Journal of Environmental Radioactivity xxx (2014) 1e8
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Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident Yuichi Onda a, *, Hiroaki Kato a, Masaharu Hoshi b, Yoshio Takahashi c, Minh-Long Nguyen d a
Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan Research Institute for Biology and Medicine, Hiroshima University, 1-2-3 Kasumi, Minami-Ku, Hiroshima 734-8553, Japan Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan d Soil and Water Management and Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Austria b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 26 May 2014 Accepted 2 June 2014 Available online xxx
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in extensive radioactive contamination of the environment via deposited radionuclides such as radiocesium and 131I. Evaluating the extent and level of environmental contamination is critical to protecting citizens in affected areas and to planning decontamination efforts. However, a standardized soil sampling protocol is needed in such emergencies to facilitate the collection of large, tractable samples for measuring gamma-emitting radionuclides. In this study, we developed an emergency soil sampling protocol based on preliminary sampling from the FDNPP accident-affected area. We also present the results of a preliminary experiment aimed to evaluate the influence of various procedures (e.g., mixing, number of samples) on measured radioactivity. Results show that sample mixing strongly affects measured radioactivity in soil samples. Furthermore, for homogenization, shaking the plastic sample container at least 150 times or disaggregating soil by hand-rolling in a disposable plastic bag is required. Finally, we determined that five soil samples within a 3 m 3-m area are the minimum number required for reducing measurement uncertainty in the emergency soil sampling protocol proposed here. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Fallout inventory map Soil sampling protocol Gamma-ray emitting radionuclides Fukushima Dai-ichi Nuclear Power Plant accident
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred after the Great East Japan Earthquake on March 11, 2011, releasing high concentrations of radionuclides into the atmosphere (e.g., Hirose, 2012; Takemura et al., 2011). The released radionuclides, such as 137Cs, 134Cs, and 131I, were widely deposited on soil surfaces, causing high dose rates and subsequent contamination of the land. Following the accident at the FDNPP, a rapid soil survey was urgently requested by the Science Council of Japan (2011) to evaluate radionuclide contamination levels and any possible effects on
* Corresponding author. E-mail address: [email protected] (Y. Onda).
human health. The request included the collection of 1500e15,000 soil samples from within a 30 km radius of the power plant to evaluate FDNPP-derived radionuclides. To collect this number of samples, an emergency soil sampling protocol was also requested (Science Council of Japan, 2011). Several international (IAEA, 2004; ICRU, 2006; ISO, 2002, 2007), European (Khomutinin et al., 2004; Theocharopoulos et al., 2001), and Japanese (MEXT, 1983, 2011a) guidelines exist for standard environmental sampling and sample preparation protocols; however, very few standardized soil sampling protocols can deal with very large numbers of samples that can be easily processed for measuring gamma-emitting radionuclides (e.g., Onda, 2013). The difficulty in obtaining sufficient sampling equipment has also been a restriction in emergency situations. Additionally, any potential sampling protocol is required to be simple to facilitate consistency between data collectors.
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Iodine-131 is toxic to humans because it is taken up rapidly by the thyroid, delivering a relatively high-radiation dose to exposed individuals in a short period of time (Kazakov et al., 1992). Also, 131I is deposited on vegetation and thus can be transferred to humans through food such as pasture-grazing animal milk (e.g., Assimakopoulos et al., 1988; Chamberlain and Dunster, 1958). Among the fission products released from reactor accidents, 131I is one of the most hazardous. However, compared to radiocesium, little published data exist regarding direct measurements of 131I contamination (e.g., Sahoo et al., 2009), though several investigations of the much more long-lived 129I have been conducted in different environments after the Chernobyl accident (e.g., Hou et al., 2003). This lack of information is mainly due to the technical difficulties involved in measuring 131I activity soon after release into the environment because of its relatively short half-life (8 d; Orlov et al., 1996). Thus, no protocol was available for investigating soil contamination by 131I fallout immediately after the reactor accident. Therefore, we adapted a plastic cylindrical container (U-8; AS ONE, Tokyo, Japan; 50 mm inner diameter and 60 mm height), which is widely used for the measurement of gamma-rays in environmental samples in Japan, for use as a soil collector and also as a container during measurements. We then used the resulting protocol to collect 2200 soil samples (Saito et al., 2014). This paper describes the sampling protocol and analytical methods that we used to create a map of radionuclide contamination following the FDNPP accident.
2. Establishment of the emergency soil sampling protocol 2.1. A preliminary study of soil collection methods Since most of the 131I and radiocesium concentrations in the soil were contained within 5 cm of the surface (Kato et al., 2012b; Ohno et al., 2012), we assumed that the radionuclide deposition flux resided in the surface soil layer (within 5 cm of the soil surface) during the collection period (JuneeJuly 2011). Therefore, we used 100-mL U-8 containers (AS ONE) outfitted with calibration radiation sources; the samples were placed in these containers so as to homogenize the radioactive material contained in the soil. It was important not to heat soil samples containing 131I since because drying the soils could cause 131I to evaporate and disperse into the laboratory. However, air-drying would be time-consuming with such a large number of samples, and we therefore measured wet soil samples sealed in the field (e.g., Kato et al., 2012b). To evaluate the accuracy and precision of the measurement of radionuclide concentrations in soil samples with a germanium detector, we undertook a preliminary test of the following three potential methods (Fig. 1): 1) Unmixed soil (control): Soil was collected by inserting a U-8 container into the surface soil layer and left unmixed. Radioactivity concentration was then measured. 2) Stirred soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was stirred with a disposable plastic knife and vibrated 150 times (see Fig. 1) after sealing. Radioactivity concentration was then measured. 3) Homogenized soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was placed in a polyethylene bag and shaken. The soil must be loosened through pressing and crushing by hand if any aggregated soil remains after shaking. Finally, the sample was transferred back to a U-8 container for storage, and the radioactivity concentration was measured.
