Article pubs.acs.org/est
Iodine Isotopes in Precipitation: Temporal Responses to Emissions from the Fukushima Nuclear Accident
129
I
Sheng Xu,*,† Stewart P. H. T. Freeman,† Xiaolin Hou,‡ Akira Watanabe,§ Katsuhiko Yamaguchi,§ and Luyuan Zhang‡ †
Scottish Universities Environmental Research Center, East Kilbride, G75 0QF, U.K. Center for Nuclear Technologies, Technical University of Denmark, 4000 Roskilde, Denmark § Geophysical Institute, Fukushima University, Fukushima 960-1296, Japan ‡
ABSTRACT: The Fukushima Dai-ichi Nuclear Power Plant accident in 2011 has released a large amount of radionuclides to the atmosphere, and the radioactive plume has been dispersed to a large area in Europe and returned to Asia. To explore long-term trend of the Fukushima-derived radioactive plume and the behavior of harmful radioiodine in the atmosphere, long-term precipitation samples have been collected over 2010−2012 at Fukushima, Japan for determination of long-lived 129I. It was observed that 129I concentrations of 1.2 × 108 atom/L in 2010 before the accident dramatically increased by ∼4 orders of magnitude to 7.6 × 1011 atom/L in March 2011 immediately after the accident, with a 129 127 I/ I ratio up to 6.9 × 10−5. Afterward, the 129I concentrations in precipitation decreased exponentially to ∼3 × 109 atom/L by October 2011 with a half-life of about 29 days. This declining trend of 129I concentrations in precipitation was interrupted around October 2011 by a new input of 129I to the atmosphere following a second exponential decrease. Such a cycle has occurred three times until the present. This temporal variation can be attributed to alternating 129I dispersion and resuspension from the contaminated local environment. A 129I/131I atomic ratio of 16 ± 1 obtained from rainwater samples is comparable with a value estimated for surface soil samples. 129I results from Denmark suggest an insignificant effect of 129I released from Fukushima to the 129 I levels in Europe.
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INTRODUCTION A massive earthquake of M9.0 magnitude occurred in Japan on March 11, 2011, causing a tsunami that seriously damaged the Fukushima Dai-ichi Nuclear Power Plant (Fukushima NPP). As a result, large amounts of radioactive substances were released into the atmosphere and to the Pacific Ocean. It was estimated that 160 PBq of 131I and 15 PBq of 137Cs were released into the atmosphere, mainly on March 15−16, 2011.1 Although major atmospheric emissions of radionuclides from the Fukushima NPP have apparently ceased, further investigation is urgently needed to reconstruct the emission history of the Fukushimaderived radionuclides and assess their impact on the environment. The Fukushima-derived radionuclides caused serious radioactive contamination not only in the area near the power plant, but also extended to a large area of eastern Japan.2 In addition, radionuclides released into the atmosphere have been dispersed eastward to the Pacific Ocean, through North America to Europe and further back to Asia.3−7 Numerous data on Fukushima-derived radionculides in the environment have been published recently. Among them, 134Cs, 137Cs, and 131I with half-lives of 2 years, 30 years, and 8 days, respectively, are of major concern. Studies on long-lived radionuclide 129I (half-life: 1.6 × 107 years) in environmental and food samples have not © 2013 American Chemical Society
been undertaken except for a few studies of seawater and soil near the Fukushima NPP.8−10 131I is regarded as one of the most important radionuclides, because of its high fission yield and very short half-life. When it is taken up through food and inhalation, it mainly concentrates in the thyroid gland, resulting in a considerable radiation dose to this organ. However, 131I can hardly be detected several months after release, thus, an assessment of the radiation risk of 131I is only possible by knowing the ratio of 131I to that of other long-lived radionuclides. Although 137Cs can be potentially considered as a proxy for 131I, different chemical properties and behavior in the environment make 137Cs an imperfect choice for such studies.11 A better assessment of 131I would rely on the calculation based on the 129I/131I ratio. In the case of the Chernobyl accident, Kutschera et al. obtained a 129I/131I ratio from a rainwater sample collected in Munich.12 For Fukushima emissions and fallout, a 129I/131I ratio of 22.3 ± 6.3 was obtained by analyzing 131I and 129I from 27 surface soil samples distributed 4−59 km away from the Fukushima NPP.8 It is Received: Revised: Accepted: Published: 10851
