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Characteristic of Pu from urban wetland and lacustrine sediments in Suzhou Industrial Park, China
Journal of Environmental Radioactivity 213 (2020) 106134
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Characteristic of Pu from urban wetland and lacustrine sediments in Suzhou Industrial Park, China Yongjin Guan a, Jingyu Mai a, b, Jiawei Xu b, Zhiyong Liu b, * a
Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Centre of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, 215123, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Suzhou Industrial Park China Wetland Plutonium source Distribution Deposition
In this study, plutonium activity concentrations in the urban wetlands and lacustrine sediment of Suzhou In dustrial Park (SIP) are studied for the first time. Results show 239þ240Pu activity concentrations in the wetland surface soils of SIP range from 0.035 to 0.426 mBq/g and the 240Pu/239Pu atom ratio ranges from 0.171�0.024 to 0.226�0.049. Judging from the atom ratio of 240Pu/239Pu, the main source of Pu in the wetland is global fallout. The correlations of Pu between organic matter and heavy metals are also studied. The correlation coefficients show Pu has significant positive correlations with Cu, Sn and Pb but negative correlation with As. Unlike dis tributions of Pu in other places, Pu in SIP has weak correlation with organic matter content. A sediment core from Lake Yangcheng is also analyzed to investigate the historical record of Pu deposition. The atom ratios of each layer in the sediment core indicate the area is mainly influenced by global fallout. Using Pu as a discrete-time maker, the deposition rate in Lake Yangcheng is 0.396�0.019 cm/yr. The calculated inventory of 239þ240Pu is 58.5 Bq/m2, which is in the range of inventories of the corresponding latitudes according to UNSCEAR.
1. Introduction Pu is an artificial radionuclide associated with nuclear industry including nuclear weapon and energy industry. Common anthropogenic Pu are in the form of 239Pu (T1/2 ¼ 2.411 � 104 yr), 240Pu (T1/2 ¼ 6563 yr), 241Pu (T1/2 ¼ 14.4 yr), 242Pu (T1/2 ¼ 3.73 � 105 yr) and 244Pu (T1/2 ¼ 8.08 � 107 yr). Anthropogenic Pu in the environment mostly comes from the series of nuclear weapon tests from the mid-1940s to the 1980s. The atmospheric nuclear weapon tests conducted by countries such as the former Soviet Union, the United States and China have emitted about 1.1 � 1016Bq of 239þ240Pu into the environment, and Pu was spread globally due to atmospheric dispersion (Harley, 1980). The 240Pu/239Pu atom ratio differs for different sources such as nu clear test, military weapons or industrial sources. Therefore, the atom ratio is often used to identify the sources of radioactive pollutants in the environment (Krey et al., 1976; Warneke et al., 2002). Specifically, 240 Pu/239Pu atom ratio of weapon-grade Pu ranges from 0.01–0.07 (Lindahl et al., 2017), while nuclear reactor-grade Pu exhibits higher atom ratios, ranging from 0.24–0.8 (Warneke et al., 2002; Ketterer and Szechenyi, 2008). Atom ratio values of the Pacific proving ground (PPG) Pu are usually 0.306–0.36, which are higher than the average value of
240
Pu/239Pu atom ratios 0.178�0.019 (0–30� N) and 0.180�0.014 (30–71� N) from the global fallout (Krey et al., 1976; Kelley et al., 1999). As 239Pu and 240Pu have a high radiotoxicity and long-term persis tence in the environment, risk assessment for Pu in the environment is indispensable. More and more researchers are trying to determine the characteristics of distribution, sources and fate of Pu in different layers of the earth, including atmosphere, water and sediments (Hirose and Povinec, 2015; Zhang et al., 2018). Regional studies on 239þ240Pu activity concentration in the surface soils and its vertical distribution covers many part of China. However, 239þ240 Pu activities and inventories of the research areas are quite distinct from each other. The 239þ240Pu activities in the surface soil range from 0.023�0.003 mBq/g (Gansu, Lanzhou, northwestern China) to 1.30�0.05 mBq/g (Wuling, Chongqing, southwestern China) and the distinct 239þ240Pu activities in China might be caused by the differences of soil particle sizes, organic matter, regional climatic factors and other factors (Zheng et al., 2009; Bu et al., 2014). In the vertical distribution of 239þ240 Pu, most Pu depositions (more than 80%) concentrate on the top 10 cm layer of soils (Dong et al., 2010; Bu et al., 2014). However, some soil core analyses show opposite results and the most Pu inventories are in the deeper layers (Zheng et al., 2009; Zhang and Hou, 2019) largely
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.jenvrad.2019.106134 Received 24 August 2019; Received in revised form 2 December 2019; Accepted 9 December 2019 Available online 16 December 2019 0265-931X/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Environmental Radioactivity 213 (2020) 106134
due to soil erosion and downward percolation of fine particles. Studies also show that the inventories of 239þ240Pu in the estuary sediments are much larger than in soils, which might be caused by scavenging of Pu from the seawater and the higher sedimental deposition rates in the estuaries areas (Hao et al., 2018; Liu et al., 2011; Zhang et al., 2018). The 240Pu/239Pu atom ratios have also been investigated in different regions and the results show that China has been affected by different sources of Pu and the global fallout is the main source of Pu in mainland (Huang et al., 2019). Studies have shown that the mainland of China, such as the northwest has been affected not only by the global fallout but
also the Lop Nor nuclear tests conducted in the 1980s (Bu et al., 2015). Also, some results indicate that China coast including the marginal seas, such as the East China Sea, the Yellow Sea and the South China Sea, has been influenced by both the global fallout and the PPG (Pacific Proving Ground) close-in fallout (Dong et al., 2010; Liu et al., 2011, 2013; Wu et al., 2014; Zhang et al., 2018). It has been demonstrated that Pu derived from the PPG can be transported over long distances to the western Pacific Ocean and to its adjacent marginal seas (Liu et al., 2011; Wu et al., 2014). However, information on the distribution and char acteristics of Pu deposition in the wetlands and lakes in the eastern
Fig. 1. The location of the sampling sites in the areas of SIP, Suzhou, China. 2
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China are very limited. Since China operates nuclear power plants (NPPs) along the east coast, investigating the historical deposition and migration mechanisms of Pu in urban wetlands and lakes is vitally important to understand and evaluate possible future pollution with radionuclides. In this study, the spatial distributions of 239þ240Pu activity of the urban wetlands in Suzhou Industrial Park (SIP) were measured for the first time and the sources of Pu in this area were investigated to see whether other sources such as Chinese nuclear tests or the Fukushima NPP accident have affected this area. In addition, a sediment core taken from Lake Yangcheng was also analyzed to investigate the historical information of Pu deposited in the lake. 2. Samples and methods
washed with 50 ml 10 M HCl to remove Th. Pu is eluted with 50 ml 0.1 M NH4I - 8.5 M HCl. 1 ml concentrated HCl is added and the eluted solution is evaporated to dryness. After evaporation, 3 ml HCl–H2O2 is added into the residue and the solution is heated by water bath at 40 � C for 1 h. In the second Pu fraction stage, the solution is washed on the column (AG MP-1M). Then the column continues to be washed with 20 ml 8 M HNO3 and 10 ml 10 M HCl and Pu in the solution is eluted with 16 ml HBr. After that, 1 ml concentrated HNO3 is added into the solution and evaporated to dryness. The final step is measurement. In this step, the eluted solutions need to be prepared for measurement. 1 ml 4% HNO3 is added to dissolve the final eluted solution and the samples are prepared for analyzing. The final measurements were conducted with a sector-field ICP-MS (Fin nigan Element 2, Bremen, Germany).
