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In this study, we applied a new methodology that utilized stable isotopic concentrations of water and SF6 concentrations of the air to investigate temporal variations in the spring water mean transit time (MTT) of a forested headwater catchment in Japan. We monitored the area from June 2010 to November 2021, including the forest thinning that occurred in October 2011. The SF6 concentrations were used to estimate the apparent age of the spring water, which was used to initiate a parameter calibration that determined the best possible MTT for each study period. The MTT was evaluated by estimating the d-excess variations of the spring water stable isotopes using the exponential piston flow model. The MTT ranged from 40 to 55 months and tended to increase soon after the forest was thinned. In contrast, the MTT was shorter by approximately 6 years after thinning occurred. The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest management practices improved the results and can be used for better forest and subsurface water resource management. [ABSTRACT FROM AUTHOR]

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

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]

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]

Summary We examined the effect of strip thinning on rainfall interception ( E i ) in a 32-year-old Japanese cypress plantation in central Japan. Strip thinning was conducted in the catchment in October 2011; that removed 50% of the stems. The gross precipitation ( P g ), throughfall ( TF ) and stemflow ( SF ) were monitored in a 12-m × 13-m plot before and after thinning. E i was calculated as the difference between P g and the sum of TF and SF . The interception processes were illustrated using the revised Gash model with quantifying interception parameters. The results showed that the Gash model successfully predicted E i on a rainy-season basis in both pre- and post-thinning periods. Thinning altered the interception components whereas the largest part during and after rainfall accounted for similar proportion in both periods. Additionally, after thinning, the annual TF rate was increased from 61.4% to 73.0%, whereas the annual SF rate was decreased from 9.8% to 6.1%, and the annual E i rate was decreased from 28.7% to 20.8%. The summarized findings of previous studies indicate that the degree of decline in the E i caused by thinning is related to P g and the thinning ratio. These results provide useful information for understanding the changes in interception processes induced by thinning, and for acquiring a more accurate forecast of the effects of forest management practices on water resources in the watershed. The response in rainfall partitioning to strip thinning can also help us to acquire an integrated understanding of the changes in canopy water balance under different forest practices. [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]

To determine the effects of the characteristics of litter and surface soil on the infiltration capacity in bamboo groves, infiltration rates were measured on a plot-scale using an oscillating-nozzle rainfall simulator at seven Moso bamboo ( Phyllostachys pubescens) sites and one Madake ( P. bambusoides) site in Japan. The maximum final infiltration rate (FIR), which was defined in this study as the infiltration capacity, was calculated, and the correlation between FIR and earth surface properties was analyzed. The FIR ranged from 9 to 308 mm h, showing little correlation with litter as surface cover, and a weak correlation with bulk density (0.3-0.9 g cm) and organic matter content (10.4-45.9%). These results suggest that the bulk density and organic matter content of surface soil, rather than surface cover, can be the controlling factors of infiltration capacity in bamboo groves. The bulk density of surface soil correlated significantly with the soil organic matter content, represented by bamboo rhizomes, suggesting that the infiltration capacity can increase with the amount of root networks present, related to macropore formation. The stand density showed a weak positive correlation, although not significant, with the soil organic matter content and a positive correlation with FIR. Thus, the infiltration capacity of bamboo groves can increase as the stand density of culms and bamboo rhizomes increase. [ABSTRACT FROM AUTHOR]

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

The article presents a study on the estimation of the medium-term erosion rate of semi-arid grassland hillslope in Japan. The study intends to evaluates the extent to which a resurvey of the magnetite distribution on the hillslope can provide insight into the fine soil particles' movement in natural rainfall events. The rainfall-simulation experiments were performed to be able to determine the travel distance of the magnetite.

The article presents a study which examines the connections between watershed and flush parameters and determines whether flush exists at both the seasonal and event levels. The study quantifies three parameters that describe the characteristics of the flush and investigates flush characteristics of chemicals from four forested watersheds in Japan at both seasonal and event scales. Results show that the flush characteristics are complicated and depended on the elements of the hydrochemistry.

The article presents a study on the role of the bedrock groundwater and the subsurface flow in the rainfall-runoff process with the use of a tracer at Shiranui in Kumamoto, Japan. The study intends to determine the contribution of the bedrock groundwater to rainfall-runoff processes. The study reveals the interrelation of the groundwater table's location to the bedrock groundwater and soil water.

