Introduction

Polycyclic aromatic hydrocarbons (PAHs) have received increasing attentions in recent years due to their mutagenic and carcinogenic properties (Yang 2000; Lima et al. 2003; Zhou et al. 2008). The US Environmental Protection Agency (EPA) has listed 16 PAHs as priority pollutants for the control in the environment (Brown and Maher 1992; Pereira et al. 1996). PAHs are primarily aroused by the incomplete consumption/burning of wood, coal, petroleum and natural gas (Lima et al. 2003), spill of petroleum or petroleum products (Boehm and Farrington 1984) and discharge of the traffic exhausts (Naes et al. 1995). PAHs can be detected in almost all environmental medias including air, water and soil (sediment) (Fleming and Di Giulio 2011; Huang et al. 2012; Zhao et al. 2012). The sediments are considered as a primary reservoir of PAHs in aquatic systems. PAHs in water will be rapidly absorbed into the particles and then deposit into the sediments, due to their hydrophobic property and strong particulate-oriented behavior (Latimer and Quinn 1996; Tolosa et al. 1996). Furthermore, PAHs can be bio-accumulated by shellfish and some benthic organisms, leading to an access to humans (Hartmann et al. 2004).

The Pearl River delta is situated in the southern part of Guangdong Province, one of the most prosperous economically region in China. According to annual runoff, the Pearl River is the second largest river in China. With the rapid development of the economy, urbanization and industrialization, the Pearl River delta has experienced significant contaminant inputs (such as heavy metals and organic pollutants). Chlorinated and non-chlorinated hydrocarbons (such as PAHs) have been detected in both water and the sediment samples in the Pearl River and estuary (Mai et al. 2002). However, the dynamics of PAHs in the Pearl River estuary are still unclear.

In the paper, it aimed to investigate the distribution of PAHs in the Pearl River estuary, including Guangzhou channel, Lingding Bay and the adjacent coastal areas, and the potential sources of PAHs in the Pearl River estuary were also discussed.

Materials and methods

Sampling

Surface sediments were collected with a stainless steel grab sampler in December 2012. Total 17 stations in the Pearl River estuary were selected for analysis (Fig. 1). A1–A3 stations were located in Guangzhou channel; 9 stations (A4–A10, A12 and A13) were located in Lingding Bay; and the rest stations (A11, A14–A17) were situated in the adjacent coastal areas of the Pearl River estuary. All sediments samples were placed under −20 °C.

Fig. 1
figure 1

Map of the study area and sampling stations in the Pearl River estuary

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Sample extraction and clean-up

All sediment samples were freeze-dried for 48 h, and then sieved through a 0.2 mm screen. The sediment samples (about 5 g) together with PAH surrogates (naphthalene-d8; acenaphthene-d10; phenanthrene-d10; chrysene-d12 and perylene-d12) were then placed in a Soxhlet-extraction thimble with about 2 g sodium sulphate. The samples were extracted with methylene chloride for 48 h at 70 °C at 4 cycles/h. The extracts were desulphurised through activated copper powder and then concentrated to about 2 ml by rotary evaporation. The extracts were charged to a 1:2 alumina/silica gel glass column for clean-up and fractionation. The first fraction, containing aliphatic hydrocarbons, was eluted with 10 ml of hexane. The following fraction, containing PAHs, was collected by eluting 5 ml hexane and 70 ml of methylene chloride: hexane (3:7, v/v). The PAHs fraction was then concentrated to 1 ml under a stream of pure nitrogen gas.

