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Temporal changes of the ambient dose rate in the forest environments of Fukushima Prefecture following the Fukushima reactor accident
Journal of Environmental Radioactivity 210 (2019) 106058
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Temporal changes of the ambient dose rate in the forest environments of Fukushima Prefecture following the Fukushima reactor accident
T
Hiroaki Kato∗, Yuichi Onda, Toshiro Yamaguchi Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fukushima Dai-ichi nuclear power plant accident Forest Ambient dose rate Regional survey
Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011–2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014–2016 and 2011–2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10–20% faster than that induced solely by physical decay of radiocesium for the observation period 2011–2016.
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant accident resulted in the release of an enormous amount of radiocesium (~20 PBq for 137Cs) into the atmosphere (Chino et al., 2011; Amano et al., 2012; Hirose, 2012; Aoyama et al., 2016). The subsequent atmospheric deposition of radiocesium contaminated large areas of the terrestrial environment in both Fukushima and the neighboring prefectures (Butler, 2011; MEXT, 2011; NRA, 2017a,b). The results of airborne monitoring surveys and model calculations simulating the atmospheric transport of contaminants estimated that approximately 22% of the total radiocesium released into the atmosphere was deposited onto the land area in Fukushima and its neighboring prefectures (Morino et al., 2011, 2013; MEXT, 2011). Fukushima Prefecture area accumulated 2.1 PBq of 137Cs in total following the accident (Kato and Onda 2018). Although the contaminated area included a wide range of environments and land uses (e.g., Kitamura et al., 2014), approximately 70% of the total contaminated land surface consisted of forest cover (Hashimoto et al., 2012). Therefore, monitoring radiocesium contamination and predicting the gamma radiation dose in forest environments following the accident are important tasks to assess external exposure to radiation for
∗
DOI of original article: https://doi.org/10.1016/j.jenvrad.2018.08.009 Corresponding author. 1-1-1 Tennodai, Tsukuba, Ibaraki, 203-0006, Japan. E-mail address: [email protected] (H. Kato).
https://doi.org/10.1016/j.jenvrad.2019.106058
Available online 17 October 2019 0265-931X/ © 2018 Elsevier Ltd. All rights reserved.
human and wildlife (e.g., Fesenko et al., 2005). However, the extent of the radiocesium contamination and temporal evolution of the ambient dose rate (ADR) in different types of forest have not been assessed sufficiently. The ambient dose rate following the Fukushima Dai-ichi Nuclear Power Plant accident has been monitored across a large area of Fukushima and its neighboring prefectures by means of in-situ measurements (MEXT, 2011; Saito et al., 2015; Mikami et al., 2015a, 2015b) and airborne and car-borne surveys (e.g., NRA, 2015). The results of those monitoring surveys have determined the spatial pattern of the ambient dose rate in the Fukushima landscape and its neighboring prefectures, with the measured ambient dose rate being closely related to the initial amount of 137Cs deposition (Saito et al., 2015; Mikami et al., 2015a). The decreasing trend in the ambient dose rate has been assessed to determine the cumulative external radiation dose to local citizens and for long-term prediction of ambient dose rates in various land uses. The in-situ measurement of ambient dose rates at 1 m above the ground surface using a survey meter at flat and sparsely vegetated locations revealed that the relative percentage reduction in the air dose rate was 10% greater than that resulting solely from the radioactive decay of radiocesium between 2011 and 2012 (Mikami et al., 2015a).
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the Fukushima terrestrial region was performed by Gonze et al. (2016) using dynamic, spatially distributed, and process-based models. Their results demonstrated that air dose rates, either within or above forests, were very sensitive to both the detector height and vertical location of radiocesium in the system. Therefore, airborne surveys might not reflect dose rates at ground level in forest systems. Thus, statistical analyses based on ambient dose rates measured in situ in various forests are necessary to determine the actual temporal evolution of the ambient dose rate in Japanese forest environments following the Fukushima accident. The purpose of this study was to determine the temporal evolution of the ambient dose rate in various forest types. Repeated measurements of the ambient dose rate were conducted in 2014 and 2016 at the same measurement sites as those used by MAFF in 2011. The influence of forest type on the temporal evolution of the ambient dose rate was determined based on a statistical analysis of the regional measurements.
Such a rapid decrease in the ambient dose rate can be attributed to an increase in the relaxation depth of radiocesium in soil profiles rather than the loss of radiocesium due to natural weathering processes. On the other hand, the results of a car-borne survey indicated that the magnitude of the decreasing trend in the ambient dose rate was dependent on the land use in a particular area, with the ambient dose rate measured in forest areas decreasing more slowly than those in urban areas and over water (Kinase et al., 2014; Andoh et al., 2015; Kinase et al., 2017). Most previous studies were based on the ambient dose rate measured from aircraft or along major roadways, with very little monitoring data available regarding the temporal changes in the ambient dose rate in forest environments. The Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) conducted regional measurements of the ambient dose rate from September to November of 2011, targeting forests within Fukushima Prefecture (MAFF, 2011a,b). A total of 391 locations, containing various forest types, were selected for the measurements. The extent of radioactive contamination and ambient dose rates in forest environments were thus determined during the early phases of the Fukushima accident. However, further detailed analyses and repeated measurements at the same location were not conducted, and consequently the temporal evolution of the ambient dose rate in forest environments has not been determined. Officials from Fukushima Prefecture have conducted repeated measurements of the ambient dose rate in the forests of Fukushima Prefecture (Fukushima Prefecture, 2016). The decreasing trend in the ambient dose rate determined in 362 locations was similar to the reduction expected due to the decay of radiocesium. The results obtained in Fukushima Prefecture have been presented as the average value of all forest types, i.e., evergreen, deciduous, and secondary mixed forests. The results of in-situ measurements of ambient dose rate suggest that temporal changes in the ambient dose rate in forests differ depending on the predominant tree species (Imamura et al., 2015). Further analysis is required to determine the influence of forest type on the long-term trend in the ambient dose rate in Japanese forest environments. Radionuclides deposited in forested areas by either wet or dry processes encounter the forest canopy. Most radiocesium (> 90%) deposited onto the canopy of evergreen conifers is intercepted and retained by tree needles and branches (Hoffman et al., 1995; Pröhl and Hoffman, 1996; Kinnersley et al., 1996, 1997). On the other hand, most of the atmospherically deposited radiocesium tends to deposit directly onto the forest floor, particularly during the leafless winter season in deciduous forests (e.g., Melin et al., 1994; Kato et al., 2017). The canopy-intercepted radiocesium is subsequently transferred to the forest floor as a result of