Water Research 36 (2002) 1457–1468

Distribution and sources of polycyclic aromatic hydrocarbons in the sediment of a sub-tropical coastal Gene J. Zheng, Ben K.W. Man, James C.W. Lam, Michael H.W. Lam*, Paul K.S. Lam

Centre for Coastal Pollution and Conservation, Department of Biology & Chemistry, City University of , 83 Tat Chee Avenue, Kowloon, Hong Kong

Received 10 October 2000; received in revised form 25 June 2001; accepted 23 July2001

Abstract

Sediment cores (0–35 cm below surface) from twelve sampling stations in the Mai Po and Inner Deep BayRamsar Site of Hong Kong were retrieved in the period March–December 1999. Vertical profiles of 15 priority polycyclic aromatic hydrocarbons (PAHs) in each sediment core were determined. Ranges of total PAH concentration, [SPAH], in the wetland sediment were 0.18–0.83 ðx% ¼ 0:36Þ mg=g dried sediment (mudflats) and 0.63–0.96 ðx% ¼ 0:77Þ mg=g dried sediment (). A decreasing trend in depth averaged [SPAH] was observed from the landward end towards the seaward end of the Marshes. On the mudflats, vertical profiles of the PAHs were quite uniform. At the fringe of the Mai Po mangroves, significantlyhigher concentration of all PAHs was observed at the upper 0 to À8 cm layer. No significant difference in the distribution patterns of the 15 priorityPAHs in summer and winter was observed. This indicates that distribution of PAHs in the sediment of the is not verysensitive to sub-tropical climatic changes of the region. Two PAH isomer ratios, [Phen]/([Phen]+[Anthra]) and [Pyrene]/([Pyrene]+[Fluoran]), were used to identifypotential sources of PAH contamination in the wetland. Results revealed that local deposition is a more important source than long-range atmospheric transportation. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: PAHs; Coastal ; Sediment; Mai Po; Hong Kong

1. Introduction Marshes, the sixth largest coastal wetland in China, has aroused increasing scientific and public attentions Coastal wetlands are among the most biologically since its being formallydesignated a Wetland of important and productive ecosystems on earth. International Importance under the Theyprovide food, habitats and spawning grounds to in 1995 [8–11]. Similar to situations faced byother a wide varietyof infauna and act as important coastal wetlands that are close to denselypopulated staging areas for migratorywaterfowl and other birds cities and/or industrial facilities, the Mai Po Marshes is [1–3]. Coastal wetlands also possess high socioeconomic suffering from increasing environmental stresses due to values. Theyplayimportant roles in the protection of the rapid urbanization and industrialization of the shorelines and banks from erosion and can be region. To identifyand assess the risk caused by used to treat domestic and industrial wastewaters anthropogenic pollution on the ecosystem of the Mai [4–7]. In Hong Kong, conservation of the Mai Po Po Marshes, a detail understanding of the sources and distribution of all contaminants in the local environment *Corresponding author. Tel.: +852-2788-7329; fax: +852- is necessary. For hydrophobic organic pollutants, 2788-7406. sediment is the most important phase where this E-mail address: [email protected] (M.H.W. Lam). information can be obtained as these pollutants

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0043-1354(01)00363-3 1458 G.J. Zheng et al. / Water Research 36 (2002) 1457–1468

Fig. 1. The Mai Po and Inner Deep BayRamsar Site of Hong Kong. have high tendencyin associating with water- and air- hydrocarbons (PAHs) in the sediment of the Mai Po borne particulates which will eventuallysettle onto the Marshes and to evaluate the importance of potential sediment. The objective of this work is to studythe sources and routes bywhich PAHs reach the coastal spatial and vertical distribution of polycyclic aromatic wetland. G.J. Zheng et al. / Water Research 36 (2002) 1457–1468 1459

