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Article Cite This: ACS Omega 2018, 3, 18028−18038
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Determination and Comparison of the Strontium-90 Concentrations in Topsoil of Fukushima Prefecture before and after the Fukushima Daiichi Nuclear Accident Mitsuyuki Konno†,‡ and Yoshitaka Takagai*,†,§ †
Faculty of Symbiotic Systems Science, Cluster of Science and Technology and §Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan ‡ Environmental Radiation Monitoring Centre, Fukushima Prefecture, 45-169 Sukakeba, Kaibama, Haramachi, Minamisoma, Fukushima 975-0036, Japan ACS Omega 2018.3:18028-18038. Downloaded from pubs.acs.org by 146.185.203.120 on 12/24/18. For personal use only.
S Supporting Information *
ABSTRACT: To precisely understand the status of scattered strontium-90 (90Sr) after the 2011 accident at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) of Tokyo Electric Power Company (TEPCO), the measurement of the soil samples collected both before and after the day of the accident from the same sampling locations is necessary. However, very few reports have investigated the background contaminant data before the accident even though several studies have been conducted to investigate the effects of the F1-NPP accident. To address the lack of the passed 90Sr information and reestablished baseline, this study focuses on the stored topsoil samples that are collected from the same sampling locations from the Fukushima Prefecture before and after the F1-NPP accident, which are analyzed for obtaining the 90Sr concentrations. The results of our investigation exhibited that the 90Sr concentrations in the Fukushima Prefecture soils ranged from 0.2 to 20.4 Bq/kg in the samples that were collected before the accident and from 1.37 to 80.8 Bq/kg in the samples that were collected after the accident from identical sampling locations. Further, the soil samples that were collected from 30 out of 56 locations displayed significant differences in terms of concentrations before and after the accident. In addition, the relations between the 90Sr concentrations and the soil properties of the samples (organic content, pH, water content, and composition) were investigated, and it was found that the organic content and water content had a positive correlation with 90Sr concentrations and, in contrast, the sandiness was shown to have a negative correlation with 90Sr concentrations. The depth characteristics were also investigated. The aforementioned results indicate that this tendency would be observed even in the future.
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detector.14−19 Meanwhile, radiostrontium (90Sr) (half-life: 28.79 y20) is a pure β-ray-emitting nuclide that does not emit γ-rays, which makes it necessary to chemically isolate it for measuring β-rays because the β-ray spectra overlap. In particular, it is imperative to monitor 90Sr over a long period because it will require several decades to decommission F1NPP. In Japan, instead of a few literature concerning the development of a rapid analytical means,21−25 radiochemical analysis using milking-low background gas-flow counter (milking-LBC) is adopted as the official analysis method for analyzing 90Sr because of good sensitivity and/or highprecision analysis in low concentration levels in the environ-
INTRODUCTION
A large amount of radioactive materials was scattered throughout the environment (ocean, atmosphere, land, and so on) because of the accident that occurred on March 11, 2011 at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) that was owned by Tokyo Electric Power Company Holdings, Inc. (TEPCO).1−3 Seven years have passed by since the accident, and research institutes around the world have been monitoring the influence of the environmental dynamics of radionuclides that have been released.4−13 More specifically, there have been several environmental monitoring reports regarding β-ray-emitting nuclides, such as radioiodine and radiocesium, because multiple samples can be analyzed in a relatively short time using certain types of instruments such as a germanium semiconductor detector, a sodium iodide scintillator detector, and a lantern bromide scintillator © 2018 American Chemical Society
Received: October 3, 2018 Accepted: December 10, 2018 Published: December 21, 2018 18028
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ment.26 This method requires considerable amount of time and effort to pretreat the analysis as compared to those required by the γ-ray measurement method. Although various studies have been vigorously conducted,27−33 the study related to the scattering of 90Sr is not as advanced as compared to that related to the γ-ray-emitting nuclides such as radiocesium. To precisely understand the status of scattered 90Sr after an incident of nuclear accident, the samples collected both before and after the day of the accident should be measured, thereby distinguishing from the fallout of atmospheric nuclear tests (20th century’s) that have been conducted in the past. So far, the survival ratios of nuclides with short half-lives in samples have been employed in several studies.34 However, this technique cannot track the long-term process because it becomes difficult to evaluate the nuclides that exhibit a short decrease in half-lives. The optimal method for addressing these issues is to measure the radioactive concentrations of 90Sr in soil that is collected at identical locations before and after the accident. However, few examples exhibited the presence of 90Sr in soil before the F1-NPP accident, which was completely unexpected. Fortunately, we already possessed analytical data related to the 90Sr concentrations in soil samples that were collected before the accident with precise sampling locations throughout the Fukushima Prefecture (not published). Therefore, in this study, we succeeded in estimating the exact amount of 90Sr deposition before and after the F1-NPP accident. When performing the long-term observation, understanding the background level of 90Sr before the accident was observed to be considerably important for understanding the environmental radioactivity and the environmental dynamics or the usage of 90Sr as a tracer. In this study, we measured the radioactivity concentrations of 90Sr in the topsoil at the same locations in the Fukushima Prefecture before and after the accident and obtained the background levels of 90Sr before the F1-NPP accident. Thus, we revealed the deposition status of 90Sr before and after the accident. We also investigated the correlation between the soil properties and 90Sr to determine the status of deposition of 90 Sr on the topsoil in Fukushima prefecture (Figure 1).
