Title

Concentration of radioactive materials in small mammals collected from a restricted area in Fukushima, Japan since 2012

Authors

Hiroko Ishiniwa^1,a, Tsukasa Okano^1,b, Akira Yoshioka², Masanori Tamaoki², Yasushi Yokohata³, Junji Shindo⁴, Noriko Azuma^1,c, Nobuyoshi Nakajima¹, Manabu Onuma^1*

Affiliations

¹Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies, Ibaraki, Japan

²Fukushima Branch, National Institute for Environmental Studies, Fukushima, Japan

³Graduate School of Science and Engineering, University of Toyama, Toyama, Japan

⁴School of Veterinary Medicine, Kitasato University, Aomori, Japan

Present addresses

^aInstitute of Environmental Radioactivity, Fukushima University, Fukushima, Japan

^bGihu Prefecture, Gihu, Japan

^cMokoto, Abashiri, Hokkaido, Japan

^*Corresponding author: Manabu Onuma

Address: 16-2, Onogawa, Tsukuba, Ibaraki 305-8506, Japan

Tel: +81-29-850-2498

Fax: +81-29-850-2673

E-mail: monuma@nies.go.jp

Abstract

Fukushima Daiichi Nuclear Power Plant was struck by a huge earthquake and tsunami, which ultimately released a large amount of radioactive material owing to steam explosion in March 2011. This accident affected extensive area and resulted in soil contamination, which would prevail over a long period. The contaminants included various types of nuclear materials, among which cesium (^134,137Cs) has a particularly long half-life. Organisms living in the contaminated area are presumably exposed to radiation caused by the presence of these radioactive contaminants in their environment, in addition to internal exposure owing to the intake of these contaminants through the food web. The purpose of our data collection is to monitor accumulation of radioactive materials in small mammals for a long period and to predict its effects. Reasons for focusing on small mammals include: i) mammals are the most sensitive to radiation, ii) small mammals have a smaller home range compared to large mammals; therefore, accumulation of radioactive materials in small mammals is considered to reflect the condition of the surrounding environment, and iii) among the small mammals, rodents are used as laboratory animals and provide a lot of information, from which we can expect to acquire much knowledge based on investigations. The dataset contains measurements of ^134,137Cs and potassium (⁴⁰K) concentrations in small terrestrial mammals collected in areas contaminated with radioactive contaminants from the Fukushima Daiichi Nuclear Power Plant accident. We established two fixed study sites and three temporary sites inside the zone from which humans were evacuated owing to high levels of radiation. Annual monitoring of Cs contamination in small mammals was started in 2012 at the fixed study sites. Until 2016, we collected 547 individuals of four species (Apodemus speciosus, A. argenteus, Microtus montebelli, and Urotrichus talpoides). Cs concentrations in each individual were measured using high-purity germanium detectors against a ground body after removing the internal organs and the head. Our data provide valuable information about temporal changes in ¹³⁴Cs and ¹³⁷Cs concentrations in small mammals following the Fukushima Daiichi Nuclear Power Plant accident. The study is continuing and the present dataset will be updated in the future.

Key words

• Accumulation of radioactive materials
• Difficult-to-return zone
• Evacuation zone
• Fukushima Daiichi Nuclear Power Plant
• Cesium-134
• Cesium-137
• Potassium-40
• Rodentia
• Apodemus
• Soricomorpha

Introduction

A large amount of radioactive materials was discharged from the Fukushima Daiichi Nuclear Power Plant (FDNPP) owing to the hydrogen explosion accident following the Great East Japan Earthquake on March 2011. Consequently, the coastal areas of Fukushima Prefecture, including towns, agricultural fields, and forests, were heavily contaminated with radionuclides. Although humans were evacuated from contaminated area where the annual dose level exceeded the regulatory level of radioactivity defined by Japanese government (20 mSv y^-1), wild animals and plants, which have remained there, have been chronically exposed to high levels of radiation. In 646 km² of the area heavily contaminated with Cesium-134 and -137 (^{134, 137}Cs) ≥ 1,000 kBq m^-2 had 66 % (428 km²) forest cover (Hashimoto et al. 2012). Modeling studies conducted immediately after the accident reported dose rates over the first 30 days after the accident as 1 mGy d^-1 for plants, 1.5 mGy d^-1 for birds, 2.3 mGy d^-1 for soil invertebrates, and 3.9 mGy d^-1 for forest rodents (Garnier-Laplace et al. 2011). These dose rate values were sufficiently high for reducing reproductive success among plants, birds, and rodents, according to the established paradigms in radiation dose and effect studies reviewed by the International Commission on Radiological Protection (ICRP 2008). Moreover, Strand et al. (2014) calculated the dose rate to wildlife and indicated that the dose rates might have been as high as 1 mGy h^-1 for some organisms over short periods, owing to short-lived radioisotopes, such as Iodine-132 and Terium-132. From 3 months to 1 year after the accident, the major dose-contributing isotopes changed to ¹³⁴Cs and ¹³⁷Cs, which have longer half-lives (T_1/2 = 2 and 30 years, respectively). Dose rates decreased with time, but a potential risk to individuals of certain species in limited areas might have persisted (Strand et al. 2014). From these reports and situations, it is apparent that long-term research on exposure, dose, and effects of radiation to wildlife is an important research topic that should not be missed.

