Environ. Sci. Technol. 2006, 40, 709-714
Distribution and Mass Inventories of Polycyclic Aromatic Hydrocarbons and Organochlorine Pesticides in Sediments of the Pearl River Estuary and the Northern South China Sea S H E - J U N C H E N , †,‡ X I A O - J U N L U O , † BI-XIAN MAI,† GUO-YING SHENG,† J I A - M O F U , † A N D E D D Y Y . Z E N G * ,† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Science, Guangzhou 510640, China, and Graduate School, Chinese Academy of Sciences, Beijing 100049, China
Surface sediment (0-5 cm) samples were collected from the Pearl River Estuary (PRE) and the adjacent northern South China Sea (SCS) in July 2002 and analyzed for 25 polycyclic aromatic hydrocarbons (PAHs) and 8 organochlorine pesticides (OCPs) including dichlorodiphenyltrichloroethanes (DDTs), hexachlorocyclohexanes (HCHs), and heptachlor. The total PAHs and OCPs concentrations were 138-1100 and 0.18-3.57 ng/g dry weight, respectively. Compositional pattern analysis suggested that PAHs in the PRE were derived from both pyrogenic and petrogenic sources, whereas most PAHs in the northern SCS were pyrogenically originated. The concentrations of both PAHs and OCPs were higher in the PRE than in the northern SCS, and a decreasing trend with the distance from the estuary to the open sea was observed. In addition, perylene was a predominant component in all samples and clustered with PAH compounds with high log Kow values (from phenanthrene). These findings indicated that river outflows were the major source of contamination in the offshore sediments. A preliminary assessment suggested that atmospheric deposition contributed only a minor portion of PAHs or OCPs in the sediments of the northern SCS. The sediment (0-5 cm) mass inventories were 126 and 423 metric tons for PAHs and were 0.4 and 1.4 metric tons for OCPs in the PRE and the northern SCS, respectively. Clearly, contaminated sediments of the northern SCS may be a potential source of PAHs and OCPs to the global oceans.
Introduction Polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs), most of which are categorized as persistent organic pollutants (POPs) (1), are of environmental significance due to their widespread distribution in the environment and potential toxicity to organisms. PAHs originate mainly * Corresponding author phone: +86-20-85291421; fax: +86-2085290706; e-mail:
[email protected]. † Guangzhou Institute of Geochemistry. ‡ Graduate School, Chinese Academy of Sciences. 10.1021/es052060g CCC: $33.50 Published on Web 01/04/2006
2006 American Chemical Society
from anthropogenic sources such as combustion of fossil fuels and direct release of oil and oil products (2). Many of the PAHs with four or more rings are carcinogenic and mutagenic because of their metabolic transformation capability. OCPs, such as hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), are known endocrine-disrupting chemicals (3). HCHs and DDTs were widely used in China between the 1950s and the 1980s. The amounts of HCHs and DDTs produced in China were 4.9 and 0.4 million metric tons, respectively, accounting for 33% and 20% of the total worldwide productions (4). After the ban on their usage in 1983, the concentrations of OCPs have decreased significantly in foodstuffs (5) and human breast milk (6) during the past 10-20 years in China. By contrast, elevated levels of HCHs and DDTs were reported in seawater and sediments of Daya Bay of southern China (7), and newly discharged DDTs were frequently detected in air, water, and sediments in recent years (7-9). These results suggest that significant sources of DDTs remain active in China, and further research is needed to assess how these toxic chemicals are transported from sources to the surrounding regions. The Pearl River Delta (PRD), one of the most economically developed regions in China, contains rich waterways and numerous entrances to the coastal ocean. A large number of rainfalls occur annually due to its subtropical location and weather conditions. The average annual water flux from the PRD is 330 billion cubic meters (10), flowing into the Pearl River Estuary (PRE) and the adjacent South China Sea (SCS) (Figure 1). The rapid economic development and urbanization in the PRD during the last 2 decades have resulted in significant air and water pollution (11, 12). Contaminants from the PRD may also enter the PRE and the SCS through surface runoff, creating long-term adverse effects on the coastal resources. Previous studies have shown that sediments of the PRE were the main reservoir for OCPs (8, 13) and PAHs (8, 14). Although sediments of the northern SCS have not been surveyed, they are expected to be an important “sink” for pollutants generated in the PRD because continental shelf sediments in general have been identified as a significant destiny for polychlorinated biphenyls, another class of POPs (15). On the other hand, the year-round mild to high temperatures typically encountered in this subtropical region could turn the SCS sediments into an important source of pollutants to the global environment, as a result of the global distillation/fractionation mechanism (16). The role of oceans in dictating the global transport and fate of POPs has also been emphasized (17, 18). Therefore, the POPs originated from the PRD region may find their way to the global oceans via the SCS. This project aimed to carry out a survey of sediments of the PRE and the northern SCS to determine the concentration levels and spatial distribution of selected PAHs and OCPs. The sampling plan was so designed as to allow the determination of riverine influences to the continental shelf of the SCS and the sediment inventories of the target pollutants.
