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An extensive investigation of particulate radiocaesium in suspended solids and dissolved radiocaesium in river water was undertaken at 30 sites in Fukushima and Miyagi Prefectures in December 2012, and their relationships with catchment inventory and the solid/liquid distribution coefficient (Kd) were evaluated. Rivers located in the coastal region on the north side of the Fukushima Dai-ichi Nuclear Power Plant exhibited relatively higher particulate radiocaesium concentrations. Significant correlations were found between concentrations of particulate/dissolved radiocaesium and average catchment inventories, indicating that the concentrations of particulate/dissolved radiocaesium could be approximated from the catchment inventory. Particulate radiocaesium concentration was significantly correlated with dissolved radiocaesium concentration (with the exception of concentrations measured in estuaries), and the geometric mean Kd was calculated as 3.6 × 10(5) with a 95% confidence interval of 2.6-5.1 × 10(5)., (Copyright © 2014 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 occurrence of (137)Cs in size fractionated samples in river water from the Abukuma River system, (the Kuchibuto and Abukuma Rivers, five sampling events for three sites) was studied from June 2011--approximately some three months after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident until December 2012. The total concentration of (137)Cs (mBq/L) in river water was generally high at the upper stream site in the Yamakiya District within the evacuation/off-limits zone. The (137)Cs concentration was about 1Bq/L for the first sampling campaign (June 2011) at all sites, but then decreased substantially to about one-tenth of that by the time of a second sampling campaign (November or December 2011). The (137)Cs in the <0.45 μm fraction was present exclusively as a dissolved species rather than as a species adsorbed on suspended solids or complexed with organic materials. The contribution of the dissolved fraction ranged from 1.2 to 48.9% (averaged 20%) of the total concentration of (137)Cs throughout the observation period. The maximum contribution of (137)Cs was found in the silt size fraction (3-63 μm), which can be explained by the relatively large Kd values and the suspended solids (SS) concentration of this size fraction. Although the concentration (Bq/g) of (137)Cs in each size fraction did not show any significant trends and/or variations for any of the sampling campaign, Kd values for each site increased with time. Furthermore, it was found that the Kd values decreased with distance from the headstream in the off-limits zone. Thus, the data acquired in this study give an overview of the radiological situation for Fukushima including temporal and spatial variation of radiocaesium in a natural riverine system, within a few years after the accident., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

