Introduction

Due to the carcinogenic and mutagenic potential, polycyclic aromatic hydrocarbons (PAHs) have been identified as priority pollutants by the US Environmental Protection Agency and have attracted a great amount of attention. (Keith and Telliard 1979). PAH compounds are composed of two or more fused aromatic rings which are mainly produced during the incomplete combustion of fossil fuels and petroleum product release. Considering the fact that the major source of PAHs are of anthropogenic origin, they are now one of the ubiquitous pollutants in the environment (Yunker et al. 2002). The PAHs are introduced to aquatic environments through atmospheric deposition (dry and wet), surface runoff, oil leakage, and waste water discharge (Johnsen et al. 2005). These PAHs could pollute sources of drinking water (Wu et al. 2011a, 2011b). Also, exposure of aquatic organisms (phytoplankton, zooplankton, benthic macrofauna, and fish) that dwell in the polluted aquatic environments to PAH, portends great danger to human health, due to possible bioconcentration and biomagnification of PAHs, as well as the critical role of these organisms in the food chain. (Patrolecco et al. 2010; Guo et al. 2011).

The Songhua River Basin (41°42′–51°38′N, 119°52′–132°31′E; Fig. 1) is the biggest tributary of the Amur River and is the third largest river basin in China, covering an area of 55.72 × 104 km2. The basin flows through Inner Mongolia Municipality, Heilongjiang, Liaoning, and Jilin Provinces. The Songhua River Basin has two headstreams: the Nen River and the Second Songhua River. The Nen River is the northern headstream and originates in the Daxinganling Mountains. The southern headstream is the Second Songhua River in the Changbai Mountains. After their confluence, the river is called the Songhua River and follows into the Heilongjiang River.

Fig. 1
figure 1

Map showing the location of sampling sites in Songhua River Basin

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The rainy season spans from June to September and accounts for 85% of annual rainfall. The ice-bound period lasts at least 5 months. The Songhua River Basin is one of the most important agricultural bases of China and is one of the three black soil terrains in the world. The Daqing Oilfield, which is the largest oilfield in China, is also located in the basin. The rivers of the Songhua River Basin are an important water resource in the area. Despite their strategic importance, rapid economic development and high-energy consumption in the region during the last several decades, and their attendant extensive human activities such as industrial and agricultural activities, urbanization, etc., have caused constant and increasing emission of pollutants into the rivers (Wang et al. 2012; Ministry of Environmental Protection of the People’s Republic of China 2015; Wang et al. 2016).

Considering the nature of anthropogenic activities that is dominant in the region, certain degree of health risk associated with PAHs exists, especially in urban areas. A few studies have been conducted on the water quality in the rivers of Songhua River Basin; however, few have focused on PAHs (Liu et al. 2007; Voulvoulis et al. 2007; Wang et al. 2013; Wang et al. 2015a, 2015b; Cui et al. 2016; Wang et al. 2016). In the past, the investigations of PAHs were mainly focused on the concentrations and distribution in the sediments and coastal water (Dickhut et al. 2000; Wang et al. 2001; Mai et al. 2002; Yim et al. 2005); few investigations have accessed the distribution and ecosystem risk of PAHs in the rivers, especially in Northeast China (Chen et al. 2004; Patrolecco et al. 2010). It has been established that exposure of PAHs to organisms and human beings via water sources is the most significant routes of exposure (Okoli et al. 2015). Considering the potential toxicity effects of PAHs, it is necessary to analyze the characteristics and assess the ecological risk of the PAHs in the aquatic environments.

The present work presents a systematic research study on the PAHs of the Songhua River Basin. The objectives of this study were (1): to evaluate the distribution characteristics of PAHs in the dissolved form in the water and in SPM in the Songhua River Basin, (2) to trace the possible sources of the PAHs by molecular diagnostic ratios and PCA, and (3) to determine the contamination status and evaluate the potential environmental risks in the aquatic system.

