Occurrence, compositional distribution, and toxicity assessment of pyrethroid insecticides in sediments from the fluvial systems of Chaohu Lake, Eastern China Ji-Zhong Wang, Ya-Shu Bai, Yakton Wu, Shuo Zhang, Tian-Hu Chen, Shu-Chuan Peng, Yu-Wei Xie & Xiao-Wei Zhang Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-015-5831-6
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Author's personal copy Environ Sci Pollut Res DOI 10.1007/s11356-015-5831-6
RECENT SEDIMENTS: ENVIRONMENTAL CHEMISTRY, ECOTOXICOLOGY AND ENGINEERING
Occurrence, compositional distribution, and toxicity assessment of pyrethroid insecticides in sediments from the fluvial systems of Chaohu Lake, Eastern China Ji-Zhong Wang 1,2 & Ya-Shu Bai 3 & Yakton Wu 1,2 & Shuo Zhang 1,2 & Tian-Hu Chen 1,2 & Shu-Chuan Peng 1,2 & Yu-Wei Xie 4 & Xiao-Wei Zhang 4
Received: 29 June 2015 / Accepted: 17 November 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Surface sediment-associated synthetic pyrethroid insecticides (SPs) are known to pose high risks to the benthic organisms in Chaohu Lake, a shallow lake of Eastern China. However, the pollution status of the lake’s tributaries and estuaries is still unknown. The present study was conducted to investigate the occurrence, compositional distribution, and toxicity of 12 currently used SPs in the surface sediments from four important tributaries, as well as in the sediment cores at their estuaries, using GC-MS for quantification. All SPs selected were detectable, with cypermethrin, es/fenvalerate, and permethrin dominant in both surface and core sediments, suggesting that these compounds were extensively applied. Urban samples contained the highest summed concentrations of the 12 SPs analyzed (Σ12SP) in both surface and core sediments compared with rural samples, suggesting that urban areas near aquatic environments posed high risks for SPs. The mean concentration of Σ12SP in surface sediments of each river was generally higher than that Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-015-5831-6) contains supplementary material, which is available to authorized users. * Ji-Zhong Wang
[email protected] 1
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
2
Laboratory for Nanomineralogy and Environmental Material, Hefei University of Technology, Hefei 230009, China
3
Third Institute of Oceanography, State Oceanic Administration of the People’s Republic of China, Xiamen 361008, Fujian, China
4
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China
found in core sediments from its corresponding estuary, perhaps implying recent increases in SP usage. Surface sediments were significantly dominated by cypermethrin and permethrin, whereas core sediments were dominated by permethrin and es/ fenvalerate. The compositional distributions demonstrated a spatial variation for surface sediments because urban sediments generally contained greater percentages of permethrin and cypermethrin, but rural sediments had significant levels of es/ fenvalerate and cypermethrin. In all sediment cores, the percentage of permethrin gradually increased, whereas es/fenvalerate tended to decrease, from the bottom sediments to the top, indicating that the former represented fresh input, whereas the latter represented historical residue. Most urban samples would be expected to be highly toxic to benthic organisms due to the residue of SPs based on a calculation of toxic units (TUs) using toxicity data of the amphipod Hyalella azteca. However, low TU values were found for the samples from rural areas. These results indicate that the bottom sediments were exposed to high risk largely by the residual SPs from urban areas. The summed TUs were mostly attributable to cypermethrin, followed by λcyhalothrin and es/fenvalerate. Despite permethrin contributing ∼28.7 % of the Σ12SP concentration, it only represented 6.34 % of the summed TUs. Therefore, our results suggest that high levels of urbanization can increase the accumulation of SPs in aquatic environments. Keywords Synthetic pyrethroid insecticides . Sediment . Chaohu Lake . Residue and toxicity assessment
Introduction Synthetic pyrethroid insecticides (SPs) are well-known manmade organic chemicals that have been widely used over the last few decades for the purpose of pest control in both
