Fig. 1 Schematic illustration of ¹³⁷Cs migration and monitoring on land
Out of 2.7 PBq of ¹³⁷Cs fell onto the ground, 67% settled in the forests, 10% in paddy fields, 7.4% in farmlands and grasslands, and 5% in urban areas. Of which, the amount of ¹³⁷Cs discharged from the ground into the ocean via rivers over six years after the accident was calculated to be about 4.8% (from June 2011 to March 2017, Abukuma River). Taking into account physical decay (13% in the same period) of ¹³⁷Cs, about 82% of ¹³⁷Cs that fell onto the Abukuma River catchment still remains.

Accumulation and migration in forests

On the land, forests were the most affected by radioactive cesium (¹³⁷Cs) released by the accident at the Fukushima Daiichi NPS. Forests accounted for 60% of the affected area (Fig. 1). In forests, radioactive cesium caused various levels of contamination due to factors including direct fallout onto the forest floor, temporary capturing by branches and leaves and transfer to the forest floor thereafter, and the difference in the species of habitant trees. We found that the difference in the species (evergreen coniferous trees or deciduous broadleaf trees) and properties of the forest (e.g., density of trunks, branches, and leaves) affect migration and temporal distribution change of ¹³⁷Cs in forests, dictating the rate of migration to the forest floor through rainfall and falling leaves and branches (Fig. 2).

In evergreen coniferous trees, the amount of ¹³⁷Cs accumulated in tree bodies (entire tree including bark, trunk, branches, and leaves) dropped by about 60% in one year after the accident. The amount kept falling after that, down to about 2% of the initial deposition eight years after the accident (Fig. 2b). In deciduous broadleaf trees, the amount of ¹³⁷Cs accumulated in tree bodies decreased to about 60% of the initial levels in one year after the accident, but after that it started to increase especially in the trunks due to absorption from roots. As a result, an amount equivalent to 91% of the initial deposition was still accumulated even eight years after the accident (Fig. 2f). Regarding the amount of ¹³⁷Cs accumulated on the forest floor eight years after the accident, we found that 54% of the initial deposition remained within 2 cm of the soil surface for cedar forests and 59% for deciduous broadleaf forests.

As a result of continuous monitoring, it was discovered that the amount of ¹³⁷Cs discharged from the forested catchment via river water or soil in a year was less than 0.3% of the initial deposition, and almost all of the initial deposition still remains in the forest ecosystem.

Fig. 2 Migration and temporal distribution change of radioactive cesium in forests
In “Inventory by part” (in kBq m^–2), figures in parentheses are percentages to the total inventory. The “Percent contribution of migration pathway” refers to the percent contribution of each pathway to the migration of ¹³⁷Cs from the tree canopy to the forest floor.

Downward migration in soil

¹³⁷Cs strongly adsorbs to soil particles. For that reason, immediately after the accident, 99% of ¹³⁷Cs deposited by the accident existed in the top 10 cm of the soil surface layer. Since soil itself has a radiation shielding effect, air dose rates on the ground are greatly affected by how ¹³⁷Cs is distributed and migrates in the top 10-cm layer.

The depth distribution of ¹³⁷Cs in soil can be approximated by an exponential function formula, and the larger coefficientβin the formula means deeper migration. The coefficientβis called relaxation mass depth, and it was found that its value has been gradually increasing, i.e. cesium has been migrating downward, over time since the accident (Fig. 3d). The rate of the downward migration was the highest in abandoned paddy fields with about 1.3 cm per year: after three years after the fallout, the concentration of ¹³⁷Cs was found to be significantly lower in the surface soil layer and nearly consistent in the range of 10 cm from the soil surface (Fig. 3b). The concentration of ¹³⁷Cs in cultivated paddy fields was approximately 15% for the depth of 2 cm from the surface as of 2012, exhibiting a significant decrease in the concentration (Fig. 3c).

