Anomalous Plutonium Isotopic Ratios in Sediments of Lake Qinghai

ARTICLE pubs.acs.org/est

Anomalous Plutonium Isotopic Ratios in Sediments of Lake Qinghai from the Qinghai-Tibetan Plateau, China Fengchang Wu,*,†,^ Jian Zheng,*,§,^ Haiqing Liao,† Masatoshi Yamada,§ and Guojiang Wan‡ †

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China § Office of Biospheric Assessment for Waste Disposal, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan ‡ State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry of the Chinese Academy of Sciences, Guiyang 550002, China

bS Supporting Information ABSTRACT: The vertical profiles of 239+240Pu and 137Cs activities and 240Pu/239Pu isotopic ratios are determined for three sediment cores of Lake Qinghai from the Qinghai-Tibetan Plateau, China, and compared with those in sediments of another three lakes (Lakes Bosten, Sugan, and Shuangta), the only existing ones closest to Lop Nor area, China’s nuclear weapons test site in the northwestern part of the country. The mean inventory of 47.7 ( 18.7 MBq km
2 for 239+240Pu activity in Lake Qinghai is comparable to the average value of global fallout expected at the same latitude, yet the mean inventory of 1112.0 ( 78.0 MBq km
2 for 137Cs is slightly lower than that of global fallout. Anomalously low 240Pu/239Pu isotopic ratios (0.038
0.125) were found in the 3
6.5 cm deep sediment layers, indicating the trace Pu input from early nuclear weapons research activities at Atomic City in the lake’s watershed during the 1950
60s. Model calculation indicated that the Pu input accounted for approximately 5
16% of the total Pu inventory. The observation of low 240Pu/239Pu ratio in the deep sediment layer provided a new time marker for recent sediment dating in the lake and around the area. The results are of great significance to the further understanding of sources, records, and environmental impacts of global and regional nuclear activities in the environment and provide important chronological information for further studies on the water eutrophication process and climatic change, and reconstruction of pollution history of organic contaminants and heavy metals in the watershed of Lake Qinghai.

’ INTRODUCTION It is of great importance to obtain information on accurate dating of recent lacustrine sediments for environmental studies, such as retrospectively investigating the variation of nutrients, reconstructing the pollution history, and elucidating the variation process of biological productivities. There are mainly two kinds of dating methods for recent lake sediment chronology. One is the 210Pb dating using unsupported 210Pb derived from the atmospheric fallout of 222Rn daughters to calculate the sedimentation rate or accumulation rate.1 That is, however, sustained by the assumptions that either sediment surface activity or flux to the sediment surface is relatively constant.2,3 The other is the use of fallout radionuclides, such as 137Cs, 239+240Pu, and 241Am, in the sediment to obtain chronological information. These radionuclides were assumed to be derived from stratospheric fallout due to the atmospheric testing of nuclear weapons.3
6 Considering the relatively short half-life of 30.2 years, the sediment dating using 137Cs will be impractical because more than 60% of the original inventory of global fallout 137Cs has decayed.2 The use of global fallout radionuclides for sediment dating is based on the fact that the obvious peak of activity of these r 2011 American Chemical Society

radionuclides can be found in sediments as a time marker for the years 1963
1964 due to a series of large-scale atmospheric nuclear tests conducted by the former Soviet Union in 1961
1962.7 A weak activity peak of radionuclides signed to the year of 1953 may be recorded in well-preserved sediment cores due to atmospheric nuclear tests in the early 1950s.8 Deposition of global/local fallout radionuclides derived from nuclear incidents can also contribute reliable time markers to sediment dating. For example, the deposition of 137Cs and Pu resulted from the nuclear plant explosion at Chernobyl in lake sediments provided a time maker of 1986 in some European countries.2,9,10 Moreover, atmospheric fallout of 238Pu was deposited in the Antarctic sediments and ice cores as a result of the burn up of a US satellite, which contained about 1 kg of 238 Pu in auxiliary nuclear power, in 1964.11,12 In addition to activities of radionuclides, the unique atomic ratio of Pu isotopes Received: July 6, 2011 Accepted: September 28, 2011 Revised: September 23, 2011 Published: September 28, 2011 9188

