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Environmental Science: Processes & Impacts View Article Online
DOI: 10.1039/C5EM00077G
The Yangtze Estuary is located in the Yangtze River Delta, heavy population density and the construction of great dam (the Three Gorges Dam) on the upstream Yangtze river make this region become a respective area affected by anthropogenic activity in Asia and even in the world. Recently, the flow rate and river sediment discharge have dramatically changed due to the construction of the Three Gorges Dam. Simultaneously, atmospheric non-point precipitation (vehicle exhaust and fossil fuel combustion), side discharge all contribute to polluting the estuary. This change certainly influences the seasonal and spatial variations of pollutants in the Yangtze Estuary.
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1
Spatial variation and sources of polycyclic aromatic hydrocarbons
2
(PAHs) in surface sediments from the Yangtze Estuary, China
3
Ying Wang*1, Chanchan Shen1, Zhenyao Shen1, Di Zhang1, John C. Crittenden2
4 5
1 The Key Laboratory of Water and Sediment Sciences, Ministry of Education, School of
6
Environment, Beijing Normal University, Beijing 100875, P.R. China
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2 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta,
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Georgia 30332-0595, United States
9 10 11 12 13 14 15 16 17 18 19 20 *
Corresponding author
Tel.: +86-10-5880 0398; Fax: +86-10-5880 0398 E-mail: [email protected]
1
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DOI: 10.1039/C5EM00077G
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Abstract The spatial distributions and sources of polycyclic aromatic hydrocarbons (PAHs)
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in surface sediments from the Yangtze Estuary were systematically analyzed. The
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results indicated significant spatial variations. The mean of ∑PAHs in different
25
sampling time in a year varied from 128.5±51.4 to 307.8±108.9 ng g-1. Samples
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collected during the flood season showed higher PAH concentrations and larger PAH
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fluctuations compared with those collected during the dry season. This variation was
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mainly ascribed to the change in river flow rate. Higher values of ∑PAHs were
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observed in the inner estuary than the adjacent coastal area over a year because of
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diffusion and degradation effects. Analysis of PAH abundance revealed a predominant
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proportion of light PAHs with two to three rings, with phenanthrene being the most
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abundant. Based on principal component analysis, vehicular emissions, coal and
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biomass combustion were the main sources of PAHs.
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Keywords
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Polycyclic aromatic hydrocarbons; spatial distribution; Yangtze Estuary; sources
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1 Introduction
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The Yangtze River, the world's fifth largest river in terms of water discharge and
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historically the fourth largest in terms of sediment discharge1, carries approximately
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9200×106 tons of water and 480 ×106 tons of suspended matter to the East China Sea
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every year. Over half of the sediment carried by the Yangtze River is deposited in the
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Yangtze Estuary2, causing an accumulation of pollutants in the estuary. In recent years,
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the flow rate and river sediment discharge from the upstream and middle reaches of
2
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the Yangtze River have dramatically decreased due to the construction of the Three
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Gorges Dam. With the impoundment of the Three Gorges Dam from November to
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April every year, the flow rate and river flux obviously decreased. The Yangtze
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Estuary climate can be characterized as subtropical monsoon, which has four distinct
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seasons and changes in seasonal wind directions. This climate makes atmospheric
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non-point precipitation play a seasonal role in conveying pollutants over long
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distances in the aquatic environment. Seasonal changes in ocean currents (e.g., the
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Taiwan Warm Current) also lead to the directional changes in the river flow in the
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estuary. These combined effects influence the transport and transformation of
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pollutants in the estuary.
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The Yangtze Estuary is located in the Yangtze River Delta, where the economy
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and the population density have developed very rapidly in recent years. A world-class
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city cluster has been constructed in the Yangtze River Delta, covering an area of
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99600 km2 with a population of 75 million people. The local industries are prosperous
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and include petrochemical, metallurgical, chemical and shipping. The economy in this
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area accounts for more than 20% of China's GDP. The urbanization of this area has
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resulted in an increased input of industrial effluents into the river and the discharge of
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domestic sewage. There are more than 30 branches that flow through the Yangtze
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Estuary, including the Baimao River, Yanglintang River, Liuhe River, Gujing River,
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Huangpu River and Xinjianghai River. The flow of multiple tributaries3,4 and
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discharge from industrial processes5 into the estuary and vehicle exhaust and fossil
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fuel combustion6 in the surrounding area all contribute to polluting the estuary.
