Hydrogeology Journal (2018) 26:2315–2325 https://doi.org/10.1007/s10040-018-1814-2

REPORT

Sources and contamination characteristics of PAHs in environmental media in a karst underground river system (southern China) Li Lu 1,2 & Zhe Wang 1,2 & Jianguo Pei 1,2 Received: 16 November 2017 / Accepted: 5 June 2018 / Published online: 20 June 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The aim of this study was to determine the sources and contamination characteristics of polycyclic aromatic hydrocarbons (PAHs) in various environmental media in a karst underground river system. For this purpose, air, underground river water, sediment, and soil samples were collected from a typical underground river in southern China in the dry and wet seasons of 2013– 2014, and the compositional spectra, distribution, and ratio characteristics of 16 PAHs were determined for comparative analysis. The results show that three 2–3-ring PAHs (naphthalene, phenanthrene, and fluoranthene) mainly occur in air and underground river water. In sediments and soils, 4–6-ring PAHs are the main components. The PAH concentrations in the air in the wet season are clearly greater than those in the dry season, while it is the opposite in the underground water. Seasonal differences in the concentration of PAHs in the sediments and soils are minor. The concentrations of PAHs in the environmental media overall showed variation in the following order: upstream < midstream < downstream, and this is related to pollutant discharge, adsorption, etc. The main source of PAHs in the upstream area is the combustion of grass, wood, and coal, while it is petroleum in the midstream area, and combustion of grass, wood, coal, and petroleum near the outlet of the underground river. It is necessary to change the energy structure in the study area, improve the efficiency of environmental protection facilities, reduce the emission in vehicle exhaust, and control pollution by PAHs at their sources. Keywords PAHs . Karst . Environmental media . Contamination . China

Introduction Polycyclic aromatic hydrocarbons (PAHs) are a type of persistent organic pollutant that are mainly derived from the incomplete combustion of fossil and biomass fuels (Xu et al. 2006). They pose serious hazards to the environment and human health as they are carcinogenic, teratogenic, mutagenic, and nonbiodegradable. With economic development and increases in human activities, PAH pollution is gradually becoming more serious. The US Environmental Protection Agency (EPA) identified 16 PAHs as priority control pollutants (Dugay et al. 2002).

* Zhe Wang [email protected] 1

Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China

2

Key Laboratory of Karst Dynamics, MLR & GZAR, Guilin 541004, China

Underground rivers are the main source of water in many karst regions, providing water supply to urban and rural areas (Qin et al. 2007). However, because of their unique water-bearing structure and point-source recharge features, such as sink holes and underground river entrances, pollutants easily enter the aquifer directly without any filtration; thus, karst underground rivers are extremely vulnerable to pollution, and recovering water quality after pollution is difficult. Once PAHs enter underground rivers, they reside there for a long time and cause persistent pollution, owing to their own characteristics and the special environmental characteristics of underground spaces, sometimes turning underground rivers into sewage channels; therefore, it is important to study PAH pollution in underground rivers. At present, research on PAHs mainly focuses on surface water (Su et al. 2014; Shi et al. 2005; Mitra and Bianchi 2003; Zhang et al. 2004; Farooq et al. 2011; Xu et al. 2011; Guo et al. 2012), air (Liu and Wang 2007; Zhu et al. 2001; Yang et al. 2014; Ravindra et al. 2008; Zhang et al. 2014a, b), and soil (Zhu et al. 2014; Lau et al. 2012; Peng et al. 2012; Zhang et al. 2014a, b) in nonkarst areas. Even in

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karst areas, most scholars only study PAHs in a single environmental medium (Kong et al. 2011a, b; Lang et al. 2014); therefore, this study focused on the characteristics of PAHs in multiple environmental media, rather than a single medium, in an underground river system. The Qing-shui Spring underground river, a typical underground river in Nanning City (southern China), was selected as the study area, and the pollution levels, chemical components, distribution characteristics, and main sources of PAHs in air, underground river water, sediments, and soils were analyzed. The analysis showed that from upstream to downstream, the source of pollution changed from the single sources of grass, wood, and coal combustion or petroleum to multiple sources, and the concentrations increased gradually.