2. Experimental Marshes are two satellite industrial/residual towns, namelyYuen Long and Tuen Mun, respectively,of 2.1. Site description Hong Kong. Population in these satellite communities has alreadyexceeded 0.7 million. Local traffic is heavyin The Mai Po Marshes is located at the northwestern these areas as theyare major import/export gateways part of the , Hong Kong, and is the between Hong Kong and Mainland China. The Shan largest remaining wetland system in southern China. Pui River and its branches, which pass through these The wetland is in the vicinityof the Inner Deep Bay, towns, are moderatelypolluted byindustrial and which is located on the eastern side of the Pearl River municipal wastewaters. The metropolitan region of Estuary. The marshes is also fed by two smaller rivers, Hong Kong with over 3 million population is located namelythe River and . It on both sides of the Victoria Harbour, some 24 km comprises some 180 ha of intertidal mudflats, 85 ha of southwest of the Mai Po Marshes. sub-tropical mangroves as well as a series of man-made fishponds (Fig. 1). Tidal regime of the wetland is diurnal 2.2. Sampling and the mudflats and mangroves are submerged during flood tide. Ambient temperature ranges from 30731Cin Sediment core samples were retrieved from 12 summer (April–August) to 10731C in winter (October– sampling stations in the Mai Po Mudflat and January). Rainfall is mainly concentrated in summer. areas within the period March–December 1999 (Fig. 2). Nature of the sediment of the wetland is mainly clayey, Distribution of the sampling stations was limited bythe with o63 mm particle size fraction constituting 43.3– accessibilityof the intertidal mudflats and the mangrove 94.5% (w/w) of total sediment. Deeper sediments forest. Stations 1–4 represent a transact extending from appeared to contain slightlyhigher silt & clay the edge of the Mai Po mangroves to the Inner Deep (o63 mm particle size) content. Sediment in the man- Bay. Station 5 was also located close to the Inner Deep grove forest also contains slightlyhigher silt & clay Baynear the mouth of Shan Pui River. Stations 6–9 fraction than that of the intertidal mudflats. Redox- represent another transact extending from the north- imorphologyof the wetland sediment was studied by eastern part of the mudflats near the Shenzhen River to analyzing the amount of dissolved iron (Fe) and the southwestern part near Tsim Bei Tsui. Stations 10– manganese (Mn) in the sediment porewater [11,12]. 12 were within the mangrove area. Two sampling The dissolved [Fe] and [Mn] profiles suggested that the exercises were undertaken, sampling at Stations 1–5 redoximorphologyof the Mai Po Marshes are quite was carried out in summer while sampling at Stations 2, uniform and do not fluctuate much throughout the year 3, 6–12 was carried out in winter. Stations 2 and 3 were despite the marked summer/winter ambient temperature sampled in both exercises in order to check for possible differences. In the intertidal mudflats, an oxic layer is seasonal discrepancyin PAH profiles. No significant found in the upper 0 to À8.5 cm of the surface sediment. difference in the PAH profiles of both the summer and From À8.5 to À20.0 cm, sediment becomes sub-oxic winter samples at these two stations were observed where reduction of oxides-hydroxides of [Fe] and [Mn] is (more discussions are given in later sections). This observed. Beyond À20.0 cm, sediment is characteristi- suggested that concentrations and distribution of PAHs callyanoxic. For the mangrove sediments, the oxic layer in the Mai Po Marshes do not fluctuate much is either absent or overlapped with the sub-oxic layer throughout the year. [13,14]. The sub-oxic layer begins immediately beneath At Stations 1–9, two sediment cores (30–35 cm) were the sediment–water interface. retrieved per station using a 200 i.d. sediment corer with Mai Po Marshes is surrounded bydeveloped com- polyvinychloride (PVC) liners. All sampled core sedi- mercial and industrial communities. The Shenzhen ments were immediatelycapped, stored in upright Special Economic Zone of China is located about 6 km position and transported back to the laboratorywithin north to the wetland while the Shekou Special Economic 2 h. Each sediment cores were sectioned into regular Zone of China is located about 12 km to the west. Both 8 cm intervals for the first 24 cm, then a 6 cm interval and economic zones have undergone rapid industrialization a 5 cm interval, if the length of the sediment core and urbanization in the recent decade with hundreds permitted. Sectioned core sediment samples were care- and thousands of small to medium size factories for fullytransferred into pre-cleaned containers, and stored manufacturing, electroplating, dyeing and textile, me- at the À101C for subsequent analyses. At Stations 10– tallurgy, etc., already established. The Shenzhen River 12, onlythe surface 0–8 cm sediment was sampled using and the Inner Deep Bayhave alreadybeen heavily the core sampler. Again, two samples per station were polluted byindustrial effluents. Automobile emissions retrieved. Two coastal water samples were collected and fossil fuel combustion constitutes a significant, if during flood tide on the edge of the Mai Po Mudflats not major, source of PAHs in the local environment. To towards the Inner Deep Bayin September 1999. the south (6 km) and southeast (12 km) of the Mai Po Suspended particulates in each water sample were 1460 G.J. Zheng et al. / Water Research 36 (2002) 1457–1468

