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Journal of Environmental Radioactivity xxx (2014) 1e8

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Soil sampling and analytical strategies for mapping fallout in nuclearemergencies based on the Fukushima Dai-ichi Nuclear Power Plantaccident

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, Japanb Research Institute for Biology and Medicine, Hiroshima University, 1-2-3 Kasumi, Minami-Ku, Hiroshima 734-8553, Japanc Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima739-8526, Japand Soil and Water Management and Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of NuclearSciences and Applications, International Atomic Energy Agency, Austria

a r t i c l e i n f o

Article history:Received 30 January 2014Received in revised form26 May 2014Accepted 2 June 2014Available online xxx

Keywords:Fallout inventory mapSoil sampling protocolGamma-ray emitting radionuclidesFukushima Dai-ichi Nuclear Power Plantaccident

* Corresponding author.E-mail address: onda@geoenv.tsukuba.ac.jp (Y. Ond

http://dx.doi.org/10.1016/j.jenvrad.2014.06.0020265-931X/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Onda, Y., etFukushima Dai-ichi Nuclear Power Plj.jenvrad.2014.06.002

a b s t r a c t

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in extensive radioactivecontamination of the environment via deposited radionuclides such as radiocesium and 131I. Evaluatingthe extent and level of environmental contamination is critical to protecting citizens in affected areas andto planning decontamination efforts. However, a standardized soil sampling protocol is needed in suchemergencies to facilitate the collection of large, tractable samples for measuring gamma-emitting ra-dionuclides. In this study, we developed an emergency soil sampling protocol based on preliminarysampling from the FDNPP accident-affected area. We also present the results of a preliminary experimentaimed to evaluate the influence of various procedures (e.g., mixing, number of samples) on measuredradioactivity. 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 dis-aggregating soil by hand-rolling in a disposable plastic bag is required. Finally, we determined that fivesoil samples within a 3 m � 3-m area are the minimum number required for reducing measurementuncertainty in the emergency soil sampling protocol proposed here.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accidentoccurred 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 radionu-clides, such as 137Cs, 134Cs, and 131I, were widely deposited on soilsurfaces, causing high dose rates and subsequent contamination ofthe land.

Following the accident at the FDNPP, a rapid soil survey wasurgently requested by the Science Council of Japan (2011) to eval-uate radionuclide contamination levels and any possible effects on

a).

al., Soil sampling and analytiant accident, Journal of

human health. The request included the collection of 1500e15,000soil samples from within a 30 km radius of the power plant toevaluate FDNPP-derived radionuclides. To collect this number ofsamples, 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 standardenvironmental sampling and sample preparation protocols; how-ever, very few standardized soil sampling protocols can deal withvery large numbers of samples that can be easily processed formeasuring gamma-emitting radionuclides (e.g., Onda, 2013). Thedifficulty in obtaining sufficient sampling equipment has also beena restriction in emergency situations. Additionally, any potentialsampling protocol is required to be simple to facilitate consistencybetween data collectors.

cal strategies for mapping fallout in nuclear emergencies based on theEnvironmental Radioactivity (2014), http://dx.doi.org/10.1016/

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 bagby hand, respectively. Five soil core samples were collected from the forest floor,grassland, and paddy field on May 21, 2011.

Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e82

Iodine-131 is toxic to humans because it is taken up rapidly bythe thyroid, delivering a relatively high-radiation dose to exposedindividuals in a short period of time (Kazakov et al., 1992). Also, 131Iis deposited on vegetation and thus can be transferred to humansthrough 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 isone of the most hazardous. However, compared to radiocesium,little published data exist regarding direct measurements of 131Icontamination (e.g., Sahoo et al., 2009), though several in-vestigations of the much more long-lived 129I have been conductedin different environments after the Chernobyl accident (e.g., Houet al., 2003). This lack of information is mainly due to the tech-nical difficulties involved in measuring 131I activity soon afterrelease into the environment because of its relatively short half-life(8 d; Orlov et al., 1996). Thus, no protocol was available for inves-tigating soil contamination by 131I fallout immediately after thereactor accident.