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Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants prevalent in the
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sediments of marine and freshwater environments. These pollutants are carcinogenic,
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mutagenic and genotoxic to both aquatic and terrestrial organisms. Currently, the main
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sources of PAHs in aquatic environments are anthropogenic. Due to their hydrophobic
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nature (logKow, 3-8), once PAHs are introduced into the estuarine environment, they
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are rapidly adsorbed onto suspended particles and bottom sediment7,8. Estuarine
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sediment, which contains large pools of organic matter, can be a significant repository
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for PAHs. In our previous study9, an assessment of PAHs in surface sediment from
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near-shore and farther shore zones of the Yangtze Estuary was conducted.
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Experimental studies have reported the concentration distribution and fate of PAHs in
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the Yangtze Estuary. However, most of the published data have been obtained without
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any temporal monitoring and provide inaccurate indications of the temporal variations
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of PAHs. Understanding the temporal variations of PAHs in the Yangtze Estuary is
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essential for the protection of the estuary’s water quality.
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In this study, samplings were collected to analyze the variation of PAHs in the
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sediment from both the inner estuary and the adjacent sea of the Yangtze Estuary in a
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year. The combined effects of water currents (e.g., river runoff, coastal current),
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monsoon, discharge from shore-side areas were also analyzed to determine the
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possible sources of PAHs in the Yangtze Estuary.
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2. Materials and Methods
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2.1 Study area and sampling
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Figure 1 shows the study area of the Yangtze Estuary, which begins from
4
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Xuliujing and ends at the -20-m isobath and is located between 30.5–32ºN and 121
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–122.6ºE. The Yangtze Estuary is divided into two branches by Chongming Island:
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the North Branch and the South Branch. The South Branch receives more than 95% of
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the total estuarine runoff and is further separated into two parts by Changxing and
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Hengsha Islands: the northern part is called the north bay, and the southern part
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extends throughout the Jiuduansha Islands and is divided into the North Passage and
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the South Passage. There are more than 30 branches in the Yangtze Estuary, including
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the Baimao River, Yanglintang River, Liuhe River, Gujing River, Huangpu River and
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Xinjianghai River. The Huangpu River, which flows through the highly populated and
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industrialized Shanghai City, flows into the southern branch of the Yangtze Estuary.
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Additionally, three wastewater outlets are located along the south channel. The
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sedimentation rate of the Yangtze estuary is 6.3-6.6 cm yr-1. The climate in the
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Yangtze estuary is subtropical monsoon with four distinct seasons. Annual average
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temperatures range from 16-18℃ and annual average rainfall ranges from 800 to
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1600 mm.
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Nineteen samples from the inner Yangtze estuary and 11 samples from the
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adjacent East China Sea (along the -10 m and -20 m isobaths) were collected in
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August and November 2010 and in February and May 2011, respectively. The surface
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sediment was sampled at a depth of 0-2 cm using a Van Veen stainless steel grab
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sampler (Eijkelamp, Netherlands).
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2.2 Analytical methods
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All sediment samples were freeze-dried and ground to pass through a 100-mesh
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sieve. A freeze-dried sample (5 g) was extracted with a 1:1 mixture of n-hexane and
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acetone (66 mL) by an accelerated solvent extractor (Dionex ASE 300). It was heated
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to 100 ℃ for 5 min, which was followed by 5 min of standing. The extract was then
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purged using N2 and reduced to 1-2 mL by rotary evaporation.2 The concentrated
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extract was passed through a 2:1 silica/Na2SO4 polytetrafluoroethylene column (1-cm
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i.d.) with 8 mL hexane and then eluted with 10 mL dichloromethane/n-hexane
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(v:v=1:1). Finally, the elution containing the PAHs was concentrated to about 1 mL,
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which was then dissolved in the methanol and the final volume was 2.0 mL for HPLC
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analysis10.
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PAHs were analyzed
by a Dionex U-3000
high-performance liquid
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chromatography (HPLC) system (Dionex, USA). Separation was performed on an
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Ultimate PAH column (4.6×250 mm, particle size 5 µm, Varian, USA) at a constant
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solvent flow rate of 1.0 mL min-1 using a gradient elution program starting with 75%
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methanol followed by a 17.2-min linear gradient to 100% methanol for 25 min. A
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linear gradient (1 min) back to 75% methanol was followed by a 3-min pre-run to
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achieve equilibration for each subsequent run. The majority of the PAHs (15 total)
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were identified by a fluorescence detector; acenaphthylene was quantified by UV
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detector at 229 nm.