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Table 1. Typical Plutonium Isotopic Ratios of Various Sources in the Environment 240

Pu/239Pu atom ratio

global fallout

0.178 ( 0.019 (0
30N)

241

Pu/239Pu activity ratio

238

Pu/239+240Pu activity ratio

2.7

0.025

references 15, 16

0.180 ( 0.014 (30
71N) weapon-grade plutonium

0.01
0.07

5.8
13

0.015
0.42

17, 18

reactor-grade plutonium

0.24
0.80

>130

>2.1
2.9

19
21

Chernobyl accident

0.31
0.40

135
213

0.49
0.56

20, 22

Pacific proving ground

0.306
0.36

27a

0.001
0.014

13, 23

Nagasaki atomic bomb

0.028
0.037

1.21b

0.074

24, 25

Thule hydrogen bomb debris Semipalatinsk nuclear test site

0.0551 0.03
0.05

0.87c


0.0161 0.0064
0.085

26 27, 28

a 241 +240

Pu/239+240Pu activity ratio, reference date of 241Pu: 1952
1954. b 241Pu/239+240Pu activity ratio, reference date of 241Pu: August 1945. c 241Pu/239 Pu activity ratio, reference date of 241Pu: October 2001.

and activity ratio of 239+240Pu/137Cs are also useful for the source identifications and dating in the aquatic sediments, especially in lakes close to nuclear testing sites.6,13,14 In particular, the isotopic composition of Pu is characteristic for various Pu sources (Table 1),13,15
28 and therefore accepted as a good indicator for identifying Pu sources in the environment. However, information on the distribution, inventory, and sources of fallout radionuclides in China is very limited, which severely hampered the study of recent lake sediment dating needed in China for the reconstruction of the pollution history of organic pollutants and heavy metals. It is known that Chinese atmospheric nuclear tests were conducted at the Lop Nor nuclear test site in northwestern China since 1964. However, the early nuclear weapon research and development activities performed at Atomic City in Lake Qinghai’s watershed remain unknown.29,30 This research center, located in Haibei Prefecture, about 20 km northeast of Lake Qinghai was constructed in 1958 for China’s independent development and assembly of nuclear weapons.29 It was here that China’s first atomic and hydrogen bombs were designed and developed in the 1950s and 1960s. It was completely shut down in 1987 following an official environmental safety evaluation and was officially declassified in 1995. It is now open to the public as a National Patriotism Education Demonstration Base. Since studies are very limited, it is not clear whether there were sediment records of radionuclides resulting from the early nuclear activities in Lake Qinghai. If so, the deposited Pu may bear a unique isotope composition different from that of global fallout source Pu, thus provide a new tool for recent sediment dating in the area. In the present work, in order to illustrate the possible influence of global fallout, the early nuclear activities conducted in the watershed, and close-in fallout from Lop Nor in Lake Qinghai, three sediment cores in the lake are collected, and vertical profiles of 239+240Pu and 137Cs activities and 240Pu/239Pu isotopic ratios are investigated and compared with those of another three lakes (Lakes Bosten, Sugan, and Shuangta), the only existing ones closest to Lop Nor area, China’s nuclear weapon test site. For the first time, the Pu input from the early nuclear research and development activities in Atomic City with a 240Pu/239Pu atom ratio well below typical global atmospheric fallout levels was found. The unique 240Pu/239Pu isotopic ratios and the vertical profiles of 239+240Pu and 137Cs activities in the sediments were found to be important for the further understanding of possible environmental impact of regional nuclear activities and useful for recent sediment dating which is needed for investigation of pollution histories in the lake and its watershed.

Figure 1. Map showing the locations of Lake Qinghai, Lake Bosten, Lake Sugan, and Lake Shuangta, and sampling sites in Lake Qinghai and Atomic City.

