The deposition of insoluble radiocesium bearing microparticles (CsMPs), which were released from the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident in March 2011, has resulted in the widespread contamination of eastern Japan. Obviously, these deposited insoluble CsMPs may become the secondary contamination sources by atmospheric migration or other environmental transferring process, however, the understanding of the transport mechanism remains non-elucidation, and the relevant evidence has not been directly provided. This study, for the first time, provides the direct evidence for the resuspension of these insoluble CsMPs to the atmosphere from 1) proximity of 137Cs radioactivity and resemblance of the morphology and the elemental compositions of CsMPs in the samples of soil and aerosol derived from the same sampling site, 2) the special characteristics of the resuspended CsMPs of which the ratios of Na/Si, K/Si and/or Cs/Si were smaller than those from the initially released CsMPs collected at either long distance or near F1NPP, which can be ascribed to the slowly natural corrosion of CsMPs by the loss of the small amount of soluble contents in CsMPs and 3) high CsMPs concentration of 10 granules/gram in the surface soil of our sampling site and the observed resuspension of surface soil dust at wind gust speed higher than 4 m s-1. Specifically, fifteen single CsMPs were successfully isolated from the aerosol filters collected by unmanned high-volume air samplers at a severely polluted area in Fukushima Prefecture, about 25 km away from F1NPP, from January 2015 to September 2019. The mean diameter of these CsMPs was 1.8 ± 0.5 μm, and the average 137Cs radioactivity was 0.35 ± 0.23 Bq/granule. The contribution rate of the resuspended CsMPs to the atmospheric radiocesium was estimated from the ratio of 137Cs radioactivity of a single CsMP to that of the aerosol filter to be of 23.9 ± 15.3%. There has been no considerable decreasing trend in the annual CsMP resuspension frequency.
Research Article
Atmospheric Resuspension of Insoluble Radioactive Cesium Particles Found in the Difficult-to-Return Area in Fukushima
https://doi.org/10.21203/rs.3.rs-832399/v1
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Resuspension
Radiocesium
Water-insoluble radiocesium bearing microparticles (CsMPs)
Fukushima accident
Aerosol
1.1. Accident
The Fukushima Daiichi Nuclear Power Plant (F1NPP) accident was triggered by the 9.0-magnitude Tohoku-Oki earthquake and the subsequent tsunami on March 11, 2011,1, 2 which resulted in the complete loss of all power and over-pressurization of Units 1–3 (there are 6 Units in F1NPP and Units 4–6 had been “shut down” for regular maintenance prior to the earthquake3). Over the subsequent several days, although venting operations were conducted to depressurize Units 1–3 (starting at 10:40 JST on March 12,2 10:15 JST on March 133 and 08:41 JST on March 12,4 respectively), hydrogen explosions could not be avoided at Units 1 and 3 (at 15:36 JST on March 12 and 11:01 JST on March 141). Thus, a large quantity of radioactive materials was released into the atmosphere,5 the land,6, 7 and the Pacific Ocean.8 Specifically, the amount of total atmospheric release of 137Cs was up to 15–21 PBq (1 PBq = 1015 Bq), and 3–6 PBq was directly deposited onto the land, and the rest of the 137Cs was distributed over the North Pacific was quantified to be15–18 PBq.9 Moreover, Steinhauser et al.10 reported that the highly contaminated area, with 137Cs levels exceeding 185 kBq m− 2, spread approximately 1700 km2 in Fukushima Prefecture.
1.2. Radiocesium-Bearing Microparticles
Water-insoluble radiocesium-bearing microparticles (hereafter CsMPs)11, 12 were released and deposited in the environment by the F1NNP accident. In recent several years, research on the morphology, elemental compositions, classifications, and emission sources of CsMPs has been comprehensively reported.11–14 Specifically, Adachi et al.11 first reported that spherical CsMPs mainly contained O, Fe, and Zn as well as other trace elements. These CsMPs were referred to as the released and deposited CsMPs in early stage of the F1NPP accident.15 Based on the difference of size and structure as well as the difference in the isotopic ratio of 134Cs/137Cs, the CsMPs have been classified into type A (possibly derived from Unit 2 or 3 with a size of less than 10 µm and 134Cs/137Cs ≈ 1.0) and type B (perhaps released from Unit 1 with a size of more than 100 µm and 134Cs/137Cs ≈ 0.9).12, 13 There are also reports suggesting that type A CsMPs were derived from Unit 216 or Unit 1.17 Moreover, the glassy state of CsMPs implied water insolubility (discussed in detail in references11, 18) and comparatively persistent toxicity and hazards in the terrestrial environment19 (137Cs has a half-life of 30.2 years20). The high-dose internal radiation exposure by the inhalation of radioactive aerosols and the consumption of food associated with radioactive material is the most concerning health hazard.21–23 In order to assess and reduce the risk of radiocesium inhalation24 and the intake of food25, 26 contaminated by CsMPs, it is necessary to understand its distribution and transport in the secondary processes.