Meanwhile, the rate of downward migration in undisturbed soil at urban areas was about 0.3 cm per year (about 0.3 g cm^–2 by mass depth conversion taking into account the soil density) and affecting a reduction in air dose rates, but the amount of ¹³⁷Cs that migrated to sections deeper than 10 cm (down to 20 cm) was less than 10% as of 2017. In forests, supply of ¹³⁷Cs continues due to falling leaves, and ¹³⁷Cs continued to remain around the soil surface layer (Figs. 3a and 3d).

Fig. 3 Temporal change in depth distribution of ¹³⁷Cs concentration and relaxation mass depthβin various soils

Migration of radioactive materials due to erosion

Measurement of radioactive cesium discharged from paddy fields, farmlands, grasslands, urban areas and forests told us that the concentration of ¹³⁷Cs that flows into rivers as particulates due to soil erosion varies depending on the type of land use and time elapsed after the accident (Fig. 4).

In paddy fields, while the normalized ¹³⁷Cs concentration (Sc) was one order of magnitude higher than that in other land use areas for the first six months after the accident, it rapidly dropped after that and became the same level as other land use areas in one year after the accident. For cultivated farmlands, it is likely due to repeated mixing of surface soil (high in ¹³⁷Cs concentration) with deeper soil (low in ¹³⁷Cs concentration) by cultivation and washing of ¹³⁷Cs exposed at the surface. As for discharge from urban areas, the normalized particulate ¹³⁷Cs concentration was 2.4 times the concentration in the forested catchment as of one year after the accident, but the concentration then rapidly fell and lowered to 1.5 times the concentration in the forested catchment four years after the accident (Fig. 4).

Meanwhile, the normalized concentration of ¹³⁷Cs discharged from forests did not fall very fast (the rate of reduction was about 60% of that from the urban areas) (Fig. 4).

Fig. 4 Temporal change in normalized concentration of ¹³⁷Cs contained in soil released from various sources after the Fukushima Daiichi NPS accident

Discharge of ¹³⁷Cs from rivers

Regarding transportation of ¹³⁷Cs via rivers after the accident at the Fukushima Daiichi NPS, long-term wide-area monitoring has been conducted led by the University of Tsukuba and Fukushima Prefecture. As a result, it was found that the total amount of ¹³⁷Cs discharged to the ocean via rivers was about 4.8% of the initial land deposition (from June 2011 to March 2017, Abukuma River), of which 96.5% was transported as particulates (Fig. 1).

It was also discovered that land use of the catchment deeply affects the temporal change in the concentration of ¹³⁷Cs that flows along rivers. Especially, the concentration sharply dropped in rivers with high catchment coverage for the sphere of life (= paddy field, farmland, and urban area (PFU)) in one year after the accident (Fig. 5A). This trend was also observed in the survey for the concentration of ¹³⁷Cs discharged from paddy fields and urban areas into rivers. After going through a steady gradual reduction trend thereafter, six years after the accident, the particulate ¹³⁷Cs concentration in the water of Abukuma River became 2% of the figures that were observed immediately after the accident (Fig. 5B).

Fig. 5 Temporal change in ¹³⁷Cs transition in rivers after the accident at the Fukushima Daiichi NPS

• A. Dissolved ¹³⁷Cs concentration normalized by the initial deposition onto the catchment
• B. Particulate ¹³⁷Cs concentration normalized by the initial deposition onto the catchment
• C. Loss of ¹³⁷Cs from the catchment (every four months)

Nuclides other than ¹³⁷Cs

Nuclides other than radioactive cesium were also released into the environment by the accident at the Fukushima Daiichi NPS. Such nuclides include radioactive strontium (Sr) and plutonium (Pu), which are found in Fukushima, but the quantity is relatively small. For example, the concentration of 90Sr is 1/100 to 1/10,000 of that of ¹³⁷Cs in soils within the range of 50 km from the Fukushima Daiichi NPS. In addition, it was found that about 7% of Pu found in Fukushima originates from the accident at the Fukushima Daiichi NPS, and the rest comes from atmospheric nuclear tests conducted in the past.

Comparison with the Chernobyl disaster

The natural conditions (e.g., characteristics of forests, land use, topography, precipitation) of Fukushima are different from Chernobyl, and researchers have predicted that the dynamics of ¹³⁷Cs in the land area would also be different. However, no comparative study has been made to date on the dynamics of ¹³⁷Cs in these regions based on comprehensive monitoring surveys.