’ EXPERIMENTAL SECTION Sample Collection. Lake Qinghai (99360
100160 E,

36320
37150 N) is the largest inland salt lake in China with a lake surface area of 4278 km2, a watershed area of 16 570 km2, a lake surface height of 3193 m above sea level, a maximum depth of 26.5 m, a water pH value of 9.2 on the average, a mineralization degree of 12.3 g L
1, and a salinity of 14.2 %.31,32 Lake Qinghai is situated on the Qinghai-Tibetan Plateau in Qinghai Province, China, and is about 1000 km away from Lop Nor, China’s nuclear weapons test site, together with locations of Lake Bosten, 9189

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Figure 2. Vertical profiles of 239+240Pu and 137Cs activities and 240Pu/239Pu atom ratios in the sediments of Lake Qinghai.

Shuangta, and Sugan. China’s nuclear weapons research and development center is located in Haibei Prefecture, about 20 km northeast of Lake Qinghai in the watershed (Figure 1). Three sediment cores were sampled in Lake Qinghai in 2006 (Figure 1). Three sediment cores were collected in the center area of Lakes Bosten, Sugan, and Shuangta in 2006
2007, respectively. Each sediment core was sectioned at 0.5-cm intervals in the field. The sediment samples were weighed immediately after collection. They were kept cool until brought to a laboratory and were then freeze-dried, weighed, and ground to 120 mesh for further analyses of 137Cs and 239+240Pu activities and 240Pu/239Pu isotopic ratios. Analytical Procedure. 137Cs activity was determined using gamma-spectrometry on a Canberra S-100 multichannel spectrometer with a GC5019 H
P Ge coaxial detector (efficiency 50%) at the Institute of Geochemistry of the Chinese Academy of Sciences. The 137Cs peak used to determine its activity was 661.6 KeV. Liquid standard (Catalog No. 7137) supplied by the Institute of Atomic Energy of the Chinese Academy of Sciences, was used. For 137Cs activity measurements, the relative standard deviation was within 10%.33 Activities of 239Pu and 240Pu were determined at National Institute of Radiological Sciences, Japan. Pu isotope separation and analytical methods were described in our previous reports.34,35 Briefly, 0.5
2.0 g samples were digested using HNO3 to extract Pu isotopes. AG 1-X8 and AG MP1-M anion resins were used in the separation and purification process. The analytical procedure was characterized by 75% Pu chemical recovery and a U decontamination factor of 105. 239Pu and 240Pu were analyzed using a sector-field ICP-MS (Element 2,

Finngan, Germany) combined with a high efficiency sample introduction system (APEX-Q). Sediment standard reference materials IAEA-368 (marine sediment standards, International Atomic Energy Agency) and SRM-4354 (freshwater lake sediment standards, American National Standards Institute of Technology) were used for analytical method validation. The results obtained for the two reference materials clearly show the suitability of the analytical procedure for the analysis of sediment samples (Table 1 in Supporting Information).

’ RESULTS AND DISCUSSION Vertical Profiles of Pu Isotopes and

137

Cs in Sediments. The vertical profiles of Pu and Cs activities in sediments of Lake Qinghai are characterized by a single-peak distribution pattern (Figure 2). The peak values of 239+240Pu and 137Cs activities appear at the mass depths of 0.459 and 0.716 g cm
2 in 2006QH-2 and 2006QH-3, respectively. 239+240Pu activities are as high as 4.74 and 6.99 mBq g
1, and 137Cs activities are 94.62 and 112.58 mBq g
1, respectively. These vertical profiles are consistent with those found in other lakes in China3,36 and are also similar to those in other lacustrine and marine sediments.37
39 This suggests that the single peak of 239+240Pu and 137Cs activities in the sediments corresponds to the 1963
1964 global fallout. In the sediment from the surface to the layer of peak deposition in 1964, 240Pu/239Pu isotopic ratios range from 0.169 ( 0.007 to 0.228 ( 0.042 (Figure 2; Table 2 in Supporting Information); the inventory-weighted mean 240Pu/239Pu atom ratios are 0.181, 0.175, and 0.170 for 2006QH-1, 2006QH-2, and 239+240

9190

137

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Figure 3. Vertical profiles of 239+240Pu and 137Cs activities and 240Pu/239Pu atom ratios in the sediments of Lake Bosten, Lake Sugan, and Lake Shuangta (redrawn from Wu et al.6,41).

