Temporal trends of hydrocarbons in sediment cores from the Pearl River Estuary and the northern South China Sea

Temporal trends of hydrocarbons in sediment cores from the Pearl River Estuary and the northern South China Sea

Available online at www.sciencedirect.com Environmental Pollution 156 (2008) 442e448 www.elsevier.com/locate/envpol Temporal trends of hydrocarbons ...

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Available online at www.sciencedirect.com

Environmental Pollution 156 (2008) 442e448 www.elsevier.com/locate/envpol

Temporal trends of hydrocarbons in sediment cores from the Pearl River Estuary and the northern South China Sea Xianzhi Peng a,*, Zhendi Wang b, Yiyi Yu a, Caiming Tang a, Hong Lu a, Shiping Xu a, Fanrong Chen a, Bixian Mai a, Shejun Chen a, Kechang Li a, Chun Yang b a

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehau Street, Guangzhou 510640, China b Environmental Science and Technology Centre, Environment Canada, 335 River Road, Ottawa, Ontario K1A 0H3, Canada Received 13 October 2007; received in revised form 21 January 2008; accepted 21 January 2008

The accumulation rates of pyrogenic PAHs have significantly increased in the northern South China Sea in the recent decades. Abstract Concentrations and fluxes of unresolved complex mixture of hydrocarbons (UCM) and polycyclic aromatic hydrocarbons (PAHs) were analyzed for two 210Pb dated sediment cores from the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS). Compoundspecific stable carbon isotopic compositions of individual n-alkanes were also measured for identification of the hydrocarbon sources. The historical records of PAHs in the NSCS reflected the economic development in the Pearl River Delta during the 20th century. PAHs in the NSCS predominantly derive from combustion of coal and biomass, whereas PAHs in the PRE are a mixture of petrogenic and pyrogenic in origins. The isotopic profiles reveal that the petrogenic hydrocarbons in the PRE originate predominantly from local spillage/leakage of lube oil and crude oils. The accumulation rates of pyrogenic PAHs have significantly increased, whereas UCM accumulation has slightly declined in the NSCS in the recent three decades. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Hydrocarbon accumulation; Dated sediment cores; Compound-specific isotopic fingerprinting; The South China Sea

1. Introduction Aliphatic and polycyclic aromatic hydrocarbons are ubiquitous sedimentary contaminants due to their tendency to accumulate in sediments (Wakeham et al., 2004). Sedimentary aliphatic hydrocarbons (AHCs) have both natural (including biogenic and petrogenic) and anthropogenic sources. Anthropogenic AHCs in sediment originate mainly from petroleum residues and generally exhibit an unresolved complex mixture (UCM) of branched and cyclic hydrocarbons (Doskey, 2001). Polycyclic aromatic hydrocarbons (PAHs), known carcinogens and mutagens, derive mainly from both petroleum leakage and combustion of fossil fuels, municipal wastes, and biomass * Corresponding author. Tel.: þ86 20 8529 0191; fax: þ86 20 8529 0706. E-mail address: [email protected] (X. Peng). 0269-7491/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.01.037

(Khalili et al., 1995; Mai et al., 2001; Yunker et al., 2002). UCM and PAHs are introduced into aquatic environment through a variety of routes including oil spill/leakage, riverine runoff, and atmospheric deposition, and tend to persist in environment (Doskey, 2001;Tolosa et al., 1996; Wang et al., 1998; Reddy et al., 2002). It has been proved that accumulations of sedimentary UCM and PAHs are effective for evaluating and reconstructing the historical anthropogenic impact on the environment because they are both persistent and mainly anthropogenic in origin (Pereira et al., 1999; Fernandez et al., 2000; Yamashita et al., 2000). Previous research has revealed an increase trend of UCM and PAHs concentrations/ fluxes in sediment cores from lakes, estuaries, river basins, and continental shelf since the Industrial Revolution (Doskey, 2001; Wakeham et al., 2004). Chronic profiles of PAHs are generally closely correlated with the economic development

