decrease in the order of soybean > buckwheat > brown rice. The effective half-lives for 137 Cs in these crops were estimated to be between 1 and 2.5 years for the period 2011 2016, and longer than 5-7 years after 2016. It was found that these data comply with the Chernobyl related data obtained for similar conditions and complement of international documents on radionuclide transfer in agricultural environment such as the IAEA TRS 472., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the 137 Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of 137 Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured 137 Cs activities adjacent to the 1F NPP. River discharge rates of 137 Cs were estimated by multiplying simulated river flow rates by the dissolved 137 Cs activities, which were estimated by an empirical function. Inflow of 137 Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of 137 Cs activity was large, the simulated results were compared with the annual averaged observed 137 Cs activity distribution. Normalized annual averaged 137 Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated 137 Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted 137 Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic 137 Cs activity was small compared to the effect of directly released 137 Cs., 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 © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The effects of a 50% forest thinning intensity on Fukushima-derived 137 Cs deposition by litterfall and its discharge by runoff in hillslope coniferous forest were monitored using four litterfall traps and a hillslope erosion plot. The observation was underway during the pre-and post-thinning periods. Results demonstrated that during the pre-thinning period a total 150 ± 13 g m -2 of litterfall deposited about 924 ± 69 Bq m -2 of 137 Cs. This accounts for 11% of the local 137 Cs fallout recorded for the study site in the aftermath of the accident. After thinning, both litterfall and 137 Cs increased by more than six- and two-fold, respectively. This is possibly owing to the slow individual tree recovery rate assisted by the change on the running space provided by canopy openings, which can accelerate even the normal gust wind to gain damaging power on the unshielded mechanically injured parts of the contaminated residual trees. In both cases, litterfall generally transferred about 37% (3 ± 0.2 kBq m -2 ) of the local 137 Cs fallout onto the forest floor over the observation period. The eroded litter-associated 137 Cs increased by about a factor of two after thinning, which only accounted for less than 1% of 137 Cs deposited by litterfall. This implies that the forest floor retains 137 Cs and remains contaminated regardless of the size of the eroded litter material. But this could become a potential secondary contamination source for the downstream resources such as water bodies and villages, especially at the time of flooding, which in turn calls a serious attention in designing decontamination schemes., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Since headwater catchments are the source areas of 137 Cs for downstream river systems, 137 Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and 137 Cs concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed. The normalized 137 Cs concentrations ( 137 Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10 -6  m 2 /L, 10 -2  m 2 /kg, and 10 -1  m 2 /kg, respectively, before 2013 and declined to around 10 -8  m 2 /L, 10 -4  m 2 /kg, and 10 -2  m 2 /kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas 137 Cs concentrations in SS declined significantly and the total flux of 137 Cs decreased. Although the clear effect of land use on decline trend in normalized 137 Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them., (Copyright © 2019 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.)

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.)

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.)

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of 137 Cs from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of 137 Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of 137 Cs in litter and soil layers for 6 y (2011-2017) following the previous study (2011-2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The 137 Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80-95% of the deposited 137 Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of 137 Cs inventory in the litter layer to the total 137 Cs inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22-0.44 y -1 . The slower migration was observed in the YC site, probably because of higher initial interception of 137 Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the 137 Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the 137 Cs concentration in the litter within 2-3 y of the accident. The 137 Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of 137 Cs concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y -1 . In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of 137 Cs and its factors regarding both forest and soil characteristics., (Copyright © 2018 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.)