2006QH-3, respectively. These values are close to 0.180 ( 0.014, the value reported for the global atmospheric fallout.16 This provides further isotopic evidence for the source of 239+240Pu and 137 Cs in the surface sediments, i.e., they originated from global atmospheric fallout. However, in the deep sediment layers below the 1964 peak (below mass depth of 1.60 g 3 cm
2 in 2006QH-1, 0.46 g 3 cm
2 in 2006QH-2, and 0.72 g 3 cm
2 in 2006QH-3, respectively), 240Pu/239Pu ratios range from 0.038 to 0.159, which are significantly lower than those in the surface sediments, and they also decrease as depth increases. Previous studies have shown that 240Pu/239Pu ratios from nuclear weapon-grade materials ranged from 0.01 to 0.07,17 those from nuclear reactors ranged from 0.24 to 0.80, and the global atmospheric fallout prior to 1963 was slightly higher than 0.18.13,21,40 Therefore, the low 240 Pu/239Pu atom ratios in the deep sediment layers may suggest the possible occurrence of weapon-grade Pu input in the lake, which may not originate from the global atmospheric fallout. We also analyzed 241Pu/239Pu activity ratio in sediment cores of 2006QH-2 and 2006QH-3. Although the obtained 241Pu/239Pu activity ratios have relatively high RSD (25
68%) due to the low activity of 241Pu, they ranged from 2.35 to 3.04, close to the value of 2.7 for global fallout,15,16 in the sediment layers above the 1964

maximum deposition peak while low values of 0.55 to 2.08 were seen in the layers below the 1964 peak (Table 2 in Supporting Information). The plot of 240Pu/239Pu atom ratio vs 241Pu/239Pu activity ratio for 2006QH-2 and 2006QH-3 sediment cores (Figure 1 in Supporting Information) indicated the mixing of local source Pu input and the global fallout source in Lake Qinghai. Another possibility for the low 240Pu/239Pu atom ratios in Lake Qinghai is the influence of China’s nuclear weapons tests. China’s nuclear weapons test site, where the first nuclear test took place on October 16, 1964, is Lop Nor which is 1000 km from Lake Qinghai.29 240Pu/239Pu atom ratios in the sediments of Lakes Bosten, Sugan, and Shuangta near Lop Nor are generally within the global atmospheric fallout range (Figure 3) except at two incident mass-depth layers of 2.83 g 3 cm
2 in 07Bosten 10
2 (Lake Bosten) and 0.90 g cm
2 in 2007SG-2 (Lake Sugan) with ratios of 0.080 ( 0.016 and 0.103 ( 0.009, respectively.6,41 Those two layers are located at or above the 239+240Pu peak layer. The ratios are significantly lower than the global fallout value of 0.180 but slightly higher than the nuclear weapon-grade characteristic value (0.01
0.07), showing the limited influence of China’s atmospheric nuclear weapon tests. As the observed 240 Pu/239Pu atom ratios in the deep layers of Lake Qinghai 9191

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Figure 4. Inventories of 239+240Pu and 137Cs in the sediments of Lake Qinghai (note: 137Cs activity was corrected back to July 1998; for comparison, the average values of global atmospheric fallout in 30
40N latitude bands are also provided).