Material and methods

Chemicals and materials

The 16 EPA priority PAHs measured in this study included naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fle), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benzo[a]anthracene (B[a]A), chrysene (Chr), benzo[b]fluoranthene (B[b]F), benzo[k]fluoranthene (B[k]F), benzo [a] pyrene (B[a]P), indeno[1,2,3-cd]pyrene (Ipy), Dibenzo[a,h]anthracessne (DBA), and benzo[g,h,i]perylene (BPE). A mixed standard solution of 16 EPA priority PAHs, internal standards (2-fluorobiphenyl and p-terphenyl-d14) and surrogate PAH standards (naphthalene-D8, acenaphthene-D10, phenanthrene-D10, chrysene-D12, and perylene-D12) were purchased from Accustandard Inc. (New Haven, CT, USA). Hexane, dichloromethane (DCM), and methanol were purchased from Merck Inc. (New Jersey, NJ, USA). Neutral silica gel and aluminum oxide (Al2O3) were purchased from Silicycle Incorporation (Quebec, Canada). J.T. Baker C18 solid-phase extraction (SPE) cartridges (200 mg/6 mL) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). All glassware and Whatman glass fiber filters (GF/Fs, 0.47 μm) were heated at 450 °C for 5 h before use.

Sample collection

The surface water sampling sites were along the main streams of the Songhua River Basin, the Second Songhua River, the Nen River, and their major tributaries (Fig. 1). A total of 48 river water samples, including 16 from main streams and 32 from tributaries, were collected using the baked glass bottles on July 2010. Sites 1–5 were located in the main stream of the Songhua River (SHR), sites 6–9 in the Second Songhua River (SSHR), sites 10–16 in the Nen River (NR), sites 17–27 in the tributaries of the Songhua River (TSHR), sites 28–30 in the tributaries of the Second Songhua River (TSSHR), and sites 31–48 in tributaries of the Nen River (TNR). The sum of 16 PAH concentrations is designated ΣPAH.

Sample pretreatment

All samples were divided into two phases, the dissolved phase and SPM, via filtration with 0.47-μm glass fiber filters (GF/Fs); the volume of each sample was 2 L, and the surrogates were added before filtration. Each phase was analyzed for PAHs. Extraction of the dissolved phase PAHs was processed following the previously method (Hu et al. 2014). In brief, dissolved PAHs in the water were extracted with C18 SPE cartridges. The SPE columns were activated with 5-mL methanol, followed by 2.5-mL deionized water, and 12% methanol (V/V) was added to amend the water samples (to eliminate possible adsorption of PAHs on the walls of the sample containers) before passing through the cartridges. The flow rate was controlled at 6 mL/min with a vacuum. After the extraction process, the cartridges were dried with a N2 stream (to eliminate evaporation of low weight PAHs and achieve high recoveries) and thereafter eluted three times with 3 mL of DCM. The eluates were later combined and concentrated to 2 mL under a gentle N2 stream. After exchanging the solvent to 10 mL hexane, the eluate was concentrated to approximately 0.5 mL. Prior to the instrumental analysis, the internal standards were added.

The SPM-loaded filters extraction was conducted as previously reported (Mai et al. 2002). In brief, after being freeze-dried and weighed, the SPM-loaded filters were shredded and the surrogate standards were spiked. Then the filters were Soxhlet extracted for 48 h with 250 mL DCM. After the DCM was concentrated to 2 ml, the extract solvent was replaced with hexane. Then, the extract was cleaned by passing through an alumina/silica gel column. The column was first eluted with 15 mL hexane and discarded the first fraction eluted from the column, while the second fraction eluted with DCM/hexane (30:70, v/v) was collected and concentrated to 0.5 mL. Prior to GC-MS analysis, the internal standards were added.

