Atmospheric Fallout of 129I in Japan before the Fukushima Accident

Article pubs.acs.org/est

Atmospheric Fallout of 129I in Japan before the Fukushima Accident: Regional and Global Contributions (1963−2005) Chiaki Toyama,*,† Yasuyuki Muramatsu,† Yasuhito Igarashi,‡ Michio Aoyama,‡ and Hiroyuki Matsuzaki§ †

Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan § Department of Nuclear Engineering and Management, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan ‡

S Supporting Information *

ABSTRACT: Atmospheric 129I deposition was studied in different locations of Japan (Akita, Tsukuba, Tokyo, and Ishigaki Island) with samples collected between 1963 and 2005 in order to understand the distribution and sources of this nuclide and provide a reference deposition level prior to the Fukushima accident. Over this time period, the deposition pattern of 129I in Tsukuba and Tokyo (on the Pacific side) differed from that of Akita (on the Japan Sea side). The primary source of deposition in Tsukuba and Tokyo is related to the 129I discharge from domestic reprocessing in Tokai-mura. In contrast, the time-series pattern of deposition in Akita seems to have been influenced by 129I discharges from reprocessing facilities in Europe and the transport of this radionuclide by westerly winds to coastlines of the Japan Sea. The 129I deposition in Ishigaki (one of the southernmost islands in Japan) is influenced primarily by oceanic air masses (easterly winds), and deposition was 1 order of magnitude lower than that observed in Tsukuba and Tokyo. Cumulative 129I deposition in Tokyo before the Fukushima accident was estimated at 13 mBq/ m2. The results of this study on deposition contribute to understanding the deposition levels of 129I prior to the accident.

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distribution of this nuclide in the environment.2,9−11,14 It is also important to understand its temporal variations.6,15,16 However, there is still a lack of data on the atmospheric deposition of 129I in Japan, because of analytical difficulties. It is also difficult to obtain appropriate time-series samples to estimate deposition of this nuclide. In our previous paper,17 we examined chemical separation procedures for the determination of 129I from dried precipitation materials for low-level detection of 129I. The method was applied to atmospheric fallout samples collected in Tokyo between 1963 and 2003 by Japan’s Meteorological Research Institute (MRI). The 129I/127I ratios in atmospheric fallout samples were analyzed by accelerator mass spectrometry (AMS) at Micro Analysis Laboratory, Tandem accelerator (MALT) at The University of Tokyo. Results indicated the 129 127 I/ I ratios in the atmosphere during 1963−1977 in Tokyo ranged from 1 × 10−8 to 2 × 10−8. These values are roughly 4 orders of magnitude higher than preatomic level, which is close to 1.5 × 10 −12 . 10−12 Calculated monthly atmospheric deposition rates of 129I after 1963 indicated that the recent variations in 129I deposition in Tokyo are not influenced exclusively by nuclear bomb testing and spent fuel reprocessing facilities in Europe. Moreover, the variation of estimated annual 129 I deposition in Tokyo showed a close relationship between

INTRODUCTION Iodine has only one stable isotope (127I) and more than 30 radioisotopes. Among them, two radioiodine isotopes, 129I and 131 I, are important from the viewpoint of environmental safety. They have been introduced into the environment as a result of human nuclear activities such as weapons testing,1 spent fuel reprocessing,2−4 and nuclear accidents including the Fukushima Daiichi nuclear plant accident.5−8 Approximately 250 kg of 129I in the environment is naturally occurring; produced both by decay of uranium and cosmic ray reaction with 130Xe.9 The natural 129I/127I ratio in a marine environment has been assumed to be 1.5 × 10−12 in several studies.10−12 Because the main source of atmospheric iodine is derived from the ocean, it is thought that the natural 129I/127I ratio in the atmosphere should be similar to the ratio in the ocean prior to the atomic age. A comparison of contemporary 129I/127I ratios in rivers and lakes in coastal areas with nearby shallow marine seawater confirms that the isotopic ratios are conserved even though the absolute concentration of 129I varies between shallow marine, atmospheric, and surface waters.13 Iodine-131 is regarded as the radioiodine isotope of greatest immediate concern in the event of nuclear accidents, because of its high fission yield. However, 131I decays out after a few months due to its short half-life (8 days). In contrast, 129I fission yield is not high. But this nuclide is retained in the environment for much longer time periods due to its long halflife (1.57 × 107 years). Because of the persistence of 129I in surface reservoirs, it is necessary to know the levels and © XXXX American Chemical Society