Therefore, we adapted a plastic cylindrical container (U-8; ASONE, Tokyo, Japan; 50 mm inner diameter and 60 mm height),which is widely used for the measurement of gamma-rays inenvironmental samples in Japan, for use as a soil collector and alsoas a container during measurements. We then used the resultingprotocol to collect 2200 soil samples (Saito et al., 2014). This paperdescribes the sampling protocol and analytical methods that weused to create a map of radionuclide contamination following theFDNPP 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 soilwere contained within 5 cm of the surface (Kato et al., 2012b; Ohnoet al., 2012), we assumed that the radionuclide deposition fluxresided in the surface soil layer (within 5 cm of the soil surface)during the collection period (JuneeJuly 2011). Therefore, we used100-mL U-8 containers (AS ONE) outfitted with calibration radia-tion sources; the samples were placed in these containers so as tohomogenize the radioactive material contained in the soil. It wasimportant not to heat soil samples containing 131I since becausedrying the soils could cause 131I to evaporate and disperse into thelaboratory. However, air-drying would be time-consuming withsuch a large number of samples, and we therefore measured wetsoil samples sealed in the field (e.g., Kato et al., 2012b). To evaluatethe accuracy and precision of the measurement of radionuclideconcentrations in soil samples with a germanium detector, weundertook a preliminary test of the following three potentialmethods (Fig. 1):

1) Unmixed soil (control): Soil was collected by inserting a U-8container into the surface soil layer and left unmixed. Radioac-tivity concentration was then measured.

2) Stirred soil: Soil was collected by inserting a U-8 container intothe surface soil layer, after which it was stirredwith a disposableplastic 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-8container into the surface soil layer, after which it was placed ina polyethylene bag and shaken. The soil must be loosenedthrough pressing and crushing by hand if any aggregated soilremains after shaking. Finally, the sample was transferred backto a U-8 container for storage, and the radioactivity concentra-tion was measured.

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2.2. Preliminary sampling results

Fig. 2 shows 137Cs concentrations obtained using the threemixing techniques for samples collected in a paddy field in theYamakiya region, Fukushima Prefecture (sampled onMay 21, 2011).Our preliminary tests showed that soil samples collected frompaddies and grasslands measured using method 1, in which thedistribution of radionuclides in the sample was not uniform, pro-duced some measurement errors arising from the application of acalibration gamma source that assumed a homogeneous distribu-tion of radionuclides, while stirring the samples as in method 2allowed soil to spill from the containers. Measurement variabilitywas less than in method 1, but a scatterplot indicated that sampleswere still not sufficiently homogenized. Fig. 2 clearly indicates thatsoil mixed outside the containers, as in method 3, was in anadequately homogeneous state and radioactivity concentrationmeasurements had little statistical scattering. Using these results,we decided to use method 3, stirring the sample in a polyethylenebag, for further soil collection.

2.3. Effects of forests on radionuclide concentrations

To identify possible sampling locations, we collected soil bothinside and outside a Japanese cedar forest (Fig. 3). We established atransect that crossed the boundary between the cedar forest andthe grassland, and collected four core samples in each differentland-use area on May 21, 2011. We also recorded the ambientequivalent dose rate level at a height of 1 m above the ground usinga CsI(TI) scintillator (PA-1000; Horiba, Tokyo, Japan). The 137Cs in-ventory outside the forest was significantly higher than inside theforest (Fig. 4a), but the 131I inventory did not differ between areas(Fig. 4b). These data strongly suggest that a significant amount of137Cs 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 watermovement (e.g., Kato et al., 2012a). While the dose rate at a heightof 1 m was significantly higher outside than inside the forest(Fig. 4), Fig. 5 indicates that forest soil accumulated much lessatmospherically deposited radiocesium. In contrast, the 131I in-ventory differed slightly between forest and grassland soils, andforest soils showed significant variability. Although we cannotdiscuss the results based on statistical error because of the smallsampling number, these results clearly indicate that initial emer-gency samples should not be collected inside forests.

cal strategies for mapping fallout in nuclear emergencies based on theEnvironmental Radioactivity (2014), http://dx.doi.org/10.1016/

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 radioactivityin the five soil samples. Soil core samples were collected on May 21, 2011.

Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e8 3

2.4. Selection of soil collection locations

To monitor long-term changes in the fallout inventory ofradioactive material, collecting multiple samples from the samelocations is vital. Therefore, we selected locations with no antici-pated disturbances. We attempted to avoid sampling points withvegetation, but this was not always possible, and some soil coresamples were taken together with aboveground vegetation.

Because we anticipated variation in soil radioactivity concen-trations, we collected five soil samples from each study site, asmuch as possible within a range of 3 m. Where sampling locationsfell inside high-radiation dose-rate areas (e.g., evacuation zones),only one to three samples were taken to avoid prolonged exposureof sampling workers to radiation.