Instrumental analysis

The chromatographic analysis was performed on an Agilent 6890 gas chromatography and 5972B mass selective detector, equipped with a DB-5 capillary column (60 m × 0.25 mm × 0.25 μm). In total, 1.0 μL sample was injected by an auto sampler. The oven temperature program was: a 2-min hold at 80 °C, followed by an increase to 180 °C at 10 °C/min, an increase to 220 °C at 2 °C/min, and an increase to 290 °C at 8 °C/min (Xu et al. 2014).

Quality control and assurance

Procedural blank, spiked blank, and spiked samples were used for each set of samples for quality control. The recoveries of the surrogate standards were 62.8 ± 3.2% for naphthalene-d8, 77.1 ± 5.6% for acenaphthene-d10, 81.1 ± 11.2% for phenanthrene-d10, 81.3 ± 10.3% for chrysene-d12, and 75.5 ± 12.1% for perylene-d12. The average recoveries of the 16 PAHs varied from 71.6 to 101% in spiked blanks and from 63.2 to 103% in spiked samples. Method detection limit was dervied with statistical analysis of repeatability by the low concentration standard addition experiments; the field blank and procedure blanks were deducted. The instrument detection limits were 1–5 ng/ml for 16 EPA PAHs. The procedural blank was used to assess the effect of the experimental procedure on the concentration of PAHs, while the spiked blank and spiked samples were used to monitor the recovery efficiency in the present study. The concentrations of PAHs were not corrected with blanks and recoveries because the average recovery values of the 16 PAHs obtained in the present study were within the analytically acceptable range (70–120%) (Okoli et al. 2016; Bispo et al. 2011; Fajgelj and Ambrus 2000).

Statistical analyses

Principal component analysis (PCA) is a useful tool of multivariate analysis that uses an orthogonal transformation to convert the original observations into a set of values called principal components. The number of original variables is reduced to the number of principal components without an obvious loss in the total variance. The principal component has the possible variance in turn. Grouping factors were selected when the components contributed more than 10% of the total variance, and each factor was associated with an emission source. The most representative PAH compounds in each factor were identified based on loading values. Factors were selected based on loading values greater than 0.5 (Ho et al. 2002; Singh et al. 2004). The software SPSS 20.0 was used for the PCA analysis. Other statistical analyses were performed with Origin 8.0.

Results and discussion

PAH concentrations in the dissolved form and SPM

ΣPAH in the dissolved form and SPM in the surface water from the main streams of the SHR, the SSHR, the NR, and their major tributaries are presented in Tables 1 and 2 respectively. Not all of the 16 priority PAHs were detected in all of the surface water samples. The high molecular weight PAHs of BbF, BkF, Ipy, BaP, BPE, and DBA were not detected in the dissolved form in surface water, mainly because they are hydrophobic and tend to associate with SPM or colloids. As shown in Table 1, ƩPAH in the dissolved form from surface water ranged from 13.9 to 305.5 ng L−1, and the average ƩPAH in the dissolved form in the SHR, SSHR, NR, TSHR, TSSHR, and TNR were 33.7 ± 5.8, 51.9 ± 5.6, 34.1 ± 9.8, 65.1 ± 81.1, 92.1 ± 68.8, and 65.2 ± 17.9 ng L−1, respectively. The ƩPAH in the dissolved form from TSSHR was significantly higher than that from the other rivers in the Songhua River Basin, and the ƩPAH in the dissolved form from SHR was the lowest. A large variability in samples collected from TSSHR and TSHR was observed because ƩPAH from TSHR and TSSHR in different sample sites exhibited significant difference which have made the range of concentrations in TSHR and TSSHR get so large compared to the other rivers and tributaries. As shown in Fig. 1, the SSHR is the shortest and SHR is the longest among the three rivers; the number of sampling sites were 3 in TSSHR, 11 in TSHR, and 18 in TNR respectively. Most of the sampling sites in TSSHR and TSHR were near the villages or cities, which exposes the rivers to more contamination through pollutant chemicals discharge and disposal. The ƩPAH in some sample sites are obviously higher due to the special nature of the sampling site, as indicated by the visible outliers shown in Fig. 2. Among the main streams, ƩPAH in the dissolved form in the SHR was the lowest, while ƩPAH in the SSHR was the highest. This may be due to the different amount of runoff. The mean annual runoff in the SHR, NR, and SSHR were 76.2, 22.73, and 16.23 billion cubic meters, respectively. The results implied that the PAH concentrations were diluted more with more runoff. The same trend was observed in the tributaries of the Songhua River Basin. Most of the average ƩPAH values in the tributaries were higher than that in the main streams of the SHR, NR, and SSHR (Fig. 3). Hence, the PAHs in the tributaries may have contributed to the main stream.

