Paper
IMAGING PLANT LEAVES TO DETERMINE CHANGES IN RADIOACTIVE CONTAMINATION STATUS IN FUKUSHIMA, JAPAN Hiroo Nakajima,* Mamoru Fujiwara,† Isao Tanihata,† Tadashi Saito,‡ Norihiro Matsuda,§ and Takeshi Todo* AbstractVThe chemical composition of plant leaves often reflects environmental contamination. The authors analyzed images of plant leaves to investigate the regional radioactivity ecology resulting from the 2011 accident at the Fukushima No. 1 nuclear power plant, Japan. The present study is not an evaluation of the macro radiation dose per weight, which has been performed previously, but rather an image analysis of the radioactive dose per leaf, allowing the capture of various gradual changes in radioactive contamination as a function of elapsed time. In addition, the leaf analysis method has potential applications in the decontamination of food plants or other materials. Health Phys. 106(5):565Y570; 2014 Key words: imaging
137
Cs; Chernobyl; contamination, environmental;
INTRODUCTION THE UNPRECEDENTED M9 East Japan Earthquake and accident at the Fukushima No. 1 nuclear power plant (NPP) occurred on 11 March 2011. The subsequent hydrogen explosion at the reactor building dispersed water-soluble radioactive materials such as highly radioactive iodine (131I) and radioactive cesium (134Cs and 137Cs). Water containing these radioactive materials evaporated into the atmosphere. Pollutants were transported by wind and rain, which eventually resulted in the deposition of high-level pollutants northeast from the Fukushima No. 1 NPP (Fujiwara et al. 2011; MEXT 2011).
*Department of Radiation Biology and Medical Genetics, Graduate School of Medicine; †Research Center for Nuclear Physics; ‡Radioisotope Research Center, Osaka University; §Headquarters of Fukushima Partnership Operations, Japan Atomic Energy Agency. The authors declare no conflicts of interest. For correspondence contact: Hiroo Nakajima, Department of Radiation Biology and Medical Genetics (B4), Graduate School of Medicine, Osaka University, 2-2, Yamada-Oka, Suita, Osaka 565-0871, Japan, or email at [email protected]. (Manuscript accepted 26 August 2013) 0017-9078/14/0 Copyright * 2014 Health Physics Society DOI: 10.1097/HP.0000000000000020
Fukushima has abundant natural areas, and buildings, soil, and open grounds such as fields, mountainous areas, and plant leaves suffered widespread radioactive contamination. Thus, by investigating plant leaves, variations in regional contamination could be identified. Because the visualization of radioactive pollution gives an intuitive perception, reports of image analyses of radioactive contamination in plants (Bersina et al. 1995; Nakajima et al. 1998; Soudek et al. 2006; Sawidis et al. 2010) and animals (Yamaguchi et al. 2012) increased after the Chernobyl accident. However, certain drawbacks of the image analysis, such as its inability to measure doses of radioactivity accurately and identify radionuclides, led to the use of image analysis as a minor method. In the present work, the authors did not employ conventional evaluation methods, which use large numbers of samples to measure radiation levels on leaves. Instead, they performed image analysis of leaf samples to monitor temporal changes in contamination and showed the advantages of using image analysis. MATERIALS AND METHODS Leaf samples taken from radioactively contaminated areas were placed on a BAS imaging plate (20 cm 25 cm, 20 cm 40 cm; Fujifilm Corporation, Tokyo, Japan) and were stored in cool, dark conditions for exposure periods of 3 or 7 d. Latent imaging was conducted using a scanner-type image analysis device (Typhoon FLA 7000, GE Healthcare) to visualize the distribution of radioactivity on leaf surfaces. The intensity of radioactivity was measured as the PSL (photo-stimulated luminescence) value at 50 mm pixelj1. The PSL values ranged from 12,798 (background) to 65,535 (upper limit). In the images, areas of high radioactivity on leaf surfaces were represented as darker patches (Figs. 1, 2, and 3). To visualize the semi-quantification of radioactivity on a leaf, a concentration series of 137Cs solutions (0.5, 1, 5, 10, and 50 Bq per 5 mL) was prepared as a scale of radioactivity levels. Each solution was dropped onto 565
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Fig. 1. Distribution of radioactive cesium on bamboo leaves sampled at Iitate village located 42 km from the Fukushima No. 1 NPP. Samples were taken 109 d after the NPP accident. Panel (a) shows old leaf; panel (b) shows young leaf.
water-absorbent paper (diameter 6 mm) and was exposed to the imaging plate containing leaf and wood samples. The PSL values at 50 mm pixelj1 measured by imaging the plates were converted to a color gradient (blue, green, yellow, and red) using image analysis software (Win Roof, Mitani Corporation, Tokyo, Japan), and this is shown at the right side of Fig. 4 and 5. The colors from blue to red represent levels of radioactivity ranging from background to high levels. A series of 137Cs radioactivity images were visualized using a color scale with units of Bq per area (6 mm in diameter circles) at the bottom right of Figs. 4 and 5. To confirm the major radioactive contaminants of the samples, gamma spectrometry of leaves and trees was performed using Standard Electrode Coaxial Ge Detectors GC3018 (CANBERRA Industries Inc., Connecticut, USA), and 137Cs and 134Cs were identified as major nuclides with an approximate mass ratio of 1:0.98.