April 9, 2013 August 26, 2013 September 3, 2013 September 3, 2013 dx.doi.org/10.1021/es401527q | Environ. Sci. Technol. 2013, 47, 10851−10859
Environmental Science & Technology
Article
USA) were added as a carrier. NaOH pellets were added to the sample to achieve a NaOH concentration of 0.30 mol/L. The solution in the beaker was covered by a watch glass and heated on a hot plate at about 120 °C for 3 h to digest and convert organic 129I to inorganic form. After cooling to room temperature, 0.5 mL of 1.0 mol/L NaHSO3 was added and the solution was adjusted to pH = 1−2 using HNO3 to convert iodate to iodide. After transferring the sample solution to a separation funnel, CHCl3 was added, and then 1.0 mL of 1.0 mol/L NaNO2 was added to oxidize iodide to I2, which was extracted to CHCl3 phase by shaking. The extraction was repeated once and the CHCl3 phase was combined and transferred to another separation funnel, iodine in the CHCl3 phase was back-extracted to the water phase by addition of 15 mL of water and 0.2 mL of 0.05 mol/L NaHSO3. These extraction and back-extraction steps were repeated again, and the final back-extracted iodide in water was transferred to a 10mL centrifuge tube, 0.5 mL of 1.0 mol/L AgNO3 was added, and the formed AgI was separated by centrifuge. After being dried at 70 °C, the AgI precipitate was ground to fine powder for 129I measurement. Procedure blanks were prepared by treatment of two 30-mL aliquots of deionized water (18.2 MΩ cm) using the same procedure as the samples. It should be noted that, because some part of the organic iodine might have not decomposed and been converted to inorganic form by heating of samples in 0.3 mol/L NaOH, the measured 129I concentration in the precipitation is only inorganic and part of organic 129I, therefore the total 129I in the precipitation might be underestimated. The AMS measurement of 129I was modified from our previous reported procedure,14 and a brief description of the method is summarized here. The prepared iodine as AgI was mixed with high-purity Ag powder (100 mesh, 99.95%, Assure) with a mass ratio 1:2 for AgI:Ag and then pressed into an aluminum target holder with a 1-mm diameter. Negative I− ions were extracted by a Cs-sputtering ion source. Three MV was chosen as the terminal voltage and the I5+ was chosen for detection. The 127I5+ was detected using a Faraday cup mounted at the exit of high-energy analyzing magnet, while 129 5+ I was counted using an ionization detector with 100-nm thickness SiN detector window. Although the 97Mo4+, which were produced by dissociation of the injected MoO2− and had a similar magnetic rigidity (ME/q2) to 129I5+, may interfere with 129 5+ I , they can be completely separated from 129I5+ in the detector. The measured 129I/127I ratios were corrected against a standard material with 129I/127I ratio of 1.138 × 10−10 prepared by 127I addition to the original NIST 4949B standard. The measured 129I/127I ratios in samples were 10−11−10−10, which are more than 2 orders of magnitude higher than that of procedure blank (10−13). Due to possible memory effects in the ion source, care was taken to measure samples in sequence from low to high 129I/127I ratios. Repeat measurements of secondary standards indicated better than 2% reproducibility.
evident that this ratio has a large uncertainty and therefore more data are required. This work aims to investigate the behavior of Fukushimaderived 129I in the atmosphere by analyzing 2-year time-series of precipitation samples collected at Fukushima University (37°41′00″ N, 140°27′16″ E), located ∼60 km northwest of the Fukushima NPP. A snow sample collected from Mt. Azuma (37°45′26″ N, 140°16′28″ E) in Japan and rainwater samples collected at Risø, Denmark (55°41.63′ N, 12°5.15′ E) were also analyzed in order to investigate the transport of iodine in the atmosphere.