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2.3 Quality assurance
128
Quantifications were performed using external standards, and the correlation
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coefficients for calibration curves were greater than 0.999. Before the onset of the
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extraction and analysis program, recovery experiments were conducted by spiking the
6
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16 PAH standard solutions with sediment samples. The six parallel experiments
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conducted indicated that the recoveries for the 16 PAHs were 60%-120% for all
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sediments except Napthalene, whose recovery was 30±5%. For each batch of 12
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sediment samples, a method blank (solvent), a spiked blank (standards spiked into
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solvent) and a sample duplicate were processed. The respective relative standard
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deviations (RSDs) ranged from 3.13% to 16.06% for all samples. The PAH detection
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limits for the 16 PAHs ranged from 0.34 to 4.05 ng g-1 for sediment samples. Method
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blank analysis revealed no detectable amounts of PAH contamination.
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2.4 Data analyses
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Principal components analysis (PCA) followed by multiple linear regression
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analysis was used to analyze the data set to quantify the profiles of the possible PAH
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sources. Data analysis was performed on the correlation matrix using SPSS 18.0
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software.
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3 Results and discussion
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3.1 Variations in PAHs concentrations
146 147
Figure 1
148
Table 1
149 150
The sum of the concentrations of all measured PAHs (∑PAHs) at each sampling
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site of the Yangtze Estuary in different sampling time in a year is shown in Fig. 1. Tab.
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1 shows the concentration range, median and mean of ∑PAHs in the surface sediment
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from the Yangtze Estuary. The results show that the values of ∑PAHs in the Yangtze
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Estuary varied greatly in a year. The mean of ∑PAHs (307.8±108.9 ng g-1 for all
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samples) was the highest in August 2010 (the wet season). The mean value of ∑PAHs
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in May 2011 was 128.5±51.4 ng g-1 (for all samples), which was only 38% of the
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mean value calculated in August 2010. This phenomenon might be explained by the
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different river flow rates that occur in different months in a year. In August 2010, the
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average flow rate was at its maximum value (55300 m3/s); thus, higher concentrations
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of PAHs were carried into the estuary by the river runoff from the upper and middle
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reach and tributaries. During the dry season, the average flow rates decreased
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drastically to 17880 m3/s and 13990 m3/s in November 2010 and February 2011,
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respectively, because of the water is impounded by the Three Gorges Dam during
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November and December. Smaller amounts of PAHs in the upper and middle reach of
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the Yangtze River were transported and precipitated in the estuary. In May 2011,
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although the flow rate increased slightly (18970 m3/s), it was the driest May in 50
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years. The amount of PAHs in the estuary further decreased due to lower scouring and
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degradation. According to Zhang et al. 11, the amount of PAHs in the tidal flat during
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the dry season was higher than that in the wet season, in contrast to our results; this
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discrepancy may be due to a differences in sources. The amount of PAHs in the tidal
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flat was mainly influenced by the side discharge (such as petrochemical, metallurgical,
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chemical plants). However, in our study, river runoff from the upper and middle reach
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and tributaries was the main reason for the change in the river channel and adjacent
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coastal area, which resulted in an increase in PAH concentrations during the wet
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season.
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During different sampling time in a year, the mean concentration values were
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higher in the inner estuary than in the adjacent coastal area, which suggests that great
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dispersal and degradation occurred when pollutants were introduced into the adjacent
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coastal area. The mean value of ∑PAHs in the northern adjacent coastal area was
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lower than that in the southern coastal area in all months except February 2011 due to
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the directional change of the Yangtze river in different time. Mixing of the Taiwan
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Warm Current and Yangtze dilute the PAHs concentrations and generates a strong
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plume that blocks the eastward expansion of the diluted water. Thus, making it move
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toward the northeast and southeast after it leaves the mouth of the river. The main
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directional flow of the Yangtze diluted water was influenced by the river runoff. When
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the runoff increased, the main axis of the Yangtze diluted water expanded toward the
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southeast, causing pollutant deposition in that direction. In contrast, when the runoff
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decreased, the Yangtze diluted water flowed toward the northeast and accumulated
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pollutants in that area. In our study, the runoff in February 2011 was at a minimum;
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thus, the main directional flow of the Yangtze diluted water moved from south to
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north, which led to the concentration of pollutants being higher in the northern
192
adjacent coastal area (S1-S5).
