Vertical distributions of global fallout 137Cs and 14C in a Japanese forest soil profile and their implications for the fate and migration processes of Fukushima-derived 137Cs

Effect of soil organic matter on the fate of 137Cs vertical distribution in forest soils

Understanding the fate of the vertical distribution of radiocesium (¹³⁷Cs) in Japanese forest soils is key to assessing the radioecological consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The ¹³⁷Cs behavior in mineral soil is known to be primarily governed by interaction with clay minerals; however, some observations suggest the role of soil organic matter (SOM) in enhancing the mobility of ¹³⁷Cs. Here we hypothesized that soil organic carbon (SOC) concentration profile determines the ultimate vertical pattern of ¹³⁷Cs distribution in Japanese forest soils. In testing this hypothesis, we obtained soil samples that were collected before the FDNPP accident at four forest sites with varying SOC concentration profiles and quantified the detailed vertical profile of ¹³⁷Cs inventory in the soils roughly half a century after global fallout in the early 1960 s. Results showed that the higher the SOC concentration in the soil profile, the deeper the ¹³⁷Cs downward penetration. On the basis of the data for surface soils (0-10 cm), the ¹³⁷Cs retention ratio for each of the 2-cm thick layers was evaluated as the ratio of ¹³⁷Cs inventory in the target soil layer to the total ¹³⁷Cs inventory in and below the soil layer. A negative correlation was found between the ratio and SOC concentration of the layer across all soils and depths. This indicates that the ultimate fate of ¹³⁷Cs vertical distribution can be predicted as a function of SOC concentration for Japanese forest soils, and provides further evidence for SOM effects on the mobility and bioavailability of ¹³⁷Cs in soils.

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.

Accumulation of 137Cs in aggregated organomineral assemblage in pasture soils 8 years after the accident at the Fukushima Daiichi nuclear power plant

Despite the presence of minerals that allow Cs fixation in soils, ¹³⁷Cs remains available to crops for several years after its deposition, particularly in pasture soils. Larger amounts of organic matter derived from herbage residues are accumulated in pasture soils than in tilled farmland soils. As the above-ground part of herbage crops initially received airborne ¹³⁷Cs during the accident at Fukushima Daiich nuclear power plant (FDNPP), the organic matter originated from the contaminated herbage should play an important role in the fate of ¹³⁷Cs in soils. To evaluate the role of organic matter on ¹³⁷Cs distribution between potentially mobile and immobile fractions, we compared the distribution of ¹³⁷Cs and stable ¹³³Cs, which are differently associated with organic matter, by sequential extraction and density fractionation. Soil samples were collected 8 years after the accident from Andosols in pasture fields located about 160 km southwest of FDNPP. More than 90% of ¹³⁷Cs was not extracted even after oxidative digestion of organic matter, suggesting that most ¹³⁷Cs was strongly associated with soil minerals. Density fractionation results showed that the ¹³⁷Cs/¹³³Cs ratio was highest in the density fraction of 1.6-1.8 g cm^-3, in which organic matter -including fragmented and decomposed plant detritus -was associated with minerals. Mineral-free organic matter, mostly composed of fresh plant detritus (<1.6 g cm^-3), had a higher ¹³⁷Cs/¹³³Cs ratio than that of crops harvested in the same year of soil sampling. Thus, the transfer of ¹³⁷Cs from soil to plants decreased with cultivation cycles. Our results demonstrate that plant-available ¹³⁷Cs in pasture soil decreased with aging time, not only through increased ¹³⁷Cs fixation in mineral-dominated fractions but also through its physical sequestration in aggregates.

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.

Assessment of vertical radiocesium transfer in soil via roots

Several years after the Fukushima Daiichi Nuclear Power Plant accident, the surface mineral soil layer is believed to be the main reservoir of radiocesium (¹³⁷Cs) in forest ecosystems in Japan. Dissolved ¹³⁷Cs combines with clay minerals in the soil, and hence, it is not expected to easily infiltrate over time. However, previous studies have indicated that ¹³⁷Cs derived from the older global fallout migrated deeper than that of the Chernobyl accident, and this cannot be explained by only the dissolved ¹³⁷Cs vertical migration in the soil. Considering the carbon and nutrient dynamics in the forest floor, the ¹³⁷Cs transfer process in soil via roots may alter its vertical distribution on a decadal scale. Therefore, in this study, we investigated the ¹³⁷Cs activity concentrations in both roots and soil matrix, by considering four (0-20 cm) or six (0-30 cm) mineral soil layers taken at every 5 cm at seven study sites dominated by one of the six plant species (three coniferous forests, one deciduous forest, two deciduous forests covered by Sasa, and one bamboo forest) in eastern Japan in 2013. Comparing the results of ¹³⁷Cs activity concentrations between roots and soil matrix taken at the same soil layer, roots at the surface (0-5 cm) layer often showed lower values than the soil matrix. However, roots deeper than 5 cm had higher activity concentrations than the soil matrix, conversely. The ¹³⁷Cs inventories ratio of roots to soil matrix are about 1% at the 0-5 and 5-10 cm soil layer, and about 2% at the soil layers deeper than 10 cm. These results suggest that decomposition of root litter little affect the short-term vertical migration of ¹³⁷Cs in the forest soil. However, it indicates that continuous production and mortality of roots with relatively high ¹³⁷Cs activity concentrations have an important role for changing the vertical distribution of ¹³⁷Cs on time scale of decades, particularly at deeper soil layers.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified ¹³⁷Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated ¹³⁷Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the ¹³⁷Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of ¹³⁷Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1-2 years after the accident) for Japanese forest ecosystems.

