After the accident that occurred at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, the Japanese government delineated a Special Decontamination Zone (SDZ) in the areas located within a 20 km radius of the FDNPP or in those areas where the cumulative dose 1 year after the accident was expected to exceed 20 mSv⋅yr−1. In most of SDZ except for the difficult-to-return-zone corresponding to the most severely contaminated land located in the close vicinity of FDNPP, topsoil (0–5 cm) was removed from 50% of agricultural land. This soil was taken where 137Cs concentrations exceeded 5000 Bq⋅kg−1 and where it was replaced with a 5 cm layer of “clean” material often corresponding to mountainous sand [Evrard et al. 2019]. Then, the coarse material showing a sandy texture was thoroughly mixed with residual soils in depth to prepare the soil for restarting cultivation by the returnees. These decontamination activities decreased the 137Cs concentrations in topsoil by about 80% compared to the original levels [Kurokawa et al. 2019]. In contrast, decontaminated forested areas within the SDZ was restricted to those zones lying within 20 m around residential areas and along roads [Ministry of the Environment 2013].
In October 2019, Typhoon Hagibis stuck the eastern part of Fukushima Prefecture and generated the widespread overflow of the rivers flowing across the SDZ. Sediment transported by the typhoon-generated flooding, which was referred to as flood sediment (FS), got deposited in the decontaminated agricultural fields at many locations along the rivers. Evrard et al.  estimated that approximately 40% of the sediment deposited during the flood induced by the Hagibis typhoon originated from forested areas. Since most of the forest soils in the SDZ were virtually not decontaminated and maintained high levels of 137Cs concentrations [Ondaet al. 2020], the deposition of sediment following the typhoon-induced flooding of decontaminated fields along the river network may increase the transfer potential of 137Cs from soil to crops in these decontaminated agricultural fields. However, to the best of our knowledge, neither the mobility of 137Cs found on FS nor the transfer potential of 137Cs to the crops in these previously decontaminated fields has been investigated so far.
Therefore, this study investigated total and exchangeable radiocesium (137Cs) content, and acid-extractable potassium (AE-K) contents for FS and DS in agricultural fields in the SDZ to discuss the impact of FS deposition on the potential migration of 137Cs from decontaminated agricultural land to crops. The AE-K, which is generally referred to as Nex-K in soil science, provides a good indicator to predict the inhibitory effect of K on 137Cs uptake by plants. Kurokawa et al.  determined that the soils with Nex-K > 50 mg K2O 100 g−1 showed sufficiently low soil-to-rice transfer risk of 137Cs in Fukushima.
2. Materials and methods
2.1. Sampling sites and sampling preparation of flood sediments and decontaminated soils
FS and DS samples were collected along Niida River and Mano River within Iitate Village, Fukushima, Japan, which is located about 40 km northwest from the FDNPP. These rivers overflowed at many locations following Typhoon Hagibis (typhoon 19) in October 2019. Their catchments include upstream mountainous plateaus at an altitude of 700–900 m above sea level connected to a flat coastal plain by deep and strongly incised rivers [Chartin et al. 2017]. About two-thirds of the land in these catchments is occupied by forested areas. Most of the agricultural fields along these rivers were used as paddy fields before the 2011 nuclear accident. Geologically, granite and granodiorite are dominant in the area, with a minor occurrence of sedimentary rocks and basalts [AIST Geological Survey of Japan website 2022].
DS samples were collected using a shovel from the upper 0–15 cm layer of surface soil from 18 decontaminated fields in March 2019. In each field, subsamples of soil were collected from five points (at the center and the four vertices of a square with a side length of 15 m) and mixed together well. Also, FS samples were collected from 19 points where river flooding occurred in the close vicinity of the DS sampling sites after the Hagibis typhoon in October 2019 (Figure 1). The FS samples correspond to fresh sediments deposited as a fine material draped on the upper part of the embankment along the Mano River (n = 6) and Niida River (n = 13). FS samples were collected from the uppermost layer of about 0–1 cm of fresh sediments using a shovel. The DS and FS samples were dried at room temperature and sieved to <2.0 mm.
2.2. Radiocesium analyses
Total and exchangeable 137Cs contents in FS or DS were determined following the method of Kurokawa et al. . Approximately 20–40 g of the soil sample was filled in a polyethylene vial (20 cm3). The vial was set in an auto gamma counter with a NaI (Tl) detector (2480WIZARD2, PerkinElmer, Waltham, Massachusetts, USA, Radioisotope Research Center of Kyoto Prefectural University) to determine the gamma activity (between 589 and 735 keV). To determine the exchangeable 137Cs content, duplicates of 50 g of sample were shaken with 250 mL of 1 mol⋅L−1 ammonium acetate (CH3COONH4) for 24 h at 20 °C. The extracts were heated on a hot plate and concentrated to about 20 mL. The concentrated solution was measured with NaI (TI) detector in the same way as for total 137Cs content. The data were calibrated by comparing the measured values with those obtained using a Ge semiconductor detector connected to a multichannel analyzer system (GC-4018, Canberra, Japan, Radioisotope Research Center of Fukushima University). Details of this procedure are provided by Kurokawa et al. .
