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Radiocesium decontamination of a riverside in Fukushima, Japan
Journal of Environmental Radioactivity 177 (2017) 58e64
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radiocesium decontamination of a riverside in Fukushima, Japan Tatsuhiro Nishikiori*, Satoshi Suzuki Fukushima Prefectural Centre for Environmental Creation, 10-2 Fukasaku, Miharu Town, Fukushima 963-7700, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2017 Received in revised form 25 May 2017 Accepted 4 June 2017
Extensive decontamination measures have been implemented in the area affected by the Fukushima Daiichi nuclear disaster. Typical decontamination measures, such as removing topsoil of several centimeters in depth, are not suitable for rivers where contaminated sediments have been deposited. A decontamination measure was tested that considered the spatial distribution of radiocesium at the lower part of a tributary of the Abukuma River in Fukushima. The radiocesium distribution in the flood channel was vertically and horizontally highly heterogeneous. In some parts, the activity concentration was high (>10 kBq/kg for 137Cs) even at depths of 25 cm in the sediment. This may be due to plant growth in the flood channel favoring the deposition of sediment with high activity concentration. On the basis of the radiocesium distribution, the flood channel sediment was removed to a depth of 15e35 cm, which accumulated the most radiocesium (>3.0 kBq/kg for the sum of 134Cs and 137Cs). The upper 5 cm of soil was removed from the dike slopes. The river bed was not decontaminated because the activity concentration was low (<1 kBq/kg) in the river bed sediment and because the water shields gamma rays emitted from the sediment. The test decontamination measure reduced the air dose rate by a factor of approximately two, demonstrating the effectiveness of our measures. Annual external doses were calculated for when this part of the dike and the flood channel is used for commuting to school and outdoor education. The doses during the activities at the test site accounted for only 1e2% of the value during daily life in the surrounding area, indicating that radiation exposure during riverside activities is limited. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Radiocesium River Decontamination Sediment Air dose rate
1. Introduction During the Fukushima Dai-ichi Nuclear Power Plant disaster in March 2011, several tens of petabecquerels (1 PBq ¼ 1015 Bq) of radiocesium (134Cs and 137Cs) were released into the atmosphere (Buesseler et al., 2016) and contaminated a wide area of eastern Japan (Morino et al., 2011). The physical half-life of 134Cs is relatively short (2.06 years), whereas that of 137Cs is longer (30.2 years), which may cause prolonged contamination similar to the Chernobyl nuclear accident (International Atomic Energy Agency, 2006). An extensive decontamination program has therefore been implemented over a wide area of eastern Japan to reduce radiation exposure (Ministry of the Environment, Government of Japan (MOE), 2017). The decontamination measures focus on living areas such as houses, schools, roads, parks, agricultural land, and forest adjacent to inhabited areas (MOE, 2013). Because radiocesium is characterized by strong adsorption to clay minerals
* Corresponding author. E-mail address: [email protected] (T. Nishikiori). http://dx.doi.org/10.1016/j.jenvrad.2017.06.005 0265-931X/© 2017 Elsevier Ltd. All rights reserved.
(Nakao et al., 2012), most radiocesium in soil is retained in the top layer to a depth of a few centimeters (Matsuda et al., 2015). The MOE accordingly recommends soil decontamination measures, including topsoil removal, deep plowing, and covering of soil surfaces (MOE, 2013). According to the MOE decontamination guidelines (MOE, 2013), living areas also include frequently used riverside parks and playing fields. Sediments containing radiocesium may be deposited on riversides during floods; thus, the radiocesium inventory is sometimes greater on these sediments than in the surrounding area (Burrough et al., 1999; Konoplev et al., 2016). However, sediment deposition is highly heterogeneous and sediment erosion occurs continuously. Consequently, the radiocesium distribution on river banks and reservoirs is expected to be vertically and horizontally heterogeneous. For such conditions, standard soil decontamination measures, where only the top layer is removed, are not suitable for riversides, and appropriate decontamination measures have not been established. In this study, we tested a decontamination measure based on the vertical and horizontal distribution of radiocesium at a river in Fukushima.
T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
2. Materials and methods 2.1. Site description The test site was 55 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (N37460 200 , 140 340 4900 ) and at the lower part of the Kami-Oguni River (Fig. 1a and b), a tributary of the Abukuma River, which is the largest river in Fukushima. The total deposition density of 134Cs and 137Cs was 440 kBq/m2 (decay correction date: July 2, 2011), and the air dose rate at 1 m above the ground was 0.9 mSv/h on November 7, 2014, according to the
59
airborne monitoring survey (Nuclear Regulation Authority, 2017). The annual mean temperature and precipitation in the period 2006 to 2015 were 12.7 C and 1160 mm, respectively, observed at the AMeDAS Yanagawa station 9 km north of the test site (Japan Meteorological Agency (JMA), 2017). The area and the elevation of the Kami-Oguni River watershed are 13.2 km2 and 110e580 m, respectively (Fig. 1b), calculated from data provided by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT, 2017). The lower parts of the catchment are used as residential areas and for agriculture, and the hilly area is covered with forest (Fig. 1c). The residential land, agricultural land, and forest account for 2%, 22%,
Fig. 1. Map of the Kami-Oguni River watershed (a), topographic map of the watershed (b), land-use map of the watershed (c), the decontamination area in the test site (d), and schematic cross-section of the test site (e). Radiocesium deposition densities (decay correction date: July 2, 2011) are derived from the third airborne monitoring survey data (Ministry of Education, Culture, Sports, Science and Technology, 2011). Various landscape elements are cited from data provided by MLIT (2017).