Despite continued interest in stream water temperature (Tw) analysis, there are few studies of Tw response to rainfall events at forested watersheds. We examined 61 sets of data on Tw for 21 rainfall events at 16 watersheds with various slope gradients (from 0.08 to 0.56) in four regions of Japan from June 2004 to December 2005. The investigation focused on the changes of specific discharge (ΔQs) and ΔTw at medium-sized watersheds (0.5–100 ha). The results clearly demonstrated different flow patterns expressed by Qs vs. Tw hysteretic loops. Those were clockwise in Period I (April–September) and counterclockwise in Period II (October–March), except for lower slope gradient at Aichi, where counterclockwise loops were observed in both periods. These differences in hysteretic loops could be explained by the differences in Tw and in response times to rainfall between surface/subsurface and groundwater flows. The response times were probably determined by the slope gradient and the vertical level of groundwater. We also found that the changes in air temperature (ΔTa) influenced ΔTw to a lesser degree than Qs. The average rainfall intensities in Period I and Period II (9.3 ± 1.7 and 5.4 ± 0.2 mm/h, respectively) affected the average values of ΔQs and ΔTw (6.62 ± 4.08 mm/h and 1.7 ± 0.4°C; 0.85 ± 0.68 mm/h and 0.9 ± 0.3°C, respectively). This indicates that slope gradient and Qs influenced ΔTw by changing the relative proportions of flow paths. In addition, the water table changes influenced the percentage of groundwater flow to the stream. [ABSTRACT FROM AUTHOR]

The article offers information on a study conducted on the increasing forest degradation in Japan due to poor management. It states that in this study, the ecological characteristics have been examined including soil water content (SWC) and leaf water content of healthy and degraded forests. Moreover, the study shows that the SWC of the degraded Japanese cypress plantations was lower than those being well-managed Japanese cypress plantations.

The article focuses on a field investigation conducted on the effects of rainfall and slope angle on splash detachment, applying 27 splash cups in a Japanese cypress plantation in southern Japan. The study shows that the kinetic energy of throughfall was constant and independent of rainfall intensity, suggesting that splash detachment can be associated to soil slope and throughfall intensity. Moreover, there is no consistent relationship between the slope angle and splash detachment.

The article presents a study to quantify the impact of the abandonment of forest management practices on the runoff of surface-derived suspended sediment at the catchments scale. In this study, the relative contributions of soil erosion among Japanese cypress plantation catchments was compared with different stand densities and surface coverage. Moreover, the magnitude of surface soil erosion and the particular runoff of surface soil was affected by the light condition of the forest floor.

The article offers information on the studied flow pathways applying geochemical tracers and hydrometric observations in a 4.9 hectare cypress watershed in Japan. It states that there are measurably significant amounts of overland flow to all types of hillslopes was indicated in the hydrometric observations, regardless of the underwater vegetation's condition. Furthermore, the overland flow imparted around 30% of storm runoff in specific small catchments with thin understorey vegetation.

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

This article discusses the relationship between runoff and soil water repellency under Japanese cypress (Chamaecyparis obtusa) forests in Japan. The authors combined the critical surface tension (CST) test with a spraying experiment in which the simulated rainfall infiltration rates of water and ethanol solutions sprayed over plots were compared. They found that water repellency and the resultant surface runoff depended on the history of rainfall at the site. They concluded that relationships between soil water content and soil water potential indicate that changes in the soil water repellency and consequently surface runoff could occur during a single storm event.

We investigated the characteristics of nutrients (N and P) and dissolved organic carbon (DOC) concentrations during high, intense typhoon storm and base flow periods from three watersheds (W1, W2 and W3) in central Japan. Concentrations in base flow were low and relatively constant. In contrast, concentrations of N, P, and DOC in runoff during typhoon storms became highly dynamic. Particulate nitrogen (PN) and particulate phosphorus (PP) concentrations in runoff were 4-50 times greater than those in base flow for all storm events. Because PN and PP concentrations were significantly correlated with suspended sediment concentrations, particulate nutrient transport may be associated with overland flow in forested hillslopes. Dissolved organic nitrogen (DON), NO3-N, DOC and dissolved phosphorus (DP) concentrations in runoff were 1-10 times greater than those in base flow. All nutrients showed a clockwise, discrete hysteresis pattern, which implied that the sources of nutrients were from forested hillslopes. Additionally, the time constant (t1/e) calculated from a decreasing function suggested that particulate nutrients were mainly sourced from surface flow, whereas dissolved nutrients were provided from both surface flow and subsurface flow. The ratios of DOC : nitrate and UV : DOC suggested that higher values occurred in the early period of a storm event and was potentially related to the occurrence of overland flow. Our findings suggested that investigation of the parameters (e.g. DOC:nitrate and UV:DOC) were potentially useful for identifying sources of nutrients and potential applications of hydrological separation. Our results also contribute towards understanding the nutrient dynamics during intense storm events when various hydrological processes such as surface and subsurface runoffs govern the nutrient concentrations in headwater streams. [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]