Article
RESULTS AND DISCUSSION Sr in Soil before and after the Accident throughout Fukushima. The 90Sr concentrations in soil before the F1NPP accident (2005) are presented in Table 1, which also presents the sampling dates, global positioning system (GPS) coordinates, and the concentrations of 134Cs and 137Cs. The concentrations of 90Sr, 134Cs, and 137Cs that are presented in Table 1 are the values that are decay-corrected on the sampling day. In this case, the detection limits (DLs) for 90Sr, 134Cs, and 137 Cs are 0.2, 0.9, and 0.9 Bq/kg, respectively. At several sampling locations, 134Cs was below the DLs. However, the measured values of 90Sr and 137Cs exhibited significant differences from those DLs, and the 90Sr concentrations before the F1-NPP accident were in the range of 0.2 to 20.4 Bq/kg. The measurement error of the mean of these values before the accident (relative standard deviation, RSD) was observed to be 5.3%. The radioactive concentrations of 90Sr and 137Cs before the F1-NPP accident (2005) became clear because of this result. However, these 90Sr concentrations were detected at levels that were similar to those found in other areas of Japan. For example, the 90Sr concentrations in soil collected in the same year in Ibusuki-city [Kagoshima Prefecture (south Japan)], Imaichi-city [Tochigi Prefecture (neighbor of Fukushima Prefecture)], and Takeda-city [Oita Prefecture (south Japan)] were 14−15 Bq/kg.38 The 90Sr concentrations in soil before the accident (2005) (data in Table 1) were plotted on a map according to the concentration range, as depicted in Figure 2. The distribution of 90Sr in soil was observed to be high in the southwestern part of Fukushima before the F1-NPP accident. It was assumed that 90Sr was associated with the fallout of the atmospheric nuclear tests that were conducted in the past.34 Meanwhile, the amount of 90Sr deposition in Japan from 1995 through 2005 was reported to be less than 0.5 MBq/km2 per month at the maximum and was observed to be undetectable in most areas.38 Therefore, it can be assumed that 90Sr that remained in the soil was the one that was deposited before 1995. In addition, 90Sr in soil tended to increase in the order of the eastern, central, and western regions in Fukushima. It can be assumed that this difference can be attributed to snow accumulation. During average years, there are a few snowfalls in the eastern region, whereas more than 5 m of snow accumulation during winter in the western region and several tens of centimeters of snowfall in the central region have been recorded.39 The topsoil stays in place during winter because of the large quantity of snowfall, and the mass transfer of the topsoil decreases as compared to that in the rainy season. Therefore, it can be assumed that the 90Sr concentrations tend to be high in the western region where there is more snowfall. The 90Sr concentrations in the soil samples that are collected from the same locations after the F1-NPP accident (2011) are presented in Table 2. The sampling point (GPS coordinates) in Table 2 is the same as that in Table 1 (sampling point). The concentrations of 90Sr, 134Cs, and 137Cs in the Table 2 are also the values that are decay-corrected on the day of sampling. Further, the DLs for 90Sr, 134Cs, and 137Cs are 0.2, 11, and 13 Bq/kg, respectively. The DLs for 90Sr that was contained in soil collected from the same locations were 1.37−80.8 Bq/kg. The RSD after the accident was observed to be 9.6%. Additionally, the measurements before the accident (2005) that were reported in Table 1 were decay-corrected to August 1, 2011 90
Figure 1. Sampling was conducted at 56 points which were chosen from the whole area of the Fukushima Prefecture. 18029
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Table 1. Radioactivity Concentration of 90Sr and 134Cs, 137Cs in Surface Soil Were Collected before Accident of Fukushima Nuclear Power Plants (the Whole Fukushima Prefectural Area 2005) location informationb a
radioactivityc
no.
