3.1 Pollutant identification
From the suspect screening, 55 chemicals were identified in the GH River (detailed information is provided in Table S4). The total intensity and quantity of pollutants detected at each site are shown in Figure 1a. Significant variations in the total response intensity were observed, with sites G-3 to G-6 exhibiting notably higher values compared to sites G-1 and G-2. For instance, the total response intensity of the sample from G-6 was 10 times greater than that at G-2. The CIP was situated between sites G-4 and G-5, potentially contributing to the heightened response intensities at G-3 to G-6. Across all sites, the distribution patterns in both the number of pollutants detected and their respective response displayed similar trends from G-1 to G-10. The 55 chemical substances detected were classified based on their applications (Figure 2) and identified 8 bactericides, 3 plasticizers, 1 dye, 12 herbicides, 5 insecticides, 11 pharmaceuticals, 7 pharmaceutical or dye intermediates, 1 pesticide, and 7 chemicals with miscellaneous uses, with pesticides (bactericides, herbicides, insecticides, and plant growth regulators) comprising 47.3% of the total. Furthermore, among the detected chemicals, 13 substances were listed in the List of Hazardous Chemicals in China (Table S4). The presence of these hazardous chemicals highlights the potential risks to both the environment and public health. Continuous monitoring and effective management strategies are crucial to mitigate the impact of these pollutants on the river ecosystem and surrounding communities. Detailed analyses of the chemical profiles and their potential sources can provide valuable insights for developing targeted remediation efforts.
The suspect screening results showed the presence of 55 chemical substances in the samples collected from the ZB River (Table S5). The total intensity of response peaks and quantity of pollutants detected at each site are illustrated in Figure 1b. Significantly higher total response intensity was observed at ZB-4 compared to other points, it was 13 times greater than that at ZB-6, possibly due to ZB-4 being situated within the CIP and experiencing a strong impact as a result. This elevated response intensity at ZB-4 highlights the need for further investigation into local sources of contamination and their potential impacts on the river's ecosystem. When classified according to their use, the 55 substances detected included 12 pharmaceuticals, 12 herbicides, 8 bactericides, 1 dye, 5 insecticides, 3 plasticizers, 8 pesticides, pharmaceutical, or dye intermediates; 1 plant growth regulator; and 5 chemicals with other uses (Figure 2). This diverse array of chemicals underscores the complexity of pollution sources affecting the river. Among these, 15 substances were listed on the List of Hazardous Chemicals in China (Table S5). The identification of these hazardous chemicals raises concerns about potential risks to aquatic life and human health, necessitating ongoing monitoring and intervention strategies. Detailed chemical profiling and source tracking are essential steps towards effective pollution control and ensuring the long-term health of the river and its surrounding environments. Collaboration between regulatory bodies, local industries, and the scientific community is crucial to address and mitigate the adverse effects of these pollutants.
The upper section of the XY River, located above the confluence of the GH River, was divided into three segments - north, middle, and south - each equipped with a sluice. Sites within the sluices, namely XY-1, XY-3, and XY-5, were compared to sites outside the sluices, specifically XY-2, XY-4, and XY-6. The suspect screening analysis revealed 51 chemical substances present in the XY River samples (Table S6). The total intensity of response peaks and quantity of pollutants detected at each site are depicted in Figure 1c. Notably, the response intensity was lower inside the sluice compared to outside, indicated by XY-1 < XY-2, XY-3 < XY-4, and XY-5 < XY-6. The number of pollutants detected at each site aligned with the corresponding intensity ranking, suggesting that the river sluice had a beneficial effect on reducing the inflow of pollutants. This finding underscores the importance of sluice management in controlling pollutant distribution in river systems.
Classification of the 51 detected substances by use showed that there were 9 bactericides, 9 pharmaceuticals, 12 herbicides, 6 insecticides, 3 plasticizers, 5 pesticides, pharmaceutical, or dye intermediates, 1 plant growth regulator, and 6 chemicals with other uses (Figure 2); of these, 54.9% were pesticides. The high percentage of pesticides highlights the extensive agricultural impact on the river’s chemical profile. A total of 14 substances were listed on the List of Hazardous Chemicals in China (Table S6), raising concerns about their potential environmental and health risks.
