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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. Measurement(s) Cesium-137 Gamma Radiation • Cesium-134 Gamma Radiation • Latitude • Longitude • grain size • radiocesium flux • radioactivity concentration Technology Type(s) gamma-ray spectroscopy • GPS navigation system • laser diffraction particle size analyzer • Calculation Factor Type(s) geographic location Sample Characteristic - Environment suspended sediment Sample Characteristic - Location Abukuma River • Fukushima Prefecture • Miyagi Prefecture Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.13055315 [ABSTRACT FROM AUTHOR]

Purpose: Spectrocolourimetric measurements provide a relatively inexpensive, quick and non-destructive alternative to the analysis of geochemical and organic matter properties. When used in the analysis of sediments and their potential sources, these colour parameters may provide important information on the dominant processes (i.e. erosion) occurring in the Critical Zone. Here, they are used to investigate whether eroded sediment is derived from forest (i.e. natural), cultivated (i.e. anthropogenic) or subsoil sources in order to assess their potential to monitor the effect of decontamination in regions impacted by fallout from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Materials and methods: Fifteen spectrocolourimetric properties (L*, a*, b*, C*, h, x, y, z, L, a, b, u*, v*, u', v') were measured in potential source (n = 37) and sediment (n = 400) samples collected during 13 campaigns from 2011 to 2017 after major flood events in two catchments (total surface area of 450 km2) draining the main FDNPP radioactive pollution plume. Potential sources included topsoil from forest and cultivated sources along with subsoil material originating from landslides, channel banks and the decontamination of cultivated areas. The optimum set of parameters used in the mixed linear model to calculate the sediment source contributions was obtained through the use of a range test, the Kruskal–Wallis H test and a linear discriminant analysis. Results and discussion: Nine selected colour parameters correctly classified 100% of the source samples (i.e. forest, subsoil and cultivated sources). The results illustrate that cultivated landscapes were the main source of sediment to these river systems (mean 56%, SD 34%) followed by subsoil (mean 26%, SD 16%) and forest sources (mean 21%, SD 24%). However, these contributions varied strongly over time, with a peak of subsoil contributions (mean 57%, SD 17%) in Fall 2015, coinciding with the occurrence of a typhoon after the remediation works. These results were consistent with monitoring studies conducted in the same area that showed the major impact of typhoon Etau in September 2015 on sediment and radiocaesium fluxes. Conclusions: These original results demonstrate that spectrocolourimetric measurements may contribute to the routine monitoring of the effectiveness of remediation works in this post-accidental context. Owing to the inexpensive, rapid and non-destructive analyses, spectrocolourimetric-based tracing methods have significant potential to provide information on the dominant erosion processes occurring in the Critical Zone. [ABSTRACT FROM AUTHOR]

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 137Cs 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 137Cs 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. [Display omitted] • Soil moisture content and air dose rate were estimated using effective rainfall. • Air dose rate changes were estimated using measured soil moisture content changes. • Magnitude of soil shielding effect was elevated by preceding rainfall. • This method might reveal influences besides rainfall on air dose rate changes. [ABSTRACT FROM AUTHOR]

The Fukushima Dai-ichi nuclear power plant accident released limited amounts of actinides on soils of Japan. Characterisation of these particles is essential to determine the fate of actinides in the environment. The method presented in this paper, based on α-tracks detections, microscope observations and mass-spectrometry measurements, was designed to identify and characterize actinide-bearing particles in soil samples. The method was tested on a road dust sample collected in the main radioactive plume of the Fukushima region. Accordingly, α-tracks detection was demonstrated to provide a powerful technique to localise these particles and prepare their morphological, elemental and isotopic characterization. [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]