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In August 2010, the highest ∑PAHs value was observed at site A3 due to the
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input from the Lvhua dock in the southern near-shore of Chongming Island. The
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burning and leakage of fossil fuels and the discharge of sewage during shipping led to
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PAH accumulation at site A3. The peak total concentration level in November 2010
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was observed at site N2, near the outlet of the North Branch; the concentration likely
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peaked due to the site’s location within the river-sea boundary zone. In the area,
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organic-rich and fine-grain suspended particles tend to settle due to the ‘double layer
200
compression by seawater and flocculation’, which causes higher PAH deposition. In
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the inner part of the Yangtze Estuary, water currents are mainly caused by river runoff
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and tidal currents. In the dry season, runoff decreased. Water is impounded by the
203
Three Gorges Dam during November and December; therefore, the runoff volume and
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number of suspended particles in the lower reach of the watershed declined drastically
205
as compared with those observed over the last decade. Therefore, the river-sea
206
boundary zone was mainly influenced by tidal currents, especially in the outlet of the
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North Branch, where the river runoff was lower than that of the South Branch. That is,
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higher evidence of seawater encroachment in the North Branch led to a higher
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accumulation of PAHs.
210
In February 2011, the value of ∑PAHs was the highest at site C2, located near
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two large wastewater outlets (Zhuyuan and Bailong-gang) of Shanghai City. Because
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the runoff in February 2011 was at its lowest point, pollutants from the wastewater
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outlets were not dispersed as much. The lower flow rate resulted in the deposition of
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PAHs. In May 2011, the highest value of ∑PAHs was observed at site D4, in the mud
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area and the outlet of the South Branch. More fine particles entered the overlying
216
water, which indicated that the ‘double layer compression’ and coagulation led to the
217
accumulation of PAHs.
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3.2 Variation in PAHs composition
219 220
Figure 2
221
Table 2
222 223
Fig. 2 and Tab. 2 shows the variation of PAH abundance in different sampling
224
time in a year. The coefficients of variation for each PAH monomer were the highest
225
in August 2010 (Fig. 2(a)), which indicated that the concentrations of PAH monomers
226
varied greatly between different sites in the wet season. Phenanthrene (Phe) was the
227
primary component. In August 2010, the average relative abundance of Phe was the
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highest, accounting for 27.4% of ∑PAHs. The percentage of Phe decreased from the
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inner estuary (37.9%) to the adjacent sea (20.8%), which demonstrated that
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degradation and dilution effects occurred during the transportation of Phe from the
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estuary to the sea.
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Based on the number of rings in the PAHs, the 16 PAHs can be divided into three
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groups: (2 + 3)-ring, 4-ring and (5 + 6)-ring components, representing low-, medium-
234
and high-molecular-weight PAHs, respectively.12 Ternary plots showing the PAH
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abundance categorized by the number of rings are shown in Fig. 2(b). The abundance
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2-3-ring PAHs in ∑PAHs was higher than the average abundance as well as the
237
abundance of high-molecular-weight PAHs (Tab. 2), which indicates that the
238
dominant components of PAHs in the Yangtze Estuary were 2-3-ring PAHs
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(LM-PAHs). Low-molecular-weight PAHs made up more than 50% of the total PAHs
11
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in 93% of the samples of the Yangtze Estuary in August 2010 and in 77% of the
241
samples in November 2010. In February and May 2011, the abundance of 4-6-ring
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PAHs in ∑PAHs increased. LM-PAHs (2-3-ring PAHs) may be derived from
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petrogenic sources (fossil), and they are also major constituents of petroleum, whereas
244
MM-PAHs
245
(high-temperature combustion) 13,14. However, some studies reported that Flu, Phe,
246
Nap and Ant are related to the emission of coal and biomass combustion (Dong and
247
Lee, 2009). In February 2011, the flow rate clearly decreased. Shore-side discharge
248
had a dominant effect on PAH accumulation. That is, more PAHs derived from
249
industrial wastewater may be discharged into the Yangtze Estuary. So the proportion
250
of 4-6-ring PAHs (MM-PAHs and HM-PAHs) increased in the sediments in the dry
251
season.