We monitored the vertical distribution of (137)Cs in soil profiles under eight different land uses for the 2 y after the Fukushima Dai-ichi Nuclear Power Plant accident, and discussed the temporal changes in the early-stage of the migration and the determinants of the initial distribution. The soil samples were collected for four surveys using a scraper plate at each study site, which consisted of three forests (mixed forest, mature cedar, and young cedar), two grasslands (pasture and meadow) and three abandoned agricultural fields (farm land, tobacco field, and paddy field). The land use patterns have a large influence on some soil properties and the migration processes of (137)Cs above ground, resulting in different distribution of (137)Cs in those soil profiles. Specifically, the secondary deposition of (137)Cs from the coniferous canopy, retention of (137)Cs by litter layer, and the homogenization of (137)Cs concentrations in surface soil by natural soil mixing such as the disturbance by cattle grazing, roots growing and the formation of needle ice were important to cause redistribution of the deposited (137)Cs. Only in the paddy field, the (137)Cs inventory in subsurface soils (5-10 cm) gradually increased and comprised 26% of the total (137)Cs in 2 y, showing the downward migration of (137)Cs to subsurface soil. In the other sites, it was considered that (137)Cs were strongly adsorbed by soil particles and rarely migrated downward as soluble form. Vertical distributions during the first survey were able to be used as the initial distributions and were well fitted to the exponential equation. The distribution parameters α (relaxation depth) and β (relaxation mass depth), calculated by the exponential equation were correlated with RIP (r = -0.806, p < 0.05), macro pore (r = 0.651, p = 0.11), and dispersible fine particle content (r = 0.856, p < 0.05). It indicated that the initial distribution would be influenced by the Cs fixation ability of soil, and the penetration process of water and particles in soils., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Artificial radionuclides including radiocesium (134 Cs and 137 Cs) and radiosilver (110mAg) were released into the environment following the Fukushima Dai-ichi nuclear power plant accident in March 2011. These particle-bound substances deposited on soils of north-eastern Japan, located predominantly within a ∼3000 km2 radioactive fallout plume and drained by several coastal rivers to the Pacific Ocean. The current dataset (Evrard et al., 2021), which can be accessed at 10.1594/PANGAEA.928594, compiles gamma-emitting artificial radionuclide activities measured in 782 sediment samples collected from 27 to 71 locations across catchments draining ∼6450 km2 during 16 fieldwork campaigns. These campaigns were conducted in Japan between November 2011 and November 2020 in river catchments draining the main radioactive plume. This database may be useful to evaluate and anticipate the post-accidental redistribution of radionuclides in the environment and for the spatial validation of models simulating the transfer of radiocesium across continental landscapes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Sediment connectivity characterizes the physical transfer of sediment through different geomorphic compartments in catchments due to sediment detachment, transport and deposition. Quantifying and modelling sediment connectivity is therefore a key prerequisite to improving our understanding of the dispersion of particle-borne contaminants, especially in catchments exposed to highly erosive climates. The objective of this study is to provide novel insights into typhoon impacts on sediment connectivity from hillslopes to rivers. The dispersion of particle-bound caesium-137 (137Cs) was investigated in two coastal catchments draining the main contamination plume from the Fukushima Daiichi Nuclear Power Plant accident. Five sampling campaigns were carried out from November 2011 to November 2015, after each typhoon season. The spatial and temporal evolution of 137Cs contamination was investigated through the calculation of 137Cs enrichment ratios in sediment relative to nearby soils. Rainfall erosivity (EI30) associated with the main typhoons that occurred prior to each sampling campaign were computed, mapped, and finally used to improve a topographic-based index of connectivity. From 2011 to 2015, mean contamination levels in Mano and Niida catchments decreased from 11.9 kBq kg−1 to 3.3 kBq kg−1 and from 34.1 kBq kg−1 to 8.0 kBq kg−1, respectively. Regional mean EI30 ranged from 262 MJ mm ha−1 h−1 for typhoon Jelawat (in 2012) to 1695 MJ mm ha−1 h−1 for typhoon Roke (in 2011). Typhoons Roke (2011) and Etau (2015) showed the highest connectivity from contaminated sources to the rivers, and induced a significant export of sediment to the ocean. In 2013 a slight increase in 137Cs levels in river sediments occurred, likely resulting from initial decontamination works and the occurrence of two consecutive typhoons. Importantly, this research provides new insights into the connectivity of the main sources of sediments contaminated with radiocaesium in Fukushima Prefecture and their temporal evolution, which will help with ongoing decontamination efforts. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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 (T eff ) 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. [ABSTRACT FROM AUTHOR]

The Fukushima Daiishi nuclear power plant (FDNPP) accident led to a massive radionuclide deposition mainly onto Japanese forest canopies. In our previous study, an improved double exponential (IDE) equation including rainfall intensity was proposed to estimate the 137 Cs hydrological transport from evergreen canopies to the ground. This equation used two types of parameters, kinetic ( k 1 and k 2 ) and leachable stock ( A 1 and A 2 ). Those parameters have been estimated by adjusting them in the IDE equation in order to accurately describe the measured cumulative leached 137 Cs from canopies ( k 1 = 4.2E-04–5.0E-04 d −1 , k 2 = 1.2E-02–1.7E-02 d −1 , A 1 = 62–99 kBq/m 2 , A 2 = 25–61 kBq/m 2 ). In this study, we linked the total leachable stock ( A leachable , a parameter of the IDE equation corresponding to A 1 + A 2 ) to a physiological criteria (the canopy closure CC, which can be measured with a simple camera equipped with a fish-eye objective). Furthermore, the kinetic parameters measured for Japanese cedar ( k 1 = 5.0E-04 d −1 , k 2 = 1.2E-02 d −1 , and r 12 = 0.22 ( r 12 = A 1 / A 2 ) could also be used for two other coniferous species: Japanese cypress and spruce. This suggests that these parameters could be constants for coniferous forests. [ABSTRACT FROM AUTHOR]