Received: April 12, 2013 Revised: June 25, 2013 Accepted: July 4, 2013

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dx.doi.org/10.1021/es401596z | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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the annual atmospheric discharges of 129I from the Tokai reprocessing plant, located about 130 km northeast of Tokyo. Although no marked influence of 129I from European reprocessing plants was observed for the deposition in Tokyo, it is not clear whether other cities in Japan receive contributions from overseas sources. In this study, we carried out analyses of the atmospheric fallout samples collected at different localities in Japan (Akita, Tsukuba, and Ishigaki Island) between 1963 and 2005. We also used additional samples from Tokyo between 2004 and 2005, which were not previously published. At the time of the Fukushima accident (March 12, 2011) caused by the tsunami, approximately 1.6 × 1017 Bq of 131I was released into the atmosphere.18 A total amount of 129I released from the accident was estimated to be 8.06 × 109 Bq by Hou et al.19 Detailed reconstruction of the dispersal and deposition of 131 I at the time of the accident has been hindered by difficulties in collecting field samples immediately following the Fukushima disaster and the time constraints of measuring this shortlived radioisotope. Nonetheless, 131I is a critical nuclide to be considered for dose assessment following the accident, and it is essential to reconstruct the regional distribution of this nuclide away from the reactor. For this purpose, soil analysis of 129I, which was also released by the accident, has been used for assessment of regional 131I deposition, since both isotopes are released during nuclear accidents.7,20 In order to use this approach, it is necessary to subtract the amount of 129I accumulated in soil prior to the release event to evaluate the event-related deposition of 129I. The purpose of this paper is to observe the temporal variation of 129I deposition in different places of Japan to understand the depositional distribution and source of this nuclide, including overseas sources, before the Fukushima accident.

Figure 1. Locations of sampling stations.

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METHODS To analyze 129I by accelerator mass spectrometry (AMS), iodine was isolated from the fallout samples through four steps: (1) separation of iodine from the samples, (2) measurement of stable iodine by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500 and 7700) for a separated aliquot, (3) purification of iodine by solvent extraction, and (4) measurement of the 129I/127I ratio by AMS. (1) About 100 mg of the dried fallout sample was mixed with V2O5 in a ceramic combustion boat and placed in a quartz tube. Then the sample was heated at 1000 °C under constant oxygen flow. By this procedure, most iodine in the sample was volatilized and collected in a trap solution containing tetramethylammonium hydroxide (TMAH) solution and Na2SO3 solution.24 (2) For the analysis of 127I (stable iodine) concentration by ICP-MS, a part of the trap solution was appropriately diluted with deionized water. (3) A known amount of iodine (usually 2 mg as I−) was added to the rest of the trap solution as a carrier. The iodine fraction was then purified by solvent extraction and back-extracted with CCl4. (4) The purified iodine fraction was finally precipitated as AgI, well dried and mixed with Nb powder, and then it was pressed into the cathode cone to prepare the AMS target. Details on the methods were published by Toyama et al.,17 which is based on the method developed by Muramatsu and co-workers24,25 and on the measurement conditions published by Matsuzaki et al.26 129I/127I ratio and 129I concentrations of samples were calculated from the independent 127I and 129I/127I ratio measurements by AMS (see Supporting Information, Table S1), and the specific activity of 129I (6.53 mBq/ng) was used to calculate the 129I concentration in millibecquerels per gram. A decay constant of 1.4 × 10−15 s−1 was applied to convert 129I atoms to activity. The deposition rate of solid particulates in grams per square meter per month was then used to calculate the deposition rate of 129I in millibecquerels per square meter per month. Based on the analytical precision errors of both ICP-MS and AMS, the maximum error for 129I deposition is 12.6% and most errors are below 3%. We validated our method by analyzing a standard reference material soil, IAEA-375 (Chernobyl soil). Our results (129I concentration 1.5 ± 0.1 mBq/kg) were within the range of the reference value (129I concentration 1.7 ± 0.4 mBq/kg) recommended by IAEA. Sampling variability and measurement