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of 137 Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ 1 ) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of 137 Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of 137 Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported 137 Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ 2 ) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with 137 Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios ( 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 242 Pu/ 239 Pu) and for 137 Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, 236 U/ 238 U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Radionuclide contamination of groundwater causes critical impacts on water resources, human lives, and ecosystems. The intrusion of radionuclides into the groundwater flow system in Fukushima, Japan, could be illuminated by determining groundwater age and mixing processes. To do this, periodical field surveys were conducted in catchments contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Sampling began in May 2011, which was 2 months after the disaster, and continued through June 2012. Chlorofluorocarbon (CFCs), tritium, and oxygen and hydrogen stable isotopes were used as environmental tracers. The observed tritium concentrations suggested that the water contained accident-derived radionuclides that exceeded the natural background baseline. Groundwater ages in the selected two headwater catchments were estimated to be between 10 and 26 years by combined use of multiple CFCs concentrations. In addition, the governing groundwater flow system was mostly approximated by a piston flow model; however, modern water fraction was also suggested based on the relationship between CFC-11 and CFC-12. The estimated water age and isotopic signals among stream water, spring water, and groundwater revealed that the intrusion of radionuclides into the groundwater was caused by the mixing between groundwater and modern water sources such as soil water and precipitation with relatively high radionuclide concentrations. This mixing was facilitated by a weathered and fractured granite bedrock and a thin unsaturated subsurface layer in the study area. Continued long-term monitoring of radionuclides in the groundwater will be necessary for water resources management in the future. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration., (Copyright © 2019 Elsevier B.V. All rights reserved.)

An excessive supply of sediment is observed in numerous rivers across the world where it leads to deleterious impacts. Information on the sources delivering this material to waterbodies is required to design effective management measures, and sediment tracing or fingerprinting techniques are increasingly used to quantify the amount of sediment derived from different sources. However, the current methods used to identify the land use contributions to sediment have a limited discrimination power. Here, we investigated the potential of environmental DNA (eDNA) to provide more detailed information on the plant species found in sediment source areas as a next generation fingerprint. To this end, flood sediment deposits (n = 12) were collected in 2017 in two catchments impacted by the Fukushima radioactive fallout along differing river sections draining forests, cropland or a mix of both land uses. Conventional fingerprints (i.e. fallout radionuclides and organic matter properties) were also measured in these samples. The conventional fingerprint model results showed that most sediment samples contained a dominant proportion of subsoil material. Nevertheless, the eDNA information effectively discriminated the three above-mentioned groups of sediment, with the dominance of tree, shrub and fern species in sediment sampled in rivers draining forests versus a majority of grass, algae and cultivated plant species in sediment collected in rivers draining cropland. Based on these encouraging results, future research should examine the potential of eDNA in mixed land use catchments where the contribution of topsoil to sediment dominates and where the cultivation of land has not been abandoned in order to better characterize the memory effect of eDNA in soils and sediment., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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

Dynamics of the Fukushima-derived radiocesium and distribution of the natural stable isotope 133 Cs in Japanese cedar (Cryptomeria japonica D. Don) forest ecosystems were studied during 2014-2016. For the experimental site in Yamakiya, Fukushima Prefecture, we present the redistribution of radiocesium among ecosystem compartments during the entire observation period, while the results obtained at another two experimental site were used to demonstrate similarity of the main trends in the Japanese forest ecosystems. Our observations at the Yamakiya site revealed significant redistribution of radiocesium between the ecosystem compartments during 2014-2016. During this same period radionuclide inventories in the aboveground tree biomass were relatively stable, however, radiocesium in forest litter decreased from 20 ± 11% of the total deposition in 2014 to 4.6 ± 2.7% in 2016. Radiocesium in the soil profile accumulated in the 5-cm topsoil layers. In 2016, more than 80% of the total radionuclide deposition in the ecosystem resided in the 5-cm topsoil layer. The radiocesium distribution between the aboveground biomass compartments at Yamakiya during 2014-2016 was gradually approaching a quasi-equilibrium distribution with stable cesium. Strong correlations of radioactive and stable cesium isotope concentrations in all compartments of the ecosystem have not been reached yet. However, in some compartments the correlation is already strong. An increase of radiocesium concentrations in young foliage in 2016, compared to 2015, and an increase in 2015-2016 of the 137 Cs/ 133 Cs concentration ratio in the biomass compartments with strong correlations indicate an increase in root uptake of radiocesium from the soil profile. Mass balance of the radionuclide inventories, and accounting for radiocesium fluxes in litterfall, throughfall and stemflow, enabled a rough estimate of the annual radiocesium root uptake flux as 2 ± 1% of the total inventory in the ecosystem., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Spatial patterns of atmospherically deposited radiocesium on the forest floor and the temporal evolution were measured in two Japanese cedar stands and a secondary mixed broad-leaved forest in the early phase of the Fukushima Daiichi Nuclear Power Plant accident. In situ measurements of the 137 Cs gamma count were made using a portable germanium gamma ray detector. These measurements revealed that the forest floors were contaminated with radionuclides derived from the accident. In the cedar stands, the inter-canopy area had higher 137 Cs count rate relative to the under-canopy area, whereas no clear relationship was found between the radiocesium pattern and canopy cover in the mixed broad-leaved forest. Repeated radiocesium measurements revealed that the spatial pattern of radiocesium activity on the forest floor did not substantially change following additional deposition inputs. Furthermore, the magnitude of canopy cover partially explained spatial variability of the 137 Cs on the forest floor in cedar stands. These results suggest that canopy structure affected the genesis of the horizontal variability of atmospherically deposited radiocesium on the forest floor during the early phase of the Fukushima accident., (Copyright © 2017 Elsevier B.V. All rights reserved.)