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T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
and 75% of the watershed area, respectively. Decontamination has been implemented in the living areas, but not in most of the agricultural land and forest. The parent rock material of the watershed comprises Cretaceous granodiorite and Miocene non-alkaline mafic volcanic rocks (Geological Survey of Japan, 2015). The test site consisted of a river, which is approximately 170 m long and 2e6 m wide, a flood channel with a width of up to 10.5 m, and dikes, which are approximately 3 m wide and 2 m high (Fig. 1d and e). The dikes are partly covered with concrete. The left bank, the right bank, and the flood channel were used as a school route, an orchard, and a place for outdoor education, respectively. The school route on the left dike was decontaminated before the experiment was performed. We chose this riverside as the test site because the air dose rate exceeded the MOE decontamination criterion value (>0.23 mSv/h (MOE, 2013)) and the residents requested decontamination. 2.2. Decontamination test process We decontaminated the flood channel and the dike slopes from August to November 2014. Fig. 2 shows the test process. First, we removed all plants from the flood channel from August to September, and then removed a soil layer approximately 5 cm thick
from the dike slopes with an excavator. Turf was put on the decontaminated slope from September to October 2014. We removed sediments from the flood channel from October to November 2014, and adjusted the removal depth to the vertical distribution of radiocesium, described later. The removed plants were taken to a waste incineration plant. The removed soil and sediments were packed into flexible container bags and taken to a temporary storage site for decontaminated soil. 2.3. Sampling and measurements We set lines across the river at 10 m intervals in the test site and measured air dose rates at 1 and 100 cm above the ground by using a NaI scintillation survey meter (TCS-172B, Hitachi Aloka Medical, Ltd. Tokyo, Japan) without a collimator. The measurements were taken before decontamination, at the completion of decontamination, and 3 months after decontamination (February 2015). We collected sediments in the flood channel and in the river bed and soil from the dike slopes just before the decontamination. The sampling depths were 20e40 cm in the flood channel, 15 cm in the river bed, and 5 cm in the dike slopes. The samples were collected with a trowel at seven points in the flood channel, seven points in the river bed, and one point in each of the dike slopes. We took
Fig. 2. Photographs of the decontamination test process.
T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
gravel with a diameter of >5 mm out of the samples, packed the treated samples into 100 mL plastic containers (50 mm inner diameter, 62 mm in height, U-8, As-one Co. Ltd. Osaka, Japan), and analyzed the samples for radiocesium. After the analysis, the sediment was sieved with water through a 63 mm mesh, dried at 105 C, and weighed to calculate the mud fraction (<63 mm). We measured the activities of 134Cs and 137Cs in the samples by using coaxial germanium detectors (GEM20-70 or GMX20200, ORTEC, Oak Ridge, TN). The counting error was <10%. We analyzed the gamma-ray spectra with Gamma Studio software (Seiko EG&G Co. Ltd. Tokyo, Japan), and used MX033U8PP (The Japan Radioisotope Association, Tokyo, Japan) as a standard source for efficiency calibration. The measured activity was corrected for radioactive decay at each sampling date.
61
2.4. Calculation of external exposure dose We calculated the additional annual external exposure during commuting and outdoor education at the test site before and after the decontamination. The dose was estimated by multiplying the time spent on the dikes and the flood channel and the average air dose rates at a height of 100 cm on the left dike and the flood channel. A value of 0.04 mSv/h measured before the accident (Minato, 2006) was subtracted from the average air dose rates to account for the natural background. The times were 35 h per year for commuting (10 min per day and 210 days per year) and 24 h for outdoor education (2 h per day and 12 days per year).
Table 1 Air dose rate before and after decontamination test. Decontamination process
Air dose rate (mSv/h)
Measurement date 1 cm above the ground
Before decontamination Completion of decontamination 3 months after decontamination a
Aug. 23, 2014 Nov. 16, 2014 Feb. 27, 2015
100 cm above the ground a
Range
Mean ± SD
Range
Mean ± SDa
0.20e1.99 0.15e0.95 0.15e1.20
0.78 ± 0.41 0.34 ± 0.15 0.34 ± 0.17
0.29e1.17 0.18e0.78 0.19e0.77
0.66 ± 0.22 0.34 ± 0.11 0.34 ± 0.12
SD is a standard deviation.