Fig. 1. Schematic diagram of the three sampling procedures. The terms “Control”, “Knife”, and “Bag” indicate no treatment, mixed with a knife, and mixed in a plastic bag by hand, respectively. Five soil core samples were collected from the forest floor, grassland, and paddy field on May 21, 2011.
2.2. Preliminary sampling results Fig. 2 shows 137Cs concentrations obtained using the three mixing techniques for samples collected in a paddy field in the Yamakiya region, Fukushima Prefecture (sampled on May 21, 2011). Our preliminary tests showed that soil samples collected from paddies and grasslands measured using method 1, in which the distribution of radionuclides in the sample was not uniform, produced some measurement errors arising from the application of a calibration gamma source that assumed a homogeneous distribution of radionuclides, while stirring the samples as in method 2 allowed soil to spill from the containers. Measurement variability was less than in method 1, but a scatterplot indicated that samples were still not sufficiently homogenized. Fig. 2 clearly indicates that soil mixed outside the containers, as in method 3, was in an adequately homogeneous state and radioactivity concentration measurements had little statistical scattering. Using these results, we decided to use method 3, stirring the sample in a polyethylene bag, for further soil collection. 2.3. Effects of forests on radionuclide concentrations To identify possible sampling locations, we collected soil both inside and outside a Japanese cedar forest (Fig. 3). We established a transect that crossed the boundary between the cedar forest and the grassland, and collected four core samples in each different land-use area on May 21, 2011. We also recorded the ambient equivalent dose rate level at a height of 1 m above the ground using a CsI(TI) scintillator (PA-1000; Horiba, Tokyo, Japan). The 137Cs inventory outside the forest was significantly higher than inside the forest (Fig. 4a), but the 131I inventory did not differ between areas (Fig. 4b). These data strongly suggest that a significant amount of 137 Cs was intercepted by the tree canopy (e.g., Bunzl et al., 1989; Hoffman et al., 1995; Kato et al., 2012a; Kinnersley et al., 1997), but that 131I was quickly removed from the canopy by water movement (e.g., Kato et al., 2012a). While the dose rate at a height of 1 m was significantly higher outside than inside the forest (Fig. 4), Fig. 5 indicates that forest soil accumulated much less atmospherically deposited radiocesium. In contrast, the 131I inventory differed slightly between forest and grassland soils, and forest soils showed significant variability. Although we cannot discuss the results based on statistical error because of the small sampling number, these results clearly indicate that initial emergency samples should not be collected inside forests.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 2. Effect of mixing method on the measured radioactivity in soil samples (paddy field). Refer to Fig. 1 for each treatment. Error bars denote the range of measured radioactivity in the five soil samples. Soil core samples were collected on May 21, 2011.
2.4. Selection of soil collection locations To monitor long-term changes in the fallout inventory of radioactive material, collecting multiple samples from the same locations is vital. Therefore, we selected locations with no anticipated disturbances. We attempted to avoid sampling points with vegetation, but this was not always possible, and some soil core samples were taken together with aboveground vegetation. Because we anticipated variation in soil radioactivity concentrations, we collected five soil samples from each study site, as much as possible within a range of 3 m. Where sampling locations fell inside high-radiation dose-rate areas (e.g., evacuation zones), only one to three samples were taken to avoid prolonged exposure of sampling workers to radiation.
3. Protocol for assessing radionuclide contamination in soil samples 3.1. General information Guidelines for soil sampling preparation and for the recording of basic information are listed and described below. 1) Select sampling locations and points: Flat topography is preferable to minimize the effects of the redistribution of radionuclides. Verify that the terrain is flat and that no large obstacles (such as vehicles or buildings) exist within a 5 m range of the sampling location. Open areas, such as croplands or paddy fields, are preferred, and forested areas should be avoided as
Fig. 3. Soil samples were collected along a 40-m transect extending from pastureland to Japanese cedar forest. The bag-mixing method was used for soil sampling. Soil core samples were collected on May 21, 2011.
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Fig. 4. Relationships between distance from the pastureeforest boundary and (a)
most of the fallout is trapped in tree canopies. Paddy fields can be used before irrigating (Takahashi et al., 2014). 2) Soil and land cover maps are useful for designing a sampling strategy (location and density of sampling points) since soil types and land uses can influence the extent of radioactive contamination in soils. 3) The geographic position of each sampling point within each location should be recorded using a global positioning system (GPS). 4) Use protective clothing and gloves for handling soil samples.