sizes of fallout particles scattered by the first explosion at the NPP were reflected in each radioactive spot. In contrast, no black spots are present on the image of the more recent leaves, which grew after all airborne radioactive fallout was washed out. The results indicate that leaf growth subsequent to the accident was generally not exposed to radioactive fallout. However, small leaf veins
RESULTS AND DISCUSSION Imaging of the collected leaves Fig. 1 shows an image of bamboo leaves sampled from a mountainous site at Iitate village along Highway 115, located 42 km from the Fukushima NPP. The samples were collected 109 d after the NPP accident (12 MarchY29 June 2011). The upper panel (a) shows old leaf growth prior to the accident, and the lower panel (b) shows young leaf growth after the accident. Several black punctated areas that adhered to the surface of the old leaves are due to radioactive fallout. This might have occurred also because of rain washing out airborne radioactive materials that subsequently dried on the leaf surface. Individual spot radioactivity on the leaves was estimated roughly using the dose response curve of the PSL values of the prepared 137Cs scale. The dose range of measurable spots was 1.5Y16.0 Bq. It seemed that the
Fig. 2. Distribution of radioactive cesium on sorrel leaves sampled at Fukushima city located 62 km from the Fukushima No. 1 NPP. From the top panel, leaves were sampled at the same location on days 78 (a), 119 (b), and 133 (c).
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Fig. 3. Distribution of radioactive cesium on bamboo leaves at Futaba town located 10 km from the Fukushima No. 1 NPP. Samples were taken 389 d after the accident. Panel (a) shows the old leaf; panel (b) shows the young leaf.
and surrounding areas are slightly visible in the images, showing a diffuse spreading of radioactive material to every part of the leaf. The presence of radioactive materials in the old leaves is explained by the scattering of radioactive materials. The presence of radioactive materials in new leaves is explained by the uptake of radioactive materials from the soil through the roots and subsequent migration to the leaf veins (Baeza et al. 1999; Tagami et al. 2012). Fig. 2 shows an image of sorrel leaves sampled from the front area of Fukushima railway station that is of higher sensitivity than the image in Fig. 1. The site is located 62 km from the Fukushima NPP and was not classified as an evacuation zone. The three leaf samples were obtained from the same location on days 78 (29 May 2011, a), 119 (9 July 2011, b), and 133 (23 July 2011, c). Fig. 2 indicates lower radioactive contamination than the images of bamboo leaves shown in Fig. 1. The contamination level decreases over time, but the contamination pattern is similar to that of the bamboo leaves (Fig. 1a); the images of day 78 show more black spots due to adhered radioactive fallout (Fig. 2a). There are fewer black spots in the images of days 119 (Fig. 2b) and 133 (Fig. 2c); however, the leaf vein and surrounding areas become homogeneous, suggesting that low-level radioactivity spreads throughout the leaf (Baeza et al. 1999; Tagami et al. 2012).
In both Figs. 1 and 2, high concentrations of radioactive fallout are observed mainly on the surface of leaves sampled soon after the accident, but surface contamination declines over time because of dispersion by wind and rain. However, it is understood that radioactive material in the upper soil profile accumulates internally in plant leaves via uptake by plant roots (White et al. 2000; Zhu et al. 2000; NCRP 2006). Fig. 3 shows an image of bamboo leaves sampled at the town of Futaba, located 10 km from the NPP and hence within the 20 km exclusion zone. The imaging measurement started 389 d (5 April 2012) after the NPP accident. Panel (a) shows an old leaf grown prior to the accident; panel (b) shows leaf growth after the accident. Even 1 y after the accident, surface contamination within the highly polluted area shows little decline (Fig. 3a). On the other hand, the levels of radioactive materials in the leaf veins have not yet increased (Fig. 3b). Fig. 4 shows images of oak leaves (a) sampled at a highly contaminated area (Masani village, Belarus) in 1997, 11 y after the accident at the Chernobyl NPP. These images are compared with images of the bamboo leaf sample (b) from Iitate village (upper panel in Fig. 1) and the sorrel leaf sample (c) from Fukushima City (upper panel in Fig. 2). All the images used the same graphics processing
Fig. 4. Comparison of oak leaf (a), sampled in an area of high radioactivity (Masani village, Belarus) in 1997, 11 y after the Chernobyl NPP accident; bamboo leaf (b) from Iitate village; and sorrel leaf (c) from Fukushima City. www.health-physics.com
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Fig. 5. Comparison of cedar trees in high-contamination areas of Fukushima. Tree trunks were sampled at 0 km (left panel a), 5 km (center panel b), and 10 km (right panel c) from Fukushima NPP on day 389.
method. The radioactivity in each leaf can be semiquantitatively compared using the color scale presented at the bottom right of Fig. 4 (the color circles indicate Bq per circle area). Significant differences were observed in radiation intensity, distribution, and contamination status. In particular, the Chernobyl leaf sample does not show any punctated contamination pattern. Radioactivity was observed in the leaf veins with decreasing concentration toward the tip, suggesting the accumulation of contaminants due to root uptake. Thus, it is likely that a similar pattern of contamination will be observed in leaves from the contaminated Fukushima areas in future. Detailed observations from the images revealed the contamination pattern of radiocesium and suggest in vivo dynamics similar to those of alkaline potassium (White et al. 2000; Zhu et al. 2000; NCRP 2006). The changes in the contamination pattern were similar to the aforementioned, which is easy to capture using radioactivity image analysis because it is not measured in kilograms of cumulative dose (the detectable contamination patterns are: dose distribution plane, dose distribution inside and outside the living tissue, pharmacokinetics, and changes over time of dose distribution). Further, considering the characteristics of leaves, it is understood that accumulation of radioactivity in evergreen trees may be assessed over a long period. However, pulsatile and short-term contamination may be assessed in deciduous trees and annual grasses. In areas of low contamination, the detection sensitivity is increased by measuring the cumulative dose in evergreen trees. The analysis of imaging plates enables the efficient quantification of radiation levels, because the detection sensitivity is from 10 to several hundred times greater than that obtained by a photographic method (autoradiogrphy). In the case of 100 Bq cmj2 of 137Cs contamination, a 10-min exposure to the imaging plate is sufficient to produce an image of the radioactivity pattern. Furthermore, image analysis permits easy monitoring of radioactive contamination in materials other than leaves.
From contamination of leaves to future expectation From the images shown in Figs. 1b, 2b, and 3b, it is understood that after the accident, the progression of internal radioactive contamination in young leaf growth occurred via transportation from the surface of old leaves or uptake from the contaminated soil. However, unlike surface contamination, the currently identified internal contamination is due to absorption that progresses extremely slowly. This might be because radioactive cesium fallout has a strong affinity to clay soil, making it difficult for plants to absorb. Additionally, the transfer coefficient to grains and vegetables was 0.01Y0.026 (Nisbet and Woodman 2000; IAEA 1994). The transfer coefficient is equal to the concentration in agricultural edible parts (Bq gj1 raw weight) per unit concentration in the soil (Bq gj1 dry weight). Since cesium has a strong affinity to the soil, the downward penetration of radioactive cesium into the deep soil layer is very slow. In soil samples taken from Fukushima in June 2011, contamination levels at a depth of 4 cm were less than one-tenth of those at the soil surface. A similar result was reported for soil from the Chernobyl area, where 95% of the radioactive cesium remained within a 5 cm depth even 10 y after the incident (Nakajima et al. 2000, 2008). However, further studies are required to assess whether a similar pattern will occur in Fukushima, where precipitation is greater than that in other areas. The authors consider that, following the accident, livestock and wild herbivores in the radioactive areas initially ate leaves with high concentrations of surface contaminants but subsequently ate young leaves with low concentrations of contaminants. In a previous study, drinking water containing 137Cs was given to mice (1 kBq gj1 body weight) and the resulting internal contamination was evaluated. It was found that the approximate concentration of 137 Cs in all internal organs reduced the initial concentration to half after 2 d. However, the concentration in muscles peaked after more than 2 d and then reduced by half after day 13 (Nakajima 2011). Thus, in cases where excretion
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Imaging plant leaves to determine contamination c H. NAKAJIMA
exceeds intake, 137Cs is not accumulated in the animal’s body, and the total amount of contamination in the body gradually decreases. On the other hand, cesium radioactivity strongly absorbed in soil gradually accumulates in shallow-root shrubs and annual weeds. When these plants die, the plant matter forms a compost layer. Thereafter, 137Cs is absorbed by large plants directly from the compost layer. It is important to understand levels of plant contamination in Fukushima because plants form the first link in the food chain for animals and humans. Application of image analysis It is well known that radioactive cesium is absorbed by plants and that this phenomenon is employed in soil decontamination methods. Members of the Amaranthaceae family, such as Amaranthus retroflexus, Amaranthus lividus, and Achyranthes japonica, and sunflowers from the family Compositae are candidate plants with high absorption efficiency for 137Cs (Dushenkov et al. 1999; Fuhrmann et al. 2003; Soudek et al. 2006). However, because the root lengths and sizes differ among plants, appropriate plant species must be selected carefully and adjusted according
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to the contamination status in order to maximize the absorption efficiency of radioactive cesium from the shallow soil layer. It is also necessary to avoid disruption to the current ecosystem due to the introduction of non-native plants. Image analysis of leaves can be used not only to evaluate surface contamination of radioactive cesium but also to assess leaf contamination to identify candidate plants from the existing Fukushima flora that show good internal absorption. This method can be used to identify plants with distinct biological or chemical ability to accumulate radioactive cesium in the inedible parts as a means of efficient radioactive decontamination in combination with productive farming. In addition, the contamination of building materials has become a serious public concern in Japan, because waste materials from the lumber industry may not undergo detailed contamination checks. Fig. 6 compares images used to analyze contamination status of cut pine trees taken in 1997 from non-contaminated (Ribzky village), moderately contaminated (Babchin village), and highly contaminated (Masani village) areas after the Chernobyl accident. Since the 137Cs, unlike radioactive 14C, was not accumulated by biological organisms, it was not incorporated into the annual tree-growth rings following the accident in
Fig. 6. Comparison of pine trees sampled at areas of high contamination (Masani village: a, b), moderate contamination (Babchin village: c, d), and no contamination (Ribzky village: e, f ) in 1997, 11 y after the Chernobyl NPP accident. www.health-physics.com
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1986. These cesium radioactivities have pharmacokinetics similar to that of potassium. Therefore, they are not accumulated in internal growth rings consisting of inactive cells; however, they are accumulated in the bark, which consists of active and dividing cells and has more electrically conductive ducts and sieve tubes (Nakajima et al. 1998). Fig. 5 shows the contamination status of cedar trees in the highly contaminated areas of Fukushima (tree trunks were sampled 0, 5, and 10 km from Fukushima No.1 NPP in 2012, 389 d after the accident). These images show that strong radio-contamination appears only at the wood surface and that the internal uptake of radiocesium seen in the Chernobyl pines has not yet been observed at Fukushima (see Fig. 6). The radioactivity in each tree can be semi-quantitatively compared using the color scale presented at the bottom right of Fig. 5 (the color circles indicate Bq per circle area). The colors from blue to red represent levels of radioactivity from background to high levels. Even in the case of trees that grew in heavily contaminated areas, a cross-sectional image analysis of contamination status would indicate that the trees could be decontaminated by scraping off the tree bark. CONCLUSION Plants are the first link in the food chain for animals and humans. Therefore, it is important to understand contamination levels within plants in the Fukushima area. Timely response to the constantly changing contamination status is the most effective approach to minimizing contamination damage. Therefore, it is anticipated that the image analysis of plants will play a major role in minimizing radioactive contamination, although the image analysis has a drawback in that it does not identify the radionuclides. REFERENCES Baeza A, Paniagua JM, Rufu M, Sterling A, Barandica J. Radiocaesium and radiostrontium uptake by turnips and broad beans via leaf and root absorption. Appl Radiat Isot 50:467Y474; 1999. Bersina G, Brandt R, Vater P, Hinke K, Schu¨tze M. Fission track autoradiography as a means to investigate plants for their contamination with natural and technogenic uranium. Radiat Meas 24:277Y282; 1995. Dushenkov S, Mikheev A, Prokhnevsky A, Ruchko M, Sorochinsky B. Phytoremediation of radiocesium-contaminated soil in the vicinity of Chernobyl, Ukraine. Environmental Sci Technol 33:469Y475; 1999. Fuhrmann M, Lasat M, Ebbs S, Cornish J, Kochian L. Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments. J Environ Qual 32: 2272Y2279; 2003.
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Fujiwara M, Tanihata I, Saito K. Distribution map of radiation dose in Fukushima Eonline^. 2011. Available at http://www.rcnp.osakau.ac.jp/dojo/. Accessed 3 August 2013. International Atomic Energy Agency. Handbook of parameter values for the prediction of radionuclide transfer in temperate environments. Vienna: IAEA; Technical Reports Series No. 364; 1994. Ministry of Education, Culture, Sports, Science and Technology (MEXT), Distribution map of radiation dose around Fukushima Dai-ichi & Dai-Ni NPP Eonline^. 2011. Available at http://www.mext.go.jp/english/incident/1305901.htm. Accessed 10 August 2013. Nakajima H, Ryo H, Yamaguchi Y, Saito T, Yeliseeva KG, Piskunov EV, Krupnova EV, Voitovich AM, Nomura T. Biological concentration of radionuclides in plants and animals after the Chernobyl catastrophe. In: Sato F, Yamada Y, Onodera J, eds. Biological effects of low dose radiation. Aomori, Japan: Institute for Environmental Sciences; 2000: 199Y205. Nakajima H, Saito T, Ryo H, Nomura T. Ecological decrease and biological concentration of radionuclides in plants and animals after the Chernobyl catastrophe. In: Miura T, Kinoshita N, eds. Proceedings of the Eighth Workshop on Environmental Radioactivity. High Energy Accelerator Research Organization (KEK) Proceedings 2007-16; 2008: 113Y118. Nakajima H. How to convey the information of radiation impact correctly to the general public. Radiat Biol Res Commun. 46:120Y126; 2011. Nakajima H, Ryo H, Nomura T, Saito T, Yamaguchi Y, Yeliseeva KG. Radionuclides carved on the annual rings of a tree near Chernobyl. Health Phys 74:265Y267; 1998. National Council on Radiation Protection and Measurements. Cesuim-137 in the environment: radioecology and approaches to assessment and management. Washington, DC: NCRP; Report No. 154; 2006. Nisbet AF, Woodman RFM. Soil-to-plant transfer factors for radiocaesium and strontium in agricultural systems. Health Phys 78:279Y288; 2000. Sawidis T, Tsigaridas K, Tsikritzis L. Cesium-137 monitoring using lichens from W. Macedonia, N. Greece. Ecotoxicol Environ Safety 73:1789Y1796; 2010. Soudek P, Valenova´ Sˇ, Vavrˇ´ıkova´ Z, Vaneˇk T. 137Cs and 90Sr uptake by sunflower cultivated under hydroponic conditions. J Environ Radioact 88:236Y250; 2006. Tagami K, Uchida S, Ishii N, Kagiya S. Translocation of radiocesium from stems and leaves of plants and the effect on radiocesium concentrations in newly emerged plant tissues. J Environ Radioact 111:65Y69; 2012. White PJ, Broadley MR. Mechanisms of caesium uptake by plants. New Phytol 147:241Y256; 2000. Yamaguchi T, Sawano K, Furuhama K, Mori C, Yamada K. An autoradiogram of skeletal muscle from a pig raised on a farm within 20 km of the Fukushima Daiichi nuclear power plant. J Vet Med Sci 75:93Y94; 2012. Zhu YG, Smolders E. Plant uptake of radiocaesium: a review of mechanism, regulation and application. J Exp Bot 51: 1635Y1643; 2000.
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¡¡
IMAGING PLANT LEAVES TO DETERMINE CHANGES IN RADIOACTIVE CONTAMINATION STATUS IN FUKUSHIMA, JAPAN Hiroo Nakajima,* Mamoru Fujiwara,† Isao Tanihata,† Tadashi Saito,‡ Norihiro Matsuda,§ and Takeshi Todo* AbstractVThe chemical composition of plant leaves often reflects environmental contamination. The authors analyzed images of plant leaves to investigate the regional radioactivity ecology resulting from the 2011 accident at the Fukushima No. 1 nuclear power plant, Japan. The present study is not an evaluation of the macro radiation dose per weight, which has been performed previously, but rather an image analysis of the radioactive dose per leaf, allowing the capture of various gradual changes in radioactive contamination as a function of elapsed time. In addition, the leaf analysis method has potential applications in the decontamination of food plants or other materials. Health Phys. 106(5):565Y570; 2014 Key words: imaging
137
Cs; Chernobyl; contamination, environmental;
INTRODUCTION THE UNPRECEDENTED M9 East Japan Earthquake and accident at the Fukushima No. 1 nuclear power plant (NPP) occurred on 11 March 2011. The subsequent hydrogen explosion at the reactor building dispersed water-soluble radioactive materials such as highly radioactive iodine (131I) and radioactive cesium (134Cs and 137Cs). Water containing these radioactive materials evaporated into the atmosphere. Pollutants were transported by wind and rain, which eventually resulted in the deposition of high-level pollutants northeast from the Fukushima No. 1 NPP (Fujiwara et al. 2011; MEXT 2011).
*Department of Radiation Biology and Medical Genetics, Graduate School of Medicine; †Research Center for Nuclear Physics; ‡Radioisotope Research Center, Osaka University; §Headquarters of Fukushima Partnership Operations, Japan Atomic Energy Agency. The authors declare no conflicts of interest. For correspondence contact: Hiroo Nakajima, Department of Radiation Biology and Medical Genetics (B4), Graduate School of Medicine, Osaka University, 2-2, Yamada-Oka, Suita, Osaka 565-0871, Japan, or email at [email protected]. (Manuscript accepted 26 August 2013) 0017-9078/14/0 Copyright * 2014 Health Physics Society DOI: 10.1097/HP.0000000000000020
Fukushima has abundant natural areas, and buildings, soil, and open grounds such as fields, mountainous areas, and plant leaves suffered widespread radioactive contamination. Thus, by investigating plant leaves, variations in regional contamination could be identified. Because the visualization of radioactive pollution gives an intuitive perception, reports of image analyses of radioactive contamination in plants (Bersina et al. 1995; Nakajima et al. 1998; Soudek et al. 2006; Sawidis et al. 2010) and animals (Yamaguchi et al. 2012) increased after the Chernobyl accident. However, certain drawbacks of the image analysis, such as its inability to measure doses of radioactivity accurately and identify radionuclides, led to the use of image analysis as a minor method. In the present work, the authors did not employ conventional evaluation methods, which use large numbers of samples to measure radiation levels on leaves. Instead, they performed image analysis of leaf samples to monitor temporal changes in contamination and showed the advantages of using image analysis. MATERIALS AND METHODS Leaf samples taken from radioactively contaminated areas were placed on a BAS imaging plate (20 cm 25 cm, 20 cm 40 cm; Fujifilm Corporation, Tokyo, Japan) and were stored in cool, dark conditions for exposure periods of 3 or 7 d. Latent imaging was conducted using a scanner-type image analysis device (Typhoon FLA 7000, GE Healthcare) to visualize the distribution of radioactivity on leaf surfaces. The intensity of radioactivity was measured as the PSL (photo-stimulated luminescence) value at 50 mm pixelj1. The PSL values ranged from 12,798 (background) to 65,535 (upper limit). In the images, areas of high radioactivity on leaf surfaces were represented as darker patches (Figs. 1, 2, and 3). To visualize the semi-quantification of radioactivity on a leaf, a concentration series of 137Cs solutions (0.5, 1, 5, 10, and 50 Bq per 5 mL) was prepared as a scale of radioactivity levels. Each solution was dropped onto 565
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Fig. 1. Distribution of radioactive cesium on bamboo leaves sampled at Iitate village located 42 km from the Fukushima No. 1 NPP. Samples were taken 109 d after the NPP accident. Panel (a) shows old leaf; panel (b) shows young leaf.
water-absorbent paper (diameter 6 mm) and was exposed to the imaging plate containing leaf and wood samples. The PSL values at 50 mm pixelj1 measured by imaging the plates were converted to a color gradient (blue, green, yellow, and red) using image analysis software (Win Roof, Mitani Corporation, Tokyo, Japan), and this is shown at the right side of Fig. 4 and 5. The colors from blue to red represent levels of radioactivity ranging from background to high levels. A series of 137Cs radioactivity images were visualized using a color scale with units of Bq per area (6 mm in diameter circles) at the bottom right of Figs. 4 and 5. To confirm the major radioactive contaminants of the samples, gamma spectrometry of leaves and trees was performed using Standard Electrode Coaxial Ge Detectors GC3018 (CANBERRA Industries Inc., Connecticut, USA), and 137Cs and 134Cs were identified as major nuclides with an approximate mass ratio of 1:0.98.
sizes of fallout particles scattered by the first explosion at the NPP were reflected in each radioactive spot. In contrast, no black spots are present on the image of the more recent leaves, which grew after all airborne radioactive fallout was washed out. The results indicate that leaf growth subsequent to the accident was generally not exposed to radioactive fallout. However, small leaf veins
RESULTS AND DISCUSSION Imaging of the collected leaves Fig. 1 shows an image of bamboo leaves sampled from a mountainous site at Iitate village along Highway 115, located 42 km from the Fukushima NPP. The samples were collected 109 d after the NPP accident (12 MarchY29 June 2011). The upper panel (a) shows old leaf growth prior to the accident, and the lower panel (b) shows young leaf growth after the accident. Several black punctated areas that adhered to the surface of the old leaves are due to radioactive fallout. This might have occurred also because of rain washing out airborne radioactive materials that subsequently dried on the leaf surface. Individual spot radioactivity on the leaves was estimated roughly using the dose response curve of the PSL values of the prepared 137Cs scale. The dose range of measurable spots was 1.5Y16.0 Bq. It seemed that the
Fig. 2. Distribution of radioactive cesium on sorrel leaves sampled at Fukushima city located 62 km from the Fukushima No. 1 NPP. From the top panel, leaves were sampled at the same location on days 78 (a), 119 (b), and 133 (c).
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Fig. 3. Distribution of radioactive cesium on bamboo leaves at Futaba town located 10 km from the Fukushima No. 1 NPP. Samples were taken 389 d after the accident. Panel (a) shows the old leaf; panel (b) shows the young leaf.
and surrounding areas are slightly visible in the images, showing a diffuse spreading of radioactive material to every part of the leaf. The presence of radioactive materials in the old leaves is explained by the scattering of radioactive materials. The presence of radioactive materials in new leaves is explained by the uptake of radioactive materials from the soil through the roots and subsequent migration to the leaf veins (Baeza et al. 1999; Tagami et al. 2012). Fig. 2 shows an image of sorrel leaves sampled from the front area of Fukushima railway station that is of higher sensitivity than the image in Fig. 1. The site is located 62 km from the Fukushima NPP and was not classified as an evacuation zone. The three leaf samples were obtained from the same location on days 78 (29 May 2011, a), 119 (9 July 2011, b), and 133 (23 July 2011, c). Fig. 2 indicates lower radioactive contamination than the images of bamboo leaves shown in Fig. 1. The contamination level decreases over time, but the contamination pattern is similar to that of the bamboo leaves (Fig. 1a); the images of day 78 show more black spots due to adhered radioactive fallout (Fig. 2a). There are fewer black spots in the images of days 119 (Fig. 2b) and 133 (Fig. 2c); however, the leaf vein and surrounding areas become homogeneous, suggesting that low-level radioactivity spreads throughout the leaf (Baeza et al. 1999; Tagami et al. 2012).
In both Figs. 1 and 2, high concentrations of radioactive fallout are observed mainly on the surface of leaves sampled soon after the accident, but surface contamination declines over time because of dispersion by wind and rain. However, it is understood that radioactive material in the upper soil profile accumulates internally in plant leaves via uptake by plant roots (White et al. 2000; Zhu et al. 2000; NCRP 2006). Fig. 3 shows an image of bamboo leaves sampled at the town of Futaba, located 10 km from the NPP and hence within the 20 km exclusion zone. The imaging measurement started 389 d (5 April 2012) after the NPP accident. Panel (a) shows an old leaf grown prior to the accident; panel (b) shows leaf growth after the accident. Even 1 y after the accident, surface contamination within the highly polluted area shows little decline (Fig. 3a). On the other hand, the levels of radioactive materials in the leaf veins have not yet increased (Fig. 3b). Fig. 4 shows images of oak leaves (a) sampled at a highly contaminated area (Masani village, Belarus) in 1997, 11 y after the accident at the Chernobyl NPP. These images are compared with images of the bamboo leaf sample (b) from Iitate village (upper panel in Fig. 1) and the sorrel leaf sample (c) from Fukushima City (upper panel in Fig. 2). All the images used the same graphics processing
Fig. 4. Comparison of oak leaf (a), sampled in an area of high radioactivity (Masani village, Belarus) in 1997, 11 y after the Chernobyl NPP accident; bamboo leaf (b) from Iitate village; and sorrel leaf (c) from Fukushima City. www.health-physics.com
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Fig. 5. Comparison of cedar trees in high-contamination areas of Fukushima. Tree trunks were sampled at 0 km (left panel a), 5 km (center panel b), and 10 km (right panel c) from Fukushima NPP on day 389.
method. The radioactivity in each leaf can be semiquantitatively compared using the color scale presented at the bottom right of Fig. 4 (the color circles indicate Bq per circle area). Significant differences were observed in radiation intensity, distribution, and contamination status. In particular, the Chernobyl leaf sample does not show any punctated contamination pattern. Radioactivity was observed in the leaf veins with decreasing concentration toward the tip, suggesting the accumulation of contaminants due to root uptake. Thus, it is likely that a similar pattern of contamination will be observed in leaves from the contaminated Fukushima areas in future. Detailed observations from the images revealed the contamination pattern of radiocesium and suggest in vivo dynamics similar to those of alkaline potassium (White et al. 2000; Zhu et al. 2000; NCRP 2006). The changes in the contamination pattern were similar to the aforementioned, which is easy to capture using radioactivity image analysis because it is not measured in kilograms of cumulative dose (the detectable contamination patterns are: dose distribution plane, dose distribution inside and outside the living tissue, pharmacokinetics, and changes over time of dose distribution). Further, considering the characteristics of leaves, it is understood that accumulation of radioactivity in evergreen trees may be assessed over a long period. However, pulsatile and short-term contamination may be assessed in deciduous trees and annual grasses. In areas of low contamination, the detection sensitivity is increased by measuring the cumulative dose in evergreen trees. The analysis of imaging plates enables the efficient quantification of radiation levels, because the detection sensitivity is from 10 to several hundred times greater than that obtained by a photographic method (autoradiogrphy). In the case of 100 Bq cmj2 of 137Cs contamination, a 10-min exposure to the imaging plate is sufficient to produce an image of the radioactivity pattern. Furthermore, image analysis permits easy monitoring of radioactive contamination in materials other than leaves.
From contamination of leaves to future expectation From the images shown in Figs. 1b, 2b, and 3b, it is understood that after the accident, the progression of internal radioactive contamination in young leaf growth occurred via transportation from the surface of old leaves or uptake from the contaminated soil. However, unlike surface contamination, the currently identified internal contamination is due to absorption that progresses extremely slowly. This might be because radioactive cesium fallout has a strong affinity to clay soil, making it difficult for plants to absorb. Additionally, the transfer coefficient to grains and vegetables was 0.01Y0.026 (Nisbet and Woodman 2000; IAEA 1994). The transfer coefficient is equal to the concentration in agricultural edible parts (Bq gj1 raw weight) per unit concentration in the soil (Bq gj1 dry weight). Since cesium has a strong affinity to the soil, the downward penetration of radioactive cesium into the deep soil layer is very slow. In soil samples taken from Fukushima in June 2011, contamination levels at a depth of 4 cm were less than one-tenth of those at the soil surface. A similar result was reported for soil from the Chernobyl area, where 95% of the radioactive cesium remained within a 5 cm depth even 10 y after the incident (Nakajima et al. 2000, 2008). However, further studies are required to assess whether a similar pattern will occur in Fukushima, where precipitation is greater than that in other areas. The authors consider that, following the accident, livestock and wild herbivores in the radioactive areas initially ate leaves with high concentrations of surface contaminants but subsequently ate young leaves with low concentrations of contaminants. In a previous study, drinking water containing 137Cs was given to mice (1 kBq gj1 body weight) and the resulting internal contamination was evaluated. It was found that the approximate concentration of 137 Cs in all internal organs reduced the initial concentration to half after 2 d. However, the concentration in muscles peaked after more than 2 d and then reduced by half after day 13 (Nakajima 2011). Thus, in cases where excretion
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Imaging plant leaves to determine contamination c H. NAKAJIMA
exceeds intake, 137Cs is not accumulated in the animal’s body, and the total amount of contamination in the body gradually decreases. On the other hand, cesium radioactivity strongly absorbed in soil gradually accumulates in shallow-root shrubs and annual weeds. When these plants die, the plant matter forms a compost layer. Thereafter, 137Cs is absorbed by large plants directly from the compost layer. It is important to understand levels of plant contamination in Fukushima because plants form the first link in the food chain for animals and humans. Application of image analysis It is well known that radioactive cesium is absorbed by plants and that this phenomenon is employed in soil decontamination methods. Members of the Amaranthaceae family, such as Amaranthus retroflexus, Amaranthus lividus, and Achyranthes japonica, and sunflowers from the family Compositae are candidate plants with high absorption efficiency for 137Cs (Dushenkov et al. 1999; Fuhrmann et al. 2003; Soudek et al. 2006). However, because the root lengths and sizes differ among plants, appropriate plant species must be selected carefully and adjusted according
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to the contamination status in order to maximize the absorption efficiency of radioactive cesium from the shallow soil layer. It is also necessary to avoid disruption to the current ecosystem due to the introduction of non-native plants. Image analysis of leaves can be used not only to evaluate surface contamination of radioactive cesium but also to assess leaf contamination to identify candidate plants from the existing Fukushima flora that show good internal absorption. This method can be used to identify plants with distinct biological or chemical ability to accumulate radioactive cesium in the inedible parts as a means of efficient radioactive decontamination in combination with productive farming. In addition, the contamination of building materials has become a serious public concern in Japan, because waste materials from the lumber industry may not undergo detailed contamination checks. Fig. 6 compares images used to analyze contamination status of cut pine trees taken in 1997 from non-contaminated (Ribzky village), moderately contaminated (Babchin village), and highly contaminated (Masani village) areas after the Chernobyl accident. Since the 137Cs, unlike radioactive 14C, was not accumulated by biological organisms, it was not incorporated into the annual tree-growth rings following the accident in
Fig. 6. Comparison of pine trees sampled at areas of high contamination (Masani village: a, b), moderate contamination (Babchin village: c, d), and no contamination (Ribzky village: e, f ) in 1997, 11 y after the Chernobyl NPP accident. www.health-physics.com
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1986. These cesium radioactivities have pharmacokinetics similar to that of potassium. Therefore, they are not accumulated in internal growth rings consisting of inactive cells; however, they are accumulated in the bark, which consists of active and dividing cells and has more electrically conductive ducts and sieve tubes (Nakajima et al. 1998). Fig. 5 shows the contamination status of cedar trees in the highly contaminated areas of Fukushima (tree trunks were sampled 0, 5, and 10 km from Fukushima No.1 NPP in 2012, 389 d after the accident). These images show that strong radio-contamination appears only at the wood surface and that the internal uptake of radiocesium seen in the Chernobyl pines has not yet been observed at Fukushima (see Fig. 6). The radioactivity in each tree can be semi-quantitatively compared using the color scale presented at the bottom right of Fig. 5 (the color circles indicate Bq per circle area). The colors from blue to red represent levels of radioactivity from background to high levels. Even in the case of trees that grew in heavily contaminated areas, a cross-sectional image analysis of contamination status would indicate that the trees could be decontaminated by scraping off the tree bark. CONCLUSION Plants are the first link in the food chain for animals and humans. Therefore, it is important to understand contamination levels within plants in the Fukushima area. Timely response to the constantly changing contamination status is the most effective approach to minimizing contamination damage. Therefore, it is anticipated that the image analysis of plants will play a major role in minimizing radioactive contamination, although the image analysis has a drawback in that it does not identify the radionuclides. REFERENCES Baeza A, Paniagua JM, Rufu M, Sterling A, Barandica J. Radiocaesium and radiostrontium uptake by turnips and broad beans via leaf and root absorption. Appl Radiat Isot 50:467Y474; 1999. Bersina G, Brandt R, Vater P, Hinke K, Schu¨tze M. Fission track autoradiography as a means to investigate plants for their contamination with natural and technogenic uranium. Radiat Meas 24:277Y282; 1995. Dushenkov S, Mikheev A, Prokhnevsky A, Ruchko M, Sorochinsky B. Phytoremediation of radiocesium-contaminated soil in the vicinity of Chernobyl, Ukraine. Environmental Sci Technol 33:469Y475; 1999. Fuhrmann M, Lasat M, Ebbs S, Cornish J, Kochian L. Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments. J Environ Qual 32: 2272Y2279; 2003.
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Fujiwara M, Tanihata I, Saito K. Distribution map of radiation dose in Fukushima Eonline^. 2011. Available at http://www.rcnp.osakau.ac.jp/dojo/. Accessed 3 August 2013. International Atomic Energy Agency. Handbook of parameter values for the prediction of radionuclide transfer in temperate environments. Vienna: IAEA; Technical Reports Series No. 364; 1994. Ministry of Education, Culture, Sports, Science and Technology (MEXT), Distribution map of radiation dose around Fukushima Dai-ichi & Dai-Ni NPP Eonline^. 2011. Available at http://www.mext.go.jp/english/incident/1305901.htm. Accessed 10 August 2013. Nakajima H, Ryo H, Yamaguchi Y, Saito T, Yeliseeva KG, Piskunov EV, Krupnova EV, Voitovich AM, Nomura T. Biological concentration of radionuclides in plants and animals after the Chernobyl catastrophe. In: Sato F, Yamada Y, Onodera J, eds. Biological effects of low dose radiation. Aomori, Japan: Institute for Environmental Sciences; 2000: 199Y205. Nakajima H, Saito T, Ryo H, Nomura T. Ecological decrease and biological concentration of radionuclides in plants and animals after the Chernobyl catastrophe. In: Miura T, Kinoshita N, eds. Proceedings of the Eighth Workshop on Environmental Radioactivity. High Energy Accelerator Research Organization (KEK) Proceedings 2007-16; 2008: 113Y118. Nakajima H. How to convey the information of radiation impact correctly to the general public. Radiat Biol Res Commun. 46:120Y126; 2011. Nakajima H, Ryo H, Nomura T, Saito T, Yamaguchi Y, Yeliseeva KG. Radionuclides carved on the annual rings of a tree near Chernobyl. Health Phys 74:265Y267; 1998. National Council on Radiation Protection and Measurements. Cesuim-137 in the environment: radioecology and approaches to assessment and management. Washington, DC: NCRP; Report No. 154; 2006. Nisbet AF, Woodman RFM. Soil-to-plant transfer factors for radiocaesium and strontium in agricultural systems. Health Phys 78:279Y288; 2000. Sawidis T, Tsigaridas K, Tsikritzis L. Cesium-137 monitoring using lichens from W. Macedonia, N. Greece. Ecotoxicol Environ Safety 73:1789Y1796; 2010. Soudek P, Valenova´ Sˇ, Vavrˇ´ıkova´ Z, Vaneˇk T. 137Cs and 90Sr uptake by sunflower cultivated under hydroponic conditions. J Environ Radioact 88:236Y250; 2006. Tagami K, Uchida S, Ishii N, Kagiya S. Translocation of radiocesium from stems and leaves of plants and the effect on radiocesium concentrations in newly emerged plant tissues. J Environ Radioact 111:65Y69; 2012. White PJ, Broadley MR. Mechanisms of caesium uptake by plants. New Phytol 147:241Y256; 2000. Yamaguchi T, Sawano K, Furuhama K, Mori C, Yamada K. An autoradiogram of skeletal muscle from a pig raised on a farm within 20 km of the Fukushima Daiichi nuclear power plant. J Vet Med Sci 75:93Y94; 2012. Zhu YG, Smolders E. Plant uptake of radiocaesium: a review of mechanism, regulation and application. J Exp Bot 51: 1635Y1643; 2000.
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