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 137Cs inventories in the layers of deposited organic material and mineral soil at two different spatial scales (hillslope [60 m2] and small [1 m2]). The results showed that 137Cs 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 137Cs inventory in each layer tended to show a lognormal distribution. The correlation between the 137Cs inventories in deposited organic material and mineral soil in the same sampling grid was weak. The controlling mechanisms of the 137Cs 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 137Cs inventory in the deposited organic layer at each sampling point. In contrast, the 137Cs 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 137Cs 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 137Cs inventory estimations in the cedar stand. The results indicated that a soil sampling area greater than 0.06 m2 at the hillslope scale and 0.008 m2 at the small scale enabled the mean 137Cs inventory to be estimated with an uncertainty of less than 20% in the cedar stand. [Display omitted] • A grid sampling experiment revealed 137Cs distribution on forest floor. • Hillslope and small scale 137Cs spatial patterns were determined. • The peripheral area of the canopy accumulated more 137Cs. • The initial 137Cs input pattern was preserved in forest soil. • The optimized sampling guideline for forest 137Cs was presented. [ABSTRACT FROM AUTHOR]

We collected fluvial suspended sediments in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and analyzed the Cs concentration in bulk and size-fractioned samples to investigate the particle-size-dependent distribution of radiocesium. The Cs concentration in bulk suspended sediments decreased from August to December 2011, possibly reflecting a decrease of radiocesium concentration in its source materials. Smaller particles had higher radiocesium concentrations, reflecting larger specific surface areas. Silt- and sand-size fractions occupied more than 95 % of the total Cs in the suspended sediments. The contribution of clay-size fractions, which had the highest Cs concentration, was quite small because of their low frequency. A line of the data showed that the particle size distribution of radiocesium was essential to evaluate the migration and distribution of radiocesium in river systems where radiocesium is mainly present as particulate form after the FDNPP accident. [ABSTRACT FROM AUTHOR]

We investigated the biological accumulation of radiocesium in tadpoles [Rana (Pelophylax) porosa porosa] in rice paddies with and without decontamination practice at Fukushima. Radiocesium was accumulated in surface part of soils both in the control and decontaminated paddies one year after decontamination. Mean 134Cs and 137Cs concentrations in tadpoles in the control and decontaminated paddies were 3000 and 4500, and 600 and 890 Bq/kg dry weight, respectively. Radiocesium concentrations in surface soil (0–5 cm depth) and tadpoles in the decontaminated paddy were five times smaller than in the control paddy. These results suggest that decontamination practice can reduce radiocesium concentrations in both soil and tadpoles. However, at the decontaminated paddy, radiocesium concentrations in surface soils became 3.8 times greater one year after decontamination, which indicates that monitoring the subsequent movement of radiocesium in rice paddies and surrounding areas is essential for examining contamination propagation. [Copyright &y& Elsevier]

Abstract: 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 131I 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 131I 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 131I 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 137Cs 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 137Cs. However, further analysis is required to clarify the effect of clay content on the initial penetration depth of deposited 137Cs in soil profile. [Copyright &y& Elsevier]

The article presents an estimation of suspended sediment sources with the use of 137Cs and 210Pbex in unmanaged cypress plantation watersheds in southern Japan. The 137Cs and 210Pbex were detected with the use of gamma-ray spectrometry in the surface soil of forest floor, stream bank and others. The multivariate sediment-mixing model that use 137Cs and 210Pbex showed that the contribution of the forest floor changes in various subwatersheds. Results show that the forest floor should be recognized as a source of suspended sediment in unmanaged cypress plantation watersheds.

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

It is the conventional understanding that rain removes aerosols from the atmosphere. However, the question of whether rain plays a role in releasing aerosols to the atmosphere has recently been posed by several researchers. In the present study, we show additional evidence for rain-induced aerosol emissions in a forest environment: the occurrence of radiocaesium-bearing aerosols in a Japanese forest due to rain. We carried out general radioactive aerosol observations in a typical mountainous village area within the exclusion zone in Fukushima Prefecture to determine the impacts and major drivers of the resuspension of radiocaesium originating from the nuclear accident in March 2011. We also conducted sampling according to the weather (with and without rain conditions) in a forest to clarify the sources of atmospheric radiocaesium in the polluted forest. We found that rain induces an increase in radiocaesium in the air in forests. With further investigations, we confirmed that the fungal spore sources of resuspended radiocaesium seemed to differ between rainy weather and nonrainy weather. Larger fungal particles (possibly macroconidia) are emitted during rainy conditions than during nonrainy weather, suggesting that splash generation by rain droplets is the major mechanism of the suspension of radiocaesium-bearing mould-like fungi. The present findings indicate that radiocaesium could be used as a tracer in such research fields as forest ecology, meteorology, climatology, public health and agriculture, in which fungal spores have significance. [ABSTRACT FROM AUTHOR]