The 137Cs exchangeable fraction (%), i.e., the percentage of exchangeable 137Cs content compared to the total 137Cs content, was calculated using the following equation:
[137Cs exchangeable fraction (%)] = [Exchangeable 137Cs content in FS and DS (Bq⋅kg−1)]/[Total 137Cs content in FS and DS (Bq⋅kg−1)] × 100.
2.3. Particle size distribution
Particle size distribution, i.e., respective content in coarse sand (2000–200 μm), fine sand (200–50 μm), and silt and clay ( < 50 μm) in FS and DS, was assessed by pipetting and sieving after removing soil organic matter with oxidizing reagents.
2.4. Total carbon content
The total carbon (TC) content in sample was measured, as an indicator of total organic matter content, determined by the dry combustion method with an NC analyzer (NC-95A, Smika Chem. Anal. Service, Tokyo, Japan) and gas chromatography (GC-8A, Shimadzu, Kyoto, Japan) after fine grinding with agate mortar.
2.5. X-ray diffraction (XRD) analysis
XRD diffractogram was determined using the matrix flushing technique [Chung 1974]. 1.0-g soil sample was mixed with 7-mL methanol and 0.25-g 𝛼-Al2O3 (Baikalox 3.0CR, Bakowski, France) as internal standard and finely-ground to a powder using a grinding machine (XRD-Mill Mcrone, Retsch, German) for 10 min. The mixed powder was packed into a holder to ensure a random sample orientation and then irradiated from 5° to 65° two-theta using CuK𝛼-radiation, with 0.02° steps and a counting rate of 10° per min by XRD (MiliFlex 600, Rigaku, Tokyo, Japan). The diffraction peak at 8.8° two-theta corresponded to the 1.0 nm lattice spacing associated with the presence of non-expanding mica layers [Kitayama et al. 2020, Ogasawara et al. 2019]. As K in these non-expanding layers is the primary source of Nex-K, XRDmicas provided a rough index of the amount of Nex-K present. Therefore, XRDmicas was calculated by the following equation:
2.6. Non-exchangeable K as an indicator of acid-extractable K
Nex-K could be used as a good proxy of K phytoavailable fraction especially in mica-rich soil [Mengel and Rahmatullah 1994, Surapaneni et al. 2002]. Kurokawa et al.  and Wakabayashi et al.  reported that the Nex-K reduced RCs crop migration as well as that of exchangeable K (Ex-K). The Nex-K content of the FS and DS samples was determined by subtracting Ex-K from the estimation of K concentration determined as the elemental content extracted with the hot nitric acid method [Helmke & Sparks 1996]. The Nex-K is often used as an indicator of the phytoavailable K fraction, which is not extractable with ion-exchange reaction. A 2.50-g soil sample was gently heated using a hot plate (EA-DC10, ZOJIRUSHI, Osaka, Japan) at nearly 100 °C for 15 min with 25.0 mL of 1 mol⋅L−1 HNO3 after boiling started. After cooling, the extract was filtered and filled up to 100 mL with 0.1 mol⋅L−1 HNO3. The K content of the extract solution was determined via Atomic Absorption Spectroscopy (AAS) (AA -6200, Shimadzu, Kyoto, Japan). To determine the Ex-K content, the DS and FS samples were shaken with 1 mol⋅L−1 CH3COONH4 at pH 7.0 in a soil:solution ratio of 1:5 for 30 min at 20 °C. The suspension was centrifuged, and the supernatant was collected by filtration. These processes were repeated three times. Approximately 75 mL of supernatant was brought up to a total volume of 100 mL with 1 mol⋅L−1 CH3COONH4, and the K content of this solution was determined using the same instruments.
3. Results and discussion
3.1. Total 137Cs content in flood sediment and its controlling factors
The total 137Cs content in FS reached on average 4.2 kBq⋅kg−1 (Table 1). This observed value was lower than that obtained in FS collected along the same two rivers in 2011 and 2015 [Chartin et al. 2017]. Previous research indicated that the amount of 137Cs had strongly decreased in river sediment and in the soil surface in 2019 compared to 2011 [Evrard et al. 2021, Funaki et al. 2019, Iwagami et al. 2017, Yoshimura et al. 2016], which further supports the results obtained in the current study.