The Fukushima Daiichi nuclear power plant (FDNPP) accident in March 2011 resulted in the fallout of significant quantities of radiocesium over the Fukushima region. After reaching the soil surface, radiocesium is quickly bound to fine soil particles. Thereafter, rainfall and snowmelt run-off events transfer particle-bound radiocesium downstream. Characterizing the precipitation regime of the fallout-impacted region is thus important for understanding post-deposition radiocesium dynamics. Accordingly, 10 min (1995-2015) and daily precipitation data (1977-2015) from 42 meteorological stations within a 100 km radius of the FDNPP were analyzed. Monthly rainfall erosivity maps were developed to depict the spatial heterogeneity of rainfall erosivity for catchments entirely contained within this radius. The mean average precipitation in the region surrounding the FDNPP is 1420 mm yr-1 (SD 235) with a mean rainfall erosivity of 3696 MJmmha-1 h-1 yr-1 (SD 1327). Tropical cyclones contribute 22% of the precipitation (422mmyr-1) and 40% of the rainfall erosivity (1462 MJmmha-1 h-1 yr-1 (SD 637)). The majority of precipitation (60 %) and rainfall erosivity (82 %) occurs between June and October. At a regional scale, rainfall erosivity increases from the north to the south during July and August, the most erosive months. For the remainder of the year, this gradient occurs mostly from northwest to southeast. Relief features strongly influence the spatial distribution of rainfall erosivity at a smaller scale, with the coastal plains and coastal mountain range having greater rainfall erosivity than the inland Abukuma River valley. Understanding these patterns, particularly their spatial and temporal (both inter- and intraannual) variation, is important for contextualizing soil and particle-bound radiocesium transfers in the Fukushima region. Moreover, understanding the impact of tropical cyclones will be important for managing sediment and sediment-bound contaminant transfers in regions impacted by these events. [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]

This study deals with the description of the vertical distribution of radiocaesium ( 137 Cs and 134 Cs) in a representative coniferous forest soil, investigated 10 months after the Fukushima radioactive fallout. During soil sampling, the forest floor components (understory plants, litter (Ol-) and fermented layers (Of)) were collected and treated separately. The results indicate that radiocesium is concentrated in the forest floor, and high radiocesium transfer factor observed in the undergrowth plants (3.3). This made the forest floor an active exchanging interphase for radiocesium. The raw organic layer (Ol + Of) holds 52% (5.3 kBq m −2 ) of the Fukushima-derived and 25% (0.7 kBq m −2 ) of the pre-Fukushima 137 Cs at the time of the soil sampling. Including the pre-Fukushima 137 Cs, 99% of the total soil inventory was in the upper 10 cm, in which the organic matter (OM) content was greater than 10%, suggesting the subsequent distribution most likely depends on the OM turnover. However, the small fraction of the Fukushima-derived 137 Cs at a depth of 16 cm is most likely due to the infiltration of radiocesium-circumscribed rainwater during the fallout before that selective adsorption prevails and reduces the migration of soluble 137 Cs. The values of the depth distribution parameters revealed that the distribution of the Fukushima-derived 137 Cs was somewhat rapid. [ABSTRACT FROM AUTHOR]

We analyzed fresh and dead leaves collected in forests in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using autoradiography. Both fresh and dead leaves of Cryptomeria japonica were contaminated by radionuclides (Cs and Cs). Contamination of the fresh leaves was possibly attributed to interception of radionuclides by tree canopies, whereas the dead leaves indicated the direct deposition of radionuclides by fallout and/or washout of radionuclides intercepted by tree canopies. Translocation of radiocesium from a contaminated branch to new leaves growing after the FDNPP accident was not clearly observed, although transfer of radiocesium from leaf parts to male flowers occurred. Fallen leaves of Quercus serrata, which started growing after the FDNPP accident, did not show radioactivity, indicating that significant amounts of translocation from other parts to new leaves did not occur. Fallen leaves of Q. serrata collected from a litter showed hot spots originating from direct fallout. Needles of Pinus densiflora were also contaminated by fallout. Leaching with pure water removed soluble fractions of radiocesium and hot particles from the surface of the contaminated leaves, but significant amounts of radioactivity remained. This means that foliar absorption occurred in both fresh and dead leaves. Further leaching experiments using surfactant and acetone could not remove the remaining radiocesium from the leaves. The leaching experiments indicate that radiocesium in the contaminated leaves is strongly fixed in leaf tissues and is not readily released unless leaf tissues are decomposed. [ABSTRACT FROM AUTHOR]