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agriculture and hygiene. In particular, because some highly toxic organophosphate insecticides (methamidophos, monocrotophos, parathion, methyl parathion, and phosphamidon) were gradually phased out, the usage of these SPs has drastically increased. The global sales volume of SPs was estimated to increase from 52.7 million dollars in 1960 up to 1.33 billion dollars in 2005, with an average annual growth rate of 7.4 % (Zhang et al. 2011). Consequently, the increasing application of SPs has caused their significant accumulation in the environment (Feo et al. 2010; Mehler et al. 2011; Weston and Lydy 2010), as well as in animal (Alonso et al. 2012) and human tissues (Babina et al. 2012; Feo et al. 2012). Despite SPs generally being considered to have lower degrees of toxicity to mammals compared to other insecticides such as organophosphate, they are known to exhibit high toxicity in fish (Ural and Sağlam 2005). Recently, studies on the carcinogenic, neurotoxic, immunosuppressive, and reproductive potential toxicity of pyrethroids also confirmed their adverse effects in mammals (Jin et al. 2012; Scollon et al. 2011). Furthermore, reversible symptoms such as headache, dizziness, nausea, irritations of the skin and mucosa, and paresthesia have been generally reported for populations that were highly exposed to SPs (He et al. 1988). Therefore, the occurrence and potential environmental impact of SPs have garnered considerable attention. Aquatic sediments are vulnerable to contamination with SPs due to their high hydrophobicity. Our previous study documented the high levels of residual SPs in sediments from Chaohu Lake, East China, occasionally with extremely high values of toxic units (TUs) in analyzed samples (Wang et al. 2011). SPs have been primarily considered as predictors of urbanization (Li et al. 2011; Wang et al. 2011; Weston et al. 2005; Weston and Lydy 2010). However, aquatic ecosystems influenced by runoff from agricultural land have also been found to contain SPs at elevated concentrations (Weston and Lydy 2010; Weston et al. 2004). Definitive identification of the sources of SPs in complex aquatic systems that receive both urban and agricultural streams, such as Chaohu Lake, is a major challenge. The critical reason for this challenge is due, in part, to an incomplete understanding of the sources, runoff characteristics, and fates of SPs in the watershed. The present study sought to determine the occurrence of 12 currently used SPs in surface sediments in four tributaries and core sediments at their estuaries around Chaohu Lake. Special attention was paid to compositional distributions and spatial/ vertical variations.
Materials and methods Study area and sample collection Chaohu Lake, the fifth largest freshwater lake in China, is located between the Huaihe River watershed and the
Yangtze River Delta, with a total area of ∼700 km2 and an average water depth of ∼3.0 m. The lake steadily receives water input from a total of 33 rivers and tributaries. Among its tributaries, Nanfei River (NFR) and Hangbu River (HBR) on the western flank and Tangyang River (TYR) and Yuxi River (YXR) on the eastern flank of the lake are particularly important owing to their large amounts of riverine runoff. Furthermore, NFR passes through a megacity (Hefei City) and is usually considered to be the critical source of contaminants in Chaohu Lake (Wang et al. 2012a). The other three rivers generally go through areas significantly influenced by agricultural cultivation. Surface sediment samples were collected from the four rivers using a stainless grab during September–October 2012 and from four sediment cores at the river estuaries of Chaohu Lake using a gravity core sampler in July 2012 (Fig. 1). In total, 44 surface sediments and 84 core sediments were obtained. ENF, EHB, ETY, and EYX denote the core sediments collected from the estuaries of NFR, HBR, TYR, and YXR, respectively. All cores were sectioned immediately into 1 cm slices. A global positioning system was employed to determine the geographical coordinates of all the sampling sites. All samples were stored frozen prior to laboratory processing. Preparation of materials n-Hexane, methylene chloride, acetone, and methanol at grades suitable for pesticide residue analysis were purchased from Oceanpak Alexative Chemical Co., Ltd (Gothenburg, Sweden). Twelve SPs, namely tefluthrin, bioallethrin, prallethrin, tetramethrin, permethrin, cyfluthrin, cypermethrin, fenvalerate, deltamethrin, allethrin, λ-cyhalothrin, and esfenvalerate, were purchased from Accustandard, Inc. (New Haven, USA). 4,4′-Dibromooctafluorobiphenyl, 2,3′,4,5tetrachlorobiphenyl (PCB 67), 2,3,3′,4,4′,5′,6heptachlorobiphenyl (PCB 191), and decachlorobiphenyl (PCB 209), used as surrogate standards, were obtained from Accustandard, Inc. Parathion-ethyl-d 10 , 2,2′,3,3′,4pentachlorobiphenyl (PCB 82) and 2,3,3′,4,4′,5,5′heptachlorobiphenyl (PCB 189), supplied by the laboratory of Dr. Ehrenstorfer (Augsburg, Germany), were selected as internal standards. Dual-layer solid-phase extraction (SPE) cartridges packed with granular black carbon and primary/ secondary amine (300 mg/600 mg) were purchased from Supelco of Sigma-Aldrich Corp (Bellefonte, PA, USA) for the purpose of sample cleanup. Absorbent cotton and filter paper were Soxhlet extracted with methanol and methylene chloride for 48 h, respectively, prior to use. Sodium sulfate was baked at 450 °C and stored in sealed containers. Copper sheets were activated with ∼10 % HCl in an ultrasonic bath to remove surface copper oxide and then washed with tap water, deionized water, and acetone. All laboratory spoons, scissors, forceps, mortars, and pestles were cleaned with acetone before