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, continuous monitoring of the detailed vertical distribution of radiocesium in soil is required to evaluate the fate of radiocesium and establish strategies for remediation and management of the contaminated land. It is especially important to investigate paddy soil because little knowledge has been accumulated for paddy soil and wetland rice is a major staple in Japan. Therefore, we monitored the vertical distribution of 137 Cs in abandoned paddy soil in a planned evacuation zone from June 2011 to March 2016. The decontamination works (i.e., 5 cm of surface soil removal and re-covering with uncontaminated soil) were conducted by the government in 2015. As a result of monitoring, the 137 Cs gradually migrated downward with time and the 137 Cs concentration in the 0-10 cm soil was almost homogenous in October 2014, although it was non-cultivated. The liner relationship was obtained between the median depth, which is the thickness of a soil layer containing half of the total 137 Cs inventory, and the time after the accident, indicating the migration rate was constant (1.3 cm y -1 ) before the decontamination works. After the decontamination works, the 137 Cs concentration in the uppermost surface layer was reduced by 90%, however the total 137 Cs inventory was reduced by only 50-70%. It was shown that the efficiency of 137 Cs removal by the decontamination works decrease linearly over time in fields like the studied paddy, in which the homogenization of 137 Cs concentration occurred. Conversely, the downward migration of 137 Cs to subsurface layers deeper than 10 cm (i.e., plowpan layer) with low permeability rarely occurred. It is expected that these unique trends in distribution and migration of 137 Cs would be found in abandoned paddy soils with properties similar to the studied soil, sandy loam but poorly drained because of the low permeable plowpan layer, although further validation is necessary., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The Fukushima nuclear accident in Japan resulted in the deposition of radiocesium over forested and rural landscapes northwest of the power plant. Although there have been several investigations into the dynamics of contaminated river sediment, less attention has been paid to the sources of deposited particulate matter in dams and reservoirs. In the Fukushima Prefecture, there are 10 significant dams and over a 1000 reservoirs for both agricultural and surface water management. These reservoirs may have trapped a significant volume of radiocesium-contaminated sediment. Therefore, characterizing the sources of contaminated particulate matter is important for the ongoing management of contamination in the region. Accordingly, the composition of particulate matter deposited in the Mano Dam reservoir, approximately 40km northwest of the power plant, was investigated with the analyses and modelling of carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N), total organic carbon (TOC) and total nitrogen (TN) concentrations. Four sediment cores, with lengths ranging 29-41cm, were sampled in the Mano Dam. Source samples from 46 forest soils, 28 cultivated soils and 25 subsoils were used to determine the source contributions of particulate matter. Carbon and nitrogen parameters were analyzed on all samples and a concentration-dependent distribution modelling approach was used to apportion source contributions. Three of the four cores sampled in the Mano Dam reservoir had distinct radiocesium peaks representative of the initial post-accident wash-off phase. Cultivated sources were responsible for 48±7% of the deposited fine particulate matter whereas forests were modelled to contribute 27±6% and subsoil sources 25±4%. Ongoing decontamination of cultivated sources in the Fukushima region should result in a decrease of contaminated matter deposition in reservoirs., (Copyright © 2017 Elsevier B.V. All rights reserved.)