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MATERIALS AND METHODS Precipitation samples were collected at the roof of the main monitoring building on the Fukushima University campus at Fukushima city (Figure 1). The samples included single
Figure 1. Sampling location of precipitation (star) and the Fukushima Dai-ichi nuclear power plant, redrawn on a map of soil 137Cs.2
precipitation events, and in most cases, monthly events. Using a rainwater collector (RS-20, Miyamoto Riken Ind. Co., Japan), the rainwater samples were filtered through a 0.45μm membrane filter to remove particles and stored in dark environment until processing to avoid loss of iodine during storage. The pack snow sample was collected by burying a U8 container (internal diameter 4.75 cm, height 6 cm) into the snow from the surface. Danish rainwater was collected on the Risø campus of the Technical University of Denmark using a 1m2 precipitation collector. 127 I was analyzed only for samples that had a sufficient volume of water. 127I concentrations in the precipitation samples were determined using inductively coupled plasma mass spectrometer at the Technical University of Denmark. Detailed procedures are described elsewhere.13 129I concentrations were determined by chemical extraction of iodine from the water combined with determination using a 5 MV tandem accelerator mass spectrometer (AMS) in the Scottish Universities Environmental Research Center. Water samples (20−35 mL) were transferred to a beaker, 200 Bq of 125I as a yield tracer and 0.5 mg of stable 127I (prepared by dissolving iodine in a crystal from Woodward Inc.,
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RESULTS AND DISCUSSION Table 1 lists 127I and 129I concentrations, and 129I/127I ratios measured for 27 precipitation samples from Fukushima, and for 3 rainwater samples from Denmark. Figure 2 shows temporal variations of 129I and 129I/127I of these samples from November 2010 to December 2012. The measured 127I concentrations varied from 0.6 to 2.3 μg/ kg with an average of 1.4 ± 0.7 μg/kg, which falls into the 10852
dx.doi.org/10.1021/es401527q | Environ. Sci. Technol. 2013, 47, 10851−10859
10853
a
Fukushima
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Risø
Mt. Azuma
55°41′29″, 12°06′05″
37°45′26″, 140°16′28″
37°41′00″, 140°27′16″
latitude (N), longitude (E)
I Concentrations and
129
8
1374
243
Denmark
66.9 71.0
97.4 94.4 322.3 126.7 24.8 63.3 167.3 51.1 31.7 120.0 185.7 128.0
22.3 64.6 57.3 45.5 49.4
Japan
precipitation (mm) I (μg/kg)
± ± ± ± ± ± ± ± ±
± ± ± ±
0.014 0.018 0.010 0.014 0.016 0.016 0.012 0.010 0.016
0.012 0.014 0.002 0.004
2.357 ± 0.020 2.768 ± 0.009 1.494 ± 0.002
0.564 ± 0.008
0.854 1.430 0.836 0.808 2.717 1.600 1.137 0.921 2.188
1.036 2.230 0.917 0.943
2.321 ± 0.018
127
I concentration measured on April 11, 2011.22
b131
Apr 1−6, 2011 Apr 7−14, 2011 Jul 1−31, 2012
Nov 5−Dec 1, 2010 Dec 1−31, 2010 Dec 31, 2010−Jan 31, 2011 Feb 1−Mar 2, 2011 Mar 3−31, 2011 Apr 1−20, 2011 Apr 21−May 9, 2011 Jun 3, 2011 Jun 15, 2011 Jun 30, 2011 Jul 15, 2011 Jul 16−Aug 5, 2011 Aug 6− Sep 2, 2011 Sep 3−Oct 1, 2011 Oct 2−Nov 4, 2011 Dec 1−31, 2011 Dec 31, 2011− Feb 5, 2012 Feb 5−Mar 14, 2012 Mar 14−Apr 6, 2012 Apr 6−May 2, 2012 May 2−Jun 1, 2012 Jun 1−Jul 3, 2012 Jul 3−31, 2012 Sep 1, 2012 Oct 2−Nov 1, 2012 Nov 1− Dec 1, 2012 May 6, 2011
sampling dates ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.001 0.001 0.001 0.001 1.99 0.21 0.28 0.093 0.175 0.011 0.034 0.012 0.012 0.006 0.007 0.005 0.005 0.002 0.002 0.004 0.002 0.002 0.002 0.006 0.001 0.001 0.003 0.208 ± 0.003 0.340 ± 0.004 0.231 ± 0.003
0.012 0.017 0.028 0.010 75.84 17.90 10.37 3.889 7.618 1.713 2.098 1.081 1.074 0.345 0.329 0.442 0.446 0.136 0.090 0.351 0.161 0.132 0.127 0.513 0.055 0.036 0.146
129 I (1010 atom/L)
3277.8
131 b I (Bq/L)
I/127I Ratios of Precipitation Samples from Fukushima, Japan and Risø, Denmarka
altitude (masl)
129
Sample 27 is snow and the rest of the samples are rainwater.