Study area The Qing-shui Spring is located in Pu-miao town, Yongning District, Nanning City, Guangxi (Fig. 1). The average annual precipitation over 20 years in the area is 1,235.2 mm. The data come from the Nanning meteorological station, which is located in the south of Nanning City, at an elevation of 30 m above sea level (asl). The geomorphology is characterized by karst hills with an elevation of 20–60 m asl, and development of a regional subsurface water-flow system. The study area is about 55 km2 and the exposed strata mainly consist of Carboniferous and Middle and Upper Ordovician thickly bedded limestones and clastic dolostones (C 2 , C 3 ), Cretaceous lower-grade siltstones (K 1 ), and Tertiary-Paleocene coarse sandstones (E 1 ). Underground river outlets, roof-collapse features (skylights), and sinkholes are more developed in the exposed limestone area. The underground river flows from east to west (traced by skylights QS06, QS09, and QS11 in Fig. 1), with the water flowing out of the ground to form of a stream at QS16 (underground river outlet). The underground river is 7 km in length, and the flow at the outlet in the dry season (from November to March the next year) is 900 L s−1, while the flow in the wet season (from May to August) is 2,300 L s−1. A water collection plant with a water supply capacity of 50,000 m3 day−1 has been built at the outlet, and has become an important source of water for Nanning. Pollution sources in the study area are divided into industrial, domestic, and agricultural. Industrial pollution is attributed to companies operating paper mills and a cement plant, which includes three elevated chimneys, and a gas station. Domestic pollution originates from domestic sewage and domestic waste produced by 10,000 residents in the study area, while agricultural pollution originates from pesticides and fertilizers used in agriculture and wastewater discharged from farms.

Hydrogeol J (2018) 26:2315–2325

Materials and methods Sample collection Air, water, sediment and soil samples were collected during the periods of the dry season and the wet season. The dry season was sampled from November 2013 to January 2014, while the wet season sampling was concentrated in June 2014. Based on the distribution of pollution sources and local wind direction (prevailing wind direction is easterly), the best air sampling point was selected such that the point represents the air conditions in the study area; this air collection point was at the underground river outlet (KQ01). In the dry season (the winter), the lower rainfall leads to the decline of the water level or even disconnection of flow at some of the original sampling points. Considering the difficulty in sampling, the characteristics of the upstream and downstream of the study area, and the distribution of pollution sources, five underground river water sampling points (QS01, QS02, QS04, QS09, and QS16), two sediment sampling points (QS04-CJ and QS09-CJ), and three soil sampling points (NT1, NT2, and NT3) were selected along the flow direction. All sampling points are shown in Fig. 1. Air samples were collected using a polyurethane foam (PUF) passive sampler (Jaward et al. 2004; Garban et al. 2002; Kong et al. 2012c). The PUF sampler has the following specifications: round-pie disc shape, 7 cm radius, 1.5 cm thickness, 154 cm2 surface area, 231 cm3 volume, and 4.5 g weight. Underground river water samples were collected directly at the water sampling points using 1-L brown fine bottle and sealed with polytetrafluoroethylene seal membrane. One kilogram sediment samples were collected in a brown jar with a grab bucket from the bottom of the river, and soil samples were collected from surface soil at a depth of 0–20 cm using a stainless steel shovel. After sample collection, the air samples were sealed in the polyethylene plastic bags, the underground river water samples were sealed in the 1,000-ml organic brown bottles, the sediment samples were placed in the brown jars, and the soil samples were placed in the polyethylene bags. All samples were refrigerated at 4 °C in a car refrigerator and transported to the laboratory within 1 day and pretreatment was completed within 7 days.

Reagents and materials Methylene chloride, n-hexane and acetone reagents used in the analysis, whose use reflects the presence of pesticide residues, were purchased from Fisher, USA. Anhydrous sodium sulfate (analytical grade) was placed in a muffle furnace oven burning at 480 °C for 5–6 h, and then placed in a cooling dryer as stand-by. Silica gel and alumina were activated at 180 and 250 °C, respectively, for 24 h and then allowed to cool to room temperature. Ultra-pure water equivalent to 3% of the mass of

Hydrogeol J (2018) 26:2315–2325

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Riv

iang

C1 Bachi

C1

j Yo n g

Tuanjie village

R iv er cement plant

E1

E2

r ve

N

C1

C1 R iv e r

Yongjiang

iang

C1 Q4

C1

Ri

er

j Yo n g

E1

C1

E2

Yongning District

C1

E1

K1 C2

C1

Nalian villiage

Q4

C3

E1

paper mill paper mill gas station

QS05 QS10 NT3 Ganhuai village QS08 QS04 QS03 QS02 QS16 QS06 QS09 C 3 Nabei village KQ01 QS11 QS07 QS13 NT2 QS12