weathering by rainwater and wind (Bunzl et al., 1989; Bonnet and Anderson, 1993). Some radiocesium is readily removed from plant surfaces by rainfall, but the remaining is strongly absorbed by leaf, branch, and bark surfaces (Rauret et al., 1994). The removal rate of radiocesium from plant tissue is affected by several factors, such as the elapsed time since the initial atmospheric deposition, tree species and age, and climatic conditions. The ambient dose rate in forest environments is extremely variable in space and time. The vertical distribution of the measured ambient dose rate has been found to vary among different forest types, representing differences in radiocesium accumulation in canopies. Furthermore, the decrease in the ambient dose rate at 1 m above the ground surface occurred more slowly than the decrease measured at canopy height in cedar stands. The ambient dose rate in a forest growing on a slope was found to be highly heterogeneous and influenced by topography. These studies have suggested that the spatial distribution of atmospherically deposited radiocesium varies significantly among different forest stands; nevertheless, those differences in the initial distribution and subsequent transfer of radiocesium have not been linked to the temporal evolution of the ambient dose rate in forest environments. A numerical assessment of air dose rates induced by radiocesium in
2. Materials and methods 2.1. Measurement of the ambient dose rate in forests 2.1.1. Data measured by MAFF in 2011 (MAFF, 2011a, 2011b) MAFF measured the ambient dose rate in forests at 391 locations within Fukushima Prefecture (Fig. S-1 in the Supplementary Information document). It should be noted that white spot on the map is Lake Inawashiro. Each measurement site was designated to become part of the national survey of the biodiversity of forest ecosystems in Japan, with all sites distributed inside a 4-km grid within an 80-km radius of the power plant and inside a 10-km interval grid outside of the 80-km radius. The exact location of each measurement site was recorded using GPS, given a geographical coordinate, and marked with yellow or green poles (Fig. S-2 in the Supplementary Information document). The ambient dose rate at 1 m above the ground surface was measured using a NaI(TI) scintillation survey meter (TCS-172B, Hitachi, Ltd., Tokyo, Japan) at each measurement site. The measurements were conducted during between September 16 and November 9, 2011. The measured ambient dose rate was decay-corrected to the date of October 1, 2011. 2.1.2. Data measured by the Nuclear Regulation Authority (NRA) in 2014 Additional measurements of the ambient dose rate were conducted by authors as a part of the commission research project supported by NRA, at the same measurement sites in 2011. However, the measurements were reduced to 285 locations due to limited site accessibility caused by forest road collapse during typhoon event, deforestation and decontamination of the measurement site. The ambient dose rate at 1 m above the ground surface was measured using the same measurement protocol employed in 2011 by MAFF. It should be noted that the survey meters were checked and calibrated before field monitoring survey by a dosimeter company (Chiyoda Technol Corporation, Tokyo, Japan). The measurements were conducted between September and November 2014. The measured ambient dose rate was decay-corrected to the date of October 1, 2014. 2.1.3. Data measured in this study in 2016 Measurements of the ambient dose rate were conducted at 100 sites of the exact locations used by MAFF in 2011 and NRA in 2014. The number of measurement site was reduced to nearly one third of the measurements in 2014 due to the limitation of human resources and research fund. Criteria for the selection of repeated measurement site of ambient dose rate was as follows; 1) the ambient dose rate was measured in both 2011 and 2014, 2) the measurement location was easy to access, 3) the measurement site was covered by major forest type in Fukushima Prefecture (e.g., cedar (Cryptomeria japonica), konara oak (Quercus serrata), and secondary deciduous broad-leaved mixed forest 2
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with red pine (Pinus densiflora)), and 4) initial 137Cs deposition density of the measurement site was greater than 5 kBq/m2. The ambient dose rate at 1 m above the ground surface was measured using the same measurement protocol as that used in previous measurements in 2014. The measurement was conducted between September and November 2016. The measured ambient dose rate was decay-corrected to the date of October 1, 2016.
deposition density of radiocesium in the area outside of this radius were derived from the results of the fifth airborne monitoring survey (Measurement period: June 22–28, 2012). The 137Cs inventory was decaycorrected to the reference date of July 2, 2011. A detailed information on the initial 137Cs deposition density map can be found in Kato and Onda (2018). The relationship between the initial atmospheric deposition density of 137Cs and the measured ambient dose rate was expressed as the following equation:
2.2. Data preparation and statistical analysis
ADR = α·Ainitial
The measured ambient dose rate was classified into three different forest types: evergreen coniferous forest (EGC), mixed broad-leaved forest (MBL), and deciduous broad-leaved forest (DBF). It should be noted that there are in total 439 fixed monitoring sites for the national survey of forest inventory by MAFF in Fukushima Prefecture (in 4-km grid mesh), in which EGC, MBL, and DBF account for 29% (125 sites), 13% (58 sites), and 36% (192 sites) of the total 439 monitoring sites, respectively. These forest types are common in the area affected by the radioactive contamination from the Fukushima accident. The EGC in the Fukushima area consists mainly of Japanese cedar (Cryptomeria japonica) or cypress (Chamaecyparis obtusa) plantation, whereas the MBL is typically secondary forest, dominantly with a mixture of konara oak (Quercus serrata) and Japanese red pine (Pinus densiflora). The DBF consists of deciduous broad-leaved species such as konara oak (Quercus serrata), beech (Fagus crenata), and chestnut (Castanea crenata) trees. Minor forest types and undefined sites were excluded from the analysis. The total numbers of measurement sites for each forest type in each measurement year are shown in Table 1. Total numbers of ambient dose data for the target forest type in 2011, 2014, and 2016 were 328, 202, and 87, respectively. In addition, the stand properties of the forest sites are summarized in Tables S–1 in the Supplementary Information document. The measured ambient dose rate includes gamma radiation from global fallout, radiocesium derived from the Fukushima accident, and the natural background. The radiation dose from the natural background in the Fukushima area is 0.03 μSv/h (Akabane et al., 2013); therefore, the equivalent dose rate was subtracted from the measured ambient dose rate. On the other hand, Saito et al. (2015) determined that about 70.9% of the external effective dose was due to 134Cs, and 28.1% was due to 137Cs, and the contributions from the other nuclides were less than 1% in the middle of June 2011. Therefore, influences of short-lived radionuclides and the 137Cs derived from global fallout to the measured ADR could be negligible during the observation periods of this study. The initial 137Cs deposition density following the Fukushima accident was determined from the results of an airborne monitoring survey (MEXT, 2011). The results of the third airborne monitoring survey (Measurement period: May 31-July 2, 2011) were used for the area within an 80-km radius of the power plant, whereas the aerial
