Aomori , Japan. Japan. *Corresponding author:

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1 Radiation Protection Dosimetry (2012), Vol. 152, No. 1 3, pp Advance Access publication 29 August 2012 doi: /rpd/ncs222 RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHI NUCLEAR POWER PLANT IN THE TERRESTRIAL ENVIRONMENT H. Tazoe 1, *, M. Hosoda 2, A. Sorimachi 1, A. Nakata 1, M. A. Yoshida 1, S. Tokonami 1 and M Yamada 1 1 Hirosaki University, Institute of Radiation Emergency Medicine, 66-1 Hon-cho, Hirosaki, Aomori , Japan 2 Hirosaki University, Graduate School of Health Sciences, 66-1 Hon-cho, Hirosaki, Aomori , Japan *Corresponding author: tazoe@cc.hirosaki-u.ac.jp Major contaminants from venting and hydrogen explosions at the Fukushima Daiichi nuclear reactors between 12 and 15 March 2011 were transported northwestward and deposited on soil and plants via precipitation. Surface soils and plant leaves were sampled at 64 sites in the Fukushima Prefecture. The highest concentrations of 134 Cs (84.4 kbq kg 21 ) and (82.0 kbq kg 21 ) in surface soils were observed at Nagadoro in Iidate village located 32 km northwest from the Fukushima Daiichi nuclear power plant. Furthermore, 131 I, 129 Te, 129m Te, 110m Ag and 140 La were detected in the same samples. Outer surface of plant leaves, such as bamboo, cabbage and grasses were highly contaminated at the high-dose rate areas of Tsushima and Minami-Tsushima in Namie town. Mugwort leaves that grew after the pollution event had extremely low concentration of radionuclides; however, the plant/soil radiocaesium ratio was It is anticipated that decomposition of fallen leaves will promote recycling of radionuclides in the environment. INTRODUCTION A catastrophic earthquake (9.0 Mw) and tsunami occurred on 11 March 2011, which caused major destruction in northeastern Japan and severe damage at the Fukushima Daiichi nuclear power plant. Electrical generators were shut off and three nuclear reactors suffered explosions due to hydrogen gas after the cooling system failed. Residents within 20- km radius of the Fukushima Daiichi nuclear power plant were evacuated; densely populated areas such as the cities of Fukushima and Koriyama were highly contaminated. Major contaminants originated from venting and hydrogen explosions at reactors from 12 March to 15 March Total depositions over the Japanese islands were estimated to be.1.0 PBq (1, 2). Fission products were transported northwestward and deposited on the ground via precipitation, which resulted in heterogeneous contamination and geographical distribution of ratios of radionuclides such as 131 I/, 134 Cs/ and 110m Ag/ (3). In addition, hydrogen explosions and metrological conditions such as wind direction and precipitation caused, 131 I, 110m Ag and 129m Te in the surface soil to vary. MATERIALS AND METHODS Surface soils (0 1 cm) and plant leaves were collected from 64 sites in the Fukushima Prefecture (Fig. 1) on four sampling expeditions from 12 to 16 April, 26 to 28 April, 6 to 10 June and 15 to 16 June 2011, and then these samples were analysed for radionuclides. For the soil analysis, stones and plant roots were removed by handpicking and soil was transferred to a 100-ml polystyrene container. To compare the deposition to soils and leaves and the plant uptake after deposition, leaves of Japanese mugwort that grew after the pollution event were also collected. We collected leaves from the upper part of the plant to minimise contamination from the soil. Plant leaves were cut into 11 cm with a scissor. The concentrations of 134 Cs, 136 Cs,, 131 I, 110m Ag, 129 Te and 129m Te were determined by gamma-ray spectroscopy. Gamma-ray emissions at energies of 460 kev ( 129 Te), 604 kev ( 134 Cs), 636 kev ( 131 I), 662 kev ( ), 730 kev ( 129m Te) and 885 kev ( 110m Ag) were measured for 1000 up to s by a high-purity coaxial Ge gamma-ray detector (ORTEC GEM-40190, SEIKO-EG&G). Mixed gamma standard sources with different sample heights distributed from the Japan Radioisotope Association were used for efficiency correction. Because the measurements were started in the middle of May 2011, some short half-life radionuclides such as 131 I(T 1/2 ¼8.02 d) and 136 Cs (T 1/2 ¼13.16 d) could not be detected. RESULTS AND DISCUSSION Heterogeneous deposition of radionuclides on the ground The highest concentration of 134 Cs (84.4 kbq kg 21 ) in surface soil was observed at the Nagadoro (S54) in Iidate village located 32 km northwest from the Fukushima Daiichi nuclear power plant (Table 1). In addition, (82.0 kbq kg 21 ), 131 I (924 kbq # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com

