Depth distribution of 137Cs, 134Cs, and 131I in soil profile after Fukushima Dai-ichi Nuclear Power Plant Accident

In-situ gamma-ray measurement to estimate depth distribution of 137Cs in farmland in Fukushima Prefecture

Radioactive cesium released into the atmosphere caused by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 has contaminated the surrounding area. We confirmed the applicability of in-situ methods to evaluate the depth distribution of 137Cs by employing the ratio of Compton-scattering and photo-peak components (rC) obtained from measured gamma-ray spectra. In the present study, we applied the in-situ method to farmlands in Fukushima Prefecture whose sites were disturbed by decontamination and plowing operations. rC and the net count of the 662-keV photo-peak, npeak, were obtained from gamma-ray spectra measured using a portable CsI detector. Reasonable rC was obtained by removing the contribution of naturally occurring radioactive materials through a simple and versatile procedure. The depth distribution of 137Cs measured using the conventional sampling method was reproduced using the Fermi distribution function. The concentration of 137Cs on the ground surface, N(0), and the depth at which the concentration becomes half of N(0), d1/2, can be described by simple functions of npeak and rC, respectively. We also confirmed that the Monte Carlo simulation is useful to reproduce the present results, taking into account the contribution of 134Cs and the detection system properly.

Construction of a realistic voxel phantom of the Japanese red fox (Vulpes vulpes japonica) based on MRI imaging and estimation of its background external radiation dose rate from environmental radionuclides

In this study, we selected the Japanese red fox (Vulpes vulpes japonica) as a representative mid-size mammal from the forests near the spent nuclear fuel reprocessing plant at Rokkasho. A fox voxel phantom was constructed based on magnetic resonance imaging of a female red fox caught in Rokkasho. This phantom consisted of 264 × 321 × 383 voxels (each voxel size: 0.78 × 0.78 × 2 mm) with internal organs. The external radiation dose rate to the voxel phantom by beta and gamma rays from environmental radionuclides was estimated using a Monte Carlo code (EGS4 and UCPIXEL). We estimated the dose rates to the phantom on the ground and in an average fox burrow in the Rokkasho forest, which were 11 and 27 nGy h-1, respectively. Assuming that the animal on the ground was irradiated by cosmic rays of 27 nGy h-1, the total external dose rate was evaluated to be 38 nGy h-1. Based on the assumption that the fox lives on the ground for 12 h and in the burrow for 12 h, the dose rate was estimated to be 33 nGy h-1.

Impacts of farmland decontamination on 137Cs transfers in rivers after Fukushima nuclear accident: Evidence from a retrospective sediment core study

Following the Fukushima Daiichi Nuclear Power Plant disaster in March 2011, the Japanese government initiated an unprecedented decontamination programme to remediate 137Cs-contaminated soils and allow population return. This programme involved the removal of topsoil under farmland and residential land, and its replacement with "fresh soil" composed of granitic saprolite. However, decontamination was limited to these two land uses, without remediating forests, which cover 70 % of the surface area in the affected region. In this unprecedented context, the specific impact of this unique decontamination programme on 137Cs transfers in river systems remains to be quantified at the catchment scale. In this study, based on the analysis of a sediment core collected in June 2021 in the Mano Dam reservoir draining a decontaminated catchment, the effects of soil decontamination on particle-bound 137Cs dynamics and sediment source contributions in response to a succession of extreme precipitation events were retrospectively assessed. The sequence of sediment layer deposition and its chronology were reconstructed through the analysis of several diagnostic properties (organic matter, elemental geochemistry, visible colourimetry, granulometry) and contextual information. During abandonment (2011-2016), cropland contribution decreased (31 %). Concurrently, 137Cs activity and deposition flux decreased (19 and 29%year-1, respectively). Following decontamination (2017), sediment transfer increased (270 %) in response to increased contributions from decontaminated cropland and "fresh soil" (625 % and 180 % respectively). Meanwhile, forest contributions remained stable. In contrast, 137Cs activity dropped (65 %), although 137Cs deposition flux remained constant. Forests acted as a stable source of 137Cs. Accordingly, 137Cs deposition flux after decontamination (2016-2021) was similar to that observed during the 5-years period of land abandonment (2011-2016), as a result of the regrowth of spontaneous vegetation over farmland, protecting soil against erosion. Future research should further investigate the impact of longer land abandonment that prevailed in some regions decontaminated lately on the 137Cs fluxes in the rivers.

A Water-Soluble Cryptophane Decorated with Aromatic Amine Groups Shows High Affinity for Cesium and Thallium(I)

An anti-cryptophane decorated with three aromatic amine and three phenol groups shows a high affinity for the cesium and thallium cations in LiOH/H2O (0.1 M). The formation of the complexes was studied by 133Cs NMR and by 205Tl NMR spectroscopy at different temperatures. Characteristic signals for caged cesium and thallium were observed at a high field with respect to the signals of the free cations present in the bulk. Isothermal titration calorimetric experiments performed in LiOH/H2O (0.1 M) and NaOH/KCl buffer (pH = 13) allowed us to determine the parameter of complexation and to ascertain the high affinity of this cryptophane for cesium and thallium. A comparison with other cryptophanes that bind these two cations shows that the introduction of nitrogen atoms into the cryptophane backbone has an effect on the binding properties. The affinity for cesium and thallium(I) ions is in the following order of substitution: OH > NH2 > OCH2COOH. This study paves the way to the design of new efficient host molecules for the extraction of these two cations in aqueous solution.

Response of personal dosemeters on various age-specific anthropometric phantoms under external irradiation applied to areas affected by the 2011 Fukushima Daiichi nuclear accident

We experimentally obtained the responses of two personal dosemeters (PDs, D-shuttle and Dose-i) attached on five age-specific phantoms under rotational irradiation geometry, which simulated an environment that was radiologically affected by the 2011 Fukushima nuclear accident using of a 137Cs source. Although the PD responses showed an angular phase shift by the PD position on the phantoms, the angular dependence was small when the contamination was widely distributed. The PD responses decreased as much as ~10% with the increase in the phantoms' body size. Although there were ~17% variations in the PD/ADE (ambient dose equivalent) ratio depending on the different PDs, this variation was due to the fact that D-shuttle was calibrated with the inclusion of a safety margin. The PD/ADE ratios were similar to the effective dose to ADE ratios for corresponding age-specific phantoms. Our results suggest that these two PDs can provide reasonable estimates for age-dependent effective doses.