Q4 C2

C3

Renmei villiageC 3 C2

Nawang villiage

Tunli village

QS14

E1 QS01

NT1

Naba village

0

Nasao village

K1

QS15

Q4

2km

Russia Shiguang village

Kazakhstan Mongolia

Zhoutong village

P1

K1

Legend K1 E2

Spring

Well

Quarry

Chimney

Air sampling point

Soil sampling point

South Korea

Bhutan

India

India

Burma

Study area

m

Sinkhole

os

Underground river outlet

China

Nepal

na

Skylight

La

Stratum boundary

et Vi

Boundary of study area

North Korea

Kyrgyzstan Tajikistan Pakistan

Fig. 1 Location map showing sampling points in the study area, and regional distribution of geological units

the gels was added to reduce activity, and after equilibration, n-hexane was added and the gels stored in a desiccator for later use. Dichloromethane was extracted through filtration using a filter paper and cotton wool for 72 h, after which it was air-dried and then sealed. Sixteen priority control PAHs identified by the EPA were purchased from Dr. Ehrenstorfer Germany GmbH. PAH recovery indicators were deuterated naphthalene, deuterated acenaphthene, deuterated phenanthrene, deuterium, and deuterated perylene, and the internal standard was hexamethylbenzene, purchased from Supeco, USA.

Sample pretreatment Air sample pretreatment First, the air sample was weighed and the PAH recovery indicators were added to the sample. In accordance with the Soxhlet extraction method, dichloromethane was used in continuous distillation of the samples for 48 h. The extract was concentrated to 5 ml, and then dissolved and exchanged to n-hexane. Separation and purification were performed using alumina and silica columns with a volume ratio of 1:2. The column was packed according to the wet

method: 10-cm silica gel, 5-cm alumina, and 2-cm anhydrous sodium sulfate were sequentially loaded from bottom to top. Then the column was leached with 25-ml mixed liquid of dichloromethane and n-hexane with a volume ratio of 2:3. The column liquid was concentrated to 0.5 ml and transferred to 2-ml cell flasks, flushed to 0.2 ml with nitrogen, added with 4 μl of hexamethylbenzene, and placed in a refrigerator for measurement. Underground river-water-sample pretreatment The samples were kept standing for 24 h before pretreatment. The supernatant was poured into a separatory funnel, and 1-L water samples were filtered through a glass fiber filter, added with 25 ml of dichloromethane and PAH recovery indicators. The mixture was shaken and allowed to stand still until it layered. The liquid was then concentrated to 2 ml, and subjected to the same processing steps as for the air samples. Sediments sample pretreatment The sediment samples were freeze-dried, ground, and sieved. Then, 20 g samples were weighed and the PAH recovery indicators were added to each sample. According to the Soxhlet extraction method,

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Hydrogeol J (2018) 26:2315–2325

dichloromethane was used in the continuous distillation of the samples for 48 h. The extract was concentrated to 5 ml, and subjected to the same processing steps as for the air samples. Soil sample pretreatment The soil samples were passed through a 100-mesh-stainless-steel sieve after freeze-drying and grinding. Similar to the sediment samples, 20 g samples were weighed, the Soxhlet extraction method was performed, the extract was concentrated, and then the same processing steps as for the air samples were applied.

PAHs analysis and testing PAHs were detected by a 7890A gas chromatograph-mass spectrometer (Agilent, USA) with a capillary column: HP5MS (30.00 m × 0.32 mm × 0.25 μm). The inlet temperature was 270 °C, and during the heating process, the initial temperature of 40 °C was held continuously for 4 min, and then raised to 300 °C for 12 min at a rate of 5 °C min−1. High-purity helium was loaded at a flow rate of 1 ml min−1 without shunt injection, and the injection volume was 1 μl. An internal standard method and multi-point calibration curve were used to qualitatively analyze the PAHs. Those tested are 2–3-ring PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, Table 1

phenanthrene, anthracene, and fluoranthene), 4-ring PAHs (pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, and benzo[k]fluoranthene), and 5–6-ring PAHs (benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, and benzo[ghi]perylene).

Quality control and quality assurance The analytical instrument control process in these experiments is consistent with the EPA requirements, and the quality control and quality assurance measures are described in the literature Lin et al. (1999) and Kong et al. (2012a, b, c). Quality assurance and quality control measures for the entire process were undertaken using blank samples, spiked samples, spiked parallel samples, and parallel samples, where no target compound was detected in the blank sample, and the relative standard deviations of spiked parallel and parallel samples were less than 10%. A recovery indicator was added to each sample before extraction to detect the sample recovery rate. In this study, the recovery rate is 75–95% (average 84.0%) for air samples, 83–105% (average 94.5%) for underground river water samples, 78–97% (average 85.5%) for sediment samples, and 81–102% (average 89.6%) for soil samples. The detection limits for air, underground water, sediment

Concentration of PAHs in various environmental media in the dry season of the Qing-shui Spring underground river system Air (ng m−3)

Underground river water (ng L−1)

KQ01

QS16

QS09

QS04

QS02

Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo[a]anthracene Chrysene Benzo[b]fluoranthene