Materials and Methods Sample Collection. Sediments were collected from 33 sampling sites along four transects (Figure 1), of which 8 sites were located within the PRE and 25 around the northern SCS. Top 5-cm surface sediments were taken in July 2002 using a stainless steel grab sampler and placed in precleaned glass jars. The samples were cooled in a refrigerator (0 °C) during transport to the laboratory where they were stored at -20 °C until further analysis. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map of sampling sites. L1-L8 represent sites within the Pearl River Estuary, and others are sites around the northern South China Sea. Arrows 1, 2, and 3 indicate the direction and magnitude of water flows. Sample Extraction. The sample extraction procedures have been described in detail elsewhere (12). Briefly, a freezedried sediment sample was spiked with surrogated standards (PCB 15 for OCPs and five perdeuterated PAH compounds for PAHs) and was Soxhlet extracted with methylene chloride. Activated copper was added for desulfurization. The extract was concentrated and solvent-exchanged to hexane using a rotary evaporator. The hexane extract was subject to a 1:2 alumina/silica gel glass column for cleanup and fractionation. The column was eluted with 15 mL of hexane, and the eluate was discarded. The second fraction containing PAHs and OCPs was eluted with 70 mL of methylene chloride/hexane (30:70). Target analytes include 25 PAH compounds, i.e., naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, acenaphthylene, acenaphthene, fluorene, phenanthrene, methylphenanthrenes, dimethylphenanthrenes, trimethylphenanthrenes, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[i+k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, benzo[g,h,i]perylene, and retene, and 8 OCP components, including R-, β-, γ-, and δ-HCH, p,p′-DDD, -DDE, and -DDT, and heptachlor. Instrumental Analysis. Concentrations of PAHs were determined with a Hewlett-Packard (HP) 5890 Series II gas chromatograph (GC) equipped with an HP-5972 mass selective detector (MSD) and an HP-5 capillary column (50 m × 0.32 mm × 0.17 µm). OCPs were measured using an HP-5890 Series II GC, equipped with an electron capture detector (ECD) and an HP-5 silica fused capillary column (50 m × 0.32 mm × 0.17 µm). The chromatographic conditions, as well as the procedure for qualification and quantification of PAHs and OCPs, were the same as those detailed elsewhere (12). All concentrations were normalized to dry sediment weight and were not surrogate recovery corrected. 710
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Organic Carbon Measurement. Sediment total organic carbon (TOC) was measured with an Elementar Vavio EL III elemental analyzer (Hanau, Germany) after removal of carbonates with 1 N HCl. Quality Control and Quality Assurance. A degradation check solution obtained from Ultra Scientific (North Kingstown, RI) was analyzed daily and the extent of degradation had to be less than 15% before the analysis of DDTs could proceed. Surrogate standards were added to all samples prior to extraction to quantify the procedural recoveries. The mean surrogate recoveries in the sediment samples were 49.3 ( 14.2% for naphthalene-d8, 74.8 ( 9.8% for acenaphthened10, 79.4 ( 9.8% for phenanthrene-d10, 95.8 ( 12.6% for chrysene-d12, 86.3 ( 13.9% for perylene-d12, and 91.8 ( 12.5% for PCB 15. For each batch of 10 field samples, a method blank (solvent), a spiked blank (standards spiked into solvent), a matrix spike (standards spiked into pre-extracted sediment), a sample duplicate, and a National Institute of Standards and Technology (NIST) standard reference material (SRM 1941) sample were processed. The method blanks contained no detectable amounts of the target analysts except for naphthalene. The average