Distribution Dynamics of Phthalate Esters in Surface Water and Sediment of the Middle-Lower Hanjiang River, China

Phthalate esters (PAEs) are endocrine-disrupting chemicals that pose potential risks to human health. Water and sediments are crucial carriers and storage media for the migration and transformation of PAEs. In this study, six congeners of PAEs were measured in water and sediment samples to elucidate their spatial distribution, congener profiles, and ecological risks in the middle-lower Hanjiang River during the wet and dry seasons. The concentration of the Σ6PAEs ranged from 592 to 2.75 × 103 ng/L with an average of 1.47 × 103 ng/L in surface water, while the concentration of the Σ6PAEs ranged from 1.12 × 103 to 6.61 × 103 ng/g with an average of 2.69 × 103 ng/g in sediments. In general, PAE concentrations were ranked as sediment > water, and dry season > wet season. DEHP and DBP were the dominant PAEs in the middle-lower Hanjiang River in surface water and sediments. SPSS analysis showed that dissolved organic carbon (DOC) in surface water was significantly correlated with the concentration of DBP, DEHP, and the ∑6PAEs, while organic matter (OM) was significantly correlated with the concentration of the ∑6PAEs in sediments. The concentrations of PAEs were irregularly distributed and varied significantly in surface water and sediments. Compared with other regions at home and abroad, the pollution levels of surface water and sediments in the middle-lower Hanjiang River were relatively low and not enough to have a negative impact on the local water’s ecological environment. However, the supervision of land-based discharge should still be strengthened.


Background
Phthalate esters (PAEs) are a group of chemical compounds that are widely used as nonreactive plasticizers not only in polyvinyl plastics (PVC) but also in a broad range of industrial processes and consumer products, including cosmetics, building materials, insect repellents, automobile parts, and food packaging [1]. PAEs are easily released into the environment during the processes of manufacturing and application via evaporation and leaching from domestic and industrial effluents [2][3][4][5]. Global PAE production exceeds 8.0 million tons annually, and it has been reported that PAE consumption in 2011 reached approximately 2.2 million tons in China [6]. Previous studies have shown that PAEs are endocrine-disrupting chemicals that pose potential health risks to humans and other organisms; for example, they can disrupt the hormonal balance of mammalian species [7]. Six PAE monomers are listed as priority control pollutants by the European Union (EU) and the Environmental Protection Agency in the United States (USEPA) [4]. PAE-containing products in industry and households have led to PAEs being ubiquitous in various environments, including air, water, sediments, soil, and food [7][8][9].
in environmental media regarding pollution sources, sediment adsorption, and watersediment relationships. Further, this work provides the first documentation of surface water and sediment contamination by PAEs in the middle-lower Hanjiang River, thereby enabling a monitoring program to study the pollution status and changing trends of PAEs over space and time, which is also beneficial for water environmental management and protection in the economic belt of the Hanjiang River.

Reagents and Chemicals
All solvents, including hexane, methylene chloride, acetone, methanol, and ethyl acetate, which had purity levels of at least 99%, were of HPLC grade and obtained from Fisher Chemical Co. (Fair Lawn, OH, USA). Six PAE standard solutions used in this study including dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP), di-2-Ethylhexyl phthalate (DEHP), and di-n-octyl phthalate (DNOP) were supplied by AccuStandard Inc. (New Haven, CT, USA, 99%). Solid-phase extraction membranes (ENVI-18 DSK) were obtained from Sigma-Aldrich Chemistry Co. (St. Louis, MO, USA). Anhydrous sodium sulfate (analytical grade, Beijing Chemical Reagent Co., Shanghai, China) was baked in a furnace oven (FP-40, China) at 550 • C for 5 h to remove any organics or water, then kept in a sealed desiccator prior to use. Water was prepared from a Milli-Q system (Millipore; Bedford, MA, USA). Glassware was cleaned with acetone first and then n-hexane.

Study Area
The sampling sites along the middle-lower Hanjiang River are plotted in Figure 1. Detailed information is listed in Table A1. Water and sediment samples were collected from the middle-lower Hanjiang River in June 2019 (wet season) and in January 2020 (dry season).  Downstream of the Danjiangkou Reservoir, six cascade reservoirs are located (as shown in Figure 1), of which Wangpuzhou Reservoir, Cuijiaying Reservoir, and Xinglong Reservoir are in operation, while the other three reservoirs, Xinji Reservoir, Yakou Reservoir, and Nianpanshan Reservoir, are still under construction.