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and shifts of energy structures (Lima et al., 2003; Kannan et al., 2005; Van Metre and Mahler, 2005; Guo et al., 2006). The Pearl River Delta (PRD) is one of the most urbanized and developed areas in China. Unfortunately, the heavy urbanization and rapid economic development have severely compromised the environment (Mai et al., 2001, 2003; Zhang et al., 2002). Previous works have demonstrated that sediments of the Pearl River Estuary (PRE) and the South China Sea are the main reservoirs for pollutants originated from the PRD (Chen et al., 2006; Peng et al., 2007). This work aims to: (1) delineate the temporal trends of concentrations and fluxes of UCM and PAHs in the PRE and the adjacent northern South China Sea (NSCS) recorded by sedimentary cores based on 210Pb dating; (2) assess the anthropogenic impacts on the estuarine and marine environment through rough estimation of environmental burdens and accumulation rates of hydrocarbon contaminants in the PRE and NSCS during the 20th century; and (3) trace the sources and environmental fate of hydrocarbons in the PRE and NSCS by combination of molecular and isotopic fingerprinting. 2. Materials and methods 2.1. Study area and sampling The study area and collecting of sediment cores were detailed elsewhere (Peng et al., 2007). Briefly, core #1 (N22 16.00 , E113 42.00 , seawater depth of 20 m) and core #2 (N21 43.00 , E113 56.00 , seawater depth of 39 m) were taken in the PRE and the adjacent NSCS in October and April 2003 respectively (Fig. 1). Both cores were 30 cm long. The cores were sectioned on board (2 cm interval for core #1 and 1 cm interval for core #2), wrapped in baked (450  C) aluminum foil, and transported on ice to the laboratory, where they were stored at 20  C until further treatment. Crude oil Walio (originating from Indonesia) was sampled from Guangzhou Petroleum and Chemical Plant that is one of the largest local petrochemical plants. Crude oils Masila (originating from Yemen) and Benchamas (originating from Thailand) were kindly provided by Dr. Gan Zhang. These three oils were included based on the knowledge that crude oils used in petrochemical plants in the PRD are imported mainly from the Southeastern Asia and the Middle East (personal communication). The lube oil was bought from the local market. Three crude oils originating from China were also included for comparison. They are Shengli oil, Lunnan oil 1, and Lunnan oil 2 that were collected from oil fields located in North China.

2.2. Extraction and instrumental analysis Freeze-dried sediment was accurately weighed, homogenized, quantitatively transferred into a pre-cleaned extraction thimble, and spiked with o-terphenyl and a mixture of four deuterated PAHs (acenaphthene-d10, phenanthrene-d10, benz(a)anthracene-d12, perylene-d12) as recovery surrogates. The sample was then Soxhlet extracted with 200 mL of dichloromethane (DCM) for 24 h at a rate of 5 cycles per hour. The raw extracts were filtered, concentrated, and solvent-exchanged to hexane. The cleanup and fractionation of the concentrated extract was achieved using silica gel chromatographic technique as described in reference (Wang et al., 1994). The aliphatic and aromatic fractions were successively eluted with 15 mL of hexane and DCM respectively. The fractions were concentrated under a gentle flow of high-purity nitrogen to appropriate volumes, spiked with appropriate internal standards (5a-androstane for the aliphatic and terphenyl-d14 for the aromatic fraction respectively), and then adjusted to accurate pre-injection volumes (1.00 mL) for instrumental analysis. The AHC fraction was subjected to an Agilent 6890 GC-FID analysis using a DB-5 capillary column (30 m length  0.25 mm i.d  0.25 mm film thickness). The oven temperature started at 50  C, ramped to 300  C at

Fig. 1. Study area and the coring stations (:) in the Pearl River Estuary and the northern South China Sea. (C) are locations of petrochemical plants in the Pearl River Delta.