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FALLOUT SAMPLES Atmospheric fallout samples (precipitation containing airborne particulate dust) have been collected monthly by MRI since the 1960s at their different research stations in Japan.21,22 The samples have already been analyzed for radiocesium and radiostrontium within a framework organized by the MRI.23 In this study, we selected a subset of samples from four different sites: Akita, located on the Japan Sea side; Tsukuba, located between Tokai reprocessing plant (TRP) and Tokyo; Tokyo, located on the Pacific Ocean side; and Ishigaki, a small island west of Okinawa in Japan’s southernmost prefecture (Figure 1). The archived samples of precipitation were collected every month between 1963 and 2005, using a constantly open surface collector with a surface area of 1 or 0.5 m2, and then they were evaporated to dryness in an evaporation dish or in a glass flask by use of a rotary vacuum evaporator. Since the analysis of 129I is time-consuming, we selected one annual sample (generally from the month of May) for all locations excluding Tokyo and Tsukuba. For these cities, an additional sample was selected from those collected in autumn (generally September). Residue samples were weighed after drying in an oven at 110 °C and then were transferred to a plastic container. Subsets of samples collected in May (or June and July) and in September (or August and October) were selected in this study. In Tsukuba, monthly samples from the year 1986 were analyzed separately in order to examine the possible enhancement of 129I deposition in April and May due to the effect of the Chernobyl accident in 1986. B



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Figure 2. 129I deposition and gaseous discharge of 129I from TRP.3,29,30 The amounts of annual 129I deposition (millibecquerels per square meter per year) in Tsukuba and Tokyo were roughly estimated by averaging the monthly values for May and September and multiplying the average by 12. The data for Tokyo from 1963 to 2003 are from Toyama et al.17 Note that the scale for deposition is different for panels a, b, and c.

discussion, the time series of Tokyo used is a combination of data analyzed in this study and our previous one. The mean value of 129I in Akita from 1963 to 1978 (2.2 × 10−2 mBq·m−2·month−1) is twice that of Tokyo (1.1 × 10−2 mBq·m−2·month−1). The timing of a pronounced increase in 129 I deposition over Tokyo (at 1978) is earlier than that of Akita (at 1980). It is interesting to note that the time variations of 129I deposition during 1991−2005 in Tsukuba is quite similar to that of Tokyo, while the time-series pattern for Akita (Japan Sea side) is somewhat different from that of Tokyo (Pacific side). This suggests that these cities receive 129I from different input sources, both domestically and outside Japan. The 129I deposition in Ishigaki Island (one of the southernmost islands in Japan) was consistently 1 order of magnitude lower than that in Akita, Tsukuba, and Tokyo (Figure 1).

of particulate deposition are not included in the error estimations, because we have only one sample for the atmospheric fallout of each month.

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RESULTS Results of the 129I/127I ratios, 127I concentrations, and 129I deposition in atmospheric fallout samples in Akita, Tsukuba, and Ishigaki from 1963 to 2005 and Tokyo from 2004 to 2005, along with other relevant information such as the amount of monthly deposition mass, are reported in Table S1 (Supporting Information). Ratios of 129I/127I in fallout samples collected from Akita, Tsukuba, and Ishigaki were (0.58−8.25) × 10−8, (0.66−23.4) × 10−8, and (0.08−1.64) × 10−8, respectively. Those of Tokyo, including the data of our previous study,17 were (0.23−10.7) × 10−8. Levels of 129I in the fallout samples collected in Akita, Tsukuba, Tokyo, and Ishigaki were (0.06−12.6) × 10−2, (0.13− 11.5) × 10−2, (0.39−13.3) × 10−2, and (0.02−0.81) × 10−2 mBq·m−2·month−1, respectively. Time series of 129I deposition in Akita, Tsukuba, and Ishigaki is shown in Figure S1 (Supporting Information) with that in Tokyo17 to compare results for each region. For further

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DISCUSSION Influence of Nuclear Weapons Testing and Nuclear Accident. The levels of 129I/127I ratio in the fallout samples collected from four different sites in Japan are shown in Figure S2 (Supporting Information). As can be seen in this figure, the ratio varies between 10−10 and 10−7. Compared to the levels of C



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Figure 3. 129I deposition in Akita and Ishigaki and discharge of 129I into the atmosphere and the ocean from European spent fuel reprocessing plants.2,3,34−36 Total 129I discharge represents the sum of values for Sellafield and La Hague.