Fig. 3. Maps of air dose rate 1 and 100 cm above the ground before and after decontamination test.
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T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
3. Results and discussion 3.1. Air dose rate and activity concentration before decontamination The air dose rates at 1 and 100 cm above the ground before decontamination varied up to 10-fold and 4-fold, respectively (Table 1). The value at 1 cm above the ground reflects the ground surface contamination at the measurement point; thus, the wide range of values at a height of 1 cm suggests a highly heterogeneous radiocesium distribution. On the flood channel, relatively high values of >1 mSv/h were observed, mainly on the right bank (Fig. 3), indicating selective accumulation of radiocesium. Fig. 4a and Table S1 presents the vertical distribution of the radiocesium concentration in the flood channel sediment before the decontamination. The depth profiles varied from point to point. At points 2, 3, and 4, sediments had high activity concentrations (>10 kBq/kg for 137Cs) to a depth of 15e25 cm, whereas at point 5, the activity concentration was low to moderate (1.0 to 5.0 kBq/kg for 137Cs) at all depths. At points 1 and 7, the activity concentration peaked at the top layer and decreased with depth. These conditions indicate that the vertical and horizontal radiocesium distributions were highly heterogenous in the flood channel. The activity concentration was moderate (4.1 to 5.5 kBq/kg for 137Cs) in the soil in the dike slopes (Fig. 4b), and was low in the river bed sediments (Fig. 4c and Table S1), 70% of which was <1.0 kBq/kg for 137Cs. Considering these distributions, we adjusted the removal depth of the flood channel sediment to 15e35 cm to remove most radiocesium (>3.0 kBq/kg for the sum of 134Cs and 137Cs). We did not
decontaminate the river bed because the activity concentration in the sediment was low and the water shields the gamma rays emitted from the sediment. In Fig. 5, the 137Cs activity concentration in the total sediments is plotted against the mud fraction in the sediments of the flood channel and the river bed before decontamination. The activity concentration is positively correlated with the mud fraction in the sediments. This is probably because of the strong adsorption of radiocesium to clay minerals (Nakao et al., 2012). Because the sediment in the flood channel was rich in mud, the high level of radiocesium in the lower layer (Fig. 4a and Table S1) was likely caused by sediment deposition rather than downward migration of radiocesium leaching from the upper layer. Heavy rainfalls probably caused the sediment deposition at the test site. Eleven rainfall events of 50 mm were observed at AMeDAS Yanagawa station 9 km north of the test site (JMA, 2017) from the Fukushima nuclear accident to the start of our test. The rainfall events were defined as independent from each other when they were separated by a period without rainfall of >6 h. Other reports (Mikami et al., 2014; Saegusa et al., 2016) have shown more radiocesium- and mud-rich sediments in flood channels than in river beds. They suggested that mud tends to be swept off river beds by the constant flow of water, and that plant communities in flood channels reduce flow velocity during floods, favoring the deposition of sediment with mud. A dense plant community grew on the flood channel in this test site (Fig. 2), meaning that these factors likely caused differences in the accumulation of radiocesium in the flood channel and the river bed. The mud fraction distribution in the flood channel sediments was also vertically and horizontally heterogeneous (Fig. 4a and
Fig. 4. Vertical distribution of the radiocesium activity concentration and the mud fraction in flood channel sediment (a), dike slope soil (b), and river bed sediment (c).
T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
63
is limited. 4. Conclusions Typical decontamination measures, such as removing topsoil to a depth of several centimeters, are not appropriate for rivers where contaminated sediments have been deposited. In this paper, we tested a decontamination measure that reflects the spatial distribution of radiocesium at a riverside in Fukushima, where part of the dike and the flood channel were used for commuting to school and for outdoor education, respectively. The methods and effectiveness are as follows.
Table S1), leading to the high heterogeneity of the radiocesium distribution. The deposited particle size depends on the flow velocity, and flow velocity probably varied with the sampling points because the plant density and microtopography were not uniform. Flow velocity also varied with flood event magnitude even at the same sampling points. These variations in flow velocity were probably caused by the high heterogeneity of the mud and radiocesium distribution in the sediments. Sediment erosion may also enhance the high heterogeneity.
1. In the flood channel, radiocesium distribution was highly heterogeneous vertically and horizontally, and accordingly, the sediment decontamination depth was adjusted to 15e35 cm to remove most radiocesium. This heterogeneity may be due to plant growth on the flood channel favoring the deposition of more radiocesium- and mud-rich sediment. 2. The dike slopes were decontaminated with a typical measure (topsoil removal). The river bed was not decontaminated, considering the low activity concentration in the river bed sediment and that the water shields gamma rays emitted from the sediment. 3. The decontamination measure reduced the air dose rate by a factor of approximately two. 4. Estimated annual external doses during the activities at the test site accounted for only 1e2% of the value during daily life in the surrounding area, indicating that radiation exposure during riverside activities is limited.