Table 1 Concentration of PAHs in surface water from the Songhua River basin (ng L−1)
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Table 2 Concentration of PAHs in SPM from the Songhua River basin (μg g−1)
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Fig. 2
figure 2

Total PAHs in Songhua River Basin in water (a), SPM (b) and in water + SPM

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Fig. 3
figure 3

The composition pattern of PAHs in water and in SPM

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As shown in Table 2, the ƩPAH in the SPM from the Songhua River Basin varied from 0.3 to 109.7 μg g−1. The average ƩPAH in the SPM from SHR, SSHR, NR, TSHR, TSSHR, and TNR were 2.9 ± 2.2, 18.3 ± 22.1, 12.4 ± 10.6, 13.5 ± 32.0, 0.9 ± 0.8, and 8.4 ± 6.8 μg g−1, respectively. The trends observed in the SPM were as similar to those of the dissolved form. The average ƩPAH in SPM in the TSSHR was significantly higher than that from the other rivers in the Songhua River Basin. Among the main streams, the average ƩPAH in the SPM form the SHR was the lowest, while the average ƩPAH from the SSHR was the highest. This observation may be due to the different amount of runoff. The ƩPAH variations in the suspended particles from the tributary are affected by complex combination of series of factors: the runoff volume of the rivers, population density, distance of the sampling sites from the city and factories, and other possible emission sources. During the flood, some containers and cans from factories were poured into the rivers, which serve as input sources to the rivers.

The PAH concentrations at each sample site in the Songhua River Basin are presented in Fig. 2a–c. The results indicated that the higher concentration of PAHs was obviously related to urban sources contribution. The highest concentration of PAHs in the dissolved phase and in the suspended particle matter was observed at sampling sites 26 and 27, respectively, which are situated near the Mudanjiang City (Fig. 2a, b). Mudanjiang River is one of the biggest tributaries of SHR, and Mudanjiang City is the important regional central cities of northeast China, and with growing industrial and urban development in the past few years, sewage discharges, urban runoff, contamination emission of vehicular exhaust, and shipping activities were intense in Mudanjiang City. While higher concentration of PAHs in the dissolved phase was found at site S30, the sampling site 30 is situated in the Yinma River which is one of the biggest tributaries of SSHR and pass through six counties. Another study has reported that the deterioration of water quality of Yinma River had an evident influence on SSHR (Sun et al. 2011). Similarly, the higher PAH concentration in the suspended particle matter was observed at site 7 which is situated near the Songyuan City in SSHR. There are eight wharf around Songyuan City, and the Jilin oilfield which is the one of the four major production oil fields of China is located in Songyuan City. The urban and oilfield sources showed significant contribution to the PAHs at site 7.

As discussed earlier, the amount of annual runoff maybe has a significant effect on the ƩPAH in the rivers. The amount of annual runoff in the main streams was obviously higher than that in the tributaries in the Songhua River Basin; however, there were a complex set of factors causing unexpected results. In general, the PAHs in the tributaries maybe one of the main contributors to the main stream of the SHR.