6  C min1. UCM, which appears as the ‘‘envelope’’ or ‘‘hump’’ between the solvent baseline and the curve defining the base of resolved peaks, was determined by integrating the total GC area with subtraction of the resolved peaks and using the average response factor of n-alkanes obtained from the n-alkane standard mixture during the instrumental calibration. Analyses of PAHs were performed on an Agilent 6890 GC system equipped with an Agilent 5973 mass spectrometer operating in selective ion monitoring mode using an HP-5 MS column (30 m length  0.25 mm i.d  0.25 mm film thickness). Samples were injected in splitless mode with the injector temperature at 280  C. Helium was used as carrier gas at a flow rate of 1 mL min1. For analyses of parent PAHs: start at 90  C with 1 min hold, ramp to 160  C at 15  C min1, and then to 290  C at 8  C min1 with a final hold of 8 min. For analyses of alkylated PAHs: start at 60  C with 2 min hold and ramp to 300  C at 6  C min1 with a final hold of 13 min. The quantified compounds included 20 parent PAHs and their alkylated homologues, which were naphthalene, biphenyl, acenaphthylene, acenaphthene, fluorene, dibenzothiophene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthe, benzo(k)fluoranthene, benz(e)pyrene, benz(a)pyrene, perylene, indeno(1,2,3-cd)pyrene, dibenz(ah)anthrance, benzo(ghi)perylene, 2-methylnaphthalene, 1-methylnaphthalene, 2,6-methylnaphthalene, 2,3,5-trimethylnaphthalene, C1-phenanthrene, C2-phenanthrene, C3-phenanthrene, and C4-phenanthrene. A procedural blank and a spiked matrix were set with each batch of 10 samples respectively. Only trace amount of fluorene and fluoranthene were detected in the blanks. The average recoveries from the matrix spikes were 50.1e69.4% for 2 and 3-ring PAHs (from naphthalene to phenanthrene) and 84.9e112.2% for high molecular PAHs (from fluoranthene to benzo(ghi)perylene). The recoveries of the surrogate standards from the sediment samples were determined to be 72.3  13.0, 74.5  11.6, 96.1  11.1, and 93.8  9.6% for acenaphthene-d10, phenanthrene-d10, benz(a)anthracene-d12, and perylene-d12 respectively. The reported concentrations of the

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UCM and PAHs were based on dry weight of sediments and not recoverycorrected.

2.3. Compound specific stable carbon isotopic analysis of n-alkanes Another aliquot of about 20 g of each segment of the sediment core #1 was Soxhlet extracted and fractionated as described above. An appropriate amount of each oil sample was also fractionated by silica gel column. The aliphatic fractions of sediments and oils were subjected to the urea inclusion paper layer chromatography to separate n-alkanes from their branched/cyclic homologues following the procedure described in detail by Xu and Sun (2005). The stable carbon isotope analyses of individual n-alkanes were performed using an Agilent 6890 GC connected with a VG Isochrom II isotope ratio mass spectrometer (GC-IRMS) via a high temperature combustion interface. An HP-5 capillary column (60 m  0.32 mm  0.25 mm) was used to separate the n-alkanes. The carrier gas was helium. The oven was held isothermally for 5 min at 70  C, ramped to 110  C at 15  C min1 and held for 2 min, and then to 290  C at 7  C min1 with a final hold of 20 min. The combustion furnace was run at 850  C. Carbon isotope ratios for individual n-alkanes were calculated using CO2 as the reference gas that was automatically introduced into the IRMS in triplicate at the beginning and the end of each analysis. The data were reported in & relative to the PDB standard (Mazeas and Budzinski, 2002). A calibration solution containing n-alkanes (n-C12en-C32) with known d13C values was injected in duplicate everyday prior to sample analysis to test the instrumental performance. The standard deviation of replicate analyses is within 0.3&.

2.4. Dating of the sediment cores Dating of the sedimentary cores has been described in detail elsewhere (Peng et al., 2007). Briefly, 210Pb activities in sediment are determined by analyzing the a radioactivity of its decay product 210Po. Supported 210Po was obtained by indirect determination of the a radioactivity of the supported parent 236Ra. Constant initial 210Pb concentration model was adopted, obtaining average sedimentation rates of 0.97 and 0.34 cm yr1 for cores #1 and #2 respectively.

2.5. Data statistics The deposition fluxes (F ), inventories (I ), and environmental loadings (L) were calculated using the equations, F ¼ Cirigi, I ¼ SCiridi, and L ¼ A  I, respectively, where Ci (ng g1), ri (g cm3), and di (cm) refer to the concentration, the dry mass density, and the thickness of the segment i, g (cm yr1) is the sedimentation rate, and A (km2) is the surface area of the study region.