A substantial quantity of radiocesium was deposited on the Fukushima prefecture, Japan in 2011 because of the Fukushima Daiichi nuclear power plant accident. On farmland, contaminated surface soil was removed to reduce the radiation dose rate because radiocesium is strongly attached to fine soil particles. Instead of the removed soil, decomposed granite, which has larger soil particles and less organic materials, was added. Such large-scale land use change can affect the sediment transport system. Soil erosion plot is generally applied to quantify the soil erosion, but this method cannot reveal how the sediment is detached, transported and deposited. In order to understand soil particle movement and soil redistribution, Unmanned Aerial Vehicle – Structure from Motion (UAV-SfM) method, which enables to obtain 3D data of the soil surface and detect soil surface changes, can be used. Radio Frequency Identification (RFID) tag can also be applied to trace soil particle movement (Parsons et al., 2014). It can show how the sediment transport system is different in the plot. However, it takes so much time to locate the RFID tag with a total station. This issue would be solved by means of high-resolution orthomosaic image derived with UAV-SfM method.This research has two objectives: (1) assessing the accuracy of UAV-SfM measurement and the survey of the RFID tag location with the orthomosaic image. (2) applying UAV-SfM method and soil particle tracing with the RFID tag to study the sediment transport processes on decontaminated slope farmland. Study sites were located in Kawamata town (decontaminated) and Date city (not decontaminated), Fukushima prefecture, Japan. USLE plots (22.13 m × 5 m) were installed on each slope farmland. These plots were kept with no vegetation and no cultivation. Water and sediment discharge were measured at the outlet of each plot. The accuracy of 3D data derived from the UAV-SfM method was assessed by comparing the data with those derived from 3D laser scanning. Locations of RFID tags were determined by using Orthoimage derived from UAV-SfM method and were compared with those measured with a total station. We found that the accuracy of UAV-SfM can be minimized as 3.3 mm (S.D.). RFID tags also can be located with an accuracy of 28 mm (RMSE). The results of UAV-SfM surveys showed no rill erosion in the decontaminated plot (DP) and the development of micro-terraces, and the result of RFID tracing showed a short transport distance in inter-rill zones at DP. After the decontamination works, the runoff and sediment yield decreased in DP. Our results suggest that the combination of these methods can provide new insights to understand sediment transport processes, and the decontamination works affected the water and sediment discharge. [ABSTRACT FROM AUTHOR]

• The prokaryotic density was 103 to 106 cells/mL in groundwater and spring water. • Groundwater with longer residence time contained thermophilic bacteria. • The community constituents had been changed among the groundwater flow processes. • Prokaryotic information can reflect changes in groundwater flow processes. Besides surface water systems, groundwater also provides a habitat for prokaryotes. The prokaryotes inhabiting groundwater are affected by the environment that they grow in. Owing to the environmental specificity, some prokaryotic community constituents might thus provide information on local subsurface growth conditions as well as advective transport processes in case of rapid dislocation. In this study, we investigated the spatiotemporal distribution of prokaryotic communities in groundwater and spring water to examine the relationship between the groundwater flow system and prokaryotic dynamics. We sampled and monitored the water in a mountainous forested catchment in Kawamata Town, Fukushima Prefecture, Japan. Seven piezometers and a spring located across a ridge, hillslope, and valley were monitored. Prokaryotic information, such as density estimated by total direct counting, community constituents and diversity using the next-generation sequencing methods, physicochemical parameters, general water quality, and groundwater mean residence time using sulfur hexafluoride (SF 6) were analyzed. Our results showed that the mean residence time of groundwater and spring water ranged from less than 1 year to 30 years. The prokaryotic density ranged from 5.37 × 103 cells/mL to 2.01 × 106 cells/mL (n = 60). As the mean residence time increased, the density of prokaryotes in the shallow subsurface also increased, whereas that of the prokaryotes in the deep subsurface decreased. The contribution of deep groundwater to the examined groundwater is inferred using the groundwater residence time and prokaryotic information, including density and specific constituents. The stable water isotopes did not suggest the deep groundwater contribution to the groundwater at the ridge in the high-rainfall period; however, the SiO 2 concentration indicated that. Elucidating the prokaryotic information along with existing hydrological tracers may enable a better understanding of groundwater flow processes. [ABSTRACT FROM AUTHOR]