2 RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHI NUCLEAR POWER PLANT Figure 1. Map of the soil sampling locations and total radioactive caesium concentrations ( 134 Csþ ). The soil samples observed high radiocaesium concentration ( 134 Csþ.100 kbq kg 21 ) are shown on the map and Table 1. kg 21, concentration is corrected to March 11), 110m Ag (0.40 kbq kg 21 ), 129 Te (56.3 kbq kg 21 ) and 129m Te (89.4 kbq kg 21 ) were detected for the same sample on March and April. Furthermore, the extremely high-dose rates after dispersion from the nuclear power plant were thought to be mainly caused by 132 Te and its daughter 132 I based on the in situ gamma-ray spectrum (3). High radiocaesium concentrations were observed in the northwestern Namie town (S46) and southwestern Minamisohma city near the Tetsuzan dam (S12), which are located northwest of the Fukushima Daiichi nuclear power plant. This is consistent with the results of aircraft monitoring by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT, Fukushima city is located on the major contamination path that also shows heterogeneous geographical distribution, particularly anomalous high concentrations for S63 (Table 1). This resulted from geographical features such as the surrounding mountains and the setting of buildings and trees, because Fukushima city is densely populated. Concentration ratios against for 134 Cs and 129m Te are mostly constant (Fig. 2), although those for the 131 I and 129m Te are observed different trends. 131 I/ ratios decrease from 32 to 10 with concentration (,10 kbq kg 21 ). On the contrary, 110m Ag/ increase for high samples (.50 kbq kg 21 ). These fractionations were caused by the physical and/or chemical properties for each nuclide during transportation and deposition processes. Hosoda et al. (4) reported that the highest ambient-dose rate in air was 36 mgy h 21 at Hirusone, Namie town (37.558N, E), on the basis of the carborne survey conducted on 12 April Regional heterogeneity in radiocaesium distribution was observed at Minami-Tsushima, Namie town (Fig. 3). Concentrations of collected from the three paddies (S19, S27 and S36) widely vary, ranging from 0.24 to 9.9 kbq kg 21.In the same paddy, much lower radiocaesium concentrations are observed in samples S19-1 and S36-1 relative to the other soil samples. The heterogeneous distribution of concentration can reflect both the initial deposition in the surface soil and the postdepositional redistribution, such as transportation adsorbed soil particles by the rainfall, in the soil. The soil sample from inside a greenhouse in Tsushima, Namie town, had much lower concentration than the soil outside the greenhouse (Fig. 4). This indicates that the major deposition took place during wet precipitation and that the inside was not contaminated. The radiocaesium deposition primarily occurs in association with precipitation (3). This indicates that the intrusion of radiocaesium indoors could be prevented easily. Deposition and translocation of radionuclides to the plant Figure 5 shows the concentration of radionuclides in soils (S12 S14) and plant samples (L11 L13) near the Tetsuzan dam, corrected to 12 April. Plant samples L11, L12 and L13 are leaves of Japanese ceder, bamboo and clinopodium gracile, respectively. The concentrations of radionuclides in S12 are nearly twice of that in others. On the other hand, plant samples show much wider variation in radiocaesium concentration. Japanese ceder leaves have low radiocaesium concentrations ( 134 Cs¼1.18 kbq kg 21 and ¼1.18 kbq kg 21 ). Bamboo leaves have concentrations several folds higher than those of soils and other plants. In cryptomeria, this could 199

3 Table 1. Radioactivities for highly contaminated soil samples ( 134 Cs >50 kbq kg 21 ) and distance from the Fukushima Daiichi Nuclear power plant. Radioactivies are corrected to 11 March Te 129m Te 110m Ag 131 I 134 Cs Date Collection Measurement Sample Latitude Longitude Distance from the FD1NPP (km) S N E Apr May S N E Apr May S N E June July S N E June July S N E June July S N E June July S N E June July H. TAZOE ET AL. depend on the height of the branch. Since Japanese ceder leaf samples were collected from the lower part of trees overlapped with other branches, the deposition of radionuclides was inhibited and the observed radiocaesium concentrations were low. On the other hand, bamboo leaves did not overlap with each other and the radionuclides were well deposited on the entire leaf. Furthermore, the low moisture content of the bamboo leaves resulted in higher radioactive caesium. Plant samples were collected in Namie town in 13 and 14 April. The samples were located close to the town, and the radiocaesium concentration considerably fluctuated (Fig. 6). For example, the concentration in the outer leaves of cabbages was 50 times higher than that in the inner leaves. The radionuclides, except 134 Cs and, in the inner cabbage leaves were below the detection limit in the gammaray measurements. The translocation from the outer leaves caused 0.90 kbq kg 21 of 134 Cs and 1.01 kbq kg 21 of which originated from the nuclear accident. For the period between March and June 2011, in soils and plants from Koriyama city was compared (Fig. 7). Bamboo, clover and pine leaves were collected on 17 March. Japanese mugwort was collected in 16 June. Japanese mugwort is a perennial plant and only the underground stem is overwintering. In the part that grew after the nuclear plant accident, the concentration reflects absorption via the root from the soil. The root of the Japanese mugwort splits at shallow depths. The deposited radiocaesium was transferred to the rest of the plant by root uptake and translocation. Radioactive caesium was compared between soils and plants (Table 2). Plants were mainly collected from Japanese mugwort, except for clover from the Zenpo pond and grass leaves from the Keiko park, in June and July Although in the soil samples ranges from 6.3 to 27.1 kbq kg 21, the Japanese mugwort shows extremely low concentrations,,0.5 Bq kg 21,of. Only grass leaf shows high radioactive caesium. The concentration ratios between plant and soil, raw plant / soil, for mugwort were relatively constant ( ). For the radiocaesium uptake from agricultural soils, the transfer factors range up to three orders of magnitude even for individual soil crop combinations (5). The high raw plant / soil ratio of in the grass samples could be caused by direct adsorption of radioactive caesium deposited on the leaves and root uptake, which spreads the roots to ground surface similar to an open net. The clover samples also have a shallow root system and spreads roots. It was difficult to distinguish whether radiocaesium originated from root uptake or direct adsorption on the leaves just after the nuclear accident. It is expected that in future, radioactive caesium will 200