3. Results and discussions 3.1. Vertical profiles of UCM abundances and fluxes The top three sections of the core #2 (corresponding to about 1993e2003) were missing and therefore not analyzed. A unresolved complex mixture of hydrocarbons, or UCM ‘‘hump’’, was present in all samples, being predominated by the boiling range of n-C13en-C21. Another smaller ‘‘hump’’ from n-C25 to n-C33 was also observed in some sections of the core #1. The UCM concentrations range from 12.9 to 42.7 mg g1 in the PRE sediments, comparable to those of the northern Rio de la Plata Estuary, Argentina (Colombo et al., 2005). Concentrations and fluxes of the UCM were determined to be 4.0e35.9 mg g1 and 4.4e11.8 mg cm2 yr1 respectively in the NSCS sediments, which are higher than those reported for the northwestern Mediterranean continental shelf (Tolosa et al., 1996) and Cretan Sea (Gogou et al., 2000). The UCM concentrations in the PRE and the NSCS sediment cores peaked in the 1970s, obviously declined in the 1970se 1980s, and then fluctuated in a zigzag manner (Fig. 2), which is quite different from those in the northwestern Mediterranean continental shelf that showed upcore increase with surface maxima (Tolosa et al., 1996). The widespread presence of UCM may indicate the petrogenic pollution in NSCS since the early 1900s. It is not clear if the peak concentrations of UCM in the 1970s core layers correspond to any significant oil spill events as oil spills were poorly documented in the PRE and the NSCS at that time. The decline of UCM fluxes in the recent decades may suggest the decreasing input of petrogenic hydrocarbon pollutants in the PRE and NSCS. 3.2. Temporal trend of PAH concentrations and deposition fluxes The 16 US EPA priority PAHs have been used as reference to evaluate the anthropogenic pollution in the environment, and have also been measured in many other comparable studies (Tolosa et al., 1996; Khim et al., 1999; Pereira et al., 1999;

Fig. 2. Vertical profiles of concentrations and fluxes of UCM and 16 US EPA priority PAHs (S16PAHs) in the sediment cores from the Pearl River Estuary and the northern South China Sea.

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Lima et al., 2003). The total of 16 US EPA priority PAHs (S16PAHs) was determined to range from 312 to 486 ng g1 in the PRE, comparable to those of the northern and southern Rio de la Plata Estuary, Argentina (Colombo et al., 2006), Mashan Bay and Yeongil Bay mean S16PAHs of 353 and 309 ng/ g respectively) in Korea (Khim et al., 1999; Koh et al., 2006), higher than that of San Francisco Estuary (Oros and Rose, 2004), but lower than those of the central Rio de la Plata Estuary (Colombo et al., 2006) and Malaysian estuaries (Zakaria et al., 2002). Concentrations and fluxes of S16PAHs range from 69 to 216 ng g1 and 21 to 52 ng cm2 yr1 respectively in the NSCS, which are similar to those of the Ebro continental shelf of the northwestern Mediterranean (Tolosa et al., 1996), Cretan Sea (Gogou et al., 2000), and East China Sea (Guo et al., 2006), but lower than the Rhone continental shelf of the northwestern Mediterranean (Tolosa et al., 1996). Concentrations and fluxes of S16PAHs in the core #1 increased from 385 to 486 ng g1 from the bottom to the mid1980s, and then fluctuated in a zigzag manner (Fig. 2). Chen et al. (2007) reported similar vertical pattern of polybrominated diphenyl ethers at this station, suggesting the fluctuation to be likely caused by the sediment disturbance and mixing due to strong fluvial input and complicated hydrodynamics. In core #2, on the other hand, the vertical profiles of S16PAHs tracked closely the historical development of economy in both Hong Kong and the Pearl River Delta on the mainland China side (Fig. 2). The S16PAHs concentration did not vary significantly from the late 1910s to the mid-1940s (from 69 to 85 ng g1), which might be correlated with the slow economic development in both Hong Kong and mainland China during this period (Lu and Lu, 2002). Concentrations of the S16PAH increased from 85 in 1946 to 108 ng g1 in 1951, corresponding to the post-war growth of the economy in both Hong Kong (35% increase in economic development rate from 1947 to 1951, Lu and Lu, 2002) and in the mainland China after the People’s Republic of China was established in 1949. In the following years, Hong Kong suffered another severe economic depression when the Embargo Policy toward China was implemented by the United State and European countries from 1950 to 1954. In addition, mainland China experienced ‘‘the three toughest years’’ from 1959 to 1961. During this time, both the concentrations and sedimentary fluxes of S16PAHs consequently declined bymore than 30%. Concentrations and fluxes of the S16PAHs began to increase again after 1963 and reached 113 ng g1 and 31 ng cm2 yr1 respectively in 1969. In the following ten years from 1966 to 1975, Chinese economy was severely depressed by the Cultural Revolution. The economy in Hong Kong was also affected by the oil embargo of the Organization of Petroleum Exporting Countries (OPEC) in 1973. As a result, the S16PAHs concentrations and fluxes decreased by ~30% from 113 to 79 ng g1 and 31.0 to 21.5 ng cm2 yr1 respectively. As the bridgehead to initiate the China’s ‘‘Reform and Open’’ policy, the PRD region has experienced the fastest economical development and industrialization in the country since 1978, accelerating even further after 1990. Correspondingly, an almost linear increase in the concentration of the S16PAHs at a rate of 3.1 ng g1 yr1 was

