[en] 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: 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.

[en] Highlights: • Solid ¹³⁷Cs wash-off were observed using eight erosion plots during 2011–2014. • There was no significant decreasing trend in ¹³⁷Cs concentration in most of the plots. • The ¹³⁷Cs concentration in sediment significantly decreased on bare soil. • The ¹³⁷Cs concentration in sediment decreased only due to significant soil erosion. - Abstract: Understanding solid ¹³⁷Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid ¹³⁷Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of ¹³⁷Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in ¹³⁷Cs activity concentration in sediment and solid wash-off processes of ¹³⁷Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized ¹³⁷Cs solid entrainment coefficient, defined as Sc (m² kg⁻¹), was calculated by dividing ¹³⁷Cs activity concentration in sediment by initial ¹³⁷Cs deposition. A particle size-corrected Sc, defined as Sc_correct (m² kg⁻¹), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sc_correct, ranged from 0.0072 to 0.084 m² kg⁻¹ and 0.0052–0.078 m² kg⁻¹, respectively. Annual wash-off rates of solid ¹³⁷Cs were 0.0029–12% year⁻¹. There was no significant decreasing trend in Sc or Sc_correct on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sc_correct significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in ¹³⁷Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity.

[en] Highlights: • 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: 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.

[en] Highlights: • Large-scale environmental monitoring elucidated temporal changes in contamination conditions in Fukushima. • Air dose rates in living environments decreased to levels lower than expected from physical decay by a factor of 2–3. • Causes of air dose rate reduction were physical decay, vertical and horizontal radiocesium movement, and decontamination. • Radiocesium movement was modified significantly by type of land use and human activity. - Abstract: We summarized temporal changes in air dose rates and radionuclide deposition densities over five years in the 80 km zone based on large-scale environmental monitoring data obtained continuously after the Fukushima Nuclear Power Plant (NPP) accident, including those already reported in the present and previous special issues. After the accident, multiple radionuclides deposited on the ground were detected over a wide area; radiocesium was found to be predominantly important from the viewpoint of long-term exposure. The relatively short physical half-life of ¹³⁴Cs (2.06 y) has led to considerable reductions in air dose rates. The reduction in air dose rates owing to the radioactive decay of radiocesium was more than 60% over five years. Furthermore, the air dose rates in environments associated with human lives decreased at a considerably faster rate than expected for radioactive decay. The average air dose rate originating from the radiocesium deposited in the 80 km zone was lower than that predicted from radioactive decay by a factor of 2–3 at five years after the accident. Vertical penetration of radiocesium into the ground contributed greatly to the reduction in air dose rate because of an increase in the shielding of gamma rays; the estimated average reduction in air dose rate was approximately 25% with penetration compared to that without penetration. The average air dose rate measured in undisturbed fields in the 80 km zone was estimated to be reduced owing to decontamination by approximately 20% compared to that without decontamination. The average deposition density of radiocesium in undisturbed fields has decreased owing to radioactive decay, indicating that the migration of radiocesium in the horizontal direction has generally been slow. Nevertheless, in human living environments, horizontal radiocesium movement is considered to contribute significantly to the reduction in air dose rate. The contribution of horizontal radiocesium movement to the decrease in air dose rate was estimated to vary by up to 30% on average. Massive amounts of environmental data were used in extended analyses, such as the development of a predictive model or integrated air dose rate maps according to different measurement results, which facilitated clearer characterization of the contamination conditions. Ecological half-lives were evaluated in several studies by using a bi-exponential model. Short-term ecological half-lives were shorter than one year in most cases, while long-term ecological half-lives were different across the studies. Even though the general tendency of decrease in air dose rates and deposition densities in the 80 km zone were elucidated as summarized above, their trend was found to vary significantly according to location. Therefore, site-specific analysis is an important task in the future.

[en] Highlights: • Recent progress of numerical simulations on radioactive cesium adsorption on clay minerals is reviewed. • Brief review of experiments of adsorption of radiocesium on clay minerals is given. • Key remaining technical topics and some directions toward future solutions of waste soil management are discussed. - Abstract: Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai–ichi nuclear power plant accident. In particular, computer–based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force–field molecular dynamics on large–scale realizations composed of many thousands of atoms including water molecules to first–principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