Sample Collection
All surface water samples (about 20 cm below the water's surface) were sealed in 5 L glass containers and carried back to the laboratory within the day and stored at 4 • C in a refrigerator before further research. On the same day, 4 L surface water was passed through a 0.45 µm glass fiber membrane at the laboratory, and then the water sample was passed through a methanol-activated solid-phase extraction membrane (SPE) to isolate the PAEs [20,21]. Finally, the solid-phase extraction membrane that contained the PAEs was wrapped in foil and then refrigerated for further analysis. The other 1 L from the initial 5 L water sample was obtained to analyze the concentrations of dissolved organic carbon (DOC), total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH 3 -N). Moreover, the pH, dissolved oxygen (DO), temperature (T), and oxidation-reduction potential (ORP) were field-measured with a multiparameter water quality probe (USA, YSI EXO2).
The surface sediments (0-5 cm) from 15 sampling sites were collected with stainless steel Peterson grab samplers and put into a glass petri dish. The samples were stored in ice on their way to the laboratory, where they were stored at −20 • C until further processing. Each sample was divided into two sub-samples: one sub-sample was thawed for TN, TP, and organic matter (OM) analysis, the other sub-sample was freeze-dried and then carefully homogenized over 100 mesh for PAE analysis in sediments.

The Extraction Process of Water and Sediment
Refer to the extraction methods of our previous study [22]. For water samples, SPE extraction disks were eluted twice with ethyl acetate and dichloromethane-ethyl acetate (1:1, v:v). Then, the extracts were combined and dried over anhydrous sodium sulphate. The extracts were concentrated to 0.1 mL in a rotary evaporator (R-210, Buchi, Flawil, Switzerland) and a thermovap sample concentrator (N-EVAP, Organomation, Berlin, MA, USA). Finally, the samples were redissolved in 1 mL of n-hexane for analysis.
The sediment was lyophilized by a freeze dryer (FD5-series, SIM, Newark, DE, USA) for 72 h and then crushed and passed through a 100-mesh sieve. Approximately 2 g of the dry sample was accurately weighed and then transferred to a 50 mL extract tube containing 25 mL of acetone-hexane (1:1, v:v) organic extraction solvent. Each mixture was extracted with a microwave digestion system (Mars6, CEM, Matthews, NC, USA) wherein the temperature changed at a rate of 10 • C/min from room temperature to 120 • C; the 120 • C temperature was maintained for 30 min. After the microwave extraction was completed, the supernatant was passed through anhydrous sodium sulphate. Then each sample was extracted and purified in a Florisil SPE Cartridge (1 g, 6 mL/30 pcs). Finally, the eluent from the Florisil SPE Cartridge was evaporated to 0.1 mL using a rotary evaporator and nitrogen flow, and then re-dissolved in 1 mL of n-hexane for measurement.

GC/MS Analysis
Analytical detection of PAEs was performed using a gas chromatography-mass spectrometer (GC-MS) (7890B/5977A, Agilent, Palo Alto, CA, USA) equipped with a DB-5 ms fused silica capillary column (30 m × 0.25 mm × 0.25 µm, Agilent, Palo Alto, CA, USA). High-purity helium (99.999%) served as the carrier gas at a flow rate of 1.0 mL/min. The transfer line and the inlet temperature were maintained at 300 and 250 • C, respectively. The injection volume was 1.0 µL, and the splitless injection mode was used. The oven temperature program for the PAEs was as follows: initial temperature of 70 • C held for 2 min; increased to 130 • C at 20 • C/min; increased to 200 • C at 5 • C/min; and increased to 300 • C at 15 • C/min, held for 5 min [22].

Quality Control
Quality control and quality assurance in the sample analyses were implemented according to the regulations for water environmental monitoring of China (SL 219-2013). The quantitative standards for PAEs in the samples were determined using a combination of external standard methods. The linearity correlation coefficients were between 0.999 and 1.00 for the 6 PAE monomers. The limits of detection (LOD) were determined based on the concentrations that existed at three times the signal-to-noise ratio. The LOD of the water samples ranged from 0.120 to 0.920 ng/L, whereas the LOD of the sediment samples ranged from 0.250 to 1.85 ng/g [22]. The recoveries of 6 PAE monomers for the water samples were 86.9-110%, while the recoveries of 6 PAE monomers for the sediment samples were 63.9-75.5%. In total, 6 PAE monomers were quantitatively analyzed using Mass Hunter data acquisition software on an Agilent 7890B/5977A GC/MS instrument. To avoid the injection contamination caused by the analysis process, a sample blank was made after every 10 samples analyzed. The sample blank value was below LOD. Any datum that was under the LOD was calculated as not detected (ND) [23].