3.2. Soil sampling and analysis Detailed procedures for soil sampling and laboratory analysis are listed below. Disposable gloves should be used to avoid cross-contamination. 1) Five soil samples, 5 cm deep, should be collected within a 3 3m2 area at the selected sampling location. Ideally, we recommend the four corners and center of the square as sampling points. The depth is recommended because most of the recent and current radioactive contamination remains in this layer (Kato et al., 2012b; Ohno et al., 2012). 2) The total area of soil sampled (98 cm2) when taking five samples is sufficiently larger than the 50-cm2 sampling area suggested by Khomutinin et al. (2004). 3) Measure the ambient equivalent dose rate (mSv/h) using a portable dosimeter at a height of 1 m. At all locations, slowly move the survey meter 3 m in all directions from the center to confirm the absence of any singular points with sudden spikes in air-dose rates.
137
Cs and (b)
131
I inventories in soil samples.
4) Try to locate sites with open space, and if possible, areas not covered by vegetation. Do not remove small fragments of leaves and organic layers because they may contain 137Cs and 134Cs. In such cases, litter or organic matter should be collected with soil. 5) Soil samples can be collected using one of two methods: 1) Using a measurement container, for emergency purposes or soft soil (Fig. 6): Each U-8 measurement container should be weighed before sampling and clearly marked. Insert the U-8 container gently into the soil and use it as a scoop. Cut the surface with a disposable plastic knife and mix well in the plastic bag before sealing. 2) Using a core sampler for hard soil (Fig. 7): Use a core sampler 50 mm in diameter or larger to a depth of 50 mm. The samples should be placed in plastic bags, and then mixed well by shaking the outside of the plastic bag and packed into U-8 containers (when using a 100-mL core sampler). Metal samplers can be used at the same sampling site after cleaning with alcohol in situ, but never use the same sampler for different locations to prevent crosscontamination. Samplers should be cleaned after returning to the lab. 3) Soil water content may be measured in the field using a portable time-domain reflectometer (TDR). This procedure is optional if oven-drying is impossible due to the need to avoid iodine sublimation at higher temperatures. 4) Because of the possible spatial variability, all five soil samples should be measured. All sampling containers should be properly labeled with weight, soil depth, GPS reference number, and land-use type, and hermetically sealed. Wipe the outside of the container with alcohol-impregnated tissue paper to decontaminate and take a photograph to distinguish the soil color and type.
Fig. 5. Comparison of 134Cs, 137Cs, and 131I inventories in surface soil. The number in the figure represents the mean inventory for pasture and forest soils. The box bar denotes the value of the first and third quartile, whereas the whisker bar indicates the maximum and minimum values of the measured inventories.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 6. Emergency procedure for the investigation of radioactive contamination of the soil using the U-8 container.
Fig. 7. Emergency procedure for the investigation of radioactive contamination of compacted soil using the 100-mL soil core sampler.
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5) Each sample should be placed inside a new plastic bag and zip-locked by a person who has not touched the soil (expected to be uncontaminated). The five samples from each location should also be zip-locked in a larger bag and transported to a laboratory in secured containers labeled with radioactive signs. 6) The national radiation safety regulations should be observed at all times (i.e., do not exceed 5 mSv/h at the surface of the transportation container as proposed in the Japanese L package standard). 7) In the laboratory, information regarding GPS coordinates (region, latitude, and longitude), land use, soil types, digital photographs, sampling dates, and other relevant comments should be entered into a computer database. 8) The U-8 soil containers should be sealed again in zip-lock bags or plastic film by two persons (one person to touch the container, and another to cover the container without touching its surface), to avoid contamination of the Gedetector. 9) The bulk density should be calculated using net sample weights and field soil moisture contents. If oven-drying is possible (i.e., 131I level is low), soil can be dried and the bulk density can be calculated. 10) To convert the amount of radioactive contamination per kilogram of soil to the amount of radioactive contamination per 1 m2 of land (Bq/m2), average the radioactive contamination (Bq/kg soil) and the bulk density values of the five subsamples. 11) Before measurement, the sample container should again be shaken well to mix the large amounts of 137Cs in the surface soil. Fig. 8 shows the relationship between the number of times a sample was shaken and the measured concentration of radiocesium and 131I. The shaking treatment was performed reciprocally by several operators to correct individual differences. Results indicate that shaking samples at least 150 times is necessary to homogenize soil materials in the containers. 12) Given the expected high concentrations of radionuclides, the counting time will be limited by the counting statistics error of 137Cs, 134Cs, and 131I, which should be a maximum of 5% (ideally 3%). 13) In some locations, to confirm the validity of sampling to depth of 5 cm, incremental scraper plate sampling (Zapata, 2003; unit depths of 5 mm to 1 cm) should be conducted to identify the profile distribution of the radionuclides.