Compared with PAHs pollution levels reported previously in the literature (Table 3), the ƩPAH values in the Songhua River Basin were similar to those in the Moscow River and Mississippi River (Eremina et al. 2016; Zhang et al. 2007) and lower than in the Yangtze River (Yu et al. 2016), Tianjin River (Shi et al. 2005), Daliao River (Zheng et al. 2016), Pear River(Liu et al. 2014), Gaoping River (Doong and Lin 2004), Cauca River (Sarria-Villa et al. 2016), and Gomti River (Malik et al. 2011).The ƩPAH concentrations in the Songhua River Basin are relatively low. There are two possible reasons. First, during the sampling time in 2010, there was heavy flooding in the Songhua River Basin; it was the most serious floods over the past 15 years. According to hydrology record, the flood increased the river discharge greatly to the Songhua River basin, and the PAHs have been diluted by the flood water (Song et al. 2015). The flooding also changed the PAH processes of dissolution, adsorption, and deposition in the water. The second reason is that the average population density is approximately 97 indiv km−2 throughout the river basin, and the population is mainly concentrated between Changchun City and Harbin City. The population density is much lower than the average population density (approximately 135 indiv km−2) in China, so the anthropogenic contributions were likely relatively lower.

Table 3 Comparison of PAHs concentrations in rivers worldwide
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PAH composition profiles

The mean compositional profiles of the PAHs in the main streams of the SHR, SSHR, and NR and their major tributaries are shown in Fig. 3. The 2- and 3-ring PAHs were the most abundant PAHs in both the water and SPM. In the water samples, the mean percentage of low molecular weight PAHs (2- and 3-ring) was 96.2%, (range from 94.5 to 97.4%), and the proportion of 4-ring PAHs ranged from 2.3 to 4.9% of ƩPAH, with a mean of 3.6%. Few 5- and 6-ring PAHs were detected in the surface water.

The proportion of PAHs in the SPM was also dominated by lower ring number PAHs except in SSHR. The percentage of 2- and 3-ring PAHs ranged from 39.2 to 82.1%, with a mean value of 70.3%. The mean percentage of 4-ring PAHs was 20.9%, with a range from 16.5 to 31.7%. The 4-ring PAHs make up the largest percentage of the high-ring number PAHs (4-, 5-, and 6-ring). In SSHR, the proportion of higher ring number PAHs was higher in the SPM than in other rivers in the Songhua River Basin. As discussed above, there were visible outliers of higher concentration of PAHs both in the dissolved phase and in the suspended particle found, due to contributions of urban and oilfield sources. This observation was also the reason for the higher proportion of higher ring number PAHs in this SSHR.

The results, which the low molecular weight PAHs (2- and 3-ring) were dominant in the surface water, have been found in other studied area (Fernandes et al. 1997; Li et al. 2010; Song et al. 2013) indicating a relatively recent local source of PAHs in the study area. The observed trend is also a result of the solubility of PAHs. The solubility of PAHs has a negative trend with the molecular weight of PAHs. Low molecular weight PAHs are more likely to remain in solution than their higher molecular weight counterparts. Hence, high molecular weight PAHs are preferentially adsorbed onto SPM and fine particles and sink in the sediments due to the low aqueous solubility and hydrophobic nature which results in lower levels of higher molecular weight PAHs in the water samples (Zhu et al. 2004).

