3.1 Level of PAHs in the sediment
The PAH concentrations of 18 sediments samples are presented in Table 2. The concentrations of total 16 PAHs (Σ16PAHs) ranged from 188.64 to 1060.39 ng/g, with an average of of 472.62 ng/g. The PAH concentration was similar with the studies conducted by Yuan Zhang(Zhang et al. 2012) and Bingli Lei(Lei et al. 2014), but showed a decrease than the level reported by Yuqiang Tao(Tao et al. 2010).The concentrations of total 7 carcinogenic PAHs (Σ7PAHs) ranged from 58.13 to 504.76 ng/g, with an average of 192.04 ng/g, which account for 40.63% of the Σ16PAHs. Among 16 PAHs, Nap was the main pollutant with mean concentration of 93.65 ng/g, whereas the concentration of Fluo, Acy and DbA were below the detection limit in almost samples.
3.2. Spatial distributions of PAHs
Figure 1 depicted the spatial distribution of Σ16PAHs in the sampling sites. The highest concentration of Σ16PAHs (1060.39 ng/g) was detected at WTP2, followed by 912.15ng/g at S12, 850.68 ng/g at S13, 749.74 ng/g at S7 and 586.84 ng/g at S16. The lowest concentration of Σ16PAHs (188.64ng/g) was observed at S6 in the Yongan river, one of the upstream rivers.
The WTP2 is a wastewater treatment plant of Yixing city nearby chemical industrial park, which is responsible for treating surrounding chemical wastewater, suggesting that the highest level of PAHs at site WTP2 might be associated with chemical plant emission. Sites S12, S13 and S16 are located in the lake estuary, where many chemical plants were concentrated, which may attribute to the chemical industrial activities. S7 is located at the Beijing-Hangzhou Canal of Changzhou, which is near the printing and dyeing industries, suggesting that wastewater from printing and dyeing operations might be the main source of PAH pollution. (Liu et al. 2016, Wang et al. 2018). The geographic distribution of PAHs showed that Σ16 PAH concentrations in the downstream rivers were higher than those in adjacent upstream rivers, which could be associated with domestic industrial wastewater and adhesion of upstream PAH pollution in sediment(Wang et al. 2018).
The composition patterns of the PAHs by ring size in the 18 samples were depicted in Fig. 2. The two-ring PAHs (Nap) accounted for 0–43.28% of the total PAH content, three-ring PAHs 14.17–31.88% (Acy, Ace, Fluo, Phe, Ant), four-ring PAHs 15.87–39.08% (Flua, Pyr, BaA, Chr), five-ring PAHs 4.89–39.30% (BbF, BkF, BaP, DbA), and six-ring PAHs 0–22.73% (InP, BgP).(Lin et al. 2018). Except for S4, S5, S6, S10 and S11, high- molecular- weight (HMW) PAHs (4–6 rings) were the predominant compounds in most sample sites (51.71%~72.29%). Indeed, due to the high water solubility and benthic recycling in aquatic environment, low- molecular- weight (LMW) PAHs (2–3 rings) were more likely to dissolve and degrade, while HMW PAHs were more resistant to degradation and easier to accumulate in the sediment(Liu et al. 2015, Montuori et al. 2016). In general, the composition of PAHs in the sediments is dominated by high rings, indicating that the combustion at high temperature is the major source of PAH pollution in sediment.
3.3. Source identification
3.3.1 Diagnostic ratio charts
Based on the distribution levels, the diagnostic ratios of Flua / (Flua + Pyr), Ant / (Ant + Phe), BaA / (BaA + Chr), and InP / (InP + BgP) were used to identify the possible sources(Yunker MB 2002). The results of diagnostic ratio for the sampling sites were shown in Fig. 3. In this study, ratio of Ant / (Ant + Phe) > 0.1 was found at most of the sample sites except S6, suggesting that the pollution of PAHs was attributed to combustion origin. Sample sites with Flua / (Flua + Pyr) < 0.4 accounted for 44.4%, and sites with > 0.5 accounted for 55.6%, which indicated that PAH pollution were mainly from source of direct petroleum spillage and combustion of biomass and coal. And similarly, the BaA / (BaA + Chr) of all of the samples were > 0.35 in this study, indicating the source of biomass and coal combustion. The ratio of InP / (InP + BgP) were < 0.2 at 77.8% of the sample sites, and only 22.2% of the sites were > 0.5, indicating the direct petroleum spillage pollution(Bemanikharanagh et al. 2017, Bortey-Sam et al. 2014, Yunker MB 2002). Therefore, these results showed that the PAH pollution was mainly sourced from and fuel combustion direct and petroleum spillage.