Fig. 2. Locations of sediment core sampling stations.

obtained byfiltering 5 l of the coastal water through a methylene chloride/n-hexane mixture (20 : 80 v/v). The membrane filter of 0.45 mm pore size. The average latter fraction was collected and volume reduced to suspended solid concentration of the water samples 100 ml for gas chromatographic analysis. was 85712 mg/l. The suspended particulates collected A GC-MS system consisted of a Hewlett Packard were freeze-dried immediatelyand were kept at À101C 5890 series II gas chromatographywith a 30 m Ultra-2 until analysis. fused silica capillarycolumn (0.2 mm diameter and 0.33 mm thickness film; 95% dimethyl- 5% diphenyl- polysiloxane) connected to a Hewlett Packard 5970 2.3. PAH analysis series Mass Selective Detector was used for PAH determination. Selected ion monitoring (SIM) mode Analytical procedures essentially followed those was employed for identification and quantification of methods reported byZheng and Quinn [15] and Zheng the priorityPAHs [17]. Standard reference materials and Richardson [16]. Before each extraction, m-terphe- (HS-6, NRC, Ottawa, Canada) as well as pristine nyl was spiked into 30.0 g of freshly thawed sediment. sediment samples spiked with known concentrations of For suspended particulate analysis, m-terpheyl was PAHs were used for analytical method validation. spiked into accuratelyweighted, fleshlythawed particu- Practical quantitation limits and recoveries for all late sample. The spiked sample was allowed to age for PAH analytes were 0.01 ng/g (dried sediment) and 1 h before refluxed with 120 ml methanol for 2 h. After >85%, respectively. Variation in PAH concentration cooling to room temperature, the mixture was filtered of replicated samples was between 5.8% and 24.9%. through pre-cleaned glass filter paper, and the aqueous- methanol solution was re-extracted with 3 Â 30 ml2 distilled n-hexane. The n-hexane extracts were pooled and volume reduced to approximately1 ml under 3. Results and discussion reduced pressure. The extract was cleaned up bycolumn chromatographyusing a column of 1 g activated copper Vertical profiles of the 15 priorityPAHs in the coastal powder on top of 6 g of activated silica. The column was waters and sediments of the Mai Po Marshes are first eluted with 15 ml n-hexane followed by20 ml of a tabulated at Table 1. Resolution of the vertical profiles Table 1 Vertical profiles of the 15 priorityPAHs in the Mai Po sediment

Stationa Averaged sedimentaryPAH b,c content (in ng/g-dried sediment) Nap Acen Acen0 Fluoren Phen Anthra Fluoran Pyrene Chry B[k]f B[b]f B[a]p Indeno Dibenzo B[ghi]p

