Article pubs.acs.org/est
Vertical Distributions of Radionuclides (239+240Pu, 240Pu/239Pu, and 137 Cs) in Sediment Cores of Lake Bosten in Northwestern China Haiqing Liao,† Wenting Bu,‡,§ Jian Zheng,*,‡ Fengchang Wu,*,† and Masatoshi Yamada⊥ †
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China ‡ Research Center for Radiation Protection, National Institute of Radiological Sciences, Chiba 263-8555, Japan § State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China ⊥ Institute of Radiation Emergency Medicine Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan S Supporting Information *
ABSTRACT: Artificial radionuclides (137Cs, 239+240Pu, 241Pu, 241 Am) deposited in lacustrine sediments have been used for dating as well as radionuclide source identification. In the present work, we investigated the vertical distributions of 239+240 Pu and 137Cs activities, 240Pu/239Pu atom ratios, and 239+240 Pu/137Cs activity ratios in sediment cores collected from Lake Bosten, which is the lake closest to the Lop Nor Chinese Nuclear Weapon Test site in northwestern China. Uniformly high concentrations of 239+240Pu and 137Cs were found in the upper layers deposited since 1964 in the sediment cores, and these were controlled by the resuspension of soil containing radionuclides from the nearby land surface. As the Chinese nuclear tests varied remarkably in yield, the mixing of the tropospheric deposition from these tests and the stratospheric deposition of global fallout has led to a 240Pu/239Pu atom ratio that is similar to that of global fallout and to a 239+240Pu/137Cs activity ratio that is slightly higher than that of global fallout. However, a low 240Pu/239Pu atom ratio of 0.080 and high 239+240Pu/137Cs activity ratio of 0.087, significantly different from the global fallout values, were observed in one sediment core (07BS10-2), indicating the inhomogenous tropospheric deposition from the Chinese nuclear tests in Lake Bosten during 1967−1973. These results are important to understand the influence of the CNTs on the radionuclide contamination in Lake Bosten.
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INTRODUCTION Extensive nuclear weapon tests were conducted by the United States and the former Soviet Union in the mid-20th century, and tests on a smaller scale were done later by the United Kingdom, France, China, and India. Most of these tests led to stratospheric (or global) fallout of radionuclides, including 90Sr, 137 Cs, and plutonium, etc. The nuclear weapon tests also generated tropospheric or close-in fallout of radionuclides in areas close to test sites.1,2 These fallout radionuclides were thereafter recorded in aquatic sediments and have been used to date recent sediments as well as trace geochemical processes.3,4 During the period from October 16, 1964, to July 29, 1996, 23 atmospheric and 22 underground nuclear tests were conducted at Lop Nor, the Chinese Nuclear Test (CNT) site, which is located in the southern part of Xinjiang Province.5 These nuclear tests varied considerably in type and yield and resulted in global and local dispersal of radioactive debris. Efforts have been done to evaluate the levels of radioactive contamination in the downwind area from the CNT site. In a previous work, we investigated Pu isotopes and 137Cs in Lake Sugan located ca. 500 km southeast of Lop Nor and Lake © 2014 American Chemical Society
Shuangta located ca. 600 km east of Lop Nor (Figure S1, Supporting Information). More than 25% of the 137Cs and 239+240 Pu inventories in Lake Sugan and Lake Shuangta could be attributed to the Lop Nor nuclear weapon tests.6 The contributions of 137Cs in Lake Sugan and Shuangta were obviously lower than that (ca. 70%) in the Jiuquan area, Gansu Province, China.7 Another study found a low 240Pu/239Pu atom ratio of 0.16 in the soil of Gansu Province.8 These results indicated that Gansu Province, which is southeast (downwind area) of Lop Nor, may have received close-in fallout radionuclides from the CNTs. However, little information is available to evaluate the environmental impact of fallout radionuclides produced by the CNTs in the upwind areas of Lop Nor. Olivier et al. reported that plutonium from global stratospheric fallout rather than from the direct tropospheric input was recorded in an ice core from the Belukha Glacier, Received: Revised: Accepted: Published: 3840
December 1, 2013 February 12, 2014 March 1, 2014 March 1, 2014 dx.doi.org/10.1021/es405364m | Environ. Sci. Technol. 2014, 48, 3840−3846
Environmental Science & Technology
Article
Figure 1. Map showing the locations of sediment core collection sites in Lake Bosten.