Data Collection and Processing
Hydrologic and sediment data were collected from the Changjiang River Sediment Bulletin [24]. All the figures were constructed using the software Origin (version 2019) and Surfer (version 16.0). Pearson correlation analysis of PAEs, TP, CODMn, Chl-a, TN, NH 4 -N, DOC, DBP, and DEHP (data normally distributed) in surface water and Pearson correlation analysis of PAEs, OM, TN, TP, DBP, and DEHP (data normally distributed) in sediments were carried out using SPSS (IBM SPSS Statistics 22.0).

Surface Water
The exposure concentrations of the six PAE monomers in the middle-lower Hanjiang River are presented in Table A2, and detailed information is listed in Table A3. The total concentrations of the six PAE monomers in the water samples were detected in the range of 592-2.75 × 10 3 ng/L (the mean value was 1.47 × 10 3 ng/L). The PAE concentrations indicated significant seasonal variation. DBP and DEHP in the surface water samples of the middle-lower Hanjiang River did not exceed the standard limits of 3.00 × 10 3 and 8.00 × 10 3 ng/L, respectively, which are based on the National Surface Water Environmental Quality Standards (GB3838-2002) in China. The exposure concentrations of the ∑ 6 PAEs in the wet season were much lower than those in the dry season, with the mean concentration of DEP, DBP, BBP, DEHP, and DNOP in the dry season about 1.20-5.61 orders of magnitude higher than that in the wet season. This may be because the rainfall and the high-speed flow in the river acted as diluents during the wet season, improving its self-purification ability [25]. In addition, during the wet season, the volume of the water body increases significantly, thus diluting the concentrations of PAEs [26]. Similar seasonal changes have occurred in the main stream of the Yangtze River [25] and Poyang Lake [27].
The distributions of the six PAE monomers in different seasons are shown in Figure 2A,B. Combined with monitoring data results, the exposure concentrations of PAEs in different sampling sites exhibited significant differences in the wet season and dry season. Higher PAE concentrations were mainly concentrated in urban areas and agricultural planting areas, for example, the Danjiangkou urban section (S2), Laohekou urban section (S4), downstream of the Xinglongba gate (S9, agricultural planting area), and the Qianjiang urban section (S10), while the concentration of PAEs in other sampling sites was relatively low. These sites with higher PAE concentrations were mostly influenced by human activities, inevitably receiving household wastes and industrial wastewater containing PAEs from surrounding residents, chemical reagent factories, or fine chemical mills. The relative composition profiles of PAEs in the surface water samples from the mid dle-lower Hanjiang River were different depending on the sampling locations (Figur 2C,D). Generally, DBP and DEHP dominated, with an average contribution of up to 60.0% of the ∑6PAEs in the wet season or dry season. Almost one-half of the sampling site showed that DBP was the dominant compound with a relative contribution of >50.0% t the ∑6PAEs, while DEHP dominated at one-half of the sampling sites, accounting fo >20.0% of the ∑6PAEs in the wet season or dry season. The most abundant PAE in th water samples was DBP, accounting for 35.3-72.9% of the ∑6PAEs, followed by DEHP (10.6-47.9%). DBP and DEHP were the most prevalent PAEs in the water body, which wa consistent with those found in the Wuhan Section of the Yangtze River [28,29] and th Yangtze River Estuary in a previous study [30].
The water pollution levels of the middle-lower Hanjiang River were compared with The relative composition profiles of PAEs in the surface water samples from the middlelower Hanjiang River were different depending on the sampling locations ( Figure 2C,D). Generally, DBP and DEHP dominated, with an average contribution of up to 60.0% of the ∑ 6 PAEs in the wet season or dry season. Almost one-half of the sampling sites showed that DBP was the dominant compound with a relative contribution of >50.0% to the ∑ 6 PAEs, while DEHP dominated at one-half of the sampling sites, accounting for >20.0% of the ∑ 6 PAEs in the wet season or dry season. The most abundant PAE in the water samples was DBP, accounting for 35.3-72.9% of the ∑ 6 PAEs, followed by DEHP (10.6-47.9%). DBP and DEHP were the most prevalent PAEs in the water body, which was consistent with those found in the Wuhan Section of the Yangtze River [28,29] and the Yangtze River Estuary in a previous study [30].
The water pollution levels of the middle-lower Hanjiang River were compared with the pollution levels of other rivers in China and abroad (Table A4). The mean concentrations of the ΣPAEs in the middle-lower Hanjiang River were approximately 4 times higher than those in the Three Gorges Reservoir [22] and were approximately 1.6 times higher than those in the Jiangsu section of the Yangtze River [31] and the Changjiang River Estuary [32]. However, the mean concentrations of the ΣPAEs in the middle-lower Hanjiang River were about 4 and 13 times lower than those in the Jiulong River [33] and the Songhua River [9], separately. For the main PAE monomers, the maximum DBP measured in the middle-lower Hanjiang River (1.48 × 10 3 ng/L) was about 10-20 times lower than that in the Songhua River (11.8 × 10 3 ng/L) [9], and the mid-lower reaches of the Yellow River (26.0 × 10 3 ng/L). DBP was commonly recorded in the Kaveri River from India (250 ng/L) [2] and the Jiangsu section of the Yangtze River (max. value 286 ng/L) [31], approximately five times lower than that in the middle-lower Hanjiang River in this study. The pollution levels of DEHP in the middle-lower Hanjiang River were 10-20 times lower than those of rivers in China and abroad, such as the Humber River [34], the midlower reaches of the Yellow River, the Songhua River [9], and the Jiulong River [33]. Meanwhile, the maximum pollution level of DEHP in the middle-lower Hanjiang River was approximately 2 times higher than that in the Three Gorges Reservoir [22]. Overall, compared with the levels in other rivers in China and abroad, the pollution levels of the ΣPAEs in surface water were relatively low along the middle-lower Hanjiang River.