recoveries in six spiked blanks and matrix spikes varied from 52% to 98% for PAHs (16 components) and from 44.4% to 103.9% for OCPs. Measured concentrations of target analytes in the NIST SRM 1941 were within 80-120% of the certified values. Statistical Analysis. Correlation analysis was performed using Origin 7.5 (OriginLab Corporation, Northampton, MA) to examine whether TOC was the major factor controlling the distribution of PAHs or OCPs in the study area. The same software was also used to estimate the far-reaching distances of PAHs and OCPs in the northern SCS. In addition, principal component analysis was conducted with SPSS 12.0 for Windows (SPSS Inc., Chicago, IL) to explore the similarity and difference in the entry modes of PAHs to the northern
TABLE 1. Concentrations (ng/g) and Inventories (Metric Tons) of Total PAHs (t-PAHs) and Total OCPs (t-OCP), as Well as Total Organic Carbon (TOC) and Diagnostic Ratios of Individual PAHs and OCPs, in Surface Sediments from the Pearl River Estuary and Northern South China Sea concentration site
inventory Fl/ DDT/ ttTOC MP/ (Fl + (DDD + DDD/ area ttPAHs OCPs (%) Pa Py)b DDE)c DDEc (km2)d PAHs OCPs
L1 446 L2 294 L3 854 L4 695 L5 950 L6 877 L7 780 L8 1100 subtotal A1 498 A2 255 A3 213 A4 283 A5 227 B1 251 B2 313 B3 311 B4 278 B5 226 B6 244 B7 171 C1 442 C2 480 C3 330 C4 272 C5 161 C6 138 D1 359 D2 206 D3 346 D4 299 D5 252 D6 380 D7 209 subtotal
1.59 0.76 3.12 2.63 2.19 3.95 2.97 1.87
0.06 0.15 0.94 0.79 0.92 0.93 0.94 1.02
1.57 1.19 1.53 1.30 1.89 1.42 1.76 1.69
0.46 0.45 0.46 0.46 0.47 0.47 0.47 0.50
0.32 0.28 0.36 0.29 0.25 0.40 0.31 0.07
1.49 1.72 1.93 1.96 1.00 1.93 1.59 1.37
95 143 191 239 286 334 298 430
3 3 12 12 20 22 17 35 126
0.01 0.01 0.04 0.05 0.05 0.10 0.07 0.06 0.38
2.42 0.98 0.70 1.04 0.63 1.49 1.20 0.65 0.96 0.64 0.45 0.46 1.47 1.47 0.87 0.92 0.86 0.18 2.07 f 1.14 1.27 1.15 0.75 0.70
0.70 0.47 0.29 0.37 0.36 0.37 0.64 0.58 0.37 0.54 0.57 0.33 0.73 0.70 0.58 0.36 0.44 0.25 0.61 0.56 0.66 0.64 0.63 0.64 0.38
1.06 1.13 0.97 1.13 0.96 1.00 0.64 0.95 1.08 0.98 0.75 0.92 0.89 0.89 1.06 1.00 0.90 1.07 0.77 0.88 1.03 0.95 1.07 0.86 0.84
0.55 0.61 0.64 0.63 0.62 0.58 0.57 0.63 0.65 0.63 0.62 0.57 0.57 0.64 0.63 0.63 0.62 0.61 0.62 0.60 0.53 0.62 0.60 0.64 0.59
0.65 0.21 0.28 0.30 0.44 0.40 0.32 0.41 0.36 0.76 0.36 0.60 0.49 0.51 0.57 0.57 0.20 e 1.05 f 0.42 0.81 0.31 0.15 0.33
1.31 1.14 0.51 0.68 1.13 0.85 0.82 0.67 0.43 1.07 0.38 0.64 1.13 1.19 1.01 1.01 0.99 0.59 1.51 f 1.18 0.84 1.01 0.98 0.43
620 620 725 1530 2220 495 561 602 619 1090 1860 2190 513 578 674 963 963 706 387 414 569 675 604 572 587
23 12 12 32 38 9 13 14 13 19 34 28 17 21 17 20 12 7 10 6 15 15 11 16 9 423
0.11 0.05 0.04 0.12 0.10 0.06 0.05 0.03 0.04 0.05 0.06 0.08 0.06 0.06 0.04 0.07 0.06 0.01 0.06 0 0.05 0.06 0.05 0.03 0.03 1.38
a MP/P ) methylphenanthrene/phenanthrene. b Fl/(Fl + Py) ) fluoranthene/(fluoranthene + pyrene). c Only p,p′-isomers were included. d Area of each compartment represented by a specific sampling site. e DDT was nondetectable. f All OCP target analytes were nondetectable.
SCS and further confirm the role of TOC in dictating the distribution patterns of sediment PAHs.
Result and Discussion Concentration Levels and Spatial Distribution. Concentrations of total PAHs (t-PAHs; sum of the 2-6-ring PAHs) in sediments of the northern SCS varied from 138 to 498 ng/g with an average of 286 ng/g, considerably lower than those found in the PRE sediments (294-1100 ng/g with an average of 749 ng/g; Table 1). The highest t-PAHs concentration was found in the sediment from station L8 with the highest TOC content (Table 1), likely because it situates within the riversea boundary zone where organic-rich, fine-grain suspended particles tend to deposit due to the effect of so-called “marginal filter” (19). The general level of PAHs in the