19.38 4.82 6.52 10.86 17.62 1.34 19.96 21.30 ND 10.54 ND

32.94 2.89 3.91 3.30 26.02 2.19 32.29 38.54 11.04 26.44 11.39

12.66 1.85 4.34 16.24 35.12 3.86 17.89 29.16 5.06 24.18 5.53

34.37 2.89 3.58 ND 17.43 3.00 28.03 18.45 6.80 12.32 7.75

Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Dibenzo[a,h]anthracene Benzo[ghi]perylene 2–3-ring 4-ring 5–6-ring ∑PAHs

ND ND ND ND ND 80.50 31.84 0.00 112.34

7.23 9.20 ND 4.80 7.78 103.54 94.64 21.78 219.96

7.23 6.28 ND 3.30 4.54 91.96 71.16 14.12 177.24

5.07 6.21 ND 4.50 5.54 89.30 50.39 16.25 155.94

PAHs

ND not detected

Underground river sediments (ng g−1)

Soil (ng g−1)

QS01

QS09-CJ

QS01-CJ

NT3

NT2

NT1

27.14 2.86 3.42 ND 18.24 2.77 28.06 16.17 12.54 9.95 7.91

31.17 2.86 3.84 ND 22.65 2.85 21.22 10.53 11.76 10.18 9.92

6.19 1.10 1.85 2.86 6.77 0.55 4.80 8.82 2.38 11.76 3.54

4.10 1.02 1.74 2.71 5.67 0.73 16.44 4.10 3.51 2.13 3.98

2.27 0.16 0.23 0.45 2.65 0.21 3.88 4.22 1.25 4.70 2.02

1.17 0.14 0.27 0.38 1.65 0.27 3.91 0.32 0.50 0.34 0.95

3.15 0.09 0.24 0.42 2.01 0.23 2.03 0.46 1.69 0.98 2.01

5.28 5.98 ND 2.20 5.67 82.49 51.85 13.85 148.19

6.40 5.94 ND 2.60 4.95 84.59 48.79 13.49 146.87

ND 2.49 5.23 1.06 2.35 24.12 26.50 11.13 61.75

3.09 2.02 3.63 1.52 ND 32.41 16.81 7.16 56.38

1.50 1.77 3.83 1.10 2.32 9.84 13.69 9.01 32.55

1.25 1.87 3.29 1.76 1.81 7.79 3.36 8.73 19.87

1.20 1.40 ND 0.64 1.31 8.17 6.34 3.35 17.85

Hydrogeol J (2018) 26:2315–2325 Table 2

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Concentration of PAHs in various environmental media in the wet season of the Qing-shui Spring underground river system

PAHs

Air (ng m−3)

Underground river water (ng L−1)

KQ01

QS16

QS09

QS04

QS02

QS01

Underground river sediments (ng g−1)

Soil (ng g−1)