1_a n.d.d 9.4 1.2 8.1 32.7 102.4 17.9 82.2 14.3 219.5 55.9 103.2 85.2 31.0 69.3 1_b n.d. 3.0 n.d. 2.6 19.3 61.9 24.5 59.1 10.2 111.9 32.9 103.3 17.3 4.2 38.4 1_c n.d. 9.4 1.2 12.7 82.3 49.0 29.1 96.9 10.2 58.2 37.7 60.0 34.2 8.4 41.8 2_a n.d. 5.0 4.0 5.1 42.1 46.8 42.9 89.3 5.0 91.2 9.2 33.9 11.1 1.7 11.0 2_b n.d. 4.7 n.d. 4.4 23.5 35.9 48.6 90.6 15.5 104.9 12.4 45.3 17.8 2.2 16.4 2_c n.d. 10.2 1.3 3.0 25.5 34.3 42.4 65.6 9.3 109.0 11.6 31.4 14.2 2.0 10.9 2_d n.d. 5.5 n.d. 2.6 21.4 26.1 43.2 91.1 10.3 103.0 10.3 40.7 15.0 2.5 9.3

2_e n.d. 13.8 0.8 8.8 47.4 31.4 53.2 97.5 6.9 99.6 13.5 50.0 6.4 0.4 7.4 Zhen G.J. 3_a 0.9 16.6 2.5 8.7 22.5 30.7 56.2 96.9 7.0 92.9 9.7 39.1 9.6 7.3 11.8 3_b n.d. 6.4 n.d. 2.3 19.5 34.0 53.6 95.5 10.3 86.5 11.3 39.3 13.7 5.6 13.7

3_c n.d. 13.0 n.d. 0.4 20.5 34.5 37.4 89.7 2.9 80.5 7.3 28.4 8.3 3.5 6.8 g

3_d n.d. 15.4 0.1 8.5 18.0 24.1 41.6 92.7 9.0 82.2 8.7 29.2 9.9 5.0 7.6 1457–1468 (2002) 36 Research Water / al. et 3_e n.d. 12.8 1.3 5.6 27.0 35.0 44.8 91.1 4.0 92.5 13.2 31.6 18.2 3.1 12.0 4_a n.d. 6.3 n.d. 6.1 29.9 28.5 73.4 75.1 10.8 115.3 12.1 46.4 15.8 2.5 3.0 4_b FFF F F F F F FFFFF F F 4_c n.d. 0.5 0.2 2.7 19.9 11.0 3.2 74.6 0.6 69.1 6.6 24.4 4.9 0.6 4.2 4_d FFF F F F F F FFFFF F F 4_e n.d. 14.9 1.0 9.8 26.7 35.4 46.9 97.6 8.4 98.9 10.6 40.9 5.8 2.1 1.6 5_a n.d. 12.2 n.d. 17.7 43.9 17.5 24.7 47.2 2.7 21.2 0.0 24.7 n.d. n.d. n.d. 5_b FFF F F F F F FFFFF F F 5_c n.d. 2.1 1.1 9.5 37.7 24.8 2.4 56.7 2.3 35.1 6.3 22.3 4.9 n.d. n.d. 5_d FFF F F F F F FFFFF F F 5_e n.d. 2.5 0.9 4.8 33.8 20.3 4.0 58.9 2.6 25.0 4.5 22.4 n.d. n.d. 1.6 2_a0e n.d. 5.8 2.5 6.9 37.8 17.4 77.7 108.7 14.9 47.1 14.7 17.2 4.9 14.2 20.1 2_b0 FFF F F F F F FFFFF F F 2_c0 1.0 2.8 4.2 8.9 41.7 11.8 72.1 122.0 13.2 27.0 11.5 8.4 2.8 9.5 11.4 2_d0 FFF F F F F F FFFFF F F 2_e0 0.5 2.6 3.3 8.8 38.8 7.6 35.7 87.0 12.9 22.1 9.1 1.6 2.5 10.2 7.4 3_a0 0.8 5.4 5.4 17.4 33.9 23.3 67.3 96.4 32.5 29.7 17.3 7.4 1.7 15.0 10.5 3_b0 FFF F F F F F FFFFF F F 3_c0 0.6 6.8 6.5 18.1 32.7 18.1 54.5 103.9 37.8 25.8 13.2 9.9 1.6 15.8 13.1 3_d0 FFF F F F F F FFFFF F F 3_e0 0.7 4.6 3.7 23.6 34.5 19.1 63.7 101.6 36.7 23.6 17.9 9.3 3.0 13.8 14.7 6_a 0.5 3.3 1.8 4.5 20.6 10.5 24.7 80.1 22.5 22.8 18.1 8.8 5.2 8.1 14.3 6_b FFF F F F F F FFFFF F F 6_c 0.3 4.2 3.0 5.1 23.8 13.7 27.3 76.6 19.2 20.9 14.8 9.0 7.2 10.8 16.5 6_d FFF F F F F F FFFFF F F 6_e 0.4 3.8 2.9 4.8 23.5 11.9 27.1 82.2 20.1 18.2 18.1 9.9 6.9 9.3 14.0