A wide area of paddy fields was contaminated by radiocesium derived from the Fukushima nuclear accident. Implement of agricultural countermeasures is one of the principle methods to reduce the contamination of rice plants. In this study, the effect of topsoil removal measure and fertilizer application on radiocesium uptake by rice plants was investigated over a four-year period. Some other available countermeasures carried out after the Fukushima nuclear accident were also summarized. The results indicate that the effect of topsoil removal measure on the accumulation of radiocesium in rice plants was effective, but the concentration ratio of radiocesium activity concentration between rice plant and soil increased. This may be correlated with the radiocesium imported from irrigation water and relatively high exchangeable radiocesium proportion of plowing soil in the topsoil removal paddy. We summarized four year's data to further confirm that potassium and nitrogen fertilizers had an opposite effect on the accumulation of radiocesium in rice plants. Increasing potassium and reducing nitrogen fertilizer conditions tended to inhibit the radiocesium uptake by rice plants. Moreover, of all the available countermeasures applied in the paddy fields, the most effective countermeasure was the application of phlogopite. However, further analyses are required to confirm the effectiveness of application of phlogopite considering the limited available information., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Radiocesium ( 137 Cs) migration from headwater forested areas to downstream rivers has been investigated in many studies since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which was triggered by a catastrophic earthquake and tsunami on 11 March 2011. The accident resulted in the release of a huge amount of radioactivity and its subsequent deposition in the environment. A large part of the radiocesium released has been shown to remain in the forest. The dissolved 137 Cs concentration and its temporal dynamics in river water, stream water, and groundwater have been reported, but reports of dissolved 137 Cs concentration in soil water remain sparse. In this study, soil water was sampled, and the dissolved 137 Cs concentrations were measured at five locations with different land-use types (mature/young cedar forest, broadleaf forest, meadow land, and pasture land) in Yamakiya District, located 35 km northwest of FDNPP from July 2011 to October 2012. Soil water samples were collected by suction lysimeters installed at three different depths at each site. Dissolved 137 Cs concentrations were analyzed using a germanium gamma ray detector. The dissolved 137 Cs concentrations in soil water were high, with a maximum value of 2.5 Bq/L in July 2011, and declined to less than 0.32 Bq/L by 2012. The declining trend of dissolved 137 Cs concentrations in soil water was fitted to a two-component exponential model. The rate of decline in dissolved 137 Cs concentrations in soil water (k 1 ) showed a good correlation with the radiocesium interception potential (RIP) of topsoil (0-5 cm) at the same site. Accounting for the difference of 137 Cs deposition density, we found that normalized dissolved 137 Cs concentrations of soil water in forest (mature/young cedar forest and broadleaf forest) were higher than those in grassland (meadow land and pasture land)., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The role of particulate organic matter in radiocesium transfers from soils to rivers was investigated in areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. Suspended and deposited sediments, filtered water, macro organic debris and dead leaves were sampled along the six most contaminated coastal river catchments of the Fukushima prefecture in the early autumns 2013 and 2014. Radiocesium concentrations of river samples and total organic carbon concentrations in suspended and deposited sediments were measured. Radiocesium concentrations of suspended and deposited sediments were significantly correlated to 137 Cs inventories in soils and total organic carbon. The distributions of radiocesium between the organic and mineral phases of both types of sediment were assessed by using a modelling approach. The results suggest that, during the early autumn season, the organic fraction was the main phase that carried the radiocesiums in deposited sediments and in suspended sediments for suspended loads <25mg·L -1 . For higher suspended loads like those occurring during typhoon periods, the mineral fraction was the main carrier phase. Thus, high apparent