Effects of land use types on the depth distribution of selected soil properties in two contrasting agro-climatic zones

The depth distribution of soil properties are governed by several interacting factors including land use types (LUT) and agro-climate (AgC) factors. Yet, there is little information on the effects of LUT, AgC and their combination on soil properties along depth, which this study aimed to investigate. We collected a total of 36 composite soil samples using the manual percussion of a steel core tube layer by layer vertically up to 30 cm in sites representing both highland and lowlands, and analyzed for selected soil properties. A significant main effects of LUT on the depth distribution of bulk density (BD), Ca, Na, K and Cu, and AgC on soil texture, pH, EC, Ca, Na, K, P, Mn, Fe and Cu were noted. The two-way ANOVA analysis also revealed the significant effects of both LUT and AgC on the depth distribution of BD, Na, K, Cu and EC, reflecting their influences on the paths associated to bio-geo-recycling processes. Compared to crop and forestlands, the average SOC and Fe were lower while EC, CEC, Ca, Na, K, P, Mn and Zn were higher in homegarden located in highland than lowland, possibly the acid nature of the highland soil may make the extractable cations available. SOC was not significantly influenced by AgC, LUT and their interaction effect. Based on the Elemental Enrichment Ratio (EER), the SOC was concentrated in the upper surface soil in forest and cropland located both in highland (1.79, 1.33) and lowland (1.80, 1.57), respectively. The reverse propagation pattern SOC depth distribution in soils under homegarden with EER of 0.7 (highland) ad 0.8 (lowland) showed that implementing such system can accelerate carbon sinking and safely store it in subsoil. Also, diversified species composition associated with respective root architectures in the homegarden system, make it an efficient soil nutrient management, which should be widely promoted.

New Approach To Understanding the Experimental 133Cs NMR Chemical Shift of Clay Minerals via Machine Learning and DFT-GIPAW Calculations

Structural determination of adsorbed atoms on layered structures such as clay minerals is a complex subject. Radioactive cesium (Cs) is an important element for environmental conservation, so it is vital to understand its adsorption structure on clay. The nuclear magnetic resonance (NMR) parameters of ¹³³Cs, which can be determined from solid-state NMR experiments, are sensitive to the local neighboring structures of adsorbed Cs. However, determining the Cs positions from NMR data alone is difficult. This paper describes an approach for identifying the expected atomic positions on clay minerals by combining machine learning (ML) with experimentally observed chemical shifts. A linear ridge regression model for ML is constructed from the smooth overlap of atomic position descriptor and gauge-including projector augmented wave (GIPAW) ab initio data. The constructed ML model predicts the GIPAW data to within a 3 ppm root-mean-squared error. At this stage, the ¹³³Cs chemical shifts can be instantaneously calculated from the Cs positions on any clay layers using ML. The inverse analysis, which derives the atomic positions from experimentally observed chemical shifts, is developed from the ML model. The input data for the inverse analysis are the layer structure and the experimentally observed chemical shifts. The Cs positions for the targeted chemical shifts are then output. Inverse analysis is applied to montmorillonite, and the resultant Cs positions are found to be consistent with previous results (Ohkubo, T.; et al. J. Phys. Chem. A 2018, 122, 9326-9337). The Cs positions on saponite clay are also clarified from experimentally observed chemical shifts and inverse analysis.

Highly selective adsorption and lattice process of cesium by cubic cyanide-based functional materials

Cesium (Cs) is a byproduct of nuclear bombs, nuclear weapons testing, and nuclear fission in nuclear reactors. Cs can enter the human body through food or air and cause lasting damage. Highly efficient and selective removal of ¹³⁷Cs from low-level radioactive effluents (LLREs), which contain many radionuclides and dissolved heavy metal species, is imperative for minimizing LLRE volume, and facilitating their final disposal. Prussian blue analogs (PBAs) have received much attention as materials for the removal of radioactive Cs because of their affinity for adsorbing Cs⁺. In this study, an inexpensive and readily available cyanide-based functional material (PBA_Cu) exhibiting high efficiency and excellent selectivity toward Cs capture was designed through a facile low-temperature co-precipitation process. Nano-PBA_Cu, crystallizing in the cubic space group (Fm-3m (225)), has an average pore size of 6.53 nm; consequently, PBA_Cu can offer abundant atomic occupation sites for capturing and incorporating Cs. Here, the pseudo-second-order kinetic model and Langmuir model fitted well with the adsorption of Cs ⁺ on PBA_Cu, with a maximum capture capacity of 95.75 mg/g within 5 min, confirming that PBA_Cu could rapidly capture Cs ions. PBA_Cu strongly and selectively interacted with Cs even in a simulant containing large Na⁺, NH₄⁺, Ca²⁺, and Mg²⁺ ion concentrations in an aqueous solution. The process of Cs ⁺ adsorption by cyanide-based functional crystals was confirmed to involve the entry of Cs⁺ into cyanide-based functional crystals to replace K⁺ and finally achieve the lattice incorporation of Cs. The current results broaden the lattice theory of radionuclide Cs removal and provide a promising alternative for the immobilization of Cs from radioactive wastewater.

Radiocesium accumulation in Lake Kasumigaura by riverine input and migration following the Fukushima Dai-ichi nuclear power plant accident

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 (¹³⁷Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the ¹³⁷Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The ¹³⁷Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating ¹³⁷Cs migration. However, the rate of decrease subsequently slowed due to the ¹³⁷Cs supply from the river. The ¹³⁷Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate ¹³⁷Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in ¹³⁷Cs concentrations will be smaller. The ¹³⁷Cs concentration in the Koise River will continue to decrease; thus, the difference in the ¹³⁷Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of ¹³⁷Cs in the entire lake were 3.72 × 10¹² Bq in December 2012 and 4.18 × 10¹² Bq in August 2016. The accumulated amount of ¹³⁷Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate ¹³⁷Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of ¹³⁷Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes.