ICRP defined a set of reference species and recommended dosages to assess and manage radiation exposures in wild organisms (ICRP 2008). According to the ICRP, mammals are the most sensitive to radiation among all organisms and may show some symptoms when exposed to > 0.1 mGy d^-1. Rodents are a key species in radiation studies owing to their use in laboratory studies and extrapolation of dose-effect relationships to humans. Most of the rodent data were generated over short periods under high radiation exposures in the laboratory. However, little is known about the effects of radiation from chronic exposures to more moderate dose rates, as is occurring at Fukushima. In Japan, the large Japanese field mouse (Apodemus speciosus) is a good candidate as a reference rodent species. This species commonly inhabits a wide variety of habitats, from open fields to secondary lowland forests. The home range size is reported to be 500 to 1,400 m² (Oka 1992). Because it rarely climbs trees, insects and plants near the ground surface are the main foods of this species (Ohdachi et al. 2009). Therefore, the large Japanese field mouse could be directly affected by radiation from local areas, which might reflect local changes in radiation dose.

We selected two fixed monitoring sites inside the most contaminated areas, from where humans were evacuated (i.e., the difficult-to-return zone), and started sampling in 2012. In addition, one-shot sampling sites were also established in 2012 and 2014. Our main aim is to track changes in accumulation of radionuclides in the rodents and to investigate the biological effects of long-term radiation exposure in the large Japanese field mouse. In addition, during the investigation, we captured other species, such as the small Japanese field mouse (Apodemus argenteus), Japanese field vole (Microtus montebelli), and Japanese shrew-mole (Urotrichus talpoides). Accumulation of radionuclides in these species was compiled in our dataset, because this data is important and cannot be obtained twice. Among the species collected, the small Japanese field mouse feeds on insects and plants inhabiting from ground surface to arboreal, the Japanese field vole mainly feeds on plants, and the Japanese shrew-mole feeds mainly on insects, earthworms, spiders, and plants (Ohdachi et al. 2009). These variations in feeding habit may result in species-specific differences in radionuclide accumulation rate.

The present data paper provides information on the accumulation of ¹³⁴Cs and ¹³⁷Cs in small rodents collected from the same sites each year since the FDNPP. For some individuals, we show the data on the accumulation of potassium-40 (⁴⁰K), which naturally constitutes 0.012 % of the total amount of potassium, because it is a chemical analog of Cs and useful for discussions related to the effects of ^{134, 137}Cs. These data may contribute to future radiation research by showing changes in radionuclide accumulation by wildlife after the accident.

Metadata

1. TITLE

Concentration of radioactive materials in small mammals collected from a restricted area in Fukushima, Japan since 2012

2. IDENTIFIER

ERDP-2018-06

3. CONTRIBUTOR

A. Dataset owner

Manabu Onuma

Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies

Address: 16-2, Onogawa, Tsukuba, Ibaraki 305-8506, Japan

Tel: +81-29-850-2498

Fax: +81-29-850-2673

E-mail: monuma@nies.go.jp

B. Contact person

Manabu Onuma

Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies

Address: 16-2, Onogawa, Tsukuba, Ibaraki 305-8506, Japan

Tel: +81-29-850-2498

Fax: +81-29-850-2673

E-mail: monuma@nies.go.jp

4. GEOGRAPHICAL COVERAGE

A. Geographical description

Namie city, Fukushima Prefecture, Japan

B. Boundary coordinates

West:140° 43' 15" E

East:140° 49' 36" E

North:37° 36' 02" N

South:37° 32' 57" N

5. TEMPORAL COVERAGE

The earliest sampling date: August 10, 2012

The latest sampling date: September 2, 2016

6. METHODS

A. Sampling sites

The samples were collected during the breeding season (July to September) of the large Japanese field mouse from five sites in Fukushima Prefecture (Fig. 1). Site name and geographic coordinates of the sampling sites are shown in Table 1. Sampling at two sites, N1 and N2, was conducted beginning in 2012. Sampling at three other sites was conducted in a single year (site N3 in 2012, and sites N4 and N5 in 2014).