This study seeks to better understand the vertical variation in the transport and fate of radiocesium via branchflow and stemflow in the aftermath of the Fukushima nuclear power plant accident. Working in both a coniferous forest (Cryptomeria japonica, young Japanese cedar stands, average height is 14.0 m) and a mixed deciduous broadleaved forest (Quercus serrata, Japanese oak stands, average height is 13.3 m), we employed isotopic tracers to help determine the fate of radiocesium transported by branchflow and stemflow from the upper and lower portions of the canopy. Branchflow was harvested and examined from the upper canopy layers (including younger foliage, dead foliage, and live branches), whereas stemflow was collected in both the upper and lower portions of the canopy (with varying portions of live and dead branches). Particular attention was paid to the washoff, leaching, adsorption, transport, and storage (stem and bark) of radiocesium. The preliminary results showed radiocesium leaching (Cs-137 concentration) was greater for branchflow that received washoff and leachate from the dead foliage than the branchflow receiving radiocesium inputs from mixed and young foliage. For the tree trunk, radiocesium leached more in stemflow from the lower part of the canopy as compared to the upper canopy. We also found that the isotopic composition of branchflow was generally enriched in δ18O and δD compared to open rainfall and throughfall, however, the differences in enrichment between branchflow and stemflow remains unclear. Further work should examine the effect of tree architecture on the cycling of radiocesium both stemflow and branchflow. [ABSTRACT FROM AUTHOR]

A substantial quantity of radionuclides was deposited on the surrounding environment of the Fukushima prefecture, after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Radionuclides are discharged through river systems resulting in downstream contamination of irrigation water, paddy fields, ponds, and sea. In order to reduce the effects of such contamination, the Japanese government has engaged in decontamination works. In farmland and grassland, decontamination was performed by removing the contaminated topsoil and vegetation and recovering the area with uncontaminated soil. Along with the significant reduction of radiation levels, decontamination works have contributed to a critical change in the land-use and soil properties of decontaminated areas. Several decontaminated grassland and farmland are used now as temporal storage sites for the contaminated soils. Such land-use change can influence multiple parameters that control the hydrological, soil erosion, and sediment transport processes like soil infiltration capacity, soil erodibility, surface roughness, and the local capacity to store water and sediment. In this study, we monitored the sediment, water and radioactive Cesium (137Cs) discharges, at plot and basin scale, in order to assess the impacts of land cover and land-use change on soil erosion, hydrological processes and 137Cs discharge. Two USLE-type plots with a length of 22.13 m and a width of 5 m, were installed in 2011 and 2013. Those plots present the use of a grassland area before and after the decontamination works. Four forested headwater catchments with different grassland fractions (0 %, 1.5 %, 19 %, and 23%) were selected to investigate the impacts of land cover on 137Cs discharge. In one of those catchments (the Iboishi-yama catchment), decontamination works were conducted in the grassland area in December 2013. This catchment was served as a case of study to investigate the impacts of land-use change at the basin scale. The results of the present study provide clear evidence of the sensitivity of the hydrological process, soil erosion and 137Cs discharge to the land cover and the land-use change. First, the declining trend of the dissolved (in stream water) and particulate (attached to suspended sediment or coarse organic matter) 137Cs, in the four headwater catchments, was demonstrated with a two-component exponential and a one-component exponential model, respectively. The 137Cs was mainly discharged by suspended sediments (96 ~ 99 %) and the decline rate of the dissolved 137Cs was related to the grassland area extent in the different catchments. Second, we found that the decontamination works resulted in a radical decrease in the 137Cs discharge, a change in the critical radio-cesium source areas, and an increase in sediment yield and surface runoff in the Iboishi-yama catchment and the decontaminated plot. The event-based analysis of sediment flux from the Iboishi-yama catchment showed an increase in sediment yield by 5 to 8.5 times after the decontamination works. Third, the plot experiments showed a significate increase in sediment discharge (~ 40 times) after the decontamination works. Finally, the relationship between sediment and 137Cs discharge fluxes suggested the potential use of 137Cs as a good tracer of sediment source. [ABSTRACT FROM AUTHOR]