PAHs analyses

Identification and quantification of PAHs were accomplished on an Agilent 7890A GC with a 5975C mass selective detector (MSD) (Agilent Technologies, USA) (Mai et al. 2002). The 7890A gas chromatography was used for quantification of PAHs with a GC column (HP-5MS 30 m long, 0.25 mm internal diameter, 0.25 μm coating) and the 5975C mass spectrometer for identification of PAHs. Column temperature was programmed at 80°C for 2 min, increasing to 220 °C at 20 °C/min, and a hold for 10 min at 220 °C, increasing to 240°C at 2 °C/min, and a hold for 5 min at 240 °C, then increasing to 300 °C at 4 °C/min, and a hold for 5 min at 300 °C. In the progress, helium was the carrier gas at a flow of 1.0 ml/min. Selected ion monitoring (SIM) mode was employed for identification and quantification of the 16 PAH compounds. The mass scanning ranged between m/z 50 and m/z 500. External standards containing equal amounts of the 16 PAHs analyzed were used for the quantification of PAHs. Blanks (solvent), spiked blanks (standards spiked into solvent), and sample duplicates were analyzed.

Sediment characteristics

The method of K2Cr2O7 heating was used to determine the total organic carbon contents of the samples (Walkley and Black 1934). The grain size of the sediment was analyzed using a Mastersizer 2000 particle size analyzer (Malvern, England), and the particle was divided into three size fraction: <63, 63–2000 and >2000 μm (Evans et al. 1990).

Source analysis of PAHs

LMW/HMW ratio: LMW PAHs (with the m/z < 202) usually come from crude oil contamination, and HMW PAHs (with the m/z ≥ 202) are from fuel-combustion. If the values of LMW/HMW are lower than 1, the sediments are mainly polluted by pyrolytic inputs. When the values are higher than 1, the petrogenic origin is considered as a major source (Yuan et al. 2001; Tam et al. 2001).

Isomer PAHs ratio (Ant/Phe, Flu/Pyr and IcdP/BghiP): Petroleum often contains more phenanthrene than anthracene and phenanthrene is a more thermodynamically stable tricyclic aromatic isomer than anthracene, therefore a high Phe/Ant value is a symbol for petrogenic pollution of PAHs. On the contrary, a low value means the source of pyrolytic contamination. Budzinski et al. (1997) suggested that the source of samples with Phe/Ant >10 were mainly the petrogenic pollution, and Phe/Ant <10 was symbol of the pyrolytic source. On the other hand, fluoranthene/pyrene ratio can also present the source of the PAHs. Sicre et al. (1987) concluded that Flu/Pyr values of greater than 1 were obviously associated with the pyrolytic source, and the values less than 1 were attributed to the petrogenic pollution. Values of BaA/(BaA + Chry) (Benzo[a]anthracene to Benzo[a]anthracene plus Chrysene) and IP/(IP + BP) (Indeno[1,2,3-cd]pyrene to Indeno[1,2,3-cd]pyrene plus) can present more details about the sources of PAHs. A value of BaA/(BaA + Chry) <0.2 indicates the petroleum, between 0.2 and 0.35 means the mixed sources of petroleum and combustion, and the PAHs derive from pyrolytic (combustion) contamination if it is more than 0.35. Values with IP/(IP + BP) <0.2 imply petroleum, from 0.2 to 0.5 petroleum combustion, and also imply wood and coal combustion if more than 0.5 (Yunker et al. 2002).

Statistical analysis

All results were analyzed and calculated using SPSS 13.0 (SPSS Inc, Chicago, IL, USA) and Excel 2003 for Windows. Additionally, principal component analysis (PCA) was also employed to evaluate the sources of PAHs in the sediments of the Pearl River estuary.

Results and discussion

Distribution of PAHs in surface sediments

16 EPA priority PAHs were determined, and the total PAHs concentrations ranged from 119.49 to 3828.58 ng/g dry wt with an average of 563.78 ng/g (Table 1). The highest PAHs concentrations were recorded in Guangzhou channel, followed by Lingding Bay and the adjacent coastal areas of the Pearl River estuary. Being near the cities, the highest PAHs in Guangzhou channel (such as A1) was expected. The present data indicated that anthropogenic activity acted as a major contributor to PAHs accumulation in the Pearl River estuary. The declined PAHs in Lingding Bay and the adjacent costal areas might partly be ascribed to the dilution of the seawater, as well as biodegradation of PAHs in the progress of migration and diffusion (Brown and Maher 1992).