3.3.2 Principal component analysis (PCA)
PCA was applied to further explore the PAH sources in this study(Bemanikharanagh et al. 2017, Lin et al. 2018, Zheng et al. 2016). Researches show that PAHs with LMW are abundant in petrogenic and low-temperature pyrolytic sources (e.g., petroleum spillage and incomplete combustion), while those with HMW are abundant in compounds from pyrolytic sources(Li et al. 2015). Flua, Phe, Ant, Pyr usually imply coal combustion. Ace and Acy are the foremost product of coke burning. BaA, Chr and BaP are regarded as typical pollutants of biomass and coal combustion. BkF and BbF are typical pollutant of diesel emissions, and InP, BgP and DbA are typical markers of traffic emission(Kannan et al. 2005, Liu et al. 2017, Liu et al. 2016).
Barttlet's sphericity test was used to verify if the PCA was applicable in this study, and the P-value was < 0.01, indicating the applicability of PCA here (Zheng et al. 2016). The factor loadings for PAH concentration by variamax rotation were shown in Table 3. Two components (PC1 and PC2) were extracted from sediment data responsible for 81.88% of the total variation of PAHs. The PC1 explained 71.87% of the total variance, which was highly loaded with BaA, Chr, Ant, Ace and Phe and relatively highly loaded of Flua, Pyr, BbF, BaP and BkF, which suggest that the PAH pollution were mainly from the pretroleum spillage and incomplete combustion of coal, coke and biomass, while traffic emission (e.g. gasoline and diesel exhaust) is also an an important factor for PAH pollution(Zheng et al. 2016). PC2 account for 10.01% of the total variance, and InP and BgP account for high loadings, indicating that traffic emission was predominant in PC2, such as gasoline combustion and diesel combustion (Fig.S1).
Table 3
Factor loadings for PAHs concentritions with varimax rotation
PAHs
|
Ring
|
Principal component
|
PC1
|
PC2
|
NaP
|
2
|
0.59
|
0.51
|
Ace
|
3
|
0.93
|
0.25
|
Phe
|
3
|
0.87
|
0.23
|
Ant
|
3
|
0.87
|
0.19
|
Flua
|
4
|
0.79
|
0.32
|
Pyr
|
4
|
0.82
|
0.33
|
BaA
|
4
|
0.95
|
0.23
|
Chr
|
4
|
0.94
|
0.18
|
Bap
|
5
|
0.75
|
0.56
|
BbF
|
5
|
0.75
|
0.46
|
InP
|
5
|
0.06
|
0.96
|
BgP
|
6
|
0.37
|
0.77
|
BkF
|
5
|
0.73
|
0.52
|
Variance(%)
|
|
71.87
|
10.01
|
Cumulative(%)
|
|
71.87
|
81.88
|
Therefore, these findings further confirmed that PAH pollution were mainly sourced from mixture of petroleum spillage and fuel combustion, such as coal, coke, biomass, gasoline and diesel. which may be associated with intensive traffic (e.g. shipping), discharge of urban sewage and industrial wastewater(Li et al. 2015).
