Heavy Metals Pollution and Pb Isotopic Signatures in Surface Sediments Collected from Bohai Bay, North China

To investigate the characteristics and potential sources of heavy metals pollution, surface sediments collected from Bohai Bay, North China, were analyzed for the selected metals (Cd, Cr, Cu, Ni, Pb, and Zn). The Geoaccumulation Index was used to assess the level of heavy metal pollution. Pb isotopic compositions in sediments were also measured to effectively identify the potential Pb sources. The results showed that the average concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were 0.15, 79.73, 28.70, 36.56, 25.63, and 72.83 mg/kg, respectively. The mean concentrations of the studied metals were slightly higher than the background values. However, the heavy metals concentrations in surface sediments in Bohai Bay were below the other important bays or estuaries in China. The assessment by Geoaccumulation Index indicated that Cr, Zn, and Cd were classified as “the unpolluted” level, while Ni, Cu, and Pb were ranked as “unpolluted to moderately polluted” level. The order of pollution level of heavy metals was: Pb > Ni > Cu > Cr > Zn > Cd. The Pb isotopic ratios in surface sediments varied from 1.159 to 1.185 for 206Pb/207Pb and from 2.456 to 2.482 for 208Pb/207Pb. Compared with Pb isotopic radios in other sources, Pb contaminations in the surface sediments of Bohai Bay may be controlled by the mix process of coal combustion, aerosol particles deposition, and natural sources.


Introduction
Bohai Bay is a semienclosed bay located in the western region of the Bohai Sea and is one of the four major compositions in Bohai Sea, North China. It covers an area of 1.60 × 104 km 2 with a population of about 70 million. The average depth is 12.5 m with the maximum of 32 m. Bohai Bay is important harbor and marine lines of economic development in North China. With the rapid development of urban economy and industry around sea, Bohai Bay receives both industrial and domestic wastewater discharges from Beijing, Tianjin, and Hebei province. All the wastewater through rivers and channels drains into the near-shore waters of Bohai Bay directly. This process produced a certain degree of heavy metal pollution for ocean environment [1]. In fact, sediments can receive and absorb metal pollutants from natural weathering, erosion, and anthropogenic activities. Heavy metal concentrations in ocean sediments are important indicators which can reflect the heavy metal pollution of ocean environment [2]. Moreover, heavy metals may be recycled via chemical and biological processes within these sedimentary compartment and back to the water column [3]. The accumulation of metal contaminants in sediments can pose serious environmental problems to the ocean environment. Therefore, identifying the sources of heavy metals is of key importance for making decisions concerning site remediation. Up till now, most researchers have identified the metal sources in sediments by using variations of metal concentrations and enrichment factors relative to natural inputs. However, recent studies have proved that the Pb stable isotopic signatures were a useful tool to effectively identify various sources of Pb pollution in water environment [4][5][6].
In the recent years, the researches of heavy metals in Bohai Bay had mainly focused on the estuarine and the intertidal zone [7][8][9]. The scale of whole Bay consisting of the estuarine, the intertidal zone, and centre of the Bay was  not fully researched for the characteristics of heavy metals pollution. The objectives of this present study were (1) to determine concentrations and distribution characteristics of heavy metals in surface sediments collected from the Bohai Bay, (2) to evaluate the degree of heavy metal contamination, and (3) to identify the metal pollution sources using Pb isotopic signature. We hope our research on heavy metals contamination and Pb isotopic signatures will be useful to assess the environmental impact of urbanization and economic development on ocean water environment, while providing scientific information for environmental management and restoration in this region.

Sampling Sites.
Eighteen surface sediments (0-10 cm) were collected from Bohai Bay, North China. A map of the sampling sites is shown in Figure 1. After sampling, the sediment samples were taken back to indoor laboratory and dried at a temperature below −40 ∘ C, crushed, and sieved to less than 200 m before the chemical measurements were taken as described below.