7_a 0.7 5.2 2.8 9.5 44.3 28.2 54.5 95.2 27.0 22.9 18.4 7.7 11.9 6.2 15.2 1461 7_b FFF F F F F F FFFFF F F 1462

Table 1 (continued)

Stationa Averaged sedimentaryPAH b,c content (in ng/g-dried sediment) Nap Acen Acen0 Fluoren Phen Anthra Fluoran Pyrene Chry B[k]f B[b]f B[a]p Indeno Dibenzo B[ghi]p

7_c 0.5 5.7 2.5 9.3 45.8 31.3 47.0 95.8 24.1 23.9 17.8 9.4 12.4 13.2 15.2

7_d FFF F F F F F FFFFF F F Zhen G.J. 7_e 0.9 4.4 3.9 11.7 37.1 37.7 41.1 92.3 39.1 28.7 17.9 8.3 10.6 8.6 12.0 8_a 0.5 2.8 2.7 13.4 50.4 22.0 56.6 94.7 27.1 19.2 17.4 15.2 5.4 8.5 15.2 FFF F F F F F FFFFF F F

8_b g

8_c 0.1 1.6 3.9 11.1 45.4 20.0 52.0 93.5 26.5 24.7 12.3 9.5 4.1 12.8 17.2 1457–1468 (2002) 36 Research Water / al. et 8_d FFF F F F F F FFFFF F F 8_e n.d. 1.4 2.8 9.5 40.6 18.5 47.6 87.7 26.7 19.9 10.0 7.8 4.1 9.6 11.3 9_a 0.6 4.6 1.4 16.9 55.0 18.3 36.6 101.4 38.9 23.4 18.8 13.5 8.4 16.3 21.4 9_b FFF F F F F F FFFFF F F 9_c 0.6 3.5 3.1 12.1 43.2 10.8 36.6 99.1 32.0 20.0 19.3 13.1 9.9 10.4 21.3 9_d FFF F F F F F FFFFF F F 9_e 0.7 4.3 1.0 13.9 55.0 16.2 36.0 95.5 28.1 19.9 17.4 12.2 9.9 11.2 18.2 10_a 38.6 15.5 0.6 16.5 60.4 64.3 210.1 281.4 114.8 49.3 51.3 13.1 7.7 19.4 15.2 11_a 65.8 12.0 0.4 17.2 75.2 37.2 76.0 126.9 77.1 41.2 55.1 11.7 6.5 3.6 20.6 12_a 58.6 15.6 0.7 19.7 54.6 55.0 72.1 122.9 86.3 74.4 79.5 27.3 10.4 4.1 35.4 Deep_Bay87.1 307.1 100.0 484.7 830.6 483.5 428.2 724.7 165.9 215.3 201.2 429.4 149.4 35.3 74.1

a The letter after the station number represents the depth section at which sediment samples were located: a (0 to À8.0 cm); b (À8.0 to À16.0 cm); c (À16.0 to À24.0 cm); d (À24.0 to À30.0 cm); e (À30.0 to À35.0 cm). b Nap=Naphthalene; Acen=Acenaphthylene; Acen0=Acenaphthene; Fluoren=Fluorene; Phen=Phenanthrene; Anthra=Anthracene; Fluoran=Fluoranthene; B[k]f=Ben- zo(k)fluoranthene; B[b]f=Benzo(b)fluoranthene; B[a]p=Benzo(a)pyrene; Indeno=Indeno(123–cd)pyrene; Dibenzo=Dibenzo(1256)anthracene; B[ghi]p=Benzo(ghi)perylene. c n ¼ 4: d n.d.=below practical quantitation limit. e Stations 20 and 30 stand for samples obtained from Stations 2 and 3, respectively, during the winter sampling exercise. G.J. Zheng et al. / Water Research 36 (2002) 1457–1468 1463