QS09-CJ

NT3

QS01-CJ

NT2

NT1

Naphthalene

23.94

20.81

10.74

29.25

22.94

28.21

5.83

4.17

1.48

1.20

1.76

Acenaphthylene

3.31

2.12

1.36

3.25

3.80

2.10

1.93

1.78

0.28

0.25

0.16

Acenaphthene Fluorene

3.95 13.58

2.87 ND

3.19 1.94

3.20 ND

3.08 ND

2.82 ND

1.24 2.30

2.05 2.75

0.41 0.81

0.49 0.68

0.43 0.76

Phenanthrene

20.60

15.73

20.94

13.94

11.71

16.64

7.29

4.92

2.24

1.17

1.29

Anthracene Fluoranthene

ND 31.15

2.15 18.42

1.85 10.97

1.71 15.74

1.32 15.57

1.70 12.29

1.09 4.27

1.57 9.52

0.80 2.92

0.61 3.15

0.50 1.36

Pyrene Benzo[a]anthracene

37.96 9.33

25.32 6.50

21.43 4.28

13.95 7.30

12.92 9.21

9.94 7.78

9.44 1.37

6.12 2.89

4.20 1.85

1.14 1.90

0.69 2.34

Chrysene

19.71

18.60

19.07

9.05

10.44

9.11

11.57

4.03

3.96

1.01

1.36

Benzo[b]fluoranthene Benzo[k]fluoranthene

4.63 1.86

8.37 5.31

5.06 5.42

5.69 3.73

5.81 3.88

7.29 5.70

4.10 ND

3.97 2.41

2.64 2.70

1.71 1.53

1.46 1.16

Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Dibenzo[a,h]anthracene

ND ND ND

3.76 ND 3.66

5.31 ND 2.42

3.56 ND 3.31

2.39 ND 1.62

2.36 ND 1.91

2.36 5.15 1.85

3.13 4.34 1.66

3.18 2.29 1.98

2.36 1.92 1.17

1.51 ND 0.58

Benzo[ghi]perylene 2–3-ring

0.75 96.53

3.12 62.11

3.47 50.98

2.07 67.09

2.17 58.42

1.64 63.76

3.12 23.95

ND 26.76

1.18 8.95

1.25 7.55

1.06 6.26

4-ring 5–6-ring

73.49 0.75

64.11 10.53

55.27 11.20

39.72 8.94

42.27 6.18

39.82 5.91

26.47 12.48

19.42 9.13

15.34 8.63

7.29 6.71

7.01 3.16

∑PAHs

170.77

136.74

117.45

115.75

106.87

109.50

62.91

55.31

32.91

21.55

16.42

ND not detected

and soil samples are 0.04–0.4 ng m−3, 0.01–0.09 ng L−1, 0.01–0.018 ng g−1, and 0.08–0.60 ng g−1, respectively.

Results and discussion Concentration and composition of PAHs in various environmental media The composition and concentration of PAHs in various environmental media from the dry season and the wet season in the Qing-shui Spring underground river system are given in Fig. 2 Component spectrum of 16 PAHs in the environmental media of the study area in the dry season

Tables 1 and 2. The concentrations of ∑PAHs in the air of the dry and wet season were 112.34 and 170.77 ng m−3, respectively. The 2–3-ring PAHs naphthalene, phenanthrene, and fluoranthene represent the main components. For the dry season, the detection rate of PAHs in the air was low; seven PAHs were not found above the detected limit—benzo[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, and benzo[ghi]perylene. In the wet season, only four PAHs were not detected in the air samples. In the dry and wet seasons, the average sum concentrations of PAHs (∑PAHs) in the underground water were 175.50 and 118.81 ng L−1, respectively.

PAHs% 25 20 15

Underground river water Underground river sediments

10

Soil Air

5

Nap Acy Ace Flu Phe Ant FlA Pyr BaA Chr BbFBkF BaP InP DaABgP

2320 Fig. 3 Component spectrum of 16 PAHs in the environmental media of the study area in the wet season

Hydrogeol J (2018) 26:2315–2325 PAHs% 25 20 15

Underground river water Underground river sediments

10

Soil Air

5

Nap Acy Ace Flu Phe Ant FlA Pyr BaA Chr BbFBkF BaP InP DaABgP

Naphthalene, phenanthrene, and fluoranthene, with 2–3 rings, account for 54.84 and 52.20%, respectively, of the main components. The detection rate of 2–3-ring PAHs was 100%, except for fluorene and indeno[1,2,3-cd]. In general, 2–3-ring PAHs are the main components for noncontaminated or slightly polluted underground river water, whereas 4–6-ring PAHs are the main components for seriously polluted water (Kong et al. 2011b). The characteristics of the concentration and content of PAHs in the underground water in the study area were similar to those discussed above, which was slightly polluted by 2–3-ring PAHs; however, in some slightly polluted water, under the action of microbes, the 2–3-ring PAHs are easy to biodegrade and generate other organic pollutants, thus increasing the proportion of 4–6-ring PAHs. Slight pollution mentioned here refers to the concentrations of ∑PAHs ranging from 50 to 250 ng L−1, irrespective of the number of PAH rings, which is the definition in China’s sanitary standard for drinking water in 2006. In sediment samples, the average concentrations of ∑PAHs of the dry and wet season were 61.79 and 61.87 ng g−1, respectively, with 4–6-ring PAHs being the main components. Except for the detection rate of benzo[k]fluoranthene and benzo[ghi]perylene, that of other PAHs was 100%. In soil samples, the average concentrations of ∑PAHs of the dry and wet season were 24.61 and 24.33 ng g−1, respectively, Fig. 4 Distribution of PAHs in the underground river water

with 4–6-ring PAHs accounting for 65.06 and 68.82% respectively of the main components. All 16 PAHs were detected in the soil samples, except for indeno[1,2,3-cd]pyrene in sample NT1. In summary, air and underground river water were mainly polluted by naphthalene, phenanthrene, and fluoranthene, while sediments and soils were mainly polluted by 4–6-ring PAHs. Figures 2 and 3 show that the component spectra of detected PAHs are similar for various environmental media, indicating that their composition is highly consistent. From the viewpoint of the severity of the pollution, according to the low biological impact value of the PAHs risk standard determined by Long et al. (1995), the China sanitary standard for drinking water in 2006 and the China environmental air quality standard in 2012, the concentrations of ∑PAHs in the air, underground river water, sediments, and soils of the study area were less than 500 ng m−3, 250 ng L−1, 4,022 ng g−1, and 4,022 ng g−1, respectively, such that the concentrations of ∑PAHs are still at low levels.