Identification of PAH sources by diagnostic ratios

The anthropogenic source of PAHs in the environment is typically generated via petrogenic process (petroleum oil and refined products) and pyrogenic process (combustion of organic matter, coal). Generally speaking, PAHs of petrogenic origin are abundant in 2- and 3-ring PAHs; however, PAHs from pyrogenic origin have higher molecular weight PAH compounds. Different PAH sources are marked with different ratios of some individual PAHs (Zhang and Tao 2008). Several ratios of specific individual PAHs are used to infer possible sources (Yunker et al. 2002, Ping et al. 2007; Wang et al. 2015a, 2015b; Parinos and Gogou 2016). In present study, the ratios of Fla/(Fla + Pyr) vs. Ant/(Ant + Phe) and Ipy/(Ipy + BPE) vs. BaA/(BaA + Chr) were used to identify the sources of PAHs in the Songhua River Basin. To avoid misdiagnosis of PAH origins caused by PAH partitioning between dissolved form and SPM, total PAH concentrations (dissolved + particle) were used to calculate the diagnostic ratios. The ratio of Fla/(Fla + Pyr) < 0.4 is characteristic of petroleum contamination, and ratios between 0.4 and 0.5 indicate that the PAHs derived from the petroleum combustion and the combustion of grass, wood, and coal result in a ratio >0.5. The ratio of Ant/(Phe + Ant) with 0.1 is usually used to distinguish petrogenic and pyrolytic origins. Petroleum products usually have low Ant/(Phe + Ant) (<0.1), while an Ant/(Phe + Ant) > 0.1 indicates that the origin is petrogenic (Yunker et al. 2002).

In the present study, the ratio of Ant/(Phe + Ant) at most sites was less than 0.1, while Fla/(Fla + Pyr) varied from 0.32 to 0.68(Fig. 4a), indicating the PAHs in the Songhua River Basin originated from combined sources, and the combustion of coal, petroleum, and biomass being the most important origins of PAHs in this region.

Fig. 4
figure 4

Cross-plots for the isomeric ratios of: a Fla/(Fla + Pyr)vs. Ant/(Ant + Phe) and b Ipy/(Ipy + BaP) vs. BaA/(BaA + Chr) in Songhua River Basin

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Ratios of BaA/(BaA + Chr) < 0.20 and Ipy/(Ipy + BPE) < 0.2 are characteristic of a petroleum source. The ratio of BaA/(BaA + Chr) ranged from 0.20 to 0.35, and Ipy/(Ipy + BaP) from 0.20 to 0.50 indicate PAHs originated from petroleum combustion. BaA/(BaA + Chr) > 0.35 and Ipy/(Ipy + BPE) > 0.50 indicate PAHs derived from coal, wood, and grass combustion (Yunker et al. 2002). In Fig. 4b, the samples show evidence of a combination sources of biomass combustion, petroleum-derived pollution, and coal combustion.

The distribution of Fla/(Fla + Pyr) vs. Ant/(Ant + Phe), and Ipy/(Ipy + BPE)vs. BaA/(BaA + Chr) indicated that sources of the PAHs in of SHR, SSHR, NR, TSHR, TSSHR, and TNR were a combination of biomass combustion, petroleum-derived pollution, and coal combustion(Fig. 4).

In summary, the diagnostic ratios indicated that the PAHs in the Songhua River Basin originated from combined sources. Combustion of biomass, coal, and petroleum was the dominant source of PAHs in the Songhua River Basin. However, degradation of some PAHs is expected to take place during the river water that navigates its course downstream, which would lead to uncertainty about the trace sources. Having provided the qualitative identification with diagnostic ratios, PCA is necessary to provide the quantitative assessments.

Source identification of PAHs by PCA

Three principal components contributed more than 79.9% of the total variances (Fig. 5).

Fig. 5
figure 5

Plots with PC1, PC2, and PC3 from principal component analysis for PAHs from the Songhua River Basin

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PC1 was characterized by high loadings of Fla, Pyr, BaA, Chr, BbF, BkF, and DBA and explained 40.6% of the variance. BkF are typical markers of gasoline and vehicle diesel emissions, while BbF and Chr are indicators of coal combustion. The high loadings for Fla, Pyr, and Chr indicated coal combustion (Harrison et al. 1996; Khalili et al. 1995; Simcik et al. 1999). PC1 represented combination of diesel vehicle emissions, gasoline emissions, coal combustion, and petroleum combustion.