I/127I ratio before 1977 (about 10−8), those after 1978 generally increased. Considering the 129I/127I ratio in the environment before the nuclear tests of 1.5 × 10−12,10−12 the ratios in the atmosphere appear to have increased by 4 orders of magnitude or more. Since a large amount of 129I was discharged into the atmosphere by nuclear testing and the reprocessing facility by 1977, as shown in Table S2 (Supporting Information),1,2 the rise in levels of 129I/127I ratio is likely due to the effects of atmospheric nuclear weapons tests and nuclear fuel reprocessing plants. It is useful to look at our 129I deposition results in the context of the deposition of other anthropogenic radioisotopes over the same time period. Figure S3 (Supporting Information) compares the monthly 129I deposition in Tsukuba in 1986 with 90Sr and 137Cs deposition during the same period.22 On April 26, 1986, the Chernobyl accident released various radionuclides into the atmosphere. In Japan, the increase in deposition of 90Sr and 137Cs from the accident was observed in May 1986 (Figure S3b, Supporting Information).27,28 However,

no appreciable influence of the Chernobyl accident on 129I deposition was observed in May (Figure S3a, Supporting Information). There was, however, a peak in 129I deposition observed in July of the same year, although we do not think that is directly related the Chernobyl accident because no 137Cs and 90Sr peaks were observed in the same month. The estimated amount of 129I released from the Chernobyl accident (1.3 kg)5,6 was considerably smaller than the annual release from European reprocessing plants; that is, total amounts of atmospheric release from La Hague, Sellafield, and Marcoule are around 15 kg/year in the 1980s.9 Therefore, the global 129I inventory is not significantly influenced by Chernobyl, relative to other sources. Comparison of 129I Depositions and Discharge from Reprocessing Plants. As discussed previously, the high 129I values observed in the 1980s most likely indicate the contribution from nuclear spent fuel reprocessing plants. Two countries are operating commercial spent nuclear fuel reprocessing plants, the United Kingdom (Sellafield facility)

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patterns of 129I deposition in Akita, we consider the possible influence of nuclear spent fuel reprocessing plants in Europe, by comparing the monthly 129I deposition with the annual discharge of 129I from the Sellafield (United Kingdom) and La Hague (France) reprocessing facilities (Figure 3).2,4,34−36 Since the 129I discharge from these European reprocessing plants to the sea is a few orders of magnitude higher than to the atmosphere, we considered both discharge pathways and compared them to the 129I deposition in Akita. As shown in Figure 3a,c, the 129I discharge into the atmosphere from these plants increased from 1980 and decreased in the 1990s. The monthly 129I deposition pattern in Akita is also shown in Figure 3a,b. Although there is no clear agreement about the comparison of monthly deposition with the annual discharge, the patterns of 129I discharge into the atmosphere from reprocessing facilities in Europe exhibit similarities with those of 129I deposition in Akita. On the other hand, the discharge pattern into the ocean from European reprocessing facilities is quite different from the 129I deposition pattern in Akita. These results suggest that the discharge of 129I into the atmosphere from European reprocessing plants is the major contributor of 129 I in Akita. In addition, we compared 129I deposition fluxes obtained from the Fiescherhorn glacier in the Swiss Alps37 with the monthly 129I deposition observed in Akita (Figure S5, Supporting Information). It is reported by Reithmeier et al.37 that the variation of 129I deposition at the Fiescherhorn glacier is in good agreement with that of the total 129I releases into the atmosphere from the European reprocessing facilities and from atmospheric nuclear weapons tests. As shown in this figure, the pattern of 129I deposition in Akita parallels the 129I deposition in the Fiescherhorn glacier. The air masses from Europe pass over Honshu (the main island of Japan) in the spring.38,39 The 129I discharged from Europe should be transported by westerly winds and arrive at the eastern shore of the Japan Sea, where Akita is located. The high mountains in the middle of the Japanese main island of Honshu exert an influence on the transport of radionuclides and other airborne materials by the westerly winds, such that deposition is higher on the Japan Sea side than on the Pacific side. It is reported by many previous studies that the deposition of other nuclides was also much greater on the Japan Sea side of Japan than on the Pacific Ocean side.21,40,41 The deposition of 129I in Akita is therefore likely influenced by the discharge from overseas reprocessing facilities, such as La Hague and Sellafield. In the case of Ishigaki, the 129I deposition pattern does not correspond to the discharge patterns from TRP and or from European reprocessing facilities (Figures 2c and 3b,d). The 129I deposition in Ishigaki was 1 order of magnitude lower than that in Akita, Tsukuba, and Tokyo, and no large fluctuations in the time series were seen. Ishigaki Island is located in the Pacific, and the spring air masses that pass over Ishigaki come from the Pacific Ocean.33 Therefore, Ishigaki is mainly affected by the atmosphere of the open ocean, and the influence of continental air masses is small. In addition to this, distance from the domestic reprocessing facility is very large. We conclude that the atmospheric deposition in Ishigaki is only minimally influenced by the reprocessing facilities in Japan or in other countries. The Ishigaki samples have relatively low 129I/127I ratios when compared with other locations. This is likely due to the influence of marine aerosols, which contain relatively greater concentrations of stable iodine, from the sea