atom ratio of the CNTs is needed to distinguish local fallout from global fallout and for accurate dating of recent sedimentation. In this work, we determined the vertical distributions of Pu isotopes and 137Cs in sediment cores of Lake Bosten, the only existing freshwater lake in the upwind area and closest to the Lop Nor test site. In addition, 210Pb was also measured in two sediment cores to establish sediment chronologies that would allow the radioactive contamination sources in Lake Bosten to be studied. The aims of this work are (1) to measure 239+240Pu, 137 Cs activities and 240Pu/239Pu atom ratios in the sediments to evaluate the characteristics of these radionuclides in Lake Bosten and (2) to identify the local source and global fallout source by the deposition history of radionuclides and understand the influence of the CNTs on the radionuclide contamination in Lake Bosten.
which is ca. 1300 and 600 km from Lop Nor and Semipalatinsk, respectively (Figure S1, Supporting Information).9 Yamamoto et al. determined Pu isotopes and 137Cs activities in soils collected from the Kazakhstan−Chinese border, which is ca. 1200 km from Lop Nor, and they observed additional 239+240Pu input as the local fallout from the Semipalatinsk or Lop Nor test sites, but which test site contributed dominantly in the Pu contamination in soil was not identified.10 The upwind area close to Lop Nor, therefore, needs to be studied to evaluate the environmental impact of Chinese nuclear weapons tests. Depending on the emission sources, Pu isotope ratios, in particular the 240Pu/239Pu atom ratio, vary significantly in the environment.11,12 Previous studies showed that 240Pu/239Pu atom ratios from nuclear weapon-grade materials ranged from 0.01 to 0.07.13 The 240Pu/239Pu ratios from Semipalatinsk, Nevada, and the Pacific Proving Ground test sites were found to be 0.043, 0.054, and 0.36, respectively.14−17 The 240Pu/239Pu ratios from nuclear reactors ranged from 0.24 to 0.80 and that of the global atmospheric fallout was about 0.18.18−20 High ratios of 240Pu/239Pu for the Chernobyl and the Fukushima Daiichi Nuclear Power Plant accidents were reported to be 0.43 and 0.33, respectively.1,21 Our previous work also found that the 240Pu/239Pu atom ratio for the early nuclear weapons development research activities at Atomic City near Lake Qinghai may be lower than 0.038.22 As for the 239+240Pu/137Cs activity ratio, the global fallout value was reported to be 0.029 ± 0.003 (137Cs decay was corrected to July 1, 1998),23,24 and those for the Chernobyl accident and the Fukushima Daiichi Nuclear Power Plant accident were about 0.009 and 6.76 × 10−8, respectively.1,21 However, information about the characteristic Pu isotopes and 137Cs for the Lop Nor test site is quite limited. Jin et al. observed a 240Pu/239Pu atom ratio of 0.16 in a soil in northwestern China.8 We also found a low 240Pu/239Pu ratio of 0.103 in sediments of Lake Sugan which is located ca. 500 km southeast of Lop Nor.6 These results indicated that the 240 Pu/239Pu atom ratio from the CNTs might be lower than that of global fallout. However, another report implied that plutonium for the CNTs had a 240Pu/239Pu ratio higher than that of global fallout.25 More information on the 240Pu/239Pu