Further analysis and continuous monitoring are essential to understand the sources and long-term effects of these chemicals on the river ecosystem. Effective regulatory measures and targeted pollution control strategies must be developed to protect the river and surrounding habitats. Additionally, collaboration with local agricultural sectors to promote sustainable practices could significantly reduce the input of hazardous chemicals into the river. Public awareness and education initiatives regarding the impact of chemical pollutants can also play a crucial role in fostering a more environmentally conscious community.
The suspect screening conducted at the coast (YS) detected a total of 26 chemical substances (Table S7), with the intensity of response peaks and number of pollutants detected at each site depicted in Figure 1d. Notably, the response intensity was elevated at YS-5, a site distant from the river estuary, potentially influenced by water flow direction or discharges from other sources. YS-3 and YS-4 also exhibited higher response intensities compared to YS-1 and YS-2, likely due to seawater dilution effects. The elevated levels at YS-3 and YS-4 could also indicate localized contamination sources. YS-5 had the lowest number of substances detected, leading to a relatively high average response intensity per substance, indicating potential pollution sources unrelated to the CIP or GH river.
The 26 identified substances were classified into 3 bactericides, 4 pharmaceuticals, 13 herbicides, 1 insecticide, 4 pesticides, pharmaceuticals, or dye intermediates, and 1 miscellaneous chemical, with pesticides representing 65.4% of the total. This high percentage of pesticides suggests significant agricultural runoff impacting the coastal water quality. Importantly, none of the substances identified at YS were listed as hazardous chemicals in China, distinguishing this site from others in the study. This finding highlights the variable nature of chemical pollution sources along the coast, which may differ significantly from inland river sources.
Further investigations are warranted to identify the specific sources contributing to the elevated pollutant levels at YS-5 and to understand the broader impacts on coastal ecosystems. Regular monitoring and comprehensive assessments are necessary to track changes over time and evaluate the effectiveness of implemented pollution control measures. Additionally, engaging local communities and industries in pollution reduction efforts can enhance environmental protection and sustainability. Enhanced public awareness and educational programs about the importance of preserving coastal water quality can further support these initiatives, fostering a collaborative approach to mitigating chemical pollution.
3.2 Analysis of the frequently detected pollutants
The suspect screening results revealed that across the GH River, ZB River, XY River, and YS sampling sites, 12, 13, 23, and 2 chemical substances were detected, respectively (Figure 3). Upon classification by use, it was determined that 75%, 61.5%, 69.6%, and 100% of the substances in the GH River, ZB River, XY River, and YS were pesticides, respectively. Notably, certain hazardous chemicals listed in China were found in specific rivers, such as 2-nitrophenol, 2,4-dichlorophenol, and tributylamine in the GH River; 2-nitrophenol, nonylphenol, and tributylamine in the ZB River; and 2,4-dichlorophenol, 2,4,6-trichlorophenol, nonylphenol, and 2-phenylphenol in the XY River. Phenolic derivatives, common organic water pollutants, have the potential for conversion into substitute compounds in sewage treatment and natural water environments (Abaide et al. 2019; Arasteh et al. 2010). These substances exhibit toxicity even at low environmental concentrations, with nonylphenol acting as an endocrine disruptor capable of interfering with biological hormone systems (Soares et al. 2008). A notable compound, tributylamine, a strong Lewis base, finds extensive use in catalysts, extractants, and pesticides (Tian et al. 2020; Wang et al. 2008).
The analysis of substances detected frequently in the GH River (Figure 4) revealed varying response intensities at different sampling sites. Specifically, the response intensities were notably higher at sites G-3 to G-6, and relatively lower at G-1 and G-2 (Figure 4). Notably, the response intensities of pesticides and pesticide intermediates were significantly elevated at sampling points near the chemical park, indicating a strong influence of the Chemical Industrial Park (CIP) on the concentrations of these substances in the GH River. However, prometryne and tributylamine exhibited distinct patterns. Prometryne, a selective herbicide widely used in controlling annual grasses, exhibited the highest response intensity at G-10, with a noticeable increase at the site closest to the CIP compared to other locations. Prometryne is a selective herbicide of the striazine family that inhibits photosynthesis in plants and is commonly used to control annual grasses in developing countries including China (Chen et al. 2010; Jin et al. 2012; Tian et al. 2020). Prometryne poses challenges in terms of biodegradation and accumulates readily in aquatic organisms (such as fish, shrimp, and shellfish) (Chen et al. 2013 Saka et al. 2018; Yang et al. 2021;). On the other hand, tributylamine was consistently detected at all sampling sites, but with relatively low response intensity, showing no clear association with environmental exposure near the chemical parks. Additionally, 16 chemical substances, including nonylphenol, metribuzin, and pentachlorophenol, were not detected at G-1 and G-2 but were found at sites beyond G-3, suggesting a potential link to CIP emissions (Figure S1).
3.3 Comparative analysis
The analysis of pollutants detected in the GH River, ZB River, industrial wastewater, and WWTP effluents found that a total of 33 substances were consistently present across all these sources (Liu et al. 2020). Among these substances, pesticides accounted for 16, or 48.5%, of the detected pollutants (Figure 5). Three specific pollutants - Amitraz metabolite, Sulpiride, and Thiamethoxamn - were exclusively found in the GH River. Furthermore, the presence of Amitraz metabolite was detected at sites G-1, G-3, G-4, and G-5. Detection at site G-1 could be attributed to the use of Amitraz in cross-strait agricultural production, while the absence of detection at G-2 may be due to dilution from incoming tributaries. The presence of N’-(2,4-Dimethylphenyl)-N-methylformamidine at sites G-3, G-4, and G-5 may be linked to the metabolism of Amitraz under specific conditions, as identified in enterprise wastewater and WWTP effluent (Saito et al. 2008).
Sole detection of Sulpiride occurred at G-6, while Thiamethoxam was exclusively found at three sites near the Chemical Industrial Park (CIP), indicating potential point source pollution. These pollutants found in the CIP water bodies were also detected in other surrounding water bodies, suggesting a spread of contaminants from the industrial area. Dinoctyl phthalate (DNOP) and Ranitidine were absent in the enterprises’ wastewater and WWTP effluent but were present in the river and surrounding water, raising concerns about non-industrial sources or transformation processes occurring in the environment. Phthalates, like DNOP, are commonly used as plasticizers in PVC production, where they are physically mixed with raw materials to form plastic without chemical bonding, leading to their widespread detection in soil, water, and the atmosphere (Kuang et al. 2010; Sarkar et al. 2013; Wu et al. 2010).
Various phthalate compounds, such as N-octyl phthalate, Dibutyl phthalate, and Diethyl phthalate, were identified in the enterprises’ wastewater and WWTP effluent, indicating their pervasive use and release in industrial processes. Ranitidine, an H-2 receptor antagonist used to treat peptic ulcers, esophageal reflux, and Zollinger-Ellison syndrome, is generally well-tolerated. Ranitidine hydrochloride, which can be converted to Ranitidine under specific conditions (Bens et al. 2004; Isidori et al. 2009), was detected in the WWTP effluent and may be linked to the presence of Ranitidine in the irrigation river. This highlights the complexity of pollutant sources and the need for comprehensive monitoring to understand the full extent of chemical contamination. Further studies should focus on the pathways and transformations of these substances in the environment to develop effective mitigation strategies. Collaborative efforts between regulatory bodies, industries, and scientific communities are essential to address and reduce the impact of these pollutants on the ecosystem and human health.
The XY River, a tributary of the GH River, exhibited similar pollutant profiles to the GH River (Figure 6), indicating comparable sources of contamination. This suggests that pollutants discharged on both banks of the XY River mirrored those released into the GH River, or that materials could have been transported between the two rivers due to tidal fluctuations, with sluices potentially failing to act as a barrier. The potential influence of tidal actions underscores the need for a comprehensive understanding of hydrodynamic conditions in the region to mitigate pollutant dispersion effectively.
We further analyzed the 43 substances detected inside and outside the sluices, revealing three substances exclusively found outside (Barban, Amitraz metabolite, and Sulpiride) and five substances exclusively detected inside (Bifenthrin, Carbofuran, Pentachlorophenol, 4-Chloro-o-tolyloxyacetic acid, and Sulbactam acid). This differentiation in pollutant distribution highlights the partial effectiveness of the sluices in controlling pollutant flow and points to possible localized sources of contamination within the sluiced sections.
Barban, Amitraz metabolite, and Sulpiride, found exclusively outside the sluices, suggest that these contaminants may have been introduced from external sources such as agricultural runoff or upstream industrial activities. Conversely, the presence of Bifenthrin, Carbofuran, Pentachlorophenol, 4-Chloro-o-tolyloxyacetic acid, and Sulbactam acid inside the sluices indicates internal sources, which could be due to localized usage or the presence of point source discharges within the sluiced areas.
Further detailed investigations are required to pinpoint the exact origins of these substances and to understand the mechanisms driving their distribution. Monitoring programs should include more frequent sampling and advanced analytical techniques to capture the temporal and spatial variations of these pollutants. Additionally, collaborative efforts involving local authorities, industrial stakeholders, and agricultural sectors are crucial to implement effective pollution control measures. Public engagement and awareness campaigns can also play a vital role in reducing pollutant inputs by encouraging best practices in waste management and sustainable agricultural practices.
Understanding the specific pathways and behaviors of these pollutants in the aquatic environment will aid in developing targeted strategies to minimize their impact. Continued research and policy development are essential to safeguard the water quality of the XY and GH Rivers, ensuring the protection of both ecological and human health in the region. Of the substances detected inside and outside the sluice of the XY River and GH River, two substances, Bifenthrin and Sulbactam acid, were exclusively found inside the sluice, while one substance, Barban, was solely detected outside the sluice. Additionally, Barban and Cyprodinil were found outside the sluice but not in the GH River.
Barban is an herbicide, and Cyprodinil is an anilinopyrimidine plant bactericide (Karadag and Ozhan 2015; Schirra et al. 2009), which may affect the cardiac development of vertebrates through its interaction with the aromatic hydrocarbon receptor (AhR) (Medjakovic et al. 2014; Tang et al. 2020). The exclusive detection of these substances outside the sluice suggests that they may originate from agricultural activities in the surrounding areas, highlighting the need for improved agricultural practices to prevent runoff into water bodies.
The presence of Bifenthrin and Sulbactam acid inside the sluice indicates potential localized contamination sources, such as specific agricultural applications or untreated wastewater discharges. Bifenthrin, a pyrethroid insecticide, is widely used for pest control and has been shown to persist in the environment, affecting aquatic life (Spurlock and Lee 2008). Sulbactam acid, a beta-lactamase inhibitor, is commonly used in combination with antibiotics to treat bacterial infections, and its presence in the water may be indicative of pharmaceutical residues entering the aquatic system.
Further detailed investigations are required to pinpoint the exact origins of these substances and understand the mechanisms driving their distribution. Monitoring programs should include more frequent sampling and advanced analytical techniques to capture the temporal and spatial variations of these pollutants. Additionally, collaborative efforts involving local authorities, industrial stakeholders, and agricultural sectors are crucial to implement effective pollution control measures. Public engagement and awareness campaigns can also play a vital role in reducing pollutant inputs by encouraging best practices in waste management and sustainable agricultural practices.
Understanding the specific pathways and behaviors of these pollutants in the aquatic environment will aid in developing targeted strategies to minimize their impact. Continued research and policy development are essential to safeguard the water quality of the XY and GH Rivers, ensuring the protection of both ecological and human health in the region.
3.4 Analysis of the sources of pollutants in the GH River and ZB River in the CIP
3.4.1 GH river
Principal Component Analysis (PCA) was conducted to explore the potential sources of pollutants identified in all sampling sites along the GH River. Following varimax rotation, two main principal components, PC1 and PC2, were discerned, explaining 59.28% and 25.50% of the total variance, respectively (Figure 7). The first component (Figure 7a) was primarily influenced by anileridine, atrazine, azoxystrobin, isoprothiolane, metolachlor, paclobutrazol, triazophos, and tricyclazole, all of which are pesticides except anileridine, a pharmaceutical. These pesticides were predominantly detected at sites G-3 to G-6, exhibiting higher levels compared to other locations. This indicates a significant impact from agricultural activities and potential point sources near these sites.
Figure 7b illustrated that pollutants detected at G-3 to G-6, both upstream and downstream of WWTPs, shared similar sources. The presence of anileridine and other compounds in WWTP effluents suggests influences from land-based sources, consistent with previous research (Liu et al. 2020). This finding underscores the role of WWTPs as conduits for both pharmaceutical and pesticide contaminants entering the river system.
Sites G-1 and G-2, situated in the upper GH River far from industrial zones, primarily showed pesticide contamination possibly originating from agricultural activities on both sides of the river. The detection of these substances in such locations highlights the pervasive nature of agricultural runoff and its capacity to affect even remote sections of the river.
Pollutants detected at sites G-7 to G-10 might have stemmed from effluents released by additional industrial and sewage treatment facilities downstream in the river. The variation in pollutant types and concentrations across these sites suggests diverse sources, including industrial discharges and urban runoff, contributing to the pollution load in the lower reaches of the GH River.
The results of the PCA provide a clearer understanding of the spatial distribution and potential sources of pollutants along the GH River. This analysis underscores the need for targeted pollution control measures tailored to specific sections of the river. For instance, enhanced agricultural practices and runoff management could mitigate pesticide contamination in the upper river, while stricter regulation and improved treatment technologies at WWTPs and industrial facilities could address pollutant sources in the mid to lower river sections.
Further studies should focus on identifying the exact pathways through which these contaminants enter the river system. This could involve a combination of field surveys, modeling studies, and advanced analytical techniques. Collaborative efforts between environmental agencies, agricultural stakeholders, and industrial operators are crucial to develop and implement effective strategies for reducing pollutant loads and improving water quality in the GH River.
Understanding the specific pathways and behaviors of these pollutants in the aquatic environment will aid in developing targeted strategies to minimize their impact. Continued research and policy development are essential to safeguard the water quality of the XY and GH Rivers, ensuring the protection of both ecological and human health in the region.
3.4.2 ZB River
Principal Component Analysis (PCA) was utilized to investigate the potential sources of pollutants exclusively detected in the ZB River. Following varimax rotation, three primary principal components (PC) were determined, explaining 53.71% (PC1), 24.24% (PC2), and 16.30% (PC3) of the total variance (Figure 8).
In Figure 8a, PC1 was largely influenced by 2-nitrophenol, anileridine, carbendazim, dibutyl phthalate, diethyl phthalate, and tributylamine. All substances, except anileridine, were found in the chemical enterprise, with two-thirds detected in WWTP influents. The response intensity of these substances at site ZB-4 was significantly higher than at other sites, suggesting a specific source at this location.
Figure 8b indicated that ZB-4 was distinct from other sampling sites, potentially representing a point source pollution, such as from WWTPs. The elevated levels of these pollutants at ZB-4 highlight the significant influence of a nearby WWTP or other industrial activities.
The varied sources of pollutants at different sites, as well as the detection of pharmaceuticals and pesticides, could be linked to activities in the chemical industrial park, agricultural practices, and residential activities in the surrounding areas. The presence of 2-nitrophenol, dibutyl phthalate, and diethyl phthalate suggests contributions from industrial processes and possibly from the use of these compounds in various manufacturing activities. Carbendazim, a fungicide, points to agricultural runoff as a source of contamination. Tributylamine, used in various chemical syntheses, further supports the influence of industrial activities.
The findings emphasize the importance of site-specific monitoring and pollution control measures. For instance, enhanced treatment processes at WWTPs and stringent regulation of industrial discharges could mitigate the impact of pollutants at ZB-4. Moreover, agricultural best practices should be promoted to reduce pesticide runoff into the river (Krier J, 2022).
Understanding the specific pathways and behaviors of these pollutants in the aquatic environment will aid in developing targeted strategies to minimize their impact. Continued research and policy development are essential to safeguard the water quality of the ZB River, ensuring the protection of both ecological and human health in the region.
Further studies should aim to identify the precise sources and pathways of these contaminants. This can be achieved through a combination of advanced analytical techniques, field surveys, and modeling studies. Collaborative efforts among environmental agencies, industrial stakeholders, and the agricultural community are crucial for developing and implementing effective pollution control strategies. Public engagement and awareness campaigns can also play a vital role in encouraging best practices and reducing pollutant inputs.