252
3.3 Identification of PAHs sources in a year
253
3.3.1 The isomeric ratios of PAHs
and
HM-PAHs
are
generated
mainly
by
pyrogenic
sources
254 255
Figure 3
256 257
Diagnostic ratios of PAHs have been widely used to identify the possible sources
258
of PAHs15. In our study, An/178 and InP/(InP + Bghip) were plotted versus. Fla/(Fla +
259
Pyr) (An: Anthracene, InP: Indeno[1, 2, 3-cd] pyrene, Bghip: Benzo [ghi]perylene,
260
Fla: Fluoranthene, Pyr: Pyrene). A ratio of An/178>0.1 generally suggests a
261
combustion origin, whereas a ratio
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Processes & Impacts Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. wang, C. shen, Z. shen and J. C. Crittenden, Environ. Sci.: Processes Impacts, 2015, DOI: 10.1039/C5EM00077G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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The Yangtze Estuary is located in the Yangtze River Delta, heavy population density and the construction of great dam (the Three Gorges Dam) on the upstream Yangtze river make this region become a respective area affected by anthropogenic activity in Asia and even in the world. Recently, the flow rate and river sediment discharge have dramatically changed due to the construction of the Three Gorges Dam. Simultaneously, atmospheric non-point precipitation (vehicle exhaust and fossil fuel combustion), side discharge all contribute to polluting the estuary. This change certainly influences the seasonal and spatial variations of pollutants in the Yangtze Estuary.
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Spatial variation and sources of polycyclic aromatic hydrocarbons
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(PAHs) in surface sediments from the Yangtze Estuary, China
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Ying Wang*1, Chanchan Shen1, Zhenyao Shen1, Di Zhang1, John C. Crittenden2
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1 The Key Laboratory of Water and Sediment Sciences, Ministry of Education, School of
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Environment, Beijing Normal University, Beijing 100875, P.R. China
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2 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta,
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Georgia 30332-0595, United States
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Corresponding author
Tel.: +86-10-5880 0398; Fax: +86-10-5880 0398 E-mail: [email protected]
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Abstract The spatial distributions and sources of polycyclic aromatic hydrocarbons (PAHs)
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in surface sediments from the Yangtze Estuary were systematically analyzed. The
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results indicated significant spatial variations. The mean of ∑PAHs in different
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sampling time in a year varied from 128.5±51.4 to 307.8±108.9 ng g-1. Samples
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collected during the flood season showed higher PAH concentrations and larger PAH
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fluctuations compared with those collected during the dry season. This variation was
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mainly ascribed to the change in river flow rate. Higher values of ∑PAHs were
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observed in the inner estuary than the adjacent coastal area over a year because of
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diffusion and degradation effects. Analysis of PAH abundance revealed a predominant
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proportion of light PAHs with two to three rings, with phenanthrene being the most
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abundant. Based on principal component analysis, vehicular emissions, coal and
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biomass combustion were the main sources of PAHs.
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Keywords
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Polycyclic aromatic hydrocarbons; spatial distribution; Yangtze Estuary; sources
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1 Introduction
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The Yangtze River, the world's fifth largest river in terms of water discharge and
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historically the fourth largest in terms of sediment discharge1, carries approximately
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9200×106 tons of water and 480 ×106 tons of suspended matter to the East China Sea
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every year. Over half of the sediment carried by the Yangtze River is deposited in the
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Yangtze Estuary2, causing an accumulation of pollutants in the estuary. In recent years,
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the flow rate and river sediment discharge from the upstream and middle reaches of
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the Yangtze River have dramatically decreased due to the construction of the Three
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Gorges Dam. With the impoundment of the Three Gorges Dam from November to
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April every year, the flow rate and river flux obviously decreased. The Yangtze
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Estuary climate can be characterized as subtropical monsoon, which has four distinct
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seasons and changes in seasonal wind directions. This climate makes atmospheric
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non-point precipitation play a seasonal role in conveying pollutants over long
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distances in the aquatic environment. Seasonal changes in ocean currents (e.g., the
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Taiwan Warm Current) also lead to the directional changes in the river flow in the
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estuary. These combined effects influence the transport and transformation of
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pollutants in the estuary.
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The Yangtze Estuary is located in the Yangtze River Delta, where the economy
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and the population density have developed very rapidly in recent years. A world-class
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city cluster has been constructed in the Yangtze River Delta, covering an area of
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99600 km2 with a population of 75 million people. The local industries are prosperous
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and include petrochemical, metallurgical, chemical and shipping. The economy in this
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area accounts for more than 20% of China's GDP. The urbanization of this area has
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resulted in an increased input of industrial effluents into the river and the discharge of
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domestic sewage. There are more than 30 branches that flow through the Yangtze
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Estuary, including the Baimao River, Yanglintang River, Liuhe River, Gujing River,
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Huangpu River and Xinjianghai River. The flow of multiple tributaries3,4 and
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discharge from industrial processes5 into the estuary and vehicle exhaust and fossil
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fuel combustion6 in the surrounding area all contribute to polluting the estuary.
