[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: • 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: •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: • Sequential or simultaneous measurement methods were adopted. • No significant difference was observed between the results by the two methods. • The evaluated radiocesium deposition densities agreed within 5%–6% in CV. • The ¹³⁴Cs/¹³⁷Cs ratio in deposition density agreed within 1%–2% in CV. • Guidance for intercomparison methods was proposed. - Abstract: Five intercomparisons of in situ gamma spectrometry by 6–7 participating teams were conducted between December 2011 and August 2015 at sites in Fukushima Prefecture that were affected by the fallout from the accident at the Fukushima Dai-ichi Nuclear Power Plant in March 2011. The evaluated deposition densities agreed within 5% and 4% in terms of the coefficient of variation for radiocesium (¹³⁴Cs and ¹³⁷Cs) and ⁴⁰K, respectively, and the ratio of ¹³⁴Cs/¹³⁷Cs in deposition density agreed within 1% in terms of the coefficient of variation, by our best achievement through five intercomparisons. Two different methods for intercomparison were conducted: 1) simultaneous measurements in a narrow area within a 3 m radius; and 2) sequential measurements at an identical point. In a comparison between the two methods at a site where radiocesium was almost homogeneously distributed, no significant difference was observed between the results. The guidance for intercomparison method was proposed based on our experience, and are expected to be used effectively to ensure the reliability of in situ spectrometry.

[en] Highlights: • A new method to estimate realistic doses of returning residents based on detailed air dose rate measurements was devised. • The correlation between indoor and outdoor air dose rate was investigated. • The proportion of exposure dose at home to total exposure dose was investigated. - Abstract: A method was devised to estimate the realistic external doses of residents expected to return to their homes after evacuation orders are lifted. For 196 residents expected to return to six towns and villages, surveys were administered in FY 2014–2016. After interviewing residents about their expected life patterns after their return, detailed air dose rate measurements were taken along their reported personal routes representing patterns of movement in their daily life. The maximum and average annual additional external effective dose were estimated at 4.9 mSv and 0.86 mSv, respectively. Although the mean values and dispersion of external effective doses differed depending on the level of evacuation, for 93.3% of all residents, the estimated annual additional external effective doses were less than 2 mSv. Measuring air dose rates inside houses was found to be important to obtain a realistic evaluation of the external exposure dose because our research revealed that the average time spent indoors was 87% of the total living time. Moreover, the average exposure dose at home accounted for 66.8% of the annual exposure dose.

[en] Highlights: • ¹³⁷Cs concentrations were monitored at headwater catchments after FDNPP accident. • Normalized ¹³⁷Cs concentrations declined 1-2 order during 2011–2016. • ¹³⁷Cs concentrations in suspended sediment declined after the decontamination work. • Total flux of ¹³⁷Cs decreased after the decontamination work. - Abstract: Since headwater catchments are the source areas of ¹³⁷Cs for downstream river systems, ¹³⁷Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and ¹³⁷Cs concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed. The normalized ¹³⁷Cs concentrations (¹³⁷Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10⁻⁶ m²/L, 10⁻² m²/kg, and 10⁻¹ m²/kg, respectively, before 2013 and declined to around 10⁻⁸ m²/L, 10⁻⁴ m²/kg, and 10⁻² m²/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas ¹³⁷Cs concentrations in SS declined significantly and the total flux of ¹³⁷Cs decreased. Although the clear effect of land use on decline trend in normalized ¹³⁷Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them.

[en] Highlights: • Walk surveys within an 80-km radius of the Fukushima Dai-ichi Nuclear Power Plant. • Decrease in the air dose rates evaluated by walk surveys in 4 y. • Focused in land use categories and evacuation order areas. • Comparison with car-borne surveys and undisturbed flat ground measurements. - Abstract: Ambient dose equivalent rates in various environments related to human lives were measured by walk surveys using the KURAMA-II systems from 2013 to 2016 within an 80-km radius of the Fukushima Dai-ichi Nuclear Power Plant. The dose rate of the locations where the walk survey was performed decreased to about 38% of its initial value in the 42 months from June 2013 to the December 2016, which was beyond that attributable to the physical decay of radiocaesium. The ecological half-life of the slow decreasing component was evaluated to be 4.1 ± 0.2 y. The air dose rates decreased depending on the level of the evacuation areas, and the decrease in the dose rates was slightly larger in populated areas where humans are active. The dose rates as measured by walk surveys exhibited a good correlation with those by car-borne surveys, suggesting that car-borne survey data are reflecting the air dose rates in living environments surrounding roads. The comparison of walk survey data with car-borne survey data indicated that the air dose rate varies largely even within a 100 m square area, and the variation is enhanced by human activities. The dose rates measured by the walk surveys were estimated to be medial of those along roads and those of undisturbed flat ground, and they were found to be decreasing quickly compared with the air dose rate from the flat ground fixed-point measurements.

[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.