sediments are even lower than those found in Lake Bosten and Lake Sugan, and the sediment layer of low 240Pu/239Pu atom ratios were below the 1964 peak layer, the possibility of the influence of China’s atmospheric nuclear tests can be excluded. This further suggests that the low Pu ratios observed in the deep layers in Lake Qinghai originated from the early nuclear weapon research and development activities, e.g., trial tests and/or waste discharges made within Atomic City in the watershed. The Net 239+240Pu Inventories from Atomic City. Inventories of 239+240Pu and 137Cs (decay corrected to July 1998) in the sediments of Lake Qinghai are shown in Figure 4. 239+240Pu inventories are 68.4 ( 2.7, 31.9 ( 0.9, and 42.8 ( 0.9 MBq km
2 in 2006QH-1, 2006QH-2, and 2006QH-3, respectively, with an average of 47.7 ( 18.7 MBq km
2, which is slightly higher than the average value of global atmospheric fallout expected at the same latitude (42 MBq km
2).15 Inventories of 137Cs in 2006QH-2 and 2006QH-3 are 933.9 ( 81.1 and 1112.0 ( 78.0 MBq km
2, respectively, significantly lower than the average value of global atmospheric fallout at the same latitude (1923 MBq km
2, decay corrected to July 1998).42 This may be related to the geographical location of the lake in the area with dry weather, low annual precipitation, and strong prevailing winds, which resulted in the low atmospheric particle settlement.43 A low 137Cs inventory in sediments is also observed in Lake Sugan, where 239+240Pu inventory is lower than the average value of global atmospheric fallout at this latitude (Figure 2 in Supporting Information). The 239+240Pu/137Cs inventory ratios in the sediments of the four studied lakes are higher than the value of global atmospheric fallout (0.029 ( 0.003, decay corrected back to July 1998) (Figure 3 in Supporting Information),44 and the highest value comes from Lake Qinghai although the other three lakes are closer to Lop Nor. In 2006QH-2 and 2006QH-3, 239+240Pu/137Cs ratios are 0.034 and 0.038, respectively. The differential deposition of Pu and 137Cs, the so-called fractionation effect that fallout particles bearing mainly Pu were removed from the nuclear cloud at earlier times or at shorter distances than fallout particles bearing mainly 137Cs, has been reported by Simon et al.,45 and they found that 239+240Pu/137Cs ratio decreased with increasing distance to the nuclear weapons test site in the Marshall Islands. Thus, if the four lakes under discussion were all influenced by nuclear weapons test activities in Lop Nor, 239+240Pu/137Cs ratios in the far-away Lake Qinghai should be lower than those in the near-by Lakes Bosten, Sugan, and

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Figure 5. The relative contributions of 239+240Pu inventories from both global fallout and local input origins.

Shuangta. However, the present observations are just the opposite. This observation further supports above discussion that Lake Qinghai did not receive direct close-in fallout Pu from Lop Nor. The higher 239+240Pu/137Cs ratios in the lake can be attributed to the trace, but unique, Pu input from activities in Atomic City. The two-component mixing model 11,46 was used to estimate contributions of 239+240Pu input from Atomic City to the total Pu inventory, taking 0.18 as the 240Pu/239Pu ratio of global fallout origin and the minimum ratio of 0.038 in the sediments as the local input origin. The contributions are 5.1%, 15.7%, and 9.2% in 2006QH-1, 2006QH-2, and 2006QH-3, respectively, and the 239+240Pu inventories are 3.38, 5.02, and 3.92 MBq 3 km2, with an average of 4.11 ( 0.84 MBq 3 km2 (Figure 5). Based on the lake area, the total 239+240Pu inventory is approximately estimated to be 17.6 ( 3.6 GBq in the lake. During the period from 1964 to 1980, 22 atmospheric nuclear weapons tests were conducted at Lop Nor. Because little information on the Chinese nuclear weapons tests and related nuclear activities is available, the environmental impact, in particular, the possible radioactive contamination in northwestern China, has been a great concern. This study for the first time reveals that Lake Qinghai did not receive significant direct close-in fallout Pu from the Chinese atmospheric nuclear weapons tests at Lop Nor. The Pu input from the early nuclear activities in Atomic City is only 5
16% of the total Pu inventory, which is close to the expected Pu inventory from global fallout; therefore, the radiation effect on the local population would be expected to be negligible. Dating Sediments. Although the local Pu input recorded in the lake is very small, the unique and significantly low 240Pu/239Pu ratios in the deep sediment layers provide a new indicator for the dating of recent sediments, making it possible to estimate more accurately the sedimentation rate. The lowest 240Pu/239Pu ratios are 0.125 ( 0.018, 0.051 ( 0.019, and 0.038 ( 0.018 in the vertical profiles of 2006QH-1, 2006QH-2, and 2006QH-3, respectively (Table 2 in Supporting Information). These low ratios appear at mass depths of 2.52, 1.25, and 1.77 g cm
2 in 2006QH-1, 2006QH-2, and 2006QH-3, respectively (Figure 2). These depths were assumed to correspond to the initial period of nuclear weapon research and development activities in Atomic City in 1958, i.e., the date for the initial local-origin Pu input in Lake Qinghai, and from them the sedimentation rates are calculated as 0.143, 0.099, and 0.125 g cm
2 a
1 in 2006QH-1, 2006QH-2, and 2006QH-3 between 1958 and 1964. They are almost 9 times the 9192