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Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants prevalent in the
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sediments of marine and freshwater environments. These pollutants are carcinogenic,
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mutagenic and genotoxic to both aquatic and terrestrial organisms. Currently, the main
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sources of PAHs in aquatic environments are anthropogenic. Due to their hydrophobic
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nature (logKow, 3-8), once PAHs are introduced into the estuarine environment, they
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are rapidly adsorbed onto suspended particles and bottom sediment7,8. Estuarine
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sediment, which contains large pools of organic matter, can be a significant repository
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for PAHs. In our previous study9, an assessment of PAHs in surface sediment from
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near-shore and farther shore zones of the Yangtze Estuary was conducted.
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Experimental studies have reported the concentration distribution and fate of PAHs in
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the Yangtze Estuary. However, most of the published data have been obtained without
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any temporal monitoring and provide inaccurate indications of the temporal variations
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of PAHs. Understanding the temporal variations of PAHs in the Yangtze Estuary is
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essential for the protection of the estuary’s water quality.
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In this study, samplings were collected to analyze the variation of PAHs in the
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sediment from both the inner estuary and the adjacent sea of the Yangtze Estuary in a
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year. The combined effects of water currents (e.g., river runoff, coastal current),
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monsoon, discharge from shore-side areas were also analyzed to determine the
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possible sources of PAHs in the Yangtze Estuary.
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2. Materials and Methods
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2.1 Study area and sampling
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Figure 1 shows the study area of the Yangtze Estuary, which begins from
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Xuliujing and ends at the -20-m isobath and is located between 30.5–32ºN and 121
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–122.6ºE. The Yangtze Estuary is divided into two branches by Chongming Island:
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the North Branch and the South Branch. The South Branch receives more than 95% of
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the total estuarine runoff and is further separated into two parts by Changxing and
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Hengsha Islands: the northern part is called the north bay, and the southern part
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extends throughout the Jiuduansha Islands and is divided into the North Passage and
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the South Passage. There are more than 30 branches in the Yangtze Estuary, including
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the Baimao River, Yanglintang River, Liuhe River, Gujing River, Huangpu River and
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Xinjianghai River. The Huangpu River, which flows through the highly populated and
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industrialized Shanghai City, flows into the southern branch of the Yangtze Estuary.
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Additionally, three wastewater outlets are located along the south channel. The
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sedimentation rate of the Yangtze estuary is 6.3-6.6 cm yr-1. The climate in the
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Yangtze estuary is subtropical monsoon with four distinct seasons. Annual average
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temperatures range from 16-18℃ and annual average rainfall ranges from 800 to
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1600 mm.
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Nineteen samples from the inner Yangtze estuary and 11 samples from the
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adjacent East China Sea (along the -10 m and -20 m isobaths) were collected in
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August and November 2010 and in February and May 2011, respectively. The surface
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sediment was sampled at a depth of 0-2 cm using a Van Veen stainless steel grab
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sampler (Eijkelamp, Netherlands).
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2.2 Analytical methods
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All sediment samples were freeze-dried and ground to pass through a 100-mesh
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sieve. A freeze-dried sample (5 g) was extracted with a 1:1 mixture of n-hexane and
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acetone (66 mL) by an accelerated solvent extractor (Dionex ASE 300). It was heated
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to 100 ℃ for 5 min, which was followed by 5 min of standing. The extract was then
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purged using N2 and reduced to 1-2 mL by rotary evaporation.2 The concentrated
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extract was passed through a 2:1 silica/Na2SO4 polytetrafluoroethylene column (1-cm
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i.d.) with 8 mL hexane and then eluted with 10 mL dichloromethane/n-hexane
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(v:v=1:1). Finally, the elution containing the PAHs was concentrated to about 1 mL,
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which was then dissolved in the methanol and the final volume was 2.0 mL for HPLC
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analysis10.
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PAHs were analyzed
by a Dionex U-3000
high-performance liquid
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chromatography (HPLC) system (Dionex, USA). Separation was performed on an
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Ultimate PAH column (4.6×250 mm, particle size 5 µm, Varian, USA) at a constant
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solvent flow rate of 1.0 mL min-1 using a gradient elution program starting with 75%
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methanol followed by a 17.2-min linear gradient to 100% methanol for 25 min. A
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linear gradient (1 min) back to 75% methanol was followed by a 3-min pre-run to
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achieve equilibration for each subsequent run. The majority of the PAHs (15 total)
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were identified by a fluorescence detector; acenaphthylene was quantified by UV
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detector at 229 nm.