3.2. Effect of decontamination
Acknowledgements
3.2.1. Air dose rate after decontamination Table 1 and Fig. 3 show the air dose rates after the decontamination. The values decreased by a factor of approximately two. Further reduction probably required decontamination in the orchard on the right bank, which was not decontaminated, or shielding the gamma rays emitted from the orchard. Therefore, our decontamination measure was effective in this river and is probably applicable to other riversides, except from upstream watersheds, which have few flood channels in mountain valleys. There were no rainfall events of 50 mm recorded at AMeDAS Yanagawa station during the 3-month period following the completion of the decontamination (JMA, 2017). Thus, the air dose rate remained nearly constant in this period. Recontamination probably depends on the amount of mud deposition by subsequent flood events. This factor may be affected strongly by plant regrowth, and further investigation is necessary to understand recontamination.
We would like to thank the following people for valuable advice: Dr. Reiko Fujita (Japan Science and Technology Agency), Dr. Seiji Hayashi (National Institute for Environmental Studies), Dr. Kazuki Iijima (Japan Atomic Energy Agency), Dr. Takeshi Iimoto (The University of Tokyo), Dr. Tadashi Inoue (Central Research Institute of Electric Power Industry), Mr. Junichiro Ishida (Japan Atomic Energy Agency), Dr. Alexei Konoplev (Fukushima University), Dr. Hisao Nagabayashi (Nihon University), Dr. Kenji Nanba (Fukushima University), Dr. Yasuo Onishi (Washington State University), Dr. Gerhard Proehl (International Atomic Energy Agency), Dr. Satoru Tanaka (The University of Tokyo), Dr. Oleg Voitsekhovich (International Atomic Energy Agency), and Dr. Satoshi Yoshida (National Institute of Radiological Sciences). We would also like to express our gratitude to Dr. Akihiro Kitamura (Japan Atomic Energy Agency) for data analysis support.
Fig. 5. Comparison of 137Cs activity concentration with mud fraction in sediment in flood channel and river bed before decontamination.
3.2.2. External exposure dose The additional annual external exposure dose during commuting and outdoor education in the test site was estimated to be 0.029 and 0.016 mSv before and after the decontamination, respectively. In Kami-Oguni and Shimo-Oguni districts, almost the same as the area of our test watershed, annual external exposures were determined for daily life based on glass dosimeter measurements. The average additional external exposure during the period from July 2013 to June 2014 was 1.2 mSv (Date City, 2015). The dose during the activities at the test site (commuting and outdoor education) accounted for only 2.3% and 1.3% of the total value before and after the decontamination, respectively. This underlines that the additional radiation exposure during activities at the riverside
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2017.06.005. References Buesseler, K., Dai, M., Aoyama, M., Benitez-Nelson, C., Charmasson, S., et al., 2016. Fukushima Daiichi-derived radionuclides in the ocean: transport, fate, and impacts. Annu. Rev. Mar. Sci. 9 (1), 1e31. Burrough, P.A., van der Perk, M., Howard, B.J., Prister, B.S., Sansone, U., Voitsekhovitch, O.V., 1999. Environmental mobility of radiocesium in the Pripyat catchment, Ukraine/Belarus. Water Air Soil Pollut. 110, 35e55. Date City, 2015. Reconstr. recovery news at Date City (translated by autors) 22, 1e4. http://www.city.date.fukushima.jp/soshiki/12/763.html (in Japanese, accessed 13.02.17). Geological Survey of Japan, 2015. Seamless Digital Geological Map of Japan 1: 200000, May 29, 2015 Version. Geol. Surv. Jpn., Natl. Inst. Adv. Ind. Sci. Technol.