study area was comparable to that of Masan Bay, Korea (20) but was lower than that of the northwestern Mediterranean Sea (21) and higher than those of the Cretan Sea and Chesapeake Bay (22, 23). Concentrations of total HCHs (t-HCHs; sum of R-, β-, γ-, δ-HCH isomers) ranged from 0.08 to 1.38 ng/g with a mean value of 0.36 ng/g (not shown), which were comparable to those of Kyeonggi Bay, Korea (0.5 are suggestive of wood and coal combustion (31). In the present study, MP/P ratios in the open sea sediments were mostly less than 1 with a range of 0.77-1.13, whereas they varied between 1 and 2 in the estuary sediments. Fl/(Fl + Py) ratios in the open sea sediments were mostly greater than 0.5 with a range of 0.53-0.65, whereas they ranged from 0.45 to 0.50 in the PRE (Table 1). This suggests that the majority of PAHs in open sea sediments was pyrogenically originated, while PAHs found in the PRE appeared to derive from both petrogenic and pyrogenic sources. Because various transformation pathways are possible for DDT residues (33, 34), the ratios of DDT/(DDD + DDE) and DDD/DDE (all are p,p′-isomers) can be used to trace the degree of DDT decomposition and to identify any recent input of DDT (24). For example, the compositional percentage of DDT declined gradually with a steady increase in the percentages of DDT metabolites if there was no new input of DDT (24, 35). The present study obtained DDT/(DDD + DDE) ratios in the range of 0.07-1.05 with a mean value of 0.41. By comparison, another sampling of suspended matter in the PRE obtained DDT/(DDD + DDE) values mostly greater than 1 in summer but all smaller than 1 in spring (36). These findings suggest that new inputs of DDTs are possible in summertime during the busy farming season, but the amount of newly discharged DDTs is minor relative to that already present in sediments. Ratios of DDD/DDE were generally larger than 1 in the PRE and nearshore areas of the northern SCS, whereas they were largely smaller than 1 in offshore areas (Table 1). This suggests a different degree of persistency for the metabolites (DDE and DDD) of DDTs. DDE tends to be more resistant to further degradation than DDD in the environment (35, 37). Implications of Diagenetic Processes. High concentrations of perylene have been reported in anoxic aquatic sediments with high biological productivity and often have been associated with terrestrial inputs from rivers and estuaries (38). Relative concentrations of perylene higher than 10% of the total penta-aromatic isomers indicate a probable diagenetic input, whereas those less than 10% suggest a pyrolytic origin of the compound (39). In our study area, perylene occurred at elevated levels (21.7-192 ng/g) and was the most abundant component in all but station L1 (Figure 2). In addition, the percentage of perylene relative to the penta-aromatic isomers was at least 30% (Figure 3), indicating a diagenetic impact in our study area. Low perylene concentrations at station L1 may have been resulting from the absence of natural precursors of perylene due to the extremely low TOC content in the same sample (0.06%; Table 712
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FIGURE 3. Concentrations of perylene and its relative abundances to the sum of 5-ring PAHs in surface sediments from the Pearl River Estuary and the northern South China Sea. L1-L8 represent sites within the Pearl River Estuary, and the other labels indicate sites around the northern South China Sea.