Spatial and temporal distribution of PAHs in various environmental media It is known from Tables 1 and 2 that the concentrations of PAHs in the air in the wet season are obviously higher than

PAHs/ng L-1 250

200

150

Dry season Wet season

100

50

0 0

3000

6000 9000 Distance from the downstream/m

12000

Hydrogeol J (2018) 26:2315–2325 Fig. 5 Distribution of PAHs in sediments and soils

2321 PAHs/ng g-1 70 60 Dry season Wet season Sediment sample Soil sample

50 40 30 20 10 0 0

3000

6000

9000

12000

15000

Distance from the downstream/m

those in the dry season, and they are related to the local pollution sources, wind direction and temperature. The air sampling point (KQ1) is located downwind of the paper mill, and, with the low temperature in winter, the plume containing the PAHs does not have enough power to reach the sampling point; however, in the summer, the opposite is true. The concentrations of PAHs in the underground water in the dry season are obviously greater than those in the wet season. This is because the flow of the underground river is greatest in the wet season, and the dilution of PAHs in the underground river is high, which leads to the low concentration of PAHs in the river. The seasonal differences in the concentration of PAHs in the sediments and soils were small, because pollutants in sediments and soils are the result of long-term accumulation, and are less affected by different seasons. As shown in Figs. 4 and 5, the distribution characteristics of PAHs in underground water, sediment and soils of Qing-

Fig. 6 Percentage of PAHs composition with different numbers of rings in the underground water in the dry season

shui Spring system follow the order: upstream < midstream < downstream. The gradual increase in concentrations is mainly due to the increasing number of pollution sources (chimneys, gas stations, quarries, etc.) from upstream to downstream. This leads to PAHs in underground river water, sediments, and soils, accumulating and increasing in concentration. This is particularly evident in the underground river, with less pollution in the midstream and upstream, the slow increase in concentration of PAHs, the increase in the pollution sources in the downstream, and the rapid increase of the concentrations of PAHs. However, the distribution characteristics of PAHs in the underground river in the dry season and the wet season are slightly different, and the increase in the concentrations of PAHs from midstream to downstream in the dry season is greater than those in the wet season. There are two possible reasons for this—on the one hand, the flow rate in the downstream of the wet season is large, and the dilution effect

PAHs/% 70 60 Percentage of 2-3-ring PAHs

50

Percentage of 4-6-ring PAHs 40 30 20 10 0 0

3000

6000 9000 Distance from the downstream/m

12000

15000

2322 Fig. 7 Percentage of PAHs composition with different numbers of rings in the underground water in the wet season

Hydrogeol J (2018) 26:2315–2325 PAHs/% 70 60 50 Percentage of 2-3-ring PAHs Percentage of 4-6-ring PAHs

40 30 20 10 0 0

3000

6000

9000

12000

15000

Distance from the downstream/m

reduces the increase in the concentration of PAHs smaller, while on the other hand, the sampling period of the dry season is winter, which is accompanied by low temperatures and poor solar radiation. Such conditions are not conducive to the evaporation and environmental degradation of PAHs. Figures 6 and 7 show that the range in the percentages of 2– 3-ring PAHs in the underground river midstream and upstream (QS1, QS2, and QS3) is 54.66–58.23%, while in the downstream, the percentages of 2–3-ring PAHs begins to decrease. At the underground river outlet (QS16), the percentage of 2–3ring PAHs has decreased by about 10%, and 4–6-ring PAHs have become the main components. This phenomenon may be related to the incorporation of PAHs from downstream pollution sources (quarries, gas stations, etc.). In general, most of the pollutants discharged from gasoline vehicles and industrial coal combustion are mainly 4–6-ring PAHs, while the pollutants discharged from the residential combustion of coal and wood are mainly 2–3-ring PAHs (Lin et al. 2015). The paper

Fig. 8 Percentage of PAHs composition with different numbers of rings in the soils and sediments in the dry season

mill and quarries in the downstream of the study area are industrial coal-fired enterprises, and the gasoline in cars does not burn completely; thus, the PAHs in the downstream region are mainly composed of 4–6-rings. The main source of pollution in the upstream is coal and firewood combustion, which leads to the PAHs in the upstream being mainly composed of 2–3rings. The tendency of PAHs in sediments and soils is similar to that of the underground river water. The percentage of 2–3ring PAHs from upstream to downstream in the dry season and the wet season decreased by 17.99 and 10.30%, respectively in sediments, and decreased by 15.52 and 10.93%, respectively in soils. In addition to the continuous pollution sources along the course of the river, the other reason is that the n-octanol-water partition coefficient of 4–6-ring PAHs is higher (lgKow 5.18– 6.75), and PAHs are more likely to be adsorbed by solid media such as sediments and soils Figs. 8 and 9. By comparing the percentage of PAHs in underground water, sediments, and soils in the same area, it was found that the