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 137Cs and 131I 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. • The sampling protocols applied just after the FDNPP accident have been reviewed and the protocols for emergency the soil and vegetation sampling were proposed. • As a result of Fukushima's outcome, vegetation sampling will be recommended when the additional dose rate (above background) is higher than 0.1 μSv/h. • Predetermined sampling points for soil and vegetation sampling and measurement exercises are highly recommended. [ABSTRACT FROM AUTHOR]

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]

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. • Ambient dose rate in Fukushima forests was repeatedly measured. • Ambient dose rates in mixed broad-leaved and deciduous forests were higher than evergreen conifers. • Ambient dose rate in evergreen conifer decreased slower than mixed broad-leaved and deciduous broad-leaved forests for the period of 2011–2014. • Decline of dose in evergreen conifer during the period of 2011–2014 was slower than that induced solely by physical decay of radiocesium. • Decline of dose in mixed broad-leaved and deciduous broad-leaved forest was 10–20% faster than the physical decay of radiocesium. [ABSTRACT FROM AUTHOR]

Since headwater catchments are the source areas of 137Cs for downstream river systems, 137Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and 137Cs 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 137Cs concentrations (137Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10−6 m2/L, 10−2 m2/kg, and 10−1 m2/kg, respectively, before 2013 and declined to around 10−8 m2/L, 10−4 m2/kg, and 10−2 m2/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas 137Cs concentrations in SS declined significantly and the total flux of 137Cs decreased. Although the clear effect of land use on decline trend in normalized 137Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them. • 137Cs concentrations were monitored at headwater catchments after FDNPP accident. • Normalized 137Cs concentrations declined 1-2 order during 2011–2016. • 137Cs concentrations in suspended sediment declined after the decontamination work. • Total flux of 137Cs decreased after the decontamination work. [ABSTRACT FROM AUTHOR]

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of 137Cs 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 137Cs in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of 137Cs 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 137Cs concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80–95% of the deposited 137Cs on the forest floor distributed in mineral soil layers by 2017. The percentage of 137Cs inventory in the litter layer to the total 137Cs 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 137Cs fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the 137Cs concentration in the first few cm of mineral soil surface gradually increased and became higher than the 137Cs concentration in the litter within 2–3 y of the accident. The 137Cs concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of 137Cs 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 137Cs and its factors regarding both forest and soil characteristics. • 137Cs concentration in soil was fitted by exponential equation at least for 6 y. • Migration from litter to mineral soil was more rapid than in the case of Chernobyl. • 137Cs concentration in surface soil surpassed that of litter within 2–3 y. • Relaxation depth in the two cedar forests has gone down with 0.08 cm y−1. • In contrast, the downward migration in the mixed forest was not clearly observed. [ABSTRACT FROM AUTHOR]

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 137Cs 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. • Initial 137Cs fallout map was established based on airborne monitoring surveys. • Reduction of the measured inventories between the third and fifth airborne surveys varied between different landuses. • Forest area accumulated 72% of total 137Cs deposition on land. • Total 137Cs deposition on land area of Japan was calculated as 2.5 PBq. [ABSTRACT FROM AUTHOR]

Understanding solid 137Cs 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 137Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of 137Cs 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 137Cs activity concentration in sediment and solid wash-off processes of 137Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized 137Cs solid entrainment coefficient, defined as Sc (m2 kg−1), was calculated by dividing 137Cs activity concentration in sediment by initial 137Cs deposition. A particle size-corrected Sc , defined as Sc correct (m2 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 m2 kg−1 and 0.0052–0.078 m2 kg−1, respectively. Annual wash-off rates of solid 137Cs 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 137Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity. • Solid 137Cs wash-off were observed using eight erosion plots during 2011–2014. • There was no significant decreasing trend in 137Cs concentration in most of the plots. • The 137Cs concentration in sediment significantly decreased on bare soil. • The 137Cs concentration in sediment decreased only due to significant soil erosion. [ABSTRACT FROM AUTHOR]

