Sedimentary Record of Polycyclic Aromatic Hydrocarbons from the Shuanglong Catchment , Southwest China

1School of Geography Science, Nanjing Normal University, 1 Wenyuan Road, Qixia, Nanjing 210023, China 2Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, China 3Centre of Lake Engineering & Technology, State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China


Introduction
Polycyclic aromatic hydrocarbons (PAHs) are classical persistent organic pollutants (POPs).They are pervasive in the environmental medias, including atmosphere, soil, sediments, plants, and human bodies [1][2][3][4].They have the characteristics of being carcinogenic, teratogenic, and mutagenic.The deposited PAHs may come from anthropogenic activities (burning processes of wood, coal and grass, incomplete combustion of fossil fuels, vehicle emissions, and leakage of crude oil) as well as natural sources (forest fires, volcanic eruptions, and biosynthesis) [5,6].As a result of their low solubility and hydrophobicity, PAHs can be easily absorbed by fine particulates once they get into the aquatic environment and accumulate in the sediments with deposition.Therefore, lake sediments and organic matter (OM) are considered as the main fate of PAHs [7,8].As a consequence, PAHs in the sediments are primarily associated with OM which plays an essential role in affecting the distribution of PAHs [9,10].
Due to their potential toxicity to living organisms, it is urgent to investigate the distributions and potential risks of PAHs.Plenty of studies have been carried out in the surface water and sediments of seas, oceans [1,5,[11][12][13][14][15][16], and atmosphere [17][18][19][20], while the food investigations [17,21] have also been conducted extensively worldwide.However, studies on the small catchment which were affected greatly by the human activities were deficient.
The source and distribution of PAHs are of great environmental significance.Owing to the complicated source of PAHs, difficulties in distinguishing the origins exist.In the past decades, growing attention has been paid to the PAHs environmental behavior.Various tools have been developed to identify the PAHs source [22][23][24][25][26][27].Among all the approaches, the Ant/(Ant + Phe), Fla/(Fla + Pyr), BaA/(BaA + Chry), and IcdP/(IcdP + BghiP) ratios (MDRs) have been applied extensively all over the world [5,[28][29][30][31].Because isomers show substantial intrasource variability and intersource similarity, sources are difficult to be distinguished clearly.Criticism about these indexes exists [32][33][34][35].Even the same sources are not always characterized by the same PAHs emissions profile, and, between different sources, there is significant overlapping in MDRs [36].They do not respond to know differences in source strength and types between sites (on rural-urban gradients) in atmospheric emission sources [37].Some researchers [38,39] commented that use of MDRs for sewage sludge PAHs was unfeasible.Even if different sourcesoriginated PAHs took place during the entire wastewater treatment process.
By dating sediment cores [40,41] and discriminating the source of PAHs via diagnostic rates, the time horizons associated with PAH trends, source variation, and quantification of flux into sediments can be determined.Most PAHs corresponded to the development of industrialization and urbanization.Thus, the profile distribution of PAHs in the sediments could elucidate the relationship between the historical trends of PAHs pollution and the socioeconomic development [42][43][44][45].Many studies have focused on the temporal trends of PAHs in the sediments [16,46].However, investigations on the PAHs historical variation and effects of human activities in the small catchment turned out to be few.
The Dianchi watershed is located in the Yunnan province, southwest of China.Over the past decades, with the development of economic and society, the population grew rapidly and the chemical fertilizer was applied excessively, resulting in serious environmental problems [47,48].Recent studies in the Dianchi watershed focused mainly on Dianchi Lake, especially water eutrophication [49,50].However, the PAHs distribution and source identification in the small catchment of the Dianchi watershed are still poorly understood.In order to control the PAHs pollution in the Dianchi watershed, the sedimentary record of PAHs in the small catchment should be focused on.The purpose of this study were as follows: (1) to investigate the pollution level of PAHs in the Shuanglong catchment; (2) to identify the sources of PAHs; (3) to explore the influence factors on PAHs distribution.