Sediments
The exposure concentrations of DMP, DEP, DBP, BBP, DEHP, and DNOP in the sediments of the middle-lower Hanjiang River are presented in Table A5. The concentrations of the ∑ 6 PAEs in the sediments ranged from 1.12 × 10 3 to 6.61 × 10 3 ng/g (with a mean value of 2.69 × 10 3 ng/g). The ranges of DMP, DEP, DBP, BBP, DEHP and DNOP concentrations in the sediments were 2.30-190 ng/g, 2.00-620 ng/g, 159-4.33 × 10 3 ng/g, ND-21.1 ng/g, 341-1.89 × 10 3 ng/g, and ND-416 ng/g, respectively. The average concentrations of the six PAE monomers followed the order: DEHP (1.11 × 10 3 ng/g) > DBP (920 ng/g) > DNOP (140 ng/g) > DEP (132 ng/g) > DMP (10.8 ng/g) > BBP (5.40 ng/g) in the wet season, whereas DBP (2.01 × 10 3 ng/L) > DEHP (735 ng/L) > DEP (196 ng/L) > DNOP (63.8 ng/L) > DMP (49.3 ng/L) > BBP (5.90 ng/L) were the average concentrations in the dry season. The exposure concentrations of the ∑ 6 PAEs in the wet season were much lower than those in the dry season. Similar to the results of the water samples, DBP and DEHP were also most frequently detected in sediment samples with 100% detection frequencies and the highest mean contamination levels of the six PAE monomers. This result was consistent with the findings in previous studies [22,28,29,35] where DBP and DEHP were also the dominant components in sediments.
In general, the distribution trend of the ΣPAEs in the sediments of the middlelower Hanjiang River in the wet season was not consistent with that in the dry season ( Figure 3A,B). However, in the middle-lower Hanjiang River, the higher concentrations of PAEs in the sediments were mainly concentrated in urban river sections, such as the Zhupi River (S8), downstream of the Xinglongba gate (S9), and Wuhan city urban area (S13, S14, and S15), which were likely caused by a stronger influence of urban activities, indicating that PAEs mainly came from point source pollution.
The compositions of six PAEs in different seasons are illustrated in Figure 3C,D. Similar to those in the water samples, DBP and DEHP dominated, with an average contribution of up to 85.0% of the ∑ 6 PAEs, which was consistent with those found in the Wuhan Section of the Yangtze River [28,29] and the Yangtze River Estuary [32] in a previous study. Specifically, the results showed that DBP and DEHP were the major components of PAEs in the surface water of the middle-lower Hanjiang River with a proportion of 60.7-95.3% (average: 86.7%) in the wet season and 83.7-99.2% (average: 89.8%) in the dry season. Following DEP, DNOP contributed to the total PAE concentration by 1.21-19.9% (average: 6.30%) in the wet season and 0.100-10.1% (average: 3.90%) in the dry season. The concentrations of PAEs were irregularly distributed and varied significantly along the middle-lower Hanjiang River in surface water and sediments, and the patterns of different distributions of PAEs varied substantially along the river without clear trends in surface water and sediments (Figures 2 and 3). The compositions of six PAEs in different seasons are illustrated in Figure 3C,D. Sim ilar to those in the water samples, DBP and DEHP dominated, with an average contrib tion of up to 85.0% of the ∑6PAEs, which was consistent with those found in the Wuha Section of the Yangtze River [28,29] and the Yangtze River Estuary [32] in a previou study. Specifically, the results showed that DBP and DEHP were the major componen of PAEs in the surface water of the middle-lower Hanjiang River with a proportion 60.7-95.3% (average: 86.7%) in the wet season and 83.7-99.2% (average: 89.8%) in the d season. Following DEP, DNOP contributed to the total PAE concentration by 1.21-19.9 (average: 6.30%) in the wet season and 0.100-10.1% (average: 3.90%) in the dry seaso The concentrations of PAEs were irregularly distributed and varied significantly alon the middle-lower Hanjiang River in surface water and sediments, and the patterns of d ferent distributions of PAEs varied substantially along the river without clear trends surface water and sediments (Figures 2 and 3). The sediment pollution levels of the middle-lower Hanjiang River were compared with the pollution levels of other rivers in China and abroad (Table A4). The mean concentrations of the ΣPAEs in the sediments found in the middle-lower Hanjiang River were approximately 4.80-6.80 times lower than those at areas such as the Changjiang River Estuary [32] and the Songhua River [35] and were approximately equal to those of areas such as Cochin estuary [4] and the Three Gorges Reservoir [22] and remarkably higher than that in the Xijiang River [36]. In general, the ΣPAEs' sediment pollution levels were low in the middle-lower Hanjiang River.