Late phase radiocesium dynamics in Fukushima forests post deposition

The long term dynamics of radiocesium in typical forest ecosystems was studied in the radioactive contaminated areas in Fukushima Prefecture. Six observations sites located in Yamakiya Village (Kawamata Town; since 2014), Tsushima Village (Namie Town, since 2015), and Tomioka Town (since 2017) were monitored. The forests consisted of artificial plantations of Japanese cedar (Cryptomeria japonica) at Yamakiya Village, Tsushima Village, and Tomioka Town. Tsushima Village also had a natural mixed forest dominated by Japanese red pine (Pinus densiflora), and Tomioka Town had a young and a mature artificial plantation of Japanese cypress (Chamaecyparis obtuse). Concentrations of ¹³⁷Cs were monitored in the samples collected from the main aboveground biomass compartments, fresh litterfall, forest litter, and soil. Concentrations of exchangeable forms of ¹³⁷Cs and stable K were measured in soil samples. During the observation period, the litter radiocesium inventories at all sites decreased significantly to approximately 1% or less of the total ground deposition. Approximately 80% of the total radiocesium inventory is localized in the upper 5-cm layer of soil and there is little downward migration of radiocesium. At the sites with the longest monitoring series (Yamakiya and Tsushima), the radiocesium expectation depths and expectation mass depths were relatively constant at 2-3 cm and 5-6 kg m^-2, respectively. Aboveground biomass compartments showed similar decreasing trends in radiocesium aggregated transfer factors, T_ag, in the compartments that were exposed to atmospheric fallout in March 2011 (old foliage, small branches, and outer bark). The mean T_ag in cedar stand compartments currently are in the range of 10^-3-10^-2 m² kg^-1 dw. However, the mean T_ag and their dynamic trend significantly differed in the wood compartments of the cedar stands, which may indicate root uptake differences of orders of magnitude between observation sites. The difference in radiocesium concentration in wood between the sites becomes less pronounced when normalized by the ratio of exchangeable ¹³⁷Cs/K in the soils.

Fukushima and Chernobyl: Similarities and Differences of Radiocesium Behavior in the Soil-Water Environment

In the wake of Chernobyl and Fukushima accidents, radiocesium has become a radionuclide of most environmental concern. The ease with which this radionuclide moves through the environment and is taken up by plants and animals is governed by its chemical forms and site-specific environmental characteristics. Distinctions in climate and geomorphology, as well as ¹³⁷Cs speciation in the fallout, result in differences in the migration rates of ¹³⁷Cs in the environment and rates of its natural attenuation. In Fukushima areas, ¹³⁷Cs was strongly bound to soil and sediment particles, with its bioavailability being reduced as a result. Up to 80% of the deposited ¹³⁷Cs on the soil was reported to be incorporated in hot glassy particles (CsMPs) insoluble in water. Disintegration of these particles in the environment is much slower than that of Chernobyl-derived fuel particles. The higher annual precipitation and steep slopes in Fukushima-contaminated areas are conducive to higher erosion and higher total radiocesium wash-off. Among the common features in the ¹³⁷Cs behavior in Chernobyl and Fukushima are a slow decrease in the ¹³⁷Cs activity concentration in small, closed, and semi-closed lakes and its particular seasonal variations: increase in the summer and decrease in the winter.

FIXED-POINT OBSERVATION AND CHARACTERIZATION OF RADIOACTIVE CS IN SOIL COLLECTED AT KAWASAKI, JAPAN

Radioactive Cs derived from the Fukushima Daiichi Nuclear Power Plant accident was detected in soils sampled at Kawasaki, Japan. Radioactive Cs adsorbed on soil is hard to be removed. Fixed-point observation of radioactive Cs concentration was performed on two observation sites of Meiji University. The soil samples were also analyzed for the distribution of radioactive Cs concentration and chemical form. 10 years after the accident, the activity concentration of radioactive Cs in soil samples at the Ikuta Campus of Meiji University ranged from 127 to 448 Bq kg-1. The sequential extraction method results indicated that most radioactive Cs in the surface soil exists in a poorly soluble form.

Estimation of rooting depth of 137Cs uptake by plants

Understanding the soil-to-plant transfer process of ¹³⁷Cs is essential for predicting the contamination levels of plants in contaminated areas. The rooting depth is considered one of the key factors explaining the difference in the activity concentration of ¹³⁷Cs in different plant species. In this study, the distributions of ¹³⁷Cs and ¹³³Cs in soils and plants were investigated, and the plants' rooting depth of ¹³⁷Cs uptake was estimated using the ¹³⁷Cs/¹³³Cs ratios in exchangeable fractions of soils and biological samples. The results showed that different plant species accumulate different levels of ¹³⁷Cs and ¹³³Cs. The ¹³⁷Cs/¹³³Cs ratios were fairly constant in plants of the same species. The average ¹³⁷Cs/¹³³Cs ratios in bamboo grasses and ferns were 0.015 ± 0.009 (n = 5) and 0.13 ± 0.04 Bq ng^-1 (n = 10) in Yamakiya, respectively. The percentage of ¹³⁷Cs in the exchangeable fraction of the uppermost soil layer was lower than that in the deeper soil layers. The activity concentrations of ¹³⁷Cs in the soil profiles decreased sharply with depth, whereas the depth distributions of ¹³³Cs were uniform. Therefore, the ¹³⁷Cs/¹³³Cs ratios were driven mainly by the ¹³⁷Cs activity concentrations in soil. The plants' rooting depths of ¹³⁷Cs uptake were estimated on the basis of the relationships between the averaged ¹³⁷Cs/¹³³Cs ratio in the soil layer and the ¹³⁷Cs/¹³³Cs ratio in the plant. The results indicate that the deeper-rooted species such as bamboo grasses have a lower accumulation of ¹³⁷Cs than the superficial-rooting species such as ferns. The soil-to-plant transfer factors would be determined using rooting depth by calculating the averaged activity concentration of ¹³⁷Cs within the estimated rooting depth.