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International Atomic Energy Agency, 2006. Environmental Consequences of the Chernobyl Accident and Their Remediation: Twenty Years of Experience Report of the Chernobyl Forum Expert Group ‘environment’. Int. At. Energy Agency, Vienna. Japan Meteorological Agency, 2017. Data and Material. http://www.jma.go.jp/jma/ menu/menureport.html (in Japanese, accessed 13.02.17). Konoplev, A., Golosov, V., Laptev, G., Nanba, K., Onda, Y., et al., 2016. Behavior of accidentally released radiocesium in soilewater environment: looking at Fukushima from a Chernobyl perspective. J. Environ. Radioact. 151, 568e578. Matsuda, N., Mikami, S., Shimoura, S., Takahashi, J., Nakano, M., et al., 2015. Depth profiles of radioactive cesium in soil using a scraper plate over a wide area surrounding the Fukushima Dai-ichi Nuclear Power Plant. Jpn. J. Environ. Radioact. 139, 427e434. Mikami, T., Maie, N., Shimada, H., Kakizaki, T., Takamatsu, R., 2014. Influence of Microtopography on the accumulation of radiocesium in a waterside land: a case study of a secondary branch of the Abukuma River flowing through Fukushima Prefecture. J. Jpn. Soc. Water Environ. 37 (6), 259e264. Minato, S., 2006. Distribution of terrestrial g ray dose rates in Japan. J. Geogr. 115 (1), 87e95 (in Japanese with English abstract). Ministry of Education, Culture, Sports, Science and Technology, 2011. Results of
Deposition of Radioactive Cesium of the Third Airborne Monitoring Survey. http://emdb.jaea.go.jp/emdb/en/portals/b224/ (accessed 13.02.17). Ministry of Land, Infrastructure, Transport and Tourism, 2017. National Land Numerical Information. http://nlftp.mlit.go.jp/ksj-e/index.html (accessed 13.02.17). Ministry of the Environment, Government of Japan, 2013. Decontamination Guidelines, second ed. Ministry of the Environment, Government of Japan, 2017. Environmental Remediation. http://josen.env.go.jp/en/ (accessed 13.02.17). Morino, Y., Ohara, T., Nishizawa, M., 2011. Atmospheric behavior, deposition, and budget of radioactive materials from the Fukushima Daiichi nuclear power plant in March 2011. Geophys. Res. Lett. 38 (7). Nakao, A., Funakawa, S., Tsukada, H., Kosaki, T., 2012. The fate of caesium-137 in a soil environment controlled by immobilization on clay minerals. SANSAI Environ. J. Glob. Community 6, 17e29. Nuclear Regulation Authority, 2017. Extension site of distribution map of radiation dose. http://ramap.jmc.or.jp/map/eng/ (accessed 13.02.17). Saegusa, H., Ohyama, T., Iijima, K., Onoe, H., Takeuchi, R., Hagiwara, H., 2016. Deposition of radiocesium on the river flood plains around Fukushima. J. Environ. Radioact. 164, 36e46.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radiocesium decontamination of a riverside in Fukushima, Japan Tatsuhiro Nishikiori*, Satoshi Suzuki Fukushima Prefectural Centre for Environmental Creation, 10-2 Fukasaku, Miharu Town, Fukushima 963-7700, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2017 Received in revised form 25 May 2017 Accepted 4 June 2017
Extensive decontamination measures have been implemented in the area affected by the Fukushima Daiichi nuclear disaster. Typical decontamination measures, such as removing topsoil of several centimeters in depth, are not suitable for rivers where contaminated sediments have been deposited. A decontamination measure was tested that considered the spatial distribution of radiocesium at the lower part of a tributary of the Abukuma River in Fukushima. The radiocesium distribution in the flood channel was vertically and horizontally highly heterogeneous. In some parts, the activity concentration was high (>10 kBq/kg for 137Cs) even at depths of 25 cm in the sediment. This may be due to plant growth in the flood channel favoring the deposition of sediment with high activity concentration. On the basis of the radiocesium distribution, the flood channel sediment was removed to a depth of 15e35 cm, which accumulated the most radiocesium (>3.0 kBq/kg for the sum of 134Cs and 137Cs). The upper 5 cm of soil was removed from the dike slopes. The river bed was not decontaminated because the activity concentration was low (<1 kBq/kg) in the river bed sediment and because the water shields gamma rays emitted from the sediment. The test decontamination measure reduced the air dose rate by a factor of approximately two, demonstrating the effectiveness of our measures. Annual external doses were calculated for when this part of the dike and the flood channel is used for commuting to school and outdoor education. The doses during the activities at the test site accounted for only 1e2% of the value during daily life in the surrounding area, indicating that radiation exposure during riverside activities is limited. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Radiocesium River Decontamination Sediment Air dose rate
1. Introduction During the Fukushima Dai-ichi Nuclear Power Plant disaster in March 2011, several tens of petabecquerels (1 PBq ¼ 1015 Bq) of radiocesium (134Cs and 137Cs) were released into the atmosphere (Buesseler et al., 2016) and contaminated a wide area of eastern Japan (Morino et al., 2011). The physical half-life of 134Cs is relatively short (2.06 years), whereas that of 137Cs is longer (30.2 years), which may cause prolonged contamination similar to the Chernobyl nuclear accident (International Atomic Energy Agency, 2006). An extensive decontamination program has therefore been implemented over a wide area of eastern Japan to reduce radiation exposure (Ministry of the Environment, Government of Japan (MOE), 2017). The decontamination measures focus on living areas such as houses, schools, roads, parks, agricultural land, and forest adjacent to inhabited areas (MOE, 2013). Because radiocesium is characterized by strong adsorption to clay minerals
* Corresponding author. E-mail address: [email protected] (T. Nishikiori). http://dx.doi.org/10.1016/j.jenvrad.2017.06.005 0265-931X/© 2017 Elsevier Ltd. All rights reserved.