3.3. Results of preliminary sampling following the FDNPP accident Fig. 9 shows the spatial pattern of the 137Cs inventory after the FDNPP accident, based on the emergency soil sampling protocol proposed in this study. Five soil samples (No. 1eNo. 5) were collected within a 3 m 3-m area at each sampling site (56 sites in total). The influence of the number and combination of samples at each site in the 137Cs inventory mapping results are discussed below. Our map shows unfavorable variation in the 137Cs inventory when a single soil sample is selected from the five. In maps based on soil samples No. 1 and No. 3, an area with a relatively high 137Cs inventory, appearing northwest of the reactor in the soil sample No. 5 map, is missing. Using multiple samples reduces the differences in 137Cs inventory patterns among maps, but large inconsistencies still exist among maps based on combinations of three regularly selected soil samples. Finally, the 137Cs inventories of the five soil samples were averaged and used to produce an inventory map. The
Fig. 8. Effect of shaking on the measured concentrations of radiocesium and 131I. Soil sampling was conducted on an artificial hillslope at the Terrestrial Environmental Research Center of the University of Tsukuba, Tsukuba, Ibaraki, Japan, on April 16, 2011.
spotlike distribution of the 137Cs inventory was averaged when the five samples were combined, and the resulting map is largely consistent with the results of the Third Airborne Monitoring Survey of Radioactivity (MEXT, 2011b). The results of this study indicate that collecting and combining at least five soil samples within a 3 m 3-m area is the minimum number required to produce a precise fallout inventory map of radionuclides from the FDNPP accident. The emergency sampling protocol proposed in this study should be considered in case of emergency situations following nuclear hazards.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 9. Influence of the number of samples at each sampling site on the
4. Conclusions In this study, we developed an emergency soil sampling protocol based on preliminary sampling from the FDNPP accidentaffected area. We also presented the results of preliminary experiments testing how different soil sampling methods (e.g., mixing, number of samples) affect measured radioactivity. The results of this study demonstrate that sample mixing strongly influences the measured radioactivity in soil samples. For homogenization, soil samples in U-8 containers must be shaken at least 150 times or hand-rolled in a disposable plastic bag to disaggregate soil. In addition, five soil samples within a 3 m 3-m area is the minimum
7
137
Cs inventory mapping results.
number necessary to reduce measurement uncertainty for the emergency soil sampling protocol proposed in this study. Acknowledgments This project was financially supported by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). We would like to express our thanks to everyone who directly and indirectly supported the project, without whom this large-scale project would have been impossible to achieve within such a short time following the Fukushima Dai-ichi Nuclear disaster. We thank Dr. Nakamura and all the members of the Committee on the
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Construction of Maps for Radiation Dose Distribution for their helpful and encouraging comments. References Assimakopoulos, P.A., Ioannides, K.G., Pakou, A.A., 1988. The environmental behavior of 131I in northwestern Greece following the nuclear reactor accident at Chernobyl. Health Phys. 55 (5), 783e791. Bunzl, K., Schimmack, W., Kreutzer, K., Schierl, R., 1989. Interception and retention of Chernobyl-derived 134Cs, 137Cs and 106Ru in a spruce stand. Sci. Total Environ. 78, 77e87. Chamberlain, A.C., Dunster, H.J., 1958. Deposition of radioactivity in north-west England from the accident at Windscale. Nature 182, 629e630. Hirose, K., 2012. Fukushima Daiichi nuclear power plant accident: summary of regional radioactive deposition monitoring results. J. Environ. Radioact. 111, 13e17. Hoffman, F.O., Thiessen, K.M., Rael, R.M., 1995. Comparison of interception and initial retention of wet-deposited contaminants on leaves of different vegetation types. Atmos. Environ. 29 (15), 1771e1775. Hou, X.L., Fogh, C.L., Kucera, J., Andersson, K.G., Dahlgaard, H., Nelsen, S.P., 2003. Iodine-129 and Caesium-137 in Chernobyl contaminated soil and their chemical fractionation. Sci. Total Environ. 308, 97e109. IAEA (International Atomic Agency), 2004. Soil Sampling for Environmental Contaminants. IAEA-TECDOC, 1415, p. 76. ICRU (International Commission on Radiation Units and Measurements), 2006. Sampling for radionuclides in the environment. J. ICRU 6. http://dx.doi.org/ 10.1093/jicru/ndl001. Report 75. ISO (International Organization for Standardization), 2002. Soil Quality e Sampling e Part 1: Guidance on the Design of Sampling Programs. ISO 10381-1. ISO (International Organization for Standardization), 2007. Measurement of Radioactivity in the Environment e Soil e Part 2: Guidance for the Selection of the Sampling Strategy, Sampling and Pre-treatment of Samples. ISO 18589-2. Kato, H., Onda, Y., Gomi, T., 2012a. Interception of the Fukushima reactor accidentderived 137Cs, 134Cs, and 131I by coniferous forest canopies. Geophys. Res. Lett. 39, L20403. http://dx.doi.org/10.1029/2012GL052928. Kato, H., Onda, Y., Teramage, M., 2012b. Depth distribution of 137Cs, 134Cs, and 131I in soil profile after Fukushima Daiichi nuclear power plant accident. J. Environ. Radioact. 111, 59e64. Kazakov, V.S., Demidchik, E.P., Astakhova, L.N., 1992. Thyroid cancer after Chernobyl. Nature 359, 21. Khomutinin, Y.V., Kashparov, V.A., Zhebrovska, K.I., 2004. Sampling Optimization when Radioecological Monitoring. UIAR, ISBN 966-646-034-3, p. 133. Kinnersley, R.P., Goddard, A.J.H., Minski, M.J., Shaw, G., 1997. Interception of caesium-contaminated rain by vegetation. Atmos. Environ. 31 (8), 1137e1145. MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan), 1983. Sampling methods for environmental materials. In: Methods for Radioactivity Analysis. http://www.kankyo-hoshano.go.jp/series/lib/No16.pdf.
MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan), 2011a. Guidance on the Measurement of Dose Rate and Soil Sampling Protocol. (accessed 07.01.2014) (in Japanese). MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan), 2011b. Results of the Third Monitoring Survey by MEXT. (accessed 07.01.2014). Ohno, T., Muramatsu, Y., Miura, Y., Oda, K., Inagawa, N., Ogawa, H., Yamazaki, A., Toyama, C., Sato, M., 2012. Depth profiles of radioactive cesium and iodine released from the Fukushima Daiichi nuclear power plant in different agricultural fields and forests. Geochem. J. 46, 287e295. Onda, Y., 2013. Establishment of the sampling protocol. Radioisotopes 62, 767e773 (in Japanese). Orlov, M.Y., Snykov, V.P., Khvalenskii, Y.A., Volokitin, A.A., 1996. Contamination of the soil of the European part of the territory of the USSR with 131I after the Chernobyl nuclear accident. Atom. Energy 80 (6), 439e444. Sahoo, S.K., Muramatsu, Y., Yoshida, S., Matsuzaki, H., Ruhm, W., 2009. Determination of 129I and 127I concentration in soil samples from the Chernobyl 30-km zone by AMS and ICP-MS. J. Radiat. Res. 50, 325e332. 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., 2014. Detailed deposition density maps by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Daiichi nuclear power plant accident. J. Environ. Radioact. http://dx.doi.org/ 10.1016/j.jenvrad.2014.02.014 (in this issue). Science Council of Japan Great East Japan Earthquake Task Force, 2011. Regarding the Necessity of the Investigation of Radiation Levels after the Accident of the Fukushima Daiichi Nuclear Power Plant. The 2nd emergency recommendation regarding the response to the Great East Japan Earthquake. (accessed 07.01.2014). Takahashi, J., Tamura, K., Suda, T., Matsumura, R., Onda, Y., 2014. Vertical distribution and temporal changes of 137Cs in soil profiles under various land uses after Fukushima Dai-ichi nuclear power plant accident. J. Environ. Radioact. (in this issue). Takemura, T., Nakamura, H., Takigawa, M., Kondo, H., Satomura, T., Miyasaka, T., Nakajima, T., 2011. A numerical simulation of global transport of atmospheric particles emitted from the Fukushima Daiichi nuclear power plant. SOLA 7, 101e104. Theocharopoulos, S.P., Wagnerb, G., Sprengartb, J., Mohrb, M.-E., Desaulesc, A., Muntaud, H., Christoua, M., Quevauvillere, P., 2001. European soil sampling guidelines for soil pollution studies. Sci. Total Environ. 264, 51e62. Zapata, F. (Ed.), 2003. Handbook for the Assessment of Soil Erosion and Sedimentation Using Environmental Radionuclides. Kluwer, Dordrecht, The Netherlands, p. 219.
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Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident Yuichi Onda a, *, Hiroaki Kato a, Masaharu Hoshi b, Yoshio Takahashi c, Minh-Long Nguyen d a
Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan Research Institute for Biology and Medicine, Hiroshima University, 1-2-3 Kasumi, Minami-Ku, Hiroshima 734-8553, Japan Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan d Soil and Water Management and Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Austria b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 26 May 2014 Accepted 2 June 2014 Available online xxx
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in extensive radioactive contamination of the environment via deposited radionuclides such as radiocesium and 131I. Evaluating the extent and level of environmental contamination is critical to protecting citizens in affected areas and to planning decontamination efforts. However, a standardized soil sampling protocol is needed in such emergencies to facilitate the collection of large, tractable samples for measuring gamma-emitting radionuclides. In this study, we developed an emergency soil sampling protocol based on preliminary sampling from the FDNPP accident-affected area. We also present the results of a preliminary experiment aimed to evaluate the influence of various procedures (e.g., mixing, number of samples) on measured radioactivity. Results show that sample mixing strongly affects measured radioactivity in soil samples. Furthermore, for homogenization, shaking the plastic sample container at least 150 times or disaggregating soil by hand-rolling in a disposable plastic bag is required. Finally, we determined that five soil samples within a 3 m 3-m area are the minimum number required for reducing measurement uncertainty in the emergency soil sampling protocol proposed here. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Fallout inventory map Soil sampling protocol Gamma-ray emitting radionuclides Fukushima Dai-ichi Nuclear Power Plant accident
1. Introduction The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred after the Great East Japan Earthquake on March 11, 2011, releasing high concentrations of radionuclides into the atmosphere (e.g., Hirose, 2012; Takemura et al., 2011). The released radionuclides, such as 137Cs, 134Cs, and 131I, were widely deposited on soil surfaces, causing high dose rates and subsequent contamination of the land. Following the accident at the FDNPP, a rapid soil survey was urgently requested by the Science Council of Japan (2011) to evaluate radionuclide contamination levels and any possible effects on
* Corresponding author. E-mail address: [email protected] (Y. Onda).
human health. The request included the collection of 1500e15,000 soil samples from within a 30 km radius of the power plant to evaluate FDNPP-derived radionuclides. To collect this number of samples, an emergency soil sampling protocol was also requested (Science Council of Japan, 2011). Several international (IAEA, 2004; ICRU, 2006; ISO, 2002, 2007), European (Khomutinin et al., 2004; Theocharopoulos et al., 2001), and Japanese (MEXT, 1983, 2011a) guidelines exist for standard environmental sampling and sample preparation protocols; however, very few standardized soil sampling protocols can deal with very large numbers of samples that can be easily processed for measuring gamma-emitting radionuclides (e.g., Onda, 2013). The difficulty in obtaining sufficient sampling equipment has also been a restriction in emergency situations. Additionally, any potential sampling protocol is required to be simple to facilitate consistency between data collectors.