Fig. 3. Soil samples were collected along a 40-m transect extending from pastureland to Japawere collected on May 21, 2011.

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3. Protocol for assessing radionuclide contamination in soilsamples

3.1. General information

Guidelines for soil sampling preparation and for the recording ofbasic information are listed and described below.

1) Select sampling locations and points: Flat topography is pref-erable to minimize the effects of the redistribution of radionu-clides. Verify that the terrain is flat and that no large obstacles(such as vehicles or buildings) exist within a 5 m range of thesampling location. Open areas, such as croplands or paddyfields, are preferred, and forested areas should be avoided as

nese cedar forest. The bag-mixing method was used for soil sampling. Soil core samples

cal strategies for mapping fallout in nuclear emergencies based on theEnvironmental Radioactivity (2014), http://dx.doi.org/10.1016/

Fig. 4. Relationships between distance from the pastureeforest boundary and (a) 137Cs and (b) 131I inventories in soil samples.

Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e84

most of the fallout is trapped in tree canopies. Paddy fields canbe used before irrigating (Takahashi et al., 2014).

2) Soil and land cover maps are useful for designing a samplingstrategy (location and density of sampling points) since soiltypes and land uses can influence the extent of radioactivecontamination in soils.

3) The geographic position of each sampling point within eachlocation 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 analysisare listed below.

Disposable gloves should be used to avoid cross-contamination.

1) Five soil samples, 5 cm deep, should be collected within a 3 � 3-m2 area at the selected sampling location. Ideally, we recom-mend the four corners and center of the square as samplingpoints. The depth is recommended because most of the recentand 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 samplesis sufficiently larger than the 50-cm2 sampling area suggestedby Khomutinin et al. (2004).

3) Measure the ambient equivalent dose rate (mSv/h) using aportable dosimeter at a height of 1 m. At all locations, slowlymove the survey meter 3 m in all directions from the center toconfirm the absence of any singular points with sudden spikesin air-dose rates.

Fig. 5. Comparison of 134Cs, 137Cs, and 131I inventories in surface soil. The number in the figuvalue of the first and third quartile, whereas the whisker bar indicates the maximum and

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4) Try to locate sites with open space, and if possible, areas notcovered by vegetation. Do not remove small fragments of leavesand organic layers because they may contain 137Cs and 134Cs. Insuch 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-8measurement container should beweighed before sampling and clearly marked. Insert the U-8container gently into the soil and use it as a scoop. Cut thesurface with a disposable plastic knife and mix well in theplastic bag before sealing.

2) Using a core sampler for hard soil (Fig. 7): Use a coresampler 50 mm in diameter or larger to a depth of 50 mm.The samples should be placed in plastic bags, and thenmixed well by shaking the outside of the plastic bag andpacked into U-8 containers (when using a 100-mL coresampler). Metal samplers can be used at the same samplingsite after cleaning with alcohol in situ, but never use thesame sampler for different locations to prevent cross-contamination. Samplers should be cleaned after return-ing to the lab.

3) Soil water content may be measured in the field using aportable time-domain reflectometer (TDR). This procedureis optional if oven-drying is impossible due to the need toavoid iodine sublimation at higher temperatures.

4) Because of the possible spatial variability, all five soil sam-ples should be measured. All sampling containers should beproperly labeled with weight, soil depth, GPS referencenumber, and land-use type, and hermetically sealed. Wipethe outside of the container with alcohol-impregnated tis-sue paper to decontaminate and take a photograph todistinguish the soil color and type.

re represents the mean inventory for pasture and forest soils. The box bar denotes theminimum values of the measured inventories.

cal strategies for mapping fallout in nuclear emergencies based on theEnvironmental Radioactivity (2014), http://dx.doi.org/10.1016/

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.

Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e8 5

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Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e86

PlFuj.j

5) Each sample should be placed inside a new plastic bag andzip-locked by a person who has not touched the soil (ex-pected to be uncontaminated). The five samples from eachlocation should also be zip-locked in a larger bag andtransported to a laboratory in secured containers labeledwith radioactive signs.

6) The national radiation safety regulations should beobserved at all times (i.e., do not exceed 5 mSv/h at thesurface of the transportation container as proposed in theJapanese L package standard).

7) In the laboratory, information regarding GPS coordinates(region, latitude, and longitude), land use, soil types, digitalphotographs, sampling dates, and other relevant commentsshould be entered into a computer database.

8) The U-8 soil containers should be sealed again in zip-lockbags or plastic film by two persons (one person to touchthe container, and another to cover the container withouttouching its surface), to avoid contamination of the Ge-detector.