(Nakao et al., 2012), most radiocesium in soil is retained in the top layer to a depth of a few centimeters (Matsuda et al., 2015). The MOE accordingly recommends soil decontamination measures, including topsoil removal, deep plowing, and covering of soil surfaces (MOE, 2013). According to the MOE decontamination guidelines (MOE, 2013), living areas also include frequently used riverside parks and playing fields. Sediments containing radiocesium may be deposited on riversides during floods; thus, the radiocesium inventory is sometimes greater on these sediments than in the surrounding area (Burrough et al., 1999; Konoplev et al., 2016). However, sediment deposition is highly heterogeneous and sediment erosion occurs continuously. Consequently, the radiocesium distribution on river banks and reservoirs is expected to be vertically and horizontally heterogeneous. For such conditions, standard soil decontamination measures, where only the top layer is removed, are not suitable for riversides, and appropriate decontamination measures have not been established. In this study, we tested a decontamination measure based on the vertical and horizontal distribution of radiocesium at a river in Fukushima.
T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
2. Materials and methods 2.1. Site description The test site was 55 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (N37460 200 , 140 340 4900 ) and at the lower part of the Kami-Oguni River (Fig. 1a and b), a tributary of the Abukuma River, which is the largest river in Fukushima. The total deposition density of 134Cs and 137Cs was 440 kBq/m2 (decay correction date: July 2, 2011), and the air dose rate at 1 m above the ground was 0.9 mSv/h on November 7, 2014, according to the
59
airborne monitoring survey (Nuclear Regulation Authority, 2017). The annual mean temperature and precipitation in the period 2006 to 2015 were 12.7 C and 1160 mm, respectively, observed at the AMeDAS Yanagawa station 9 km north of the test site (Japan Meteorological Agency (JMA), 2017). The area and the elevation of the Kami-Oguni River watershed are 13.2 km2 and 110e580 m, respectively (Fig. 1b), calculated from data provided by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT, 2017). The lower parts of the catchment are used as residential areas and for agriculture, and the hilly area is covered with forest (Fig. 1c). The residential land, agricultural land, and forest account for 2%, 22%,
Fig. 1. Map of the Kami-Oguni River watershed (a), topographic map of the watershed (b), land-use map of the watershed (c), the decontamination area in the test site (d), and schematic cross-section of the test site (e). Radiocesium deposition densities (decay correction date: July 2, 2011) are derived from the third airborne monitoring survey data (Ministry of Education, Culture, Sports, Science and Technology, 2011). Various landscape elements are cited from data provided by MLIT (2017).
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T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
and 75% of the watershed area, respectively. Decontamination has been implemented in the living areas, but not in most of the agricultural land and forest. The parent rock material of the watershed comprises Cretaceous granodiorite and Miocene non-alkaline mafic volcanic rocks (Geological Survey of Japan, 2015). The test site consisted of a river, which is approximately 170 m long and 2e6 m wide, a flood channel with a width of up to 10.5 m, and dikes, which are approximately 3 m wide and 2 m high (Fig. 1d and e). The dikes are partly covered with concrete. The left bank, the right bank, and the flood channel were used as a school route, an orchard, and a place for outdoor education, respectively. The school route on the left dike was decontaminated before the experiment was performed. We chose this riverside as the test site because the air dose rate exceeded the MOE decontamination criterion value (>0.23 mSv/h (MOE, 2013)) and the residents requested decontamination. 2.2. Decontamination test process We decontaminated the flood channel and the dike slopes from August to November 2014. Fig. 2 shows the test process. First, we removed all plants from the flood channel from August to September, and then removed a soil layer approximately 5 cm thick
from the dike slopes with an excavator. Turf was put on the decontaminated slope from September to October 2014. We removed sediments from the flood channel from October to November 2014, and adjusted the removal depth to the vertical distribution of radiocesium, described later. The removed plants were taken to a waste incineration plant. The removed soil and sediments were packed into flexible container bags and taken to a temporary storage site for decontaminated soil. 2.3. Sampling and measurements We set lines across the river at 10 m intervals in the test site and measured air dose rates at 1 and 100 cm above the ground by using a NaI scintillation survey meter (TCS-172B, Hitachi Aloka Medical, Ltd. Tokyo, Japan) without a collimator. The measurements were taken before decontamination, at the completion of decontamination, and 3 months after decontamination (February 2015). We collected sediments in the flood channel and in the river bed and soil from the dike slopes just before the decontamination. The sampling depths were 20e40 cm in the flood channel, 15 cm in the river bed, and 5 cm in the dike slopes. The samples were collected with a trowel at seven points in the flood channel, seven points in the river bed, and one point in each of the dike slopes. We took
Fig. 2. Photographs of the decontamination test process.