Author's personal copy Environ Sci Pollut Res Fig. 1 Sampling map for the surface sediment samples (red circles) and sediment cores (red arrow) in the the fluvial systems of Chaohu Lake (a), in Anhui Province (b), China (c). ENF, EHB, ETYand EYX represent the estuaries of Nanfei River, Hangbu River, Tongyang River and Yuxi River, respectively
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use. All glassware was handwashed with tap water, rinsed with deionized water, baked at 180 °C over night, and rinsed with the same solvent used in the subsequent operation. Chemical analysis Fifteen grams of appropriate freeze-dried and homogenized sediment samples was spiked with 4,4′-dibromooctafluorobiphenyl, PCB 67, PCB 191, and PCB 209 and then extracted by a Soxhlet apparatus with 100 mL of acetone and hexane mixture (1:1 by volume) for 48 h. Activated copper sheets were added for the purpose of desulfurization. Each extract was concentrated and solvent exchanged to hexane and then further reduced to approximately 1 mL with a ZX98-1 rotary evaporator (Shanghai, Yukang, China). All extracts were cleaned up and fractioned using the dual-layer SPE cartridges topped by 1 cm of anhydrous sodium sulfate. The fractions containing pyrethroid were eluted with 7 mL of a mixture of hexane and dichloromethane (7:3 by volume). The extract was concentrated to 0.5 mL under a gentle flow of nitrogen. Prior to instrumental analysis, the internal standards were added. Total organic carbon analysis The total organic carbon (TOC) of sediment was determined using a Vario TOC Cube (Elementar, Germany) after removal of the inorganic carbon portion by acidification (with 1 M HCl).
Instrumental analysis Twelve SP compounds were analyzed with an Agilent 7890A gas chromatograph coupled with a 5975C mass spectrometer (Agilent Corporation, USA). A 30-m × 0.25-mm i.d. × 0.25-μm film thickness HP-5 column (Agilent Corporation, USA) was employed for chromatographic separation of the target analytes. The initial column temperature was programmed to increase from 100 °C(held for 2 min) to 220 °C at a rate of 30 °C/min, to 225 °C at a rate of 1 °C/min, to 280 °C at a rate of 5 °C/min, to 300 °C at a rate of 20 °C/min, and then held 7 min. The injection temperature was set at 300 °C. Ultra-high-purity helium (99.999 %) was used as the carrier gas, with a constant flow rate of 1 mL/min. One microliter of the sample was injected using a 7693 Autosampler (Agilent Corporation, USA). Extract injection was conducted in a splitless mode, and the split mode was activated after 0.75 min. Negative chemical ionization (NCI) was processed in the selected ion monitoring mode. Methane was employed as the reagent gas. The temperature of the ion source and quadrupole was both 150 °C, and the transfer line temperature was 280 °C. Fenvalerate and esfenvalerate could not be chromatographically separated, and the instrumental responses are possibly different between them. In order to eliminate the error, all samples were injected twice using the same methods described above. The first run was conducted to quantify 11 SPs without esfenvalerate, and the second run was for the quantification of esfenvalerate. The finally reported concentration of es/fenvalerate was averaged using the quantitative results of fenvalerate and esfenvalerate.
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Quality assurance and quality control
Results and discussion
Quantification was performed using an internal calibration method based on eight-point calibration curves with regression coefficient (R2) values greater than 0.90 for individual pyrethroids. One laboratory blank sample and one group of triplicate sample were analyzed along with each of the 12 field samples. The mean recoveries for the surrogate standards 4,4′dibromooctafluorobiphenyl, PCB 67, PCB 191, and PCB 209 from all sediment and blank samples were 62.8 ± 36, 94.2 ± 54.4, 97.3 ± 51.3, and 68.5 ± 48.9 %, respectively. In the entire study, no target compounds except for permethrin and deltamethrin were detected in the laboratory blank samples. The mean concentrations of permethrin and deltamethrin in two laboratory blank samples were 1.5 and 1.6 ng/mL, respectively, corresponding to 0.050 and 0.053 ng/g by the average sample weight (ESM 1: Table S1). Two groups of triplicates were analyzed, with mean relative deviations ranging from 0.4 to 7.8 %.