date, 2005
east longitude
north latitude
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
June 16 June 16 June 16 June 16 June 16 June 16 June 22 June 14 June 22 June 14 June 14 June 14 June 20 June 20 June 14 June 22 June 22 June 22 June 22 June 22 June 22 July 19 July 19 July 19 July 19 July 20 July 20 June 13 June 22 June 23 June 23 June 23 June 13 June 22 June 23 June 23 July 20 July 20 July 20 June 23 July 12 June 22 June 23 June 23 July 29 June 23 June 23 June 23 June 23 June 15 June 15 June 15 June 22 June 22 June 22 June 22
37.83147 37.89228 37.60592 37.58389 37.59986 37.75808 37.39153 37.30178 37.24925 37.24467 37.17203 37.10033 37.44228 37.41928 37.32064 37.24183 37.17706 37.13186 37.15275 37.06464 36.84953 37.61528 37.52889 37.47511 37.45642 37.45317 37.44722 37.58786 37.65086 37.72933 37.68531 37.56358 37.14886 37.20022 37.18378 37.33747 37.09656 37.01511 37.27792 37.31175 37.33919 37.79717 37.61683 37.49808 37.70139 36.93603 36.86456 36.90406 37.16825 37.22583 37.30628 37.33450 37.41267 37.44750 37.49792 37.51347
140.43786 140.52094 140.60603 140.33164 140.42933 140.68994 140.21478 140.19103 140.06833 140.31908 140.46781 140.61381 140.64967 140.82492 140.51503 140.12006 140.03978 140.21258 140.31700 140.25778 140.39711 140.20800 140.03256 139.98444 139.84150 139.58600 139.69622 139.79861 139.78261 139.88622 139.89747 139.87000 139.71436 139.80389 139.52889 139.86781 139.59300 139.38078 139.50250 139.28422 140.81175 140.91508 140.89258 140.76247 140.71554 140.92347 140.78075 140.71775 140.65078 141.00389 141.02258 141.01353 141.02261 141.02389 141.01797 141.02794
90
Sr [Bq/kg]
3.57 2.52 6.27 2.17 1.01 2.53 2.21 7.26 9.29 2.42 2.19 0.51 1.71 0.64 3.11 1.90 5.56 2.54 0.73 1.20 0.94 4.55 3.40 13.1 5.11 2.10 2.13 4.09 4.57 0.20 1.26 0.94 9.75 4.49 1.13 4.46 4.23 20.4 8.86 1.69 1.66 2.61 6.59 0.22 2.70 2.72 3.18 0.47 1.24 1.68 1.89 ND ND 3.04 2.99 4.49
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.22 0.19 0.29 0.18 0.12 0.19 0.18 0.32 0.35 0.19 0.17 0.10 0.15 0.10 0.20 0.16 0.28 0.19 0.11 0.14 0.10 0.21 0.18 0.3 0.21 0.14 0.14 0.19 0.21 0.06 0.11 0.10 0.28 0.20 0.11 0.20 0.20 0.4 0.29 0.13 0.12 0.17 0.24 0.06 0.18 0.17 0.17 0.09 0.12 0.14 0.19
± 0.27 ± 0.20 ± 0.28
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Cs [Bq/kg] ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND
137
Cs [Bq/kg]
18.5 8.84 38.6 45.4 1.26 22.6 39.2 26.2 54.9 16.6 3.34 ND 9.79 ND 24.5 28.1 88.8 19.3 11.6 15.0 ND 34.1 33.8 75.6 46.8 9.24 12.4 38.1 2.53 39.2 3.63 30.7 37.8 43.5 4.13 56.2 33.8 68.2 50.6 14.1 23.7 12.4 50.3 ND 27.3 4.51 38.5 7.17 ND 17.3 ND 8.81 16.7 57.6 1.87 8.72
± ± ± ± ± ± ± ± ± ± ±
0.4 0.21 0.9 1.1 0.03 0.5 0.9 0.6 1.3 0.4 0.08
± 0.23 ± ± ± ± ± ±
0.6 0.7 2.1 0.5 0.3 0.4
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.8 0.8 1.8 1.1 0.22 0.3 0.9 0.06 0.9 0.09 0.7 0.9 1.0 0.10 1.3 0.8 1.6 1.2 0.3 0.6 0.3 1.2
± ± ± ±
0.7 0.11 0.9 0.17
± 0.4 ± ± ± ± ±
0.21 0.4 1.4 0.04 0.2
a
Sampling date. bGeographical coordinate values were based on the World Geodetic System (WGS-84) datum. cThe DLs for 90Sr, 134Cs and 137Cs were 0.2, 0.9, and 0.9 Bq/kg, respectively. 18030
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Relation between Soil Properties and 90Sr. The relations between the soil properties of the collected soil samples (topsoil) and 90Sr that was deposited on the topsoil were investigated. The samples were classified into three types (clay, silt, and sand) according to Table S2 in the Supporting Information using the methods described in “Measurement of the particle size composition of soil” in the “Experiment” section. Water content, pH, and organic content of the soil samples (topsoil) were also investigated. Those results are presented in Table S3 in the Supporting Information. The sample location numbers correspond to the same locations presented in Tables 1 and 2. The water content, pH, and organic content before and after the accident, the values before the accident (2005) were presented, because there were slight changes in the granular component. Figure 5 depicts the relation between the pH and 90Sr concentrations in soil. The pH of the collected soil ranged from 4.0 to 7.3, and most of them were acidic with a pH of 4.5−6.0. There was no difference in the relation between the 90 Sr concentration and pH before and after the accident. The relation between the organic content in the topsoil and the 90Sr concentration is presented