PAHs/% 80 70

Percentage of 4-6-ring PAHs in the soil Percentage of 4-6-ring PAHs in the sediment Percentage of 2-3-ring PAHs in the soil

60 50 40

Percentage of 2-3-ring PAHs in the sediment

30 20 10 0 0

3000

6000

9000

Distance from the downstream/m

12000

15000

Hydrogeol J (2018) 26:2315–2325 Fig. 9 Percentage of PAHs composition with different numbers of rings in the soils and sediments in the wet season

2323 PAHs/% 80 70

Percentage of 4-6-ring PAHs in the soil

60

Percentage of 4-6-ring PAHs in the sediment

50

Percentage of 2-3-ring PAHs in the soil

40

Percentage of 2-3-ring PAHs in the sediment

30 20 10 0 0

3000

percentage of 2–3-ring PAHs ranked as follows: underground river water > sediments > soil, while the percentage of 4–6ring PAHs showed the opposite trend (such as at QS09, QS09CJ, and NT3). The physicochemical properties of PAHs give rise to this phenomenon. The n-octanol-water partition coefficient of 2–3-ring PAHs is small, and these PAHs mainly exist in the aqueous phase. The n-octanol-water partition coefficient of higher-ring PAHs is higher, they are hydrophobic, and their pro-granule characteristics are so strong that they are adsorbed tightly on to sediment or soil particles. The proportion of higher-ring PAHs in sediments is lower than that in soils. In general, the source of the underground river sediments consists of two parts: (1) incorporation of surface material, such as soil and sediments from the surface to the underground; and (2) the migration of underground material, such as underground river sedimentation, and physicochemical weathering. Thus, sediment composition is extremely complex, and is diluted by material with a lower proportion of high-ring PAHs. While PAH occurrence in soil is the result of long-term accumulation by sedimentation and adsorption, volatilization of high-ring PAHs in the soil is not favored, resulting in a high proportion of PAHs in the soil. Differences in soil organic matter composition may be another factor influencing this phenomenon (Ni et al. 2006).

Source analysis of PAHs in environmental media Scholars in China and abroad (Doong and Lin 2004; Sun et al. 2014; Chen et al. 2014) often use the isomer ratio method to

Table 3

6000 9000 Distance from the downstream/m

12000

15000

identify PAH sources in environmental media; therefore, this study used the ratios of benzo[a]anthracene/ (benzo[a]anthracene + chrysene), denoted by BaA/(BaA + Chr), and fluoranthene/(fluoranthene + pyrene), denoted by FlA/(FlA + Pyr), to identify the PAH sources for the Qingshui Spring underground river system. The identification standards for specific PAH sources are shown in Table 3. The results of the calculations are shown in Table 4. Table 4 shows that the ranges in the ratio of BaA/(BaA + Chr) in the dry season and the wet season are 0.17–0.63 and 0.11–0.65, respectively. The ratios indicate that the source is petroleum at QS09 and QS09-CJ, a mixture of petroleum and combustion at QS16 and NT3, and combustion at the other sampling points. QS09 and QS09-CJ are located at the skylight at the Gan-huai village. There is a gas station situated in the northeast part of the village, and gasoline, diesel spills, and oily wastewater are discharged directly into the karst skylight. KQ01, QS16 and NT3 are located at the underground river outlet, and are affected by oil-bearing underground river water (from the QS09 skylights) as well as combustion (from quarries, paper mills, and chimneys) and, therefore, the indicated sources correspond to a mixture. The combustion sources for the sampling points are located in the middle and upper reaches, and are closely related to the type of energy used. The ranges in the ratio of FlA/(FlA + Pyr) in the dry season and the wet season are 0.35–0.92 and 0.31–0.74, respectively. These ratios at QS09 and QS09-CJ clearly indicate that the source is petroleum from oil-bearing wastewater discharge.