Analytical Methods.
All chemical treatments were in the ultraclean laboratory, and all reagents were of high purity grade. A strong acid digestion method (HNO 3 + H 2 O 2 + HF) was used to dissolve heavy metals in solution [10]. The digested solutions were measured using inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer Elan DRC-e) for the concentrations of Cd, Cr, Cu, Ni, Pb, and Zn. The accuracy of the analytical procedures employed for the analysis of the metals in sediments was checked using the certified reference material of China stream sediment (GSD-12, GBW07312), obtaining good agreement with certified values.

Heavy Metal Concentrations in Surface Sediments.
The heavy metal concentrations and statistics results of all investigated sediments in Bohai Bay are summarized in Table 1. For the comparison purpose, the background values of heavy metals and heavy metals concentrations in sediments from the other Bays and Estuaries were also shown. As can be seen, the concentration ranges of heavy metals of Cr, Ni, Cu, Zn, Cd, and Pb in Bohai Bay sediments were 51.32∼ were below those in the other estuaries in China, such as Yangtze Estuary and Pearl River Estuary. In addition, the spatial distribution of heavy metals in sediments collected from Bohai Bay is shown in Figure 2 (Cd concentration is 100 times more than the actual concentration). From Figure 2, it can be seen that heavy metal concentrations in sediments close to center of Bay (number 10) were the highest among all sediments in the whole Bay ( Figure 1). Marine sediment quality (GB 18668-2002), established by China State Bureau of Quality and Technical Supervision (CSBTS, 2002), contains three standard criteria for marine sediments. The marine sediment quality I is applied to protect the habitats for marine life including natural, rare, and endangered species as well as the areas for human recreation and sports, while the marine sediment quality II is applied to 4 The Scientific World Journal regulating general industrial use and coastal tourism. Based on marine sediment quality (GB 18668-2002), the overall mean concentrations of all selected heavy metals in Bohai Bay are below those value of the marine sediment quality I, indicating that the overall sediments quality in the Bohai Bay has not been obviously impacted by these six heavy metals. However, the mean concentration of Cr was close to the marine sediment quality I and Cr concentrations in some sampling sites have higher values than the value of marine sediment quality I, especially for the number 10 ( Figure 2). Sediment quality guidelines (SQGs) have been developed to deal with environmental concerns, and a stricter criterion in Hong Kong was also chosen to assess the contamination level of individual metals in sediments of the Bohai Bay (Table 1) [13]. The concentrations of Cu, Pb Zn, and Cd in sediments from 94.4%, 100%, 44.4%, and 100% of stations, respectively, were lower than "target" values and showed no signs of contamination. The contents of Cr at 100% of stations and Ni at 66.7% of stations were higher than the "trigger" values which are regarded as the upper limit of the desired quality for fairly clean sediments, indicating Bohai Bay sediments were moderately contaminated for Cr and Ni. In fact, there were 55.6% and 16.7% sediments heavily polluted by Cr and Ni ( Figure 2 and Table 1).