Figure 1. Locations of sampling sites and ambient dose rates (µSv h^-1) recorded 1 m above the ground surface. The figure was prepared based on the data of Geospatial Information Authority of Japan and Nuclear Regulation Authority of Japan (2011).

Table1. Sampling locations and duration of sampling.

B. Sampling collection

Small mammals were captured using Sherman-type live traps baited with sunflower seeds. At each study site, 50–80 traps were set at 10 m intervals within a 1 km radius from each coordinate. Sampling was conducted once to five times a year. Each sampling period was two to three days in duration. The traps were set during daytime and checked the next morning. Captured mammals were euthanized by CO₂ asphyxiation and transported to the laboratory immediately. Each individual was weighed, head and internal organs (stomach, intestine, liver, spleen, and reproductive organs) were removed, and the carcass was stored at –20 °C.

The permissions for collection of wild rodents by entering the exclusion zone and national forests were obtained from the Fukushima Prefecture, Namie town, and Kanto Regional Forest Office, respectively. All the sampling and experiments were conducted with the approval of the National Institute for Environmental Studies for analysis and experimentation with environmental samples contaminated with radioactive materials (Permission No. H24-10-10), and carried out in accordance with the guidelines for using wild mammals of the Mammal Society of Japan (Committee of Reviewing Taxon Names and Specimen Collections in the Mammal Society of Japan 2009) and guideline for the Care and Use of Laboratory Animals of the National Institute for Environmental Studies.

C. Determination of ¹³⁴Cs, ¹³⁷Cs, and ⁴⁰K concentrations in the body of captured mammals

The body, without internal organs and head, was ground using a food processor without drying and enclosed in a 100 mL polystyrene container. Mass and volume of the samples were measured under wet conditions. ¹³⁴Cs, ¹³⁷Cs, and ⁴⁰K radioactivity levels were measured using high-purity germanium (HpGe) detectors (GMX45P4-76, Ortec, TN, or GCW7023, Camberra Industries Inc., TN). Gamma Studio (Seiko EG&G Co. Ltd., Tokyo, Japan) and Spectrum Explorer (Camberra) were used for analyzing the gamma ray spectra. A standard radiation source (MX033U8PP, The Japan Radioisotope Association) was used for calibration.

The measuring time for Cs was restricted to 50,000 s because there were a large number of environmental samples to be measured after the nuclear accident. Normally, measurement of Cs was terminated when the number of counts (cnt), gamma rays hitting the detector, exceeded 1,000. Thus, the error of count has been controlled within 3% in our experiments. If ⁴⁰K was included as a measurement, the count was terminated after the value of ⁴⁰K exceeded the detection limit and Cs cnt exceeding 1,000. The value of detection limit was calculated according to measurement guideline of Ministry of Education (1992), which followed Cooper’s method (1970). According to Cooper’s method (1970), the value of the detection limit in samples measured was set as k-fold of standard error calculated from average values of blank samples. In our experiments, k was set to three, in accordance with measurement guideline of Ministry of Education (1992), and the detection limit of each sample was calculated. The ranges of detection limit of each nucleus until 2016 were 5.6–1757.9 Bq kg^-1 wet weight (ww) for ¹³⁴Cs, 5.2–1091.4 Bq kg^-1 ww for ¹³⁷Cs, and 25.1–179.7 Bq kg^-1 ww for ⁴⁰K. The values below the detection limits were treated as 0 Bq kg^-1 ww. The value of error was calculated by dividing the standard error of true cnt of each nucleus by wet weight of sample. The standard error of cnt (d_N) was calculated as follows.

where, n_N is number of true count, n_b is the number of background counts, t_s is the measurement time of the sample, and t_b is the measurement time of the background sample. The measured radioactivity levels of ¹³⁴Cs, ¹³⁷Cs, and ⁴⁰K were corrected for radioactive decay to obtain the expected values at the time of sampling.

D. Radiation dosimetry on ground surface at the study sites

Air dose rates at the sampling sites were measured at the ground level (0 m height), where the number of small mammals alive, was recorded using a portable environmental gamma survey meter (NHE20CY3-131By-S, Fuji Electric Co. Ltd., Tokyo, Japan). Although the air dose rates were measured at one arbitrarily chosen spot at each site in 2012, they were measured at five random locations at least 10 m apart since 2013.

