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

Downward migration of 137Cs promotes self-cleaning of forest ecosystem by reducing root uptake of Japanese cedar in Fukushima

Approximately 70 % of the area highly 137Cs-contaminated by the Fukushima Daiichi Nuclear Power Plant accident is forested. Decontamination works in most of these forests have not progressed, and the forestry industry remains stagnant. Although the long-term dynamics of 137Cs in the forest ecosystem will be controlled by the amount of 137Cs absorbed by roots in the future, temporal changes in 137Cs of tree roots have rarely been reported. In the present study, we monitored the depth distribution of 137Cs in the soil and absorptive very fine (VF) roots of 0.5 mm or less in a Japanese cedar forest from 2011 to 2023. As a result, the 137Cs inventory in the mineral soil increased over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots tended to decrease since 2020, although there was a large variation. Temporal decrease in the exchangeable 137Cs fraction with fixation and temporal increase in VF root biomass with their growth were not clearly observed, the 137Cs concentration in the VF roots at 0-2 cm decreased with the decrease in 137Cs concentration in the litter layers. Although the 137Cs concentration in the VF roots below 2 cm tended to increase with increasing 137Cs concentration in the soil at the same depth, the downward migration of 137Cs within the soil can reduce the amount of 137Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, 137Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil. This natural migration process can be regarded as a "self-cleaning" of the forest ecosystem, the green and sustainable remediation using such self-cleaning should be actively adopted for the future forest management.

Effect of litter removal five years after the Fukushima accident on 137Cs uptake by Japanese cedar

Removal of litter-associated 137Cs from the forest floor (litter removal) can reduce the 137Cs uptake by plants; however, the proposed effective period for litter removal was 1-2 years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This is because the 137Cs in forest soil migrates rapidly from the litter to surface mineral soil layers in Japanese forests, and thus the effectiveness of litter removal will quickly become limited. However, it is unknown whether this approach can be applied to forests whose vertical migration of 137Cs in the forest soil is relatively slow. Herein, we compared the 137Cs activity concentration in the inner bark of the Japanese cedar (Cryptomeria japonica) between litter removal (conducted in September and October 2016, 5 years after the accident) and in control areas in Kawauchi Village, Fukushima Prefecture, where the vertical migration of 137Cs was relatively slow from the litter to surface mineral soil layers. Air dose rates (ambient dose equivalent) in the litter removal area were significantly lower than those in the control area in 2022, and the 137Cs inventory in the forest soil in litter removal area also tended to be lower than that in the control area. In Japanese cedars with similar levels of outer bark contamination, the 137Cs activity concentration in the inner bark in the litter removal area was significantly lower than that in the control area, and consistent trends were also found when comparing the 137Cs activity concentration in the leaves of Stephanandra incisa and Wisteria floribunda obtained from the same forest. Thus, the litter removal 5 years after the FDNPP accident may have reduced the 137Cs uptake in Japanese cedar in an evergreen coniferous forest where the vertical migration of 137Cs is relatively slow in the forest soil.

Estimation of spatio-temporal distribution of 137Cs concentrations in litter layer of forest ecosystems in Fukushima using FoRothCs model

The nuclear power plant accident in Fukushima had led to pollution of forest ecosystems with ¹³⁷Cs in 2011. In this study, we simulated the spatiotemporal distribution of ¹³⁷Cs concentrations of litter layer in the contaminated forest ecosystems in two decades from 2011, which is one of the key environmental components of ¹³⁷Cs migration in the environment due to the high bioavailability of ¹³⁷Cs in the litter. Our simulations showed that ¹³⁷Cs deposition is the most important factor in the degree of contamination of the litter layer but vegetation type (evergreen coniferous/deciduous broadleaf) and mean annual temperature are also important for changes over time. Deciduous broadleaf trees had higher initial concentrations in the litter layer due to the direct initial deposition on the forest floor. However, the concentrations remained higher than those in evergreen conifers after 10 years due to redistribution of ¹³⁷Cs by vegetation. Moreover, areas with lower average annual temperatures and lower litter decomposition activity retained higher ¹³⁷Cs concentrations in the litter layer. The results of the spatiotemporal distribution estimation of the radioecological model suggest that, in addition to ¹³⁷Cs deposition, elevation and vegetation distribution should also be considered in the long-term management of contaminated watersheds, which can be informative in identifying hotspots of ¹³⁷Cs contamination on a long-term scale.

