A National-Scale Study of Polycyclic Aromatic Hydrocarbons in Surface Water: Levels, Sources, and Carcinogenic Risk

Abstract

Elucidating pollution characteristics of polycyclic aromatic hydrocarbons (PAHs) in water and assessing the associated carcinogenic risks is crucial for improving public health. PAHs in the surface water of seven main river basins across China, compiled from 95 studies from 2004 to 2022, were used to investigate geographic variations of occurrence, source, and carcinogenic risk. Total PAH concentrations exhibited substantial geographic distributions ranging from 300 to 7552 ng·L⁻¹. Low molecular weight PAHs predominated, showing three-ring PAHs abundant in the north, while two-ring PAHs dominated in the south due to distinctions regarding energy consumption. The northern basins exhibited higher concentrations of PAHs than the southern owing to the synergistic impacts of low temperature, increased energy consumption, and higher industrial activities. Coal combustion and industrial emissions were the primary contributors in the northern basins, accounting for 23–44% and 20–38%, respectively, which were associated with pollutants released from heavy industries and space heating during cold periods. In contrast, vehicle exhaust emissions and petroleum leakage from river transport constituted the principal sources in the relatively economically developed southern basins, accounting for 24–35% and 31–57%, respectively. A lifetime carcinogenic risk model revealed that the highest health risks existed in adults, followed by adolescents and children. Toxic concentrations of BaP and the daily intake of water directly enhanced the PAHs’ carcinogenic risks, while body weight featured negative correlations with the risks.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs), which are a class of persistent organic pollutants widely distributed in multiple environments [1], have received significant attention due to their carcinogenic, teratogenic, and mutagenic properties [2]. As they include the characteristics of diverse sources, toxicity, and degradation persistence, PAHs pose toxicity risks to both human and environmental health [3,4]. Hence, 16 PAHs (including 7 carcinogenic compounds) were identified as priority pollutants by the U.S. Environmental Protection Agency (USEPA) [5,6,7]. PAHs released into the environment from various pathways can adhere to particles and enter into natural water bodies through long-distance atmospheric transport, as well as wet and dry deposition [8,9,10]. Moreover, PAHs in water can migrate to different environmental media through processes such as soil (sediment)–water and air–water exchanges [11], posing potential threats to human health as they accumulate in the food chain [12,13,14]. Therefore, water, as one of the important mediums for the existence of PAHs, can cause potential harm to aquatic ecosystems and human health.

PAH sources are influenced by a combination of natural and anthropogenic factors. Natural sources of PAHs typically include forest fires, volcanic eruptions, and endogenous biological synthesis [15]. Since the industrial revolution, anthropogenic sources have significantly contributed to the increased levels of PAHs in the environment. Incomplete combustion of coal and petroleum, waste incineration, biomass combustion, and petroleum leaks during extraction and transportation can all lead to severe PAH pollution [16]. Additionally, the growing energy demand, driven by economic and population growth, has led to a continuous increase in PAH levels. It was estimated that the total PAH emissions from anthropogenic sources in China was 32,720 tons, and China had the highest PAH emissions in the word, accounting for 22% of global emissions in 2004 [17]. Therefore, this study on PAH sources in water provides insights into China’s current energy utilization and holds instructive significance for guiding future human activities.

Owing to its vast territory, China exhibits both spatial heterogeneity in climate characteristics and regional economic development patterns. The natural environment and energy utilization vary significantly across different regions [18]. Unfortunately, with the rapid development of industrialization and urbanization, human activities inevitably lead to the discharge of pollutants into rivers, causing water pollution. Improper discharge of industrial wastes (waste gas, wastewater, and solid waste) and untreated domestic sewage have disrupted the water ecological system, directly influencing the environment and health of residents [19]. Hence, addressing surface water pollution caused by PAHs remains a crucial task in China. However, previous studies on PAHs in surface water in China have mainly focused on specific river sections or basins. Bai et al. (2014) [20] investigated the occurrence, state, and distribution patterns of PAHs in the water of the Liaohe River Basin in north China, which revealed that PAH compounds with two to four rings were predominant, mainly originating from a combination of petroleum and combustion sources. Xie et al. found that the surface water of the Liuxihe River Basin in south China exhibited a mild level of PAH pollution with two rings dominant, and combustion and petroleum were the major sources [21]. This relatively scattered and independent research has affected the overall understanding and treatment of water pollution in China; however, there are few studies on the national scale. PAHs in water bodies are carcinogenic and migratory, so they can enter the human body through accumulation in the food chain and pose a threat to human health, but the existing studies have little discussion on carcinogenic risk [12,13,14]. Given all that, the characteristics of PAH levels and sources in water are different between the north and south, due to dissimilarity in climate, energy utilization, and industry construction [22]. However, understanding how these factors synergistically affect the distribution, source, and carcinogenic risk of PAHs on a national scale remains limited.

To verify the hypothesis of spatial heterogeneity of PAH pollution in surface water in China, 95 articles on PAHs in surface water of seven major river basins across China, published between 2004 and 2022, were reviewed, and the dissimilarities of the northern and southern basins in pollution level, source, and health risk were systematically analyzed (Figure 1). The specific objectives were to (1) delineate the spatial variations of PAH concentration and composition; (2) identify the main sources of PAHs quantitatively and compare their contribution rates among basins; (3) evaluate the carcinogenic risks of PAHs among different age groups; and (4) explore the potential influencing factors of PAH carcinogenic risks. The results could provide valuable information for formulating surface water pollutant control policies and guiding evidence to minimize human health risks.