3.4. Risk assessment
3.4.1 Ecological risk assessment
Table 4 shows the results of ecological risk assessment in sediment, and classifies the sample sites into three different ranges: ecological risk rarely occurred (< ERL/TEL), occasionally occurred (≥ ERL/TEL and < ERM/PEL), and frequently occurred (≥ ERM/PEL). The results of SQGs showed that the concentrations were below than their respective ERM and PEL values, except for compound DbA in sample WTP2. The concentration of Ace at all of the sample sites were between ERL/TEL and ERM/PEL, indicating that the ecological risk caused by Ace may occur occasionally at all sites. The BaA and BaP concentrations were at levels where ecological risk may occur occasionally (≥ TEL and < PEL). Meanwhile, the concentration of Acy, Ant and Pyr at few sites were between the TEL and PEL level. The Nap concentration was below the ERL at most site, while TEL level was exceeded at most sites. In addition, the concentrations of Phe and Flua were lower than the ERL level at most of the sites. Therefore, these findings suggest that the pollution of Ace, BaA, BaP, Acy, Ant and Pyr may cause potential ecological risk occasionally at some sites. The mean PEL-Q values ranged from 0.09 to 0.29. Most of the sample sites had mean PEL-Q exceeded 0.1 but lower than 1.0 (< PEL-Q ≤ 1.0) and only two sites had mean PEL-Q < 0.1(S1 and WTP1), indicating that PAH contaminants may cause moderate ecological risk in most sample sites (Fig.S2).
Table 4
The summary of SQGs on PAHs in Taihu Lake.
|
|
|
Number of sampling sites
|
|
|
Number of sampling sites
|
PAHs
|
ERL
|
ERM
|
<ERL
|
(ERL,ERM)
|
>ERM
|
TEL
|
PEL
|
<TEL
|
(TEL,PEL)
|
>PEL
|
NaP
|
160
|
2100
|
17
|
1
|
-
|
34.6
|
391
|
2
|
16
|
-
|
Acy
|
44
|
640
|
16
|
2
|
-
|
5.87
|
128
|
16
|
2
|
-
|
Ace
|
16
|
500
|
0
|
18
|
-
|
6.71
|
88.9
|
-
|
18
|
-
|
Fluo
|
19
|
540
|
-
|
-
|
-
|
21.2
|
144
|
-
|
-
|
-
|
Phe
|
240
|
1500
|
18
|
-
|
-
|
41.9
|
515
|
18
|
-
|
-
|
Ant
|
85.3
|
1100
|
18
|
-
|
-
|
46.9
|
245
|
13
|
5
|
-
|
Flua
|
600
|
5100
|
18
|
-
|
-
|
111
|
2355
|
18
|
-
|
-
|
Pyr
|
665
|
2600
|
18
|
-
|
-
|
53
|
875
|
15
|
3
|
-
|
BaA
|
261
|
1600
|
18
|
-
|
-
|
31.7
|
385
|
-
|
18
|
-
|
Chr
|
384
|
2800
|
18
|
-
|
-
|
57.1
|
862
|
18
|
-
|
-
|
Bap
|
430
|
1600
|
18
|
-
|
-
|
31.9
|
782
|
-
|
18
|
-
|
DbA
|
63.4
|
260
|
17
|
-
|
1
|
6.22
|
135
|
17
|
-
|
1
|
3.4.2 Toxicity and health risk assessment
The TEF of BaP were used to evaluate TEQBaP of PAH compounds. The TEF value of PAH compounds and total their TEQBaP concentrations were shown in Table 1. In the areas, the TEQ∑16PAH values ranged from 19.76 to 208.75 ng/g, with mean of 44.10 ng/g. The site WTP2 has the highest TEQ value, followed by S13, S12 and S16. The TEQ∑7PAH values ranged from 14.84 to 190.64 ng/g, with mean of 38.70 ng/g, accounting for 87% of the TEQ∑16PAH, suggesting that the 7 PAHs were major carcinogenic contributors.
The results of carcinogenic risk of seven carcinogenic PAHs were shown in Fig. 4. In this study, the ILCRs ranged from 2.07 ×10− 4 − 2.66 × 10− 3 for children and 9.66 ×10− 5 − 1.24 × 10− 3 for adult, which is higher than the baseline of acceptable risk. The highest risk was found in site WTP2, and followed by S13, S12 and S15, which is consistent with the spatial distributions of PAHs in sediment. Therefore, the results revealed that the PAH-contaminated sediments at most of the sites posed a potential moderate cancer risk to human health via both ingestion and dermal contact pathways.