PC2 (23.6% of the total variation) was characterized by lower molecular weight PAHs (Nap, Acy, Ace, Fle, Ant, and Phe), which are indicative of non-combustion fossil fuel processes (Dachs et al. 2002). As characterized by PC2, the PAHs may be due to the emissions from oilfield drilling platforms and motor vehicles including unburnt diesel oil and gasoline in the Songhua River Basin.

High loadings of BaP and BPE and IPY contributed more than 15.6% of the total variance for PC3 which are typical markers of diesel vehicles and gasoline (Harrison et al. 1996).

The diagnostic ratios and the PCA were consistent with respect to the source of PAH contamination. The area was exposed to PAHs originating from petrogenic and pyrogenic sources, and pyrogenic source had a greater impact.

Though alkylated PAHs and dibenzothiophenes would have given more information on the sources of PAHs and their risks, the present study focused on the 16 EPA priority PAHs; due to their prevalence and established toxicity profile, the present study focused on the 16 EPA PAHs. The study of alkylated dibenzothiophenes and other parent PAHs will be undertaken in the future to have a better understanding of the pollutant behavior in the environment.

Ecological risk assessment by RQ

The PAHs in river water can be absorbed by phytoplankton and zooplankton, accumulated in the body of aquatic organisms, and enters into the food web, resulting in a potential risk to the river ecosystems. Ecological risk assessment of PAHs is important to identify the possible dangers to the aquatic ecosystem (Wu et al. 2011a, 2011b).

When assessing the potential risk of PAHs in the aquatic ecosystem from the Songhua River Basin, the concentrations of PAHs in the water were compared to their corresponding quality values, and reference values described in previous literatures were used for the assessment. The ecosystem risks of 16 individual PAHs were characterized by the risk quotients RQNCs and RQMPCs (Kalf et al. 1997; Cao et al. 2010; Dushyant et al. 2016). RQNCs and RQMPCs were calculated as follows:

$$ {\mathrm{RQ}}_{\mathrm{NCs}}=\kern0.5em \frac{\mathrm{CPAHs}}{\mathrm{CQV}\left(\mathrm{NCs}\right)} $$
$$ {\mathrm{RQ}}_{\mathrm{MPCs}}=\kern0.5em \frac{\mathrm{CPAHs}}{\mathrm{CQV}\left(\mathrm{MPCs}\right)} $$

where CPAHs is the concentration of a specific individual PAH in the medium, CQV(NCs) is the quality values of the negligible concentrations (NCs) and CQV(MPCs) is the maximum permissible concentrations (MPCs) in the medium. RQ(NCs) < 1.0 indicates that a specific PAH is risk free and the risk can be of negligible, while RQ(MPCs) < 1.0 and RQ(NCs) > 1.0 indicate concentration level of the specific PAH poses a moderate risk. To prevent further contamination, some control action should be taken. RQ(MPCs) > 1.0 indicates that the pollution with a specific PAH is severe; the ecosystem risk of the PAH is high risk, and urgent and effective control methods and remedial measure should be taken immediately.

The average values of RQ(NCs) and RQ(MPCs) in different media from different sites are listed in Table 4. The average values presented RQ(NCs) > 1.0 and RQ(MPCs) < 1.0 for Phe, Fla, Pyr, BaA, BbF, BaP, DBA, and BPE in the SHR; for Phe, Fla, Pyr, BaA, BbF, BaP, DBA, and BPE in the SSHR; for Fle, Phe, Pyr, and BbF in the NR; and for Phe BaA, BbF, BkF, and BPE in the TSHR, which indicated that these PAHs presented a medium level of ecosystem risk at these sites. However, no ecosystem risk was identified for Nap, Acy, Ace, Ant, Chr, and Ipy at any of the sampling sites. No values of RQ(MPCs) > 1.0 were found, which indicated no severe ecosystem risk of individual PAHs was at any of the sites in Songhua River Basin. In general, high molecular weight PAHs are more mutagenic and carcinogenic. The ecosystem risk is mainly due to 3- and 4-ring PAHs; the moderate molecular weight PAHs posed a much greater ecosystem risk in the Songhua River Basin.