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

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

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

Summary: We examined the effect of forest thinning on runoff generation at plot and catchment scales in headwater basins draining a Japanese cypress (Chamaecyparis obtusa) forest. We removed 58.3% of the stems (corresponding to 43.2% of the basal area) in the treated headwater basin (catchment M5), and left the control catchment (M4) untreated. In both catchments, we monitored overland flow from hillslope plots and stream runoff from catchments at basin outlets over a 2-year pre-thinning period and a 2-year post-thinning period. Paired catchment analysis revealed that annual catchment runoff increased 240.7mm after thinning. Delayed runoff increased significantly, while quick runoff followed similar patterns in the pre- and post-thinning periods. Flow duration in the ephemeral channel in catchment M5 increased from 56.9% in the pre-thinning period to 73.3% in the post-thinning period. Despite the changes in hydrological responses at the catchment scale, increases in overland flow were not significant. The increased availability of water in the soil matrix, caused by decreased interception loss and evapotranspiration, increased base flow after thinning. Based on the summarized data of previous studies together with this study, the effects of forest thinning on increases in runoff were less than partial harvesting in which the managed areas were concentrated within a watershed. We demonstrated that the effect of forest thinning was strongly scale dependent, an important finding for optimizing water and forest management in forested watersheds. [Copyright &y& Elsevier]

Abstract: Understanding the seasonal and regional patterns of nitrogen (N) components in the headwaters of forested watersheds is important for forests management. Here, we investigated the NO3-N concentration and its seasonal variations in the baseflow headwaters in Japanese coniferous forests with different N saturation status in five regions (i.e., Nagano, Tokyo, Aichi, Kochi and Mie). We found that the ratios of NO3-N to DON were significantly higher in the N-saturated regions including Nagano (14.1±2.2) and Tokyo (16.5±4.1), compared with the regions, which didn''t experience N saturation including Aichi (1.4±0.2), Kochi (5.1±1.8), and Mie (2.2±0.5). In comparison with the regions without experiencing N saturation, the relatively higher NO3-N concentration, as well as its contrastive variation between growing season and dormant season, was also characteristic of the seasonal NO3-N concentration in the N-saturated regions. Our findings have indicated that NO3-N concentration, its seasonal variations, and the ratio of NO3-N to DON, are the better and applicable indicators for evaluating N saturation status in Japanese coniferous forests. [Copyright &y& Elsevier]

A comprehensive investigation on all dissolved nitrogen and phosphorus components at both local and regional scales in the headwaters from forested watersheds is valuable to improve our understanding of the factors controlling water quality. Here, we investigated the baseflow concentrations of dissolved nitrogen and phosphorus components, N:P ratio, and their associations with region and vegetation type in forested headwaters in fives regions of Japan. We found that inorganic nitrogen and phosphorus were the dominant components in the 26 temperate forested streams, rather than organic forms. There were significant positive correlations between the concentrations of N and P components. Furthermore, the regional patterns of the concentrations of nitrate, dissolved inorganic P (DIP), and dissolved total N (DTN) and P (DTP) were similar. Our results suggest that the regional patterns of the concentrations of N and P components should be related to the regional atmospheric deposition of both N and P nutrients. We also found that the nitrate and DTN concentrations were higher in man-made evergreen conifer (EC) than those in the natural deciduous broadleaf (DB). In contrast, the DIP and DTP concentrations in EC were lower than those in DB. The uniformly higher N:P ratio in EC- than in DB-forested streams for each region suggest that EC-forested streams could be more affected by P-limited than DB-forested streams when N inputs from atmospheric sources increased. [Copyright &y& Elsevier]

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]

Approximately 70 % of the area highly 137 Cs-contaminated by the Fukushima Daiichi Nuclear Power Plant accident is forested. Decontamination works in most of these forests have not progressed, and the forestry industry remains stagnant. Although the long-term dynamics of 137 Cs in the forest ecosystem will be controlled by the amount of 137 Cs absorbed by roots in the future, temporal changes in 137 Cs of tree roots have rarely been reported. In the present study, we monitored the depth distribution of 137 Cs in the soil and absorptive very fine (VF) roots of 0.5 mm or less in a Japanese cedar forest from 2011 to 2023. As a result, the 137 Cs inventory in the mineral soil increased over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots tended to decrease since 2020, although there was a large variation. Temporal decrease in the exchangeable 137 Cs fraction with fixation and temporal increase in VF root biomass with their growth were not clearly observed, the 137 Cs concentration in the VF roots at 0-2 cm decreased with the decrease in 137 Cs concentration in the litter layers. Although the 137 Cs concentration in the VF roots below 2 cm tended to increase with increasing 137 Cs concentration in the soil at the same depth, the downward migration of 137 Cs within the soil can reduce the amount of 137 Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, 137 Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil. This natural migration process can be regarded as a "self-cleaning" of the forest ecosystem, the green and sustainable remediation using such self-cleaning should be actively adopted for the future forest management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