Stabilization/Solidification of Strontium Using Magnesium Silicate Hydrate Cement

Magnesium silicate hydrate (M–S–H) cement, formed by reacting MgO, SiO2, and H2O, was used to encapsulate strontium (Sr) radionuclide. Samples were prepared using light-burned magnesium oxide and silica fume, with sodium hexametaphosphate added to the mix water as a dispersant. The performance of the materials formed was evaluated by leach testing and the microstructure of the samples was also characterized. The stabilizing/solidifying effect on Sr radionuclide in the MgO–SiO2–H2O system with low alkalinity is demonstrated in the study. The leaching rate in a standard 42-day test was 2.53 × 10−4 cm/d, and the cumulative 42-day leaching fraction was 0.06 cm. This meets the relevant national standard performance for leaching requirements. Sr2+ was effectively incorporated into the M–S–H hydration products and new phase formation resulted in low Sr leaching being observed.

Numerical study of transport pathways of 137Cs from forests to freshwater fish living in mountain streams in Fukushima, Japan

The accident at the Fukushima Dai-ichi Nuclear Power Plant in 2011 released a large quantity of radiocesium into the surrounding environment. Radiocesium concentrations in some freshwater fish caught in rivers in Fukushima Prefecture in October 2018 were still higher than the Japanese limit of 100 Bq kg^-1 for general foodstuffs. To assess the uptake of ¹³⁷Cs by freshwater fish living in mountain streams in Fukushima Prefecture, we developed a compartment model for the migration of ¹³⁷Cs on the catchment scale from forests to river water. We modelled a generic forest catchment with Fukushima-like parameters to ascertain the importance of three export pathways of ¹³⁷Cs from forests to river water for the uptake of ¹³⁷Cs by freshwater fish. The pathways were direct litter fall into rivers, lateral inflow from the forest litter layer, and lateral transfer from the underlying forest soil. Simulation cases modelling only a single export pathway did not reproduce the actual trend of ¹³⁷Cs concentrations in river water and freshwater fish in Fukushima Prefecture. Simulations allowing a combined effect of the three pathways reproduced the trends well. In the latter simulations, the decreasing trend of ¹³⁷Cs in river water and freshwater fish was due to a combination of the decreasing trend in the forest leaves/needles and litter compartments, and the increasing trend in soil. The modelled ¹³⁷Cs concentrations within the forest compartments were predicted to reach an equilibrium state at around ten years after the fallout due to the equilibration of ¹³⁷Cs cycling in forests. The model suggests that long term ¹³⁷Cs concentrations in freshwater fish in mountain streams will be controlled by the transfer of ¹³⁷Cs to river water from forest organic soils.

Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with ¹³⁷Cs. It is important to characterize the behavior of ¹³⁷Cs after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, ¹³⁷Cs vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and ¹³⁷Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of ¹³⁷Cs could be suppressed rapidly in Japanese forests. At all sites, ¹³⁷Cs transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous ¹³⁷Cs supply from the organic layer. The diffusion coefficients of ¹³⁷Cs in the mineral soil were estimated to be 0.042-0.55 cm² y^-1, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited ¹³⁷Cs will migrate to the mineral soil but only less than 10% of the total ¹³⁷Cs inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the ¹³⁷Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

A new perspective on the 137Cs retention mechanism in surface soils during the early stage after the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination of the soil in multiple terrestrial ecosystems. Soil is a complex system where minerals, organic matter, and microorganisms interact with each other; therefore, an improved understanding of the interactions of ¹³⁷Cs with these soil constituents is key to accurately assessing the environmental consequences of the accident. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral–organic interaction characteristics using a density fractionation method, and then analyzed for ¹³⁷Cs content, mineral composition, and organic matter content. The results show that 20–71% of the ¹³⁷Cs was retained in association with relatively mineral-free, particulate organic matter (POM)-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the ¹³⁷Cs extractability of the soils, ¹³⁷Cs incorporation into the complex structure of POM is likely the main mechanism for ¹³⁷Cs retention in the surface soil layers. Therefore, our results suggest that a significant fraction of ¹³⁷Cs is not immediately immobilized by clay minerals and remains potentially mobile and bioavailable in surface layers of organic-rich soils.

Radiocesium distribution in aggregate-size fractions of cropland and forest soils affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (¹³⁷Cs) contamination in soils in a range of terrestrial ecosystems. It is well documented that the interaction of ¹³⁷Cs with soil constituents, particularly clay minerals, in surface soil layers exerts strong control on the behavior of this radionuclide in the environment; however, there is little understanding of how soil aggregation-the binding of soil particles together into aggregates-can affect the mobility and bioavailability of ¹³⁷Cs in soils. To explore this, soil samples were collected at seven sites under different land-use conditions in Fukushima and were separated into four aggregate-size fractions: clay-sized (<2 μm); silt-sized (2-20 μm); sand-sized (20-212 μm); and macroaggregates (212-2000 μm). The fractions were then analyzed for ¹³⁷Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and 69%-83% of ¹³⁷Cs was associated with macroaggregates and sand-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and approximately 80% of ¹³⁷Cs was in the clay- and silt-sized fractions. Across all sites, the ¹³⁷Cs extractability was higher in the sand-sized aggregate fractions than in the clay-sized fractions. Mineralogical analysis showed that, in most soils, clay minerals (vermiculite and kaolinite) were present even in the larger-sized aggregate fractions. These results demonstrate that larger-sized aggregates are a significant reservoir of potentially mobile and bioavailable ¹³⁷Cs in organic-rich (forest and orchard) soils. Our study suggests that soil aggregation reduces the mobility of particle-associated ¹³⁷Cs through erosion and resuspension and also enhances the bioavailability of ¹³⁷Cs in soils.