Table 4 Mean values of RQ(NCs), RQ(MPCs), RQƩPAHs(NCs), and RQƩPAHsMPCs) of PAHs in the Songhua River basin, China
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To assess the ecosystem risk more accurately and comprehensively, the ecosystem risk posed by the combination of all 16 PAHs was characterized based on the risk quotients RQƩPAHs(NCs) and RQƩPAHs(MPCs). When the values of RQ(NCs) and RQ(MPCs) of the individual PAHs were greater than 1, they were combined to calculate RQƩPAHs(NCs) and RQƩPAHs(MPCs) of ƩPAHs. RQƩPAHs(NCs) and RQƩPAHs(MPCs) were calculated as follows:

$$ RQ\sum \mathrm{PAHs}\left(\mathrm{NCs}\right)=\sum_{i=1}^{16} RQ\mathrm{NCs}\left( RQ\left(\mathrm{NCs}\right)\ge 1\right) $$
$$ RQ\sum \mathrm{PAHs}\left(\mathrm{MPCs}\right)=\sum_{i=1}^{16} RQ\mathrm{MPCs}\left( RQ\left(\mathrm{MPCs}\right)\ge 1\right) $$

The values of RQƩPAHs(NCs) and RQƩPAHsMPCs reflect the ecosystem risk levels of the ƩPAHs. RQƩPAHs(NCs) = 0 indicates that ƩPAHs in a level of risk free, while RQƩPAHs(MPCs) = 0 and range of RQƩPAHs(NCs) from 1 to 800 suggest that ƩPAHs in a level of low risk, RQƩPAHs(NCs) □ ≥ 800 and RQƩPAHs(MPCs) = 0 indicate the ƩPAHs in a level of moderate risk1, the RQƩPAHs(MPCs) ≥ 1 and RQƩPAHs(NCs) □ < 800 indicate the ƩPAHs pose a moderate risk2, RQƩPAHs(NCs) □ ≥ 800 and RQƩPAHs(MPCs) ≥ 1 indicate the ecosystem risk of the ƩPAHs is high risk.

As shown in Table 4, RQƩPAHs(NCs) in SSHR, NR, TSHR, and TNR was less than 800 but greater than 1.0, while RQƩPAHs(MPCs) was 0, which indicated that the contamination of ƩPAH in the aquatic environment of the SSHR, NR, TSHR, and TNR showed low ecosystem risk. There was no risk in the SHR and TSSHR; the PAH contamination in these rivers was negligible. RQƩPAHs(MPCs) was always less than 1.0, and RQƩPAHs(NCs) was always less than 800, which indicated that there was no moderate- or high-risk ƩPAH contamination in the aquatic environment in the entire Songhua River Basin.

Conclusion

This study focused on assessing the concentration, source, and ecological risk in the main streams of the Songhua River Basin in Northeast China. Both the water and SPM contained mostly low molecular weight PAHs. The average ƩPAH in the tributaries of the TSHR, TSSHR, and TNR was higher than in the main streams of the SHR, SSHR, and NR. Compared with PAH river pollution levels reported in the literature, the PAH concentration in the Songhua River Basin was lower than other rivers around the world. The diagnostic ratios and PCA confirmed that the PAHs in the Songhua River Basin came from combined petrogenic and pyrogenic sources, and the pyrogenic source made a greater contribution. The ecosystem risk assessment indicated the ƩPAH in the aquatic environment of the Songhua River Basin posed a low ecosystem risk, while some individual PAHs presented moderate risks. The flood events had a significant impact on the concentrations of PAHs in the rivers of the Songhua River Basin. It is suggested that researchers should pay close attention to the effect of these unpredictable events on the quality of rivers in the area in the future research.