2.1. Sample collection and preparation
2.3. Instrument preparation and analytical procedure
The locations of the sampling sites are shown in Fig. 1. As seen in Fig. 1, the sample sites are distributed in Suzhou Industrial Park (SIP), Suzhou, eastern China. Suzhou (119� 550 –121� 200 E, 30� 470 –32� 020 N) is located in the middle of Yangtze River Delta and adjacent to Yangtze River and Lake Taihu. The city is under the impact of subtropical monsoon marine climate, having ~1100 mm annual precipitation. Surface soil samples were collected in the wetlands of SIP in June 2012 (Fig. 1). The urban wetlands are wetlands under shallow water and less affected by hydrodynamic factors and samples are collected in the upper soil layers from 0–4 cm depth (Site No. 1, 2, 4, 7, 11, 12, 14, 16, 17, 18, 19, 20, 23, 25, 26, 27, 28, 29, 30). At each site, three replicates were taken, and the samples were sealed separately within clean poly ethylene bags, carried by a box corer, preserved and transported in a cooler at 4 � C. Samples of each site were mixed uniformly to assure the quality of the samples. The sediment core was collected in Lake Yangcheng close to SIP in July 2016. Site No.32 is a sediment core sample with a depth of 55 cm, collected in 10 cm diameter Plexiglas tubes. It was also preserved at a temperature of 4 � C and transported to the Key Laboratory of Radio logical and Interdisciplinary Science of Soochow University for further analysis. The sediment core was sliced into 36 sections. 0–4 cm was taken as the surface layer and the rest was divided by every 1 or 2 cm in the following of the first layer to a depth of 55 cm.
The sector-field ICP-MS in a low-resolution mode (m/Δm ¼ 300) is used for the analysis to utilize the maximal instrument sensitivity. An APEX-Q high-efficiency sample introduction system (Elemental Scien tific Inc., Omaha, NE, USA) is used to introduce the samples by a membrane desolvation unit (ACM) and a conical concentric nebulizer. By the sample introduction system, the chance of the formation of uranium hydride species in SF-ICP-MS is lowered (Zheng and Yamada, 2006). In addition, the normal skimmer cone is replaced by a high-efficiency cone (X-cone, Thermo Finnigan) to further increase the sensitivity of SF-ICP-MS. All the measurements are made in the self-aspirating mode to reduce the risk of contamination by the peri staltic pump tubing. The isotopes of interest (238U, 239Pu, 240Pu, 242Pu) are analyzed in the peak hopping mode and the peak tops of the masses are measured at 10% of their respective peak width. A standard solution of 0.1 ng ml 1 U is used daily to optimize condition of the SF-ICP-MS. Instrument interference is corrected according to the standard with a known 240Pu/239Pu atom ratio of 0.242 in Pu standard solution (NBS-947). In addition, the standard reference materials IAEA-368 and NIST-4357 are also separated and analyzed together for comparison and verification to ensure the accuracy of chemical separation experiments and measurement experiments. The sensitivity for this system is about 107 cps∙ppb 1, the detection limit of 239Pu is 0.0005 mBq/g and of 240Pu is at 0.002 mBq/g. The chemical yield in the employed sample prepa ration procedure is estimated to be in the range of 54%–77% with a mean of 62%. For the heavy metal measurement, the pre-treated samples (0.1 g) were digested by HCl–HNO3–HF–HClO4 acid mixture at the 200 � C to the near-dry status. The residues were diluted by double deionized water, filtered by membranes (0.45 μm) and measured by SF-ICP-MS in Soochow University.
2.2. Experiment standards, reagents and samples The soil samples and sediment sample went through three main steps for analysis. The first step is pretreatment, which includes drying, grinding, and sieving. The stones and fragments like biological matters were removed from the samples. Then the samples are dried at 100 � C for 24 h and passed through 20-mesh sieves to remove rhizome and coarse stones and then transferred to crucible and dried at 500 � C for over 4 h to podzolize the samples. The second step is chemical separation and extraction. The chemical separation of Pu isotopes is modified from Zheng and Yamada (2006). 2.5 g pre-treated samples were weighed out and 1.14 pg of 242Pu was added as a yield tracer. To leach the samples, 50 ml of 8 M HNO3 is added into the samples and heated at 220 � C for at least 4 h. After leaching, the solution is filtered and evaporated to wet paste condition. Concentrated HNO3 is added into the paste and the solution is adjusted to 8 M HNO3 by adding de-ionized water. Then 0.276 g NaNO2 is added into the solution and it is heated in water bath at 40 � C for 30 min. During this procedure, Pu is adjusted to Pu4þ. Separation and purifica tion of Pu isotopes are conducted with AG 1-X8 anion-exchange column (100–200 mesh) and AG MP-1M anion-exchange column (100–200 mesh). In the first Pu fraction stage, the sample solution is loaded on the column (AG 1-X8) and washed with 60 ml 8 M HNO3 to remove U, Am and other cationic matrices such as Pb, Hg and Tl. Then the column is
3. Results and discussions 3.1. 239þ240Pu activity concentration and its distribution in surface soils in Suzhou The 239þ240Pu activities and 240Pu/239Pu atom ratios in surface soil samples collected in SIP are presented in Table 1. The results show the activity concentration of 239þ240Pu in the surface soil ranges from 0.035 to 0.426 mBq/g with the average value of 0.156�0.08 mBq/g (arith metic average) and 0.126�0.013 mBq/kg (geometric average). The activity concentration values are close to the values (0.088�0.031–0.469�0.057 mBq/g) in Guangxi that Guan et al. (2018) reported. The maximum value of the activity is 0.426�0.018 mBq/g at Site No.7, which is higher than the maximum value of 0.380�0.016 mBq/g in the surface soil (0–2 cm) of Hubei Province at similar latitude (Dong et al., 2010). The higher 239þ240Pu activity concentrations can be attributed to the different soil particle diameters (Xu et al., 2017; Chen et al., 2013). In this study, the soil samples are mainly fine particles 3
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Journal of Environmental Radioactivity 213 (2020) 106134
Table 1 239þ240 Pu activity and 240Pu/239Pu atom ratio in surface soil samples of SIP. Sample number
239þ240
1 2 4 6 7 11 12 14 16 17 18 19 20 23 25 26 27 28 29 30
0.183 0.269 0.160 0.196 0.426 0.229 0.328 0.193 0.138 0.175 0.120 0.065 0.073 0.035 0.037 0.051 0.067 0.117 0.178 0.084
Pu activity (mBq/g)
� 0.014 � 0.011 � 0.011 � 0.015 � 0.018 � 0.057 � 0.081 � 0.014 � 0.025 � 0.034 � 0.008 � 0.008 � 0.005 � 0.009 � 0.008 � 0.003 � 0.014 � 0.007 � 0.015 � 0.004
240
Pu/239Pu atom ratio
0.211 � 0.226 � 0.223 � 0.219 � 0.196 � 0.223 � 0.222 � 0.218 � 0.198 � 0.191 � 0.176 � 0.178 � 0.186 � 0.176 � 0.182 � 0.181 � 0.187 � 0.183 � 0.173 � 0.171 �
0.053 0.049 0.058 0.044 0.055 0.046 0.047 0.042 0.043 0.046 0.025 0.026 0.023 0.013 0.017 0.029 0.021 0.032 0.026 0.024
Fig. 2. Distribution of 240
239þ240
Pu activity concentration in surface soil in SIP.
Pu/239Pu atom ratio and its distribution in SIP
*All given uncertainties are � two standard deviation errors.
3.2.