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MATERIALS AND METHODS Sample Collection. Lake Bosten (41°56′−42°14′ N, 86°40′−87°56′ E) is located on the southern slope of the Tianshan Mountains and lies in the southeastern part of the Yanqi Basin between the Tianshan and the Taklimakan Desert in the Tarim Basin. It is ca. 250 km northwest of Lop Nor as shown in Figure S1 (Supporting Information). Lake Bosten is the largest inland freshwater lake in northwestern China with a surface area of ca. 1000 km2, maximum water depth of 16.2 m, and an average water depth of 8 m.26 Lake Bosten is hydrologically open with 13 rivers flowing into the lake, with four major rivers (namely, Kaidu River, Wulasitai River, Huangshui River, and Qingshui River) accounting for 96% of the total water input. Water leaves the lake via the Kongque River, which is the water source of Korla City and flows to Lop Nor. Meteorological records from Yanqi County record a mean annual precipitation of 70 mm and mean annual evaporation of 2000 mm in that area. As shown in Figure 1, three sediment cores (06BS2−1, 06BS2−3, 07BS10−2) were collected in two sampling places in 2006 and 2007 by a gravitational piston core sampler. The sediment core lengths are 55, 50, and 31 cm for core 06BS2−1, 06BS2−3, and 07BS10−2, respectively. 3841
dx.doi.org/10.1021/es405364m | Environ. Sci. Technol. 2014, 48, 3840−3846
Environmental Science & Technology
Article
Figure 2. Vertical distributions of 137Cs and 239+240Pu activities, 240Pu/239Pu atom ratio, and 239+240Pu/137Cs activity ratio (137Cs decay corrected to July 1, 1998) in sediment cores of Lake Bosten. The solid circles and open circles represent the 239+240Pu activities and 137Cs activities, respectively; the dotted lines indicate the sediment layer corresponding to 1980 when China stopped atmospheric nuclear tests, and the dashed lines indicate the sediment layer corresponding to 1964 when the peak in global fallout occurred; the shaded areas indicate the global fallout ranges of the 240Pu/239Pu atom ratio and 239+240Pu/137Cs activity ratio. Pu isotope data of 07BS10−2 sediment core are cited from Wu et al.22,32
Japan.29 Sediment standard reference materials IAEA-368 (marine sediment) and NIST-4354 (freshwater lake sediment) were used for analytical method validation. As sediment cores 06BS2−1 and 06BS2−3 were collected from the same location (Figure 1), similar vertical distributions of Pu were expected. As shown in Figure 2 and Table S1 (Supporting Information), the analytical results of Pu isotopes in the two sediment cores were identical, which indicated that the 8 M HNO 3 leaching method was valid for the determination of Pu in the sediments of Lake Bosten. 137 Cs activity was determined using γ-spectrometry on a Canberra S-100 multichannel spectrometer with a GC5019 HPGe coaxial detector (efficiency 50%) at the Institute of Geochemistry, Chinese Academy of Sciences. The peak of 137 Cs used to determine its activity was 661.6 KeV. The liquid standard (catalog no. 7137) supplied by the Institute of Atomic Energy, China, was used.30 Excess 210Pb activity was obtained by subtracting 226Ra activity from total 210Pb activity which was derived from 210Po. The 210Po activity in sediment was analyzed by α-spectrometry on a Canberra S-100 multichannel spectrometer with a PIPS Si detector, and 226Ra activity was determined by γ-spectrometry on a Canberra S-100 multichannel spectrometer mated to a GCW3022 H-PR Ge well detector. Details of the analysis of 210 Po and 226Ra have been described elsewhere.31
The core sediments were undisturbed, as indicated by their clear water−sediment interface and the preservation of fine sediment laminations. Sediment samples were sectioned at 0.5 or 1 cm intervals in situ, weighed immediately after collection, and then dried using a vacuum freeze-dryer and reweighed to determine mass depths in a laboratory. Dried samples were ground powder particles of 0.2) for the CNTs.25 This could be caused by the 21st atmospheric nuclear explosions test (1−1.5 km height) which was the largest yield Chinese thermonuclear test (4 Mt) conducted on November 17, 1976, because a high atom ratio of 240Pu/239Pu