The relationship between the isomer ratios of PAHs and their sources

PAHs ratio

Source of PAHs, ratio value

BaA/BaA + Chr FlA/FlA + Pyr

Petroleum, <0.2 Petroleum, <0.4

Mixture of petroleum and combustion, 0.2–0.35 Combustion of petroleum, 0.4–0.5

Combustion, >0.35 Grass, wood, and coal, >0.5

2324 Table 4

Hydrogeol J (2018) 26:2315–2325 Isomer ratios of PAHs in various environmental media from the Qing-shui Spring underground river system

Sampling time

Dry season Wet season

PAHs ratio

Air

Underground river water

KQ01

QS16

QS09

QS04

QS02

QS01

QS09-CJ

QS01-CJ

NT3

NT2

NT1

BaA/(BaA + Chr) FlA/(FlA + Pyr)

– 0.48

0.29 0.46

0.17 0.38

0.36 0.60

0.56 0.63

0.54 0.67

0.17 0.35

0.62 0.80

0.21 0.48

0.60 0.92

0.63 0.82

BaA/(BaA + Chr)

0.32

0.26

0.18

0.45

0.47

0.46

0.11

0.42

0.32

0.65

0.63

FlA/(FlA + Pyr)

0.45

0.42

0.34

0.53

0.55

0.55

0.31

0.61

0.41

0.74

0.66

The ratios indicate that the sources at KQ01, QS16, and NT3 are combustion of petroleum-based materials, and this is consistent with the proximity of the sampling points to a highway and quarries, where incomplete combustion of gasoline and diesel fuels is common. The ratio of FlA/(PyA + Pyr) in the other sampling points ranged from 0.53 to 0.92, which indicate that the main sources are combustion of grass, wood, and coal. These sampling points are located in the upper and middle reaches of the underground river in rural areas. The sampling points are far away from the urban area, and the agricultural population accounts for the vast majority of population, with more than 80% of the fuels used by the villagers being grass, wood, and coal. The energy structure is thus consistent with the aforementioned values of the inference ratios. In addition, a paper-mill chimney is located in the upwind direction; hence, the combustion products also affect the indicated source of the PAHs. The source at QS04, QS02, and QS01 is combustion, but the variation trend in the ratios of BaA/(BaA + Chr) and FlA/ (FlA + Pyr) in the dry season and the wet season is different. In the dry season, the ratios of BaA/(BaA + Chr) and FlA/ (FlA + Pyr) show a gradually increasing trend. This may be related to the distance of the sampling points from the papermill chimney. QS01, QS04, and QS02 are located in the downwind direction of the chimney at distances of approximately 750, 2,400, and 4,300 m, respectively. Because the sampling period was in winter, which is accompanied by low temperatures, PAHs discharged from the chimney were prone to sedimentation and component differentiation, leading to ratio differences between the two groups. In the wet season, the ratios of BaA/(BaA + Chr) and FlA/(FlA + Pyr) show a relatively stable trend, because of the heavy rainfall and high wind speeds during the rainy season, PAHs in air pollution do not readily settle-out.

Conclusions Among the various environmental media in the study area, air and underground river water were mainly polluted by three 2– 3-ring PAHs: naphthalene, phenanthrene and fluoranthene. Sediments and soils were mainly polluted by 4–6-ring

Sediments

Soil

PAHs. The detection rate of PAHs in air was clearly lower than that of the other three environmental media. From the viewpoint of the severity of pollution, the concentrations of PAHs in the various environmental media of the study area were at low levels. The seasonal variation in concentrations of PAHs in various environmental media was different. The concentration of PAHs in the air in the wet season was clearly greater than that in the dry season, while it was the opposite in the underground water. The seasonal differences in the concentrations of PAHs in the sediments and soils were minor. The distribution of PAHs in the river water, sediments, and soils of the Qing-shui Spring underground river followed the order of PAHs values upstream < PAHs values midstream < PAHs values downstream, while PAHs in the sediments > PAHs in the soils. In the same area, the percentage of 2–3-ring PAHs in the underground river was the highest, followed by sediments and soils, while the tendency of 4–6-ring PAHs was the reverse. The study revealed that the PAH sources are the combustion of grass, wood, and coal in the environmental media upstream, petroleum at the skylight near Gan-huai village in midstream, and petroleum and combustion at the underground river outlet. In the upstream, the sources of pollution are mainly for the combustion of coal and firewood. The local fuel structure should be changed and the natural gas or liquefied gas should be used as fuel. In the downstream, the sources of pollution are the pollution by oil and the industrial burning of coal. It is necessary to augment the seepage control measures at the gas station to mitigate discharge to the soil and groundwater, transform the coal-fired facilities, improve the traffic network, and reduce the emission in motor vehicle exhausts. Acknowledgements We would like to thank Yongsheng Lin, Lianjie Fan, Junge Dai, and Xiaoyuan Wang of the Institute of Karst Geology, Chinese Academy of Geological Sciences, for their help in the field investigation and sampling process. Funding information This research was financially supported by the National Natural Science Foundation of China (No. 41602277), and the Geological Survey Project of the China Geological Survey (No. 1212011121164).

Hydrogeol J (2018) 26:2315–2325

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