(<150 μm), which Pu can easily attach to. There are numerous manufacturing industries in the study area, such as precision machines and electronics and telecommunication. As re ported, the wetlands in SIP have been polluted by heavy metals to different degrees (Xu et al., 2019). To examine the relationship between Pu and heavy metals, Pearson correlation coefficient is applied. The results are presented in Table 2. In these sampling sites, Pu has a high correlation with Cu, Pb and Sn (0.65, 0.65, 0.60), respectively. The high correlations indicate that Pu and the heavy metals might be influenced by same factors or they have similarly in sediments. Meanwhile, As has significant negative correlation with Pu ( 0.63), indicating the arsenic in the soil might restrain the adsorbing process of Pu and vice versa. Interestingly, the weak correlation between Pu and the organic matter (OM) is different from previous studies (Xu et al., 2008; Lujaniene_ et al., 2013). As a matter of fact, Pu in the soils are affected by combined factors, such as pH values, particle sizes, ions, organic matter and others. The weak correlation between Pu and OM in SIP is the result of multiple factors, given the fact that the urban wetlands are in the complex environment. The mechanism of how environment factors influence Pu in the soil is still unclear. The contour map (Fig. 2) shows the distribution of 239þ240Pu activity in SIP. The maximal values located in the eastern and northeastern of SIP. Different concentrations of Pu might be caused by factors like wash offs by precipitations in the river networks of the urban wetlands.
The 240Pu/239Pu atom ratios in the surface soil of SIP range from 0.171 to 0.226 with a weighted average of 0.194 (Table 1). Noted that the atom ratio range is also close to the results (0.172–0.220) of Hubei Province (Dong et al., 2010) (Table 3), which means these two places are affected by similar sources of Pu. This indicates that the Pu contami nation in SIP mostly comes from global atmospheric deposition. In the mentioned study, the deposition of close-in fallout from the Lop Nor nuclear tests is excluded because of long-distance between two loca tions. Hence the influence of Chinese nuclear tests in Lop Nor could also be neglected for the same reason. According to known studies, Pu pollution emissions from Fukushima Daiichi Nuclear Power Plant acci dent have not influenced our study area so far (Huang et al., 2019; Ni et al., 2019). 3.3. Depth profiles of 239þ240Pu activity concentration, 240Pu/239Pu atom ratio and Pu inventory of the sediment core in SIP The depth profiles of 239þ240Pu activity concentrations and Pu/239Pu atom ratios of the sediment core collected from Lake Yangcheng are shown in Table 1S and their vertical profiles are pre sented in Fig. 3. In Fig. 3, error bars correspond to two standard devi ation and comprise contributions from counting statistics and reproducibility of the ICP-MS system, with the contributions added in quadrature. The activities of 239þ240Pu range from 0.012�0.012 to 1.698�0.210 mBq/g with a mean of 0.559 mBq/g. In the surface layers
240
Table 2 Pearson correlation coefficients between Pu, OM and heavy metals. OM As Cd Co Cr Cu Ga Mn Ni Pb Sb Sn Zn Pu a b
OM
As
Cd
Co
Cr
Cu
Ga
Mn
Ni
Pb
Sb
Sn
Zn
Pu
1.000 0.036 0.279 0.543b 0.307 0.413 0.269 0.171 0.522b 0.331 0.215 0.289 0.463b 0.184
1.000 0.875a 0.031 0.239 0.461b 0.091 0.479b 0.020 0.332 0.558b 0.391 0.271 0.634a
1.000 0.396 0.423 0.188 0.217 0.270 0.323 0.125 0.606a 0.085 0.569a 0.362
1.000 0.747a 0.026 0.694a 0.436 0.846a 0.086 0.486b 0.275 0.364 0.011
1.000 0.402 0.849a 0.170 0.895a 0.443 0.842a 0.126 0.241 0.253
1.000 0.075 0.330 0.068 0.816a 0.523b 0.611a 0.473b 0.651a
1.000 0.322 0.893a 0.225 0.587a 0.363 0.338 0.149
1.000 0.293 0.080 0.095 0.377 0.090 0.235
1.000 0.091 0.633a 0.354 0.388 0.101
1.000 0.524b 0.643a 0.499b 0.653a
1.000 0.172 0.232 0.433
1.000 0.587a 0.595a
1.000 0.248
1.000
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed). 4
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Table 3 239þ240 Pu activity and 240Pu/239Pu atom ratio in soils in different regions of China. Sample location North China Northwest China Central China Southwest China South China Northeast China East China
Surface 239þ240 Pu activity (mBq/g) Beijing Lanzhou Xinjiang Jiuquan Hubei Chongqing Guizhou Guangxi Dalian Liaodong Bay Lianyungang Jinan Suzhou
0.066–0.149 0.023 0.363–0.927 0.076–1.988 0.358–0.380 0.171 0.316–1.30 0.088–0.469 0.929–0.1178 0.023–0.938 0.370–0.770 0.119–0.174 0.035–0.426
Fig. 3. Vertical profiles of 239þ240Pu activities and Lake Yangcheng, Suzhou, China.
240
Pu/239Pu atom ratio
– 0.171–0.205 – 0.059–0.186 0.135–0.220 0.170–0.206 0.175–0.209 0.171–0.198 0.177–0.187 0.145–0.245 0.177–0.181 – 0.176–0.226
239þ240
Pu inventory (Bq/m2)
10.1–35.7 32.4 76.6–321.7 13–546 44.9–86.9 19 63–114 61.1 – 44.1–86.9 – 33.9–36.4 –
Reference Sha et al. (1991) Zheng et al. (2009) Zheng et al. (2010) Bu et al. (2015) Dong et al. (2010) Bu et al. (2014) Lee et al. (1996) Guan et al. (2018) Xu et al. (2017) Xu et al. (2013) Xu et al. (2017) Sha et al. (1991) This study
Hongfeng is 0.84 cm/yr, which is higher than the rate we observed in Lake Yangcheng. The 240Pu/239Pu atom ratio is an important fingerprint for Pu source identification. In this study, 240Pu/239Pu atom ratio in sediment is ranged from 0.161�0.018 to 0.200�0.026, with a mean of 0.174. The average 240Pu/239Pu atom ratios in the core sediments observed in the present study is consistent with the global fallout value of 0.180�0.014 in the northern hemisphere (Kelley et al., 1999). This result is similar to the atom ratio of 240Pu/239Pu observed in lacustrine sediments in southwestern China (Bu et al., 2014) and coastal sediments in southern China (Guan et al., 2018) (Table 4), which are close to the global fallout value. The similar values indicate that Pu contamination of these study areas might be mainly from global fallout. Moreover, 240Pu/239Pu atom ratios of the sediment core derived from Lake Yangcheng are lower than the 240Pu/239Pu atom ratio (0.238�0.007) in the Yangtze estuary and the difference might be attributed to Pu from PPG source in the Yangtze River estuary (Tims et al., 2010). The influence from Chinese nuclear tests at Lop Nor can be ruled out because the 240Pu/239Pu atom ratios in the sediment core are smaller 0.20 and in the range of atom ratios typical for the global fallout or PPG, but studies showed that the atom ratios of thermonuclear bomb tests are higher than 0.408 (Muramatsu et al., 2000; Warneke et al., 2002). To compare the differences between different regions, the activity and distribution characteristics of 239þ240Pu in soil in some regions of China are shown in Table 4. The Pu activities in the surface soil of different regions in China are quite different. In downwind area from Lop Nor (Chinese nuclear test area) in Gansu Province, high Pu activity concentrations (exceeding 1 mBq/g) were found in the top 30 cm layers of some sampling locations (Bu et al., 2015). In the same southwestern region of Chongqing, the activity of 239þ240Pu in the surface soil was only 0.171 mBq/g, which might be caused by long distance between sampling sites and Lop Nor area (Bu et al., 2014). The inventory of 239þ240Pu is also studied in this paper and the in ventory is calculated from the dry bulk density of the sediment samples, which follows the equation below:
240
Pu/239Pu atom ratios of
(0–4 cm), the activities are from 0.104�0.021 to 0.912� 0.011 mBq/g. At the top 0–22 cm, the 239þ240Pu activity shows an overall increasing trend with the depth, then it reaches the maximum value at the 22 cm layer and decreases in the deeper layers. In the 51–55 cm layers, the activity values are stable at the average activity of 0.280 mBq/g. Zheng et al. (2008b) reported a similar depth distribution of Pu in sediment core sample obtained from Lake Hongfeng, and in this study a maximum activity was found at 33–34 cm (2.782�0.108 mBq/g), which is higher than what we observed in Lake Yangcheng. In this study, the maximum activity value was not found in the top layers but in the 20–22 cm layer. The depth of the maximum activity value is similar to the sediment core taken in Guangxi Coast (Guan et al., 2018). Since 239Pu and 240Pu are chemically stable in the natural envi ronment, they can reveal the sedimentation history in the study area after Pu entering the water body from different sources. Given the fact that the main source of 239þ240Pu in the study area is global fallout and global fallout from nuclear weapon tests peaked in 1963, the position of the maximum activity in the sediment core can be marked as a discrete-time marker. So, the deposition rate of the study area is esti mated to be 0.396�0.019 cm/yr. In previous study of Lake Hongfeng, Zheng et al. (2008b) estimated that the rate of sedimentation of Lake
n X
I¼
Ai Δmi ; i ¼ 1; 2; 3…n i¼1
Where I represents the 239þ240Pu inventory in the sediment core (Bq/ m2), Ai is the 239þ240Pu or activity of the mass depth interval (Bq/kg) and Δmi is the mass depth interval (kg/m2). The calculated inventory of 239þ240Pu is 58.5 Bq/m2 for the lacus trine sediment cores (0–55 cm) from Lake Yangcheng. The inventory value is larger than the 239þ240Pu inventory of Ji’nan (33.9–36.4 Bq/ m2), which is also in eastern China (Sha et al., 1991). Meanwhile, the inventory is in the range of Yangtze River estuary total inventory (18–387 Bq/m2) and slightly larger than the global atmospheric depo sition in the latitude between 30–40� N (42 Bq/m2) (UNSCEAR, 2000). 5
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Table 4 Vertical distributions of Pu isotopes in lacustrine sediments in different areas of China. 239þ240
Area Lake Lake Lake Lake Lake Lake Lake Lake
Hongfeng Bosten Qinghai Poyang Sugan Shuangta Chenghai Yangcheng
Pu activity (mBq/g)
0.058–2.782 0.002–2.209 0–6.99 0.104–0.665 0.004–2.12 – – 0.012–1.698
240
Pu/239Pu atom ratio
239þ240
0.162–0.213 0.080–0.219 0.170–0.181 0.185–0.192 0.157–0.193 0.178 0.195 0.168–0.200
50.7 46.6–56.4 42.8–68.4 – 20.2–24.2 240.6 35.4 58.5
4. Conclusions
Pu inventory (Bq/m2)
Reference Zheng et al. (2008b) Liao et al., 2008 Wu et al. (2011) Liao et al. (2008) Wu et al. (2010) Wu et al. (2010) Zheng et al. (2008a) This study
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In this study, we analyzed 239þ240Pu activities and 240Pu/239Pu atom ratios in surface soil samples and lacustrine sediment samples in Suzhou. The results indicate that the activity concentration of 239þ240Pu from surface soil (0–4 cm) range from 0.035 to 0.426 mBq/g, with a mean of 0.156�0.08 mBq/g. The 240Pu/239Pu atom ratio in surface soil samples ranged from 0.176 to 0.226, with a weighted mean of 0.194. The mean ratio was similar to the typical global fallout value. Using the Pearson Correlation Coefficient, Pu in the study area has higher positive corre lation with Cu, Pb and Sn and negative correlation with As. No signifi cant correlation between Pu and OM in our study area was found. The calculated inventory of 239þ240Pu was 58.5 Bq/m2 and 240Pu/239Pu atom ratio in sediment in Yangcheng Lake ranges from 0.161 to 0.200, with a mean of 0.174. The result of this study agrees with the value of contri bution of the global fallout. Additionally, the deposition rate at Lake Yangcheng was estimated to 0.396�0.019 cm/yr using the Pu concen tration depth profile. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (41773004), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvrad.2019.106134. References Bu, W.T., Zheng, J., Guo, Q., Uchida, S., 2014. Vertical distribution and migration of global fallout Pu in forest soils in southwestern China. J. Environ. Radioact. 136, 174–180. https://doi.org/10.1016/j.jenvrad.2014.06.010. Bu, W.T., Zheng, J., Guo, Q., Aono, T., Tazoe, H., Tagami, K., Uchida, S., Yamada, M., 2014. A method of measurement of 239Pu, 240Pu, 241Pu in high U content marine sediments by sector field ICP-MS and its application to Fukushima sediment samples. Environ. Sci. Technol. 48, 534–541. https://doi.org/10.1021/es403500e. Bu, W.T., Ni, Y.Y., Guo, Q.J., Zheng, J., Uchida, S., 2015. Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout. Sci. Rep. 5, 12262. https://doi.org/10.1038/srep12262. Chen, X., Li, T., Zhang, X., Li, R., 2013. A Holocene Yalu River-derived fine-grained deposit in the southeast coastal area of the Liaodong Peninsula. Chin. J. Oceanol. Limnol. 31 (3), 636–647. https://doi.org/10.1007/s00343-013-2087-1. Dong, W., Zheng, J., Guo, Q.J., Yamada, M., Pan, S.M., 2010. Characterization of plutonium in deep-sea sediments of the Sulu and South China seas. J. Environ. Radioact. 101, 622–629. https://doi.org/10.1016/j.jenvrad.2010.03.011. Guan, Y.J., Sun, S.Y., Sun, S.H., 2018. Distribution and sources of plutonium along the coast of Guangxi, China. Nucl. Instrum. Methods Phys. Res. B 437, 61–65. https:// doi.org/10.1016/j.nimb.2018.09.047.
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Wu, J.W., Zheng, J., Dai, M.H., Huh, C.A., Chen, W.F., Tagami, K., 2014. Isotopic composition and distribution of plutonium in northern South China Sea sediments revealed continuous release and transport of Pu from the Marshall Islands. Environ. Sci. Technol. 48, 3136–3144. Xu, C., Santschi, P.H., Zhong, J.Y., Hatcher, P.G., Francis, A.J., Dodge, C.J., Dodge, K.A., Roberts, C., Hung, C., Honeyman, B.D., 2008. Colloidal cutin-like substances crosslinked to siderophore decomposition products mobilizing plutonium from contaminated soils. Environ. Sci. Technol. 42 (22), 8211–8217. https://doi.org/ 10.1021/es801348t. Xu, Y., Qiao, J., Hou, X., Pan, S., 2013. Plutonium in soils from Northeast China and its potential application for evaluation of soil erosion. Sci. Rep. 3, 3506. https://doi. org/10.1038/srep03506. Xu, Y., Pan, S., Wu, M., Zhang, K., Hao, Y., 2017. Association of Plutonium isotopes with natural soil particles of different size and comparison with 137Cs. Sci. Total Environ. 581, 541–549. https://doi.org/10.1016/j.scitotenv.2016.12.162. Xu, J.W., Zhuang, Q.F., Fu, Y., Huang, Y.Y., Sun, Z.Y., Liu, Z.Y., 2019. Spatial distribution, pollution levels, and source identification of heavy metals in wetlands of Suzhou Industrial Park, China. Wetl. Ecol. Manag. 27 (5–6), 743–758. https://doi. org/10.1007/s11273-019-09691-2. Zhang, W.C., Hou, X.L., 2019. Level, distribution and sources of plutonium in the coastal areas of China. Chemosphere 230, 587–595. https://doi.org/10.1016/j. chemosphere.2019.05.094.