in Figure 6A. Therefore, the concentrations of 90Sr tended to increase according to the organic content. Data from both before and after the accident exhibited similar results. Kim et al.40 reported on the relation between the organic content and the type of radionuclide in soil throughout Korea and described that the type of radionuclide was influenced by the organic content and that the influence on 90Sr was smaller than that on 137Cs and (239+240) Pu. These reports and the results in this study revealed a similar tendency. This was also the case with respect to the soil samples in Fukushima that the 90Sr concentrations tended to be relatively higher in soil with higher organic content, which was similar to other examples. In addition, the relation between the water content in soil and the 90Sr concentration is presented in Figure 6B. Therefore, the concentrations of 90Sr tended to be high according to the water content. Data from both before and after the accident exhibited similar results. This is considered to be because of the water contained in the organic matter based on the fact that 90Sr tends to deposit on organic matter. This is also evident from Figure 7, where a clear correlation can be observed between the organic content and the water content. 90 Sr Distribution by Depth. The relation between the percentage of sand in soil and the 90Sr concentration is presented in Figure 8A. Therefore, the concentrations of 90Sr tended to reduce according to the percentage of sand. Meanwhile, as depicted in Figure 8B, the water content also tended to decrease as the percentage of sand increased. It is also known that 90Sr not only shows a tendency to deposit on organic matter but also influences the leaching depending on the particle size distribution of soil.41 Thus, we chose three representative regions in Fukushima [eastern region (Hamadori), central region (Nakadori), and western region (Aizu). F1-NPP is located on the coast of the eastern region] and the 90 Sr concentration is investigated at every soil samplecollection depth. The samples that were used in this investigation were collected before the accident (2005); therefore, these data will be useful for researchers who intend to conduct environmental research in the future, especially when 90Sr background data are required. Two sampling locations, where there were relatively high concentrations of
Figure 2. 90Sr distribution map in Fukushima Prefecture before the accident of the Fukushima Nuclear Power Plants.
and presented for comparison with the data of after the accident. Figure 3 depicts the diagram of 90Sr concentrations in soil before (2005) and after (2011) the accident at the same locations. In the figure, the plots that were observed on the straight line represented the locations where there was no change in the concentrations before and after the accident. The number of locations that only exhibited a little change in the concentration (locations without significant differences) was nine points. When the concentrations were compared before and after the accident, the 90Sr concentrations were observed to clearly increase in 38 out of 56 locations. In particular, the places that were located several kilometers from the F1-NPP displayed large increases in 90Sr, and the largest increase was observed to be 80 Bq/kg (no. 53). The locations at which the concentrations increased after the accident were considerably likely to be locations at which 90Sr was deposited after the F1NPP accident. However, at locations at which the concentrations decreased after the accident, it was assumed that the 90 Sr concentrations decreased because of some type of land modification. Figure 4 depicts the relations between 137Cs and 90Sr fallen on the ground surface. There were more locations at which the concentrations of radioactive Cs were higher than those of 90Sr both before and after the accident. In addition, the (134+137) Cs/90Sr ratios were slightly higher before the accident (2005) as compared to those after the accident. The same tendency was observed when we focused only on the locations at which the radioactive concentrations were relatively high. The slope of the (134+137)Cs/90Sr ratio approached 1 both before and after the accident (slope = 0.80 and 0.89 for 2011 and 2005, respectively). Thus, the residual levels before the accident exhibited a higher percentage of 90Sr as compared with the levels after the accident (2011). Meanwhile, in location nos. 22 and 45, it is conceivable that the highly concentrated (134+137)Cs in the highly concentrated plume originated from the explosion of the F1-NPP deposited on soil with rainfall (on March 13, 2011) and that the (134+137)Cs/90Sr ratio increased after the accident. As the reference, the relationship between the concentration of 134Cs and 137Cs in soils (2011s) is shown in Figure S1 in the Supporting Information. The ratio was 134Cs/137Cs = 1. 18031