2. Materials and Methods

2.1. Data Collection and Processing

Data of 16 USEPA PAHs in surface water of seven main river basins across China from 95 published articles were collected and compiled into statistics. Research papers were searched through Chinese and English databases such as China knowledge Network (CNKI), Web of Science, Wanfang Data, Google Academic, and Springer Link. The data was sampled within 2004–2022, and the search keywords included “China”, “PAHs”, “water”, and “river”. The criteria for literature selection were as follows: (1) 16 priority controlled PAHs were reported, i.e., naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Flu), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), dibenzo[a,h]anthracene (DahA), benzo[g,h,i]perylene (BghiP), and indeno[1,2,3-cd]pyrene (IcdP); (2) the sampling point of PAHs in the literature should be in the range of the seven basins; (3) the literature that did not clearly give specific PAH concentrations should be deleted; (4) the detection method should follow the standard method recommended by USEPA or be revised according to the standard method; and (5) if a study did not have a specific sampling time period, it was pushed forward one year according to the publication time, and this was considered as the sampling period. Specific information on the references, including sampling time, location, size, and characteristics, is described in the Supporting Information (Table S1). When the concentration of PAHs in a drain or ditch is reported as extremely high, the PauTa criterion method (3σ criterion) is used to identify and eliminate outliers to avoid the influence of outliers on the statistical results. We also ensure that the PAH concentration selected represents the level of PAH contamination for the entire river or for a specific river segment.

The research area involved in this research is the seven main river basins in China (Figure 2), including Songhua River Basin (SRB: Heilongjiang Province, most areas of Jilin Province, and Hulunbuir City and Xing’an Alliance City in Inner Mongolia); Liaohe River Basin (LRB: Liaoning Province, Chifeng City and Tongliao City in Inner Mongolia, and Siping City in Jilin Province); Haihe River Basin (HaRB: Beijing-Tianjin-Hebei region, the east of Shanxi Province, the northwest of Shandong Province, and Anyang City in Henan Province); Yellow River Basin (YRB: Ningxia Hui Autonomous region; central Inner Mongolia; southern Gansu Province; central and northern Shaanxi Province; western Shanxi Province; and smaller parts of Sichuan, Henan, and Shandong provinces); Huaihe River Basin (HuRB: northern Jiangsu, northern Anhui, southern Shandong, and eastern and southern Henan Province); Yangtze River Basin (YtRB: Sichuan and Chongqing; Hunan and Hubei provinces; Jiangxi Province; Shanghai and southern Shaanxi; southern Anhui; and southern Jiangsu and northern Guizhou provinces, as well as some areas of Gansu, Yunnan, and Zhejiang provinces); Pearl River Basin (PRB: Hainan, Guangdong, and Guangxi provinces; southeastern Yunnan; and southern Guizhou Province). According to the collected literature and data, the investigated water bodies mainly include river and lake water bodies, while reservoir water bodies account for only a small part. The specific information about population, land-use types, and basin area of the seven main river basins is exhibited in the Supporting Information (Figure S1).

2.2. Positive Matrix Factorization

Positive Matrix Factorization (PMF) is a multivariate receptor model developed on the basis of the factor analysis model [23,24]. The model is based on multivariate statistical technology to extract the factor load that characterizes the emission characteristics of pollution sources and the factor score matrix that characterizes the contribution rate of the sources. The factor score matrix is used to calculate the contribution rate of pollution sources. PMF has the advantages that it does not need to measure the source component spectrum and the factor load in the decomposition matrix is non-negative. It also improves the processing ability of missing data by introducing the uncertainty of individual concentration of the receptor sample. Therefore, it is necessary to input the concentration data matrix of PAHs and the uncertainty data matrix of the corresponding (0.5 × MDL)² before running the model. The calculation method of uncertainty data is as follows [25]:

$$5/6 \times \mathrm{MDL}{\mathsf{\chi }}_{\mathrm{i}\mathrm{k}} < \mathrm{MDL}$$

(1)

$$\sqrt{{(\mathrm{RSD}\times {\mathsf{\chi }}_{\mathrm{i}\mathrm{k}})}^2+{(0.5\times \mathrm{MDL})}^2}{\mathsf{\chi }}_{\mathrm{ik}} > \mathrm{MDL}$$

(2)

where ${\mathsf{\chi }}_{\mathrm{ik}}$ is the concentration of k compound of PAHs in the i sample. RSD is the relative standard deviation of PAH concentration; MDL is the detection limit, and 10% or 20% of the concentration can also be used as the uncertainty data for 16 compounds.

2.3. Incremental Lifetime Cancer Risk

Incremental lifetime cancer risk (ILCR) is a method used to evaluate the exposure risk of a carcinogen to the human body, that is, the incidence of cancer caused by exposure to a certain dose of carcinogens within a certain period of time (lifetime). ILCR has been widely used to assess the health risks of PAHs in various populations [26]. The calculation formula is as follows:

ILCR = CSF × TEQ_BaP × IR × EF × ED/BW × AT

(3)

where CSF is the carcinogenic slope coefficient. The carcinogenic slope coefficient of BaP is used to evaluate the comprehensive risk of PAHs, and the value is 1.0 mg·kg⁻¹·d⁻¹; TEQ_BaP is based on the toxic equivalent of BaP (ng·L⁻¹); IR is average daily drinking water; EF is exposure frequency; ED is duration of exposure, which is the number of years of exposure; BW is body weight; and AT represents the time of exposure, which is the average life span of a person.