The study investigated temporal changes in the 137Cs concentrations in vegetal and hydrological samples collected from various forests in Yamakiya District, Kawamata Town of Fukushima prefecture over six years following the Fukushima Dai-ichi nuclear power plant accident. Cesium-137 was detected in all forest environmental samples. However, the concentration in most samples decreased exponentially with time. The 137Cs concentrations in throughfall samples exhibited a double-exponential decreasing trend with time. Temporal changes in the 137Cs concentration in vegetal samples and stemflow were approximated by using a single-exponential equation. A comparison of the decline coefficient for the latter observation period (>2 y since the accident) revealed that the declining trend of 137Cs concentrations varied between foliage and the outer barks of the Japanese cedar and Japanese konara oak trees. The 137Cs concentration in cedar needles decreased exponentially while that in konara oak leaves was constant over the last six years. Conversely, the declining trend of 137Cs concentration in the outer bark of konara oak exceeded that of cedar. The results suggested that self-decontamination processes and internal recycling of 137Cs varied among tree species and different tree parts. The results indicated that the leaching of 137Cs in the throughfall in Japanese cedar was dependent on the 137Cs concentration in needles. However, a comparison of 137Cs concentrations in vegetal and hydrological samples from each sampling year showed that the leaching rate decreased with time. Conversely, the 137Cs concentrations in the stemflow were independent of the concentrations in the outer bark. The declining trend of 137Cs concentrations in litterfall (λ : 0.31–0.33 y−1) was similar to that of the mean of new/old needles (λ : 0.26–0.33 y−1) for cedar stands. With respect to the hydrological components, the 137Cs concentration in the stemflow (λ : 0.32–0.33 y−1) decreased at a slightly slower rate than that in the throughfall (λ : 0.36–0.54 y−1) for the cedar forest. The decline coefficients of 137Cs concentration in the aforementioned types of hydrological components slightly exceeded that for the vegetal samples. The results suggest that monitoring of 137Cs concentrations in hydrological components and vegetal samples can aid in further understanding the leaching mechanisms of 137Cs from trees to rainwater. • The decline trend of 137Cs concentrations in hydrological and vegetal samples over 6 y was determined for Japanese forests. • Vegetal samples exhibited a single-exponential declining trend over 6 y. • The decline in the 137Cs concentration in the throughfall was approximated via a double-exponential model. • The decline in the 137Cs concentration in the stemflow was slightly slower than that in the throughfall for a cedar forest. • The leaching rate of 137Cs from foliage to throughfall decreased with respect to time. [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]

Schematic diagram of the groundwater and soil water mixing by means of dissolved 137Cs. • Dissolved 137Cs in stream water derived mainly from soil water. • Low impact of dissolved 137Cs in groundwater was confirmed. • 137Cs stored in the shallow unsaturated area was the source of temporary increase. Numerous studies have been conducted on 137Cs transport in the environment in Fukushima, Japan, since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Many studies have monitored river and stream water, and a declining trend in dissolved 137Cs concentrations has been reported. In addition, temporary increases in dissolved 137Cs concentrations have been observed during rainstorm events. However, the source and process of such temporary increases remain poorly understood. To address these gaps in knowledge, intense multi-point sampling and analysis of dissolved 137Cs in stream water, soil water, and groundwater were conducted in a forested headwater catchment in Yamakiya district, located 35 km northwest of the FDNPP, from July 2015 to November 2016. The average dissolved 137Cs concentration in stream water during baseflow conditions was 17 mBq/L, whereas the concentration during rainstorm events was 31 mBq/L (maximum: 170 mBq/L on July 15, 2015). The dissolved 137Cs concentration in groundwater (average: 0.93 mBq/L) was lower than that in stream water. Conversely, high concentrations of dissolved 137Cs in soil water were observed (average: 81 mBq/L), particularly at lower parts of the slope near the spring point and in wet conditions throughout summer. Consequently, the dissolved 137Cs concentrations in stream water were considered to be derived mainly from soil water and diluted by groundwater. The high dissolved 137Cs concentration in soil water may be derived from litter leachate in the shallow soil layer stored on the slope. During rainstorm events, the saturated area expands upward; therefore, the temporary increase in the dissolved 137Cs concentration in stream water observed at the beginning of a rainstorm may be caused by the release of high levels of dissolved 137Cs in soil water stored in the shallow soil layer. [ABSTRACT FROM AUTHOR]