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Table 2. Radioactivity Concentration of 90Sr and 134Cs, 137Cs in Surface Soil Were Collected after Accident of Fukushima Nuclear Power Plants (2011s Whole Fukushima Prefectural Area) and Theoretical Decayed Radioactivity Values Using 2005s Data measured radioactivity (2011s soils)c noa
dateb, 2011
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
Aug. 10 Aug. 10 Aug. 10 Aug. 10 Aug. 10 Aug. 10 Aug. 26 Aug. 17 Aug. 17 Aug. 26 Aug. 26 Aug. 26 Aug. 24 Oct. 13 Aug. 26 Aug. 17 Aug. 17 Aug. 17 Aug. 17 Aug. 17 Aug. 18 Aug. 30 Aug. 30 Aug. 30 Aug. 30 Aug. 30 Aug. 30 Aug. 31 Aug. 31 Aug. 31 Aug. 31 Aug. 31 Sep. 16 Sep. 16 Sep. 15 Sep. 16 Sep. 16 Sep. 15 Sep. 15 Sep. 15 Aug. 24 Aug. 24 Aug. 24 Aug. 24 April 25 Aug. 18 Aug. 18 Aug. 18 Aug. 29 July 14 July 14 July 14 July 13 July 13 July 13 July 13
90
Sr [Bq/kg]
3.88 2.49 6.87 1.57 2.13 3.99 3.25 4.50 ND 3.42 ND ND 1.62 2.03 4.18 3.17 7.27 2.82 2.09 3.98 2.62 13.3 5.97 9.64 4.04 1.96 ND 3.70 ND 3.07 2.56 6.29 5.69 5.93 1.70 5.64 ND 20.6 9.64 1.55 3.51 2.88 7.36 1.77 18.0 4.97 2.88 5.87 2.45 2.81 2.19 1.37 80.8 14.9 4.20 1.69
± ± ± ± ± ± ± ±
0.53 0.51 0.84 0.45 0.56 0.53 0.45 0.54
± 0.64
± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.35 0.48 0.53 0.48 0.74 0.41 0.49 0.66 0.54 1.1 0.83 1.04 0.59 0.43
± 0.50 ± ± ± ± ± ± ±
0.58 0.48 0.81 0.66 0.70 0.41 0.75
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1.2 0.79 0.39 0.44 0.49 0.70 0.39 0.8 0.77 0.52 0.60 0.57 0.45 0.36 0.30 1.6 0.8 0.60 0.45
134
Cs [Bq/kg]
3440 ± 825 1780 ± 427 1340 ± 323 967 ± 232 5270 ± 1270 8260 ± 1980 768 ± 184 598 ± 144 564 ± 135 271 ± 65 14.7 ± 3.5 176 ± 42 398 ± 95.5 836 ± 201 380 ± 91 1210 ± 292 443 ± 106 1580 ± 380 203 ± 49 1050 ± 252 177 ± 42 431 ± 103 191 ± 46 144 ± 35 639 ± 153 92.7 ± 22.2 205 ± 49 372 ± 89 412 ± 99 113 ± 27.0 493 ± 118 255 ± 61.2 32.6 ± 7.8 82.5 ± 19.8 729 ± 175 93.9 ± 22.5 22.1 ± 5.3 39.8 ± 9.6 266 ± 64 68.4 ± 16.4 545 ± 131 1130 ± 271 6420 ± 1540 5410 ± 1300 3960 ± 951 579 ± 139 894 ± 214 985 ± 236 815 ± 195 178 ± 43 1810 ± 434 11 100 ± 2660 90 100 ± 21 600 2660 ± 638 1570 ± 377 127 ± 30.5
137
Cs [Bq/kg]
4220 ± 1010 2120 ± 509 1650 ± 395 1160 ± 278 6340 ± 1520 10 100 ± 2420 931 ± 223 688 ± 165 659 ± 158 316 ± 76 17.5 ± 4.2 209 ± 50 503 ± 121 1070 ± 256 485 ± 116 1480 ± 354 546 ± 131 1960 ± 471 232 ± 56 1280 ± 307 197 ± 47 600 ± 144 283 ± 68 217 ± 52 759 ± 182 123 ± 29 268 ± 64 446 ± 107 540 ± 130 150 ± 36 590 ± 142 321 ± 77 51.0 ± 12 119 ± 29 900 ± 216 155 ± 37 33.2 ± 8.0 77.8 ± 19 353 ± 85 87.3 ± 21 668 ± 160 1350 ± 325 7430 ± 1780 6600 ± 1580 4400 ± 1060 677 ± 163 1060 ± 254 1113 ± 267 980 ± 235 165 ± 40 2000 ± 480 12 500 ± 3000 99 700 ± 23 900 3080 ± 739 1900 ± 456 186 ± 45
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theoretical decayed radioactivity (2011) using data of 2005s soilsd 90
Sr [Bq/kg]
3.08 2.17 5.41 1.87 0.87 2.18 1.91 6.27 8.02 2.09 1.89 0.44 1.47 0.56 2.69 1.64 4.80 2.19 0.63 1.04 0.81 3.94 2.94 11.3 4.42 1.82 1.84 3.53 3.95 0.17 1.09 0.81 8.41 3.88 0.97 3.85 3.66 17.7 7.66 1.46 1.44 2.25 5.69 0.19 2.32 2.35 2.74 0.41 1.07 1.45 1.63 ND ND 2.62 2.58 3.88
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.21 0.19 0.29 0.17 0.12 0.19 0.18 0.32 0.35 0.19 0.17 0.10 0.15 0.10 0.20 0.16 0.28 0.19 0.11 0.14 0.10 0.20 0.17 0.3 0.21 0.14 0.14 0.19 0.20 0.06 0.11 0.10 0.28 0.20 0.11 0.20 0.20 0.4 0.28 0.13 0.12 0.17 0.24 0.06 0.18 0.16 0.17 0.09 0.12 0.14 0.19
± 0.27 ± 0.20 ± 0.28
134
Cs [Bq/kg] ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND
137
Cs [Bq/kg]
15.9 ± 3.83 7.62 ± 1.83 33.3 ± 7.99 39.2 ± 9.41 1.08 ± 0.26 19.5 ± 4.68 33.9 ± 8.13 22.6 ± 5.42 47.4 ± 11.37 14.3 ± 3.43 2.88 ± 0.69 ND 8.45 ± 2.03 ND 21.2 ± 5.08 24.2 ± 5.82 76.7 ± 18.4 16.6 ± 3.99 10.0 ± 2.41 12.9 ± 3.10 ND 29.5 ± 7.08 29.3 ± 7.02 65.4 ± 15.7 40.4 ± 9.70 7.99 ± 1.92 10.7 ± 2.58 32.9 ± 7.90 2.2 ± 0.52 33.9 ± 8.13 3.14 ± 0.75 26.5 ± 6.36 32.6 ± 7.83 37.5 ± 9.00 3.57 ± 0.86 48.5 ± 11.6 29.2 ± 7.01 59.0 ± 14.2 43.7 ± 10.5 12.1 ± 2.91 20.5 ± 4.92 10.7 ± 2.57 43.4 ± 10.4 ND 23.6 ± 5.67 14.4 ± 3.45 49.8 ± 11.9 1.61 ± 0.39 7.52 ± 1.81 3.89 ± 0.93 33.2 ± 7.96 6.18 ± 1.48 ND 15.0 ± 3.59 ND 7.61 ± 1.83
