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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 137Cs between the suspended sediment (SS) and dissolved phases, K d , was introduced to better understand the dynamic behavior of 137Cs in different systems. However, the K d values in river water, ranging from 2 × 104 to 7 × 106 L kg−1, showed large spatiotemporal variability. Therefore, the factors controlling the 137Cs partition coefficient in natural water systems should be identified. Herein, we introduce a chemical model to explain the variability in 137Cs 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 137Cs and the binding sites in SS is the isotope exchange reaction between stable Cs and 137Cs, 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 137Cs passing through a filter is significant as the dominant dissolved species of 137Cs 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 137Cs in natural water. [Display omitted] • Partition coefficients of 137Cs between water and sediment are useful parameter to understand behaviors of 137Cs in environment. • Chemical model was applied to partitioning of 137Cs between natural water and suspended sediment. • Chemical model was based on concepts of isotope exchange equilibrium and mass balance including multi-reaction sites and colloidal forms. • Chemical model could confirm that the K d in fresh waters is inversely related to stable cesium (133Cs) concentrations. • 133Cs concentrations are one of the most important factors to control the K d in natural water systems. [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]

Abstract We examined spatial variation in vertical 137Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug–4 Oct), autumn (5 Oct–30 Nov) and winter (1 Dec–27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m2 areas with 3–5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m2/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m2/month. The ratio of 137Cs flux during the survey period to litter 137Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The 137Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the 137Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20–25 times. The spatial variations in the 137Cs flux were affected positively by those in the litter 137Cs inventory and negatively by canopy openness. Highlights • We examined variation in vertical dissolved 137Cs flux through litter. • Ratio of 137Cs flux to litter inventory during the survey was less than 10%. • Spatial variation showed log-normal distribution and had 20–25 times difference. • Spatial variation was determined by litter 137Cs inventory and canopy openness. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • 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]

Abstract 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. Highlights • 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 and deciduous broad-leaved forest was 10–20% faster than the physical decay of radiocesium. [ABSTRACT FROM AUTHOR]

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a significant release of radionuclides that were deposited on soils in Northeastern Japan. Plutonium was detected at trace levels in soils and sediments collected around the FDNPP. However, little is known regarding the spatial-temporal variation of plutonium in sediment transiting rivers in the region. In this study, plutonium isotopic compositions were first measured in soils (n = 5) in order to investigate the initial plutonium deposition. Then, plutonium isotopic compositions were measured on flood sediment deposits (n = 12) collected after major typhoon events in 2011, 2013 and 2014. After a thorough radiochemical purification, isotopic ratios ( 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 242 Pu/ 239 Pu) were measured with a Multi-Collector Inductively Coupled Mass Spectrometer (MC ICP-MS), providing discrimination between plutonium derived from global fallout, from atmospheric nuclear weapon tests, and plutonium derived from the FDNPP accident. Results demonstrate that soils with the most Fukushima-derived plutonium were in the main radiocaesium plume and that there was a variable mixture of plutonium sources in the flood sediment samples. Plutonium concentrations and isotopic ratios generally decreased between 2011 and 2014, reflecting the progressive erosion and transport of contaminated sediment in this coastal river during flood events. Exceptions to this general trend were attributed to the occurrence of decontamination works or the remobilisation of contaminated material during typhoons. The different plutonium concentrations and isotopic ratios obtained on three aliquots of a single sample suggest that the Fukushima-derived plutonium was likely borne by discrete plutonium-containing particles. In the future, these particles should be isolated and further characterized in order to better understand the fate of this long-lived radionuclide in the environment. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

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. [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]