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Table 2. Sedimentation rates for Sediment Cores in Different Time-Scales, Together with the Contribution of Pu from Atomic City sediment cores 2006QH-1

time-scale

sedimentation

Pu from Atomic

rates g cm‑2 a‑1

City (MBq km‑2)

1964
2007

0.032

0.0

1958
1964

0.143

3.49

2006QH-2

1964
2007

0.011

0.0

2006QH-3

1958
1964 1964
2007

0.099 0.017

5.02 0.0

1958
1964

0.125

3.92

sedimentation rate from 1964 to the present (0.032, 0.011, and 0.017 g cm
2 a
1), suggesting a dramatic impact from unprecedented large-scale human activities in the watershed during the 1950
60s (Table 2). Therefore, the present results suggest that the sole use of 137Cs or 239+240Pu activity profiles for the estimation of recent sedimentation in Lake Qinghai is not reliable, where strong human activities have occurred in the recent decades. If the vertical profiles of 240Pu/239Pu ratios are taken into consideration, more detailed chronological evidence can be obtained for investigating historical nuclear activities in modern lake sediments in China, for reconstructing the pollution history of organic pollutants and heavy metals and for studying environmental changes. It is difficult to resolve the local source from the global fallout and evaluate the influence of local nuclear activities in areas close to nuclear research and/or test sites if only activities of radionuclides were measured. In the present work, at least two sources of plutonium could be identified in Lake Qinghai by using the vertical profiles of 239+240Pu and 137Cs activities, and 240Pu/239Pu isotopic ratios in sediment cores, combined with those in sediments of another three lakes (Lakes Bosten, Sugan, and Shuangta). For the first time, the existence of trace Pu contamination from the early nuclear weapons research and development activities in Atomic City was found in Lake Qinghai. However, from the calculation of Pu inventory of this local source, the radiation effect on the local population can be considered to be negligible. Because of its unique Pu isotopic composition, the trace Pu input recorded in sediments provides important chronological information for further studies on the water eutrophication process and climatic change, and reconstruction of pollution history of organic contaminants and heavy metals in the watershed of Lake Qinghai.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: 0081-43-206-4634; fax: 0081-43-255-0721; e-mail: [email protected] (J.Z.); [email protected] (F.W.). Author Contributions ^

F. C. Wu and J. Zheng contributed equally to this study.

’ ACKNOWLEDGMENT This work was jointly supported by the exploratory research fund, National Institute of Radiological Sciences, Japan, Chinese National Basic Research Program (2008CB418200), Natural Science Foundation of China (40903037), and the Committee of National Defense Science & Technology of China (2008-124) and has been partly supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan. ’ REFERENCES (1) Goldberg, E. D. Geochronology with 210Pb. In radioactive dating; Vienna: IAEA, 1963; pp 121
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