where, ADR is ambient dose rate (μSv/h) and α (μSv m /(kBq h))is a coefficient for determining the ambient dose rate from the initial atmospheric deposition density of radiocesium (Ainitial; kBq/m2). The value of Ainitial was determined by using the reconstructed atmospheric deposition map of 137Cs derived from the Fukushima reactor accident (refer to the nearest measurement point of the airborne survey). Therefore, the determined value of α potentially includes measurement uncertainties derived from the airborne survey. The reduction rate β in the ambient dose rate between the two measurement periods was determined from the following equation: ADRt+1 = β·ADRt
Cs deposition density (kBq/m2)
< 10 10–50 50–100 100–400 400–700 700–1000 > 1000 Total
2011
2014
3. Results and discussion The measured ambient dose rate was plotted on a map (Fig. 1). The locations northwest of the power plant had a remarkably high ambient dose rate regardless of the forest type. The ambient dose rate decreased with time, and in 2016, the dose rate was less than 0.5 μSv/h in most locations. The initial atmospheric deposition density of 137Cs was plotted against the ambient dose rate measured in 2011 (Fig. 2). The initial atmospheric deposition density and measured ambient dose rate displayed a significant correlation. The coefficient α values for the different forest types and years are listed in Table 2. The measured α in 2011 in this study was greater than that reported previously for flat open fields, with little vegetation cover (0.0054–0.0056 μSv m2/(kBq h); Saito et al., 2015; Mikami et al., 2015a). This indicated a higher ambient dose rate in forest areas than open fields for the same initial deposition of 137Cs. A similar trend in the radiation dose derived from global fallout in the northeastern United States was reported previously even for long-term period 30 years following the fallout peak in 1960's (Miller et al., 1990). For all forest types, α decreased with time due to the radioactive decay of radiocesium. The α values in 2011 for DBF were greater than that for EGC (p < 0.01) and MBL (p < 0.05), indicating that DBF tended to have higher ambient dose rates than that of EGC and MBL for the same aerial deposition density of radiocesium. The initial canopy interception of atmospherically deposited radiocesium varied depending on canopy characteristics, such as canopy biomass, leaf area index, and canopy openness (e.g., IAEA, 1996; Kato et al., 2018). Previous studies have reported a greater canopy interception of atmospherically deposited radiocesium in ECG due to its dense canopy cover throughout the year (Melin et al., 1994; Bunzl et al., 1989; Kato et al., 2012; Kato et al., 2017). The initial canopy interception of atmospherically deposited radiocesium following the Fukushima accident was reported to be 70% or greater for evergreen conifers such as Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa), whereas it was only 23% for secondary MBL (Kato et al., 2012; Kato et al., 2017). Deciduous tree species were
2016
EGC
ML
DBF
EGC
MBL
DBF
EGC
MBL
DBF
11 54 16 25 5 2 2 115
2 17 11 14 3 3 6 56
29 72 16 33 1 0 6 157
10 42 14 16 2 1 1 86
2 10 7 5 2 1 0 27
12 40 9 27 0 0 1 89
2 19 11 7 2 1 1 43
1 2 6 2 0 1 0 12
1 15 5 11 0 0 0 32
(2)
where ADRt and ADRt+1 are the ambient dose rate (μSv/h) measured by the preceding and the following campaigns, respectively. Measurement points that received an initial radiocesium deposition density of less than 10 kBq/m2 were excluded from the analysis because the effect of the global fallout of radiocesium could be significant at these locations.
Table 1 The number of ambient dose rate measurement sites in each forest type. The cesium-137 deposition density data was based on the third and fifth airborne monitoring surveys. EGC, MBL and DBF denote evergreen conifer, mixed broadleaved, and deciduous broad-leaved forest, respectively. 137
(1) 2
3
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Fig. 2. Relationship between the initial atmospheric deposition density of 137Cs and the measured ambient dose rate in forest environments in 2011.
Table 2 The relationship between the initial atmospheric deposition density of and the ambient dose rate (ADR) in each forest type in 2011.
137
Cs
Forest type
α(μSv m2/(kBq h))
R2
n
EGC MBL DBF
6.3 × 10−3 ± 1.8 × 10−4 7.7 × 10−3 ± 1.2 × 10−4 8.1 × 10−3 ± 8.4 × 10−5
0.92* 0.99* 0.99*
104 54 128
*p < 0.01.
(HAD16.0, Shimizu, 2016). For EGC in 2011–2016 and 2014–2016, a decontaminated location was excluded from the regression analysis (Fig. 3(a2, a3)). The reductions in the ambient dose rate due to the physical decay of radiocesium in 2011–2014, 2014–2016 and 2011–2016 were 0.54, 0.75, and 0.41, respectively. During 2011–2014, the value of β for EGC was greater than those for MBL (p < 0.01) and DBF (p < 0.01) (Table 3). However, the observed differences in β value for the period of 2014–2016 were not statistically significant. These results shows that the ambient dose rate decreased more slowly in EGC than in MBL and DBF during 2011–2014, while such trend was unclear during the following years (2014–2016). The ambient dose rate in EGC decreased with time, but the rate of decrease was slower than that induced by the physical decay of radiocesium for the period of 2011–2014, whereas it decreased slightly faster for the period of 2014–2016 and 2011–2016 (Table 3). The reduction in the ambient dose rate in MBL and DBF was approximately 10% for 2011–2014 and nearly 20% for 2011–2016 faster than that induced solely by the physical decay of radiocesium, though there was possible selection bias in the determination of β value for 2014 and 2016 due to lack of the measurement sites at high 137Cs deposition area. Nevertheless, slower decrease of ambient dose rates than that by physical decay of radiocesium in EGC during 2011–2014 was followed by quick reduction in the period of 2014–2016. Officials from Fukushima Prefecture have conducted repeated measurements of the ambient dose rate in the forests of Fukushima Prefecture (Fukushima Prefecture, 2016). The decreasing trend in the ambient dose rate determined at 362 locations was similar to the reduction induced solely by the physical decay of radiocesium. The results published by officials from Fukushima Prefecture were presented as the average values of all forest types, i.e., evergreen coniferous, deciduous, and secondary mixed forests. Whereas the in-situ measurements of ambient dose rates have suggested that temporal changes in the ambient dose rate in forests vary depending on the predominant
Fig. 1. Ambient dose rates (ADR) in forest environments measured by three different measurement campaigns. “EGC”, “MBL” and “DBF” denote evergreen conifer, mixed broad-leaved, and deciduous broad-leaved forest, respectively.
leafless at the time of the Fukushima accident in March 2011; therefore, a large portion of the atmospheric deposition was deposited directly on the forest floor in DBF, whereas the canopies of EGC intercepted atmospheric deposition following the accident. Consequently, a higher ambient dose rate was reported in DBF than in EGC in previous studies (e.g., Yoshihara et al., 2013; Kato et al., 2014). However, the influence of the initial canopy interception on the ambient dose rate diminished over time, because canopy-intercepted radiocesium was gradually transferred to the forest floor as a result of self-decontamination processes in EGC. Nevertheless, it is speculated that the difference in the ambient dose rate was more distinct between EGC and DBF during the very early stages of the accident. The measurement results in 2011, 2014 and 2016 were compared each other to determine the decreasing ambient dose rate during the three periods (Fig. 3). The relation between results in each years was analyzed by an iteratively reweighted least squares method with a weighting based on the Huber's function to reduce the effect of outliers 4
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Fig. 3. Comparison of the ambient dose rates (ADR) between the two different periods. A dotted line denotes a 95% confidence limit.