Natural attenuation processes control groundwater contamination in the Chernobyl exclusion zone: evidence from 35 years of radiological monitoring

The Chernobyl Exclusion Zone (CEZ) contains the vast majority of radionuclides released by the accident in nuclear fuel particle form. We present and analyze groundwater measurements collected from the monitoring network in CEZ covering key aquifers over 35 years since the accident. These new data, together with a comprehensive analysis of historical data shows that ⁹⁰Sr remains mobile in the subsurface environment, while groundwater concentrations of ¹³⁷Cs, Pu isotopes and ²⁴¹Am are relatively low, and are not of radiological concern. During the last two decades, ⁹⁰Sr and ¹³⁷Cs levels have declined or remained stable over time in the majority of monitoring locations. This is due to natural attenuation driven by gradual exhaustion of the fuel particle source, geochemical evolution of groundwater downstream from waste dumps and radionuclide retention in surface soil due to absorption and bio-cycling. Decommissioning of the cooling pond and construction of the 'New safe confinement' over Unit 4 (damaged reactor) also favored better protection of groundwater close to the Chernobyl plant site. Data from confined and unconfined aquifers, as well as rivers, evidence low radiological risks from groundwater contamination both outside the CEZ and to onsite "self-settlers". Though several groundwater contamination "hot spots" remain in the vicinity of Unit 4, "Red Forest" waste trenches and surface water bodies with contaminated bottom sediments, the findings of this study support a monitored natural attenuation approach to groundwater management in the CEZ.

Ten-year trends in vertical distribution of radiocesium in Fukushima forest soils, Japan

To elucidate interannual changes in the vertical distribution of ¹³⁷Cs in forest ecosystems contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, we investigated ¹³⁷Cs inventories in forest soils (both organic and mineral soil horizons) at 10 sampling plots with different ¹³⁷Cs deposition levels and dominant species for up to 10 years after the accident. We examined the temporal variation of the ¹³⁷Cs inventories by depth with exponential regression models (assuming that the transition and partitioning of ¹³⁷Cs are still active) and exponential offset regression models (assuming a shift to a stable ¹³⁷Cs distribution, defined as the "quasi-equilibrium steady-state" in the Chernobyl accident). In the organic horizon, the ¹³⁷Cs inventories were exponentially decreasing, and it might take more time to converge in the quasi-equilibrium steady-state at most plots. In the mineral soil horizon, most of ¹³⁷Cs was found in the surface layer of the mineral soil horizon (0-5 cm). In this layer, the inventories first increased and then become relatively constant, and the exponential offset model was selected at most plots, suggesting entry into the quasi-equilibrium steady-state over the observation period. Although we also observed exponentially increasing trends in a lower layer (5-10 cm) of the mineral soil horizon, there was no clear increasing or decreasing trend of ¹³⁷Cs inventory in the deeper mineral soil layers (10-15 and 15-20 cm). Our calculation of the relaxation depth and migration center revealed that downward migration of ¹³⁷Cs is not significant in terms of the overall ¹³⁷Cs distribution in the mineral soil horizon over 10 years.

Investigation of the chemical and radiation stability of titanium dioxide with surface arsenate groups during 90Sr adsorption

The chemical and radiation stability of titanium dioxide with surface arsenate groups during ⁹⁰Sr adsorption has been studied. The oxalate technique has been used to obtain a solution containing ⁹⁰Sr from the objects of the aquatic environment of the Chornobyl Exclusion Zone. The dependence of the strontium adsorption on the acidity of the solution and the initial activity of the solution (Bq per mL) has been shown. SEM, XRF, and EDS spectroscopy confirm the chemical resistance of 4As-TiO₂ during regeneration. According to the study with ⁹⁰Sr, the decrease in the adsorption capacity of 4As-TiO₂ during regeneration is associated with incomplete leaching of strontium from 4As-TiO₂ micropores. Using an electron accelerator, the radiation resistance of titanium dioxide with surface arsenate groups to β^- -particles with an energy of 1 MeV has been studied. The invariability of the elemental composition and adsorption properties of 4As-TiO₂ at irradiation doses of 5·10⁷Sv testifies to the high radiation resistance of 4As-TiO₂. The result obtained indicates the promise of 4As-TiO₂ for improving radiochemical methods.