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Table 2. continued a
Location number was corresponded with Table 1. bSampling date. The radioactivity was corrected to the sampling date. cND: non detection. The DLs for 90Sr, 134Cs, and 137Cs were 0.2, 11, and 13 Bq/kg, respectively. dThe theoretical decayed radioactivity was calculated using the soil data of 2005 (Table 1) to Aug. 01, 2011. The DLs for 90Sr, 134Cs, and 137Cs for 2005s soils were 0.2, 0.9, and 0.9 Bq/kg, respectively.
33%) are selected. Because the soil properties were classified according to Table S1 in the Supporting Information, the components other than sand were classified as silt and clay. The results of the investigation are presented in Figure 9, where Figure 9A depicts the eastern region (●: no. 56, ○: no. 46), Figure 9B depicts the central region (●: no. 8, ○: no. 9), and Figure 9C depicts the western region (●: no. 38, ○: no. 33). When the results are examined according to the region, the percentage of sand was observed to be approximately 50% regardless of the depth and that 90Sr was detected at almost constant concentrations up to a depth of 45 cm in the eastern region (Figure 9A). Meanwhile, in the central (Figure 9B) and western (Figure 9C) regions, the 90Sr concentrations were at their maximum in the surface layer and gradually decreased with depth from the surface; further, the depths became almost undetectable at a depth of 45 cm. At these locations, it was not possible to clearly confirm the influence of the sand content. However, the sand content was significantly different between the surface layer and the lower layer in soil, where the influence of clay and other materials was suspected. In addition, Figure S2 shows the relationship between the reduction percentage of 90Sr from surface soil and the distribution percentage of organic matter or clay matter in each depth in nine typical locations (sampling points). As we can see in the results, 90Sr fallen-out by past nuclear tests (probably the 1960s) in the air had still deposited in the soils from 2005 from the surface to a depth of 50 cm.
Figure 3. Variation of 90Sr concentration in soils (at same GPS location) of Fukushima Prefecture between before and after the accident of Fukushima NPP (2005 and 2011).
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CONCLUSIONS The 90Sr concentrations before the F1-NPP accident (2005) were in the range of 0.2−20.4 Bq/kg (RSD 5.3%). These 90Sr concentrations were within similar concentration ranges as compared to those observed in other areas of Japan. It was observed that the 90Sr concentrations in soil tended to increase in the order of the eastern, central, and western regions and that they were particularly high in the southwestern part in Fukushima before the F1-NPP accident (2005). The 90Sr concentration in soil that was collected from the same locations after the accident (2011) was in the range of 1.37−80.8 Bq/kg (RSD 9.6%). By comparing the soil samples that were collected before and after the accident, the number of locations at which the 90Sr concentrations clearly increased was 38 out of 56 locations, with a maximum increase of 80 Bq/ kg. All the soil samples that were collected before and after the accident exhibited higher radioactive Cs concentrations as compared to the 90Sr concentrations, and the (134+137)Cs/90Sr ratio before the accident (2005) was slightly higher than the ratio after the accident. Furthermore, there was no correlation between the pH of soil and the 90Sr concentrations. Meanwhile, there was a correlation between the organic content and the water content in the topsoil, and the 90Sr concentrations tended to increase as the water content increased, and this tendency was present in the soil samples that were collected before and after the accident. Further, the 90 Sr concentrations decreased as the percentage of sand in soil increased. In the eastern region, 90Sr was detected at almost constant concentrations regardless of the depth, whereas it was
Figure 4. Relationship between 90Sr concentration in soil and (134+137) Cs concentration in the whole Fukushima prefectural region before and after the accident of Fukushima NPP.