distribution coefficient values noted by various authors in Fukushima could be attributed to the high radiocesium contents of particulate organic matter. Since it is well known that organic compounds generally do not significantly adsorb radiocesium onto specific sites, several hypotheses are suggested: 1) Radiocesiums may have been absorbed into organic components at the early stage of atmospheric radioactive deposits and/or later due to biomass recycling and 2) Those elements would be partly carried by glassy hot particles together with organic matter transported by rivers in Fukushima. Both hypotheses would lead to conserve the amount of radiocesiums associated with particles during their transfers from the contaminated areas to the marine environment. Finally, such organically bound radiocesium would lead to significant deliveries of bioavailable radiocesium for living organisms at Fukushima., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Radiocesium ( 137 Cs) migration from headwaters in forested areas provides important information, as the output from forest streams subsequently enters various land-use areas and downstream rivers. Thus, it is important to determine the composition of 137 Cs fluxes (dissolved fraction, suspended sediment, or coarse organic matter) that migrate through a headwater stream. In this study, the 137 Cs discharge by suspended sediment and coarse organic matter from a forest headwater catchment was monitored. The 137 Cs concentrations in suspended sediment and coarse organic matter, such as leaves and branches, and the amounts of suspended sediment and coarse organic matter were measured at stream sites in three headwater catchments in Yamakiya District, located ∼35 km northwest of Fukushima Dai-ichi Nuclear Power Plant (FDNPP) from August 2012 to September 2013, following the earthquake and tsunami disaster. Suspended sediment and coarse organic matter were sampled at intervals of approximately 1-2 months. The 137 Cs concentrations of suspended sediment and coarse organic matter were 2.4-49 kBq/kg and 0.85-14 kBq/kg, respectively. The 137 Cs concentrations of the suspended sediment were closely correlated with the average deposition density of the catchment. The annual proportions of contribution of 137 Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction were 96-99%, 0.0092-0.069%, and 0.73-3.7%, respectively. The total annual 137 Cs discharge from the catchment was 0.02-0.3% of the deposition., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The concentration of dissolved 137 Cs in groundwater and stream water in the headwater catchments in Yamakiya district, located ∼35 km north west of Fukushima Dai-ichi Nuclear Power Plant (FDNPP), was monitored from June 2011 to July 2013, after the earthquake and tsunami disaster. Groundwater and stream water were sampled at intervals of approximately 2 months at each site. Intensive sampling was also conducted during rainstorm events. Compared with previous data from the Chernobyl NPP accident, the concentration of dissolved 137 Cs in stream water was low. In the Iboishi-yama catchment, a trend was observed for the concentration of dissolved 137 Cs in stream water to decline, which could be divided into two phases by October 2011 (a fast flush of activity as a result of rapid washoff and a slow decline as a result of soil fixation and redistribution processes). The highest 137 Cs concentration recorded at Iboishi-yama was 1.2 Bq/L on August 6, 2011, which then declined to 0.021-0.049 Bq/L during 2013 (in stream water under normal water-flow conditions). During the rainfall events, the concentration of dissolved 137 Cs in stream water increased temporarily. The concentration of dissolved 137 Cs in groundwater at a depth of 30 m at Iboishi-yama displayed a decreasing trend from 2011 to 2013, with a range from 0.039 Bq/L to 0.0025 Bq/L. The effective half-lives of stream water in the initial fast flush and secondary phases were 0.10-0.21 and 0.69-1.5 y, respectively in the three catchments. The effective half-life of groundwater was 0.46-0.58 y at Koutaishi-yama and 0.50-3.3 y at Iboishi-yama. The trend for the concentration of dissolved 137 Cs to decline in groundwater and stream water was similar throughout 2012-2013, and the concentrations recorded in deeper groundwater were closer to those in stream water. The declining trend of dissolved 137 Cs concentrations in stream water was similar to that of the loss of canopy 137 Cs by throughfall, as shown in other reports of forest sites in the Yamakiya district., (Copyright © 2016. Published by Elsevier Ltd.)