An analysis of the results obtained in this study shows that the rate of ¹³⁷Cs transition from the tree canopy to the forest floor by rainfall in forests in Fukushima shortly after the accident was higher than that in forests in Chernobyl. This is most likely attributable to higher precipitation in Fukushima (Fukushima: 1200 mm/year, Chernobyl: 650 mm/year). Meanwhile, densely planted artificial cedar forests in Japan have a large amount of biomass in the tree canopy, and the rate of transition would become the same as or slightly lower than Chernobyl in a long-term perspective. ¹³⁷Cs that transferred to the forest floor by rainfall or falling leaves swiftly migrates downward from the litter layer with time and accumulates in the soil layer. Such cesium does not easily flow out of the system and therefore the dissolved or particulate ¹³⁷Cs concentration in the river water was one order of magnitude lower than that in forest rivers in European states (Figs. 5A and 5B). In addition, forests in Fukushima are mountainous and have steep slopes, and some of fallen leaves containing ¹³⁷Cs moved lower along the slopes and Cs was discharged into rivers. Because the majority of discharges into rivers come from mountain streams, as stated above, the annual discharge rate was lower than 0.3% of the initial deposition: only a small amount was being transported out of forests (Fig. 5C).

It was also found, in Fukushima, that the amount of ¹³⁷Cs exposed on the surface reduced faster than in Chernobyl. This was because Fukushima has steep slopes and high precipitation and saw cultivation at paddy fields and farmlands, human activities in urban areas and decontamination, while the catchment in Chernobyl mostly consists of abandoned farmlands and forests with limited human activities.

For example, the relaxation mass depthβ, a parameter that indicates downward penetration of radionuclides, of ¹³⁷Cs in cultivated paddy fields in Fukushima was two to four times as much as the values in areas affected by the Chernobyl disaster (Fig. 3d). The surface soil is a source of suspended particulates in rivers, and the rate of reduction in the particulate ¹³⁷Cs concentration in rivers in Fukushima during a period of one year after the accident was about 1.6 times as much as that in the Pripyat River that runs in Chernobyl (Fig. 5B). As for the normalized dissolved ¹³⁷Cs concentration, rivers in Fukushima continue to show values about two orders of magnitude lower than the values in European rivers after the Chernobyl disaster. This means, the ¹³⁷Cs concentration in river water in Fukushima has been rapidly dropping (Fig. 5A). Currently, values observed in periodic monitoring are mostly below standard values, but sometimes ¹³⁷Cs concentrations exceeding the regulatory standard value are observed in freshwater fish, and that is known to be highly correlated to dissolved ¹³⁷Cs concentration. In this study, similar to the dissolved Cs concentrations, the contamination level of freshwater fish in Fukushima was found to be one or two orders of magnitude lower than that in Europe after the Chernobyl disaster, indicating much faster recovery than as predicted from what we learned from the Chernobyl disaster.

After the Chernobyl disaster, scientists claimed that the reduction in the dissolved ¹³⁷Cs concentration in European rivers was due to adsorption of ¹³⁷Cs onto soil particles that gradually becomes stronger. However, this doesn’t appear to be applicable to the case of Fukushima, for the facts that the land use in the catchment directly affects the particulate ¹³⁷Cs concentration in rivers and the ratio of particulate ¹³⁷Cs concentration to dissolved ¹³⁷Cs concentration (Kd) in rivers does not change with time. Summarizing these findings brings us new understanding that, at least in the case of Fukushima, a reduction in radioactive material concentrations in the sources of soil is causing a reduction in the particulate ¹³⁷Cs concentrations, resulting in a reduction in the dissolved ¹³⁷Cs concentration.

(The findings were obtained through collaboration with the University of Tsukuba and other institutions.)

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Reference

1. Yuichi Onda et al., Radionuclides from the Fukushima Daiichi Nuclear Power Plant in terrestrial systems, Nature Reviews Earth & Environment volume 1, pages644–660 (2020).