of 0.224 ± 0.002 was observed in the debris collected at 10 km height in the atmosphere.41 Several other studies have focused on the distribution of Pu and 137Cs in the lacustrine sediments and soils in northwestern China. Wu et al. investigated Pu and 137Cs characterizations in sediment cores collected from Lake Sugan and Lake Shuangta in the downwind areas of Nop Lor and took into account the sedimentation chronology information; they concluded that about 20% of the Pu and 40% of the 137Cs in the sediments came from the CNTs.6 Jin et al. determined Pu isotopic composition in soils collected from Xian and Gansu, and they got a 240Pu/239Pu atom ratio of 0.16, slightly lower than the global fallout in the soil of Gansu.8 However, the results obtained by Zheng et al. evidenced that the Pu contamination in the soil of Lanzhou, Gansu Province originated from the global fallout.27 As discussed above, Pu and 137Cs activities were distributed uniformly high in the upper layers of the sediment cores in Lake Bosten and the abnormal 240Pu/239Pu atom ratio (0.080) and 239+240Pu/137Cs activity ratio (0.087) were found in 07BS10−2 at the mass depth of 2.83 g·cm−2. Since plutonium from the Chernobyl accident happened in 1986 was mainly deposited locally as reported by Hirose,42 and no 240Pu/239Pu atom ratio higher than 0.3 was observed in the corresponding sediment layers of Lake Bosten, we concluded that there was no detectable Pu contribution originating from the Chernobyl accident in the sediments of Lake Bosten. By calculating from the 210Pbex data, the mass depth of 2.83 g·cm−2 corresponded to the period from 1967 to 1973. During this period, nine atmospheric tests were conducted in Lop Nor, including five Mt level tests and four low yield tests.34,43 The Mt level tests mainly lead to stratospheric fallout of debris, and the local fallout or the tropospheric fallout may be contributed by these low yield tests.34 Thus, the abnormal Pu and 137Cs with low 240 Pu/239Pu atom ratio and high 239+240Pu/137Cs activity ratio distributed in 07BS10−2 could be attributed to low yield tests of the CNTs conducted from 1967 to 1973. Hodge et al. investigated the 239+240Pu and 137Cs activities in soil samples collected all over the world and observed a 137 Cs/239+240Pu activity ratio of 37 ± 1 for global fallout when corrected to 1 July 1994, which is suggested to be a background value for further Pu and 137Cs source identification.24 Cizdziel et al. tested the two methods using the 240Pu/239Pu atom ratio and 239+240Pu/137Cs activity ratio to resolve Nevada test site and global fallout Pu in attic dust and soil from Nevada and Utah,
thus providing an important parameter for geochemical studies. Inventories of 239+240Pu and 137Cs in the sediments of Lake Bosten are shown in Table 1. 239+240Pu inventories in 06BS2−1 and 07BS10−2 were 46.6 ± 2.1 Bq·m−2 and 51.6 ± 4.3 Bq·m−2, respectively, which were in the range of the global fallout value (58.1 ± 18.5 Bq·m−2) and comparable with the result (62.7 ± 3.2 Bq·m−2) obtained in Lake Sihailongwan in northeastern China at the same latitude.6 Xinjiang Province has an arid climate and low precipitation (70 mm per year), and low inventories of 239+240Pu in Lake Bosten were expected, like the inventories seen in other lakes in northwestern China.6 However, the inventories of 239+240Pu in Lake Bosten, investigated in this study, were similar to the global fallout due to the deposition of resuspended particles as has been discussed above. 137Cs inventories in 06BS2−1 and 07BS10−2 were 1287.9 ± 167.8 Bq·m−2 and 970.2 ± 91.3 Bq·m−2, respectively, significantly lower than the expected global fallout value (2618 ± 353 Bq·m−2). Similarly, low inventories of 137Cs (