Study Area and Sampling.
The Shuanglong Reservoir (24 ∘ 57  ∼24 ∘ 59  N, 102 ∘ 55  ∼102 ∘ 57  E) was built in 1956, belonging to the Dianchi watershed (Figure 1).The Shuanglong catchment is located in the Shuanglong village on the west side of the Kunluo Highway, 70 km away from the Kunming city.The outflow enters from the south to north through the Dongda River into Dianchi Lake.The elevation is 1933.40 m.The rain collecting area is approximately 54 km 2 .The main dam of the reservoir is 18.50 m high, with four auxiliary dams.The total length is 795 m.As the important water source of Jinning County, the capacity of reservoir water storage is 12.16 million m 3 .In the small catchment, land use types include forest land, abandoned land, residential land, and cultivated land.Diversity of soil types such as red soil, purple soil, alluvial soil, limestone soil, and paddy soil exists.Because of the fertile soil, Jinning County has been regarded as the main producing areas of rice, broad beans, wheat, and rapeseed.Being rich in mineral resources and the well-developed mining industry make Jinning County one of the world's four major phosphorus mining zones.In addition, light industry, food, and building materials and other processing industries are developed.
The sampling time was in June, 2012.All the sampling sites were in the southern of the Shuanglong Reservoir (Figure 1).This region is under less disturbance.According to the principle of uniform distributed, three sediment cores (70 cm depth) were collected with self-weight coring equipment.The surface layers (2 cm depth) were taken away to minimize the perturbation of the surface deposited layer.The sediment core was sectioned in 1 cm intervals immediately.The subsamples were put into plastic bags and then transported to the laboratory instantly and frozen at the temperature of −50 ∘ C for 72 h.

Dating.
The 210 Pb ex radiometric technique was used with a high-resolution HPGe -spectrometry system (GWL-120-15 USA) (for details, see Wang et al., 2013) [51].The constant CRS model was applied to date the sediment core [52].
2.3.Extraction.Subsamples were lyophilized using a freeze dryer (Eyela FDU-1200, Japan).All freeze-dried samples were ground and passed through the 200-mesh sieve.200 ± 2 mg of subsample was weighed.A supersonic apparatus was used for lipid extraction with 50 ml of dichloromethane/hexane (1 : 1, v/v) solution for 1.5 h.Centrifuge for 3 times, moving the upper extraction liquid to the round-bottom flask.The extracts were then concentrated to approximately 2 ml by rotary evaporation.In order to remove the impurities and fractionate, the concentrated extracts went through the anhydrous sodium/silica columns (from bottom to the top, successively filled with 1.5 g anhydrous sodium, 1.0 g activated silica gel and 1.5 g anhydrous sodium) and then eluted with 15 ml mixture of dichloromethane/hexane (1 : 1, v/v) and 5 ml hexane to obtain the PAH fraction.Then eluent was collected in the bottle and concentrated to 0.5 ml by rotary evaporation.

Analysis Items.
The total organic carbon (TOC) concentrations were determined by a TOC analyzer (CSHCN200, Japan).The subsamples were air-dried and ground via a 200-mesh sieve.Each subsample weighted 200 mg into two containers, respectively.One was calcined at 900 ∘ C for the measurement of total carbon (TC) concentration, and the other was added with phosphoric acid at 200 ∘ C to measure the inorganic carbon (IC) concentration.Triplicate analyses of every subsample were conducted.
The PAHs identification and quantification were performed through the gas chromatograph-mass spectrometer manufactured by Shimadzu Corporation of Japan (QP2010 ultra, Japan) operating in the electron ionization (EI) mode with a DB-5 fused silica capillary column (30 mm × 0.25 mm × 0.25 m).Splitless injection of 1 l of the subsamples is executed with an autosampler with a temperature of 250 ∘ C, and the high-purity helium (99.999%) is applied as carrier gas.The temperature of GC oven was programmed initially at the thermostatic of 90 ∘ C and increased to 180 ∘ C at the rate of 12 ∘ C min −1 , keeping this condition for 5 min, and then raised to 280 ∘ C at the rate of 5 ∘ C min −1 , staying for 10 min.The data was collected using a SCAN mode.The qualitative analysis of PAHs is the ratio between relative retention time and the peak intensity of the characteristic fragment ions.The peak area method with 6-point calibration curve was used for quantitative (SIM mode).

Results and Discussion
3.1.Chronology.The 210 Pb ex concentrations in the sediment core ranged from 14.31 to 229.35 Bq kg −1 .The core covers the periods from 1867 to 2011, 144 years of deposition (Figure 2).The average rate of sedimentation was 0.49 cm year −1 , which was similar to the former research in the nearby area [48].
The values of ∑PAHs show a rising tendency on the whole.From the bottom (70 cm depth, year 1867) to the depth of 16 cm (year 1998), the concentration of ∑PAHs shows volatile ups and downs.A peak value was obtained in into Jinning County were dominated by the biomass burning and domestic coal combustion processes.In recent years, the ever increasing vehicular combustion emission has also contributed a portion of the PAH inputs.