Fig. 3. Total concentration of the 15 priorityPAHs ([ SPAH]) of each depth section of sediment cores sampled from the Mai Po Mudflats. For Stations 2 and 3, data obtained in the summer were used.

in surface sediment was limited to 5–8 cm intervals due (Station 6). Fig. 4 shows the depth averaged [SPAH] to practical consideration to ensure enough samples along the two transects on the Mai Po Marshes. The within each section for duplicate analysis. Also because overall range of [SPAH] in the mudflat sediment was of practical constraints, the 8–16 cm and 24–30 cm 0.18–0.83 (x% ¼ 0:36) mg/g (dried sediment). The uniform sediment core sections of Stations 4–9 were not vertical PAH profiles in most stations on the mudflats analysed. Judging from the uniform vertical profiles of indicate efficient mobilization and transportation of most PAHs in the Mai Po sediment, omission of these PAHs (and probablyother hydrophobicorganic pollu- two depth sections should not impose anysignificant tants) in the upper 0–35 cm sediment layer. This is most uncertaintyto all our subsequent interpretations. The probablycaused bythe effective vertical mixing of vertical distribution of PAHs in the sediment of the Mai surface sediments in the mudflats due to tidal resuspen- Po Mudflats is more clearlydepicted in Fig. 3 where sion. Dissolved organic matters (DOM) in the porewater averaged total concentration of the 15 priorityPAHs of the organic rich wetland sediments mayalso play ([SPAH]) in each depth section is shown. The [SPAH] some part in remobilizing and transporting hydrophobic profile at all stations except Station 1 was quite uniform. organic pollutants along the sediment column [18,19], At Station 1, which is closest to the fringe of the Mai Po although recent studies have shown that PAHs exhibit mangroves, the surface 0–8 cm sediment was found to more hydrophobicity than expected from their octanol– contain significantlyhigher concentration of all the 15 water partition coefficient, KOW [20–22]. Sediment PAH priorityPAHs. A general declining trend of [ SPAH] is concentration in the Mai Po mangroves was significantly observed in all depth sections along the transect from higher than that of the mudflats. Range of [SPAH] in the landward end (Station 1) towards the seaward end the mangrove sediment was 0.63–0.96 (x% ¼ 0:77) mg/g (Stations 4 and 5) of the Mai Po Mudflats. [SPAH] (dried sediment). Surface sediment layer near the fringe along the northeast–southwest transect did not varyas of the mangrove forest (Station 10) contained the much as the east–west transect. Nevertheless, [SPAH] of highest [SPAH]. The high PAH concentrations in the the northeastern part of the mudflats (Stations 7–9) surface sediment layer with respect to deeper sediments appeared to be higher than that of the southwestern part at the fringe and within the Mai Po mangroves (Stations 1464 G.J. Zheng et al. / Water Research 36 (2002) 1457–1468

Fig. 4. Depth averaged [SPAH] along the two transects on the Mai Po Marshes. Data of Station 20 were obtained from the winter sampling at Station 2.