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Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Iodine-131 is toxic to humans because it is taken up rapidly by the thyroid, delivering a relatively high-radiation dose to exposed individuals in a short period of time (Kazakov et al., 1992). Also, 131I is deposited on vegetation and thus can be transferred to humans through food such as pasture-grazing animal milk (e.g., Assimakopoulos et al., 1988; Chamberlain and Dunster, 1958). Among the fission products released from reactor accidents, 131I is one of the most hazardous. However, compared to radiocesium, little published data exist regarding direct measurements of 131I contamination (e.g., Sahoo et al., 2009), though several investigations of the much more long-lived 129I have been conducted in different environments after the Chernobyl accident (e.g., Hou et al., 2003). This lack of information is mainly due to the technical difficulties involved in measuring 131I activity soon after release into the environment because of its relatively short half-life (8 d; Orlov et al., 1996). Thus, no protocol was available for investigating soil contamination by 131I fallout immediately after the reactor accident. Therefore, we adapted a plastic cylindrical container (U-8; AS ONE, Tokyo, Japan; 50 mm inner diameter and 60 mm height), which is widely used for the measurement of gamma-rays in environmental samples in Japan, for use as a soil collector and also as a container during measurements. We then used the resulting protocol to collect 2200 soil samples (Saito et al., 2014). This paper describes the sampling protocol and analytical methods that we used to create a map of radionuclide contamination following the FDNPP accident.
2. Establishment of the emergency soil sampling protocol 2.1. A preliminary study of soil collection methods Since most of the 131I and radiocesium concentrations in the soil were contained within 5 cm of the surface (Kato et al., 2012b; Ohno et al., 2012), we assumed that the radionuclide deposition flux resided in the surface soil layer (within 5 cm of the soil surface) during the collection period (JuneeJuly 2011). Therefore, we used 100-mL U-8 containers (AS ONE) outfitted with calibration radiation sources; the samples were placed in these containers so as to homogenize the radioactive material contained in the soil. It was important not to heat soil samples containing 131I since because drying the soils could cause 131I to evaporate and disperse into the laboratory. However, air-drying would be time-consuming with such a large number of samples, and we therefore measured wet soil samples sealed in the field (e.g., Kato et al., 2012b). To evaluate the accuracy and precision of the measurement of radionuclide concentrations in soil samples with a germanium detector, we undertook a preliminary test of the following three potential methods (Fig. 1): 1) Unmixed soil (control): Soil was collected by inserting a U-8 container into the surface soil layer and left unmixed. Radioactivity concentration was then measured. 2) Stirred soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was stirred with a disposable plastic knife and vibrated 150 times (see Fig. 1) after sealing. Radioactivity concentration was then measured. 3) Homogenized soil: Soil was collected by inserting a U-8 container into the surface soil layer, after which it was placed in a polyethylene bag and shaken. The soil must be loosened through pressing and crushing by hand if any aggregated soil remains after shaking. Finally, the sample was transferred back to a U-8 container for storage, and the radioactivity concentration was measured.
Fig. 1. Schematic diagram of the three sampling procedures. The terms “Control”, “Knife”, and “Bag” indicate no treatment, mixed with a knife, and mixed in a plastic bag by hand, respectively. Five soil core samples were collected from the forest floor, grassland, and paddy field on May 21, 2011.
2.2. Preliminary sampling results Fig. 2 shows 137Cs concentrations obtained using the three mixing techniques for samples collected in a paddy field in the Yamakiya region, Fukushima Prefecture (sampled on May 21, 2011). Our preliminary tests showed that soil samples collected from paddies and grasslands measured using method 1, in which the distribution of radionuclides in the sample was not uniform, produced some measurement errors arising from the application of a calibration gamma source that assumed a homogeneous distribution of radionuclides, while stirring the samples as in method 2 allowed soil to spill from the containers. Measurement variability was less than in method 1, but a scatterplot indicated that samples were still not sufficiently homogenized. Fig. 2 clearly indicates that soil mixed outside the containers, as in method 3, was in an adequately homogeneous state and radioactivity concentration measurements had little statistical scattering. Using these results, we decided to use method 3, stirring the sample in a polyethylene bag, for further soil collection. 2.3. Effects of forests on radionuclide concentrations To identify possible sampling locations, we collected soil both inside and outside a Japanese cedar forest (Fig. 3). We established a transect that crossed the boundary between the cedar forest and the grassland, and collected four core samples in each different land-use area on May 21, 2011. We also recorded the ambient equivalent dose rate level at a height of 1 m above the ground using a CsI(TI) scintillator (PA-1000; Horiba, Tokyo, Japan). The 137Cs inventory outside the forest was significantly higher than inside the forest (Fig. 4a), but the 131I inventory did not differ between areas (Fig. 4b). These data strongly suggest that a significant amount of 137 Cs was intercepted by the tree canopy (e.g., Bunzl et al., 1989; Hoffman et al., 1995; Kato et al., 2012a; Kinnersley et al., 1997), but that 131I was quickly removed from the canopy by water movement (e.g., Kato et al., 2012a). While the dose rate at a height of 1 m was significantly higher outside than inside the forest (Fig. 4), Fig. 5 indicates that forest soil accumulated much less atmospherically deposited radiocesium. In contrast, the 131I inventory differed slightly between forest and grassland soils, and forest soils showed significant variability. Although we cannot discuss the results based on statistical error because of the small sampling number, these results clearly indicate that initial emergency samples should not be collected inside forests.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 2. Effect of mixing method on the measured radioactivity in soil samples (paddy field). Refer to Fig. 1 for each treatment. Error bars denote the range of measured radioactivity in the five soil samples. Soil core samples were collected on May 21, 2011.