FIGURE 4. Principal component (PC) loadings for PAHs in the northern South China Sea surface sediments. PC1 and PC2 explained 61% and 14%, respectively, of the data variation. 1). The lowest perylene percentage in station L8 may be a result of the high contribution of anthropogenic PAHs. In addition, progressive seaward declines of the concentrations and abundances of perylene were clearly present for all but transect D in the northern SCS (Figure 3). Principal component analysis showed that perylene clustered with TOC and all other target PAH compounds except for acenaphthylene, acenaphthene, fluorene, naphthalene, and methylnaphthalenes (Figure 4). This indicates a similar entry mode for perylene and a majority of the target PAH compounds in sediments of the northern SCS, and TOC was a major factor controlling the distribution of PAHs with high log Kow value (from phenanthrene). Decreasing concentrations of t-PAHs or t-OCPs were found with increasing distance from the PRE toward the open sea in the four transects except for PAHs in transect D (Table 1). This trend was unlikely caused by perylene, because replotting of total PAHs without perylene versus the seaward distance (not shown) also exhibited similar trends as those for t-PAHs. These results suggest that riverine and estuarial discharge, instead of atmospheric deposition (both gas and particle fluxes) and other routes, was the main source of PAH inputs to the northern SCS. The nonprogressive distribution of perylene in transect D was probably due to the influence of the southwestward ocean currents that converge with water from the PRE. Advective Transport. The decrease in the concentration of t-PAHs or t-OCPs with increasing distance from the PRE
toward the open sea (Table 1) suggests an advective transport mechanism at work. The advective transport rates of PAHs and OCPs on the continental shelf, expressed as the concentration variability with the seaward distance, were estimated with a linear regression analysis of the correlation between the concentration and seaward distance data. The aim was to evaluate the potential environmental influence of the riverine input from the PRD region on the water body of the northern SCS. Although all data were analyzed, only the results for transects B and C are presented herein as they exhibited high correlation values. Slope values gained by the linear regression analysis were the advective transport rates (ng/g km) of the selected compounds (Figure S1 in the Supporting Information). The background level (63 ng/g) of t-PAHs was estimated from previous studies in the eastern Mediterranean Sea (22, 40) where the PAHs were derived mainly from atmospheric deposition and contained similar individual PAH components as those in the present study. The background level of OCPs was set at zero. The riverine input influences reached approximately 124276 and 143-172 km from the end of the PRE for PAHs and OCPs, respectively. The range of riverine runoff influences on the northern SCS may well have been underestimated, because only the southward advection was assessed. Influenced by the Coriolis force, monsoon, and other factors, water from the PRE and other PRD outlets is normally diverted into three subflows (arrows 1, 2, and 3 in Figure 1). The intensity and direction of these flows may vary seasonally (Figure S2). Apparently, a large portion of water from the PRE flows toward the west and east, carrying large amounts of pollutants to areas other than the sampling region. Mass Inventory. To assess the potential of sediments as a new source of contamination to the oceanic environment, the mass inventories of t-PAHs and t-OCPs in the present sampling region were estimated. The PRE and the northern SCS were divided into 8 and 25 compartments, respectively. Each of the 33 sampling sites was allocated in the center of one compartment, and the sediment concentrations from this site were chosen as representatives for the entire compartment. The inventory (I, in metric tons) was calculated by the following equation
I ) ΣkCiAidF
(1)