[en] Highlights: • Distribution maps and decreasing trend of dose rates in 5 y by car-borne surveys. • Ecological half-lives of the fast and slow components were estimated. • Dependencies of dose rates decreasing on evacuation order areas and dose rate ranges. • Comparison with the measurement on flat ground with few disturbances. - Abstract: As part of the investigation of the distribution of ambient dose equivalent rates around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), car-borne surveys using Kyoto University RAdiation MApping (KURAMA) systems have been conducted over a wide area in eastern Japan since 2011. The enormous volume of measurement data collected until 2016, including those until 2012 which were reported in the previous paper, was analyzed, and dependencies of the decreasing trend of the dose rates in regions within 80 km of the FDNPP on land-use categories, evacuation order areas and magnitude of the dose rates were examined. The air dose rates within 80 km of the FDNPP tended to decrease considerably with respect to the physical decay of radiocaesium. The decrease of the dose rate in the “forest” was slower than its decrease in other regions, while that in “urban area” was the fastest. The decrease in the air dose rate from 2011 was the fastest outside the evacuation order area until 2015, and it was the slowest in the “difficult-to-return zone”. However, the decreasing trend starting from 2013 showed that the decrease in the “zone in preparation for the lifting of the evacuation order” and in the “residence restriction area” was the fastest. It was found that the air dose rates decreased depending on the magnitude of the dose rates and elapsed time from the FDNPP accident, i.e. the decrease in air dose rates in areas with relatively low dose ranges (such as 0.2–0.5 μSv/h) was the largest during a period relatively early after the accident, and the decreasing rate in the dose rate ranges of 1.9–3.8 and 3.8–9.5 μSv/h were the fastest after 2013. The averaged ratios were analyzed to obtain the ecological half-lives of the fast and slow decay components, and those in whole area within 80 km of FDNPP were estimated to be 0.44 ± 0.05 y and 6.7 ± 1 y, respectively. The ecological half-lives with respect to the land use categories, evacuation order areas and magnitude of the dose rates were also evaluated. The decrease in the dose rates obtained by the car-borne survey was larger than that obtained on flat ground with few disturbances using the NaI(Tl) survey meter during approximately 1.5 y after the FDNPP accident. Thereafter, the difference of decreasing tendencies in the air dose rates between both the measurements was negligibly small, with the ratio of dose rates by the car-borne survey to those by the fixed-point measurement of 0.72–0.77.

[en] Highlights: • Created the integrated dose rate maps from 2014 to 2016 based on walk, car and airborne survey data around the Fukushima Dai-ichi NPP. • Visualized the spatial heterogeneity of dose rate reduction in the regional scale; including decontamination and anthropologic effects. • Used the integrated map as the initialization in the data-driven dose prediction model to predict the radiation dose rate map in 2026. - Abstract: In this study, we quantify the temporal changes of air dose rates in the regional scale around the Fukushima Dai-ichi Nuclear Power Plant in Japan, and predict the spatial distribution of air dose rates in the future. We first apply the Bayesian geostatistical method developed by Wainwright et al. (2017) to integrate multiscale datasets including ground-based walk and car surveys, and airborne surveys, all of which have different scales, resolutions, spatial coverage, and accuracy. This method is based on geostatistics to represent spatial heterogeneous structures, and also on Bayesian hierarchical models to integrate multiscale, multi-type datasets in a consistent manner. We apply this method to the datasets from three years: 2014 to 2016. The temporal changes among the three integrated maps enables us to characterize the spatiotemporal dynamics of radiation air dose rates. The data-driven ecological decay model is then coupled with the integrated map to predict future dose rates. Results show that the air dose rates are decreasing consistently across the region. While slower in the forested region, the decrease is particularly significant in the town area. The decontamination has contributed to significant reduction of air dose rates. By 2026, the air dose rates will continue to decrease, and the area above 3.8 μSv/h will be almost fully contained within the non-residential forested zone.

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

[en] Highlights: •Simulations of ${\dot{H}}^{\ast }\left(10\right)$ at three suburban locations near to FDNPP with 3D models of individual buildings, trees and roads. •Correlation demonstrated between simulated and measured ${\dot{H}}^{\ast }\left(10\right)$. •${\dot{H}}^{\ast }\left(10\right)$ was on average 5.0% higher when buildings and trees were removed from the models. •Low retention of ¹³⁴Cs and ¹³⁷Cs by buildings and asphalt was on average more important than shielding by buildings. •The results help clarify the extent to which buildings, trees and asphalt affect ${\dot{H}}^{\ast }\left(10\right)$ at these sites. - Abstract: The influence of buildings, trees and paved surfaces on outdoor ambient dose equivalent rates (${\dot{H}}^{\ast }\left(10\right)$) in suburban areas near to the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was investigated with Monte Carlo simulations. Simulation models of three un-decontaminated sites in Okuma and Tomioka were created with representations of individual buildings, trees and roads created using geographic information system (GIS) data. The ¹³⁴Cs and ¹³⁷Cs radioactivity distribution within each model was set using in-situ gamma spectroscopy measurements from December 2014 and literature values for the relative radioactive cesium concentration on paved surfaces, unpaved land, building outer surfaces, forest litter and soil layers, and different tree compartments. Reasonable correlation was obtained between the simulations and measurements for ${\dot{H}}^{\ast }\left(10\right)$ across the sites taken in January 2015. The effect of buildings and trees on ${\dot{H}}^{\ast }\left(10\right)$ was investigated by performing simulations removing these objects, and their associated ¹³⁴Cs and ¹³⁷Cs inventory, from the models. ${\dot{H}}^{\ast }\left(10\right)$ were on average 5.0% higher in the simulations without buildings and trees, even though the total ¹³⁴Cs and ¹³⁷Cs inventory within each model was slightly lower. The simulations without buildings and trees were then modified to include ¹³⁴Cs and ¹³⁷Cs in the ground beneath locations where buildings exist in reality, and the inventory of paved surfaces modelled as if they had high retention of ¹³⁴Cs and ¹³⁷Cs fallout like soil areas. ${\dot{H}}^{\ast }\left(10\right)$ increased more markedly in these cases than when considering the shielding effect of buildings and trees alone. These results help clarify the magnitude of the effect of buildings, trees and paved surfaces on ${\dot{H}}^{\ast }\left(10\right)$ at the un-decontaminated sites within Fukushima Prefecture.

[en] 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 broad-leaved and deciduous broad-leaved forest was 10–20% faster than the physical decay of radiocesium. - 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.