1, 10–12) suggest that rate of input of PAHs to the to see whether seasonal change of ambient temperature upper sediment in these areas is higher than the rate of and local hydrology would affect the distribution of mobilization and transport along the sediment column. PAHs in the coastal wetland. Numerous studies have Comparing with total PAH content of coastal sediments shown seasonal fluctuation of PAHs in aquatic environ- in the rest of Hong Kong waters, [SPAH] of the Mai Po ment due to change in intensityof temperature Marshes was 50–130 times higher than that measured at dependent biodegradation processes [23–25]. Fig. 5 the pristine site at Kat O (x%½SPAH¼7:25 ng/g dried compares the summer and winter depth averaged sediment) but close to those in industrial areas such as distribution patterns of the 15 priorityPAHs at Stations Kwun Tong, Tsing Yi in the Victoria Harbor and Tolo 2 and 3. The two distribution patterns were comparable, Harbour (0.50–0.63 mg/g dried sediment) [16]. Contents especiallyfor low-molecular-weight PAHs despite their of high-molecular-weight PAHs, such as pyrene, ben- higher susceptibilityto bio- and photodegradation. This zo[k] fluoranthene and benzo[a] pyrene, in Mai Po indicates that distribution of PAHs in the sediment of sediment were exceptionallyhigh. For instance, depth Mai Po Marshes is not verysensitive to sub-tropical averaged concentration of benzo[a] pyrene at Station 1 climatic changes of the region. This maybe attributed to (88.8 ng/g dried sediment) was over 40 times higher than the persistence of PAHs in wetland sediments with that in the sediment of Inner Deep Bay(2.15 ng/g dried generallysub-oxic to anoxic redox conditions and sediment) and even over 9 times higher than that in relativelyhigh silt and clayand organic carbon contents. coastal sediments of the most polluted To Kwa Wan An important point that needs to be addressed in Wharf (9.31 ng/g dried sediment). assessing the environmental stresses imposed byPAHs As sediment samples were retrieved from the Mai Po to the Mai Po Marshes is the identification of their Marshes both in summer and in winter, it is interesting origin and potential sources. Numerous studies have G.J. Zheng et al. / Water Research 36 (2002) 1457–1468 1465

Fig. 5. Distribution patterns of PAHs at Stations 2 and 3 on the Mai Po Mudflats in summer and winter. demonstrated the usefulness of PAH isomer ratios in Sicre et al. [31] have shown that these high-molecular- source identification [20,26–29]. Pairs of PAH isomers weight PAHs, due to their low water solubility, low that are mostlyused for this purpose include phenan- microbal degradation rates and high particulate affinity threne/anthracene (tricyclic aromatics; MW 178 Da), compared to low-molecular-weight PAHs, are typically pyrene/fluoranthene (tetracyclic aromatics; MW 202 found in atmospheric particulates and urban aerosols. Da) and benzo[e] pyrene/benzo[a] pyrene (pentacyclic These airborne particles can act as carriers for long- aromatics; MW 252 Da). Since benzo[e] pyrene was not range transportation of PAHs. In order to assess determined in this study, only the former two ratios are whether local deposition or long-range transportation used here. As phenanthrene is a thermodynamically is a more important source of pyrogenic PAHs in the more stable tricyclic aromatic isomer than anthracene, Mai Po Marshes, the tricyclic and tetracyclic PAH the phenanthrene/anthracene ratio has been frequently isomer ratios, namely: [Phen]/([Phen]+[Anthra]) and used to differentiate PAHs of petrogenic origin from [Pyrene]/([Pyrene]+[Fluoran]) (where Phen=phenan- those of pyrogenic origin in the environment [26,30,31]. threne, Anthra=anthracene and Fluoran=fluor- Petrogenic PAHs are generallycharacterized bya high anthene), of the Mai Po sediment are compared with ratio (normally>15) while those of pyrogenicorigin those of coastal sediments [16] and airborne particulates possess lower ratio. The phenanthrene/anthracene ratio [33,34] in various other parts of Hong Kong (Fig. 6). of sediments in the Mai Po Mudflats and mangroves was Depth averaged concentrations of the PAH compounds found to range from 0.31 to 5.11. The relativelylow in the Mai Po sediment were used and the ([Phen]/ isomer ratio indicates that PAHs in the Mai Po Marshes ([Phen]+[Anthra]) ratio of the Mai Po sediments and are mainlygenerated bycombustion processes. The high waters was found to range from 0.39 to 0.77 (x% ¼ 0:58) composition of tetra- and pentacyclic aromatic com- while that of [Pyrene]/([Pyrene]+[Fluoran]) was found pounds in the PAH profile further suggests that PAHs to range from 0.62 to 0.84 (x% ¼ 0:68). The two PAH contamination in the Mai Po region is related to PAHs isomer ratios for coastal sediments in various other parts rich atmospheric particulates produced byanthropo- of Hong Kong were from 2 Â 10À3 to 0.68 (x% ¼ 0:28) genic combustion processes. Muel and Saguem [32] and and from 0.01 to 1.00 (x% ¼ 0:61) respectively, while 1466 G.J. Zheng et al. / Water Research 36 (2002) 1457–1468