Zhang, K.X., Pan, S.M., Liu, Z.Y., Li, G.S., Xu, Y.H., Hong, Y.P., 2018. Vertical distributions and source identification of the radionuclides 239Pu and 240Pu in the sediments of the Liao River estuary, China. J. Environ. Radioact. 181, 78–84. https://doi.org/10.1016/j.jenvrad.2017.10.016. Zheng, J., Yamada, M., 2006. Inductively coupled plasma-sector field mass spectrometry with a high-efficiency sample introduction system for the determination of Pu isotopes in settling particles at femtogram levels. Talanta 69, 1246–1253. https:// doi.org/10.1016/j.talanta.2005.12.047. Zheng, J., Liao, H.Q., Wu, F.C., Yamada, M., Fu, P.Q., Liu, C.Q., Wan, G.J., 2008. Vertical distributions of 239þ240Pu atom ratio in sediment core of Lake Chenghai, SW China. J. Radioanal. Nucl. Chem. 275, 37–42. Zheng, J., Wu, F.C., Yamada, M., Liao, H.Q., Liu, C.Q., Wan, G.J., 2008. Global fallout Pu recorded in lacustrine sediments in Lake Hongfeng, SW China. Environ. Pollut. 152, 314–321. https://doi.org/10.1016/j.envpol.2007.06.027. Zheng, J., Yamada, M., Wu, F., Liao, H., 2009. Characterization of Pu concentration and its isotopic composition in soils of Gansu in northwestern China. J. Environ. Radioact. 100, 71–75. https://doi.org/10.1016/j.jenvrad.2008.10.017. Zheng, J., Wei, J.G., Zhou, W.L., Xu, H., 2010. Pu contamination in soils around the Xinjiang nuclear test site. Chinese J. Radiolog. Med. Protext. 8, 50–52.
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Characteristic of Pu from urban wetland and lacustrine sediments in Suzhou Industrial Park, China Yongjin Guan a, Jingyu Mai a, b, Jiawei Xu b, Zhiyong Liu b, * a
Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Centre of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, 215123, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Suzhou Industrial Park China Wetland Plutonium source Distribution Deposition
In this study, plutonium activity concentrations in the urban wetlands and lacustrine sediment of Suzhou In dustrial Park (SIP) are studied for the first time. Results show 239þ240Pu activity concentrations in the wetland surface soils of SIP range from 0.035 to 0.426 mBq/g and the 240Pu/239Pu atom ratio ranges from 0.171�0.024 to 0.226�0.049. Judging from the atom ratio of 240Pu/239Pu, the main source of Pu in the wetland is global fallout. The correlations of Pu between organic matter and heavy metals are also studied. The correlation coefficients show Pu has significant positive correlations with Cu, Sn and Pb but negative correlation with As. Unlike dis tributions of Pu in other places, Pu in SIP has weak correlation with organic matter content. A sediment core from Lake Yangcheng is also analyzed to investigate the historical record of Pu deposition. The atom ratios of each layer in the sediment core indicate the area is mainly influenced by global fallout. Using Pu as a discrete-time maker, the deposition rate in Lake Yangcheng is 0.396�0.019 cm/yr. The calculated inventory of 239þ240Pu is 58.5 Bq/m2, which is in the range of inventories of the corresponding latitudes according to UNSCEAR.
1. Introduction Pu is an artificial radionuclide associated with nuclear industry including nuclear weapon and energy industry. Common anthropogenic Pu are in the form of 239Pu (T1/2 ¼ 2.411 � 104 yr), 240Pu (T1/2 ¼ 6563 yr), 241Pu (T1/2 ¼ 14.4 yr), 242Pu (T1/2 ¼ 3.73 � 105 yr) and 244Pu (T1/2 ¼ 8.08 � 107 yr). Anthropogenic Pu in the environment mostly comes from the series of nuclear weapon tests from the mid-1940s to the 1980s. The atmospheric nuclear weapon tests conducted by countries such as the former Soviet Union, the United States and China have emitted about 1.1 � 1016Bq of 239þ240Pu into the environment, and Pu was spread globally due to atmospheric dispersion (Harley, 1980). The 240Pu/239Pu atom ratio differs for different sources such as nu clear test, military weapons or industrial sources. Therefore, the atom ratio is often used to identify the sources of radioactive pollutants in the environment (Krey et al., 1976; Warneke et al., 2002). Specifically, 240 Pu/239Pu atom ratio of weapon-grade Pu ranges from 0.01–0.07 (Lindahl et al., 2017), while nuclear reactor-grade Pu exhibits higher atom ratios, ranging from 0.24–0.8 (Warneke et al., 2002; Ketterer and Szechenyi, 2008). Atom ratio values of the Pacific proving ground (PPG) Pu are usually 0.306–0.36, which are higher than the average value of
240
Pu/239Pu atom ratios 0.178�0.019 (0–30� N) and 0.180�0.014 (30–71� N) from the global fallout (Krey et al., 1976; Kelley et al., 1999). As 239Pu and 240Pu have a high radiotoxicity and long-term persis tence in the environment, risk assessment for Pu in the environment is indispensable. More and more researchers are trying to determine the characteristics of distribution, sources and fate of Pu in different layers of the earth, including atmosphere, water and sediments (Hirose and Povinec, 2015; Zhang et al., 2018). Regional studies on 239þ240Pu activity concentration in the surface soils and its vertical distribution covers many part of China. However, 239þ240 Pu activities and inventories of the research areas are quite distinct from each other. The 239þ240Pu activities in the surface soil range from 0.023�0.003 mBq/g (Gansu, Lanzhou, northwestern China) to 1.30�0.05 mBq/g (Wuling, Chongqing, southwestern China) and the distinct 239þ240Pu activities in China might be caused by the differences of soil particle sizes, organic matter, regional climatic factors and other factors (Zheng et al., 2009; Bu et al., 2014). In the vertical distribution of 239þ240 Pu, most Pu depositions (more than 80%) concentrate on the top 10 cm layer of soils (Dong et al., 2010; Bu et al., 2014). However, some soil core analyses show opposite results and the most Pu inventories are in the deeper layers (Zheng et al., 2009; Zhang and Hou, 2019) largely
* Corresponding author. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.jenvrad.2019.106134 Received 24 August 2019; Received in revised form 2 December 2019; Accepted 9 December 2019 Available online 16 December 2019 0265-931X/© 2019 Elsevier Ltd. All rights reserved.