Assessment of PAHs Pollution. Results of this study show relatively low concentration in the Shuanglong catchment.
The pollution levels were in moderate.A comparison with other reports on ∑PAHs in the sediments from different areas are listed in Table 1.The PAH concentrations in the sediments found the same extent to Dongjiang catchment [55], Shilianghe catchment [56], and Baoxianghe catchment [57].Results in this study appeared much lower than those in Shitou Koumen catchment [58], Yongding catchment [59], and Fenhe catchment [60].And they were less polluted than Xidayang catchment [61] and Feitsui catchment [62].However, the sampling sites represented a higher contamination than that in Xiaolangdi catchment [53] and Manwan catchment [54].

Identification of the PAHs Sources.
The determination of PAHs sources is to decrease the input of PAHs to the environment.The deposited PAHs may come from natural and anthropogenic activities.The profile distributions are shown in Figure 5.The ratios of Ant/(Phe + Ant) were in the range of 0.03 to 0.64 with an average of 0.56, indicating PAHs were mainly derived from pyrogenic materials.These were further confirmed by the ratios of BaA/(BaA + Chry) ranging from 0.54 to 0.99 with an average of 0.77.It can be concluded that coal and biomass combustion were attributed to the major input of PAHs, which was anastomosed with the historical development in this area.Coal was the predominant energy resource and widely used by most of industries and residents in Jinning County.The discrimination of the   On the other hand, due to the development of tourism in the surrounding area, the vehicle emission flux is increasing.Thus the incomplete combustion of gasoline and fossil fuel became more and more predominant sources of PAHs year by year.The petrogenic sources may be interpreted to the scour of rainwater.The street dust which potentially contains petrogenic PAHs (spilled fuel, asphalt, and tire-wear materials) may enter into the water body and finally get into the sediments.

Effects of the Organic Matter Transportation on the PAHs
Distribution.With respect to the relationship between the contents of PAHs and TOC, many researches had been conducted.It turned out to be a positive linear relationship between PAHs and TOC contents [1,62].The sediments are rich in organic matter (OM), which has a complex pore structure.The pore structure has a fast absorbed outer surface and a slowly absorbed inner surface, making it easily absorb the PAHs.As a result, OM became the storage depot of PAHs.PAHs migrate along with the sediment particles, so the TOC was affected significantly on the distribution of PAHs in the sediment core.The higher contents of TOC corresponded with the higher value of PAHs.In this study, the relationship between the contents of TOC and PAHs are shown in Figure 6.TOC contents had positive correlation with ∑PAHs ( 2 = 0.72,  < 0.001), 2-3 rings ( 2 = 0.44,  < 0.001), 4-ring ( 2 = 0.78,  < 0.001), and 5-6 rings ( 2 = 0.62,  < 0.001).HPAHs have the characteristics of being strongly lipophilic and hard-degraded.Therefore, HPAHs may be adsorbed by TOC easily.While LPAHs are relatively more volatile and soluble.Obviously, the correlation between TOC contents and higher PAHs contents is much higher than that of the lower ones.TOC contents played more contribution in controlling HPAHs than LPAHs.

Conclusions
In this study, the PAHs distributions, sources identification, and influence factors analysis in the Shuanglong catchment of the Dianchi watershed were investigated.Based on the results we obtained the following conclusions: (1) The ∑PAHs concentrations ranged from 102.47 to 563.24 ng g −1 , reflecting a moderate pollution level.The ∑PAHs compositions were dominated by 2-3 rings PAHs.As for individual PAH, Nap, Ace, Flu, BaA, and BaF were main components.
(2) Based on the molecular indices (Ant/(Phe + Ant), Fla/(Fla + Pyr), and BaA/(BaA + Chry)), it was suggested that the sedimentary PAHs contents in the Shuanglong catchment were dominated by the coal combustion and petrogenic materials.
(3) Significantly positive correlations between TOC and HPAHs indicated that TOC played a more important role in controlling the distributions of HPAHs than LPAHs in sediments of the Shuanglong catchment.

Figure 1 :
Figure 1: Location of sampling sites.

1 )Figure 2 :
Figure 2: Fluctuation of sediment accumulation rate with time by CRS model.

Figure 3 :
Figure 3: Vertical distribution of TOC, TN, TP, and C/N in the sediment.

Figure 6 :
Figure 6: Relationship between TOC and PAHs concentrations.

Table 1 :
Comparisons of PAHs in different sediments in China.