Figure 5. Impact of pH on 90Sr concentration on the surface of soil and the comparison between before accident (2005) and after accident (2011) of Fukushima NPP in the whole Fukushima prefectural region.
Sr, were chosen from each area (3 regions × 2 locations) as the target locations in this investigation. Additionally, it is known that 90Sr tends to deposit on organic matter; therefore, the samples having relatively similar organic content (20− 90
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Figure 6. Impact of organic matter (A) and the moisture (B) on 90Sr concentration on the surface soil and the comparison between before accident (2005) and after accident (2011) of Fukushima NPP in the whole Fukushima prefectural region.
tendency will continue in the future if there is no new 90Sr release or decontamination of the soils. Further, it is expected that the 90Sr concentrations will be attenuated based on the physical half-life.
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EXPERIMENT Sample Collection. Figure 1 depicts the map of the sampling locations using the GPS information for each location and the sampling dates that are presented in Table 1. A total of 56 locations located within a distance of approximately 1−150 km from the F1-NPP were selected as the sampling locations throughout the Fukushima Prefecture; these locations were chosen from places at which only little artificial movement of the soil, such as cultivation, land development, and other forms of development or activity, could be observed. The sampling date [before (2005) and after accident (2011)] is listed in Tables 1 and 2. Further, the fallen leaves, grass, stones, and other debris on the surface were removed before sampling, and the soil from the surface to a depth of 5 cm was collected from five or more spots in each of the 56 locations using a cylindrical SUS soil sampler (Φ 80 mm) until exceeding 2000 g. Meanwhile, soil cores were collected from the surface up to a depth of 50 cm using a motor-powered core sampler to investigate the vertical distribution of the 90Sr concentrations (Neko Drill 02, core diameter: 50 mm, Koken Boring Machine Co., LTD.). The soil core sample that was detached from the core was cut into segments every 10 cm (=divided into five portions). Further, the soil core was collected from five or more spots in each of
Figure 7. Relationship between the organic matter and moisture in collected soils.
revealed that 90Sr was detected in the surface layer in the central and western regions. To summarize, the deposition tendency of 90Sr before (2005) and after (2011) the accident was similar depending on the organic content, water content, and sandiness (in other words, clay properties). Additionally, in terms of the depth dependency, although some regional characteristics were observed, 90Sr, for the most part, remained in the topsoil in the central and western regions of Fukushima, whereas it tended to disperse at a constant rate in the eastern region. Because a similar behavior was observed both before and after the accident, it can be considered that this surface deposition
Figure 8. Impact of sand distribution on 90Sr concentration in the surface soil between before the accident (2005) and after the accident (2011) of Fukushima NPP in the whole Fukushima prefectural region (A), and the relationship between sand and moisture (B). 18034
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In addition, the measurement of the γ-ray-emitting nuclides was performed using a germanium semiconductor detector GC4020 [resolution (FMWT): 1.87 keV, relative efficiency 44%] that was manufactured by Mirion Technologies, Inc. Canberra (Tokyo, Japan). Additionally, self-absorption and sum-peak adjustments were achieved during the analysis. Reagents and Preparation of Solutions. All the acids and bases that were used in this study exhibited the toxic metal analysis grade and were supplied by Wako Pure Chemical Industries, Ltd. The strontium carrier solution was prepared by dissolving 24.15 g of Sr(NO3)2, which was heated and dried at 300 °C for 2−3 h, in deionized water and by adjusting to 1 L in a volumetric flask, resulting in a sample with 10 mg/mL (as Sr). The calcium carrier solution was prepared by adding 300 mL of deionized water to 125 g of CaCO3, with a small amount of hydrochloric acid being added to completely dissolve it. The solution was further diluted using deionized water and adjusted to 1 L in a volumetric flask, resulting in a sample with 50 mg/mL (as Ca). The iron carrier that was used for scavenging was prepared by dissolving 6.0 g of FeCl3·6H2O with 5 mL of 6 M hydrochloric acid and by diluting with deionized water in a volumetric flask to 250 mL, resulting in a sample with 5 mg/mL (as Fe). The Fe−Y mixed carrier for milking was prepared in the following manner. Y2O3 (1.270 g) was dissolved in 50 mL of 50% concentrated nitric acid while being heated and was subsequently diluted after being cooled with deionized water in a volumetric flask to 100 mL to prepare 10 mg/mL (as Y) solution. Electrolytic iron (1.000 g) was dissolved in aqua regia, and 1 mL of 10 mg/mL Y carrier solution was added using a transfer pipette. It was subsequently diluted with deionized water in a measuring cylinder to 1 L, resulting in a sample with (1 mg Fe + 10 μg Y)/mL. Analysis of 90Sr. Approximately 100 g of each soil sample was heated in an oven at 500 °C