Soil radioactivity-depth profiles from regularized inversion of borehole gamma spectrometry data

An in situ borehole gamma logging method using a LaBr₃ gamma detector has been developed to characterize a¹³⁷Cs contaminated site. The activity-depth distribution of ¹³⁷Cs was derived by inversion of the in situ measurement data using two different least squares methods, (i) Least squares optimization (LSO) and (ii) Tikhonov regularization. The regularization parameter (λ) of the Tikhonov regularization method was estimated using three different methods i.e. the L-curve, Generalized Cross Validation (GCV) and a prior information based method (PIBM). The considered inversion method variants were first validated for a¹³⁷Cs contaminated pipe, and in most of the cases, the calculated activity of ¹³⁷Cs was found to be within the acceptable range. The calculated ¹³⁷Cs activity-depth profiles from in situ measurements were also in good agreement with the ones obtained from soil sample analysis, with an R² ranging from 0.76 to 0.82. The GCV method for estimating λ appeared to perform better than the two other methods in terms of R² and root mean squared error (RMSE). The L-curve method resulted in higher RMSE than the other Tikhonov regularization methods. Instability was observed in the activity concentration depth profile obtained from the LSO method. Therefore, we recommend the Tikhonov regularization with GCV for estimating λ for estimating the activity concentration-depth profile. The site studied showed ¹³⁷Cs activity concentrations above the exemption limit down to depths of 0.50-0.90 m.

Coal Fly Ash as Raw Material for Immobilization of Sr-Contaminated Soil by Microwave Heating: Mechanism and Performance

In this work, coal fly ash, hereinafter CFA is proposed to work as raw material for immobilization of Sr-contaminated soil by microwave sintering in the path towards resource utilization of solid waste. The immobilization mechanism and performance was systemically investigated through phase evolution, microstructure, elemental distribution, and physical properties. The results shown that the Sr could be incorporated into feldspar strontian (SrAl2Si2O8) at 1300 °C for 30 min. Moreover, the maximum solid solubility of SrSO4 was more than 30 wt.%. The Sr was homogeneously distributed in the sintered matrices without substantial enrichment. The sintered matrix exhibited high density (2.53 g/cm3). Thus, microwave heating coupled with CFA could provide a new method for immobilization of Sr-contaminated soil in case of the spent nuclear reprocessing cycle in nuclear power plants or a nuclear accident emergency.

Migration behavior of radiostrontium and radiocesium in arid-region soil

The time-dependent of the average migration, depths and migration rates of radiostrontium and radiocesium were experimentally determined in sandy soil in Kuwait. The study aimed to describe the behavior of these radionuclides in typical soil types in Kuwait. Custom-made large-diameter lysimeters (30 cm) were constructed to hold about 70 kg of undisturbed soil. The top surface soil was contaminated with known activity concentrations of ⁸⁵Sr and ¹³⁴Cs radionuclides. The topsoil was wetted with 1 L of water weekly using a rain simulator tool. Gamma scanning of the lysimeters in a step of 5 cm from three sides was performed at three measurement times by a high-resolution portable gamma spectrometry system. The vertical distribution pattern of ⁸⁵Sr and ¹³⁴Cs concentration exhibited an exponential trend. The average migration depth and migration rates were calculated, and 90% of the ⁸⁵Sr and ¹³⁴Cs were situated at 10.38 and 5.73 cm in the topsoil layer, respectively. The average vertical migration rate of ⁸⁵Sr varied from 2.2 to 4.4 cm y^-1, and ¹³⁴Cs was from 0.3 to 0.9 cm y^-1. It was demonstrated that the mobility of the ⁸⁵ Sr and ¹³⁴ Cs in the sandy soil could be attributed to the physicochemical properties of the radionuclide, in particular the distribution coefficient (Kd). The obtained results could be adopted by the regions of similar climatological conditions, especially, the gulf countries region when the radiological risk assessment of contaminated sites and dose estimation is required.

Managing soils of environmental significance: A critical review

Globally, environmentally significant soils (ESSs) mainly include acid sulfate, heavy metal(loid)-contaminated, petroleum hydrocarbon-contaminated, pesticide-contaminated, and radionuclide-contaminated soils. These soils are interrelated and have many common characteristics from an environmental management perspective. In this review, we critically evaluate the available literature on individual ESSs, aiming to identify common problems related to environmental quality/risk assessment, remediation approaches, and environmental regulation for these soils. Based on these findings, we highlight the challenges to, and possible solutions for sustainable ESS management. Contaminated land has been rapidly expanding since the first industrial revolution from the industrialized Western countries to the emerging industrialized Asia and other parts of the world. Clean-up of contaminated lands and slowdown of their expansion require concerted international efforts to develop advanced cleaner production and cost-effective soil remediation technologies in addition to improvement of environmental legislation, regulatory enforcement, financial instruments, and stakeholder involvement to create enabling environments. Two particular areas require further action and research efforts: developing a universal system for assessing ESS quality and improving the cost-effectiveness of remediation technologies. We propose an integrated framework for deriving ESS quality indicators and make suggestions for future research directions to improve the performance of soil remediation technologies.

Radiation dose rate to Japanese cedar and plants collected from Okuma, Fukushima Prefecture

After the 2011 Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident, wild populations of animals and plants living in the evacuation zone received additional ionizing radiation of both internal and external radiation doses. Morphological abnormalities of pine and fir trees near the FDNPS were reported. In order to evaluate dose-effect relationships, it is necessary to quantify the radiation doses to trees and plants. In this study, the internal and external dose rates to Japanese cedar and plants collected at three sites in Okuma, approximately 4 km southwest of FDNPS were estimated applying the ERICA Assessment Tool. The activity concentrations of ¹³⁴Cs and ¹³⁷Cs in soils, cedar trunks, and plants were determined. The total dose rates to cedar ranged from 2.2 ± 1.2 to 6.1 ± 2.2 μGy h^-1. These rates were within the derived consideration reference levels (DCRLs) reported by ICRP 108 as 4-40 μGy h^-1 for pine trees. The highest estimate for plants was 7.1 ± 2.7 μGy h^-1, much smaller than the DCRLs reported for grasses and herbs (40-400 μGy h^-1). On average, the internal radiation dose rates to cedars at the two sites accounted for 5% and 29% of the external dose rates, respectively, while the value in another site was only 0.4% for cedar. This was attributed to differences in the crown area between the three sites. The trunk diameter of cedars shows a positive correlation with the ratio of internal to external radiation dose rates. It indicates that the total dose rate to cedars is easily estimated with the soil radiocaesium inventory and trunk diameter. The internal radiation dose rate to the plant varied depending on the plant species. This variation was considerably large in plants due to the presence of two species, including Solidago altissima and Artemisia indica var. maximowiczii.