Fig. 6. A plot of tricyclic vs. tetracyclic PAH isomer ratios in coastal sediments and airborne aerosols in Hong Kong. those for airborne particulate sampled in the urban activities in the vicinityof the Mai Po Marshes are the atmosphere of Hong Kong were from 0.27 to 0.42 most probable local sources of pyrogenic PAHs. The (x% ¼ 0:34) and from 0.24 to 0.48 (x% ¼ 0:38) respectively. PAH contaminants thus produced can reach the Mai Po As shown in Fig. 6, two clusters of data points can be sediment via direct atmospheric deposition (dryand wet) identified. Cluster I mainlycontains sediment stations and sedimentation of PAH rich suspended matters within the Mai Po Marshes as well as suspended generated bydeposition of airborne particulates onto particulates from Inner Deep Bay(DB SP), while nearbywater bodies, e.g. Inner Deep Bay.The latter Cluster II is composed of sediments from other parts route maybe significant, as the Inner Deep Bay of Hong Kong waters which are generallycloser to the constitutes a much larger area for atmospheric deposi- metropolitan area. The considerable separation between tion of airborne particulates generated bythe surround- the two clusters indicates that sources of PAHs in the ing pollution sources, especiallythe Shenzhen and Mai Po sediment are different from those in the coastal Shekou Special Economic Zones of China. This also sediment of other parts of Hong Kong. Data of urban explains the high PAH concentrations in the surface aerosols obtained from denselypopulated commercial sediment of the Mai Po mangroves compared to that of and residual districts of Hong Kong around the Victoria the Mai Po Mudflats. When tidal current passes through Harbour occupya region between the two clusters. a mangrove area, resistance induced bythe presence of The clustering pattern in Fig. 6 indicates that deposi- vegetations can efficientlyreduce current speed to one- tion of PAHs generated bylocal anthropogenic pyr- third or one-fourth of the original magnitude. This olytic processes is a more important source of PAH greatlyincreases deposition rate of suspended particu- contamination than long-range atmospheric transporta- lates from the water column to the mangrove sediments tion from distant urban areas to the Mai Po region. [35]. As these suspended particulate matters are carriers Automobile emissions and fossil fuel combustion related of PAHs, increase in deposition rate also increases the to the heavytraffic conditions as well as industrial flux of PAHs from the water column to the mangrove G.J. Zheng et al. / Water Research 36 (2002) 1457–1468 1467 sediment. Comparing with the mangrove forest, the and the assessment of the associated environmental mudflats is a much more energetic environment and the stresses on the ecosystem of the Mai Po Marshes. net deposition rate of suspended matters must be much slower. The gradual decreasing trend of PAH content in the Mai Po Mudflats from the landward end towards the Acknowledgements seaward end is attributable to the steadyincrease in the deposition flux of PAHs into the sediment as tidal We would like to acknowledge the Agriculture, current losses its energyas it continues to rush inshore Fisheries and Conservation Department of Hong Kong towards the Mai Po mangroves. The direct atmospheric Special Administrative Region (SAR) and the World deposition route, on the other hand, would result in even Wide Fund for Nature Hong Kong in providing distribution of PAHs over the Mai Po region and cannot assistance during sediment sampling. The work de- explain the apparent discrepancyin sedimentaryPAH scribed in this paper was partiallysupported bythe content between the Mai Po mudflat and the Mai Po Agriculture, Fisheries and Conservation Department of mangrove areas. Hong Kong SAR, and a grant from CityU (Project No. 7000972).

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