7. DATA STATUS

Latest update: March 20, 2018

8. ACCESSIBILITY

A. License

This dataset is provided under a Creative Commons Attribution 4.0 International License (CC BY 4.0; https://creativecommons.org/licenses/by/4.0/legalcode).

B. Data updates

The dataset will be updated. Please check the latest version of the dataset.

C. Reusing materials and data

Part of the data is used in an article published by us (Okano et al. 2016) for another purpose.

D. Dataset owner

Manabu Onuma

Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies

Address: 16-2, Onogawa, Tsukuba, Ibaraki 305-8506, Japan

Tel: +81-29-850-2498

Fax: +81-29-850-2673

E-mail: monuma@nies.go.jp

9. DATA STRUCTURE

A. Data table

B. Format type

The data files are in ASCII text, comma delimited (csv) files.

C. Header information

Headers corresponding to variable names (see section 9.D) are included in the first row of the data files.

D. Variable definitions

The variables are listed according to their order of appearance in the data file. The variable names are headers included as the first row in the data file.

Acknowledgments

Our special thanks to Dr. Takehiko Hiwatari and Dr. Naoki Nakajima for assistance with the measurement of the radioactivity of cesium and potassium. We are also grateful to Professor Thomas G. Hinton for checking the quality of English used, and for useful suggestion. This study was financially supported by the project "Study on the dynamics of radioactive materials in a multimedia environment" commissioned by the Ministry of the Environment, Japan from 2012 to 2015, thereafter it was financially supported by internal funds from the National Institute for Environmental Studies.

Literature cited

Committee of Reviewing Taxon Names and Specimen Collections in the Mammal Society of Japan (2009) Guidelines for using wild mammals. Mamm Sci 49:303-309 in Japanese. doi: https://doi.org/10.11238/mammalianscience.49.303

Cooper JA (1970) Factors determining the ultimate detection sensitivity of Ge (Li) gamma-ray spectrometers. Nucl Instr Methods 82:273-277. doi: 10.1016/0029-554X(70)90361-7

Garnier-Laplace J, Beaugelin-Seiller K, Hinton TG (2011) Fukushima wildlife dose reconstruction signals ecological consequences. Environ Sci Technol 45:5077-5078. doi: 10.1021/es201637c

Geospatial Information Authority of Japan, https://maps.gsi.go.jp/#5/36.104611/140.084556/&base=blank&ls=blank&disp=1&lcd=blank&vs=c1j0h0k0l0u0t0z0r0s0f1&d=vl. Available from 2013, Accessed 12 December 2017

Hashimoto S, Ugawa S, Nanko K, Shichi K (2012) The total amounts of radioactively contaminated materials in forests in Fukushima, Japan. Sci Rep 2:416. doi: 10.1038/srep00416

Ministry of Education (1992) Radioactivity measurement series 7: Gamma ray spectrometry with germanium semiconductor detector. Japan Chemical Analysis Center, Japan. (in Japanese)

Nuclear Regulation Authority (2011) Air dose rate results of the fifth airborne monitoring survey and airborne monitoring survey outside 80 km from the Fukushima Dai-ichi NPP (Decay correction: June 28, 2012) from Extension Site of Distribution Map of Radiation Dose, etc. https://ramap.jmc.or.jp/map/. Accessed 12 December 2017

Ohdachi SD, Ishibashi Y, Iwasa MA, Saitoh T (ed) (2009) The wild mammals of Japan. Shoukadoh, Kyoto, Japan

Oka T (1992) Home range and mating system of two sympatric field mouse species, Apodemus speciosus and Apodemus argenteus. Ecol Res 7:163-169.

Okano T, Ishiniwa H, Onuma M, Shindo J, Yokohata Y, Tamaoki M (2016) Effects of environmental radiation on testes and spermatogenesis in wild large Japanese field mice (Apodemus speciosus) from Fukushima. Sci Rep 6:23601. doi: 10.1038/srep23601

Strand P, Aono T, Brown JE, Garnier-Laplace J, Hosseini A, Sazykina T, Steenhuisen F, Vives i Batlle J (2014) Assessment of Fukushima-derived radiation doses and effects on wildlife in Japan. Environ Sci Technol Lett 1:198-203. doi: 10.1021/ez500019j

The International Commission on Radiological Protection (2008) Assessing effects in terms of derive consideration levels in Environmental protection: the concept and use of reference animals and plants. ICRP Publication 108.