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observed from 1975 to 1990 (n ¼ 6, r2 ¼ 0.867), with the slope becoming steeper in later years. Similar temporal trends of the S16PAHs were also reported for sediment cores from the PRD (Mai et al., 2001) and the East China Sea (Guo et al., 2006). In contrast, the S16PAHs concentrations generally peaked from the 1950s to the 1980s in America, Europe and Japan (Tolosa et al., 1996; Pereira et al., 1999; Yamashita et al., 2000; Lima et al., 2003; Wakeham et al., 2004; Van Metre and Mahler, 2005). The difference in the vertical PAH profile patterns between China and other countries and regions may reflect different economic development models and energy sources in these areas. As a whole, the accumulation of PAH in the NSCS (core #2) reflected the economic development in the Pearl River Delta (including Hong Kongand the mainland China side) during the 20th century. The contribution of PAHs from the mainland China has gradually increased after 1949 when the People’s Republic of China was established, and become dominant after the 1980s due to the booming economy in the PRD as the result of the implement of the China’s ‘‘Reform and Open’’ policy. 3.3. PAH diagnoses Anthropogenic PAHs originate mainly from combustion of biomass and fossil fuels and oil spillage. Isomeric ratios of PAHs are classical indices for apportioning sources of PAHs in the environment (Khalili et al., 1995; Wang et al., 1998; Yunker et al., 2002). The ratio of methyphenanthrenes/phenanthrene (MP/P) is frequently used to differentiate petrogenic and pyrogenic PAHs. The MP/P ratio typically ranges from 2 to 6 in unburned fossils, but is generally smaller than 1 in combustion residues (Khalili et al., 1995). MP/P ratios were determined to be 1.24e1.83 (1.54) and 0.54e1.27 (0.80) for cores #1 and #2 respectively, suggesting that the PRE had mixed petrogenic and pyrogenic PAHs, whereas the NSCS were predominated by PAHs created by combustion. It has been well known that pyrogenic and petrogenic PAHs can be readily distinguished on the basis of their alkyl group distributions (Wang and Fingas, 2006). The petrogenic PAH homologue profiles present in crude oil form a characteristic ‘‘bell-shaped’’ distribution pattern due to the relative abundance of alkylated PAHs (C1- to C3-PAHs) over the corresponding parent (C0-) and C4-PAHs. In the contrast, pyrogenic PAH homologue pro-files present in combustion residues generally form a decreasing, or ‘‘sloped’’ pattern due to the domination of the parent PAH (C0-) and decreasing abundance of each increasingly alkylated PAH homologue. The distribution of alkylated phenanthrene homologous series in the cores showed C1-P > ¼ C2-P > C3-P > ¼ C0-P > C4-P in core #1, and C0-P > C1-P > C2-P > C3-P > C4-P in core #2, respectively (Fig. 3), further confirming the mixture input of petrogenic and pyrogenic sources for PAHs in the PRE and overwhelmingly pyrogenic origins for PAHs in the NSCS. Ratios of PAH isomer pairs, such as fluoranthene/ (fluoranthene þ Pyrene) [Fl/(Fl þ Py)] and indeno(1,2,3-cd) pyrene/(indeno(1,2,3-cd)pyrene þ benzo(ghi)perylene) [(IP/ (IP þ BP)] have been used in combination to infer possible

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2006). The PRE is a busy waterway used by a great number of ships and boats. There are also large-scale petrochemical plants in the PRD that import crude oils mainly from Southeastern Asia and the Middle East (personal communication). Therefore, exhausts from low temperature combustion engines as well as accidental oil spills/leakages could have significant impact on the PAH inputs to the sediments in the PRE. Overall, the diagnostic ratios indicate that petrogenic PAHs are basically confined to the PRE. Coal is still the main energy source in Mainland China, accounting for 69% of the energy generation capability in 2000 (Guo et al., 2006). The predominance of PAHs originating from combustion of coal and biomass in the sediment of the NSCS is consequently reasonable. 3.4. Perylene