Table 1 Mean concentrations of the total PAHs in the surface sediments of the Pearl River estuary (ng/g, dry wt)
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Except A16 and A17, almost all 16 PAHs were detected in the sediments collected from the Pearl River estuary. Among these 16 PAHs, low molecule weight PAHs (e.g., 3–4 rings) exhibited relative the higher levels. Compared with the urban industrialized areas around the world (McCready et al. 2000; Notar et al. 2001; Doong and Lin 2004; Weinstein et al. 2010; Chen et al. 2012; Shang et al. 2013), the concentrations of PAHs in Lingding Bay were comparable to the gulf of Triest in Italia. The level of PAHs in Guangzhou channel was similar with the Yangtze River, but less than South Carolina and Syndey harbor. Although the present data indicated that the study area was in a low pollution level, it should be noted that there was the potential risk of PAHs pollution in Guangzhou channel.

Relationships among PAHs, TOC and grain size

The results of TOC and grain size were summarized in Table 2. The contents of TOC ranged from 0.09 to 2.83 %. The grain size varied obviously among different stations. High proportions of the sand (more than 40 %) were observed in the sediments collected from stations A2, A4, A13, A16 and A17, while the rest stations were predominantly composed by silt and clay.

Table 2 TOC and grain size of the sediments collected from the Pearl River estuary
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The concentrations of the ∑PAHs were found to be positively correlated to the contents of TOC with a correlation coefficient 0.683 (p < 0.01, n = 13) (Fig. 2). The sediments with higher TOC appeared to exhibit relative the higher PAHs. Previous studies also indicated that PAHs accumulations in the sediments were positively correlated with TOC (Sun et al. 2011). Besides, black carbon (BC) also benefited PAHs deposition within the sediments (Bucheli et al. 2004; Oen et al. 2006; Agarwal and Bucheli 2011). The grain size was also considered as an important factor influencing PAHs deposition and adsorption/desorption within the sediments (Simpson et al. 1998; Wang et al. 2001; Thorsen et al. 2004). PAHs were easier to be absorbed on particles with small size rather than large ones (Kukkonen and Landrum 1996; Wu et al. 2006; Zhao et al. 2012). Significant relation between grain size and PAHs accumulations was not found (data not shown) in this study.

Fig. 2
figure 2

Positive relationship between TOC and PAHs in the sediments collected from the Pearl River estuary (R2 = 0.6827, p < 0.01)

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Sources of PAHs in the sediments of the Pearl River estuary

Sources of PAHs can be divided into three main kinds, the pyrolytic source (fuel-combustion), the petrogenic source (crude oil contamination) and the diagenetic origin. Generally, most of PAHs were originated from the former two sources, aggravated by anthropogenic activities (e.g., waste water from factories, oil leakage and Garbage disposal) (Brown and Maher 1992; Benlahcen et al. 1997; Doong and Lin 2004; Chen et al. 2012). Tree methods (PCA, LMW/HMW and the analysis for ratio of isomer) were employed to evaluate the sources of PAHs in the Pearl River estuary. PCA analysis could provide a quantitative data reflecting the source of PAHs in the whole study areas, while the methods of LMW/HMW and isomer ratios could distinguish the sources of PAHs in the each station (Yunker et al. 2002).

The results of PCA were shown in Fig. 3. PCA of PAHs in the sediments resulted in the first two components (58.1 and 13.4 %) accounting for 71.5 % of the variance. PC1 was dominated by Pyr, BghiP, BaP, BbF, BaA, IND, Chr, Fla, DBahA, indicating that the major sources of PAHs in the Pearl River estuary were associated with the high temperature combustion. PC2 was dominated by Ant, Ace, Acy, BkF, Fle, Nap, Phe, indicating a combined component of the petrogenic source and the low temperature pyrogenic source.