The Fukushima-derived radiocesium distribution in the typical Japanese cedar (Cryptomeria japonica D. Don) forest ecosystem was determined. In four years after the Fukushima accident, about 74% of the total radiocesium inventory was localized in soil, 20% was in the litter, and only 6% was associated with the aboveground biomass. Most of the radiocesium that was initially intercepted by the tree canopies has been already transported to the ground surface. The importance of the processes for removal of radiocesium from the tree canopies decreased in the order litterfall > throughfall >> stemflow. Within the tree compartments, the largest radiocesium activity fraction, about 46%, was observed in old foliage, which indicates that the process of removal of the initial deposit from the tree crowns has not yet completed. The aggregate soil-to-wood transfer factor was 1.1⋅10 -3  m 2  kg -1  d.w., which is close to the geometric means of transfer factors recommended by IAEA for other coniferous tree species. Further studies in Fukushima forest are necessary to assess the variation of this parameter under various soil-landscape conditions. Presence of the residues of the initial deposits does not allow to obtain the accurate values of the annual radiocesium fluxes in the ecosystem. Based on the conservative assumptions, the ranges of the fluxes were estimated. Analysis of the flux structures shows that up to percents of the total radiocesium activity in the ecosystem may be involved into biogenic cycling., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

In this study, we investigated the transfer of canopy-intercepted radiocesium to the forest floor following the Fukushima Dai-ichi Nuclear Power Plant accident. The 137 Cs content of throughfall, stemflow, and litterfall were monitored in two coniferous stands (plantations of Japanese cedar) and a deciduous mixed broad-leaved forest stand (oak with red pine) from July 2011 to December 2012. The forest floor of cedar stands had received higher levels of additional 137 Cs deposition compared with the mixed broad-leaved stand during the sampling period. The cumulative 137 Cs deposition during the study period was 119 kBq m -2 for the mature cedar stand, 105 kBq m -2 for the young cedar stand, and 41.5 kBq m -2 for the broad-leaved stand. The deposition of 137 Cs to the forest floor occurred mainly in throughfall during the first rainy season, from July to September 2011 (<200 d after the initial fallout); thereafter, the transfer of 137 Cs from the canopy to forest floor occurred mainly through litterfall. A double exponential field-loss model, which was used to simulate the removal of 137 Cs from canopies, was the best fit for the temporal changes in the canopy 137 Cs inventory., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Our study focused on radiocaesium ( 137 Cs) partitioning in forests, three vegetation periods after the Fukushima Daiichi nuclear power plant accident. 137 Cs 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 137 Cs deposit for mature and young cedars, respectively.

The concentration of particulate (137)Cs in paddy fields, which can be a major source of (137)Cs entering the water system, was studied following the Fukushima Daiichi Nuclear Power Plant accident. To parametrize the concentration and to estimate the time dependence, paddy fields covering various levels of (137)Cs deposition were investigated over the period 2011-2013 (n = 121). The particulate (137)Cs concentration (kBq kg-SS(-1)) showed a significant correlation with the initial surface deposition density (kBq m(-2)). This suggests that the entrainment coefficient (m(2) kg-SS(-1)), defined as the ratio between the particulate (137)Cs concentration and the initial surface deposition density, is an important parameter when modeling (137)Cs wash-off from paddy fields. The entrainment coefficient decreased with time following a double exponential function. The decrease rate constant of the entrainment coefficient was clearly higher than that reported for other land uses and for river water. The difference in the decrease rates of the entrainment coefficient suggests that paddy fields play a major role in radiocesium migration through the water system. An understanding of the decrease rate of the entrainment coefficient of paddy fields is therefore crucial to understand the migration of radiocesium in the water system.

About half of the total paddy field area, which is the dominant agricultural land in Fukushima Prefecture, was contaminated by radiocesium released by the Fukushima Daiichi Nuclear Power Plant accident. In this study, we investigated the temporal changes of radiocesium in soil, irrigation water, and rice plant in two adjacent rice paddies, with and without surface-soil-removal, in Fukushima Prefecture for over three years (2012-2014) after the nuclear accident. Our results showed that radiocesium migrated into 24-28 cm soil layers and that the activity concentration of radiocesium in paddy soils showed a significant reduction in 2014. The newly added radiocesium to paddies through irrigation water contributed only a maximum value of 0.15% and 0.75% of the total amount present in control and decontaminated paddies, respectively, throughout the study period. The radiocesium activity concentration in suspended sediment in irrigation water exponentially decreased, and the effective half-lives (Teff) for (137)Cs and (134)Cs were 1.3 and 0.9 years, respectively. Additionally, the average suspended sediment concentration in irrigation water increased between 2012 and 2014, suggesting that enhanced soil erosion had occurred in the surrounding environment. Radiocesium accumulation in rice plant also decreased with time in both paddies. However, the concentration ratio of radiocesium for rice plant in the decontaminated paddy increased compared with control paddy, despite approximately 96% of fallout radiocesium removed in paddy soil. Further analysis is required to clarify the reasons of high concentration ratio of radiocesium for rice plant in the decontaminated paddy., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2015 Elsevier Ltd. All rights reserved.)