for approximately 12 h to break down the organic compounds that were present in the sample. After the sample was left to cool, 10 mL of the strontium carrier and 300 mL of 12 M HCl were added, and the mixture was heated (boiling water) for approximately 3 h to extract acid. The mixture was filtered through a filter paper (5C), and 5 mL of the calcium carrier and 1200 mL of deionized water were further added to the filtrate. After ensuring that the pH was adjusted to 10 or more by adding small amounts of NaOH, 30 g of NaCO3 was added, and the solution was stirred well. The precipitate that was formed by this process was centrifuged (3000 rpm, 10 min), separated, and dissolved in 12 M HCl, which was subsequently filtered through a membrane filter (0.45 μm), and 1200 mL of deionized water was added to the filtrate. Further, the solution was heated until it reached the boiling state. After boiling, 40 g of oxalic acid was added, and ammonium hydroxide (25%) was added drop-wise to adjust the pH to 4.2. The oxalate precipitate that was formed by pH adjustment was filtered, and it was thermally decomposed at 600 °C in an oven for 3 h. After thermal decomposition, the sample was dissolved using 50 mL of 0.5 M HCl, filtered through a filter paper (5C), and diluted with 0.5 M HCl in a volumetric flask adjusted to 500 mL. This sample solution was passed through a column (30 × 260 mm) filled with an anion exchange resin (Dowex 50W-X8, Muromachi Technos Co.) at a flow rate of 20 mL/min. Thereafter, 1000 mL of a mixed solution of 6 M ammonium acetate aqueous solution and methanol (1:1) was passed through an ion exchange resin that adsorbed Sr at a flow rate of 20 mL/min to elute calcium. Ammonium acetate aqueous
Figure 9. Distribution of 90Sr concentration in the depth direction of soil. (A) Eastside of Fukushima pref. (●; no. 56, ○; no. 46), (B) Center region of Fukushima pref. (●; no. 8, ○; no. 9), (C) westside of Fukushima pref. (●; no. 38, ○; no. 33). The location number is described in Tables 1 and S3 in the Supporting Information.
the 56 locations until the total amount of the core sample at each depth exceeded 1000 g. The aliquot of the soil sample was air-dried after removing the tree roots, stones, and other debris. It was subsequently sieved through a 2 mm mesh and mixed well to measure the particle size composition and pH. Meanwhile, the collected sample was thoroughly dried in a hot-air drier at 105 °C, sieved through a 2 mm mesh, and mixed well for performing the 90Sr measurement. These samples were analyzed in a laboratory that was confirmed to be 90Sr-free to prevent inadvertent contamination during analysis. Measuring Apparatus. Beta activity was measured using the LBC (LBC-4202) that was manufactured by Hitachi Aloka Medical, Ltd (Tokyo, Japan). The detector was a GM counter tube equipped with an anticoincidence circuit with center and guard counters. The influence of the cosmic rays was shielded with a cover manufactured using a 100 mm thick lead. Additionally, a polyethylene terephthalate thin film with gold was deposited on the area at which the β-ray from the sample was incident; the quenching gas to be used during the measurement was a mixed gas containing 99% helium and 1% isobutene. Further, the measurement time was 60 min. 18035
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weight. Further, the suspension in the measuring cylinder was removed by decantation, and the residue was transferred to a 500 mL tall beaker. Deionized water was poured up to 2 cm from the top of the beaker, and the contents were thoroughly stirred using a glass rod. It was left to stand for a certain time, and the suspension was further removed using a siphon beginning from a depth of 5 cm. This process was repeated until the supernatant became clear. The supernatant was eliminated by decantation, and the residue was dried in the evaporating dish and was weighed to obtain the fine sand weight. The classification of the particle size was conducted in accordance with the International Union of Soil Sciences method36 (see: Table S2 in the Supporting Information). Measurement of the Water and Organic Content in Soil. The measurement of the water and organic content in the soil samples was conducted in accordance with the bottom sediment testing method.37 As for the measurement of water content in soil, approximately 5 g of the air-dried soil was measured in a melting pot, and its weight was recorded beforehand. Further, the sample was dried in a dryer (DNF44, Yamato Scientific) at a temperature of 105 °C for 2 h and was immediately taken out and left to cool in a desiccator for 2 h. The weight of the cooled sample was measured using a precision balance (AG245, Mettler Toledo LLC, OH, USA), with the weight loss after drying being defined as the water content, which was expressed in weight percent. With respect to the organic content in the soil, the weight of the sample after the measurement of the water content was recorded beforehand, and the sample was heated in an oven (FP42, Yamato Scientific) at 600 °C for 2 h. It was further cooled to 100 °C in the oven and was left to cool to the room temperature in the desiccator, and the weight was measured using the precision balance. The weight loss after this process was defined as the organic content in soil. Measurement of the pH of Soil. The soil pH was measured following the wet method.35 Deionized water (pH 6.8,