Development and characterization of a mobile γ spectrometer and its field deployment for in situ radioactivity measurements

A mobile γ–ray spectrometer (AMESOS) has been developed using a 3”×3” NaI(Tl) detector, a custom–made mounting holder, and portable electronics to perform in situ measurements of radioactivity. The spectrometer was calibrated using standard point sources and its absolute efficiency was determined. As a field test operation, AMESOS was deployed on the premises of the University of Athens Zografou campus focusing on estimating the NORM levels. Data were analyzed and used to create radiological maps for the metropolitan UoA campus for the first time. Besides natural radioactivity levels, trace concentrations of 137Cs were also detected, attributed to the Chernobyl fallout in Greece. An overall steady performance of the spectrometer was observed throughout the field operation. AMESOS is ready to be deployed for in situ studies of environmental radioactivity and radwaste management.

National Hazards Vulnerability and the Remediation, Restoration and Revitalization of Contaminated Sites-1. Superfund

Natural hazards can be prominent and powerful mechanisms that impact the remediation and restoration of contaminated sites and the revitalization of communities associated with these sites. The potential for hazardous material releases following a natural disaster can exacerbate the impact of contaminated sites by causing the release of toxic or hazardous materials and inhibiting the restoration of the site as well as altering the long-term sustainable revitalization of adjacent communities. Disaster-related hazardous releases, particularly in population-dense areas, can create problems as difficult as the original site clean-up. Similarly, exposure of contaminated sites to natural hazards can enhance the probability of future issues associated with the site. This manuscript addresses the co-occurrence of 12 natural hazards (singly and in combination) and individual Superfund sites.

Rapid solidification of Sr-contaminated soil by consecutive microwave sintering: mechanism and stability evaluation

Consecutive microwave sintering is a method proposed in this study to dispose soil contaminated by Sr during a nuclear accident by rapidly solidifying the contaminated soil. The results show that soil contaminated with 20 wt% SrSO₄ and 30 wt% SrSO₄ can be completely solidified by microwave sintering at 1100-1200 and 1300 ℃, respectively, for 30 min. Sr was found to be cured into slawsonite (SrAl₂Si₂O₈) and glass structures. Moreover, soil sintered at 1300 ℃ has large cured solubility (30 wt.%), good uniformity, and excellent hardness (6.9-7.2 GPa) and chemical durability (below 1.46 × 10^-5 g m^-2 d^-1 at 28 d). Thus, consecutive microwave sintering technology may provide a new method for treating Sr-contaminated soil in case of a nuclear accident emergency.

Interpretation of the interaction between cesium ion and some clay minerals based on their structural features

Cesium (Cs⁺) is known to have a strong interaction with various clay minerals; however, it is not interpreted from the structure of clay minerals and the adsorption isotherm. The adsorption interactions between Cs⁺ and hydrobiotite (H-Bio), biotite (Bio), vermiculite (Verm), and exfoliated vermiculite (E-Verm) were evaluated by analyzing adsorption isotherm, basal spacing, and adsorption/desorption experiments. The Cs⁺ adsorption of H-Bio and Verm fitted well to the Langmuir adsorption isotherm, while the Cs⁺ adsorption of Bio and E-Verm fitted well to the Freundlich adsorption isotherm. The basal spacing of H-Bio and Verm was approximately 1.4 nm, while Bio and E-Verm basal spacing was 1.0 nm. The adsorption experiment results for Cs⁺ under the coexistence of Ca²⁺ and K⁺ indicated that the contribution of the interlayer sites to Cs⁺ adsorption on H-Bio and Verm was 25-40%, while the contribution of the interlayer sites to that on Bio and E-Verm was almost 0%. The adsorption isotherms reflected this interlayer contribution to Cs⁺ adsorption, which was dependent on the basal spacing. Therefore, the basal spacing of clay minerals is one of the key structural properties controlling both the adsorption capacity and the adsorption mechanism of Cs⁺ in clay minerals.

Use of thermal treatment with CaCl2 and CaO to remove 137Cs in the soil collected from the area near the Fukushima Daiichi Nuclear Power Plant

A massive amount of soils and inflammable materials of plants etc. contaminated by radiocesium are generated from decontamination work in the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident affected area. In present study, the removal experiments of ¹³⁷Cs in a soil collected from the FDNPP accident affected area were carried out in a lab-scale electrical heating horizontal furnace through thermal treatment with CaCl₂ addition over a temperature of 900 - 1300 °C. The results indicated that the average radioactive concentration of ¹³⁷Cs in the soil was 52.8 Bq/g. The removal ratio of ¹³⁷Cs in the soil treated at 1300 °C was 96.3 % when 20 % CaCl₂ was added. The addition of CaCl₂ and CaO mixture exhibited a synergistic effect on the removal of ¹³⁷Cs, relative to the addition of CaCl₂ alone. Accordingly, the addition of CaCl₂ or its mixture with CaO during thermal treatment is suggested to remove ¹³⁷Cs in the soil collected from the FDNPP accident affected area. Additionally, segregation of the soil sample to fine and coarse fraction and then treated individually are also recommended.

Meta-analysis of radiocesium contamination data in Japanese cedar and cypress forests over the period 2011-2017

Since Fukushima accident, dozens of field studies have been conducted in order to quantify and understand the behaviour of atmospheric radiocesium (¹³⁷Cs) fallouts in contaminated forests of Fukushima and neighbouring prefectures. In this paper, we carry out a detailed review of data acquired over 2011-2017 in Japanese cedar and cypress plantations, focusing on aerial tree organs, soil layers and tree-to-soil depuration fluxes. To enable comparison and reinforce the consistency between sites, radiological measurements were normalized by the deposit and interpolated onto the same spatio-temporal frame. Despite some (poorly explained) residual variability, we derived a "mean" pattern by log-averaging data among sites. These "mean" results were analysed with the help of a simple mass-balance approach and discussed in the light of post-Fukushima literature. We demonstrated that the activity levels and dynamics in all compartments were consistent and generally well reproduced by the mass balance approach, for values of the interception fraction between 0.7 and 0.85. The analysis indicated that about 5% of the initial deposit remained in the aerial vegetation after 6 years, more than two thirds of intercepted ¹³⁷Cs being transferred to the soil due to throughfall. The simulations indicated that foliar uptake might have contributed between 40% and 100% to the activity transferred to stem wood. The activity concentration in canopy organs rapidly decreased in the first few months then more slowly, according to an effective half-life of about 1.6 years. The activity level in the organic layer peaked in summer 2011 then decreased according to an effective half-life of 2.2 years. After a rapid increase in 2011, the contamination of mineral horizons continued to increase more slowly, 85% of ¹³⁷Cs incoming through the organic layer being retained in the 0-5 cm layer according to a mean residence time longer than in the upper layer (7 against 1.5 years).