While 20 % of radionuclides released from the Fukushima Daiichi Nuclear Power Plant accident had been deposited in the terrestrial environment, rivers remain the long-term source for 137Cs, primarily through particulate transfer, from terrestrial to marine ecosystems. In this study, we estimated the suspended 137Cs flux to the ocean at 11 sites in the coastal area between October 2012 and December 2020 to be 17 TBq, based on our long-term monitoring data of concentrations of suspended 137Cs and suspended solids. The cumulative loss of suspended 137Cs from each site to the ocean ranged from 0.1 % to 1.7 % of initial deposition throughout the observation period, depending on the effect of dam lakes and normalized river discharge. The higher loss is also thought to be the large outflow of runoff during the typhoon. The current level of the suspended 137Cs concentrations was lowered to 1/10–1/100 of those immediately after the accident. The average value of the decreasing trend for each site was approximated using the equation: y = a1 e −k 1 t + a2 e −k 2t. The rate constant k 1 is higher in areas with intensive land use, such as pastures, bare land, and water surfaces, and k 2 is lower in urban areas. [ABSTRACT FROM AUTHOR]

Large quantities of radiocesium were deposited across a 3000 km 2 area northwest of the Fukushima Dai-ichi nuclear power plant after the March 2011 accident. Although many studies have investigated the fate of 137 Cs in soil in the months following the accident, the depth distribution of this radioactive contaminant in rice paddy fields requires further examination after the typhoons that occurred in this region. Such investigations will help minimize potential human exposure in rice paddy fields. Radionuclide activity concentrations, organic content and particle size were analysed in 10 soil cores sampled from paddy fields in November 2013, 20 km north of the Fukushima power plant. Our results demonstrate limited depth migration of 137 Cs with the majority concentrated in the uppermost layers of soils (<5 cm). More than 30 months after the accident, between 46.8 and 98.7% of the total 137 Cs inventories was found within the top 5 cm of the soil surface, despite cumulative rainfall totalling 3300 mm. Furthermore, there were no significant correlations between 137 Cs depth distribution and the other parameters. We attributed the maximum depth penetration of 137 Cs to grass cutting (73.6–98.5% of 137 Cs in the upper 5 cm) and farming operations (tillage – 46.8–51.6% of 137 Cs in the upper 5 cm). As this area is exposed to erosive events, ongoing decontamination works may increase soil erodibility. We therefore recommend the rapid removal of the uppermost – contaminated – layer of the soil after removing the vegetation to avoid erosion of contaminated material during the subsequent rainfall events. Further analysis is required to thoroughly understand the impacts of erosion on the redistribution of radiocesium throughout the Fukushima Prefecture. [ABSTRACT FROM AUTHOR]

Vertical profiles of radiocesium concentrations were measured in sediment cores collected at various times after the 2011 Fukushima nuclear accident in five Japanese lakes (Hinuma, Kasumigaura, Kitaura, Onogawa and Sohara) with different morphological and trophic characteristics in order to investigate the sedimentation-diffusion processes. In lakes where sediments had high porosities and experienced considerable wave action due to shallowness, we observed rapid penetration of radiocesium to a certain depth just after the accident, followed by downward movement of the peak depths. In contrast, gradual downward transfers of distinct peaks were found in other types of lakes. A one-dimensional differential sediment model with water-sediments interaction processes was constructed to describe the vertical shift of radiocesium profiles. Our proposed submodels relating to the length scales of the mixing using wind-induced stress and porosity of sediments were constructed based on one measurement of the vertical distribution of radiocesium in three lakes (Hinuma, Kasumigaura and Sohara). This model was then validated using samples from those lakes in different years, as well as from two other lakes. Good agreement was obtained. We discuss our findings, the limits of model application, and future research targets. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]