9) The bulk density should be calculated using net sampleweights and field soil moisture contents. If oven-drying ispossible (i.e., 131I level is low), soil can be dried and the bulkdensity can be calculated.

10) To convert the amount of radioactive contamination per ki-logramof soil to the amount of radioactive contamination per1 m2 of land (Bq/m2), average the radioactive contamination(Bq/kg soil) and thebulkdensity valuesof thefive subsamples.

11) Before measurement, the sample container should again beshakenwell to mix the large amounts of 137Cs in the surfacesoil. Fig. 8 shows the relationship between the number oftimes a samplewas shaken and themeasured concentrationof radiocesium and 131I. The shaking treatment was per-formed reciprocally by several operators to correct indi-vidual differences. Results indicate that shaking samples atleast 150 times is necessary to homogenize soil materials inthe containers.

12) Given the expected high concentrations of radionuclides,the counting time will be limited by the counting statisticserror of 137Cs, 134Cs, and 131I, which should be amaximum of5% (ideally 3%).

13) In some locations, to confirm the validity of sampling todepth of 5 cm, incremental scraper plate sampling(Zapata, 2003; unit depths of 5 mm to 1 cm) should beconducted to identify the profile distribution of theradionuclides.

Fig. 8. Effect of shaking on the measured concentrations of radiocesium and 131I. Soilsampling was conducted on an artificial hillslope at the Terrestrial EnvironmentalResearch Center of the University of Tsukuba, Tsukuba, Ibaraki, Japan, on April 16, 2011.

3.3. Results of preliminary sampling following the FDNPP accident

Fig. 9 shows the spatial pattern of the 137Cs inventory after theFDNPP accident, based on the emergency soil sampling protocolproposed in this study. Five soil samples (No. 1eNo. 5) werecollected within a 3 m � 3-m area at each sampling site (56 sites intotal). The influence of the number and combination of samples ateach site in the 137Cs inventory mapping results are discussedbelow.

Our map shows unfavorable variation in the 137Cs inventorywhen a single soil sample is selected from the five. In maps basedon soil samples No. 1 and No. 3, an area with a relatively high 137Csinventory, appearing northwest of the reactor in the soil sample No.5 map, is missing. Using multiple samples reduces the differencesin 137Cs inventory patterns among maps, but large inconsistenciesstill exist among maps based on combinations of three regularlyselected soil samples. Finally, the 137Cs inventories of the five soilsamples were averaged and used to produce an inventory map. The

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spotlike distribution of the 137Cs inventory was averaged when thefive samples were combined, and the resulting map is largelyconsistent with the results of the Third AirborneMonitoring Surveyof Radioactivity (MEXT, 2011b).

The results of this study indicate that collecting and combiningat least five soil samples within a 3 m � 3-m area is the minimumnumber required to produce a precise fallout inventory map ofradionuclides from the FDNPP accident. The emergency samplingprotocol proposed in this study should be considered in case ofemergency situations following nuclear hazards.

cal strategies for mapping fallout in nuclear emergencies based on theEnvironmental Radioactivity (2014), http://dx.doi.org/10.1016/

Fig. 9. Influence of the number of samples at each sampling site on the 137Cs inventory mapping results.

Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e8 7

4. Conclusions

In this study, we developed an emergency soil sampling pro-tocol based on preliminary sampling from the FDNPP accident-affected area. We also presented the results of preliminary exper-iments testing how different soil sampling methods (e.g., mixing,number of samples) affect measured radioactivity. The results ofthis study demonstrate that sample mixing strongly influences themeasured radioactivity in soil samples. For homogenization, soilsamples in U-8 containers must be shaken at least 150 times orhand-rolled in a disposable plastic bag to disaggregate soil. Inaddition, five soil samples within a 3 m� 3-m area is the minimum

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number necessary to reduce measurement uncertainty for theemergency soil sampling protocol proposed in this study.

Acknowledgments

This project was financially supported by the Ministry of Edu-cation, Culture, Sports, Science, and Technology, Japan (MEXT). Wewould like to express our thanks to everyone who directly andindirectly supported the project, without whom this large-scaleproject would have been impossible to achieve within such ashort time following the Fukushima Dai-ichi Nuclear disaster. Wethank Dr. Nakamura and all the members of the Committee on the

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Y. Onda et al. / Journal of Environmental Radioactivity xxx (2014) 1e88

Construction of Maps for Radiation Dose Distribution for theirhelpful and encouraging comments.

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