where Ci is the sediment concentration at site i (ng/g), Ai is the area of the compartment represented by site i (km2), d is the thickness of sediment sampled (cm), F is the average density of the dry sediment particles (g/cm3), and k is a conversion factor. With an assumed sediment density of 1.5 g/cm3 and a sediment thickness of 5 cm, the inventories were 126 and 423 metric tons for t-PAHs and were 0.4 and 1.4 metric tons for t-OCPs within the PRE and the northern SCS, respectively. The inventories of both t-PAHs and t-OCPs in the northern SCS sampling area appeared to be 3 times greater than those in the PRE area, but the sediments sampled actually reflected deposits of different time intervals because of the different deposition rates in the two areas. Sedimentary rates in marine environments varied from 0.1 to 0.3 cm/yr (15, 21), and a median value of 0.2 cm/yr was used in the present study. Consequently, the inventories estimated for the northern SCS represent approximately 25 years of deposition. A sedimentary rate of 1 cm/yr is a reasonable estimate for the PRE from the results of a previous study (4). Therefore, the mass inventories in the PRE represent deposits in the last 5 years. If the same time interval was used to estimate the inventories in the PRE sediments and the vertical concentration fluctuation in sediments was neglected, the inventories of t-PAHs and t-OCPs in the PRE were 631 and 1.9 metric tons, respectively. The inventory of PAHs in the sampling
region of the northern SCS is slightly larger than those in the western and eastern Mediterranean basins (21) with areas 3 and 4.6 times that in the present study, respectively. The inventory of PAHs in the northern SCS could be much higher if the whole northern SCS region was taken into account. Presumably, sediments of the PRE and the northern SCS may have served as a significant reservoir of PAHs from the PRD region and at the same time a potential source of PAHs to the global oceans. Besides riverine inputs, atmospheric deposition may also play a role in transporting PAHs and OCPs to the northern SCS. However, assessment of the aerial transport is difficult because atmospherically depositional flux data for the northern SCS are scarce. A previous study obtained the average annual aerial deposition flux of PAHs (dry + wet) at 85 ng/m2 day off the coast of Hong Kong (41). In certain situations, the role of gas flux is at least as important as dry particle deposition for delivering PAHs to the water column (42, 43). Therefore, with a total air-water exchange flux (170 ng/m2 day) twice the aerial deposition flux, the aerial flux was estimated to contribute about 11% of the PAHs masses to sediments of the furthest site B7 that is supposed to receive the least riverine runoff influences. Apparently, the aerial flux contribution to the sediments of the northern SCS is minor compared to that of riverine inputs.
Acknowledgments This research was financially supported by the Chinese Academy of Sciences (Nos. KZCX2-212 and KZCX3-SW-429), the National Basic Research Program of China (No. 2003CB415002), and the National Science Foundation of China (No. 40525012). Financial support of E.Y.Z. by the “One Hundred Talents” Program of the Chinese Academy of Sciences is particularly appreciated. The authors thank the crew of Experiment 2 administrated by the South China Sea Institute of Oceanography, Chinese Academy of Science for field support in the northern SCS and Mr. T. S. Xiang for his assistance in the GC/MS analysis.
Supporting Information Available Information about the variability of PAH and OCP concentrations with the seaward distance from the PRE and the distribution of fresh water from the PRE into the northern SCS. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review October 18, 2005. Revised manuscript received November 22, 2005. Accepted November 29, 2005. ES052060G