Cancer risk assessment requires an analysis of the exposure level of PAHs, while the ILCR model calculates the exposure level based on BaP toxicity equivalent factor [27]. Using the concentration transformation theory of PAH compounds, they obtained the toxicity equivalent factor (TEF) of each PAH relative to BaP and proposed the commonly used equivalent factor conversion coefficient. Because the toxic mechanism of PAHs is similar, it is finally transformed into the equivalent concentration of BaP (TEQ), and the BaP-based toxicity equivalents of seven carcinogenic species can be calculated according to the TEF value, which is marked as TEQ_BaP. The formula is as follows [28,29]:

TEQ_BaP = ∑C_i × TEF_i

(4)

where TEQ_BaP is the toxicity equivalent based on BaP; Ci is the concentration of the i compound in the PAH compounds; and TEF_i is the toxicity equivalent factor of the i PAH compounds. The TEF values for BaA, Chr, BbF, BkF, BaP, DahA, and IcdP are 0.1, 0.01, 0.1, 0.1, 1, 1, and 0.1, respectively.

2.4. Uncertainty Analysis

Considering the uncertainty of some exposure parameters and PAH concentrations in the process of health risk assessment, in order to analyze the output value and investigate the uncertainty, the Monte Carlo method is introduced into the ILCR model to quantify the uncertainty of risk and its impact on the expected risk value [26]. The Monte Carlo simulation connects the problem solved with a certain probability model, converts the independent variable into a probability distribution function, and obtains the cumulative probability distribution map through the iterative calculation of random simulation [30]. Compared with the determined method, the interval estimation based on Monte Carlo can enhance the understanding of the environmental behavior of pollutants and show uncertainty. Therefore, the Monte Carlo method is becoming increasingly common in environmental and human health assessment [31,32]. First of all, it is necessary to determine the probability distribution of the parameters of each model [33], establish a rating model according to the formula, and finally simulate and calculate the probability distribution of cancer risk assessment for children, adolescents, and adults in the seven river basins. The number of random iterations is set to 10,000, and the confidence interval is set to 5%−95%. Sensitivity analysis determines the significance of input parameters according to the correlation coefficient between input and output values in the process of Monte Carlo simulation, from which one finally obtains the variance contribution rate of each input variable to risk [34], which is used to analyze the impact of various parameters of PAH health risk in water [35].

2.5. Statistical Analysis

OriginPro 2022 (OriginLab, Northampton, MA, USA) was used to process data, record data, analyze data, and make related tables and drawings; PMF 2022 (US EPA) was used to identify PAHs in each watershed and calculate its contribution rate; Microsoft Excel 2021 (Microsoft, Redmond, DC, USA) combined with Crystal Ball method was used to calculate the health risk of seven watersheds, carry out a probabilistic health risk assessment, and analyze sensitivity data. All the spatial distribution maps are drawn by ArcGIS 10.8 (SRI, Redlands, CA, USA).

3. Results

3.1. Geographical Distributions of PAH Concentrations

The total concentration of 16 PAHs (∑₁₆PAHs) in the water samples of seven main river basins across China ranged from 300 to 7552 ng·L⁻¹, with an arithmetic mean value, a median value, and a geometric mean value of 1868, 1073, and 1056 ng·L⁻¹, respectively. The spatial distribution of ∑₁₆PAH concentrations followed the order of SRB (7552 ng·L⁻¹), HuRB (1743 ng·L⁻¹), LRB (1391 ng·L⁻¹), YRB (1073 ng·L⁻¹), YtRB (606 ng·L⁻¹), PRB (410 ng·L⁻¹), and HaRB (300 ng·L⁻¹) (Figure 3). The large regional variability of PAH concentrations is mainly caused by the uneven distribution of human activities, especially energy use and industrial activities. Due to the vast area of the seven river basins and the different natural geographical environments in which each river basin is located, dominant human activities in each river basin are also different. In areas with more intensive industrial activities, the release of PAHs is larger, and in areas with fewer industrial activities is less, and different human activities will cause different types of PAHs to be released. Therefore, the impact of human activities on PAH release needs to be further explored. In order to facilitate the study and capture the similarities of different river basins, the Qinling Mountains-Huaihe River Line, which is the north-south boundary line of China, was taken as an important basis for dividing the two types of river basins.

Therefore, based on the geographical characteristics of the seven basins, SRB, LRB, HaRB, YRB, and HuRB were classified as northern basins, and the concentration of ∑₁₆PAHs ranged from 300 to 7552 ng·L⁻¹, with an arithmetic mean value, a median value, and a geometric mean value of 2411, 1073, and 1426 ng·L⁻¹, respectively. On the other hand, the YtRB and PRB were regarded as the southern basin, and the concentration of ∑₁₆PAHs ranged from 410 to 606 ng·L⁻¹, with an arithmetic mean value, a median value, and a geometric mean value of 508 ng·L⁻¹, 508 ng·L⁻¹, and 498 ng·L⁻¹, respectively.