dose (Imamura et al., 2017). A numerical assessment of the air dose rates induced by radiocesium in the Fukushima terrestrial region was performed using dynamic, spatially distributed, and radiocesium transfer-process-based models in forest systems and the other terrestrial
tree species (Imamura et al., 2015). The air dose rate measured on the forest floor in 2014 originates largely from the radiocesium in the litter layer and the surface mineral soil layer (< 5 cm), with the canopy radiocesium contributing less than 3% of the total measured radiation
Table 3 The decreases in the measured ambient dose rate in each forest type during different observation periods. The error denote standard deviation for the determined β values. Forest type
EGC MBL DBF
2011–2014
2014–2016
β
R
0.66 ± 0.015 0.43 ± 0.013 0.45 ± 0.007
0.96* 0.97* 0.98*
2
2011–2016
n
β
R
76 25 77
0.64 ± 0.013 0.58 ± 0.029 0.55 ± 0.030
0.98* 0.97* 0.80*
*p < 0.01. 5
2
n
β
R2
n
41 11 31
0.33 ± 0.010 0.23 ± 0.012 0.24 ± 0.018
0.95* 0.97* 0.68*
41 11 31
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Acknowledgements
environment (Gonze et al., 2016). Their results demonstrated that the air dose rates in evergreen conifers decreased more slowly than the physical decay of radiocesium. These studies suggest that the accumulation of radiocesium in association with the self-decontamination processes of canopies affects the temporal evolution of the ambient dose rate at the forest floor, particularly in EGC. In this study, a statistical analysis based on the ambient dose rates obtained by in-situ measurements in various forests determined the actual temporal evolution of the ambient dose rate in Japanese forest environments following the Fukushima accident. The slower decrease in the ambient dose rate in EGC compared with DBF and MBL was consistent with the results of previous studies (e.g., Kinase et al., 2014). Although radiocesium in forest ecosystems is dynamic, e.g., the transfer of canopy-intercepted radiocesium to the forest floor, the measured decreasing trend in the ambient dose rate in EGC during the early phase of the accident (2011–2014) was slower than the reduction caused solely by the physical decay of radiocesium. However in the EGC, the ambient dose rate declined faster than the physical decay of radiocesium during the following periods from 2014 to 2016 when the 137Cs depositional flux from the canopy to forest floor decreased significantly (Kato et al., this issue). In DBF, where atmospheric deposition accumulated on the forest floor, the measured ambient dose decreased faster than the physical decay of radiocesium. These results suggest that radiocesium transfer among the forest canopy and surface soil layers affects the measured ambient dose rate in forest environments. Thus, understanding the biogeochemical cycling of radiocesium in various forest ecosystems is essential for predicting long-term trends in the ambient dose rate in forest environments. Furthermore, the study of elemental cycling within forest systems allows us to determine the dominant processes controlling the transport of elements in the system and is consequently prerequisite to develop models for predicting longterm radiocesium transfer in different forest ecosystems. Nevertheless, the ambient dose rate and its temporal changes varied even among the same tree species. Further analysis is required to determine the influence of forest properties, such as stand density, tree height, topography, soil type, and soil redistribution on the magnitude of the ambient dose rate and its long-term decreasing trend in Japanese forest environments.
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Temporal changes in radiocesium deposition in various forest stands following the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 166, 449–457. https://doi.org/ 10.1016/j.jenvrad.2015.04.016. Kato, H., Onda, Y., Wakahara, T., Kawamori, A., 2018. Spatial pattern of atmospherically deposited radiocesium on the forest floor in the early phase of the Fukushima Daiichi Nuclear Power Plant accident. Sci. Total Environ. 615, 187–196.
4. Conclusion In this study, we investigated temporal changes in the ambient dose rate in forest environments in Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement locations used by MAFF in 2011. The repeated measurements revealed that the decreasing trend in the ambient dose rate varied among different forest types. EGC exhibited a slower decrease in the ambient dose rate than that of MBL and DBF forests for the period of 2011–2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014–2016 and 2011–2016). In MBL and DBF forests, the ambient dose rate decreased faster than that induced solely by the physical decay of radiocesium. The measured variation in the temporal changes in the measured ambient dose rate could be attributed to differences in the initial canopy interception of atmospherically deposited radiocesium and the subsequent transfer from the canopy to the forest floor. A comparison of the decreasing trend in the ambient dose rate determined using different measurement techniques indicated that measurement of the ambient dose rate in forests is necessary to precisely evaluate temporal changes in the ambient dose rate in forest environments. Therefore, continuous monitoring is required for the assessment and long-term prediction of the ambient dose rate in forest environments.
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Mikami, S., Maeyama, T., Hoshide, Y., Sakamoto, R., Sato, S., Okuda, N., Demongeot, S., Gurriaran, R., Uwamoto, Y., Kato, H., Fujiwara, M., Sato, T., Takemiya, H., Saito, K., 2015b. Spatial distributions of radionuclides deposited onto ground soil around the Fukushima Dai-ichi Nuclear Power Plant and their temporal change until December 2012. J. Environ. Radioact. 139, 320–343. https://doi.org/10.1016/j.jenvrad.2014. 09.010. Miller, K.M., Kuiper, J.L., Helfer, I.K., 1990. 137Cs fallout depth distributions in forest versus field sites: implications for external gamma dose rates. J. Environ. Radioact. 12 (1), 23–47. Morino, Y., Ohara, T., Nishizawa, M., 2011. Atmospheric behavior, deposition, and budget of radioactive materials from the Fukushima daiichi nuclear power plant in March 2011. Geophys. Res. Lett. 38, L00G11. https://doi.org/10.1029/ 2011GL048689. Morino, Y., Ohara, T., Watanabe, M., Hayashi, S., Nishizawa, M., 2013. Episode analysis of deposition of radiocesium from the Fukushima daiichi nuclear power plant accident. Environ. Sci. Technol. 47, 2314–2322. https://doi.org/10.1021/es304620x. Nuclear Regulation Authority of Japan (NRA), 2015. Project Report 2014 on Data Collection and Development of Transfer Model for Radionuclides Derived from the Fukushima Daiichi Nuclear Power Plant. (in Japanese). http://radioactivity.nsr.go. jp/ja/contents/11000/10921/36/2-1_H26forest.pdf, Accessed date: 1 July 2017. Nuclear Regulation Authority of Japan (NRA), 2017. Monitoring Information of Environmental Radioactivity Level. Monitoring Survey on Radionuclide Distribution in Environment. Available at. http://radioactivity.nsr.go.jp/ja/list/338/list-1.html, Accessed date: 1 July 2017 (in Japanese). Nuclear Regulation Authority of Japan (NRA), 2017. Monitoring information of environmental radioactivity level. Airborne Monitoring Survey Results. Available at http://radioactivity.nsr.go.jp/en/list/307/list-1.html , Accessed date: 1 July 2017. Pröhl, G., Hoffman, F.O., 1996. Rep. Radionuclide Interception and Loss Processes in Vegetation, vol. 857 International Atomic Energy Agency, Vienna, Austria 66 pp. Rauret, G., Llauradó, M., Tent, J., Rigol, A., Alegre, L.H., Utrillas, M.J., 1994. Deposition on holm oak leaf surface of accidentally released radionuclides. Sci. Total Environ. 157, 7–16. https://doi.org/10.1016/0048-9697(94)90559-2. Saito, K., Tanihata, I., Fujiwara, M., Saito, T., Shimoura, S., Otsuka, T., Onda, Y., Hoshi, M., Ikeuchi, Y., Takahashi, F., Kinouchi, N., Saegusa, J., Seki, A., Takemiya, H., Shibata, T., 2015. Detailed deposition density maps constructed by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 308–319. Shimizu, H., 2016. An introduction to the statistical free software HAD: suggestions to improve teaching, learning and practice data analysis. Journal of Media, Information and Communication 1, 59–73 (in Japanese).