Detection of individual SP insecticides in sediments of Chaohu Lake
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permethrin
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LC50(s) concentrations of tefluthrin, λ-cyhalothrin, permethrin, cyfluthrin, cypermethrin, esfenvalerate, and deltamethrin for a 10-day exposure of H. azteca to pyrethroid insecticides were obtained from the relevant literature (Amweg et al. 2005; Ding et al. 2010) and are also presented in ESM 1: Table S1. Generally, samples with summed TUs less than 1 were considered nontoxic, while those with summed TUs greater than 1.0 were considered consistently toxic (Weston et al. 2005).
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bioallethrin
The summed TU value for each sample was calculated by X Summed TUs ¼ TUi ð2Þ
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allethrin
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Measured concentrations were not corrected for the recoveries of surrogate standards. Concentrations are reported in units of nanograms per gram dry weight (dw) for sediment samples. The total concentration of the 12 SPs is denoted as Σ12SP. The normal distribution of log-based pyrethroid concentrations was checked using a Kolmogorov-Smirnov test with IBM SPSS Statistics v19.0 (New York, USA). Pyrethroid concentrations were converted to TUs to better understand the relative toxicities of each pyrethroid. The TU value of each compound was obtained as the ratio of the TOCtransferred environmental concentration (Cs(i)/TOC) and its 10-day median lethal concentration (LC 50 (s)) of Hyalella azteca (Eq. 1).
Concentration (ng/g)
Data analysis
The data on the detection of all SPs in the surface and core sediments is illustrated in Fig. 2. In surface sediments, tefluthrin and prallethrin were not detected in any of the samples, and allethrin, bioallethrin, and tetramethrin were detected only at frequencies in six, six, and one samples, respectively, with concentrations higher than the reporting limits (RLs). Twenty out of 44 surface sediments were found to contain deltamethrin and cyfluthrin. λ-Cyhalothrin was detected in 26 surface sediments. Es/fenvalerate, cypermethrin, and permethrin were found to be widely distributed (detected in more than 88 % of sediments). It should be noted that allethrin, bioallethrin, and tetramethrin were mainly detected only in samples of NFR and TYR and were not observed in any samples of HBR and YXR. In core sediments, bioallethrin was not detected in any of the estuaries of Chaohu Lake. Tefluthrin, prallethrin, allethrin, and tetramethrin were not frequently found; they were detected only in five, one, one, and four samples, respectively. Deltamethrin, λ-cyhalothrin, and cyfluthrin were also observed only at low frequencies, with just 25, 17, and 33 % of the samples, respectively, exhibiting concentrations higher than RLs. However, es/fenvalerate, cypermethrin, and permethrin were found at high frequencies (55, 65, 71, and 99 %, respectively). It should be noted that tefluthrin was predominately detected at ETY, and deltamethrin was mostly detected at ETY and EYX, both located at the
Fig. 2 The concentrations (gray bars) and detection frequencies (red stars) of individual SPs for surface sediments (a) and core sediments (b)
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(a)
10 1 .1 .01 10
Concentration (ng/g)
eastern side of the lake. Overall, individual pyrethroids were commonly detected with greater frequencies in surface sediments than in core sediments, suggesting recently increased pollutant loadings in the river system of Chaohu Lake, or the existence of significant degradation of the residual SPs in core sediments. Permethrin was the most ubiquitous insecticide in Chaohu Lake, and es/fenvalerate and cypermethrin were also widespread. However, tefluthrin, prallethrin, allethrin, bioallethrin, and tetramethrin were barely detected at concentrations higher than their RLs. The widespread occurrence of permethrin, es/fenvalerate, and cypermethrin around Chaohu Lake could be in part explained by their usage because they are widely applied as insecticides for crop protection (Jiangsu Institute of Insecticide Research 1977), industrial/domestic insect control (Wang and Li 2010), and pest repellents for medical treatment (Goldust et al. 2013), considering their stability against degradation by light in the air (Elliott 1976; Frank and Marshall 2008). The low detection frequencies of tefluthrin, prallethrin, allethrin, bioallethrin, and tetramethrin could potentially be due to their susceptibility to degradation under sunlight. Despite the fact that tetramethrin and allethrin accounted for 14 and 12 % of the total hygienic insecticide consumption, respectively (Jiang and Wang 2006), their high degree of water solubility and rapid degradation could result in the weak accumulation of these SPs in bottom sediments (ESM 1: Table S1). On the other hand, allethrin, bioallethrin, and tetramethrin were generally detected in NFR, probably because NFR mostly receives municipal sewage from Hefei City, a metropolis in Eastern China.