Fig. 3
figure 3

Results of the principal component analysis (PCA). PC1 accounts for 58.1 % of the variance and PC2 accounts for 13.4 %. Nap naphthalene, Acy acenaphthylene, Ace acenaphthene, Fle fluorine, Phe phenanthrene, Ant anthracene, Fla fluoranthene, Pyr pyrene, BaA benzo [a] anthracene, Chr chrysene, BbF benzo [b] fluoranthene, BkF benzo [k] fluoranthene, BaP benzo [a] pyrene, IND indeno [1,2,3-cd] pyrene, DBahA dibenzo [ah] anthracene, BghiP benzo [ghi] perylene

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The values of LMW/HMW were shown in Fig. 4, and the results showed that A2, A3, A4, A10, A13 and A16 exhibited a mixture of the pyrolytic origin and the petrogenic origin. The values of LMW/HMW in the rest stations were less than 1, indicating the pyrolytic source. The analyses for ratios of isomer PAHs were shown in Figs. 5 and 6. Almost all values of Phe/Ant were less than 10, indicating the pyrolytic source of PAHs in the Pearl River estuary (Fig. 5). The ratios of BaA/(BaA + Chry) and IP/(IP + BP) also showed that PAHs accumulations in the Pearl River estuary were originated from the combustion sources (Fig. 6). IP/(IP + BP) was also considered as a useful index to distinguish the different combustion sources (e.g., wood, coal and petroleum combustion) (Soclo et al. 2000). In this study, almost all ratios of IP/(IP + BP) were fluctuated around 0.5 (Fig. 6b), which could not well distinguish the different combustion sources. The present data coincided with the energy structure in the regions of the Pearl River Delta, where coal and petroleum were the main fuel sources. Wood fuel reflected the early energy structure in these areas. Besides, the dynamics of hydrology and pollution sources in the Pearl River estuary were extremely complex, simultaneously affected by anthropogenic activities and geological sedimentation.

Fig. 4
figure 4

The LMW/HMW values in different sites (LMW-low molecular weight PAHs; HMW-high molecular weight PAHs)

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Fig. 5
figure 5

Ratios of Phe/Ant against Flu/Pyr in the sediments of different areas in the Pearl River estuary. Phe phenanthrene, Ant anthracene, Flu fluoranthene, Pyr pyrene. Pink filled square the adjacent areas of the Pearl River estuary, blue filled triangle Lingding Bay, red filled circle Guangzhou channel (Color figure online)

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Fig. 6
figure 6

Ratios of Phe/Ant against BaA/(BaA + Chry) (a) or IP/(IP + BP) (b) in the sediments of the different areas in the Pearl River estuary. Pink filled square the adjacent areas of the Pearl River estuary, blue filled triangle Lingding Bay, red filled circle Guangzhou channel. BaA: benzo [a] anthracene, Chry: chrysene, IP: indeno [1,2,3-cd] pyrene, BP: benzo [ghi] perylene (Color figure online)

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In summary, the Pearl River estuary exhibited a relative low pollution level. However, the potential risk of PAHs in Guangzhou channel should be attended. Most of PAHs in the Pearl River estuary were originated from the pyrolytic sources. Nevertheless, the dynamics of PAHs in the Pearl River estuary were extremely complex, further studies including the analysis for PAHs deposition in sediment cores and isotope dating will be conducted.

Conclusions

The concentrations of total PAHs in the sediments of the Pearl River estuary ranged from 119.49 to 3828.58 ng/g with a mean value of 563.52 ng/g. The order of the PAHs accumulations was as follows: Guangzhou channel > Lingding Bay > the adjacent coastal areas of the Pearl River estuary. PAHs in the sediments were found to be positively correlated with TOC. The source analysis showed that PAHs accumulations in the surface sediments of the Pearl River estuary were mostly originated from the pyrolytic inputs. Further studies on the dynamics of PAHs of in the Pearl River estuary will be conducted.