Fig. 3. Concentrations of phenanhrene and its alkylated homologues in the sedimentary cores from the Pearl River Estuary and the northern South China Sea.

sources of PAHs (Yunker et al., 2002). IP/(IP þ BP) < 0.2 and Fl/(Fl þ Py) < 0.4 often implies petrogenic PAHs, while ratios of both greater than 0.5 suggest PAHs origin from combustion of coal and biomass. An IP/(IP þ BP) ratio of 0.2e0.5 together with Fl/(Fl þ Py) greater than 0.4 implies PAHs originate from petroleum (vehicle fuels and crude oil) combustion. Fig. 4 demonstrates that PAHs in the PRE sediments (core #1) originate primarily from petroleum combustion, whereas PAHs of the NSCS (core #2) are homogeneous over time and predominantly originate from combustion of biomass and coal. This result is consistent with the previous reports of PAHs in the surface sediment of the PRE and the coastal SCS (Mai et al., 2001, 2003; Luo et al., 2006) and the sedimentary core in the East China Sea (Guo et al.,

Fig. 4. Source-diagnostic PAH isomer pair ratios in the sediment cores from the Pearl River Estuary (core #1) and the northern South China Sea (core #2). I: petroleum origin; II: petroleum combustion; III: Coal and biomass combustion.

Perylene is thought to derive from both combustion processes of fossil fuels and biomass and diagenesis of natural organic matter in anoxic aquatic sediments with high biological productivity (Venkatesan, 1988). Relative concentration of perylene >10% of the total penta e aromatic isomers indicates a probable diagenetic input (Pereira et al., 1999; Lima et al., 2003). The percentages of perylene relative to the total penta e aromatic isomers were determined to be 58e69% (64%) in core #1, and 60e81% (71%) in core #2, respectively, suggesting the predominance of diagenetic perylene in the PRE and NSCS. 3.5. Compound-specific stable carbon isotopic profiles of n-alkanes of the sediments and oils The stable isotopic compositions of individual middle and long chain n-alkanes have shown to be able to distinguish both the origin and the thermal maturity of crude oils, and are largely unaltered by biodegradation (Pond et al., 2002). Therefore compound-specific stable isotopic fingerprinting has been considered as a promising alternative tool for identification of weathered oil residues in environment (Mazeas and Budzinski, 2002). The stable carbon isotopic compositions of individual nalkanes (C15eC33) were analyzed for each segment of the core #1 and the seven collected oils. The isotopic profiles correlate well from top to bottom of the core, suggesting the homogeneous origins of aliphatic hydrocarbons in the PRE in the past three decades. The n-alkane isotopic profiles of the core #1 were found to be very similar to that of the lube oil that was bought in the local market, and also similar to those of the crude oils Walio (originating from Indonesia) collected from the local petrochemical plant and Masila (originating from Yemen), especially for the n-alkanes between C19 and C27, but significantly different from those of the other oils (Fig. 5). As mentioned above, petrochemical plants in the PRD generally import crude oils from Southeastern Asia and the Middle East. It is suggested from the isotopic fingerprinting that the petrogenic hydrocarbons in the PRE are mainly attributable to local accidental spills/leakages of lube oil and crude oils.

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after 1978 were found to be 0.8, 1.2, and 1.6 times of the mean value, respectively. Accumulation rates of the UCM, SPAHs, and S16PAHs in the PRE were relatively constant in the past three decades. 4. Conclusion Sedimentary PAHs records in the northern South China Sea reflected the economic development in Hong Kong and the mainland China during the 20th century. Petrogenic PAHs are confined to the Pearl River Estuary, and PAHs in sediments of the northern South China Sea originate predominantly from combustion of coal and biomass. The petrogenic hydrocarbons in the Pearl River Estuary result mainly from local accidental spills/leakages of lube oil and crude oils indicated by the stable isotopic fingerprinting. The accumulation rates of PAHs in the northern South China Sea have significantly increased after 1978 due to the implement of the ‘‘Reform and Open’’ policy in China. Fig. 5. Profiles of stable carbon isotopic composition for n-alkanes of the sediment core #1 and the oils.