Many studies on shallow landslides have focused on seepage mechanisms. However, most of these studies assumed that the hillslope was parallel to the seepage direction and force; they did not consider how the seepage was changed by the topography during failure. To investigate the seepage mechanism, particularly the influence of the topographic change during failure, flume experiments using a 30° sandy slope were performed with artificial rainfall. The factor of safety (FS) equation, which considers the seepage direction and force, and the numerical simulation were also applied to analyze the effect of topographic change on the seepage mechanism. The experiments were divided into two stages: rainfall-induced experiment (RIE) and seepage-induced experiment (SIE) with rainfall. In the RIE with a rainfall intensity of 80 mm/h, seepage erosion occurred followed by undercutting, which resulted from seepage erosion, to upward direction of sandy slope. The analysis of FS in RIE are showing an instability when seepage direction is be a slope parallel with theoretical value, i ( Hydraulic gradient ). In the numerical model, when topographic changes of the surface of the sandy flume were applied, seepage flow concentrated from failure surface of the toe of flume to middle part of flume, which is affected by saturation. In SIE, multiple failures caused by the seepage in the vertically upward direction occurred with a high seepage force, and the failure mass then moved in the downward direction with the seepage flow. FS for SIE showed a high instability than those of RIE when seepage direction with high force already changed from vertically downward seepage flow to increasing slope parallel seepage flow. Applying changed topography (more deep from sandy flume surface) in numerical model is showing concentrated seepage flow at the bottom of failures. This is one of mechanism developing failure mass to downward direction to the toe of flume because soil moisture sensors installed near failures (25 cm-depth) show high value than porosity value, indicating soil mobilization. Therefore, the seepage direction and force play important roles in landslide mechanism, and these factors should be considered in landslide studies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Throughfall ( TF ) is a critical component of the hydrological and biogeochemical cycles and is greatly influenced by stand structures in forested watersheds. Previous studies have examined the relationships between TF and stand structures for different species and regions. However, there remains acknowledged difficulty in estimation of TF with changing stand structures due to forest management (e.g., thinning) for specific species and regions. This study conducted intensive thinning with 43–50% stem removal at seven experimental plots with various structures (e.g., stand density, canopy cover and basal area) across Japan that were covered by Japanese cypress ( Chamaecyparis obtusa Endl.) and cedar ( Cryptomeria japonica D. Don) plantations. TF was measured by a set of 20 tipping-bucket rain gauges placed in a lattice-like pattern within the plots before and after thinning. Results showed that during the study periods, thinning caused an increase in the mean TF rate from 63.2 ± 7.0% to 75.4 ± 6.0%. The TF rate was significantly negatively correlated with stand density, canopy cover, and basal area on a stand scale. Additionally, the slope ( a TF ) of event-based linear TF - P g (gross rainfall) equation was significantly higher in post-thinning (0.782 ± 0.077) than that in pre-thinning (0.663 ± 0.078), whereas it’s intercept ( b TF ) showed no significant difference (−0.707 ± 0.810 mm in pre-thinning and −0.441 ± 0.607 mm in post-thinning). The a TF significantly decreased with increasing stand density and canopy cover, whereas the b TF was not significantly correlated with any forest-structure variables. Further, the TF rate and a TF were estimated, respectively, by their related stand-structural variables in multiple regression models with high determination coefficients and moderate relative errors. The b TF was assumed to be a constant and its mean value obtained from all the experimental plots in each (pre- and post-thinning) period. Thus, the models of stand-scale TF rate and event-based TF amount were developed with input of commonly forest inventories. These models were validated by using the dataset of this study and earlier publications for Japanese cedar and cypress plantations, and showed good fit between the estimated and measured values. These models are practical tools that can be readily used for assessing the changes in TF with changing stand structures at both stand and event-based scales, and have also potential implications in evaluating the spatial TF patterns at catchment scale and exploiting similar models in other species and regions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [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]

Hillslope hydrology studies have shown that bedrock surface topography maybe the most important factor affecting subsurface flow, and that soil thickness and soil properties related to topography are important factors in shallow landslide prediction. In this study, the physically based H-slider (hillslope-scale shallow landslide-induced debris flow risk evaluation) model was used to evaluate the effects of topography and soil parameterisation reflecting soil depth on shallow landslide prediction accuracy. Two digital elevation models (DEMs; i.e. ground surface and bedrock surface) and three soil thicknesses (average soil thickness, soil thickness to weathered rock and soil thickness to bedrock) and physical soil parameters (cohesion and internal friction angle) at a small hillslope site in Jinbu, Kangwon Prefecture, eastern part of the Korean Peninsula, were considered. Each prediction result simulated with the H-slider model was evaluated by receiver operating characteristic (ROC) analysis for modelling accuracy. The results of the ROC analysis for shallow landslide prediction using the ground surface DEM (GSTO) and the bedrock surface DEM (BSTO) indicated that the prediction accuracy was higher using the GSTO compared to the BSTO. Moreover, the prediction accuracy based on soil parameterisation reflecting soil thickness was highest in all cases. These results imply that the effect of soil parameters on shallow landslide prediction could be larger than the effects of topography and soil thickness. Therefore, we suggest that using soil properties representing the effect of soil thickness can improve the accuracy of shallow landslide prediction, which should contribute to more accurately predicting shallow landsides. [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]

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. [ABSTRACT FROM AUTHOR]

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

To investigate the effects of vegetation canopy on surface runoff, four runoff plots (2 m × 1 m in size) were established under different types of forest canopies (S1: sawtooth oak and Japanese larch; S2: Chinese cork oak; S3: shrub; and S4: Korean pine) on a mountain slope in central Korea. Rainfall, throughfall, and surface runoff were measured from July 2007 to October 2009. During the observation period, there was no storm event with a rainfall intensity that exceeded the average value for infiltration rates at the runoff plots. Each plot showed differences in surface runoff generation (S1 > S2 ≒ S3 > S4). The plots with broad-leaf litter layers had higher maximum water storage capacities than the plot with a needle-leaf litter layer. These results imply that the water storage capacity and the leaf shape of the litter layer affected surface runoff generation. Moreover, under dry conditions (<20 mm of API 7 ), the rainfall threshold for all plots, except S4, required for surface runoff generation decreased and surface runoff from all plots increased slightly compared with relatively wet conditions (>20 mm of API 7 ). Storm runoff coefficients at all plots decreased drastically as the value of API 7 increased. This suggests that the lowering of the rainfall threshold for surface runoff generation and the resulting increase in surface runoff might be related to the development of hydrophobic conditions on the soil surface caused by dry conditions. Therefore, in addition to leaf shape and water storage capacity of the litter layer, the soil water condition also can be an important factor in plot-scale evaluations of surface runoff generation from a slope well covered with litter layer. [ABSTRACT FROM AUTHOR]

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