T. Nishikiori, S. Suzuki / Journal of Environmental Radioactivity 177 (2017) 58e64
gravel with a diameter of >5 mm out of the samples, packed the treated samples into 100 mL plastic containers (50 mm inner diameter, 62 mm in height, U-8, As-one Co. Ltd. Osaka, Japan), and analyzed the samples for radiocesium. After the analysis, the sediment was sieved with water through a 63 mm mesh, dried at 105 C, and weighed to calculate the mud fraction (<63 mm). We measured the activities of 134Cs and 137Cs in the samples by using coaxial germanium detectors (GEM20-70 or GMX20200, ORTEC, Oak Ridge, TN). The counting error was <10%. We analyzed the gamma-ray spectra with Gamma Studio software (Seiko EG&G Co. Ltd. Tokyo, Japan), and used MX033U8PP (The Japan Radioisotope Association, Tokyo, Japan) as a standard source for efficiency calibration. The measured activity was corrected for radioactive decay at each sampling date.
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2.4. Calculation of external exposure dose We calculated the additional annual external exposure during commuting and outdoor education at the test site before and after the decontamination. The dose was estimated by multiplying the time spent on the dikes and the flood channel and the average air dose rates at a height of 100 cm on the left dike and the flood channel. A value of 0.04 mSv/h measured before the accident (Minato, 2006) was subtracted from the average air dose rates to account for the natural background. The times were 35 h per year for commuting (10 min per day and 210 days per year) and 24 h for outdoor education (2 h per day and 12 days per year).
Table 1 Air dose rate before and after decontamination test. Decontamination process
Air dose rate (mSv/h)
Measurement date 1 cm above the ground
Before decontamination Completion of decontamination 3 months after decontamination a
Aug. 23, 2014 Nov. 16, 2014 Feb. 27, 2015
100 cm above the ground a
Range
Mean ± SD
Range
Mean ± SDa
0.20e1.99 0.15e0.95 0.15e1.20
0.78 ± 0.41 0.34 ± 0.15 0.34 ± 0.17
0.29e1.17 0.18e0.78 0.19e0.77
0.66 ± 0.22 0.34 ± 0.11 0.34 ± 0.12
SD is a standard deviation.
Fig. 3. Maps of air dose rate 1 and 100 cm above the ground before and after decontamination test.
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3. Results and discussion 3.1. Air dose rate and activity concentration before decontamination The air dose rates at 1 and 100 cm above the ground before decontamination varied up to 10-fold and 4-fold, respectively (Table 1). The value at 1 cm above the ground reflects the ground surface contamination at the measurement point; thus, the wide range of values at a height of 1 cm suggests a highly heterogeneous radiocesium distribution. On the flood channel, relatively high values of >1 mSv/h were observed, mainly on the right bank (Fig. 3), indicating selective accumulation of radiocesium. Fig. 4a and Table S1 presents the vertical distribution of the radiocesium concentration in the flood channel sediment before the decontamination. The depth profiles varied from point to point. At points 2, 3, and 4, sediments had high activity concentrations (>10 kBq/kg for 137Cs) to a depth of 15e25 cm, whereas at point 5, the activity concentration was low to moderate (1.0 to 5.0 kBq/kg for 137Cs) at all depths. At points 1 and 7, the activity concentration peaked at the top layer and decreased with depth. These conditions indicate that the vertical and horizontal radiocesium distributions were highly heterogenous in the flood channel. The activity concentration was moderate (4.1 to 5.5 kBq/kg for 137Cs) in the soil in the dike slopes (Fig. 4b), and was low in the river bed sediments (Fig. 4c and Table S1), 70% of which was <1.0 kBq/kg for 137Cs. Considering these distributions, we adjusted the removal depth of the flood channel sediment to 15e35 cm to remove most radiocesium (>3.0 kBq/kg for the sum of 134Cs and 137Cs). We did not
decontaminate the river bed because the activity concentration in the sediment was low and the water shields the gamma rays emitted from the sediment. In Fig. 5, the 137Cs activity concentration in the total sediments is plotted against the mud fraction in the sediments of the flood channel and the river bed before decontamination. The activity concentration is positively correlated with the mud fraction in the sediments. This is probably because of the strong adsorption of radiocesium to clay minerals (Nakao et al., 2012). Because the sediment in the flood channel was rich in mud, the high level of radiocesium in the lower layer (Fig. 4a and Table S1) was likely caused by sediment deposition rather than downward migration of radiocesium leaching from the upper layer. Heavy rainfalls probably caused the sediment deposition at the test site. Eleven rainfall events of 50 mm were observed at AMeDAS Yanagawa station 9 km north of the test site (JMA, 2017) from the Fukushima nuclear accident to the start of our test. The rainfall events were defined as independent from each other when they were separated by a period without rainfall of >6 h. Other reports (Mikami et al., 2014; Saegusa et al., 2016) have shown more radiocesium- and mud-rich sediments in flood channels than in river beds. They suggested that mud tends to be swept off river beds by the constant flow of water, and that plant communities in flood channels reduce flow velocity during floods, favoring the deposition of sediment with mud. A dense plant community grew on the flood channel in this test site (Fig. 2), meaning that these factors likely caused differences in the accumulation of radiocesium in the flood channel and the river bed. The mud fraction distribution in the flood channel sediments was also vertically and horizontally heterogeneous (Fig. 4a and
Fig. 4. Vertical distribution of the radiocesium activity concentration and the mud fraction in flood channel sediment (a), dike slope soil (b), and river bed sediment (c).