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in radiocesium fallout contaminating coastal catchments of the Fukushima Prefecture. As the decontamination effort progresses, the potential downstream migration of radiocesium contaminated particulate matter from forests, which cover over 65% of the most contaminated region, requires investigation. Carbon and nitrogen elemental concentrations and stable isotope ratios are thus used to model the relative contributions of forest, cultivated and subsoil sources to deposited particulate matter in three contaminated coastal catchments. Samples were taken from the main identified sources: cultivated ( n = 28), forest ( n = 46), and subsoils ( n = 25). Deposited particulate matter (n = 82) was sampled during four fieldwork campaigns from November 2012 to November 2014. A distribution modelling approach quantified relative source contributions with multiple combinations of element parameters (carbon only, nitrogen only, and four parameters) for two particle size fractions (<63 μm and <2 mm). Although there was significant particle size enrichment for the particulate matter parameters, these differences only resulted in a 6% (SD 3%) mean difference in relative source contributions. Further, the three different modelling approaches only resulted in a 4% (SD 3%) difference between relative source contributions. For each particulate matter sample, six models (i.e. <63 μm and <2 mm from the three modelling approaches) were used to incorporate a broader definition of potential uncertainty into model results. Forest sources were modelled to contribute 17% (SD 10%) of particulate matter indicating they present a long term potential source of radiocesium contaminated material in fallout impacted catchments. Subsoils contributed 45% (SD 26%) of particulate matter and cultivated sources contributed 38% (SD 19%). The reservoir of radiocesium in forested landscapes in the Fukushima region represents a potential long-term source of particulate contaminated matter that will require diligent management for the foreseeable future. [ABSTRACT FROM AUTHOR]

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 (137Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the 137Cs inventory in sediments increased during 2011–2014; however, few changes were observed during 2014–2016. The 137Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating 137Cs migration. However, the rate of decrease subsequently slowed due to the 137Cs supply from the river. The 137Cs 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 137Cs 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 137Cs concentrations will be smaller. The 137Cs concentration in the Koise River will continue to decrease; thus, the difference in the 137Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of 137Cs in the entire lake were 3.72 × 1012 Bq in December 2012 and 4.18 × 1012 Bq in August 2016. The accumulated amount of 137Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate 137Cs 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 137Cs 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. [Display omitted] • Riverine input and accumulation of 137Cs was measured in Lake Kasumigaura, Japan. • 137Cs inventory in sediments increased until 2014 but changed little thereafter. • Particulate 137Cs concentration in two inflowing rivers declined for 2012–2015. • 137Cs accumulation in lake sediments was close to the riverine input of 137Cs. • The lake shows high performance in trapping particulate matter. [ABSTRACT FROM AUTHOR]

Data on reduction of radioactivity in plants are highly important for making decision on emergency response and remediation of contaminated areas. Dynamics of the 131I and 137Cs 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 131I and 137Cs 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 137Cs- 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 137Cs 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. • Effective half-lives for 131I in the weed ranged 3.8–4.2 days for the first two months after the major deposition events. • Effective half-lives for 137Cs in the weed leaves ranged 7.1–13.3 days for the first two months after the deposition. • T ag to the weed observed during first year after the FDNPP accident are similar to those measures after Chernobyl accident. • First half-lives for 137Cs soil to plant C r in 2012–2016 ranged 1.0 and 2.5 years and second half-lives - 6.3–7.3 years. • Half-lives values for 137Cs C r in 2011-2016 comply with the similar data derived after the Chernobyl accident. [ABSTRACT FROM AUTHOR]