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Spatial and vertical variation of sediment SPs in Chaohu Lake
Fig. 3 Spatial variation of concentrations of es/fenvalerate (a), cypermethrin (b), permethrin(c), and Σ12SP (d) in the surface sediments of four tributaries and in the core sediments at their corresponding estuaries of Chaohu Lake, Eastern China. NFR, HBR, TYR, and YXR represent Nanfei River, Hangbu River, Tongyang River, and Yuxi River; ENF, EHB, ETY, and EYX represent estuaries of NFR, HBR, TYR, and YXR, respectively
The concentration of Σ12SP in surface sediment was 14.8 ± 14.0 ng/g for NFR, 1.92 ± 2.19 ng/g for HBR, 4.51 ± 3.63 ng/g for TYR, and 0.98 ± 0.92 ng/g for YXR (Fig. 3a). Therefore, the concentration of Σ12SP in surface sediments collected from NFR was approximately one order of magnitude higher than those observed in the other three rivers. This suggests that urban sediments contained elevated concentrations of SPs. Previous studies also demonstrated that sediments from urban areas were more severely contaminated by SPs (Li et al. 2011; Weston and Lydy 2010), and numerous SPs have been identified as the critical contributors to sediment toxicity (Li et al. 2013; Mehler et al. 2011; Yi et al. 2015). Therefore, SPs in environmental media are generally expected to comprise urban-dominated pollutants (Li et al. 2014; Wang et al. 2012b). Two surface sediment samples collected from TYR did not contain any target pyrethroid insecticides (ESM 1: Fig. S1); one site (TYR1) was located in the upstream area of TYR, and the other (TYR7) was located at the estuary of TYR (Fig. 1). Sample TYR1
was rarely affected by agricultural runoff, and sample TYR7 was possibly influenced by water dilution at the estuary. The highest concentration of Σ12SP in NFR was observed at NFR7, which was located in the upstream area of the river (ESM 1: Fig. S1). The possible reason for this could be the isolated water cycle and great urbanization in this area. For the purpose of establishing a drinking water supply, the natural stream flow was altered by construction of two reservoirs (Dongpu Reservoir and Dafangying Reservoir) (Fig. 1), which then isolated the headwater of the Nanfei River. As a result, the sampling site at NFR7 was significantly affected by the treated or untreated municipal sewage of Hefei City. High concentrations of Σ12SP were detected in the areas close to the estuaries of HBR and YXR (ESM 1: Fig. S1), which may be due to the rapid sedimentation of suspended particles there. However, the concentration of Σ12SP in TYR appeared to decrease going from its upstream region toward its estuary, indicating the significant influence of agricultural runoff in the upstream region.