Surface Water
It has been reported that the environmental behavior and fate of PAEs are affected by their physical and chemical properties as well as the characteristics of water and sediments [10,37]. Figure A1 shows the trends of conventional water quality indexes (WQIs) measured in the middle-lower Hanjiang River. It can be seen that the WQIs involved in this study display similar trends along the middle-lower Hanjiang River (including TP, Chl-a, TN, NH 3 -N). During the wet season, TP concentration at the various sections ranged between 0.01 mg/L and 0.46 mg/L. Chl-a in the Zhupi River (S8) was higher than that in other sections. The average TN concentration was 2.10 mg/L, with the highest value also being observed at the Zhupi River (7.80 mg/L). The NH 3 -N concentration among the sections varied considerably; the lowest value was observed at the Danjiangkou Reservoir (S1, 0.09 mg/L), whereas the highest was obtained at the Zhupi River (5.91 mg/L). DOC concentration did not vary substantially among the sections (mean: 6.20 mg/L).
In addition, TP, Chl-a, TN, and NH 3 -N showed Pearson correlations to each other to some extent, but their Pearson correlation with PAEs was not significant (Table A6). However, the concentration of DBP, DEHP, and the ∑ 6 PAEs in surface water was significantly positively correlated (p < 0.01) with DOC, with Pearson correlations indexes of 0.737, 0.647, and 0.814, respectively. DOC refers to the water-soluble organic matter that can pass through 0.450 µm microporous membranes, mainly including toxic organic pollutants such as PAEs. As a result, it was indicated that the concentration of DOC had significant effects on the concentration of DBP, DEHP and PAEs.

Sediments
The TN concentrations ranged from 498 to 6.16 × 10 3 mg/kg, and the TP concentrations of the sediments were between 352 mg/kg and 1.10 × 10 3 mg/kg, with averages of 1.24 × 10 3 and 667 mg/kg, respectively ( Figure A2). The organic matter (OM) concentrations in sediment ranged from 1.12 to 6.51%, with an average of 3.56%.
There were no significant Pearson correlations (p < 0.01) between PAEs and TN and PAEs and TP in sediments (Table A7), indicating that TN and TP were not factors controlling the behavior of PAEs in sediments. However, significant positive (p < 0.01) correlation coefficients were observed between the ∑ 6 PAEs and the sediments' OM concentrations (y = 1.30 + 0.29x, r = 0.700) and between DEHP and the sediments' OM concentrations (y = 0.56 + 0.16x, r = 0.678) (Figures A3 and A4). The results show that OM could impact the concentrations of ∑ 6 PAEs in sediments. Because of the hydrophobic characteristics of the ∑ 6 PAEs, they may be readily adsorbed and fixed by the precipitate OM in sediments.