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is limited. 4. Conclusions Typical decontamination measures, such as removing topsoil to a depth of several centimeters, are not appropriate for rivers where contaminated sediments have been deposited. In this paper, we tested a decontamination measure that reflects the spatial distribution of radiocesium at a riverside in Fukushima, where part of the dike and the flood channel were used for commuting to school and for outdoor education, respectively. The methods and effectiveness are as follows.
Table S1), leading to the high heterogeneity of the radiocesium distribution. The deposited particle size depends on the flow velocity, and flow velocity probably varied with the sampling points because the plant density and microtopography were not uniform. Flow velocity also varied with flood event magnitude even at the same sampling points. These variations in flow velocity were probably caused by the high heterogeneity of the mud and radiocesium distribution in the sediments. Sediment erosion may also enhance the high heterogeneity.
1. In the flood channel, radiocesium distribution was highly heterogeneous vertically and horizontally, and accordingly, the sediment decontamination depth was adjusted to 15e35 cm to remove most radiocesium. This heterogeneity may be due to plant growth on the flood channel favoring the deposition of more radiocesium- and mud-rich sediment. 2. The dike slopes were decontaminated with a typical measure (topsoil removal). The river bed was not decontaminated, considering the low activity concentration in the river bed sediment and that the water shields gamma rays emitted from the sediment. 3. The decontamination measure reduced the air dose rate by a factor of approximately two. 4. Estimated annual external doses during the activities at the test site accounted for only 1e2% of the value during daily life in the surrounding area, indicating that radiation exposure during riverside activities is limited.
3.2. Effect of decontamination
Acknowledgements
3.2.1. Air dose rate after decontamination Table 1 and Fig. 3 show the air dose rates after the decontamination. The values decreased by a factor of approximately two. Further reduction probably required decontamination in the orchard on the right bank, which was not decontaminated, or shielding the gamma rays emitted from the orchard. Therefore, our decontamination measure was effective in this river and is probably applicable to other riversides, except from upstream watersheds, which have few flood channels in mountain valleys. There were no rainfall events of 50 mm recorded at AMeDAS Yanagawa station during the 3-month period following the completion of the decontamination (JMA, 2017). Thus, the air dose rate remained nearly constant in this period. Recontamination probably depends on the amount of mud deposition by subsequent flood events. This factor may be affected strongly by plant regrowth, and further investigation is necessary to understand recontamination.
We would like to thank the following people for valuable advice: Dr. Reiko Fujita (Japan Science and Technology Agency), Dr. Seiji Hayashi (National Institute for Environmental Studies), Dr. Kazuki Iijima (Japan Atomic Energy Agency), Dr. Takeshi Iimoto (The University of Tokyo), Dr. Tadashi Inoue (Central Research Institute of Electric Power Industry), Mr. Junichiro Ishida (Japan Atomic Energy Agency), Dr. Alexei Konoplev (Fukushima University), Dr. Hisao Nagabayashi (Nihon University), Dr. Kenji Nanba (Fukushima University), Dr. Yasuo Onishi (Washington State University), Dr. Gerhard Proehl (International Atomic Energy Agency), Dr. Satoru Tanaka (The University of Tokyo), Dr. Oleg Voitsekhovich (International Atomic Energy Agency), and Dr. Satoshi Yoshida (National Institute of Radiological Sciences). We would also like to express our gratitude to Dr. Akihiro Kitamura (Japan Atomic Energy Agency) for data analysis support.
Fig. 5. Comparison of 137Cs activity concentration with mud fraction in sediment in flood channel and river bed before decontamination.
3.2.2. External exposure dose The additional annual external exposure dose during commuting and outdoor education in the test site was estimated to be 0.029 and 0.016 mSv before and after the decontamination, respectively. In Kami-Oguni and Shimo-Oguni districts, almost the same as the area of our test watershed, annual external exposures were determined for daily life based on glass dosimeter measurements. The average additional external exposure during the period from July 2013 to June 2014 was 1.2 mSv (Date City, 2015). The dose during the activities at the test site (commuting and outdoor education) accounted for only 2.3% and 1.3% of the total value before and after the decontamination, respectively. This underlines that the additional radiation exposure during activities at the riverside
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