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due to soil erosion and downward percolation of fine particles. Studies also show that the inventories of 239þ240Pu in the estuary sediments are much larger than in soils, which might be caused by scavenging of Pu from the seawater and the higher sedimental deposition rates in the estuaries areas (Hao et al., 2018; Liu et al., 2011; Zhang et al., 2018). The 240Pu/239Pu atom ratios have also been investigated in different regions and the results show that China has been affected by different sources of Pu and the global fallout is the main source of Pu in mainland (Huang et al., 2019). Studies have shown that the mainland of China, such as the northwest has been affected not only by the global fallout but
also the Lop Nor nuclear tests conducted in the 1980s (Bu et al., 2015). Also, some results indicate that China coast including the marginal seas, such as the East China Sea, the Yellow Sea and the South China Sea, has been influenced by both the global fallout and the PPG (Pacific Proving Ground) close-in fallout (Dong et al., 2010; Liu et al., 2011, 2013; Wu et al., 2014; Zhang et al., 2018). It has been demonstrated that Pu derived from the PPG can be transported over long distances to the western Pacific Ocean and to its adjacent marginal seas (Liu et al., 2011; Wu et al., 2014). However, information on the distribution and char acteristics of Pu deposition in the wetlands and lakes in the eastern
Fig. 1. The location of the sampling sites in the areas of SIP, Suzhou, China. 2
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China are very limited. Since China operates nuclear power plants (NPPs) along the east coast, investigating the historical deposition and migration mechanisms of Pu in urban wetlands and lakes is vitally important to understand and evaluate possible future pollution with radionuclides. In this study, the spatial distributions of 239þ240Pu activity of the urban wetlands in Suzhou Industrial Park (SIP) were measured for the first time and the sources of Pu in this area were investigated to see whether other sources such as Chinese nuclear tests or the Fukushima NPP accident have affected this area. In addition, a sediment core taken from Lake Yangcheng was also analyzed to investigate the historical information of Pu deposited in the lake. 2. Samples and methods
washed with 50 ml 10 M HCl to remove Th. Pu is eluted with 50 ml 0.1 M NH4I - 8.5 M HCl. 1 ml concentrated HCl is added and the eluted solution is evaporated to dryness. After evaporation, 3 ml HCl–H2O2 is added into the residue and the solution is heated by water bath at 40 � C for 1 h. In the second Pu fraction stage, the solution is washed on the column (AG MP-1M). Then the column continues to be washed with 20 ml 8 M HNO3 and 10 ml 10 M HCl and Pu in the solution is eluted with 16 ml HBr. After that, 1 ml concentrated HNO3 is added into the solution and evaporated to dryness. The final step is measurement. In this step, the eluted solutions need to be prepared for measurement. 1 ml 4% HNO3 is added to dissolve the final eluted solution and the samples are prepared for analyzing. The final measurements were conducted with a sector-field ICP-MS (Fin nigan Element 2, Bremen, Germany).
2.1. Sample collection and preparation
2.3. Instrument preparation and analytical procedure
The locations of the sampling sites are shown in Fig. 1. As seen in Fig. 1, the sample sites are distributed in Suzhou Industrial Park (SIP), Suzhou, eastern China. Suzhou (119� 550 –121� 200 E, 30� 470 –32� 020 N) is located in the middle of Yangtze River Delta and adjacent to Yangtze River and Lake Taihu. The city is under the impact of subtropical monsoon marine climate, having ~1100 mm annual precipitation. Surface soil samples were collected in the wetlands of SIP in June 2012 (Fig. 1). The urban wetlands are wetlands under shallow water and less affected by hydrodynamic factors and samples are collected in the upper soil layers from 0–4 cm depth (Site No. 1, 2, 4, 7, 11, 12, 14, 16, 17, 18, 19, 20, 23, 25, 26, 27, 28, 29, 30). At each site, three replicates were taken, and the samples were sealed separately within clean poly ethylene bags, carried by a box corer, preserved and transported in a cooler at 4 � C. Samples of each site were mixed uniformly to assure the quality of the samples. The sediment core was collected in Lake Yangcheng close to SIP in July 2016. Site No.32 is a sediment core sample with a depth of 55 cm, collected in 10 cm diameter Plexiglas tubes. It was also preserved at a temperature of 4 � C and transported to the Key Laboratory of Radio logical and Interdisciplinary Science of Soochow University for further analysis. The sediment core was sliced into 36 sections. 0–4 cm was taken as the surface layer and the rest was divided by every 1 or 2 cm in the following of the first layer to a depth of 55 cm.
The sector-field ICP-MS in a low-resolution mode (m/Δm ¼ 300) is used for the analysis to utilize the maximal instrument sensitivity. An APEX-Q high-efficiency sample introduction system (Elemental Scien tific Inc., Omaha, NE, USA) is used to introduce the samples by a membrane desolvation unit (ACM) and a conical concentric nebulizer. By the sample introduction system, the chance of the formation of uranium hydride species in SF-ICP-MS is lowered (Zheng and Yamada, 2006). In addition, the normal skimmer cone is replaced by a high-efficiency cone (X-cone, Thermo Finnigan) to further increase the sensitivity of SF-ICP-MS. All the measurements are made in the self-aspirating mode to reduce the risk of contamination by the peri staltic pump tubing. The isotopes of interest (238U, 239Pu, 240Pu, 242Pu) are analyzed in the peak hopping mode and the peak tops of the masses are measured at 10% of their respective peak width. A standard solution of 0.1 ng ml 1 U is used daily to optimize condition of the SF-ICP-MS. Instrument interference is corrected according to the standard with a known 240Pu/239Pu atom ratio of 0.242 in Pu standard solution (NBS-947). In addition, the standard reference materials IAEA-368 and NIST-4357 are also separated and analyzed together for comparison and verification to ensure the accuracy of chemical separation experiments and measurement experiments. The sensitivity for this system is about 107 cps∙ppb 1, the detection limit of 239Pu is 0.0005 mBq/g and of 240Pu is at 0.002 mBq/g. The chemical yield in the employed sample prepa ration procedure is estimated to be in the range of 54%–77% with a mean of 62%. For the heavy metal measurement, the pre-treated samples (0.1 g) were digested by HCl–HNO3–HF–HClO4 acid mixture at the 200 � C to the near-dry status. The residues were diluted by double deionized water, filtered by membranes (0.45 μm) and measured by SF-ICP-MS in Soochow University.
2.2. Experiment standards, reagents and samples The soil samples and sediment sample went through three main steps for analysis. The first step is pretreatment, which includes drying, grinding, and sieving. The stones and fragments like biological matters were removed from the samples. Then the samples are dried at 100 � C for 24 h and passed through 20-mesh sieves to remove rhizome and coarse stones and then transferred to crucible and dried at 500 � C for over 4 h to podzolize the samples. The second step is chemical separation and extraction. The chemical separation of Pu isotopes is modified from Zheng and Yamada (2006). 2.5 g pre-treated samples were weighed out and 1.14 pg of 242Pu was added as a yield tracer. To leach the samples, 50 ml of 8 M HNO3 is added into the samples and heated at 220 � C for at least 4 h. After leaching, the solution is filtered and evaporated to wet paste condition. Concentrated HNO3 is added into the paste and the solution is adjusted to 8 M HNO3 by adding de-ionized water. Then 0.276 g NaNO2 is added into the solution and it is heated in water bath at 40 � C for 30 min. During this procedure, Pu is adjusted to Pu4þ. Separation and purifica tion of Pu isotopes are conducted with AG 1-X8 anion-exchange column (100–200 mesh) and AG MP-1M anion-exchange column (100–200 mesh). In the first Pu fraction stage, the sample solution is loaded on the column (AG 1-X8) and washed with 60 ml 8 M HNO3 to remove U, Am and other cationic matrices such as Pb, Hg and Tl. Then the column is
3. Results and discussions 3.1. 239þ240Pu activity concentration and its distribution in surface soils in Suzhou The 239þ240Pu activities and 240Pu/239Pu atom ratios in surface soil samples collected in SIP are presented in Table 1. The results show the activity concentration of 239þ240Pu in the surface soil ranges from 0.035 to 0.426 mBq/g with the average value of 0.156�0.08 mBq/g (arith metic average) and 0.126�0.013 mBq/kg (geometric average). The activity concentration values are close to the values (0.088�0.031–0.469�0.057 mBq/g) in Guangxi that Guan et al. (2018) reported. The maximum value of the activity is 0.426�0.018 mBq/g at Site No.7, which is higher than the maximum value of 0.380�0.016 mBq/g in the surface soil (0–2 cm) of Hubei Province at similar latitude (Dong et al., 2010). The higher 239þ240Pu activity concentrations can be attributed to the different soil particle diameters (Xu et al., 2017; Chen et al., 2013). In this study, the soil samples are mainly fine particles 3
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Table 1 239þ240 Pu activity and 240Pu/239Pu atom ratio in surface soil samples of SIP. Sample number
239þ240
1 2 4 6 7 11 12 14 16 17 18 19 20 23 25 26 27 28 29 30
0.183 0.269 0.160 0.196 0.426 0.229 0.328 0.193 0.138 0.175 0.120 0.065 0.073 0.035 0.037 0.051 0.067 0.117 0.178 0.084
Pu activity (mBq/g)
� 0.014 � 0.011 � 0.011 � 0.015 � 0.018 � 0.057 � 0.081 � 0.014 � 0.025 � 0.034 � 0.008 � 0.008 � 0.005 � 0.009 � 0.008 � 0.003 � 0.014 � 0.007 � 0.015 � 0.004
240
Pu/239Pu atom ratio
0.211 � 0.226 � 0.223 � 0.219 � 0.196 � 0.223 � 0.222 � 0.218 � 0.198 � 0.191 � 0.176 � 0.178 � 0.186 � 0.176 � 0.182 � 0.181 � 0.187 � 0.183 � 0.173 � 0.171 �
0.053 0.049 0.058 0.044 0.055 0.046 0.047 0.042 0.043 0.046 0.025 0.026 0.023 0.013 0.017 0.029 0.021 0.032 0.026 0.024
Fig. 2. Distribution of 240
239þ240
Pu activity concentration in surface soil in SIP.