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2.3 Quality assurance
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Quantifications were performed using external standards, and the correlation
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coefficients for calibration curves were greater than 0.999. Before the onset of the
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extraction and analysis program, recovery experiments were conducted by spiking the
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16 PAH standard solutions with sediment samples. The six parallel experiments
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conducted indicated that the recoveries for the 16 PAHs were 60%-120% for all
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sediments except Napthalene, whose recovery was 30±5%. For each batch of 12
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sediment samples, a method blank (solvent), a spiked blank (standards spiked into
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solvent) and a sample duplicate were processed. The respective relative standard
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deviations (RSDs) ranged from 3.13% to 16.06% for all samples. The PAH detection
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limits for the 16 PAHs ranged from 0.34 to 4.05 ng g-1 for sediment samples. Method
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blank analysis revealed no detectable amounts of PAH contamination.
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2.4 Data analyses
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Principal components analysis (PCA) followed by multiple linear regression
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analysis was used to analyze the data set to quantify the profiles of the possible PAH
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sources. Data analysis was performed on the correlation matrix using SPSS 18.0
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software.
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3 Results and discussion
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3.1 Variations in PAHs concentrations
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Figure 1
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Table 1
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The sum of the concentrations of all measured PAHs (∑PAHs) at each sampling
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site of the Yangtze Estuary in different sampling time in a year is shown in Fig. 1. Tab.
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1 shows the concentration range, median and mean of ∑PAHs in the surface sediment
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from the Yangtze Estuary. The results show that the values of ∑PAHs in the Yangtze
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Estuary varied greatly in a year. The mean of ∑PAHs (307.8±108.9 ng g-1 for all
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samples) was the highest in August 2010 (the wet season). The mean value of ∑PAHs
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in May 2011 was 128.5±51.4 ng g-1 (for all samples), which was only 38% of the
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mean value calculated in August 2010. This phenomenon might be explained by the
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different river flow rates that occur in different months in a year. In August 2010, the
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average flow rate was at its maximum value (55300 m3/s); thus, higher concentrations
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of PAHs were carried into the estuary by the river runoff from the upper and middle
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reach and tributaries. During the dry season, the average flow rates decreased
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drastically to 17880 m3/s and 13990 m3/s in November 2010 and February 2011,
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respectively, because of the water is impounded by the Three Gorges Dam during
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November and December. Smaller amounts of PAHs in the upper and middle reach of
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the Yangtze River were transported and precipitated in the estuary. In May 2011,
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although the flow rate increased slightly (18970 m3/s), it was the driest May in 50
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years. The amount of PAHs in the estuary further decreased due to lower scouring and
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degradation. According to Zhang et al. 11, the amount of PAHs in the tidal flat during
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the dry season was higher than that in the wet season, in contrast to our results; this
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discrepancy may be due to a differences in sources. The amount of PAHs in the tidal
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flat was mainly influenced by the side discharge (such as petrochemical, metallurgical,
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chemical plants). However, in our study, river runoff from the upper and middle reach
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and tributaries was the main reason for the change in the river channel and adjacent
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coastal area, which resulted in an increase in PAH concentrations during the wet
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season.
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During different sampling time in a year, the mean concentration values were
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higher in the inner estuary than in the adjacent coastal area, which suggests that great
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dispersal and degradation occurred when pollutants were introduced into the adjacent
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coastal area. The mean value of ∑PAHs in the northern adjacent coastal area was
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lower than that in the southern coastal area in all months except February 2011 due to
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the directional change of the Yangtze river in different time. Mixing of the Taiwan
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Warm Current and Yangtze dilute the PAHs concentrations and generates a strong
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plume that blocks the eastward expansion of the diluted water. Thus, making it move
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toward the northeast and southeast after it leaves the mouth of the river. The main
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directional flow of the Yangtze diluted water was influenced by the river runoff. When
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the runoff increased, the main axis of the Yangtze diluted water expanded toward the
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southeast, causing pollutant deposition in that direction. In contrast, when the runoff
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decreased, the Yangtze diluted water flowed toward the northeast and accumulated
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pollutants in that area. In our study, the runoff in February 2011 was at a minimum;
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thus, the main directional flow of the Yangtze diluted water moved from south to
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north, which led to the concentration of pollutants being higher in the northern
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adjacent coastal area (S1-S5).