Kato, H., Onda, Y., 2018. Determining the initial Fukushima reactor accident-derived cesium-137 fallout in forested areas of municipalities in Fukushima Prefecture. J. For. Res. 23, 73–84. https://doi.org/10.1080/13416979.2018.1448566. Kato, H., Onda, Y., Saidin, Z.H., submitted, Six-year monitoring study of radiocesium transfer in forest environments following the Fukushima nuclear power plant accident. J. Environ. Radioact., (this issue). Kinase, S., Takahashi, T., Sato, S., Sakamoto, R., Saito, K., 2014. Development of prediction models for radioactive caesium distribution within the 80-km radius of the Fukushima Daiichi Nuclear Power Plant. Radiat. Prot. Dosim. 160 (4), 318–321. https://doi.org/10.1093/rpd/ncu014. Kinase, S., Takahashi, T., Saito, K., 2017. Long-term predictions of ambient dose equivalent rates after the Fukushima Daiichi nuclear power plant accident. J. Nucl. Sci. Technol. 1–10. https://doi.org/10.1080/00223131.2017.1365659. Kinnersley, R.P., Shaw, G., Bell, J.N.B., Minski, J., Goddard, A.J.H., 1996. Loss of particulate contaminants from plant canopies under wet and dry conditions. Environ. Pollut. 91 (2), 227–235. https://doi.org/10.1016/0269-7491(95)00047-X. Kinnersley, R.P., Goddard, A.J.H., Minski, M.J., Shaw, G., 1997. Interception of caesiumcontaminated rain by vegetation. Atoms Environ 31 (8), 1137–1145. https://doi.org/ 10.1016/S1352-2310(96)00312-3. Kitamura, A., Yamaguchi, M., Kurikami, H., Yui, M., Onishi, Y., 2014. Predicting sediment and cesium-137 discharge from catchments in eastern Fukushima. Anthropocene 5, 22–31. Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), 2011. Monitoring results of ambient dose rate in forest of Fukushima Prefecture. In Japanese. http://www. rinya.maff.go.jp/j/press/hozen/111227_3.html, Accessed date: 1 July 2017. Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), 2011. Annual Report on Forest and Forestry in Japan, Fiscal Year 2011. . http://www.maff.go.jp/e/data/ publish/attach/pdf/index-27.pdf, Accessed date: 1 July 2017. Melin, J., Wallberg, L., Suomela, J., 1994. Distribution and retention of cesium and strontium in Swedish boreal forest ecosystems. Sci. Total Environ. 157, 93–105. MEXT, 2011. Corrections to the Readings of Airborne Monitoring Surveys (Soil Concentration Map) Based on the Prepared Distribution Map of Radiation Doses, Etc. (Map of Radioactive Cesium Concentration in Soil) by MEXT. http://radioactivity. nsr.go.jp/en/contents/4000/3172/24/1270_083014-2.pdf, Accessed date: July 2017. Mikami, S., Maeyama, T., Hoshide, Y., Sakamoto, R., Sato, S., Okuda, N., Sato, T., Takemiya, H., Saito, K., 2015a. The air dose rate around the Fukushima Dai-ichi Nuclear Power Plant: its spatial characteristics and temporal changes until December 2012. J. Environ. Radioact. 139, 250–259. https://doi.org/10.1016/j.jenvrad.2014. 08.020.
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Temporal changes of the ambient dose rate in the forest environments of Fukushima Prefecture following the Fukushima reactor accident
T
Hiroaki Kato∗, Yuichi Onda, Toshiro Yamaguchi Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Fukushima Dai-ichi nuclear power plant accident Forest Ambient dose rate Regional survey
Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011–2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014–2016 and 2011–2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10–20% faster than that induced solely by physical decay of radiocesium for the observation period 2011–2016.
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant accident resulted in the release of an enormous amount of radiocesium (~20 PBq for 137Cs) into the atmosphere (Chino et al., 2011; Amano et al., 2012; Hirose, 2012; Aoyama et al., 2016). The subsequent atmospheric deposition of radiocesium contaminated large areas of the terrestrial environment in both Fukushima and the neighboring prefectures (Butler, 2011; MEXT, 2011; NRA, 2017a,b). The results of airborne monitoring surveys and model calculations simulating the atmospheric transport of contaminants estimated that approximately 22% of the total radiocesium released into the atmosphere was deposited onto the land area in Fukushima and its neighboring prefectures (Morino et al., 2011, 2013; MEXT, 2011). Fukushima Prefecture area accumulated 2.1 PBq of 137Cs in total following the accident (Kato and Onda 2018). Although the contaminated area included a wide range of environments and land uses (e.g., Kitamura et al., 2014), approximately 70% of the total contaminated land surface consisted of forest cover (Hashimoto et al., 2012). Therefore, monitoring radiocesium contamination and predicting the gamma radiation dose in forest environments following the accident are important tasks to assess external exposure to radiation for
∗
DOI of original article: https://doi.org/10.1016/j.jenvrad.2018.08.009 Corresponding author. 1-1-1 Tennodai, Tsukuba, Ibaraki, 203-0006, Japan. E-mail address: [email protected] (H. Kato).
https://doi.org/10.1016/j.jenvrad.2019.106058
Available online 17 October 2019 0265-931X/ © 2018 Elsevier Ltd. All rights reserved.
human and wildlife (e.g., Fesenko et al., 2005). However, the extent of the radiocesium contamination and temporal evolution of the ambient dose rate (ADR) in different types of forest have not been assessed sufficiently. The ambient dose rate following the Fukushima Dai-ichi Nuclear Power Plant accident has been monitored across a large area of Fukushima and its neighboring prefectures by means of in-situ measurements (MEXT, 2011; Saito et al., 2015; Mikami et al., 2015a, 2015b) and airborne and car-borne surveys (e.g., NRA, 2015). The results of those monitoring surveys have determined the spatial pattern of the ambient dose rate in the Fukushima landscape and its neighboring prefectures, with the measured ambient dose rate being closely related to the initial amount of 137Cs deposition (Saito et al., 2015; Mikami et al., 2015a). The decreasing trend in the ambient dose rate has been assessed to determine the cumulative external radiation dose to local citizens and for long-term prediction of ambient dose rates in various land uses. The in-situ measurement of ambient dose rates at 1 m above the ground surface using a survey meter at flat and sparsely vegetated locations revealed that the relative percentage reduction in the air dose rate was 10% greater than that resulting solely from the radioactive decay of radiocesium between 2011 and 2012 (Mikami et al., 2015a).
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the Fukushima terrestrial region was performed by Gonze et al. (2016) using dynamic, spatially distributed, and process-based models. Their results demonstrated that air dose rates, either within or above forests, were very sensitive to both the detector height and vertical location of radiocesium in the system. Therefore, airborne surveys might not reflect dose rates at ground level in forest systems. Thus, statistical analyses based on ambient dose rates measured in situ in various forests are necessary to determine the actual temporal evolution of the ambient dose rate in Japanese forest environments following the Fukushima accident. The purpose of this study was to determine the temporal evolution of the ambient dose rate in various forest types. Repeated measurements of the ambient dose rate were conducted in 2014 and 2016 at the same measurement sites as those used by MAFF in 2011. The influence of forest type on the temporal evolution of the ambient dose rate was determined based on a statistical analysis of the regional measurements.