Selected physicochemical properties of FS
|FS sample||Total 137Cs, Bq⋅kg−1||Exchangeable 137Cs, Bq⋅kg−1||137Cs exchangeable fraction, %||Coarse sand||Fine sand||Silt + clay||PIRmica||Ex-K||Nex-K||TC, g⋅kg−1|
|%||mg K2O 100 g−1|
Typhoon Hagibis caused debris flows that apparently excavated soils from contaminated topsoil layers, and much less contaminated subsurface soils as shown for FS in Miyagi Prefecture [Moriguchi et al. 2021]. Highly contaminated surface soils from forested areas were therefore likely mixed with subsurface soils from landslides and channel bank collapse events, which may consequently explain the overall low total 137Cs content in FS.
Although total 137Cs contents in FS reached an average value of 4.2 kBq⋅kg−1, they showed a wide range of values varying from 0.51 to 16 kBq⋅kg−1 (Table 1). Potential explanation factors relating to this variation were investigated by means of the Pearson correlation analysis, which revealed that the silt + clay content of FS, averaging 18% (range: 4.9 to 36%), was positively correlated with total 137Cs content (r = 0.59,p < 0.01) (Figure 2a). This may reflect the strong interaction of 137Cs with sediments, i.e., the finer mineral particles have a larger specific surface area and a higher proportion of 2:1 clay minerals, which can strongly adsorb Cs in their frayed-edge sites [Fan et al. 2014, He & Walling 1996, Nakao et al. 2012].
Organically-bound 137Cs was not considered as a major fraction of the total 137Cs in FS. We observed few macro-organic material (e.g., leaves and litter from forest vegetation) in the field affected by FS migration via flooding events. Since DS fields were not cultivated at that time, crops could not have trapped macro-organic material, which would instead have been carried away by the river water. As previously reported, 137Cs in soils and sediments was found to be bound predominantly to the soil mineral phase whereas it was found bound to a lesser extent to organic matter [Koarashi et al. 2019, Tsukada et al. 2008]. The amount of total carbon in the FS was low, with an average of 3.7 g⋅kg−1 (range: 0.47 to 8.1 g⋅kg−1), and it was found not to be significantly correlated with total 137Cs content or the 137Cs exchangeable fraction (Figure 2b). Based on these results we considered that the amount of organic matter-bound 137Cs in the FS should only provide a minor contribution, if any, to the total 137Cs content.
3.2. Comparing total 137Cs content and 137Cs exchangeable fraction between FS and DS
The total 137Cs content in DS reached on average 1.2 ± 2.0 kBq⋅kg−1 (Figure 3). The entire range of values (Table 2) was less than one-fifth of the total 137Cs content in agricultural land observed before decontamination in Iitate Village [Ministry of Agriculture 2011]. A drastic decrease in total 137Cs content after decontamination was observed in the agricultural fields in Tomioka Town [Kurokawa et al. 2019]. Thus, decontamination activities for agricultural fields in Iitate village were thus confirmed to have effectively reduced the total 137Cs content in surface soils corresponding to about 80% in a decrease of the initial contamination levels. Although total 137Cs was not significantly different between FS and DS, this value range was lower than that of FS (Figure 3). These comparisons suggest that FS deposition on DS may lead to a new increase in their total 137Cs content, at least in their surface layer. However, the increase in total 137Cs content due to FS deposition, if any, may not raise the transfer risk of 137Cs from soil to crops for the following reasons. First, our field observations in Iitate Village confirmed that the thickness of the FS layer deposited on decontaminated fields following the flooding reached at most about 1 cm in depth. Assuming that the thin FS layer contained a total 137Cs activity of 4.2 kBq⋅kg−1 mixed with 15 cm of the plowed layer with a total 137Cs content of 1.2 kBq⋅kg−1, the total 137Cs content of the entire profile would only increase by 15% to an average of about 1.4 kBq⋅kg−1. While this is a rough estimation based on field observation of the thickness of sediments deposited in several agricultural fields, it indicates that the FS incorporation into a realistic larger volume of soil will not lead to a strong increase in the total 137Cs content in decontaminated fields. It should be emphasized that the 1 cm thickness was an exceptional case for FS migration to DS. The average trend was unfortunately not quantitatively recorded in this study, but more accurate observation is needed in the future when larger volumes are involved. A comprehensive risk assessment would require a more detailed calculation of the thickness of sediment deposited onto agricultural soils.