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F_l/t), the migration center (X_c) and the relaxation depth (α) using 99 soil inventory data sets. F_l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F_l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X_c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X_c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high ¹³⁷Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of ¹³⁷Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of ¹³⁷Cs from organic to surface mineral soil layer after the second year. These results indicate that ¹³⁷Cs retention capacity of the organic layer can affect the apparent vertical migration of ¹³⁷Cs in the underlying mineral soil layer.

Global 137Cs fallout inventories of forest soil across Japan and their consequences half a century later

Japanese forests were exposed to multiple sources of radioactive contamination. To acquire scientific guidance on forest management planning, it is crucial to understand the long-term radiocesium (¹³⁷Cs) distribution (and redistribution) over time. To obtain robust evidence of the residual global fallout of ¹³⁷Cs (¹³⁷Cs-GFO) after a few decades, we determined ¹³⁷Cs-GFO inventory in forest soil at 1171 soil pits of 316 plots evenly spaced across Japan from 2006 to 2011, shortly before the Fukushima Dai-ichi Nuclear Power Plant accident. The activity concentration measurements were performed using a NaI well-type scintillation counter. The average (±SD) ¹³⁷Cs-GFO in forest soil (0-30 cm from the surface) of the National Forest Soil Carbon Inventory (NFSCI) sampling plots uniformly extracted from the entire country was estimated to be 2.27 ± 1.73 kBq m^-2 (n = 316) as of Oct. 1, 2008. A high nationwide spatial variation was found in ¹³⁷Cs-GFO, where relatively high ¹³⁷Cs-GFO was found along the Sea of Japan compared with the total annual precipitation. We also obtained a reconstructed decay-corrected cumulative ¹³⁷Cs-GFO dataset from the fallout observatories as the initial ¹³⁷Cs-GFO. The cumulative ¹³⁷Cs-GFO of fallout observatories averaged 2.47 ± 0.95 kBq m^-2 (n = 39) as of Oct. 1, 2008 and displayed spatial variation similar to that in forest soil. To identify whether ¹³⁷Cs-GFO remains in forest soil across Japan, we examined a general linear mixed-effect model comparing ¹³⁷Cs-GFO between forest soil and the observatory under normalized annual precipitation and region. The model did not indicate a significant difference, but relatively lesser ¹³⁷Cs-GFO was found in forest soil, where the least-squares mean of ¹³⁷Cs-GFO in forest soils was 79.1% of that of the observatory. The variation in ¹³⁷Cs-GFO in forest soils within NFSCI sampling plots was 1.4 times greater than that among plots. The high spatial variation in ¹³⁷Cs-GFO within a 0.1-ha plot strongly suggested the redistribution of ¹³⁷Cs-GFO within the forest catchment. The vertical distribution pattern of ¹³⁷Cs-GFO across three depth layers indicated that the ¹³⁷Cs-GFO redistributions were likely attributed to the movements of sediments and mass. Moreover, when extracting soil pits assumed to have the least soil disturbance from the vertical distribution pattern, no significant difference in ¹³⁷Cs-GFO was observed between forest soil and observatory data. These findings provide important insights into the stability of ¹³⁷Cs-GFO in the forest ecosystem. Considering the potential hotspot where ¹³⁷Cs-GFO can accumulate deeper in the soil (>30 cm in depth), most ¹³⁷Cs-GFO has remained in the forest for decades. Our study offers microscale heterogeneous ¹³⁷Cs-GFO distribution in forests for ensuring long-term forest management planning necessary for both the long-term migration and local accumulation of ¹³⁷Cs in forests.

Soil and vegetation sampling during the early stage of Fukushima Daiichi Nuclear Power Plant accident and the implication for the emergency preparedness for agricultural systems

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

Control of radiocesium recontamination by plant removal along a decontaminated riverside in Fukushima, Japan

The accident at the Fukushima Dai-ichi Nuclear Power Plant in March 2011 resulted in the widespread deposition of radiocesium in and around Fukushima prefecture. Riversides have an additional contamination risk through deposition of material containing radiocesium transported from upstream. In our previous research (Nishikiori and Suzuki, 2017) we developed a decontamination method in a river bank in 2014; however, the decontaminated site had a recontamination risk. In this study, we evaluated the effectiveness of plant removal as a countermeasure against recontamination because plants favor the deposition of highly-contaminated fine particles. Although suspended solids with high radiocesium concentrations were transported, under plant-removal conditions (2015-2016), sediment deposited in the flood channel was sandy with low radiocesium concentrations. In 2017, one year after the plant recovery, somewhat muddy sediment with high radiocesium concentrations was deposited during medium rainfall events; the plant biomass would explain the amount of fine sediment that was deposited. Additionally, large typhoons introduced sandy sediment even on the densely vegetated flood channel. Thereby, the air dose rates did not increase during the observation period. Plant removal may control the deposition of highly contaminated sediments and be an effective countermeasure for recontamination in the flood channel.

Mineral composition characteristics of radiocesium sorbed and transported sediments within the Tomioka river basin in Fukushima Prefecture

The deposited radiocesium in the Fukushima river basin is transported in the river systems by soil particles and redistributed in the downstream areas. Although predicting the behaviors of minerals that adsorb radiocesium and of radiocesium dissolved in river water within the river systems is essential, the dominant mineral species that adsorb radiocesium have not yet been comprehensively identified. We identify herein such mineral species by investigating the ¹³⁷Cs distribution and the mineral species in each size fraction that are found in the bedload sediments from an upstream reservoir to an estuary within the Tomioka river basin located east of Fukushima Prefecture in Japan. In the fine sand sediment, which is the dominant fraction in terms of the ¹³⁷Cs quantity in the river bedload, the ¹³⁷Cs concentrations of the felsic and mafic minerals are comparable to that of micas. The mafic minerals contain 62% of the ¹³⁷Cs in the fine sand fraction in the upstream area, while the felsic minerals contain the highest quantities of ¹³⁷Cs in the downstream area. These results suggest that the quantification of the mineral species and the ¹³⁷Cs concentration of each size fraction are critically important in predicting the behaviors of the minerals and radiocesium within the Fukushima river basin in the future.