Pollution Assessment.
The Geoaccumulation Index ( geo ) introduced by Müller (1979) was used to assess metal pollution in sediments of Bohai Bay [14]. Geoaccumulation Index is expressed as follows: where is the measured concentration of heavy metal ( ) in the sediment, is the geochemical background value of heavy metal ( ), and 1.5 is the background matrix correction factor due to lithogenic effects. In the present study, was selected from the literature [15]. Geoaccumulation Index includes seven grades from Class 0 ( geo ≤ 0) to Class 6 ( geo ≥ 5). The geo is associated with a qualitative scale of pollution intensity; samples may be classified as unpolluted ( geo ≤ 0), unpolluted to moderately polluted (0 ≤ geo ≤ 1), moderately polluted (1 ≤ geo ≤ 2), moderate to strongly polluted (2 ≤ geo ≤ 3), strongly polluted (3 ≤ geo ≤ 4), strongly to extremely polluted (4 ≤ geo ≤ 5), and extremely polluted ( geo ≥ 5).
Based on the geo data and the Geoaccumulation Index, the results of the geo values and pollution level of heavy metals of surface sediment in Bohai Bay are shown in Table 2. In general, the average geo values are −0.91 for Cr, 0.27 for Ni, 0.11 for Cu, −0.20 for Zn, −0.53 for Cd, and 0.59 for Pb. The order of average geo values was Pb > Ni > Cu > Cr > Zn > Cd. Among the average geo of Cr, Zn and Cd were less than zero ( geo ≤ 0), which were classified as "unpolluted" level. However, the geo value of Cr and Zn in sampling site number 10 was more than zero, which were ranked as "unpolluted to moderately polluted" level. In addition, the average geo values of Ni, Cu, and Pb were ranked as "unpolluted to moderately polluted" level (0 ≤ geo ≤ 1). However, the geo value of Ni and Cu in sampling site number 15 was less than zero. In general, the worst pollution of heavy metals occurred in sampling site number 10 and the lightest pollution of heavy metals was in sampling site number 15.   Table 3, it can be seen that sediments from all sampling sites have relative lower values of 206 Pb/ 207 Pb ratios close to those values in pollution sources, suggesting the influence of anthropogenic inputs in Bohai Bay. In order to assess Pb contamination and identify potential Pb sources of sediments, the correlation between 1/Pb concentrations and 206 Pb/ 207 Pb ratios were analyzed. The analytic results showed that the relationship was not obviously correlated ( 2 = 0.2772), indicating that Pb sources of sediments were relatively complicated and cannot be simply attributed to binary mixing process of two sources [6]. Source apportionment can also be accomplished by analyzing the Pb isotopic radios ( 206 Pb/ 207 Pb and 208 Pb/ 207 Pb) in the environmental samples and major pollution sources with a lines mixing model [6]. The comparison between 206 Pb/ 207 Pb and 208 Pb/ 207 Pb ratios in sediments and other environmental samples showed that the 206 Pb/ 207 Pb ratios in surface sediments collected from Bohai Bay were obviously higher than those from the emission of vehicle exhaust and Pb ores mining, indicating that these two anthropogenic inputs were not important factors for Pb pollution in sediments ( Figure 3). Wang et al. also have confirmed that lead concentrations of atmospheric aerosols in Tianjin were decreased significantly after the leaded gasoline ban [16]. In fact, the 206 Pb/ 207 Pb and 208 Pb/ 207 Pb ratios in sediments were significantly similar to those in natural sources, unburned coal, and aerosol samples in the city of Tianjin. In the northern part of China, coal became one of most important energy resources in urban economical and industrial development. The leaded particular matter from coal combustion and coupled with the urban dusts (contain a mass of cement material) can be transported to the Bohai Bay by atmospheric deposition and surface river runoff. Previous study has shown that a large amount of Pb was supplied by the precipitation of aerosols in coastal environments and coal burning from power generation plants and other industrial activities may be the major source of Pb in its sediments [17]. Therefore, Pb contamination in the sediments of Bohai Bay may be controlled by the mix process of coal combustion, aerosol particles deposition, and natural sources.

Conclusion
Our investigation of heavy metals (Cr, Ni, Cu, Zn, Cd, and Pb) in surface sediments collected from Bohai Bay showed that the mean concentrations of the studied metals were slightly higher than the background values. However, the heavy metals concentrations in surface sediments in Bohai Bay were below the other important bays or estuaries in China. The assessment by Geoaccumulation Index indicated that Cr, Zn, and Cd were at the "unpolluted" level, while Ni, Cu, and Pb were ranked as "unpolluted to moderately polluted" level. The pollution level of the heavy mental was Pb > Ni > Cu > Cr > Zn > Cd. The Pb isotopic ratios in surface sediments varied from 1.159 to 1.185 for 206 Pb/ 207 Pb and from 2.456 to 2.482 for 208 Pb/ 207 Pb. Pb sources in sediments from Bohai Bay were more complicated (more than two sources), and there may exist various pollution sources. Compared with the Pb isotopic radios in other sources, coal combustion, aerosol particles deposition, and natural sources may be the major sources for Pb pollution in surface sediments of Bohai Bay.