3.6. Environmental loading of hydrocarbon contaminants in the Pearl River Estuary and the northern South China Sea The total environmental loadings of UCM, total parent and alkylated PAHs except for perylene (SPAHs), and S16PAH were roughly estimated to be 15,742, 1551, and 264 tons in the PRE (with an area of 2016 km2, Chen et al., 2007) in the past three decades, and 117,958, 1245, and 469 tons in the NSCS (with an area of 21,340 km2, Chen et al., 2007) during the 20th century (1917e1993) using the inventories of the core #1 and core #2, respectively. The overall annual accumulation rates of UCM, SPAHs, and S16PAH were roughly estimated to be 525, 51.7, and 8.8 tons yr1 in the PRE and 1388, 18.8, and 6.2 tons yr1 in the NSCS (Table 1). These values are comparable to those reported for the Rio de la Plata Estuary, Argentina (Colombo et al., 2005), but 3e10 times higher than those in the northwestern Mediterranean (Tolosa et al., 1996) taking basin areas into consideration. The accumulation rates of UCM, SPAHs, and S16PAHs in the NSCS Table 1 Annual accumulation rates (tons/year) of hydrocarbons in the Pearl River Estuary and Northern North China Sea Area (km2)

UCMa

a

c d e

S16PAHc

Meand 1978þe Meand 1978þe Meand 1978þe

Pearl River 2016 525 Estuary Northern North 21,340 1388 China Sea b

SPAHb

518

51.7

51.3

8.8

9.0

1063

18.8

22.8

6.2

10.0

UCM ¼ unresolved complex mixture of hydrocarbons. SPAHs ¼ sum of all parent and alkylated PAHs except for perylene. S16PAHs ¼ sum of 16 US EPA priority PAHs. Average annual accumulation rates throughout the core. Average annual accumulation rates after 1978.

Acknowledgments This work was financially supported by the NSFC Projects (No. 40472149, No. U0633005) and the National Basic Research Program of China (No. 2003CB415002). We would like to thank Environmental Science and Technology Centre, Environment Canada for their generous support. Thanks also to Dr. Wanglu Jia from the SKLOG for his help during the GC-IRMS analysis. The comments and suggestions from the editor and the reviewers have greatly improved this paper. References Chen, S.J., Luo, X.J., Mai, B.X., Sheng, G.Y., Fu, J.M., Zeng, E.Y., 2006. Distribution and mass inventories of polycyclic aromatic hydrocarbons and organochlorine pesticides in sediments of the Pearl River Estuary and the Northern South China Sea. Environmental Science and Technology 40, 709e714. Chen, S.J., Luo, X.J., Lin, Z., Luo, Y., Li, K.C., Peng, X.Z., et al., 2007. Time trends of polybrominated diphenyl ethers in sediment cores from the Pearl River Estuary, South China. Environmental Science and Technology 41, 5595e5600. Colombo, J.C., Capelletti, N., Lasci, J., Migoya, M.C., Speranza, E., Skorupka, C.N., 2005. Sources, vertical fluxes and accumulation of aliphatic hydrocarbons in coastal sediments of the Rio de la Plata Estuary, Argentina. Environmental Science and Technology 39, 8227e8234. Colombo, J.C., Capelletti, N., Lasci, J., Migoya, M.C., Speranza, E., Skorupka, C.N., 2006. Sources, vertical fluxes, and equivalent toxicity of aromatic hydrocarbons in coastal sediments of the Rı´o de la Plata Estuary, Argentina. Environmental Science and Technology 40, 734e740. Doskey, P.V., 2001. Spatial variations and chronologies of aliphatic hydrocarbons in Lake Michigan sediments. Environmental Science and Technology 35, 247e254. Fernandez, P., Vilanova, R.M., Martinez, C., Appleby, P., Grimalt, J.O., 2000. The historical record of atmospheric pyrolytic pollution over Europe registered in the remote mountain lakes. Environmental Science and Technology 34, 1906e1913. Gogou, A., Bouloubassi, I., Stephanou, E.G., 2000. Marine organic geochemistry of the Eastern Mediterranean: 1. Aliphatic and polyaromatic hydrocarbons in Cretan Sea surficial sediments. Marine Chemistry 68, 265e282. Guo, Z., Lin, T., Zhang, G., Yang, Z., Fang, M., 2006. High-resolution depositional records of polycyclic aromatic hydrocarbons in the central

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