2.4. Selection of soil collection locations To monitor long-term changes in the fallout inventory of radioactive material, collecting multiple samples from the same locations is vital. Therefore, we selected locations with no anticipated disturbances. We attempted to avoid sampling points with vegetation, but this was not always possible, and some soil core samples were taken together with aboveground vegetation. Because we anticipated variation in soil radioactivity concentrations, we collected five soil samples from each study site, as much as possible within a range of 3 m. Where sampling locations fell inside high-radiation dose-rate areas (e.g., evacuation zones), only one to three samples were taken to avoid prolonged exposure of sampling workers to radiation.
3. Protocol for assessing radionuclide contamination in soil samples 3.1. General information Guidelines for soil sampling preparation and for the recording of basic information are listed and described below. 1) Select sampling locations and points: Flat topography is preferable to minimize the effects of the redistribution of radionuclides. Verify that the terrain is flat and that no large obstacles (such as vehicles or buildings) exist within a 5 m range of the sampling location. Open areas, such as croplands or paddy fields, are preferred, and forested areas should be avoided as
Fig. 3. Soil samples were collected along a 40-m transect extending from pastureland to Japanese cedar forest. The bag-mixing method was used for soil sampling. Soil core samples were collected on May 21, 2011.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 4. Relationships between distance from the pastureeforest boundary and (a)
most of the fallout is trapped in tree canopies. Paddy fields can be used before irrigating (Takahashi et al., 2014). 2) Soil and land cover maps are useful for designing a sampling strategy (location and density of sampling points) since soil types and land uses can influence the extent of radioactive contamination in soils. 3) The geographic position of each sampling point within each location should be recorded using a global positioning system (GPS). 4) Use protective clothing and gloves for handling soil samples.
3.2. Soil sampling and analysis Detailed procedures for soil sampling and laboratory analysis are listed below. Disposable gloves should be used to avoid cross-contamination. 1) Five soil samples, 5 cm deep, should be collected within a 3 3m2 area at the selected sampling location. Ideally, we recommend the four corners and center of the square as sampling points. The depth is recommended because most of the recent and current radioactive contamination remains in this layer (Kato et al., 2012b; Ohno et al., 2012). 2) The total area of soil sampled (98 cm2) when taking five samples is sufficiently larger than the 50-cm2 sampling area suggested by Khomutinin et al. (2004). 3) Measure the ambient equivalent dose rate (mSv/h) using a portable dosimeter at a height of 1 m. At all locations, slowly move the survey meter 3 m in all directions from the center to confirm the absence of any singular points with sudden spikes in air-dose rates.
137
Cs and (b)
131
I inventories in soil samples.
4) Try to locate sites with open space, and if possible, areas not covered by vegetation. Do not remove small fragments of leaves and organic layers because they may contain 137Cs and 134Cs. In such cases, litter or organic matter should be collected with soil. 5) Soil samples can be collected using one of two methods: 1) Using a measurement container, for emergency purposes or soft soil (Fig. 6): Each U-8 measurement container should be weighed before sampling and clearly marked. Insert the U-8 container gently into the soil and use it as a scoop. Cut the surface with a disposable plastic knife and mix well in the plastic bag before sealing. 2) Using a core sampler for hard soil (Fig. 7): Use a core sampler 50 mm in diameter or larger to a depth of 50 mm. The samples should be placed in plastic bags, and then mixed well by shaking the outside of the plastic bag and packed into U-8 containers (when using a 100-mL core sampler). Metal samplers can be used at the same sampling site after cleaning with alcohol in situ, but never use the same sampler for different locations to prevent crosscontamination. Samplers should be cleaned after returning to the lab. 3) Soil water content may be measured in the field using a portable time-domain reflectometer (TDR). This procedure is optional if oven-drying is impossible due to the need to avoid iodine sublimation at higher temperatures. 4) Because of the possible spatial variability, all five soil samples should be measured. All sampling containers should be properly labeled with weight, soil depth, GPS reference number, and land-use type, and hermetically sealed. Wipe the outside of the container with alcohol-impregnated tissue paper to decontaminate and take a photograph to distinguish the soil color and type.