and France (La Hague facility). The Sellafield and La Hague plants began operation in 1952 and 1966, respectively. In Japan, TRP, a pilot plant, was operated from 1977 to 2007 in Tokaimura (Ibaraki Prefecture), which is located about 130 km northeast of Tokyo.3,29,30 In our previous paper,17 we compared the annual changes of 129 I discharge from TRP with the atmospheric deposition of 129I collected in Tokyo. We found that the level of deposition in Tokyo increased markedly from 1979, coinciding with the timing of major discharges from TRP. The decline in deposition that followed is similar to the decline in discharge from TRP. From this evidence, we conclude that 129I deposition in Tokyo was mainly influenced by the operation of the reprocessing plant in Tokai-mura. Temporal changes in 129I deposition for these cities and the annual discharge of 129I from the TRP3,29,30 are shown in Figure 2. The amount of the annual 129I deposition (millibecquerels per square meter per year) in Tsukuba and Tokyo was roughly estimated by use of an average monthly value of May and September multiplied by 12 (Figure 2a). The resulting deposition peaks observed for Tsukuba indicate periods of major discharge from TRP, but there are samples available only from 1986, when sampling started in this city. The deposition level of 129I tends to be higher in Tsukuba than in Tokyo, likely because Tsukuba is closer to TRP. No clear peak was found in 1985 in Tokyo, although the discharge of 129I from TRP during this year was high. This might be related to the timing of the sampling (i.e., the annual deposition was calculated from one or two representative monthly samples per year and not from the yearly average). In 1983−1984, 1988−1989, 1992−1993, and after 1997, the values of 129I discharge from TRP were very low, because the amount of reprocessing greatly diminished.31 The 129I depositions in Tsukuba and Tokyo were also low for those periods. From the above-mentioned observations, we conclude that the atmospheric fallout collected from Tsukuba and Tokyo was influenced primarily by 129I discharge from TRP. The 129I deposition patterns in Akita and Ishigaki differ significantly from the discharge pattern of TRP (Figure 2b,c). When the longer distance from TRP to Akita (about 600 km) and to Ishigaki (about 2300 km) is taken into consideration, it should be reasonable that the contributions from TRP to these cities were considerably lower than those in Tsukuba and Tokyo. The 129I deposition in Akita in the 1980s was often higher than that in Tokyo, which is located closer to TRP. The dominant wind direction around TRP is northeast, which is the direction to Tokyo and opposite to Akita. This suggests that the deposition in Akita cannot be explained by the influence of TRP. Moreover, the maximum increase of 129I deposition in Akita is in 1980, while in Tokyo maximum values are reached in 1978. Therefore, higher peaks observed in Akita suggest contributions from the other reprocessing plants. The 129I deposition before the operation of TRP during 1963−1978 for Akita and Tokyo are compared. The mean deposition value calculated for Akita (2.2 Bq/m2) is about twice that of Tokyo (1.1 Bq/m2). Although the 129I deposition rate is dominated by wet deposition such as rain or snow, as reported previously by Hou et al.,32 the amount of precipitation at Akita and Tokyo in May is almost the same (Figure S4, Supporting Information).33 During the above-mentioned period, European reprocessing plants were already in operation.2,34 In order to explain the E