Linking heterogeneous distribution of radiocaesium in soils and pond sediments in the Fukushima Daiichi exclusion zone to mobility and potential bioavailability

During the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 significant amounts of radiocaesium were released into the atmosphere from the reactor units 1, 2 and 3. This caused a non-uniform deposition, in composition and direction, of ¹³⁴Cs and ¹³⁷Cs in the near field (<30 km) from the reactors. In this work, we elucidate the influence of speciation, including radioactive particles, on mobility and potential bioavailability of radiocaesium in soils and sediments from sites located in different directions and distances from the FDNPP. Samples collected in September 2016 were characterized and subjected to sequential chemical extractions and simulated gastrointestinal fluid leaching, and the ¹³⁷Cs and ¹³⁴Cs activities were determined in bulk, grain-size and extracted fractions. The results show that radiocaesium was mainly irreversibly bound and in an inert form. Combined, the two forms contained >90% of the activity present in soils and ~84% in sediments. Digital autoradiography revealed that the inert fraction was predominantly associated with heterogeneities, an indication of radioactive particles. The frequency of heterogeneities was correlated with ¹³⁷Cs activity concentrations, and both were in agreement with the ambient equivalent air doses measured in situ during sampling. Moreover, in situ gamma spectrometry measurements were used in the InSiCal software tool to derive ¹³⁴Cs and ¹³⁷Cs surface contamination. Soil activity concentrations and contamination density estimations, decay-corrected to the day of the FDNPP accident, resulted in ¹³⁴Cs/¹³⁷Cs ratios that match the reported release and deposition plumes from the reactor units. Overall, these results demonstrate the persistence of the particle contamination in the Fukushima near field and highlight the importance of including radioactive particles in environmental impact assessments.

Effective Half-life of 134Cs and 137Cs in Fukushima Prefecture When Compared to Theoretical Decay Models

On 11 March 2011, a 9.0 magnitude earthquake struck the Tohoku region of Japan. The earthquake caused a 15 m tsunami that bombarded the east coast of the island nation. Among the losses was the damage to the Fukushima Daiichi nuclear reactor that lost onsite power and was unable to cool the reactor cores. The reactors melted down and released a plume of radionuclides into the environment. Radiocesiums (Cs and Cs) are the long-lived radionuclides of concern that were deposited along the plume and were distributed on the soil. Radiological decay models are typically used to determine the reduction in external radiation dose over time. However, these radiological decay models do not take into account physical removal by wind and water erosion, or sedimentation in soil outside expected depths. Thirty-five fixed dose-rate monitors were used to record dose rates at 1 mo intervals from the time of installation in Fukushima Prefecture in April 2012 until December 2018 and were used to estimate the effective half-life for radiocesium contamination based on external radiation dose rates. The effective half-life of cesium in the environment was calculated to be 3.2 ± 0.5 y, compared to a theoretical half-life of 7.8 y.

Application of polyethylenimine-coated magnetic nanocomposites for the selective separation of Cs-enriched clay particles from radioactive soil

The separation of Cs-enriched fine particles is a highly effective way to reduce the volume and radioactivity of contaminated soil. This work demonstrated the application of polyethylenimine (PEI)-coated Fe₃O₄ nanocomposites and a mesh filter for the selective separation of clay particles from Cs-contaminated soil. The PEI coating on the Fe₃O₄ nanoparticles enhanced the binding force between the magnetic nanoparticles and clay minerals via electrostatic attraction; thus, approximately 100% of the clay particles were magnetically separated from solution by Fe₃O₄-PEI nanocomposites at a low dose (0.04 g-nanocomposite per g-clay). In separation experiments with soil mixtures, clay- and silt-sized fine particles that had been magnetized by Fe₃O₄-PEI nanocomposites were selectively separated, and the separation efficiency improved when a mesh filter was added to exclude physically large particles. The combination of magnetic and sieving separation thoroughly separated fine particles from soil by reducing the volume of the magnetic fraction. We also evaluated the magnetic-sieving separation method for the selective removal of clay particles from ¹³⁷Cs-contaminated soil. The decrease in radioactivity in the treated nonmagnetic fraction, which accounted for 87.5% of the total soil, corresponded to a high decontamination efficiency of approximately 90%. The developed separation technology offers great potential for the efficient remediation of radioactive soil.

Selective separation of Cs-enriched fine particles from contaminated soil using Fe₃O₄-PEI nanocomposites and a mesh filter.

SELECTION OF SUITABLE SAMPLING TOOL FOR RELIABLE SOIL SAMPLING NOT ONLY AFTER RADIOLOGICAL ACCIDENT

The objective of this research was to develop a sampling protocol for contaminated soils after a large radiological accident. One of the criteria for good sampling method is reproducibility and accuracy of large number of samples collected in short time. Members of the chemical laboratories of the Fire Rescue Service of the Czech Republic (FRS CR), which are included in Radiation Monitoring Network, tested four tools in different soil types. More than 100 samples were collected. Root auger and square soil sampler seem to be the best for reliable soil sampling. Modifications of the root auger decreased of relative error of sampling to 7.5%. The root auger is recommended as a first-choice sampling tool based on: (1) ease, (2) operator safety, (3) collection of a representative sample (4) applicability to most soil types and (5) enabling collection of depth-discrete samples.

Reconstruction of a Fukushima accident-derived radiocesium fallout map for environmental transfer studies

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.

Six-year monitoring study of 137Cs discharge from headwater catchments after the Fukushima Dai-ichi Nuclear Power Plant accident

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.