Such a rapid decrease in the ambient dose rate can be attributed to an increase in the relaxation depth of radiocesium in soil profiles rather than the loss of radiocesium due to natural weathering processes. On the other hand, the results of a car-borne survey indicated that the magnitude of the decreasing trend in the ambient dose rate was dependent on the land use in a particular area, with the ambient dose rate measured in forest areas decreasing more slowly than those in urban areas and over water (Kinase et al., 2014; Andoh et al., 2015; Kinase et al., 2017). Most previous studies were based on the ambient dose rate measured from aircraft or along major roadways, with very little monitoring data available regarding the temporal changes in the ambient dose rate in forest environments. The Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) conducted regional measurements of the ambient dose rate from September to November of 2011, targeting forests within Fukushima Prefecture (MAFF, 2011a,b). A total of 391 locations, containing various forest types, were selected for the measurements. The extent of radioactive contamination and ambient dose rates in forest environments were thus determined during the early phases of the Fukushima accident. However, further detailed analyses and repeated measurements at the same location were not conducted, and consequently the temporal evolution of the ambient dose rate in forest environments has not been determined. Officials from Fukushima Prefecture have conducted repeated measurements of the ambient dose rate in the forests of Fukushima Prefecture (Fukushima Prefecture, 2016). The decreasing trend in the ambient dose rate determined in 362 locations was similar to the reduction expected due to the decay of radiocesium. The results obtained in Fukushima Prefecture have been presented as the average value of all forest types, i.e., evergreen, deciduous, and secondary mixed forests. The results of in-situ measurements of ambient dose rate suggest that temporal changes in the ambient dose rate in forests differ depending on the predominant tree species (Imamura et al., 2015). Further analysis is required to determine the influence of forest type on the long-term trend in the ambient dose rate in Japanese forest environments. Radionuclides deposited in forested areas by either wet or dry processes encounter the forest canopy. Most radiocesium (> 90%) deposited onto the canopy of evergreen conifers is intercepted and retained by tree needles and branches (Hoffman et al., 1995; Pröhl and Hoffman, 1996; Kinnersley et al., 1996, 1997). On the other hand, most of the atmospherically deposited radiocesium tends to deposit directly onto the forest floor, particularly during the leafless winter season in deciduous forests (e.g., Melin et al., 1994; Kato et al., 2017). The canopy-intercepted radiocesium is subsequently transferred to the forest floor as a result of weathering by rainwater and wind (Bunzl et al., 1989; Bonnet and Anderson, 1993). Some radiocesium is readily removed from plant surfaces by rainfall, but the remaining is strongly absorbed by leaf, branch, and bark surfaces (Rauret et al., 1994). The removal rate of radiocesium from plant tissue is affected by several factors, such as the elapsed time since the initial atmospheric deposition, tree species and age, and climatic conditions. The ambient dose rate in forest environments is extremely variable in space and time. The vertical distribution of the measured ambient dose rate has been found to vary among different forest types, representing differences in radiocesium accumulation in canopies. Furthermore, the decrease in the ambient dose rate at 1 m above the ground surface occurred more slowly than the decrease measured at canopy height in cedar stands. The ambient dose rate in a forest growing on a slope was found to be highly heterogeneous and influenced by topography. These studies have suggested that the spatial distribution of atmospherically deposited radiocesium varies significantly among different forest stands; nevertheless, those differences in the initial distribution and subsequent transfer of radiocesium have not been linked to the temporal evolution of the ambient dose rate in forest environments. A numerical assessment of air dose rates induced by radiocesium in
2. Materials and methods 2.1. Measurement of the ambient dose rate in forests 2.1.1. Data measured by MAFF in 2011 (MAFF, 2011a, 2011b) MAFF measured the ambient dose rate in forests at 391 locations within Fukushima Prefecture (Fig. S-1 in the Supplementary Information document). It should be noted that white spot on the map is Lake Inawashiro. Each measurement site was designated to become part of the national survey of the biodiversity of forest ecosystems in Japan, with all sites distributed inside a 4-km grid within an 80-km radius of the power plant and inside a 10-km interval grid outside of the 80-km radius. The exact location of each measurement site was recorded using GPS, given a geographical coordinate, and marked with yellow or green poles (Fig. S-2 in the Supplementary Information document). The ambient dose rate at 1 m above the ground surface was measured using a NaI(TI) scintillation survey meter (TCS-172B, Hitachi, Ltd., Tokyo, Japan) at each measurement site. The measurements were conducted during between September 16 and November 9, 2011. The measured ambient dose rate was decay-corrected to the date of October 1, 2011. 2.1.2. Data measured by the Nuclear Regulation Authority (NRA) in 2014 Additional measurements of the ambient dose rate were conducted by authors as a part of the commission research project supported by NRA, at the same measurement sites in 2011. However, the measurements were reduced to 285 locations due to limited site accessibility caused by forest road collapse during typhoon event, deforestation and decontamination of the measurement site. The ambient dose rate at 1 m above the ground surface was measured using the same measurement protocol employed in 2011 by MAFF. It should be noted that the survey meters were checked and calibrated before field monitoring survey by a dosimeter company (Chiyoda Technol Corporation, Tokyo, Japan). The measurements were conducted between September and November 2014. The measured ambient dose rate was decay-corrected to the date of October 1, 2014. 2.1.3. Data measured in this study in 2016 Measurements of the ambient dose rate were conducted at 100 sites of the exact locations used by MAFF in 2011 and NRA in 2014. The number of measurement site was reduced to nearly one third of the measurements in 2014 due to the limitation of human resources and research fund. Criteria for the selection of repeated measurement site of ambient dose rate was as follows; 1) the ambient dose rate was measured in both 2011 and 2014, 2) the measurement location was easy to access, 3) the measurement site was covered by major forest type in Fukushima Prefecture (e.g., cedar (Cryptomeria japonica), konara oak (Quercus serrata), and secondary deciduous broad-leaved mixed forest 2
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with red pine (Pinus densiflora)), and 4) initial 137Cs deposition density of the measurement site was greater than 5 kBq/m2. The ambient dose rate at 1 m above the ground surface was measured using the same measurement protocol as that used in previous measurements in 2014. The measurement was conducted between September and November 2016. The measured ambient dose rate was decay-corrected to the date of October 1, 2016.