1.3. Resuspension of Radiocesium
Hirose27 found that the deposition of the early-stage released radiocesium was significant from March to June 2011 and that an obvious decrease in radiocesium deposition speed occurred after July 2011. Hirose28 concluded that most of the F1NPP-derived 137Cs deposited on the ground surface remained in the surface soil layer as a potential secondary source of atmospheric 137Cs, and Tanaka et al.29 reported that more than 90% of the radiocesium was fixed within the top 5 cm of the profile. Although the F1NPP accident has passed for about 10 years, most of the 137Cs is thought to remain there,30 and thus the decontamination by removal of topsoil was conducted in Fukushima Prefecture.31, 32 Resuspension of radiocesium refers to the secondary migration of the deposited radiocesium into the atmosphere from surfaces and terrestrial ecosystems by winds,33 biomass burning5 and other different anthropogenic disturbances,34, 35 which can be found in the reports about the F1NPP accident33–36 and the Chernobyl Nuclear Power Plant accident37–39. Regarding the issue Igarashi et al. (Ref.5) emphasized, through their observation and data analysis of radiocesium in air over Tsukuba, Ibaraki, that "elucidating the secondary emission processes of the FDNPP radionuclides remains an imminent scientific challenge, especially for heavily polluted areas. Secondary sources can include soil dust suspension from polluted earth surfaces, emissions from polluted vegetation and forests, and volatilization and release from combustion of polluted garbage and open field burning. Although the main emission sources are not yet well understood, this elucidation must be performed as soon as possible." Significantly, this process is an uncontrollable route that can cause widespread secondary contamination26 in previous noncontaminated or decontaminated areas. Although the radioactivity level of resuspension5, 36 was a few orders of magnitude lower than that of the early-stage released, it would be a matter of public concern, especially for the evacuated people who lived in difficult-to-return areas.
Hirose27 suggested that the second increase in atmospheric radiocesium in the spring 2012 can be attributed to the resuspension process. Moreover, seasonal variations of radiocesium in aerosol samples were observed in severely contaminated areas of Fukushima Prefecture,15, 36, 40, 41 being explained as the result of radiocesium resuspension because the observed seasonal dependence of the atmospheric radiocesium concentration cannot be interpreted from direct or delayed primary release from F1NPP accident.41 Specifically, Ishizuka et al.15 measured and modeled the particle size distribution of suspended soil dust carrying radiocesium considering different soil textures. Kinase et al.36 and Igarashi et al.41 discussed possible resuspension-hosting sources and showed that soil particles42 can be significant in the springtime and that bioaerosols such as pollens,43, 44 spores,41 and microorganisms36 can be a major host particle in summer and autumn. Kajino et al.45 modeled the resuspension of 137Cs from bare soil and forest ecosystems and suggested a similar seasonal change of the resuspension host particle to above-mentioned studies, namely, the high radiocesium concentration can be observed in warm seasons and the low can be obtained in cold seasons. They also estimated respective resuspension rates of 1 × 10− 6 day− 1 and 2 × 10− 6 day− 1.45 These results were helpful to estimate the inhalation dose.46, 47 However, research on the resuspension of radiocesium has still been insufficient, and both observational and theoretical model studies are necessary for a quantitative understanding. Notably, these studies did not discuss the resuspension of the CsMPs.
Higaki et al.48 reported that the main sources of radiocesium adhered to non-woven fabric face masks, which were worn both outdoors and indoors by 68 residents living in eastern Japan in the spring of 2012, were fugitive dust and CsMPs24 (the masks were worn by the Fukushima residents in the spring of 2013). The CsMPs were also isolated from Japanese spinach.26 Although these results suggested the possibilities of the resuspension of CsMPs from the ground to the atmosphere, there has been no evidence to indicate that it occurred naturally (without anthropogenic interferences) and no discussion about the special features of the collected CsMPs to demonstrate whether they are derived from resuspension or other reasons. This study, for the first time, provides the direct evidence of resuspension of the CsMPs in the monitoring duration at our sampling site, and explains the transport variables and mechanism of the CsMPs. Therefore, this study aims at 1) detection and isolation of highly radioactive granules from aerosol and soil samples collected at a severely polluted area to identify them as CsMPs by analyses of their morphology and elemental compositions, 2) describing the possibility of the natural resuspension of CsMPs to the atmosphere and 3) evaluating the frequency of CsMP resuspension and its contribution ratio to the whole atmospheric concentration of radiocesium.
2.1. Sample Collection
2.1.1. Sampling of Aerosols. Aerosol samples were collected at a site (37.6° N, 140.8° E and altitude 438 m) located in a highly contaminated zone that was difficult for residents to return to (approximately 25 km northwest from the F1NPP), as shown in Figure 1. Details of the sampling site are described elsewhere.15, 36, 41, 45 Aerosol samples were collected on quartz-fiber filters (QR100, Advantech, Tokyo, Japan or 2500QAT–UP, Pall Corp., NY, USA) with high volume samplers (HV samplers; 120SL, Kimoto Electric Co. Ltd., Tennoji, Osaka, Japan and HV–1000R/F, Sibata Scientific Technology, Ltd., Saitama, Japan) at a sampling air flow rate of 0.7 m3 min-1 from January 2015 to September 2019. We used the aerosol samples collected in 2015, 2016, 2018, and 2019 because the aerosol sampling was interrupted for several months in 2017. Additionally, aerosol samples with different aerodynamic diameters were also collected separately on slotted quartz-fiber filters using an HV sampler with a seven-stage cascade impactor (TE–230, Tisch Environmental Inc., OH, USA) with slotted quartz-fiber filters. The aerodynamic size-distribution ranges with a collection efficiency exceeding 50% were sampled as follows: Q1 (> 10.3 μm), Q2 (4.2–10.3 μm), Q3 (2.1–4.2 μm), Q4 (1.3–2.1 μm), Q5 (0.69–1.3 μm), Q6 (0.39–0.69 μm), and Q7 (< 0.39 μm, working as a backup filter) at a sampling air flow rate of 0.556 m3/min. Note that the aerosol samples were successively taken by 10 timer-controlled HV samplers without anthropogenic interference, except for replacing the sampling filters over the course of a few hours every 2–4 weeks. In all sampling processes, aerosol filter samples were collected at a height of 1.2 m above the ground. The sampling periods ranged from 3 days to 4 weeks.
2.1.2. Collection of Soil Samples. About 200 g of surface soil samples were collected on June 25, 2016, at the same site as the aerosol-sampling location with a 10 × 10 cm2 area and 1.5 cm depth and were transported to the laboratory and stored at ambient temperature. They were homogenized in a plastic bag49 for measurement of their radioactivity and stored for further analysis. For extracting the CsMPs, approximately 2.5 g soil sample was randomly retrieved (the radioactivity was 173.6 ± 2.00 Bq/g and 166.2 ± 0.40 Bq/g for 134Cs and 137Cs, respectively). The radioactivity data in this study were corrected back to March 11, 2011, 14:46 JST.
2.2. Detection and Isolation of Highly Radioactive Granules
An imaging plate (IP) system (CR × 25P portable computed radiography, GE Measurement & Control, Massachusetts, USA) and a micromanipulator (Axis Pro; Micro Support Corp., Shizuoka, Japan) were used to detect and isolate CsMPs from the aerosol filters. The isolation procedure of CsMPs from the aerosol filters and from the soil samples was conducted as the similar dry process (without use of water) of Adachi et al.11 and Satou et al.50, respectively.
2.3. Measurement of Gamma-Ray Spectrometry and SEM Observation
The gamma-ray peak intensities at 605 and 662 keV were used to identify and determine the radioactivities of 134Cs and 137Cs, respectively.50 Identification of the CsMPs was based on the morphology and elemental compositions of the single highly radioactive particle. The observation of the morphology and elemental compositions was conducted with an SEM (SU3500, Hitachi High-Technologies Co., Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS: X-max 50 mm, Horiba Ltd., Kyoto, Japan) for aerosol CsMPs and a field emission SEM (JSM-7800F, JEOL Ltd. Tokyo, Japan) for visualizing soil CsMPs and water-immersed aerosol CsMPs.
3.1. Identification of Resuspension of CsMPs
3.1.1. Aerosol CsMPs (A-CsMPs). Twenty highly radioactive granules were detected from the aerosol samples by using IP imaging, 15 of which have been successfully isolated, and the rest of which was lost during the isolation procedures. Their SEM observation images are shown in Figures 2 and S1, and the detailed information regarding radioactivity and elemental compositions is given in Tables 1 and S1. These results indicate that all isolated granules have characteristics corresponding to those reported for type A CsMPs12, 13 as described below.
All CsMPs were of an almost spherical or distorted spherical shape, and their diameter distribution ranged from 1.2 to 3.0 μm with the same mean and median values of 1.8 μm. The 137Cs radioactivity of the single CsMP varied from 0.13 to 0.87 Bq/granule, and the mean and median radioactives of 137Cs were 0.35 Bq/granule and 0.36 Bq/granule, respectively. The 134Cs radioactivity was less than the detection limit in most cases due to its shorter physical half-life of about 2 years. The elemental mapping images (a) and EDS spectra (b) of #11-HNVF-300601-Q are given in Figure 3. It is obvious that Cs in the particle showed multiple peaks (Cs Lα and Cs Lβ at 4.42 and 4.55 keV), and O, Si, Na, S, Cl, K, Ca, Ti, Fe, and Zn also coexisted within the particle according to the elemental mapping images of major elements. Moreover, based on EDS analysis of 15 CsMPs, the main elemental components included Si (average atomic mass fraction 36.0 ± 5.3 wt.%), O (50.1 ± 3.7 wt.%), Cs (1.0 ± 0.2 wt.%), Zn (2.7 ± 1.0 wt.%), Sn (0.2 ± 0.2 wt.%), S (0.1 ± 0.1 wt.%), Na (3.2 ± 3.3 wt.%), K (2.5 ± 2.2 wt.%), Fe (1.0 ± 0.3 wt.%), Cl (0.2 ± 0.1 wt.%), and some other trace elements in some individual particles, such as Ba, Al, P, Ca, and Ti. The elemental composition of each CsMPs is given in Table S1.
The water insolubility of radiocesium in CsMPs has been reported by Adachi et al.,11 and other reports also support this fact (CsMPs can be isolated from the wet-isolation method51, 52 and can be detected in river water53). The water insolubility of the CsMPs was also checked by immersion of the isolated A-CsMPs into water for 24 h (#11-NHVF-300601-Q) and 48 h (#12-NHVF-300906-Q), which is shown in Figure S2. There were no obvious detectable changes in their radiocesium activity and morphology before and after the immersion. So, we conclude that CsMPs exhibit very low water-solubility in short-term. Regarding the long-term solubility, there have been a few research.53, 54 The morphology, the 137Cs radioactivity, the elemental compositions and distributions, and the water insolubility of isolated CsMPs in this study were consistent with those of the reported type A CsMPs11-13, 17. We can conclude that type A CsMPs are detected in aerosol samples and refer to aerosol CsMPs as “A-CsMPs”.
3.1.2 Comparison of CsMPs Isolated from Aerosols and Soil. There are 24 CsMPs detected in soil sample (namely, ten granules per gram, which was approximately consistent with the results of Ikehara et al.55). The 137Cs radioactivity of these CsMPs mostly ranged from 0.06 to 0.95 Bq/granule, except for one that exhibited 6.70 Bq. Their mean and median values were 0.29 and 0.22 Bq/granule, respectively, and the sum of 137Cs radioactivity of these CsMPs was 13.4 Bq. Six single granules from the group of 24 CsMPs have thus far been successfully extracted from the soil samples, and three of them have been observed and analyzed by SEM-EDS. The SEM images and their elemental compositions are given in Figure 4 and Table S2, respectively. They were all spherical particles, and their diameters were 1.8 μm (S-CsMPs-1, 0.93 Bq/granule), 1.6 μm (S-CsMPs-2, 0.56 Bq/granule), and 1.8 μm (S-CsMPs-7, 0.75 Bq/granule), with an average diameter of 1.7 ± 0.1 μm (0.75 ± 0.18 Bq/granule).
Moreover, the EDS spectra of an A-CsMP (#15-HNVA-190919) and an S-CsMP (S-CsMPs-3) are shown in comparison in Figure 5. These EDS spectra showed that both A-CsMPs and S-CsMPs mainly consist of Si, O, Cs, Fe, Zn, K, Na and Cl. Specifically, the average atomic mass fraction of the main elemental compositions of the three S-CsMPs were evaluated, given as following; Si (6.4 ± 6.7 wt.%), O (29.2 ± 4.7 wt.%), Cs (1.7 ± 2.48 wt.%), Zn (3.5 ± 4.6 wt.%), Fe (1.9 ± 2.4), Na (1.0 ± 0.7 wt.%), K (0.3 ± 0.4 wt.%) and Cl (0.4 ± 0.6 wt.%). The radiocesium activities, morphologies, and elemental compositions of A-CsMPs and S-CsMPs were almost consistent with each other. These results imply that A-CsMPs were derived from the resuspension of S-CsMPs.
3.2. Possibility of Natural Resuspension of CsMPs into the Atmosphere
The possibility of CsMP resuspension from the contaminated surface soil was suggested in studies. Specifically, CsMPs were detected in face masks worn by Fukushima residents in the spring of 2013.24 The resuspended CsMPs were also found on Japanese mustard spinach in Fukushima Prefecture cultivated from August to December 2017.26 The resuspension of radiocesium caused by decontamination work has also been reported by several studies,55, 56, 57 suggesting a possibility of the resuspension of the CsMPs induced by decontamination work by the government. However, these studies have not discussed whether the resuspension of CsMPs into the atmosphere can occur naturally or not because CsMPs in these studies were found in living areas or in association with human activities. In contrast, A-CsMPs of this study were found in aerosol samples collected by unmanned-operated HV samplers at a site located in an evacuated area that was difficult for residents to return to, indicating the possibility that A-CsMPs were derived from naturally resuspended S-CsMPs, without anthropogenic activities. Furthermore, it is possible that the A-CsMPs were transported from F1NPP or areas where decontamination and other remediation activities were carried out. If a significant number of radioactive pollutants, including CsMPs, were transported to the sampling site, the atmospheric concentration of 137Cs should have increased. However, a systematic increase in atmospheric 137Cs concentration was not detected during the sampling periods of A-CsMPs, and no leakage events of radioactive pollutants from the F1NPP were reported during the periods. These results imply a low likelihood that A-CsMPs were derived from decontamination work at the F1NPP and the severely contaminated areas. On the one hand, transport of contaminated soil by dump trucks has increased along a road near the sampling site since 2018, and decontamination activities in areas adjacent to the sampling sites (within 500 m range) were also conducted between June and December 2019. On the other hand, there was no increasing trend in the detected numbers of A-CsMPs in 2015, 2016, 2018, and 2019. More specifically, only one A-CsMP was found after June 2019, as shown in Table 2. Therefore, it is unlikely that A-CsMPs in this study was derived from human activities.
Significantly, we can find that the atomic ratios of Na/Si (≈ 0.086), K/Si (≈ 0.045) and/or Cs/Si (≈ 0.031) of our isolated A-CsMPs are smaller than that of the CsMPs (~0.139 / ~0.055 / ~0.040) released in the early stage (approximately 6 months after the Fukushima accident),58 even lower than that of the CsMPs collected at far distance away from F1NNP (Cs/Si ≈ 0.208),59 herein, the concentration of Cs has been corrected back to March 11, 2011, 14:46 JST, and assuming that there is no decrease of Si. This special characteristics of our isolated A-CsMPs quantitatively exemplifies the high possibility of A-CsMPs resulted from the resuspension of S-CsMPs. This phenomenon may be attributed to the slow environmental erosion by the loss of the very small amount of Na and K, even the loss of Cs, which is consistent with the report of slow environmental weathering dissolvable CsMPs.54 This information of lowering the Cs concentration with time lapse implies mitigation of the anxieties for the hazard resulting from the resuspension of the CsMPs. Moreover, we observed a typical suspension phenomena of surface soil on April 22, 2021, in the playground of Ibaraki University, where have similarly environmental circumstances as our sampling site. It can indirectly provide the natural resuspension condition of CsMPs at the wind gust speed higher than 4 m s-1, as shown in Figure S3 and the supporting video.
3.3. Resuspension Frequencies of CsMPs
In this study, 165 aerosol filter samples collected over four years (2015, 2016, 2018 and 2019) were used to detect and isolate the CsMPs. Although they were not all aerosol samples because of time and cost considerations, the sampling periods of the used samples (165 samples) covered more than 80% of the total sampling period: 290.5(days)/365(days) in 2015, 352.5/366 in 2016, 346.5/365 in 2018, and 225.5/274 in 2019, and 1215/1461 = 0.83 overall. As mentioned above, 20 highly radioactive granules were detected in these 165 samples, and 15 single A-CsMPs have been successfully obtained. The detailed data for A-CsMPs and aerosol filters are given in Tables 1, 2, and Table S3. If all 20 CsMPs were A-CsMPs, the sampling probability of A-CsMPs at the sampling site can be estimated as 1.6 × 10−2 granule per sampling day or 1.2 × 10−5 granule per unit sampling-air volume (m−3).
Furthermore, the yearly frequency was obtained as follows: 2.1 × 10−2 granule·day−1 in 2015, 8.5 × 10−3 granule·day−1 in 2016, 8.7 × 10−3 granule·day−1 in 2018, and 1.3 × 10−2 granule·day−1 in 2019. It can be noticed that there was no considerable decreasing trend of these resuspension frequencies, which strongly suggested that long-term monitoring of resuspended CsMPs is necessary. Alternatively, the seasonal frequency variation of resuspended A-CsMPs can be found in Table 2. The number and seasonal frequencies of A-CsMPs detected in four seasons of four sampling years are as follows: a) 1 and 4.5 × 10−3 granule·day−1 in December–February (DJF), b) 8 and 2.4 × 10−2 granule·day−1 in March–May (MAM), c) 4 and 1.1 × 10−2 granule·day−1 in Jun–August, and d) 2 and 6.9 × 10−3 granule·day−1 in September–November. The seasonal frequency was lower in DJF and higher in MAM. The surface soil is mostly frozen, and snow often covers the ground at the sampling site in DJF,15 and these conditions can inhibit the resuspension of CsMPs from the surface soil in this season. Kinase et al.36 showed that soil mineral particles were major coarse aerosols at our sampling site in the springtime (MAM), indicating that the suspension of soil particles from the ground surface occurred in this season. It is reasonable and possible that this dispersion process may cause the resuspension of CsMPs. Kinase et al.36 also found that the peak of the atmospheric radiocesium concentration was consistent with maximal average wind speed in the springtime. Based on the above discussion, it obviously that the atmospheric radiocesium concentration was associated with wind-driven suspended soil in the springtime. This seasonality is consistent with the natural resuspension of the CsMPs in this study.
Similar to the dispersion of soil dust, natural resuspension of CsMPs by wind could occur when wind speed was high and soil moisture was low, which can be confirmed as discussed below. Meteorological data, including the gust, the average wind speeds, the mean soil-water-content, and the relative humidity (RH), were measured at the sampling site in 2015, when six CsMPs were sampled, as shown in Figure S4. These data from May 2015 are plotted in Figure 6. Two CsMPs were collected when the wind gust speed exceeded 4 m s-1 and the soil water content was not notably high. Significantly, wind gust speeds exceeding 4 m s-1 can be frequently observed and sometimes even exceeded 8 m s-1. These results imply the likelihood of CsMP resuspension, although dispersion of submicron dust by winds is less likely than that of larger particles. However, Wu et al.60 observed that particles (with diameters ranging from 7.0 to 42.3 μm) could be suspended occasionally at wind speeds of 4 m s-1, with more than half of the particles suspended within 2 seconds at a wind speed of 8 m s-1. Nicholson61 obtained a gentle increasing trend for suspension of particles with a 4.1 μm diameter along with increments of wind speed, and a positive correlation between PM2.5 and a wind speed higher than 3 m s-1 was also found by Wang et al.62 These characteristics of microparticle suspension can indirectly explain the resuspension of CsMPs and the low frequency of A-CsMP detection in this work. Furthermore, Ishizuka et al.15 showed that the condition of higher wind speed was favorable for dispersion of soil mineral particles, including fine soil particles, in the springtime. We speculate that CsMP resuspension needs both a strong wind and incidental phenomena such as aggregate lumps formed by attachment/collisions of CsMPs with larger dust particles scattered by winds. Moreover, because some holes that were presumably dug by wild boars were found around the sampling site (within a few hundred meters), a possibility, that the dispersion of CsMPs occurred with such animal activities, cannot be ruled out.
3.4. Contribution of the resuspended CsMPs to Atmospheric Radioactivity
Various studies55, 63 have shown that significant levels of CsMPs were released into the environment64 and that a considerable part of the deposition of radiocesium should be attributed to the released CsMPs.51, 59 Based on the results and discussion above, CsMPs can be directly resuspended into atmosphere. Herein, the contribution ratio of the resuspended CsMPs to atmospheric 137Cs can be defined as the ratio of the radioactivity of single CsMPs (137Cs) to that of the aerosol filters from which the A-CsMPs was obtained. As given in Table 2, the resuspension contribution ratios of CsMPs ranged from 5.0 to 50.6%, and the mean and median values were 23.9% and 20.5%, respectively. The remainder of the resuspension hosts of atmospheric radiocesium can be attributed to bioaerosols and soil particles bearing with low concentrations of radiocesium41, 65 as well as low-level radioactive CsMPs. Although A-CsMPs were not detected by one-hour exposure by IP, the radioactive spot was detected in the aerosol filter by IP inspection when the IP plate was exposed for tens of hours, as shown in Figure S5. Because of the excessive amount of time and human resources required for the extraction procedure, further extraction of CsMPs in these filters has not been conducted. Moreover, the average contribution ratio of A-CsMPs, which is the ratio of total 137Cs radioactivity of 15 A-CsMPs to that of all 165 aerosol samples, was estimated to be about 2.0%, as shown in Table S3. Correspondingly, the ratio of total 137Cs radioactivity of S-CsMPs to that of the surface soil sample used in this study was about 8.0% (=13.40 Bq/166.2 Bq); the contribution of A-CsMPs to the total atmospheric radioactivity, therefore, was smaller than that in the corresponding surface soil.
This study reveals that the direct resuspension of water insoluble CsMPs, composed of silicate glass, to the atmosphere without human interference. The fact can be confirmed by i) their successful extraction from aerosol samples collected by unmanned-operated samplers, ii) the nearly resembling characteristics of A-CsMPs and S-CsMPs, which were extracted from the respective aerosol and surface soil samples collected at the same sampling site in a heavily contaminated area of F1NPP and iii) the seasonality of the A-CsMP detection. The sizes and radioactivities of both A-CsMPs and S-CsMPs found in the present study were situated within the distributions of the reported type A CsMPs, but they were generally smaller than those reported for type A CsMPs (with a diameter of less than ca. 10 μm)11, 17 sampled in primary emission from the F1NPP accident in 2011. This can be used to exclude the probability that A-CsMPs may be derived from the primary release and may be explained by surface-deposited CsMPs subjected to their very slow dissolution rates in the natural environment, as suggested by Okumura et al.54 Therefore, the erosion of CsMPs in the natural environment should be a significant process to be clarified by future studies.
This study also reveals the frequency of CsMP resuspension and its contribution to atmospheric radiocesium, although this can vary with the location depending on the amount of radiocesium and CsMP deposition (surface contamination extent). The resuspension of insoluble type A CsMPs is indeed infrequent, and their absolute activity remains at the sub-Bq level. On the other hand, the detection of type A CsMPs has been continued up to 2019 and no obvious decreasing trend has been found from annual resuspension frequencies, suggesting its persistence. The CsMPs can also work as the transferring medium of fuel debris,66 trace uranium,17, 52 strontium,67 and plutonium68 into the environment, although they are at very low level, and CsMPs resuspension increases the possibility of unpredictable inhalation. If they are inhaled, they can easily reach the deep respiratory system.69 Thus, more investigation is necessary regarding the risk assessment of even the single CsMP radiation exposure19 and long-term monitoring of these resuspended insoluble type A CsMPs.
F1NPP: Fukushima Daiichi Nuclear Power Plant; CsMPs: radiocesium-bearing microparticles; A-CsMPs: aerosol radiocesium-bearing microparticles; S-CsMPs: soil radiocesium-bearing microparticles; JAEA: Japan Atomic Energy Agency; MRI: Meteorological Research Institute; TEPCO: Tokyo Electric Power Company.
Competing interests
The authors declare that they have no competing interest.
Funding
This study was financially supported by Nuclear Energy Science & Technology and Human Resource Development Project (International joint research program of nuclear decommission of Japan-UK, 30I125) and partly by Grants-in-Aid for Scientifc Research (B) (No. 20H04318 and 17H01873).
Authors’ contributions
PT and KH analyzed the samples and summarized these data in this article. PT prepared the manuscript. KK planned and carried out the measurements as well as the samplings. YI is the leader of our project team. KK and YI helped in writing the manuscript, and provided important discussions for this article. YS, KA and TK supported the measurement of the aerosol and soil CsMPs by SEM-EDS. KN and AS collected and measured radioactivity of the soil samples. YS and KA also provided important suggestions for this article. All authors read and approved the final manuscript.
Authors’ information
PT received his PhD from University of Fukui in 2020 and worked at Ibaraki University as a postdoctroal researcher and wrote this article during this period.
KK received his PhD from Tokyo University in 1990 and worked at Tokyo University until 2002 as a research assistant. He moved to Ibaraki University in 2002. His studies focus on the atmospheric O3 and NO2, the effects of black carbon (BC) on global warming, and the resuspension processes of radiocesium.
YI received his PhD from the University of Tsukuba in 1987. From 1987 to 1991, he worked at the former National Institute of Radiological Sciences and moved to the MRI in 1991. His current interests are atmospheric aerosols; their precursors, including Asian dust and PM2.5; their possible influences on climate and general environmental changes and other related phenomena.
YS received his PhD from University of Tsukuba in 2016, worked as researcher in Collaborative Laboratories for Advanced Decommissioning Sciences, Japan Atomic Energy Agency. His current interests are focused on the research of F1NPP accident released radioactive particles.
KA received his PhD from Kobe University in 2005, worked at Arizona State University between 2005 and 2011 as a postdoctoral/faculty research associate and is currently studying atmospheric aerosols at the MRI.
TK received his PhD from Ibaraki University in 2016. His inesterts are focusing on BC aerosols and Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident.
KN received his PhD from Osaka University in 2007 and worked at OsakaUniversity in 2008. He moved to JAEA and worked as a research scientist between 2009 and 2012; then, he returned to Osaka University. His mainresearch field is nuclear- and radiochemistry.
AS received his PhD from Osaka University in 1985 and then worked at the Japan Society for the Promotion of Science. He moved to Nagoya University, where he worked between 1987 and 1998. Then, he moved to Kyoto University in 1998, before moving to Osaka University in 1999. His main
research field is nuclear and radiochemistry.
Acknowledgements
We thank Ms. N. Hayashi, Mr. K Minami, Ms. H. Li and other students of the College of Science and College of Engineering, Ibaraki University, for the maintenance of the sampling instruments, special installations in the field and data collection. We especially thank the local government of Namie and the Fukushima prefectural government, who kindly provided the monitoring opportunity. We also acknowledge Mr. K. Hama (Atox Co. Ltd) and Mr. K. Obara for assistance with the activity measurements and separation of A-CsMPs.
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Table 1. Information of 15 single isolated insoluble Cs-bearing particles
(The radioactivity is corrected back to 11/03/2011 14:46, JST)
Table 2. Seasonal or yearly frequency variations of 15 single isolated insoluble CsMPs.
|
Sampling year |
Jan. |
Feb. |
Mar. |
Apr. |
May. |
Jun. |
Jul. |
Aug. |
Sep. |
Oct. |
Nov. |
Dec. |
Total |
yearly frequency |
|
|
2015 |
Number of CsMPs |
0 |
0 |
0 |
0 |
2 |
1 |
1 |
0 |
0 |
2 |
0 |
0 |
6 |
2.1×10-2 granule·day-1 |
|
Sampling period/days |
18 |
0 |
14 |
14.5 |
31 |
30 |
31 |
31 |
30 |
31 |
29 |
31 |
290.5 |
||
|
2016 |
Number of CsMPs |
1 |
0 |
1 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
3 |
8.5×10-3 granule·day-1 |
|
Sampling period/days |
31 |
29 |
31 |
30 |
31 |
30 |
31 |
31 |
30 |
31 |
30 |
17.5 |
352.5 |
||
|
2018 |
Number of CsMPs |
0 |
0 |
1 |
0 |
1 |
0 |
0 |
1 |
0 |
0 |
0 |
0 |
3 |
8.7×10-3 granule·day-1 |
|
Sampling period/days |
12.5 |
28 |
31 |
30 |
31 |
30 |
31 |
31 |
30 |
31 |
30 |
31 |
346.5 |
||
|
2019 |
Number of CsMPs |
0 |
0 |
1 |
1 |
0 |
0 |
0 |
1 |
0 |
0 |
0 |
0 |
3 |
1.3×10-2 granule·day-1 |
|
Sampling period/days |
23.5 |
0 |
30.5 |
30 |
31 |
30 |
31 |
31 |
18.5 |
0 |
0 |
0 |
225.5 |
||
|
Seasonal frequency |
|||||||||||||||
|
Dec.-Feb. (DJF) |
Number of CsMPs |
1 |
4.5×10-3 granule·day-1 |
||||||||||||
|
Sampling period/days |
221.5 |
||||||||||||||
|
Mar.-May (MAM) |
Number of CsMPs |
8 |
2.4×10-2 granule·day-1 |
||||||||||||
|
Sampling period/days |
335 |
||||||||||||||
|
|
|||||||||||||||
|
Jun.-Aug. (JJA) |
Number of CsMPs |
4 |
1.1×10-2 granule·day-1 |
||||||||||||
|
Sampling period/days |
368 |
||||||||||||||
|
Sep.-Nov. (SON) |
Number of CsMPs |
2 |
6.9×10-3 granule·day-1 |
||||||||||||
|
Sampling period/days |
290.5 |