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 137Cs 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 137Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured 137Cs activities adjacent to the 1F NPP. River discharge rates of 137Cs were estimated by multiplying simulated river flow rates by the dissolved 137Cs activities, which were estimated by an empirical function. Inflow of 137Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of 137Cs activity was large, the simulated results were compared with the annual averaged observed 137Cs activity distribution. Normalized annual averaged 137Cs 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 137Cs 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 137Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic 137Cs activity was small compared to the effect of directly released 137Cs. • The direct release rate of dissolved 137Cs was estimated by the end of 2016. • The river discharge rate of dissolved 137Cs was estimated from 2013 to 2016. • The annual averaged 137Cs activity distribution patterns were similar in each year from 2013 to 2016. • Simulated 137Cs activities attributable to direct release were in good agreement with measured activities. • The effect of river discharge of 137Cs was not detected on oceanic 137Cs distributions. [ABSTRACT FROM AUTHOR]

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. Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration. • Water samplings were implemented in radionuclide contaminated areas. • Unique tritium data in water implied the influence of Fukushima accident. • Groundwater age was estimated at 10 to 26 years by using CFCs tracers. • Young water fraction was suggested as radionuclide sources in groundwater. [ABSTRACT FROM AUTHOR]

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following theGreat East Japan Earthquake and tsunami of 11 March 2011 resulted in the release ofradioactive materials to the ocean. Measured 137Cs activities in the coastal zone adjacent tothe 1F NPP are still higher than the one before the accident because sources to the oceanare still existing. The Regional Ocean Model System (ROMS) was employed forregional-scale simulation of 137Cs activity in the ocean offshore of Fukushima, the sourcesof radioactivity being direct release, atmospheric deposition, the inflow of 137Csdeposited into the ocean by atmospheric deposition outside the domain of the model,and river discharges. Direct release of 137Cs was estimated for 7 years after theaccident by comparing simulated results and measured activities adjacent to theaccident site. In addition, riverine discharge rates 137Cs were also estimated bymultiplication between river flow simulation rate and measured 137Cs activities.Simulated atmospheric deposition to the ocean was employed by atmospheric transportmodel. Inflow of 137Cs from boundary sections was set by the results of the NorthPacific scale ocean model. Sensitivity experiments were carried out to investigate thecontributions of each source to measured 137Cs activities in the ocean. We focused on theterm from 2013 to 2016 because there were few data in the river before 2012, andalso focused on dissolved 137Cs because most of 137Cs is dissolved form in theocean. Simulated 137Cs activity attributable to direct release were in good agreement withmeasured data in the coast zone adjacent to the 1F NPP, because the effect of direct releasewas dominant from 2013 to 2016. On the other hands, simulated results attributable to inflowfrom boundary sections were slightly underestimated to the measured data offshorearea. This suggests that recirculation of subducted 137Cs to the surface layer wasunderestimated in the North Pacific model. Apparent half-life of direct releasedand river discharged 137Cs activity were estimated to be about 1 year and 2 years,respectively. And apparent half-life of inflow of 137Cs activity was much longer dueto time scale of dilution process in the North Pacific. Apparent half-life of eachsource should be similar to the measured one attributable to each source. Apparenthalf-life of measured 137Cs activity adjacent to the 1F NPP was about 1 year, on theother hand, the ones increased with increasing distance from the 1F NPP. Apparenthalf-life of measured data was about 2 years in front of the Uda river mouth whereis far from the 1F NPP. Although 137Cs activity in this area was mainly affectedby the river input, simulated 137Cs activities with river input were one order ofmagnitudes smaller than observations. There is a brackish lagoon, Matsukawa-urain front of Uda river mouth. This underestimation suggests the removal processfrom particle 137Cs to dissolved form in the brackish lagoon may be important. [ABSTRACT FROM AUTHOR]