deposition density of radiocesium in the area outside of this radius were derived from the results of the fifth airborne monitoring survey (Measurement period: June 22–28, 2012). The 137Cs inventory was decaycorrected to the reference date of July 2, 2011. A detailed information on the initial 137Cs deposition density map can be found in Kato and Onda (2018). The relationship between the initial atmospheric deposition density of 137Cs and the measured ambient dose rate was expressed as the following equation:
2.2. Data preparation and statistical analysis
ADR = α·Ainitial
The measured ambient dose rate was classified into three different forest types: evergreen coniferous forest (EGC), mixed broad-leaved forest (MBL), and deciduous broad-leaved forest (DBF). It should be noted that there are in total 439 fixed monitoring sites for the national survey of forest inventory by MAFF in Fukushima Prefecture (in 4-km grid mesh), in which EGC, MBL, and DBF account for 29% (125 sites), 13% (58 sites), and 36% (192 sites) of the total 439 monitoring sites, respectively. These forest types are common in the area affected by the radioactive contamination from the Fukushima accident. The EGC in the Fukushima area consists mainly of Japanese cedar (Cryptomeria japonica) or cypress (Chamaecyparis obtusa) plantation, whereas the MBL is typically secondary forest, dominantly with a mixture of konara oak (Quercus serrata) and Japanese red pine (Pinus densiflora). The DBF consists of deciduous broad-leaved species such as konara oak (Quercus serrata), beech (Fagus crenata), and chestnut (Castanea crenata) trees. Minor forest types and undefined sites were excluded from the analysis. The total numbers of measurement sites for each forest type in each measurement year are shown in Table 1. Total numbers of ambient dose data for the target forest type in 2011, 2014, and 2016 were 328, 202, and 87, respectively. In addition, the stand properties of the forest sites are summarized in Tables S–1 in the Supplementary Information document. The measured ambient dose rate includes gamma radiation from global fallout, radiocesium derived from the Fukushima accident, and the natural background. The radiation dose from the natural background in the Fukushima area is 0.03 μSv/h (Akabane et al., 2013); therefore, the equivalent dose rate was subtracted from the measured ambient dose rate. On the other hand, Saito et al. (2015) determined that about 70.9% of the external effective dose was due to 134Cs, and 28.1% was due to 137Cs, and the contributions from the other nuclides were less than 1% in the middle of June 2011. Therefore, influences of short-lived radionuclides and the 137Cs derived from global fallout to the measured ADR could be negligible during the observation periods of this study. The initial 137Cs deposition density following the Fukushima accident was determined from the results of an airborne monitoring survey (MEXT, 2011). The results of the third airborne monitoring survey (Measurement period: May 31-July 2, 2011) were used for the area within an 80-km radius of the power plant, whereas the aerial
where, ADR is ambient dose rate (μSv/h) and α (μSv m /(kBq h))is a coefficient for determining the ambient dose rate from the initial atmospheric deposition density of radiocesium (Ainitial; kBq/m2). The value of Ainitial was determined by using the reconstructed atmospheric deposition map of 137Cs derived from the Fukushima reactor accident (refer to the nearest measurement point of the airborne survey). Therefore, the determined value of α potentially includes measurement uncertainties derived from the airborne survey. The reduction rate β in the ambient dose rate between the two measurement periods was determined from the following equation: ADRt+1 = β·ADRt
Cs deposition density (kBq/m2)
< 10 10–50 50–100 100–400 400–700 700–1000 > 1000 Total
2011
2014
3. Results and discussion The measured ambient dose rate was plotted on a map (Fig. 1). The locations northwest of the power plant had a remarkably high ambient dose rate regardless of the forest type. The ambient dose rate decreased with time, and in 2016, the dose rate was less than 0.5 μSv/h in most locations. The initial atmospheric deposition density of 137Cs was plotted against the ambient dose rate measured in 2011 (Fig. 2). The initial atmospheric deposition density and measured ambient dose rate displayed a significant correlation. The coefficient α values for the different forest types and years are listed in Table 2. The measured α in 2011 in this study was greater than that reported previously for flat open fields, with little vegetation cover (0.0054–0.0056 μSv m2/(kBq h); Saito et al., 2015; Mikami et al., 2015a). This indicated a higher ambient dose rate in forest areas than open fields for the same initial deposition of 137Cs. A similar trend in the radiation dose derived from global fallout in the northeastern United States was reported previously even for long-term period 30 years following the fallout peak in 1960's (Miller et al., 1990). For all forest types, α decreased with time due to the radioactive decay of radiocesium. The α values in 2011 for DBF were greater than that for EGC (p < 0.01) and MBL (p < 0.05), indicating that DBF tended to have higher ambient dose rates than that of EGC and MBL for the same aerial deposition density of radiocesium. The initial canopy interception of atmospherically deposited radiocesium varied depending on canopy characteristics, such as canopy biomass, leaf area index, and canopy openness (e.g., IAEA, 1996; Kato et al., 2018). Previous studies have reported a greater canopy interception of atmospherically deposited radiocesium in ECG due to its dense canopy cover throughout the year (Melin et al., 1994; Bunzl et al., 1989; Kato et al., 2012; Kato et al., 2017). The initial canopy interception of atmospherically deposited radiocesium following the Fukushima accident was reported to be 70% or greater for evergreen conifers such as Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa), whereas it was only 23% for secondary MBL (Kato et al., 2012; Kato et al., 2017). Deciduous tree species were
2016
EGC
ML
DBF
EGC
MBL
DBF
EGC
MBL
DBF
11 54 16 25 5 2 2 115
2 17 11 14 3 3 6 56
29 72 16 33 1 0 6 157
10 42 14 16 2 1 1 86
2 10 7 5 2 1 0 27
12 40 9 27 0 0 1 89
2 19 11 7 2 1 1 43
1 2 6 2 0 1 0 12
1 15 5 11 0 0 0 32
(2)
where ADRt and ADRt+1 are the ambient dose rate (μSv/h) measured by the preceding and the following campaigns, respectively. Measurement points that received an initial radiocesium deposition density of less than 10 kBq/m2 were excluded from the analysis because the effect of the global fallout of radiocesium could be significant at these locations.
Table 1 The number of ambient dose rate measurement sites in each forest type. The cesium-137 deposition density data was based on the third and fifth airborne monitoring surveys. EGC, MBL and DBF denote evergreen conifer, mixed broadleaved, and deciduous broad-leaved forest, respectively. 137
(1) 2
3
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Fig. 2. Relationship between the initial atmospheric deposition density of 137Cs and the measured ambient dose rate in forest environments in 2011.
Table 2 The relationship between the initial atmospheric deposition density of and the ambient dose rate (ADR) in each forest type in 2011.
137
Cs
Forest type
α(μSv m2/(kBq h))
R2
n
EGC MBL DBF
6.3 × 10−3 ± 1.8 × 10−4 7.7 × 10−3 ± 1.2 × 10−4 8.1 × 10−3 ± 8.4 × 10−5
0.92* 0.99* 0.99*
104 54 128
*p < 0.01.
(HAD16.0, Shimizu, 2016). For EGC in 2011–2016 and 2014–2016, a decontaminated location was excluded from the regression analysis (Fig. 3(a2, a3)). The reductions in the ambient dose rate due to the physical decay of radiocesium in 2011–2014, 2014–2016 and 2011–2016 were 0.54, 0.75, and 0.41, respectively. During 2011–2014, the value of β for EGC was greater than those for MBL (p < 0.01) and DBF (p < 0.01) (Table 3). However, the observed differences in β value for the period of 2014–2016 were not statistically significant. These results shows that the ambient dose rate decreased more slowly in EGC than in MBL and DBF during 2011–2014, while such trend was unclear during the following years (2014–2016). The ambient dose rate in EGC decreased with time, but the rate of decrease was slower than that induced by the physical decay of radiocesium for the period of 2011–2014, whereas it decreased slightly faster for the period of 2014–2016 and 2011–2016 (Table 3). The reduction in the ambient dose rate in MBL and DBF was approximately 10% for 2011–2014 and nearly 20% for 2011–2016 faster than that induced solely by the physical decay of radiocesium, though there was possible selection bias in the determination of β value for 2014 and 2016 due to lack of the measurement sites at high 137Cs deposition area. Nevertheless, slower decrease of ambient dose rates than that by physical decay of radiocesium in EGC during 2011–2014 was followed by quick reduction in the period of 2014–2016. Officials from Fukushima Prefecture have conducted repeated measurements of the ambient dose rate in the forests of Fukushima Prefecture (Fukushima Prefecture, 2016). The decreasing trend in the ambient dose rate determined at 362 locations was similar to the reduction induced solely by the physical decay of radiocesium. The results published by officials from Fukushima Prefecture were presented as the average values of all forest types, i.e., evergreen coniferous, deciduous, and secondary mixed forests. Whereas the in-situ measurements of ambient dose rates have suggested that temporal changes in the ambient dose rate in forests vary depending on the predominant
Fig. 1. Ambient dose rates (ADR) in forest environments measured by three different measurement campaigns. “EGC”, “MBL” and “DBF” denote evergreen conifer, mixed broad-leaved, and deciduous broad-leaved forest, respectively.