Temporal distribution of Fukushima-derived 137Cs in coniferous forest soil evaluated based on compartment-exponential model

Based on the compartment and exponential models, the distribution of Fukushima-derived ¹³⁷Cs was evaluated at four sampling dates in undisturbed coniferous forest soil. The compartment model was employed to evaluate the dynamic of ¹³⁷Cs in the three sub-sections of the forest floor (FF), namely undergrowth (UG), litter layer (OL), and fragmented litter layer (OF), while the exponential model was administrated to describe its distribution below the FF. According to the compartment model, the derived ecological half-life of ¹³⁷Cs in the UG, OL, and OF layers was 0.97, 1.1, and 4.9 years, respectively, indicating ¹³⁷Cs resides much longer in the OF layer. Hence, this soil section remains a potential source of radiation dose mainly due to its high ¹³⁷Cs content associated with low attenuation effect. Below the OF layer, the ¹³⁷Cs distribution was well described by exponential model and its derived relaxation lengths were in the range of 0.8-1.4 cm, implying the migration of ¹³⁷Cs in mineral soil is very slow and almost intact during the observation time. Collectively, our results highlighted that the compartment model for the FF and the exponential model for the soil below the FF are adequate enough to generate essential information. Thus, the potential decontamination measures should have to be chosen on their effect on the FF's ¹³⁷Cs. Graphical abstract.

Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles

The time and size dependencies of particulate ¹³⁷Cs concentrations in a reservoir were investigated to evaluate the dynamics of ¹³⁷Cs pollution from a mountainous forested catchment. Sediment and sinking particle samples were collected using a vibracorer and a sediment trap at the Ogaki Dam Reservoir in Fukushima, which is located in the heavily contaminated area that formed as a result of the Fukushima Dai-ichi Nuclear Power Plant accident of 2011. The inventory of ¹³⁷Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 10¹²-3.9 × 10¹² Bq, which is equivalent to 1.1%-1.4% of the initial estimated catchment inventory. The particulate ¹³⁷Cs concentration showed a decline with time, but the exponent value between the specific surface area and the ¹³⁷Cs concentration for the fine-sized (<63 μm) particle fraction remained almost constant from the immediate aftermath of the accident. These quantitative findings obtained by reconstructing the contamination history of particulate ¹³⁷Cs in reservoir sediments and sinking particles have important implications for the evaluation of ¹³⁷Cs dynamics in mountainous forested catchments.

Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of ¹³⁷Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ₁) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of ¹³⁷Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of ¹³⁷Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported ¹³⁷Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ₂) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

EVALUATIONS OF INVENTORY AND ACTIVITY CONCENTRATION OF RADIOCESIUM IN SOIL AT A RESIDENTIAL HOUSE 3 YEARS AFTER THE FUKUSHIMA NUCLEAR ACCIDENT

Soil samples from the surface to a 5 cm depth were collected at a residential house in Koriyama City, Fukushima Prefecture using a scraper plate every three months from March 2014 to September 2014 to evaluate the vertical distribution profiles and inventories of 134Cs and 137Cs in soil. The vertical distribution profiles of radiocesium (134Cs and 137Cs) in soil showed that greater than 86% of the total radiocesium was absorbed in the upper 2 cm 3 years after the accident. Radiocesium in the surface layer seems to move to the lower layer over time. The migration of radiocesium in surface layer might be influenced by the ground surface runoff by rainfall. Radiocesium inventories in June increased significantly over the short period between March and June. In contrast, the radiocesium inventories in September did not increase significantly compared to the values in June. Radiocesium resuspension and deposition caused by decontamination work and meteorological events might be one possible reason for the increased radiocesium inventories observed in June.

Vertical distribution of 129I and radiocesium in forest soil collected near the Fukushima Daiichi Nuclear Power Plant boundary

Three soil core samples were collected from a forest located about 1.1 km south of the Fukushima Daiichi Nuclear Power Plant (FDNPP) boundary in 2017, and the vertical profiles of ¹²⁹I from the FDNPP accident were determined by the combination of TMAH (tetramethyl ammonium hydroxide) extraction and ICP-MS/MS analysis. The humus layer above the soil layer was heavily contaminated with ¹³⁴Cs (1983-5985 Bq g^-1) and ¹³⁷Cs (1947-5902 Bq g^-1) (decay-corrected to March 11, 2011). The ¹²⁹I activity concentrations decreased sharply with the soil depth, from 1894 to 34.1, from 9384 to 78.9, and from 2536 to 51.3 mBq kg^-1, for the three sites. Downward migration of ¹²⁹I was slightly faster than the one of ¹³⁴Cs. In addition, the cumulative ¹²⁹I inventories were observed to be 43.4 ± 1.0, 71.7 ± 1.8, and 56.5 ± 1.8 Bq m^-2, respectively. Subsequently, the cumulative ¹³¹I inventories were estimated to be 1.76 ± 0.06, 2.90 ± 0.11, and 2.28 ± 0.10 GBq m^-2 (decay-corrected to March 11, 2011), respectively. Finally, the total atmospheric deposition of ¹²⁹I on the land of Japan due to the FDNPP accident was estimated to be around 1.09-1.71 kg (7.11-11.2 GBq).

Spatial properties of soil analyses and airborne measurements for reconnaissance of soil contamination by 137Cs after Fukushima nuclear accident in 2011

Following Fukushima nuclear disaster, several data gathering campaigns surveyed the radionuclide propagation in the environment. However, the acquired datasets do not have the same sampling dimension. For example, the airborne measurements are some sort of averaging over a circular field of view, beneath the sensor; while the soil analyses are much more punctual. The objective of this work is to compare the soil samples and an airborne survey to investigate whether these two datasets reflect the same spatial patterns or not. This is prerequisite for combining the multiresolution data to create and update the contamination map in a post-accidental situation. The analyses were performed on square tiles of 20 km side to study large- and small-dimension variations in ¹³⁷Cs deposition. The former was modelled by fitting a plane (called trend) to the georeferenced data points; and the latter was modelled by computing the difference (called residual) between the trend and the initial data. Dip direction and dip angle of trends as well as minimum spatial correlation distance and anisotropy of residuals were computed for both the soil and airborne datasets and compared. Dip directions are compatible in 73% of the tiles and dip angles are generally close. Anisotropy directions are compatible in 49% of the tiles and minimum spatial correlation distances are significantly more marked for the airborne dataset. The soil samples and airborne measurements are therefore more in agreement in large-dimension (trend) rather than in small-dimension (residual) variations. More generally, both the datasets allow highlighting the main contamination plumes distinguishable because of high concentration values. The airborne dataset yet appears to be more powerful to quantify spatial correlations, which could be linked to the contamination mechanisms.

Groundwater age and mixing process for evaluation of radionuclide impact on water resources following the Fukushima Dai-ichi nuclear power plant accident

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. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration.