Radiation Protection Dosimetry Advance Access published May 6, 2015 Radiation Protection Dosimetry (2015), pp. 1–5
doi:10.1093/rpd/ncv278
INFLUENCE OF THE FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY IN AOMORI PREFECTURE S. Kudo*, K. Igarashi and H. Kimura Aomori Prefectural Nuclear Power Safety Center, 400-1 Kurauchi Sasazaki, Rokkasho-mura, Aomori, Japan
Radioactive nuclides with a short half-life, such as 131I and 134Cs, were detected in environmental samples collected in Aomori Prefecture after the Tokyo Electric Power Company Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. In addition, the observed 137Cs concentration was increased over the background level. The gaseous 131I concentration in air observed in April was higher than that observed in March immediately after the accident. Using a backward trajectory analysis, the authors found that the air mass had passed the vicinity of the Fukushima Dai-ichi Nuclear Power Plant when the gaseous 131I concentration in air was increasing. Maximum 131I and radioactive Cs concentrations in daily fallout samples collected in Aomori city were observed on 28 April, when 131I was also detected in air. 134Cs and 137Cs concentration ratios in pine needles and pasture grass were nearly equal to 1, which indicates that the source of these radionuclides was the nuclear power plant accident.
INTRODUCTION Large amounts of radionuclides were released into the environment by the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which occurred in March 2011. Radioactive iodine and caesium emitted into the air spread widely and were deposited not only in the Tohoku and Kanto areas but throughout Japan. A radioactivity survey in Japan carried out by the Nuclear Regulation Authority detected these radionuclides in monthly fallout in most prefectures(1). In Aomori Prefecture, monitoring of environmental radioactivity around nuclear facilities (Nuclear Fuel Cycle Facilities, Higashidori Nuclear Power Plant and a recycled fuel storage centre) is ongoing, and radioactivity survey is being conducted throughout the prefecture. After the FDNPP accident, radionuclides with a short half-life such as 131I and 134Cs were detected in air, fallout and in some plant parts such as pine needles and pasture grass. Concentrations of 137 Cs were observed to increase over the background level from past nuclear weapons tests. In this report, the authors focus on measurement results for environmental radioactivity in Aomori Prefecture influenced by the FDNPP accident and discuss the influx of radionuclides on the basis of an air-mass trajectory analysis.
METHODS Figure 1 shows the locations of the monitoring area around the nuclear facilities, Aomori city and FDNPP. The monitoring area and FDNPP are 400
km apart. Gaseous 131I in air was collected at five monitoring stations (MSs) around Nuclear Fuel Cycle Facilities and one MS at Aomori city, installed by the Aomori Prefectural Government. Daily fallout was collected at Aomori city. Pine needles and pasture grasses were collected at five sites and four sites around nuclear facilities, respectively. Gaseous 131I in air was collected continuously using a charcoal cartridge (Advantec CHC-50) after filtration (Advantec HE-40 T filter paper). The air flow rate was 50 l min21. The authors changed the charcoal cartridge every week and put the old charcoal into a plastic (U8) vessel (50 mm w` 58 mm H) for later measurement. Fallout samples (wet and dry deposition) were collected with a funnel sampler (357 mm w, SIBATAW-402) daily at 09:00 Japan Standard Time (JST), and 80 ml of each collected sample was put into a U8 vessel for measurement. If the total sample volume was ,80 ml, the authors used the entire sample. If there had been no rain, they washed the inside of the funnel with distilled water and measured this water as well. The authors collected 2-y-old pine needles from pine trees. Pasture grass samples were collected by mowing the grass 10 cm above the ground surface at nine points within a 10 ` 10 m rectangle. These biological samples were first dried at 1058C and then incinerated at 4508C for 24 h. The resulting ash was put into a U8 vessel for measurement. The authors used a germanium semiconductor detector (SEIKO EG&G, GMX-40 or GMX-25) for measuring radionuclides. Measurement time was 80 000 s, and the measured value was corrected to the sampling date by using the half-life of the radionuclide.
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at KTH Royal Institute of Technology on November 16, 2015
*Corresponding author: [email protected]
S. KUDO ET AL.
Figure 2. Concentrations of gaseous 131I in air at six MSs in 2011 (D.L.: determination limit).
RESULTS AND DISCUSSION Gaseous 131I in air Figure 2 shows the trend of the weekly concentration of gaseous 131I in air after the FDNPP accident at the
MSs. Before the accident, the measured values of samples from all MSs were less than the determination limit (0.2 mBq m23) established by the Aomori Prefectural Government. 131I was first detected in the samples collected from 22 to 28 May 2011. The maximum values were observed in the samples collected from 4 to 18 April or from 25 April to 2 May 2011 at the five MSs around the Nuclear Fuel Cycle Facilities. In Aomori Prefecture, concentrations of gaseous 131 I in air collected in April were higher than those collected in March immediately after the accident. To explain these results, the authors examined the inflow of air masses into Aomori Prefecture by carrying out a backward trajectory analysis using CGER-METEX(2), a trajectory analysis tool developed by the National Institute for Environmental Studies. The authors created four backward trajectories for each day from 13 March to 30 May 2011. The starting times were 15:00, 21:00, 03:00 and 09:00 UTC, corresponding to 00:00, 06:00, 12:00 and 18:00 JST (UTC þ 9). The starting point was 200 m above the Obuchi MS (40.978N, 141.378E), in the vicinity of the Nuclear Fuel Cycle Facilities. The trajectory length was 120 h. The authors obtained a dataset of the
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Figure 1. Location of the monitoring area in Aomori Prefecture, Japan. A 18 ` 18 latitude by longitude grid is superimposed.
FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY
showed that the air masses passed through areas of Japan south of Aomori Prefecture before arriving at Obuchi station. In fact, the authors observed both increasing concentrations of gaseous 131I in air and higher hit numbers in grids B6 and C6 from 4 to 11 April, from 11 to 18 April and from 25 April to 2 May. The authors therefore inferred that air masses that reached Aomori Prefecture during these periods came via the area around the FDNPP. In May, in contrast, concentrations of gaseous 131I in air at Obuchi station decreased continuously to below the determination limit, even though the hit numbers were high. Trend graphs in TEPCO reports(3, 4) showing that the concentration of 131I in air collected at FDNPP became lower by about one order of magnitude from April to May 2011 explain this result.
Radionuclides in daily fallout
Figure 3. Time series of 131I concentrations in air at the Obuchi MS and that of the hit numbers around the FDNPP (grid cells B6 þ C6) in the trajectory analysis results.
Figure 5 shows daily changes in the 131I concentration in fallout in Aomori city from 14 March to 8 May 2011. During this period, 131I was detected four times, on 24 March and on 3, 10 and 28 April. The maximum value was measured on 28 April, when 134 Cs and 137Cs were also detected. High concentrations of 131I in air were observed from 4 to 11 April and from 18 April to 2 May at Aomori city (Figure 2). These results are consistent with the detection of 131I in fallout on 10 and 28 April. In contrast, no 131I was detected in fallout from 18 to 25 April, even though there was precipitation during this period. The authors speculate that no precipitation occurred at the exact times that the 131I concentration in air was high. To understand the relationship between 131I in air and 131I in fallout, air samples and fallout samples should be collected at the same time.
Figure 4. Back trajectory to the Obuchi MS started from (a) 13 March 2011 3:00 and (b) 27 April 2011 3:00 (UTC).
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position, latitude and longitude, of the air mass every hour, and they called the number of hours that the air-mass position was within grid cells B6 and C6 of the 18 latitude by 18 longitude grid superimposed on eastern Japan (see Figure 1) the ‘hit number’. Figure 3 shows time series of the concentration of gaseous 131I in air at the Obuchi MS and the hit numbers in grid cells B6 and C6 around the FDNPP. Figure 4 illustrates the backward trajectories starting from 13 March and 27 April at 03:00 UTC as typical of the trajectories in each month. In March, almost all backward trajectories showed the direct inflow of air masses to Aomori Prefecture from the Asian continent via the Japan Sea (Figure 4a). In March, no hit numbers were recorded in grids B6 and C6 (Figure 3). In April, in contrast, the backward trajectories
S. KUDO ET AL.
131
I and fallout.
134
Cs þ
137
Cs in daily
Figure 7. Relationships of 134Cs and 137Cs concentrations in various samples corrected to their values on 11 March 2011.
Radioactive Cs in pine needles and pasture grass
Figure 6. Time series of 134Cs and 137Cs in pine needles and pasture grass.
Figure 6 shows time series of 137Cs and 134Cs in pine needles and pasture grass collected from April 2011 to December 2013. In Aomori Prefecture, the established determination limit of both types of biological samples is 0.4 Bq kg21. Thus, the authors regard measured values of ,0.4 Bq kg21 as 0. None of the concentrations exceeded the limits prescribed by the Food Sanitation Act or provisional permissible levels for pasture grass. Pine needles were collected twice a year, in spring and autumn. Not only 137Cs and 134Cs but other
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Figure 5. Amounts of
FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY
CONCLUSION The authors reported measurement results for environmental radioactivity in Aomori Prefecture influenced by the FDNPP accident. The concentration of gaseous 131I in air observed in April was higher than that observed in March. By a backward trajectory
analysis, they showed that the air masses that reached Aomori Prefecture in April had passed over the FDNPP area when the gaseous 131I concentration in air was increasing. Increased amounts of 131I and radiocaesium were measured in daily fallout collected at Aomori city at almost the same time that 131I was detected in air. The 134Cs/137Cs radioactivity ratios in pine needles and pasture grass were nearly equal to 1 after the values were corrected to 11 March 2011. These results suggest that radiocaesium in these samples originated from the FDNPP accident. REFERENCES 1. Nuclear Regulation Authority. Reading of environmental radioactivity level by prefecture [fallout]. http://radio activity.nsr.go.jp/ja/contents/3000/2412/24/1060_04_ gekkan.pdf (25 September 2014, date last accessed) (in Japanese). 2. Centre for Global Environmental Research National Institute for Environmental Studies: CGER-METEX. http://db.cger. nies.go.jp/metex/index.html (25 September 2014, date last accessed). 3. Tokyo Electric Power Company. The results of nuclide analyses of radioactive materials in the air at the site of Fukushima Daiichi Nuclear Power Station (52nd release). http://www.tepco.co.jp/en/press/corp-com/release/11051 608-e.html (25 September 2014, date last accessed). 4. Tokyo Electric Power Company. The results of nuclide analyses of radioactive materials in the air at the site of Fukushima Daiichi Nuclear Power Station (68th release). http://www.tepco.co.jp/en/press/corp-com/release/11060 108-e.html (25 September 2014, date last accessed). 5. Ministry of Education, Culture, Sports, Science and Technology and Ministry of Agriculture, Forestry and Fisheries in Japan. Results of the research on distribution of radioactive substances discharged by the accident at TEPCO’s Fukushima Dai-ichi NPP, No. 1, 53– 54 (2012) (in Japanese).
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radionuclides with short half-lives such as 95Nb, 110m Ag, 136Cs, 140La, 125Sb and 129mTe were detected in spring 2011, immediately after the accident, although in the pine needles collected at Obuchi and Aomori city, the 137Cs and 134Cs concentrations were very low. The sampling date at Obuchi (7 April) and that at Aomori city (14 April) were apparently before any large fallout of 137Cs and 134Cs at those sites occurred (see Figure 5). After spring 2011, measured values gradually decreased, and from autumn 2013, they were all below the determination limit. Concentrations of 137Cs and 134Cs in pasture grass also decreased gradually from spring 2011, except at Yokohama town, where the concentrations were lower in spring and higher in summer, but overall roughly uniform. More measurement data accumulated over a longer time period are needed. Figure 7 shows the relationships between the 137Cs and 134Cs concentrations in pine needles and pasture grasses. The plotted values have been corrected to the values on 11 March 2011. 137Cs and 134Cs are positively correlated in both sample types, and the 134Cs/137Cs radioactivity ratios are nearly 1. MEXT and MAFF in Japan reported that the mean 134Cs/137Cs ratio in soil samples collected in Fukushima prefecture was 0.92 on 14 June 2011(5). This value became nearly 1 when corrected to the values on 11 March 2011. These results suggest that the radiocaesium in pine needles and pasture grasses originated from the FDNPP accident.
doi:10.1093/rpd/ncv278
INFLUENCE OF THE FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY IN AOMORI PREFECTURE S. Kudo*, K. Igarashi and H. Kimura Aomori Prefectural Nuclear Power Safety Center, 400-1 Kurauchi Sasazaki, Rokkasho-mura, Aomori, Japan
Radioactive nuclides with a short half-life, such as 131I and 134Cs, were detected in environmental samples collected in Aomori Prefecture after the Tokyo Electric Power Company Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. In addition, the observed 137Cs concentration was increased over the background level. The gaseous 131I concentration in air observed in April was higher than that observed in March immediately after the accident. Using a backward trajectory analysis, the authors found that the air mass had passed the vicinity of the Fukushima Dai-ichi Nuclear Power Plant when the gaseous 131I concentration in air was increasing. Maximum 131I and radioactive Cs concentrations in daily fallout samples collected in Aomori city were observed on 28 April, when 131I was also detected in air. 134Cs and 137Cs concentration ratios in pine needles and pasture grass were nearly equal to 1, which indicates that the source of these radionuclides was the nuclear power plant accident.
INTRODUCTION Large amounts of radionuclides were released into the environment by the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which occurred in March 2011. Radioactive iodine and caesium emitted into the air spread widely and were deposited not only in the Tohoku and Kanto areas but throughout Japan. A radioactivity survey in Japan carried out by the Nuclear Regulation Authority detected these radionuclides in monthly fallout in most prefectures(1). In Aomori Prefecture, monitoring of environmental radioactivity around nuclear facilities (Nuclear Fuel Cycle Facilities, Higashidori Nuclear Power Plant and a recycled fuel storage centre) is ongoing, and radioactivity survey is being conducted throughout the prefecture. After the FDNPP accident, radionuclides with a short half-life such as 131I and 134Cs were detected in air, fallout and in some plant parts such as pine needles and pasture grass. Concentrations of 137 Cs were observed to increase over the background level from past nuclear weapons tests. In this report, the authors focus on measurement results for environmental radioactivity in Aomori Prefecture influenced by the FDNPP accident and discuss the influx of radionuclides on the basis of an air-mass trajectory analysis.
METHODS Figure 1 shows the locations of the monitoring area around the nuclear facilities, Aomori city and FDNPP. The monitoring area and FDNPP are 400
km apart. Gaseous 131I in air was collected at five monitoring stations (MSs) around Nuclear Fuel Cycle Facilities and one MS at Aomori city, installed by the Aomori Prefectural Government. Daily fallout was collected at Aomori city. Pine needles and pasture grasses were collected at five sites and four sites around nuclear facilities, respectively. Gaseous 131I in air was collected continuously using a charcoal cartridge (Advantec CHC-50) after filtration (Advantec HE-40 T filter paper). The air flow rate was 50 l min21. The authors changed the charcoal cartridge every week and put the old charcoal into a plastic (U8) vessel (50 mm w` 58 mm H) for later measurement. Fallout samples (wet and dry deposition) were collected with a funnel sampler (357 mm w, SIBATAW-402) daily at 09:00 Japan Standard Time (JST), and 80 ml of each collected sample was put into a U8 vessel for measurement. If the total sample volume was ,80 ml, the authors used the entire sample. If there had been no rain, they washed the inside of the funnel with distilled water and measured this water as well. The authors collected 2-y-old pine needles from pine trees. Pasture grass samples were collected by mowing the grass 10 cm above the ground surface at nine points within a 10 ` 10 m rectangle. These biological samples were first dried at 1058C and then incinerated at 4508C for 24 h. The resulting ash was put into a U8 vessel for measurement. The authors used a germanium semiconductor detector (SEIKO EG&G, GMX-40 or GMX-25) for measuring radionuclides. Measurement time was 80 000 s, and the measured value was corrected to the sampling date by using the half-life of the radionuclide.
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at KTH Royal Institute of Technology on November 16, 2015
*Corresponding author: [email protected]
S. KUDO ET AL.
Figure 2. Concentrations of gaseous 131I in air at six MSs in 2011 (D.L.: determination limit).
RESULTS AND DISCUSSION Gaseous 131I in air Figure 2 shows the trend of the weekly concentration of gaseous 131I in air after the FDNPP accident at the
MSs. Before the accident, the measured values of samples from all MSs were less than the determination limit (0.2 mBq m23) established by the Aomori Prefectural Government. 131I was first detected in the samples collected from 22 to 28 May 2011. The maximum values were observed in the samples collected from 4 to 18 April or from 25 April to 2 May 2011 at the five MSs around the Nuclear Fuel Cycle Facilities. In Aomori Prefecture, concentrations of gaseous 131 I in air collected in April were higher than those collected in March immediately after the accident. To explain these results, the authors examined the inflow of air masses into Aomori Prefecture by carrying out a backward trajectory analysis using CGER-METEX(2), a trajectory analysis tool developed by the National Institute for Environmental Studies. The authors created four backward trajectories for each day from 13 March to 30 May 2011. The starting times were 15:00, 21:00, 03:00 and 09:00 UTC, corresponding to 00:00, 06:00, 12:00 and 18:00 JST (UTC þ 9). The starting point was 200 m above the Obuchi MS (40.978N, 141.378E), in the vicinity of the Nuclear Fuel Cycle Facilities. The trajectory length was 120 h. The authors obtained a dataset of the
Page 2 of 5
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Figure 1. Location of the monitoring area in Aomori Prefecture, Japan. A 18 ` 18 latitude by longitude grid is superimposed.
FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY
showed that the air masses passed through areas of Japan south of Aomori Prefecture before arriving at Obuchi station. In fact, the authors observed both increasing concentrations of gaseous 131I in air and higher hit numbers in grids B6 and C6 from 4 to 11 April, from 11 to 18 April and from 25 April to 2 May. The authors therefore inferred that air masses that reached Aomori Prefecture during these periods came via the area around the FDNPP. In May, in contrast, concentrations of gaseous 131I in air at Obuchi station decreased continuously to below the determination limit, even though the hit numbers were high. Trend graphs in TEPCO reports(3, 4) showing that the concentration of 131I in air collected at FDNPP became lower by about one order of magnitude from April to May 2011 explain this result.
Radionuclides in daily fallout
Figure 3. Time series of 131I concentrations in air at the Obuchi MS and that of the hit numbers around the FDNPP (grid cells B6 þ C6) in the trajectory analysis results.
Figure 5 shows daily changes in the 131I concentration in fallout in Aomori city from 14 March to 8 May 2011. During this period, 131I was detected four times, on 24 March and on 3, 10 and 28 April. The maximum value was measured on 28 April, when 134 Cs and 137Cs were also detected. High concentrations of 131I in air were observed from 4 to 11 April and from 18 April to 2 May at Aomori city (Figure 2). These results are consistent with the detection of 131I in fallout on 10 and 28 April. In contrast, no 131I was detected in fallout from 18 to 25 April, even though there was precipitation during this period. The authors speculate that no precipitation occurred at the exact times that the 131I concentration in air was high. To understand the relationship between 131I in air and 131I in fallout, air samples and fallout samples should be collected at the same time.
Figure 4. Back trajectory to the Obuchi MS started from (a) 13 March 2011 3:00 and (b) 27 April 2011 3:00 (UTC).
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position, latitude and longitude, of the air mass every hour, and they called the number of hours that the air-mass position was within grid cells B6 and C6 of the 18 latitude by 18 longitude grid superimposed on eastern Japan (see Figure 1) the ‘hit number’. Figure 3 shows time series of the concentration of gaseous 131I in air at the Obuchi MS and the hit numbers in grid cells B6 and C6 around the FDNPP. Figure 4 illustrates the backward trajectories starting from 13 March and 27 April at 03:00 UTC as typical of the trajectories in each month. In March, almost all backward trajectories showed the direct inflow of air masses to Aomori Prefecture from the Asian continent via the Japan Sea (Figure 4a). In March, no hit numbers were recorded in grids B6 and C6 (Figure 3). In April, in contrast, the backward trajectories
S. KUDO ET AL.
131
I and fallout.
134
Cs þ
137
Cs in daily
Figure 7. Relationships of 134Cs and 137Cs concentrations in various samples corrected to their values on 11 March 2011.
Radioactive Cs in pine needles and pasture grass
Figure 6. Time series of 134Cs and 137Cs in pine needles and pasture grass.
Figure 6 shows time series of 137Cs and 134Cs in pine needles and pasture grass collected from April 2011 to December 2013. In Aomori Prefecture, the established determination limit of both types of biological samples is 0.4 Bq kg21. Thus, the authors regard measured values of ,0.4 Bq kg21 as 0. None of the concentrations exceeded the limits prescribed by the Food Sanitation Act or provisional permissible levels for pasture grass. Pine needles were collected twice a year, in spring and autumn. Not only 137Cs and 134Cs but other
Page 4 of 5
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Figure 5. Amounts of
FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT ON ENVIRONMENTAL RADIOACTIVITY
CONCLUSION The authors reported measurement results for environmental radioactivity in Aomori Prefecture influenced by the FDNPP accident. The concentration of gaseous 131I in air observed in April was higher than that observed in March. By a backward trajectory
analysis, they showed that the air masses that reached Aomori Prefecture in April had passed over the FDNPP area when the gaseous 131I concentration in air was increasing. Increased amounts of 131I and radiocaesium were measured in daily fallout collected at Aomori city at almost the same time that 131I was detected in air. The 134Cs/137Cs radioactivity ratios in pine needles and pasture grass were nearly equal to 1 after the values were corrected to 11 March 2011. These results suggest that radiocaesium in these samples originated from the FDNPP accident. REFERENCES 1. Nuclear Regulation Authority. Reading of environmental radioactivity level by prefecture [fallout]. http://radio activity.nsr.go.jp/ja/contents/3000/2412/24/1060_04_ gekkan.pdf (25 September 2014, date last accessed) (in Japanese). 2. Centre for Global Environmental Research National Institute for Environmental Studies: CGER-METEX. http://db.cger. nies.go.jp/metex/index.html (25 September 2014, date last accessed). 3. Tokyo Electric Power Company. The results of nuclide analyses of radioactive materials in the air at the site of Fukushima Daiichi Nuclear Power Station (52nd release). http://www.tepco.co.jp/en/press/corp-com/release/11051 608-e.html (25 September 2014, date last accessed). 4. Tokyo Electric Power Company. The results of nuclide analyses of radioactive materials in the air at the site of Fukushima Daiichi Nuclear Power Station (68th release). http://www.tepco.co.jp/en/press/corp-com/release/11060 108-e.html (25 September 2014, date last accessed). 5. Ministry of Education, Culture, Sports, Science and Technology and Ministry of Agriculture, Forestry and Fisheries in Japan. Results of the research on distribution of radioactive substances discharged by the accident at TEPCO’s Fukushima Dai-ichi NPP, No. 1, 53– 54 (2012) (in Japanese).
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radionuclides with short half-lives such as 95Nb, 110m Ag, 136Cs, 140La, 125Sb and 129mTe were detected in spring 2011, immediately after the accident, although in the pine needles collected at Obuchi and Aomori city, the 137Cs and 134Cs concentrations were very low. The sampling date at Obuchi (7 April) and that at Aomori city (14 April) were apparently before any large fallout of 137Cs and 134Cs at those sites occurred (see Figure 5). After spring 2011, measured values gradually decreased, and from autumn 2013, they were all below the determination limit. Concentrations of 137Cs and 134Cs in pasture grass also decreased gradually from spring 2011, except at Yokohama town, where the concentrations were lower in spring and higher in summer, but overall roughly uniform. More measurement data accumulated over a longer time period are needed. Figure 7 shows the relationships between the 137Cs and 134Cs concentrations in pine needles and pasture grasses. The plotted values have been corrected to the values on 11 March 2011. 137Cs and 134Cs are positively correlated in both sample types, and the 134Cs/137Cs radioactivity ratios are nearly 1. MEXT and MAFF in Japan reported that the mean 134Cs/137Cs ratio in soil samples collected in Fukushima prefecture was 0.92 on 14 June 2011(5). This value became nearly 1 when corrected to the values on 11 March 2011. These results suggest that the radiocaesium in pine needles and pasture grasses originated from the FDNPP accident.