28 29 30
location
sample
I and
127
Table 1. I/127I (10−6)
± ± ± ± ± ± ± ± ±
± ± ± ±
0.02 0.011 0.007 0.007 0.003 0.004 0.005 0.006 0.007
0.21 0.17 0.03 0.08
0.186 ± 0.004 0.259 ± 0.005 0.326 ± 0.007
0.548 ± 0.013
1.09 0.658 0.342 0.234 0.272 0.212 0.245 0.292 0.494
7.93 7.20 3.94 4.69
68.9 ± 1.9
129
16 ± 1
129 131 I/ I (atomic)
3.65 2.53
105 101 111 41.7 11.0 28.2 22.7 4.59 11.1 19.3 24.5 16.3
0.618 0.619 4346 815 512
129 I deposition (1010 atom/m2)
Environmental Science & Technology Article
dx.doi.org/10.1021/es401527q | Environ. Sci. Technol. 2013, 47, 10851−10859
Environmental Science & Technology
Article
This fact clearly indicates that variation of 129I/127I ratios is mainly controlled by 129I concentrations in precipitation. Temporal Variations of 129I in Precipitation. There are several trends of the 129I concentrations in precipitation during November 2010−December 2012 (Figure 2). The 129I concentrations remained fairly constant at low levels before the accident with an average value of 1.7 ± 0.8 × 108 atoms/L, which is comparable with those (3 × 108 atoms/L) determined in precipitation from Chiba in 1983, but significantly lower than those (7.9 × 1010 atoms/L in 1979−1981 and 1.5−3.7 × 109 atoms/L in 1982−1983) observed in Tokai.16 The high 129I concentrations in Tokai during 1979−1981 were attributed to the releases of 129I from a spent fuel reprocessing plant, and the reduction since 1982 was due to the installation of a new filtration system for iodine removal in the stack of this nuclear facility.16 Therefore, 129I concentrations in preaccident samples represent typical background level of 129I in Fukushima area. Four periods of change in 129I concentration after the accident can be observed in Figure 2; these are summarized in Table 2. In Period I, the 129I concentration increased
Figure 2. Temporal variations of 129I concentration and 129I/127I atomic ratio in precipitations from Fukushima, Japan and Risø, Denmark. Note: the analytical uncertainties of 129I and 129I/127I values are within the symbols, but they are too small to be visible.
Table 2. Time-Dependence of the Fukushima-Derived Radionuclidesa
reported range in the literature (0.2−12 μg/kg).15 These include those (2.5−3.4 μg/kg) obtained in precipitation samples near Tokai, 160 km south of Fukushima where a spent fuel reprocessing plant is located,16 and values (6.6 μg/ kg) observed in a non-nuclear area at Chiba, 220 km south of the Fukushima NPP.16 The lowest 127I value was observed in the snow sample from Mt. Azuma, which is comparable with previous observations in Europe and the United States.15,17 There is no apparent seasonal dependence in 127I concentration in Fukushima samples. 129 I concentrations in Fukushima samples varied widely from 1 × 108 to 8 × 1011 atoms/L. The highest value of 8 × 1011 atoms/L was obtained between March 3 and 31, 2011, while the lowest value was observed between February 1 and March 2, 2011. 129I/127I ratios in the samples collected after the accident varied from 2 × 10−7 to 7 × 10−5. The highest value reached the top of the range reported for the highly contaminated environment from nuclear reprocessing plants at Hanford and West Valley, USA during the late 1960s and early 1970s.18 The low values are comparable with those observed in precipitation samples from Tokai in 1982−1983 (1 × 10−7), but still higher than those determined in water sample from the background area at Chiba in 1983 (1 × 10−8).16 In comparison with the 129I/127I ratio in the environment before the nuclear weapons testing (10−12),19 the Fukushima 129I/127I ratios are about 5−7 orders of magnitude higher. Toyama et al. have reported 40-yr variations of 129I/127I ratios in the atmospheric fallout in Tokyo from 1963 to 2006, with the highest 129I/127I ratios of ∼1 × 10−7 in 1986−1988 and 7 × 10−8 in 2001−2002, and the low 129I/127I ratios of 1−2 × 10−8 during 1963−1980.20 In Fukushima, 129I/127I ratios of surface soil before the accident were reported to be