Selected physicochemical properties of DS
|DS sample||Total 137Cs, Bq⋅kg−1||Exchangeable 137Cs, Bq⋅kg−1||137Cs exchangeable fraction, %||Coarse sand||Fine sand||Silt + clay||PIRmica||Ex-K||Nex-K||TC, g⋅kg−1|
|%||mg K2O 100 g−1|
Second, the average 137Cs exchangeable fraction of FS was 3.7%, only about one-third of that rate obtained from DS (i.e., 9.8%) (Figure 4). Whereas the 137Cs exchangeable fraction from DS remained in the medium range of values (range: 6.5 to 16%) previously reported for 137Cs contaminated soils in Fukushima, corresponding values from FS remained close to the lowest values previously reported for contaminated soils and sediments [Kurokawa et al. 2019, Ogasawara et al. 2019, Saito et al. 2014]. These data suggested that 137Cs was assumed not to be desorbed easily from FS particles. Third, FS showed high levels of Nex-K content, with an average of 162 mg K2O 100 g−1 (range: 80 to 317 mg K2O 100 g−1), as high as in DS (i.e., on average 218 mg K2O 100 g−1). Most of these values were higher than the recommended value (i.e., 50 mg K2O 100 g−1), above which the transfer rate of 137Cs from soil to crops was confirmed to be low [Kurokawa et al. 2020]. Since 137Cs exchangeable fraction and Nex-K content in soil control soil-to-plant transfer of 137Cs rather than the total 137Cs content itself, these results may overall lead to the conclusion that FS deposition in decontaminated fields may not increase the transfer risk of 137Cs in the case of Iitate Village in Fukushima.
A missing source of 137Cs that might have been incorporated into the DS field was dissolved 137Cs in flood water. Takata et al.  indeed showed higher dissolved 137Cs activity in nearshore water after the 2019 typhoon. Although we could not measure the amount of 137Cs in DS after the typhoon, it is unlikely that dissolved 137Cs may be supplied to DS after the deposition of FS, as the 137Cs exchangeable fraction was found to be lower than 5% in the current research.
3.3. Factors relating to Nex-K content of FS and DS
The Nex-K content of FS and DS (i.e., on average 162 and 218 mg K2O 100 g−1) was relatively high, compared to those levels previously found in other areas of Fukushima Prefecture, including a mean value of 66 mg K2O 100 g−1, obtained on 173 farmland soils in Tomioka Town [Kurokawa et al. 2019]. The regional difference in Nex-K content may reflect to a large extent the type of base rock in each area. For example, Iitate Village is a locality surrounded by Abukuma granite mountains. Therefore, the fluvisols on which cropland was installed in the valleys, and the coarse material extracted from the quarries to replace the contaminated upper soil layer during remediation, both contain minerals, derived from granite or granitoid, which in turn contains biotite as a major rock-forming mineral. To investigate whether the quantity of mica defines Nex-K, we investigated the correlation between Nex-K and XRDmicas, which is an index of mica quantity, and found that Nex-K showed significantly positive correlation with XRDmicas (p < 0.001) (Figure 5). This strongly supports the hypothesis that Nex-K content was primarily controlled by micaceous minerals probably derived from granite. Granite and granodiorite widely cover the surfaces of Abukuma Highlands, which have been weathered to supply biotite to watershed areas via river water transfer. Several studies on the mineral composition of sediments have indicated the occurrence of biotite in this material [Fan et al. 2014, Konoplev et al. 2021]. Accordingly, the K release potential of FS which was found to be comparable to that of DS should be probably ubiquitous across this region. The granitic biotite in both FS and DS may contribute to long-term K resources for plants, which effectively inhibit 137Cs migration from soil to crops.
Total 137Cs content in FS along Mano and Niida Rivers caused by Typhoon Hagibis in October 2019 was about four times larger than that found in decontaminated agricultural soils in Iitate Village. However, the 137Cs exchangeable fraction of FS was low and the Nex-K content was as high as that of DS, suggesting a low potential risk of the FS to transfer 137Cs from soil to plant. In the future, it will be important to elucidate in detail the behavior and K supply capacity of RCs in the forest area, which is the origin of FS.
Conflicts of interest
Authors have no conflict of interest to declare.
We would like to thank Dr. Yuzo Mampuku (Institute for Agro-Environmental Sciences, NARO) and inhabitants from Iitate Village for helping collect the soil samples, and Dr. Kenta Ikazaki (Japan International Research Center for Agricultural Sciences) for his kind assistance in the GIS analysis. We thank the Radioisotope Research Center of Kyoto Prefectural University, where we undertook our 137Cs experiments. We also thank the anonymous reviewers and the editor for their constructive comments, which helped to improve this manuscript. The fieldwork of the French partner was funded by the AMORAD (ANR-11-RSNR-0002) project, under the supervision of the French National Research Agency (ANR, Agence Nationale de la Recherche). The fieldwork and in vitro experiments were mainly funded by JSPS bilateral program (No. 1082680). The support of CNRS (Centre National de la Recherche Scientifique, France) and JSPS (Japan Society for the Promotion of Science) through the funding of collaboration projects (grant no. PRC CNRS JSPS 2019-2020, no.10; CNRS International Research Project–IRP–MITATE Lab) is also gratefully acknowledged.