Pu/239Pu atom ratio and its distribution in SIP
*All given uncertainties are � two standard deviation errors.
3.2.
(<150 μm), which Pu can easily attach to. There are numerous manufacturing industries in the study area, such as precision machines and electronics and telecommunication. As re ported, the wetlands in SIP have been polluted by heavy metals to different degrees (Xu et al., 2019). To examine the relationship between Pu and heavy metals, Pearson correlation coefficient is applied. The results are presented in Table 2. In these sampling sites, Pu has a high correlation with Cu, Pb and Sn (0.65, 0.65, 0.60), respectively. The high correlations indicate that Pu and the heavy metals might be influenced by same factors or they have similarly in sediments. Meanwhile, As has significant negative correlation with Pu ( 0.63), indicating the arsenic in the soil might restrain the adsorbing process of Pu and vice versa. Interestingly, the weak correlation between Pu and the organic matter (OM) is different from previous studies (Xu et al., 2008; Lujaniene_ et al., 2013). As a matter of fact, Pu in the soils are affected by combined factors, such as pH values, particle sizes, ions, organic matter and others. The weak correlation between Pu and OM in SIP is the result of multiple factors, given the fact that the urban wetlands are in the complex environment. The mechanism of how environment factors influence Pu in the soil is still unclear. The contour map (Fig. 2) shows the distribution of 239þ240Pu activity in SIP. The maximal values located in the eastern and northeastern of SIP. Different concentrations of Pu might be caused by factors like wash offs by precipitations in the river networks of the urban wetlands.
The 240Pu/239Pu atom ratios in the surface soil of SIP range from 0.171 to 0.226 with a weighted average of 0.194 (Table 1). Noted that the atom ratio range is also close to the results (0.172–0.220) of Hubei Province (Dong et al., 2010) (Table 3), which means these two places are affected by similar sources of Pu. This indicates that the Pu contami nation in SIP mostly comes from global atmospheric deposition. In the mentioned study, the deposition of close-in fallout from the Lop Nor nuclear tests is excluded because of long-distance between two loca tions. Hence the influence of Chinese nuclear tests in Lop Nor could also be neglected for the same reason. According to known studies, Pu pollution emissions from Fukushima Daiichi Nuclear Power Plant acci dent have not influenced our study area so far (Huang et al., 2019; Ni et al., 2019). 3.3. Depth profiles of 239þ240Pu activity concentration, 240Pu/239Pu atom ratio and Pu inventory of the sediment core in SIP The depth profiles of 239þ240Pu activity concentrations and Pu/239Pu atom ratios of the sediment core collected from Lake Yangcheng are shown in Table 1S and their vertical profiles are pre sented in Fig. 3. In Fig. 3, error bars correspond to two standard devi ation and comprise contributions from counting statistics and reproducibility of the ICP-MS system, with the contributions added in quadrature. The activities of 239þ240Pu range from 0.012�0.012 to 1.698�0.210 mBq/g with a mean of 0.559 mBq/g. In the surface layers
240
Table 2 Pearson correlation coefficients between Pu, OM and heavy metals. OM As Cd Co Cr Cu Ga Mn Ni Pb Sb Sn Zn Pu a b
OM
As
Cd
Co
Cr
Cu
Ga
Mn
Ni
Pb
Sb
Sn
Zn
Pu
1.000 0.036 0.279 0.543b 0.307 0.413 0.269 0.171 0.522b 0.331 0.215 0.289 0.463b 0.184
1.000 0.875a 0.031 0.239 0.461b 0.091 0.479b 0.020 0.332 0.558b 0.391 0.271 0.634a
1.000 0.396 0.423 0.188 0.217 0.270 0.323 0.125 0.606a 0.085 0.569a 0.362
1.000 0.747a 0.026 0.694a 0.436 0.846a 0.086 0.486b 0.275 0.364 0.011
1.000 0.402 0.849a 0.170 0.895a 0.443 0.842a 0.126 0.241 0.253
1.000 0.075 0.330 0.068 0.816a 0.523b 0.611a 0.473b 0.651a
1.000 0.322 0.893a 0.225 0.587a 0.363 0.338 0.149
1.000 0.293 0.080 0.095 0.377 0.090 0.235
1.000 0.091 0.633a 0.354 0.388 0.101
1.000 0.524b 0.643a 0.499b 0.653a
1.000 0.172 0.232 0.433
1.000 0.587a 0.595a
1.000 0.248
1.000
Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed). 4
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Journal of Environmental Radioactivity 213 (2020) 106134
Table 3 239þ240 Pu activity and 240Pu/239Pu atom ratio in soils in different regions of China. Sample location North China Northwest China Central China Southwest China South China Northeast China East China
Surface 239þ240 Pu activity (mBq/g) Beijing Lanzhou Xinjiang Jiuquan Hubei Chongqing Guizhou Guangxi Dalian Liaodong Bay Lianyungang Jinan Suzhou
0.066–0.149 0.023 0.363–0.927 0.076–1.988 0.358–0.380 0.171 0.316–1.30 0.088–0.469 0.929–0.1178 0.023–0.938 0.370–0.770 0.119–0.174 0.035–0.426
Fig. 3. Vertical profiles of 239þ240Pu activities and Lake Yangcheng, Suzhou, China.