After the Fukushima Daiichi nuclear power plant accident, the terrestrial environment became severely contaminated with radiocesium. Consequently, the river and lake water in the Fukushima area exhibited high radiocesium levels, which declined subsequently. The partition coefficient of 137 Cs between the suspended sediment (SS) and dissolved phases, K d , was introduced to better understand the dynamic behavior of 137 Cs in different systems. However, the K d values in river water, ranging from 2 × 10 4 to 7 × 10 6  L kg -1 , showed large spatiotemporal variability. Therefore, the factors controlling the 137 Cs partition coefficient in natural water systems should be identified. Herein, we introduce a chemical model to explain the variability in 137 Cs K d in natural water systems. The chemical model includes the complexation of Cs + with mineral and organic binding sites in SS, metal exchange reactions, and the presence of colloidal species. The application of the chemical model to natural water systems revealed that Cs + is strongly associated with binding sites in SS, and a major chemical interaction between 137 Cs and the binding sites in SS is the isotope exchange reaction between stable Cs and 137 Cs, rather than metal exchange reactions with other metal ions such as potassium ions. To explain the effect of the SS concentration on K d , the presence of colloidal 137 Cs passing through a filter is significant as the dominant dissolved species of 137 Cs in river water. These results suggest that a better understanding of stable Cs dissolved in natural water is important for discerning the geochemical and ecological behaviors of 137 Cs in natural water., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

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

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

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

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

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

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

Aiming to fill a need for data regarding radiocesium transport via both branchflow and stemflow through forests impacted by radioactive fallout, this study examined the vertical variation of radiocesium flux from branchflow and stemflow through the canopies of young Japanese cedar (Cryptomeria japonica (L. f.) D. Don) and Japanese oak (Quercus serrata Murray) trees in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident. In forested areas approximately 40 km northwest of the location of the Fukushima Dai-ichi accident, the 137 Cs concentration varied significantly among sampling periods and between the two forests, with the oak stand exhibiting higher 137 Cs concentrations and depositional fluxes than the cedar stand. Expressed per unit trunk basal area, the depositional flux of 137 Cs generated from the cedar and oak stands was 375 and 2810 Bq m -2  year -1 , respectively. Of this total, 71% and 48% originated from the cedar and oak canopy, respectively, while the remainder originated from the trunk. Accordingly, the origin of radiocesium was more balanced for the oak stand with almost half of the flux coming from the canopy (48%) and the other half from the trunk (52%). Only about a quarter (29%) of the radiocesium flux originated from the trunk in Japanese cedar. Results from this work provide needed data that can enable a more thorough conceptualization of radiocesium cycling in forests. Coupling these empirical results with a physically-based model would likely lead to better forest management and proactive strategies for rehabilitating radioactively-contaminated forests and reducing the exposure risk of radiation dose rate for those that utilize forest products., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

The vertical distribution of 137 Cs in forest soil is important for predicting air dose rates and future cycling in forest ecosystems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the 137 Cs flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of 137 Cs has been conducted since 2011. As a result, the 137 Cs concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the 137 Cs inventory in the litter layer has been exponentially decreasing, the annual 137 Cs fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14-0.17% of the deposition density of 137 Cs. Comparing these 137 Cs fluxes with the apparent amounts of downward migration of 137 Cs estimated from the change in the vertical distribution of 137 Cs, the contribution rate of the infiltrated water to downward migration of 137 Cs from litter to soil was calculated to be 8.5-17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6-0.8% on a measured basis and estimated to be 3.0 ± 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of 137 Cs, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

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

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

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

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

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

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the 137 Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of 137 Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured 137 Cs activities adjacent to the 1F NPP. River discharge rates of 137 Cs were estimated by multiplying simulated river flow rates by the dissolved 137 Cs activities, which were estimated by an empirical function. Inflow of 137 Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of 137 Cs activity was large, the simulated results were compared with the annual averaged observed 137 Cs activity distribution. Normalized annual averaged 137 Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated 137 Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted 137 Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic 137 Cs activity was small compared to the effect of directly released 137 Cs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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

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

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

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