4 RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHI NUCLEAR POWER PLANT Figure 2. Concentration ratios against for soil samples. Figure Cs and concentrations for paddy soil samples collected at Minami-Tsushima, Namie town. Concentrations are corrected to the sampling data of 12 April. migrate to the environmental ecosystem. Therefore, it is necessary to investigate the processes and transfer parameters. CONCLUSIONS We investigated radionuclides for the soil and plant samples in Fukushima Prefecture just after the accident of the Fukushima Daiichi nuclear reactors Figure 4. Comparison of radiocaesium concentration in soil between the outside and inside of the greenhouse. between 12 and 15 March The pollution was generally spread to the northwestward, as reported by the MEXT survey. However, regional heterogeneities of deposition and differences between radionuclides were observed. It indicates that more detailed studies are necessary for effective decontamination. Bamboo leaves, grasses and pine trees were highly contaminated at the high dose rate areas of 201

5 H. TAZOE ET AL. Figure 5. Concentration of radioactive nuclides for plant leaves (L11, cryptomeria; L12, bamboo and L13, clinopodium gracile) and neighbouring soil collected from the Tetsuzan dam (S12 S14). Concentrations are corrected to 12 April. Figure 6. Radiocaesium concentration various plant sample on April from the Namie town. Figure 7. Comparison of the concentration for soil and plant samples collected on March and July. Tsushima and Minami-Tsushima in Namie town. Mugwort leaves that grew after the pollution event had extremely low concentration of radionuclides; however, the plant/soil radiocaesium ratio was 202 Table 2. Comparisons of radioactive caesium concentration between soil and plant samples 134 Cs/ Law plant Law Plant / Soil Location City Soil 134 Cs/ 134 Cs 134 Cs Shinobuyama Park Fukushima Mugwort Asaka Town Koriyama Mugwort Koriyama Mugwort Aasaka-Nagamori Station Kaiseizan Park Koriyama Mugwort Araike Park Koriyama Mugwort Takako Pond Date Mugwort Zenpo Pond Koriyama Clover Keiko Park Motomiya Grass

6 RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHI NUCLEAR POWER PLANT Finally, it is anticipated that the decomposition of fallen leaves will promote recycling of radionuclides in the environment. FUNDING This work was supported by MEXT/JSPS KAKENHI Grant-in-Aid for Scientific Research B ( ). REFERENCES 1. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G. and Yamazawa, H. Preliminary estimation of release amount of 131 I and accidentally discharged from the Fukushima Daiichi Nuclear Power Plant into the atmosphere. J. Nucl. Sci. Tech. 48, (2011). 2. Yasunari, J. T., Stohl, A., Hayano, R. S., Burkhart, J. F., Eckhardt, S. and Yasunari, T. Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident. Proc. Natl Acad. Sci. USA (2011). Doi: /pnas Kinoshita, N., Sueki, K., Sasa, K., Kitagawa, J., Ikarashia, S., Nishimura, T., Wong, Y., Satou, Y., Handa, K., Takahashi, T., Sato, M. and Yamagata, T. Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan. Proc. Natl Acad. Sci. USA (2011). Doi: /pnas Hosoda, M., Tokonami, S., Sorimachi, A., Monzen, S., Osanai, M., Yamada, M., Kashiwakura, I. and Akiba, S. The time variation of dose rate artificially increased by the Fukushima nuclear crisis. Scientific Reports 87, doi: /sep Nisbet, A. F. and Woodman, R. F. M. Soil-to-plant transfer factors for radio caesium and strontium in agricultural systems. Health Phys. 78, (2000). 203