leafless at the time of the Fukushima accident in March 2011; therefore, a large portion of the atmospheric deposition was deposited directly on the forest floor in DBF, whereas the canopies of EGC intercepted atmospheric deposition following the accident. Consequently, a higher ambient dose rate was reported in DBF than in EGC in previous studies (e.g., Yoshihara et al., 2013; Kato et al., 2014). However, the influence of the initial canopy interception on the ambient dose rate diminished over time, because canopy-intercepted radiocesium was gradually transferred to the forest floor as a result of self-decontamination processes in EGC. Nevertheless, it is speculated that the difference in the ambient dose rate was more distinct between EGC and DBF during the very early stages of the accident. The measurement results in 2011, 2014 and 2016 were compared each other to determine the decreasing ambient dose rate during the three periods (Fig. 3). The relation between results in each years was analyzed by an iteratively reweighted least squares method with a weighting based on the Huber's function to reduce the effect of outliers 4
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Fig. 3. Comparison of the ambient dose rates (ADR) between the two different periods. A dotted line denotes a 95% confidence limit.
dose (Imamura et al., 2017). A numerical assessment of the air dose rates induced by radiocesium in the Fukushima terrestrial region was performed using dynamic, spatially distributed, and radiocesium transfer-process-based models in forest systems and the other terrestrial
tree species (Imamura et al., 2015). The air dose rate measured on the forest floor in 2014 originates largely from the radiocesium in the litter layer and the surface mineral soil layer (< 5 cm), with the canopy radiocesium contributing less than 3% of the total measured radiation
Table 3 The decreases in the measured ambient dose rate in each forest type during different observation periods. The error denote standard deviation for the determined β values. Forest type
EGC MBL DBF
2011–2014
2014–2016
β
R
0.66 ± 0.015 0.43 ± 0.013 0.45 ± 0.007
0.96* 0.97* 0.98*
2
2011–2016
n
β
R
76 25 77
0.64 ± 0.013 0.58 ± 0.029 0.55 ± 0.030
0.98* 0.97* 0.80*
*p < 0.01. 5
2
n
β
R2
n
41 11 31
0.33 ± 0.010 0.23 ± 0.012 0.24 ± 0.018
0.95* 0.97* 0.68*
41 11 31
Journal of Environmental Radioactivity 210 (2019) 106058
H. Kato, et al.
Acknowledgements
environment (Gonze et al., 2016). Their results demonstrated that the air dose rates in evergreen conifers decreased more slowly than the physical decay of radiocesium. These studies suggest that the accumulation of radiocesium in association with the self-decontamination processes of canopies affects the temporal evolution of the ambient dose rate at the forest floor, particularly in EGC. In this study, a statistical analysis based on the ambient dose rates obtained by in-situ measurements in various forests determined the actual temporal evolution of the ambient dose rate in Japanese forest environments following the Fukushima accident. The slower decrease in the ambient dose rate in EGC compared with DBF and MBL was consistent with the results of previous studies (e.g., Kinase et al., 2014). Although radiocesium in forest ecosystems is dynamic, e.g., the transfer of canopy-intercepted radiocesium to the forest floor, the measured decreasing trend in the ambient dose rate in EGC during the early phase of the accident (2011–2014) was slower than the reduction caused solely by the physical decay of radiocesium. However in the EGC, the ambient dose rate declined faster than the physical decay of radiocesium during the following periods from 2014 to 2016 when the 137Cs depositional flux from the canopy to forest floor decreased significantly (Kato et al., this issue). In DBF, where atmospheric deposition accumulated on the forest floor, the measured ambient dose decreased faster than the physical decay of radiocesium. These results suggest that radiocesium transfer among the forest canopy and surface soil layers affects the measured ambient dose rate in forest environments. Thus, understanding the biogeochemical cycling of radiocesium in various forest ecosystems is essential for predicting long-term trends in the ambient dose rate in forest environments. Furthermore, the study of elemental cycling within forest systems allows us to determine the dominant processes controlling the transport of elements in the system and is consequently prerequisite to develop models for predicting longterm radiocesium transfer in different forest ecosystems. Nevertheless, the ambient dose rate and its temporal changes varied even among the same tree species. Further analysis is required to determine the influence of forest properties, such as stand density, tree height, topography, soil type, and soil redistribution on the magnitude of the ambient dose rate and its long-term decreasing trend in Japanese forest environments.
This work was conducted as a commissioned study from the Japanese Atomic Energy Agency, as part of the Ministry of Education, Culture, Sports, Science and Technology-funded FY2011-2012, the Nuclear Regulation Authority of Japan-funded FY 2012–2014, and the Japanese Atomic Energy Agency-funded FY2015-2016. Additionally, this work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area; #15H00969 and #2411006) and Young Scientists (B) (#16K16201) from the Japan Society for the Promotion of Science. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvrad.2019.106058. References Akabane, K., Yonai, S., Fukuda, S., Miyahara, N., Yasuda, H., Iwaoka, K., Matsumoto, M., Fukumura, A., Akashi, M., 2013. NIRS external dose estimation system for Fukushima residents after the Fukushima Dai-ichi NPP accident. Sci. Rep. 3, 1670. https://doi. org/10.1038/srep01670. 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4. Conclusion In this study, we investigated temporal changes in the ambient dose rate in forest environments in Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement locations used by MAFF in 2011. The repeated measurements revealed that the decreasing trend in the ambient dose rate varied among different forest types. EGC exhibited a slower decrease in the ambient dose rate than that of MBL and DBF forests for the period of 2011–2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014–2016 and 2011–2016). In MBL and DBF forests, the ambient dose rate decreased faster than that induced solely by the physical decay of radiocesium. The measured variation in the temporal changes in the measured ambient dose rate could be attributed to differences in the initial canopy interception of atmospherically deposited radiocesium and the subsequent transfer from the canopy to the forest floor. A comparison of the decreasing trend in the ambient dose rate determined using different measurement techniques indicated that measurement of the ambient dose rate in forests is necessary to precisely evaluate temporal changes in the ambient dose rate in forest environments. Therefore, continuous monitoring is required for the assessment and long-term prediction of the ambient dose rate in forest environments.
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