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surrounding this island. Atmospheric fallout samples in Ishigaki contained visible salt crystals arising from sea sprays. Estimation of Total 129I Depositions in Tsukuba and Tokyo. As mentioned above, 129I was deposited on the ground from the atmosphere due to anthropogenic sources such as nuclear weapon tests and routine operations of spent fuel reprocessing plants. We have estimated total 129I deposition (becquerels per square meter) in Tokyo from 1945, when the first nuclear weapon test occurred, until 2005. Because we do not have data for 1945 to 1962, the amount of 129I deposition was estimated on the basis of the value of annual 129I deposition in 1963 (3.14 × 10−2 mBq/m2); also taken into account were the annual deposition data of 137Cs by nuclear bomb tests for 1962 (146 PBq)1 and the total deposition of 137Cs (467 PBq) described in the UNSCEAR report.1 The amount of 129I deposition from 1964 to 2005 was calculated from our annual data. As a result, the total amount of 129I deposition in Tokyo is estimated to be 12.2 mBq/m2. In order to calculate the total amount of the 129I deposition until 2010, when the Fukushima nuclear accident occurred, we used the deposition value of the year 2005 (1.79 × 10−1 mBq/m2) as representative for the 2005−2010 period. The estimated amount of the deposition from 2006 to 2010 is 0.89 mBq/m2. Finally, the total amount of 129 I deposition in Tokyo from 1945 until 2010 is calculated to be about 13 mBq/m2. These results indicate that about 70% of total deposition in Tokyo was accumulated after 1977, after the 129 I discharge from TRP had started.31 Due to the long half-life of this nuclide and the high affinity of iodine for soil constituents (specifically organic matter in soil), most 129I deposited onto the ground should be retained in the surface soil.42,43 However, there should be a contribution due to resuspension of soil to the sampling vessel. Since iodine is emitted from terrestrial and aquatic environments by the effect of microbial activity,44,45 contributions of 129I from these pathways are also expected. However, proportions of 129I deposited through the resuspension and re-emission pathways are not known. This 13 mBq/m2 value is within the range of 129 I deposition density (5−39 mBq/m2), which was estimated from the concentrations of 129I in soils collected in different locations in Japan,24,46 except for the Tokai area, before the accident of the Fukushima Daiichi nuclear power plant. In order to compare the amount of the deposition in Tsukuba and Tokyo, we have estimated the total 129I deposition (becquerels per square meter) from 1986, when the atmospheric fallout sampling was initiated in Tsukuba, until 2005. In this estimate, however, data obtained for 1988, 1990, 1999, and 2002 are excluded because there are no deposition data for both May and September for these years in Tsukuba to provide a comparison to the data in Tokyo. As a result, the total amount of 129I deposition in Tsukuba is estimated to be 7.1 mBq/m2. This value is more than twice that of Tokyo (2.9 mBq/m2) for the same period, indicating the higher influence for Tsukuba due to the smaller distance from TRP. Reconstruction of the deposition of 131I in Fukushima Prefecture and other areas using the measurement of 129I in soil is needed. For this purpose, it is also necessary to know the background levels of this nuclide in soil. Our deposition results should also contribute to the understanding of 129I deposition levels prior to the release event and can be used to help validate radionuclide measurements in soils.

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ASSOCIATED CONTENT

S Supporting Information *

Tables that summarize analytical data (129I/127I ratios, 127I concentrations, and 129I deposition in atmospheric fallout samples at Akita, Tsukuba, and Ishigaki from 1963 to 2005 and Tokyo from 2004 to 2005) and amount of discharged 129I into the atmosphere in the latter half of the 1970s from main anthropogenic sources, and five figures that illustrate temporal variations of 129I deposition and 129I/127I ratios for each location as well as monthly precipitation for Akita and Tokyo. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81 3 3986 0221; fax: +81 3 5992 1029 e-mail: chiaki. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. G. Snyder (Rice University) for his useful comments and Y. Uchida and Y. Takada for their assistance in the analysis. Thanks are also due to the editor and reviewers for their valuable suggestions for improving the paper. This study was supported in part by a grant of Strategic Research Foundation Grant-aided Project from Ministry of Education, Culture, Sport, Science, and Technology, Japan (MEXT).

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REFERENCES

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Atmospheric Fallout of 129I in Japan before the Fukushima Accident

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