Radiocesium distribution caused by tillage inversion affects the soil-to-crop transfer factor and translocation in agroecosystems

This study reports the translocation of cesium-137 (¹³⁷Cs) into deep soil layers, and the ¹³⁷Cs transfer from soil to soybean in farmland under three tillage (no tillage, NT; rotary cultivation, RC; moldboard plow; MP) treatments and an undisturbed grassland (GL) at eight years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on 11 March 2011 in Japan. Tillage influences the ¹³⁷Cs distribution in the 0-30 cm of soil; the distribution of ¹³⁷Cs in the soil was uniform under RC and MP treatments, while in the grassland, most ¹³⁷Cs was concentrated on the soil surface (0-2.5 cm). The center of vertical ¹³⁷Cs radioactivity concentration (the thickness of the soil from surface which containing half of the ¹³⁷Cs inventory) in GL was 5.5 cm, which was shallower than that in farmland (9.5 cm in NT, 13.6 cm in RC and 15.2 cm in MP). Hence, the total translocation distance of ¹³⁷Cs 8 years after FDNPP accident showed the following trend: GL (2.4 cm) < NT (7.0 cm) < RC (10.0 cm) < MP (12.3 cm). Meanwhile, a significant positive correlation was observed between ¹³⁷Cs radioactivity concentration and organic carbon and nitrogen content in the soil. However, the ¹³⁷Cs radioactivity concentration in soybean grains was negatively correlated with the center of vertical ¹³⁷Cs radioactivity concentration but positively correlated with the ratio of exchangeable ¹³⁷Cs (ExCs) and K content in the soil. The ExCs/K and ¹³⁷Cs distributions in the soil were combined into a statistical model to predict the ¹³⁷Cs radioactivity concentration in soybean grain. The results revealed the magnitude of the impact of ¹³⁷Cs distribution on the ¹³⁷Cs transfer from soil to crop. The addition of the ¹³⁷Cs distribution dramatically improved the accuracy of the prediction model of ¹³⁷Cs radioactivity concentration in soybean.

Distribution of radiocesium and its controlling factors under the Japanese cedar canopies

This study investigated the spatial distribution of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident in a densely planted Japanese cedar stand. Systematic grid sampling was conducted to determine ¹³⁷Cs inventories in the layers of deposited organic material and mineral soil at two different spatial scales (hillslope [60 m²] and small [1 m²]). The results showed that ¹³⁷Cs inventories along the hillslope were heterogeneously distributed, with coefficients of variation for the deposited organic material and mineral soil layers of 46.4% and 48.9%, respectively. The ¹³⁷Cs inventory in each layer tended to show a lognormal distribution. The correlation between the ¹³⁷Cs inventories in deposited organic material and mineral soil in the same sampling grid was weak. The controlling mechanisms of the ¹³⁷Cs inventories in the litter and mineral soil layers differed due to differences in the underlying key processes, such as canopy-forest floor transfer due to hydrological and biological processes. No significant correlation was found between the distance from the nearest tree trunk and the ¹³⁷Cs inventory in the deposited organic layer at each sampling point. In contrast, the ¹³⁷Cs inventory in the soil tended to increase as the distance from the nearest tree trunk increased at both the hillslope and small scales. It was found that the initial spatial patterns of ¹³⁷Cs in the soil layer due to atmospheric deposition were preserved in the cedar stand. Finally, we tested the effects of soil sampling density on the reliability of mean soil ¹³⁷Cs inventory estimations in the cedar stand. The results indicated that a soil sampling area greater than 0.06 m² at the hillslope scale and 0.008 m² at the small scale enabled the mean ¹³⁷Cs inventory to be estimated with an uncertainty of less than 20% in the cedar stand.

Decadal trends in 137Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident

Understanding the actual situation of radiocesium (137Cs) contamination of trees caused by the Fukushima nuclear accident is essential for predicting the future contamination of wood. Particularly important is determining whether the 137Cs dynamics within forests and trees have reached apparent steady state. We conducted a monitoring survey of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites. Using a dynamic linear model, we analyzed the temporal trends in 137Cs activity concentrations in the bark (whole), outer bark, inner bark, wood (whole), sapwood, and heartwood during the 2011-2020 period. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay. The 137Cs concentrations in the whole and outer bark samples showed an exponential decrease in most plots but a flat trend in one plot, where 137Cs root uptake is considered to be high. The 137Cs concentration ratio (CR) of inner bark/sapwood showed a flat trend but the CR of heartwood/sapwood increased in many plots, indicating that the 137Cs dynamics reached apparent steady state within one year in the biologically active parts (inner bark and sapwood) and after several to more than 10 years in the inactive part (heartwood). The 137Cs concentration in the whole wood showed an increasing trend in six plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend. Overall, the results show that the 137Cs dynamics within forests and trees have reached apparent steady state in many plots, although the amount of 137Cs root uptake in some plots is possibly still increasing 10 years after the accident. Clarifying the mechanisms and key factors determining the amount of 137Cs root uptake will be crucial for predicting wood contamination.

Pre- and post-accident environmental transfer of radionuclides in Japan: lessons learned in the IAEA MODARIA II programme

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

Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of 137Cs derived from the FDNPP accident in a cedar forest soil

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