The river basins in northern China exhibited more severe PAH pollution compared with southern China. Previous studies have revealed higher pollution levels in the northern rivers compared with the southern rivers [36]. This observation indicates significant differences in energy utilization and climate between the northern and southern regions of China. The energy consumption structure significantly influenced the input of PAHs into surface water. The Pearl River Delta, which is the largest base of China’s light industry and is characterized by a predominantly light industry structure in southern China, exhibited lower PAH concentrations in surface water. In northern regions, such as SRB, the presence of large oil fields (e.g., Daqing oil field), coal mines, and numerous petrochemical industries has resulted in an increased input of PAHs into water. The high coal consumption for heating during winter months in northern areas has contributed to a higher concentration of PAHs [37].

Moreover, climate is a vital factor influencing the concentration and distribution of PAHs. Studies have shown that increased temperature, humidity, and ultraviolet radiation provide favorable conditions for the volatilization and degradation of PAHs [38]. Through the comparative analysis of the water bodies of the two regions in northern China, the concentration of PAHs in the dry season was significantly higher than that in the wet season (Yinchuan: dry season 1455.38–2538.84 ng·L⁻¹, wet season 818.69–1582.14 ng·L⁻¹ [39]; Beijing: dry season 132.5–890.8 ng·L⁻¹, wet season 164.0–230.6 ng·L⁻¹ [40]). The higher dry season concentration in northern China was mainly caused by the prolonged ice-bound period that hindered the transport of PAHs and reduced their degradation rate; also, there is the influence of the accumulation of PAHs from snowfall. Additionally, the average annual temperature in northern China is lower than 15 °C, it is 15 °C in southern regions, and some regions can reach more than 20 °C. The average annual precipitation is lower than 800 mm in northern China, and it is much higher in the south. Therefore, the high temperatures can promote the biodegradation of PAHs in water, leading to increased PAH volatility and reduced concentrations in water, while the migration of PAHs in northern China is affected by low temperature, which greatly reduces the degradation rate of PAHs [41,42,43]. Furthermore, the abundant precipitation in southern China also contributes to the dilution of PAHs in waters.

3.2. Geographical Distributions of PAH Compositions

Regarding the composition (Figure 3), PAHs with two to four rings dominated in the seven major river basins, while PAHs with five to six rings exhibited lower proportions owing to the specific arrangement of rings in PAHs present in water. PAH compounds in water exhibited high KOW values (octanol-water partition coefficient) and a strong affinity for particles owing to their large relative molecular weight [44,45]. Consequently, PAHs tended to adsorb and aggregate in sediment and organic matter in the soil. Conversely, PAHs with a low molecular weight readily underwent biodegradation by microorganisms. Thus, these PAHs featured high solubility and volatility, making them more likely to be present in water. PAHs with two to four rings mainly originated from the combustion of biomass and coal at medium to low temperatures. Additionally, crude oil and raw coal contained abundant PAHs with two to four rings. PAHs with three rings dominated the water in SRB, constituting 69% of the PAH concentrations in this region. A previous study analyzed PAHs in the Songhua River. The results revealed that the Songhua River exhibited a higher concentration of three-ring PAHs compared with other ring PAHs owing to industrial activities, oil field distribution, and winter coal combustion for heating in the SRB [46]. In PRB, two-ring PAHs exhibited the highest proportion, accounting for 58.9% of the PAH concentration. This could be attributed to the bustling port shipping activities and liquid leaks resulting from numerous vessels [47].

In the seven major basins, Phe, Nap, Acy, Ace, and Fla compounds constituted the highest proportion of ∑₁₆PAHs in water (Table S2, Figure S2). Phe and Fla served as indicators of coal combustion, Nap mainly originated from petroleum leakage, and Acy and Ace were characteristic compounds of biomass combustion. These results were consistent with the energy consumption characteristics and shipping capacity of rivers in the seven major river basins. The concentrations of seven PAH carcinogenic substances in the seven river basins followed the order of BaP (71.76 ng·L⁻¹) > BaA (67.99 ng·L⁻¹) > Chr (52.65 ng·L⁻¹) > BbF (45.13 ng·L⁻¹) > BkF (23.29 ng·L⁻¹) > DahA (19.92 ng·L⁻¹) > IcdP (14.7 ng·L⁻¹). Notably, BaP was identified as the most toxic substance. The limit of BaP specified in the US EPA surface water quality standard EPA822-Z-99-001 is 3.6 ng·L⁻¹ [48], while the limit of BaP specified in the Chinese environmental protection standard GB 3838-2002 for surface water quality is 2.8 ng·L⁻¹ [49]. The BaP concentration in the waters of the seven major river basins in China significantly exceeded both the US and Chinese environmental quality standards.

3.3. Pollution Levels of PAHs

The PAH pollution level in waters was classified into four groups based on the Σ16PAH concentrations, namely 0–100, 100–1000, 1000–5000, and >5000 ng·L⁻¹, indicating slight pollution, mild pollution, moderate pollution, and severe pollution, respectively [50]. In some foreign water studies (

Table 1. Comparison of PAH levels in waters between foreign rivers and seven major basins in China.

Study Area	Range (ng·L−1)	Mean (ng·L−1)	Reference
Nile River, Egypt	1112.7–4364.3	1877.5	61
Seine River and Estuary, France	4–36	20	51
Brisbane River, Australia	6.67–11.54	9.45	52
Cauca River, Colombian	52.1–12888.2	2344.5	62
Ganges River, India	0.05–65.9	32.5	53
Soan River, Pakistan	61–207	134.4	56
Danube River, Hungary	25–1208	122.6	57
Amu River Basin, Uzbekistan	3.19–779	98.4	55
Moscow River, Russia	50.6–120.1	—	58
Mississippi River, USA	62.9–144.7	114.9	59
Tiber River, Italy	23.9–72	43.4	54
St. Lawrence River, Canada	—	326	60
Seven River Basin, China	300–7552	1868	—

), on the whole, PAH pollution in water bodies from other countries was found to be relatively light. The highest and lowest values of total PAH concentrations in the Seine River and Estuary in France [51], Brisbane River in Australia [52], Ganges River in India [53], and Tiber River in Italy [54] were 0–100 ng·L⁻¹, all of which are slightly polluted; the Amu Darya River in Uzbekistan [55] is between slight pollution and mild pollution; the Soan River in Pakistan [56], the Danube in Hungary [57], the Moscow River in Russia [58], the Inland shallow lakes in Spain, the Mississippi River in the United States [59], and the St. Lawrence River in Canada [60] exhibited mild PAH pollution, and some sampling areas reached moderate pollution. The Nile River in Egypt [61] and the Cauca River in Colombia [62] have a large area of PAH pollution, and the peak of ΣPAHs has reached the level of severe pollution, indicating that the water body in some of the sampling point areas is seriously damaged.

Compared with rivers from other countries, the pollution levels of China’s seven major river basins are significantly higher. Combining the data of PAH content in the water bodies of the above seven major river basins, it was found that the HaRB, YtRB, and PRB are all slightly polluted, which is related to the density content of ΣPAHs in the water of the basin to a certain extent; the LRB, YRB, and HuRB all reached moderate pollution, and the population along these rivers is large, and economic and social activities contributed greatly to the PAH pollution in the water. The Songhua River Basin is heavily polluted, and the level of PAHs in its water body far exceeds the grading standard. China has a huge population and large consumption of coal, oil, and other energy sources, resulting in high levels of PAHs in China’s surface water, and it is necessary to focus on controlling PAH emissions and strengthening governance.

The establishment of pollution classification standards based on the actual nature of China’s water bodies is essential for further pollution assessment and remediation, since relatively high PAH concentrations were detected due to large population and energy consumption such as coal and oil. China has taken a series of necessary measures to control water pollution. In 2002, China made clear statements in the Environmental Quality Standards for Surface Water and specified the standard limit of BaP. In 2007, the Chinese government included the Songhua River as a key water resource protection project and launched a national science and technology project to focus on areas with high PAH pollution and strengthen control.

4. Discussion

4.1. Spatial Heterogeneity of PAH Sources

PAH sources are complex, with anthropogenic sources being the primary contributors to PAHs compared with natural sources. PAHs are byproducts of human activities. Moreover, PAHs originating from different sources exhibited diverse structures and compositions. Additionally, PAHs can maintain stability during migration and deposition processes. Therefore, PAH composition characteristics can serve as a basis for distinguishing pollution sources [63,64].

4.1.1. Characteristic Ratio

The source apportionment of PAHs using characteristic ratios was based on significant differences in the concentrations of characteristic compounds emitted from different pollution sources. Examining the ratios of these characteristic compounds enabled the identification of various emission sources of PAHs in specific regions. Commonly used ratios, such as Fla/(Fla + Pyr), IcdP/(IcdP + BghiP), and BaA/(BaA + Chr), were used for distinguishing the sources of PAHs. Fla exhibited environmental behavior characteristics similar to Pyr, InP, and BghiP, and its ratio remained relatively stable during environmental processes, enhancing the reliability of results [65]. Therefore, this study utilized the ratios of these three characteristic compounds to analyze PAH sources in different basins. Yunker et al. (2002) [65] found that a Fla/(Fla + Pyr) ratio exceeding 0.5 indicated a PAH source from the low-temperature combustion of coal and biomass. A ratio ranging between 0.4 and 0.5 indicated a PAH source from the combustion of liquid fossil fuels. A ratio below 0.4 indicated a PAH source from petroleum leakage. Regarding the IcdP/(IcdP + BghiP) ratio, a ratio exceeding 0.5 indicated a PAH source from the low-temperature combustion of coal and biomass, while a ratio ranging between 0.2 and 0.5 indicated a PAH source from fossil fuel combustion. A ratio below 0.2 signified a PAH source from petroleum leakage. Regarding the BaA/(BaA + Chr) ratio, a ratio exceeding 0.35 suggested a PAH source from the combustion of burning of coal and biomass, while a ratio below 0.2 indicated a PAH source from petroleum leakage. Moreover, a ratio ranging between 0.2 and 0.35 indicated petroleum and gas combustion [65].

Analysis of the characteristic ratios for PAH concentrations in the water of the seven major river basins revealed that the BaA/(BaA + Chr) ratio and IcdP/(IcdP + BghiP) ratio in most basins exceeded 0.2, indicating that the predominant PAH source originated from the combustion of coal, petroleum, and natural gas. The majority of the values for the Fla/(Fla + Pyr) ratio either exceeded 0.4 or ranged from 0.4 to 0.5, with only a few data points indicating a petroleum source (Figure 4). The results of the three characteristic ratios revealed differences between the northern and southern river basins. The ratios of SRB, LRB, HaRB, and HuRB predominantly indicated a PAH source from both coal and biomass combustion, with only a few ratios suggesting a PAH source from petroleum and natural gas combustion. Moreover, the ratios of YtRB and PRB indicated a higher contribution from petroleum and natural gas combustion. The observed differences in characteristic ratios may be attributed to the variations in economic development patterns and geographical conditions within the basins. Overall, the characteristic ratios in the waters of the seven major river basins were mainly concentrated in the combustion areas, indicating a PAH source involving mixed combustion of coal, biomass, and fossil fuels.

4.1.2. Positive Matrix Factorization (PMF)

Although the ratio method can provide preliminary insights into the sources of PAHs in the water of the seven major river basins, this method is sensitive to influences from atmospheric deposition and biodegradation. The PMF model, a multivariate factor analysis tool, was used to refine the results of the ratio method and quantitatively determine the sources and contributions of PAHs to water pollutants in the seven major river basins. Finally, three distinct sources of PAH were identified for SRB, while the other six river basins exhibited four main PAH factors. The fitting results of the model revealed that the majority of PAH compounds featured a slope R² value close to 1, confirming the high reliability of the model in elucidating the information contained in the original data.

The formation mechanism of PAHs is relatively complex, mainly due to incomplete combustion of carbon-containing compounds under high temperature and hypoxia conditions. Generally speaking, with the increase in molecular weight of PAHs, their melting points and boiling points increase [50]. Therefore, PAHs with two to four rings typically originated from combustion products of grass, wood, coal, and petroleum. Phe, Ant, and Fla were characteristic compounds associated with coal combustion, while BaA and Chr served as indicators for the combustion of fossil fuels such as petroleum and natural gas. PAHs with five to six rings (e.g., BbF, BkF, and IcdP) mainly resulted from vehicular exhaust emissions of gasoline and diesel fuel burned at high temperatures. A factor indicating significant contributions from indicator substances in both PAHs with two to four rings and five to six rings was categorized as an industrial emission source. This source suggests a combined combustion from coal, petroleum, natural gas, and vehicle exhaust emissions [66,67].

The fractional source concentrations and average source contributions of individual PAH species from the PMF model of seven main river basins across China are shown in Figures S3–S9. On the whole, the PMF analysis of different basins (Figure 5) revealed that SRB exhibited three evenly distributed PMF sources related to combustion. In LRB, a significant proportion of PMF sources could be attributed to coal combustion. Owing to the characteristics of the old industrial base and its location in the northeastern region, the PMF source related to coal combustion was a significant contributor to both SRB and LRB. In HaRB and YRB, the four PMF sources were distributed in a similar and uniform pattern, indicating a diversity of PMF sources. HRB was mainly influenced by coal combustion. The PAHs in the waters of YtRB and PRB primarily originated from petroleum leakage and vehicular exhaust emissions. Overall, human activities were the primary contributors to the presence of PAHs in the surface water of the seven major river basins, resulting in different patterns of PAH sources in each basin.

The contributions of different sources of PAHs to the water in different basins were determined (Figure 5 and Figure 6). Coal combustion, a globally significant activity, has emerged as the primary contributor to PAHs in Chinese water bodies. Particularly, coal combustion significantly contributed 44%, 36%, and 29% to PAH concentrations in HRB, LRB, and SRB, respectively. Additionally, coal combustion accounted for nearly a quarter of PAHs in both HaRB and YRB but contributed less than 20% to the PAH levels in YtRB and PRB. This indicates that a higher proportion of PAHs in northern China is attributed to coal combustion compared with southern China. Coal combustion is closely linked to industrial activities. Among the seven major river basins, SRB, YRB, and HaRB exhibited higher proportions of PAHs from industrial sources. In three northern China basins, industrial emissions contributed 38%, 20%, and 29% to PAH pollution, indicating the significance of industrial emissions as a primary source of PAH pollution in the northern region.

The northern region, characterized by intensive industrial activities and prominent industrial bases such as the Jing-Jin-Tang industrial base and the LiaoZhong south industrial base, played a significant role in PAH pollution. The metal-smelting and steel industries in these areas significantly contributed to the production of PAHs [22]. Particularly, the SRB in the northern region served as a representative example. Owing to its location in the historic industrial base of northeast China, including the petrochemical industry and metal processing, the sources of PAHs in the SRB water were closely linked to industrial activities. Additionally, the river basins in northern China, characterized by high latitudes and long, cold winters, require coal combustion for heating. This practice constituted another significant contributor to PAH emissions in the northern region [68].

Petroleum emissions constituted the primary source of PAHs in the water of PRB, accounting for 57% of the total PAH content, followed by YtRB and YRB, which contributed 31% and 30%, respectively. Moreover, other basins exhibited varying levels of petroleum pollution. Petroleum emissions were a dominant source of PAHs in the water across all seven major river basins but were particularly significant in PRB and YtRB. This occurrence may be attributed to the increased risk of petroleum infiltrating surface water during petroleum transportation and extraction processes in coastal and riverside areas.

Furthermore, vehicle exhaust emissions significantly contributed to PAH sources, particularly in YtRB, LRB, YRB, and PRB. In these regions, the contributions of vehicle exhaust to PAHs in the water exceeded 20%. The analysis of the contributions of vehicle exhaust to PAHs in the water across the seven basins revealed that traffic exhaust emissions were highly significant in the southern basins. This difference may be attributed to the various dominant transportation modes of northern and southern regions. Vehicle exhaust emissions mainly resulted from the combustion of gasoline and diesel in cars and ships. The southern regions, particularly the Yangtze River Delta and the Pearl River Delta, exhibit higher levels of economic development and population density. Consequently, the southern regions feature a higher number of civilian vehicles than the northern regions, and the road density in the south is nearly twice that of the north [69]. Additionally, both river basins are extensively involved in shipping activities, which contributes to higher PAH pollution levels in the water. Therefore, the contribution of vehicle exhaust as a source of PAHs exhibited a highly significant impact on the water of southern basins.

In summary, the PAHs in the water of the seven major river basins originated from diverse sources, with dominant contributions from coal combustion, petroleum emissions, and vehicle exhaust emissions. However, variations in energy structures, development status, and consumption areas influenced the emissions patterns of PAHs [22]. The PAHs in the water of the basins in northern and southern China originated from different sources. The dense industrial activities and lower temperatures in the basins of northern China led to increased contributions from both coal combustion and industrial emissions. The basins in southern China, characterized by economic developments and well-established transportation systems, were mainly influenced by PAH pollution from petroleum emissions and vehicle exhaust emissions.

4.2. Assessment of Carcinogenic Risks of PAHs

4.2.1. Distribution and Toxicity Characterization of BaP Concentrations

According to the collected concentrations and TEF of PAHs in the water of the seven major river basins, the toxic equivalent of Bap (TEQ_BaP) for seven carcinogenic PAHs in each basin was calculated (Table S4-1, Figure S10). SRB exhibited the highest TEQ_BaP (mean = 390.99 ng·L⁻¹), possibly due to the relatively high concentration of PAHs detected in this basin. In contrast, PRB featured the lowest TEQ_BaP (mean = 9.02 ng·L⁻¹). It is worth noting that the TEQ_BaP values of all seven major basins exceeded the specified limit of GHZB1-1999 (2.8 ng·L⁻¹), indicating that carcinogenic PAHs in the water of the seven major river basins might potentially pose adverse effects on the residents within these areas [49].

4.2.2. Assessment of Life-Long Carcinogenic Risks (ILCRs) in Different Basins

For a more effective assessment of the impact of the seven carcinogenic PAHs in the water of different basins on residents, this study considered three age groups, namely children (0–10), teenagers (11–20), and adults (21–70). Relevant parameters for risk assessment were collected for each age group based on recommended values by the USEPA and the Chinese Population Exposure Parameter Manual. The Kolmogorov–Smirnov test (K–S test) revealed that the probability distribution of TEQ_BaP followed a log-normal distribution. Therefore, TEQ_BaP was not treated as a fixed value, and the probability distributions of other parameters are shown in Table S4-5. The study calculated the daily exposure doses for different age groups and assessed the health risks of different basins. The Monte Carlo simulation was used to evaluate carcinogenic risks. To prevent overestimation, the 95th percentile was used as the high-end exposure estimate instead of the maximum value.

As health risk was determined by the toxic exposure level, the carcinogenic risks of the three age groups (children, teenagers, and adults) were combined to represent the life-long carcinogenic risk of PAHs in the water of each basin (Figure 7). According to USEPA guidelines, incremental lifetime carcinogenic risks (ILCRs) below 10⁻⁶ indicate a negligible carcinogenic risk. ILCRs ranging from 10⁻⁶ to 10⁻⁴ suggest an acceptable carcinogenic risk. ILCRs exceeding 10⁻⁴ indicate a significantly high carcinogenic risk, requiring considerable attention. Among the seven major basins, SRB exhibited the highest ILCR (3.89 × 10⁻⁵), followed by LRB (1.37 × 10⁻⁵), and PRB featured the lowest ILCR (7.99 × 10⁻⁷). This indicates a negligible health risk from PAHs in the water of PRB. The ILCRs in the other basins followed the order of YRB = 5.74 × 10⁻⁶, YtRB = 3.58 × 10⁻⁶, HuRB = 3.43 × 10⁻⁶, and HaRB = 1.58 × 10⁻⁶. According to the USEPA assessment standard for health risks, the carcinogenic risk of PAHs in the water bodies of PRB is negligible, and the health risks of other basins are within an acceptable range. The health risk of PAHs in the water of LRB and SRB is slightly higher, but still within the acceptable range. As depicted in Figure 5, the main sources of polycyclic aromatic hydrocarbons in SRB and LRB water are coal combustion, industrial emissions, and traffic pollution. This can be attributed to the prominence of industrial clusters in both SRB and LRB in northern China, resulting in higher PAH emissions from industrial activities [70]. This indicates that the water in SRB and LRB has a significant carcinogenic risk, and it is necessary to pay considerable attention to the control of water pollution for their high-risk situations.

The Monte Carlo simulation was used to assess the carcinogenic risk of PAHs in the water across different basins for three age groups on a national scale [71]. The results indicated that the 95th percentile carcinogenic risks for children, teenagers, and adults ranged from 1.26 × 10⁻⁷ to 5.17 × 10⁻⁶, 3.88 × 10⁻⁸ to 9.12 × 10⁻⁶, and 4.64 × 10⁻⁷ to 2.46 × 10⁻⁵, respectively. Notably, SRB exhibited the highest 95th percentile carcinogenic risks for children, teenagers, and adults, respectively. The 95th percentile carcinogenic risks for the all-age group was less than 10⁻⁴. This indicates that the carcinogenic risk for the three age groups in all the basins was within acceptable limits. The 95th percentile carcinogenic risks from PAHs in the water across the seven major river basins generally followed the order of children < teenagers < adults. This pattern mainly resulted from differences in body weight, water intake, and exposure frequency among different age groups.

4.2.3. Sensitivity and Uncertainty

To assess the influence of exposure parameters on health risks, sensitivity analysis was performed through probabilistic risk assessment using Monte Carlo simulation (Figure 6). A higher absolute sensitivity value indicated a stronger influence on risk results. A positive sensitivity value indicated a positive correlation with the final health risk results, while a negative sensitivity value indicated a negative correlation with the final health risk results.

Regarding overall exposure parameters, TEQ_BaP emerged as the most influential factor contributing to the variance in risk assessment, accounting for over 60% in each basin. The daily water intake (IR) emerged as the second-largest contributing factor, exerting a significant influence on the variance in risk assessment, with a contribution rate ranging from 40% to 60% across all basins. Other exposure parameters, including exposure frequency and exposure duration (ED), contributed less than 20% to the variance in risk assessment, while body weight (BW) exhibited negative sensitivity in risk assessment results. Regarding age groups, the sensitivity of TEQ_BaP for each age group in each basin followed the order of children < teenagers < adults. This indicates that different parameters for various age groups influenced the health risks across various age groups. IR, a primary sensitive factor, exhibited a lower impact on the health risks for children compared with teenagers and adults. IR exhibited a lower effect on the teenager group compared with the adult group. The overall sensitivity to the IR factor increased in the order of children < teenagers < adults, indicating that reducing water intake can mitigate health risks to some extent. Among all exposure parameters, BW featured a negative correlation with the overall sensitivity to IR, with the ranking of children > teenagers > adults. This suggests that groups with a lower BW exhibited a higher health risk from PAHs. As BW increased, the health risk decreased.

Therefore, reducing the IR and ED of adults to a certain extent can effectively mitigate their health risks. TEQ_BaP exhibited the highest impact on health risks. Hence, it is vital to control the levels of carcinogenic substances in water, emphasizing the need to prioritize water pollution management in the future.

Although the Monte Carlo simulation quantified health risks to some extent and reduced uncertainty in risk assessment, this study still faced limitations. First, regarding exposure routes, the health risk assessment only focused on the PAH ingestion route and did not include all three evaluation methods recommended by USEPA (ingestion, inhalation, and dermal contact). Second, regarding the exposure parameters used in the proposed health risk assessment model, the thresholds for these parameters were derived from EPA recommendations and determined through existing research conducted both domestically and internationally. This study did not account for the impact of errors on the results of health risk assessment.

4.3. Limitations

The research content of this paper is helpful for better detection and control of water pollution, but there are still limitations at the research level. First, the data were extracted from previous studies, and although we screened the data, there were differences in sampling time and laboratory analysis. Secondly, the amount of data in some river basins is small, and the distribution of sampling points is uneven, which cannot accurately reflect the level of PAH pollution at the national scale to a certain extent. Previous studies have shown that the sampling area is located in an area highly polluted with PAHs, which may lead to an overestimation of PAH contamination in China when we evaluate the data. In addition, in order to better address the effects of these constraints and verify the results obtained in this study, it is necessary to establish a long-term monitoring plan for PAH concentrations in water bodies. In addition to PAHs in water, PAHs are also present in the atmosphere, soil, and sediments, so PAHs in other media should also be studied and considered.

5. Conclusions

This study analyzed the concentrations, sources, and health risks of PAHs in the surface water of the seven major river basins in China. The results revealed that 16 PAHs were consistently present in the surface water across China, and their concentrations exceeded those in other countries. Regarding spatial distribution, SRB exhibited the highest PAH concentration in water, while HaRB featured the lowest PAH concentration in water. Overall, the river basins in northern China exhibited higher pollution levels than those in southern China. This indicates that the energy consumption structure and climate influenced the PAH concentration in the water of both southern and northern China. Regarding their composition, PAHs with different rings exhibited various Kow values. PAHs with two to three rings were predominantly found in surface water of both northern and southern China. SRB and PRB exhibited the highest proportions of PAHs with three and two rings, respectively. Despite the complexity of PAH sources in surface water across seven river basins, calculations based on compound ratios suggested that combustion sources were the dominant contributors. Additionally, the PMF model was used to determine the contribution rates of six PAH sources to each basin. In the northern basins, coal combustion and industrial emissions were identified as the primary sources owing to heavy industry and winter heating demands. Conversely, in the southern basins, PAH pollution was significantly influenced by vehicle exhaust emissions and petroleum sources owing to bustling transportation activities and potential leakage during river transport. The concentration of TEQ_Bap in surface water in China ranged from 9.02 to 390.99 ng·L⁻¹, which exceeded the specified limit set by the Chinese environmental standard for water quality, indicating a potential health threat to residents. The carcinogenic risk associated with PAHs in surface water ranged from 7.99 × 10⁻⁷ to 3.89 × 10⁻⁵ across different basins. The carcinogenic risk of PAHs in PRB water bodies is negligible, the health risk of PAHs in LRB and SRB water is slightly higher, and the health risks in all basins are within acceptable ranges. Owing to high PAH emissions in SBR, considerable attention should be focused on this basin. Additionally, Monte Carlo simulation was used to assess the health risk for populations in three age groups (children, teenagers, and adults) within the basins. The results revealed that the risk levels followed the order of children < teenagers < adults. Sensitivity analysis identified TEQ_BaP and IR as the two major factors influencing health risk, while the other factors featured a lower impact on health risk. Notably, BW featured negative correlations with health risks. Furthermore, regions with high PAH emissions exhibited severe health risks. Therefore, a comprehensive nationwide investigation is required to elucidate the migration of PAHs from water to other environmental media to address human and ecological needs.