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In August 2010, the highest ∑PAHs value was observed at site A3 due to the
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input from the Lvhua dock in the southern near-shore of Chongming Island. The
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burning and leakage of fossil fuels and the discharge of sewage during shipping led to
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PAH accumulation at site A3. The peak total concentration level in November 2010
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was observed at site N2, near the outlet of the North Branch; the concentration likely
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peaked due to the site’s location within the river-sea boundary zone. In the area,
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organic-rich and fine-grain suspended particles tend to settle due to the ‘double layer
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compression by seawater and flocculation’, which causes higher PAH deposition. In
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the inner part of the Yangtze Estuary, water currents are mainly caused by river runoff
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and tidal currents. In the dry season, runoff decreased. Water is impounded by the
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Three Gorges Dam during November and December; therefore, the runoff volume and
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number of suspended particles in the lower reach of the watershed declined drastically
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as compared with those observed over the last decade. Therefore, the river-sea
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boundary zone was mainly influenced by tidal currents, especially in the outlet of the
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North Branch, where the river runoff was lower than that of the South Branch. That is,
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higher evidence of seawater encroachment in the North Branch led to a higher
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accumulation of PAHs.
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In February 2011, the value of ∑PAHs was the highest at site C2, located near
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two large wastewater outlets (Zhuyuan and Bailong-gang) of Shanghai City. Because
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the runoff in February 2011 was at its lowest point, pollutants from the wastewater
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outlets were not dispersed as much. The lower flow rate resulted in the deposition of
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PAHs. In May 2011, the highest value of ∑PAHs was observed at site D4, in the mud
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area and the outlet of the South Branch. More fine particles entered the overlying
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water, which indicated that the ‘double layer compression’ and coagulation led to the
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accumulation of PAHs.
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3.2 Variation in PAHs composition
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Figure 2
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Table 2
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Fig. 2 and Tab. 2 shows the variation of PAH abundance in different sampling
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time in a year. The coefficients of variation for each PAH monomer were the highest
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in August 2010 (Fig. 2(a)), which indicated that the concentrations of PAH monomers
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varied greatly between different sites in the wet season. Phenanthrene (Phe) was the
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primary component. In August 2010, the average relative abundance of Phe was the
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highest, accounting for 27.4% of ∑PAHs. The percentage of Phe decreased from the
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inner estuary (37.9%) to the adjacent sea (20.8%), which demonstrated that
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degradation and dilution effects occurred during the transportation of Phe from the
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estuary to the sea.
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Based on the number of rings in the PAHs, the 16 PAHs can be divided into three
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groups: (2 + 3)-ring, 4-ring and (5 + 6)-ring components, representing low-, medium-
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and high-molecular-weight PAHs, respectively.12 Ternary plots showing the PAH
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abundance categorized by the number of rings are shown in Fig. 2(b). The abundance
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2-3-ring PAHs in ∑PAHs was higher than the average abundance as well as the
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abundance of high-molecular-weight PAHs (Tab. 2), which indicates that the
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dominant components of PAHs in the Yangtze Estuary were 2-3-ring PAHs
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(LM-PAHs). Low-molecular-weight PAHs made up more than 50% of the total PAHs
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in 93% of the samples of the Yangtze Estuary in August 2010 and in 77% of the
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samples in November 2010. In February and May 2011, the abundance of 4-6-ring
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PAHs in ∑PAHs increased. LM-PAHs (2-3-ring PAHs) may be derived from
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petrogenic sources (fossil), and they are also major constituents of petroleum, whereas
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MM-PAHs
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(high-temperature combustion) 13,14. However, some studies reported that Flu, Phe,
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Nap and Ant are related to the emission of coal and biomass combustion (Dong and
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Lee, 2009). In February 2011, the flow rate clearly decreased. Shore-side discharge
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had a dominant effect on PAH accumulation. That is, more PAHs derived from
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industrial wastewater may be discharged into the Yangtze Estuary. So the proportion
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of 4-6-ring PAHs (MM-PAHs and HM-PAHs) increased in the sediments in the dry
251
season.
252
3.3 Identification of PAHs sources in a year
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3.3.1 The isomeric ratios of PAHs
and
HM-PAHs
are
generated
mainly
by
pyrogenic
sources
254 255
Figure 3
256 257
Diagnostic ratios of PAHs have been widely used to identify the possible sources
258
of PAHs15. In our study, An/178 and InP/(InP + Bghip) were plotted versus. Fla/(Fla +
259
Pyr) (An: Anthracene, InP: Indeno[1, 2, 3-cd] pyrene, Bghip: Benzo [ghi]perylene,
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Fla: Fluoranthene, Pyr: Pyrene). A ratio of An/178>0.1 generally suggests a
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combustion origin, whereas a ratio