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Figure 2b shows the concentrations of individual SP in core sediments. Briefly, the mean concentrations of Σ12SP were 10.6 ± 11.0 ng/g for ENF, 1.18 ± 1.81 ng/g for EHB, 1.78 ± 2.25 ng/g for ETY, and 1.09 ± 1.40 ng/g for EYX, also suggesting high concentrations of SPs in urban sediments. The vertical distributions of Σ12SP are different among the four sediment cores (ESM 1: Fig. S2). For the sediment core at ENF, the peak concentrations of Σ12SP occurred at sediment depths of 22 and 7 cm, and the concentrations tended to increase recently. For EHB and ETY, the peak levels of Σ12SP occurred at sediment depths of 6 and 15 cm, respectively, but SP concentrations were gradually reduced in shallower (more recent) layers. However, for EYX, the peak concentration occurred in the subsurface sediment. Overall, es/fenvalerate was predominant mostly in the bottom core sediments and present throughout all the sediment cores, but permethrin was the main component in the top core sediments (ESM 1: Fig. S3), implying that es/fenvalerate came from long-term discharge, while permethrin came from fresh input around Chaohu Lake. A possible explanation for the concentration of Σ12SP peaking in ENF is the industrial wastewater discharge from a local insecticide plant into that river. It is estimated that 147 tons of fenvalerate was produced in Hefei Agricultural Chemicals Plant during 1984 and 1985. Therefore, the first peak in concentration of Σ12SP (dominated by es/fenvalerate), at a depth of 22 cm, could be inferred to represent production and discharge in 1984. Accordingly, the second concentration peak (dominated by permethrin) might have occurred in 2002, when the Henong Pesticide Limited Corporation of Hefei was built. The large input of wastewater containing SPs might also be expected to enhance their accumulation at EHB and ETY because the vertical distribution of Σ12SP concentrations at EHB and ETY clearly show similar trends for the peak concentrations. However, we could not draw a definite conclusion regarding the urban effect on the deposition of Σ12SP at rural sites because accurate chronologies could not be determined in the present study. These sediment cores possibly represent at least the last three decades because previous studies have suggested that the sedimentary rates in Chaohu Lake range from 0.2 to 0.4 cm/year (Chen et al. 2011; Zan et al. 2011). Additionally, increasing concentrations of Σ12SP in the upper layers of cores at ENF might imply the influence of increasing input from an urban area because the sampling site is located near an urban region. However, for EHB and ETY, the concentrations tended to decrease toward the top of the samples, which is an indication of the decreasing input of SPs from rural areas. This could be explained by the fact that the rural residents around Chaohu Lake have gradually been moving into cities (Wang 2007); as a result, the total volume of SPs applied in rural regions would have declined. However, the relatively low and consistent concentration of Σ12SP in the segments from the bottom to subsurface samples at EYX indicates that this location was
less contaminated by SPs because Yuxi River is the only outflow of Chaohu Lake, and its estuary is not expected to receive wastewater from Chaohu City (Yang et al. 2011). The peak concentration occurring at the subsurface sediment could be associated with the construction of Woniushan Park in 2005 and Guishan Park in 2009 near EYX; SPs were presumably applied for garden maintenance. The significant decrease of SP concentrations from the subsurface to surface sediments might be associated with the sediment dredging project in this area (http://news.sina.com.cn/o/2012-07-20/091724811640. shtml, accessed on July 28th, 2014). In summary, these results imply the historical production and consumption of SPs around Chaohu Lake. In comparison to core sediments, surface sediments were more significantly contaminated by cypermethrin and had higher Σ12SP levels for all rivers (Fig. 3). The surface sediments in Nanfei River contained significantly higher concentrations of permethrin, but the mean concentration of permethrin in surface sediments of HBR, TYR, and YXR was lower than that observed in core sediments of their corresponding estuaries. These results suggest that permethrin was a fresh contaminant from urban areas, and NFR was the pivotal source for this contaminant in Chaohu Lake. However, the opposite trend was observed for es/fenvalerate. Specifically, a lower concentration of this SP was observed in the surface sediment of NFR than in the core sediments of ENF, and higher concentrations were detected in the surface sediments of the other three rivers than in the core sediments of their corresponding estuaries. These results potentially imply that the sediment-associated es/fenvalerate in Chaohu Lake resulted from its long-term accumulation and its current use. Sediment organic carbon is generally considered the crucial factor controlling the accumulation of hydrophobic organic contaminants in sediment. The relationships between the Σ12SP and TOC values in surface sediments of each river and the core sediments of each estuary are illustrated in ESM 1: Fig. S4. Overall, the concentration of Σ12SP significantly correlated with sediment TOC for the surface sediments in NFR, suggesting that the TOC played an important role in SP deposition. The SP concentrations in surface sediments of HBR and TYR appeared to increase with TOC increase but without statistical significance, which could be in part explained by the small number of samples. For surface samples collected from YXR and in core sediments from the four estuaries, the concentrations of Σ12SP did not depend on the sediment TOC. YXR is the only outflow river of Chaohu Lake, and sediment TOC could be largely depleted by water flushing and sediment resuspension. The main reason on independence between SP concentrations and TOC for the core sediments could be that the estuaries were more greatly influenced by eutrophication, resulting in complex sources of residual organic matter in the bottom sediments.
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For surface sediments, cypermethrin shared the largest proportion of the Σ12SP in composition, accounting for 36.8 ± 22.7 % (ESM 1: Fig. S5), and its concentration ranged from