Fig. 5. Comparison of 134Cs, 137Cs, and 131I inventories in surface soil. The number in the figure represents the mean inventory for pasture and forest soils. The box bar denotes the value of the first and third quartile, whereas the whisker bar indicates the maximum and minimum values of the measured inventories.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 6. Emergency procedure for the investigation of radioactive contamination of the soil using the U-8 container.
Fig. 7. Emergency procedure for the investigation of radioactive contamination of compacted soil using the 100-mL soil core sampler.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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5) Each sample should be placed inside a new plastic bag and zip-locked by a person who has not touched the soil (expected to be uncontaminated). The five samples from each location should also be zip-locked in a larger bag and transported to a laboratory in secured containers labeled with radioactive signs. 6) The national radiation safety regulations should be observed at all times (i.e., do not exceed 5 mSv/h at the surface of the transportation container as proposed in the Japanese L package standard). 7) In the laboratory, information regarding GPS coordinates (region, latitude, and longitude), land use, soil types, digital photographs, sampling dates, and other relevant comments should be entered into a computer database. 8) The U-8 soil containers should be sealed again in zip-lock bags or plastic film by two persons (one person to touch the container, and another to cover the container without touching its surface), to avoid contamination of the Gedetector. 9) The bulk density should be calculated using net sample weights and field soil moisture contents. If oven-drying is possible (i.e., 131I level is low), soil can be dried and the bulk density can be calculated. 10) To convert the amount of radioactive contamination per kilogram of soil to the amount of radioactive contamination per 1 m2 of land (Bq/m2), average the radioactive contamination (Bq/kg soil) and the bulk density values of the five subsamples. 11) Before measurement, the sample container should again be shaken well to mix the large amounts of 137Cs in the surface soil. Fig. 8 shows the relationship between the number of times a sample was shaken and the measured concentration of radiocesium and 131I. The shaking treatment was performed reciprocally by several operators to correct individual differences. Results indicate that shaking samples at least 150 times is necessary to homogenize soil materials in the containers. 12) Given the expected high concentrations of radionuclides, the counting time will be limited by the counting statistics error of 137Cs, 134Cs, and 131I, which should be a maximum of 5% (ideally 3%). 13) In some locations, to confirm the validity of sampling to depth of 5 cm, incremental scraper plate sampling (Zapata, 2003; unit depths of 5 mm to 1 cm) should be conducted to identify the profile distribution of the radionuclides.
3.3. Results of preliminary sampling following the FDNPP accident Fig. 9 shows the spatial pattern of the 137Cs inventory after the FDNPP accident, based on the emergency soil sampling protocol proposed in this study. Five soil samples (No. 1eNo. 5) were collected within a 3 m 3-m area at each sampling site (56 sites in total). The influence of the number and combination of samples at each site in the 137Cs inventory mapping results are discussed below. Our map shows unfavorable variation in the 137Cs inventory when a single soil sample is selected from the five. In maps based on soil samples No. 1 and No. 3, an area with a relatively high 137Cs inventory, appearing northwest of the reactor in the soil sample No. 5 map, is missing. Using multiple samples reduces the differences in 137Cs inventory patterns among maps, but large inconsistencies still exist among maps based on combinations of three regularly selected soil samples. Finally, the 137Cs inventories of the five soil samples were averaged and used to produce an inventory map. The
Fig. 8. Effect of shaking on the measured concentrations of radiocesium and 131I. Soil sampling was conducted on an artificial hillslope at the Terrestrial Environmental Research Center of the University of Tsukuba, Tsukuba, Ibaraki, Japan, on April 16, 2011.
spotlike distribution of the 137Cs inventory was averaged when the five samples were combined, and the resulting map is largely consistent with the results of the Third Airborne Monitoring Survey of Radioactivity (MEXT, 2011b). The results of this study indicate that collecting and combining at least five soil samples within a 3 m 3-m area is the minimum number required to produce a precise fallout inventory map of radionuclides from the FDNPP accident. The emergency sampling protocol proposed in this study should be considered in case of emergency situations following nuclear hazards.
Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
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Fig. 9. Influence of the number of samples at each sampling site on the
4. Conclusions In this study, we developed an emergency soil sampling protocol based on preliminary sampling from the FDNPP accidentaffected area. We also presented the results of preliminary experiments testing how different soil sampling methods (e.g., mixing, number of samples) affect measured radioactivity. The results of this study demonstrate that sample mixing strongly influences the measured radioactivity in soil samples. For homogenization, soil samples in U-8 containers must be shaken at least 150 times or hand-rolled in a disposable plastic bag to disaggregate soil. In addition, five soil samples within a 3 m 3-m area is the minimum
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Cs inventory mapping results.
number necessary to reduce measurement uncertainty for the emergency soil sampling protocol proposed in this study. Acknowledgments This project was financially supported by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). We would like to express our thanks to everyone who directly and indirectly supported the project, without whom this large-scale project would have been impossible to achieve within such a short time following the Fukushima Dai-ichi Nuclear disaster. We thank Dr. Nakamura and all the members of the Committee on the
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Please cite this article in press as: Onda, Y., et al., Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/ j.jenvrad.2014.06.002