240
Pu/239Pu atom ratio
– 0.171–0.205 – 0.059–0.186 0.135–0.220 0.170–0.206 0.175–0.209 0.171–0.198 0.177–0.187 0.145–0.245 0.177–0.181 – 0.176–0.226
239þ240
Pu inventory (Bq/m2)
10.1–35.7 32.4 76.6–321.7 13–546 44.9–86.9 19 63–114 61.1 – 44.1–86.9 – 33.9–36.4 –
Reference Sha et al. (1991) Zheng et al. (2009) Zheng et al. (2010) Bu et al. (2015) Dong et al. (2010) Bu et al. (2014) Lee et al. (1996) Guan et al. (2018) Xu et al. (2017) Xu et al. (2013) Xu et al. (2017) Sha et al. (1991) This study
Hongfeng is 0.84 cm/yr, which is higher than the rate we observed in Lake Yangcheng. The 240Pu/239Pu atom ratio is an important fingerprint for Pu source identification. In this study, 240Pu/239Pu atom ratio in sediment is ranged from 0.161�0.018 to 0.200�0.026, with a mean of 0.174. The average 240Pu/239Pu atom ratios in the core sediments observed in the present study is consistent with the global fallout value of 0.180�0.014 in the northern hemisphere (Kelley et al., 1999). This result is similar to the atom ratio of 240Pu/239Pu observed in lacustrine sediments in southwestern China (Bu et al., 2014) and coastal sediments in southern China (Guan et al., 2018) (Table 4), which are close to the global fallout value. The similar values indicate that Pu contamination of these study areas might be mainly from global fallout. Moreover, 240Pu/239Pu atom ratios of the sediment core derived from Lake Yangcheng are lower than the 240Pu/239Pu atom ratio (0.238�0.007) in the Yangtze estuary and the difference might be attributed to Pu from PPG source in the Yangtze River estuary (Tims et al., 2010). The influence from Chinese nuclear tests at Lop Nor can be ruled out because the 240Pu/239Pu atom ratios in the sediment core are smaller 0.20 and in the range of atom ratios typical for the global fallout or PPG, but studies showed that the atom ratios of thermonuclear bomb tests are higher than 0.408 (Muramatsu et al., 2000; Warneke et al., 2002). To compare the differences between different regions, the activity and distribution characteristics of 239þ240Pu in soil in some regions of China are shown in Table 4. The Pu activities in the surface soil of different regions in China are quite different. In downwind area from Lop Nor (Chinese nuclear test area) in Gansu Province, high Pu activity concentrations (exceeding 1 mBq/g) were found in the top 30 cm layers of some sampling locations (Bu et al., 2015). In the same southwestern region of Chongqing, the activity of 239þ240Pu in the surface soil was only 0.171 mBq/g, which might be caused by long distance between sampling sites and Lop Nor area (Bu et al., 2014). The inventory of 239þ240Pu is also studied in this paper and the in ventory is calculated from the dry bulk density of the sediment samples, which follows the equation below:
240
Pu/239Pu atom ratios of
(0–4 cm), the activities are from 0.104�0.021 to 0.912� 0.011 mBq/g. At the top 0–22 cm, the 239þ240Pu activity shows an overall increasing trend with the depth, then it reaches the maximum value at the 22 cm layer and decreases in the deeper layers. In the 51–55 cm layers, the activity values are stable at the average activity of 0.280 mBq/g. Zheng et al. (2008b) reported a similar depth distribution of Pu in sediment core sample obtained from Lake Hongfeng, and in this study a maximum activity was found at 33–34 cm (2.782�0.108 mBq/g), which is higher than what we observed in Lake Yangcheng. In this study, the maximum activity value was not found in the top layers but in the 20–22 cm layer. The depth of the maximum activity value is similar to the sediment core taken in Guangxi Coast (Guan et al., 2018). Since 239Pu and 240Pu are chemically stable in the natural envi ronment, they can reveal the sedimentation history in the study area after Pu entering the water body from different sources. Given the fact that the main source of 239þ240Pu in the study area is global fallout and global fallout from nuclear weapon tests peaked in 1963, the position of the maximum activity in the sediment core can be marked as a discrete-time marker. So, the deposition rate of the study area is esti mated to be 0.396�0.019 cm/yr. In previous study of Lake Hongfeng, Zheng et al. (2008b) estimated that the rate of sedimentation of Lake
n X
I¼
Ai Δmi ; i ¼ 1; 2; 3…n i¼1
Where I represents the 239þ240Pu inventory in the sediment core (Bq/ m2), Ai is the 239þ240Pu or activity of the mass depth interval (Bq/kg) and Δmi is the mass depth interval (kg/m2). The calculated inventory of 239þ240Pu is 58.5 Bq/m2 for the lacus trine sediment cores (0–55 cm) from Lake Yangcheng. The inventory value is larger than the 239þ240Pu inventory of Ji’nan (33.9–36.4 Bq/ m2), which is also in eastern China (Sha et al., 1991). Meanwhile, the inventory is in the range of Yangtze River estuary total inventory (18–387 Bq/m2) and slightly larger than the global atmospheric depo sition in the latitude between 30–40� N (42 Bq/m2) (UNSCEAR, 2000). 5
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Journal of Environmental Radioactivity 213 (2020) 106134
Table 4 Vertical distributions of Pu isotopes in lacustrine sediments in different areas of China. 239þ240
Area Lake Lake Lake Lake Lake Lake Lake Lake
Hongfeng Bosten Qinghai Poyang Sugan Shuangta Chenghai Yangcheng
Pu activity (mBq/g)
0.058–2.782 0.002–2.209 0–6.99 0.104–0.665 0.004–2.12 – – 0.012–1.698
240
Pu/239Pu atom ratio
239þ240
0.162–0.213 0.080–0.219 0.170–0.181 0.185–0.192 0.157–0.193 0.178 0.195 0.168–0.200
50.7 46.6–56.4 42.8–68.4 – 20.2–24.2 240.6 35.4 58.5
4. Conclusions
Pu inventory (Bq/m2)
Reference Zheng et al. (2008b) Liao et al., 2008 Wu et al. (2011) Liao et al. (2008) Wu et al. (2010) Wu et al. (2010) Zheng et al. (2008a) This study
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In this study, we analyzed 239þ240Pu activities and 240Pu/239Pu atom ratios in surface soil samples and lacustrine sediment samples in Suzhou. The results indicate that the activity concentration of 239þ240Pu from surface soil (0–4 cm) range from 0.035 to 0.426 mBq/g, with a mean of 0.156�0.08 mBq/g. The 240Pu/239Pu atom ratio in surface soil samples ranged from 0.176 to 0.226, with a weighted mean of 0.194. The mean ratio was similar to the typical global fallout value. Using the Pearson Correlation Coefficient, Pu in the study area has higher positive corre lation with Cu, Pb and Sn and negative correlation with As. No signifi cant correlation between Pu and OM in our study area was found. The calculated inventory of 239þ240Pu was 58.5 Bq/m2 and 240Pu/239Pu atom ratio in sediment in Yangcheng Lake ranges from 0.161 to 0.200, with a mean of 0.174. The result of this study agrees with the value of contri bution of the global fallout. Additionally, the deposition rate at Lake Yangcheng was estimated to 0.396�0.019 cm/yr using the Pu concen tration depth profile. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (41773004), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvrad.2019.106134. References Bu, W.T., Zheng, J., Guo, Q., Uchida, S., 2014. Vertical distribution and migration of global fallout Pu in forest soils in southwestern China. J. Environ. Radioact. 136, 174–180. https://doi.org/10.1016/j.jenvrad.2014.06.010. Bu, W.T., Zheng, J., Guo, Q., Aono, T., Tazoe, H., Tagami, K., Uchida, S., Yamada, M., 2014. A method of measurement of 239Pu, 240Pu, 241Pu in high U content marine sediments by sector field ICP-MS and its application to Fukushima sediment samples. Environ. Sci. Technol. 48, 534–541. https://doi.org/10.1021/es403500e. Bu, W.T., Ni, Y.Y., Guo, Q.J., Zheng, J., Uchida, S., 2015. Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout. Sci. Rep. 5, 12262. https://doi.org/10.1038/srep12262. Chen, X., Li, T., Zhang, X., Li, R., 2013. A Holocene Yalu River-derived fine-grained deposit in the southeast coastal area of the Liaodong Peninsula. Chin. J. Oceanol. Limnol. 31 (3), 636–647. https://doi.org/10.1007/s00343-013-2087-1. Dong, W., Zheng, J., Guo, Q.J., Yamada, M., Pan, S.M., 2010. Characterization of plutonium in deep-sea sediments of the Sulu and South China seas. J. Environ. Radioact. 101, 622–629. https://doi.org/10.1016/j.jenvrad.2010.03.011. Guan, Y.J., Sun, S.Y., Sun, S.H., 2018. Distribution and sources of plutonium along the coast of Guangxi, China. Nucl. Instrum. Methods Phys. Res. B 437, 61–65. https:// doi.org/10.1016/j.nimb.2018.09.047.
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