Surface Water
Based on toxicological data and numerical calculations, the USEPA has established ambient water quality criteria for human health (USEPA, 2015), which provide guidance for states to use to establish water quality standards and ultimately provide a basis for controlling discharges or releases of PAEs.
As shown in Table 1, the human health ambient water quality criteria (AWQC) represents the maximum acceptable levels of pollutants in a water body that are not expected to cause adverse effects on human health by consumption of "water + organism" or "organism only". In this study, the concentrations of DMP, DEP, DBP, and BBP in all surface water samples from the middle-lower Hanjiang River were lower than those of the human health AWQC for the consumption of "water + organism" or "organism only" in the wet season and the dry season, which indicated a low risk to human health (Table 1). However, more than half of the sampling sites (53.3%) showed levels of DEHP that exceeded the value of the human health AWQC for the consumption of "water + organism", while 46.7% of the sampling sites exceeded the value of the human health AWQC for the consumption of "organism only" in the wet season (Tables 1 and A3). Further, 60.0% of the sampling sites showed levels of DEHP that exceeded the value of the human health AWQC for the consumption of "water + organism" and "organism only" in the dry season (Tables 1 and A3). In particular, DEHP levels were relatively high (more than 600 ng/L) at several sampling sites of the middle-lower Hanjiang River, such as the Danjiangkou urban section (S2), Laohekou urban section (S4), and downstream of the Xinglongba gate (S9) in the wet season, and the Qianjiang urban section (S10) and Hanchuan urban section (S12) in dry season. Therefore, there might be a potential impact of DEHP on human health in these areas.

Sediments
Sediment accumulates PAEs in the process of water adsorption. When different PAE monomers coexist, there may be competition in the adsorption process, and when the environmental redox conditions change, it will also lead to the release of PAEs in the sediment [38,39]. This study shows that the order of PAE concentration is sediment > water (Tables A2 and A5), which indicates that sediment is an important reservoir of PAEs. These contaminants might cause adverse ecological impacts. There have been some studies on the impact assessment of the ecological risks of PAEs [7,8,23,35,[40][41][42]. However, no unified standards have been established for the assessment of the ecological risks of PAEs in sediments.
Regarding the environmental risk levels (ERLs) of the two PAE monomers determined [23,41], the ERLs of DBP and DEHP are 700 ng/g and 1.00 × 10 3 ng/g, respectively. When the relative pollution factor (RCF = C PAEs /ERLs, C PAEs is the measured environmental concentration of PAEs) is less than 1, PAE concentrations do not cause endocrine disruption and ecotoxicity risks; when the RCF is greater than 1, PAE concentrations can cause endocrine disruption and ecotoxicity risks. In the present study, both DBP and DEHP concentrations in 60% of the sediment samples exceeded the ERLs in the wet season, while DBP concentrations in 93.3% samples and DEHP concentrations in 26.6% samples exceeded the ERLs in the dry season (Table A5 and Figure 3). It should be noted that both DBP and DEHP concentrations of the sediment samples at some sampling sites exceed the ERLs, such as those in the Qianjiang urban section (S10) and midstream of Wuhan city (S14) in the wet and dry seasons. These results indicated that DBP and DEHP might exhibit potential risks to aquatic life and human health in some urban river sections of the middle-lower Hanjiang River, which were similar to the results of some other river studies in China [1,33].
For the other PAE monomers (including DMP, DEP, BBP, and DNOP), this study used the sediment management standards from the Washington State Department of Ecology, USA, each of whose values is 610 ng/g [33]. Therefore, DMP, DEP, BBP, and DNOP did not exceed the sediment quality standard limit, indicating that the ecological risks of DMP, DEP, BBP, and DNOP were relatively low.

Source Analysis
As for surface water, DBP, DEHP, DMP, and DEHP are found in high concentrations in household wastes (e.g., toys, plastic containers, and commodities) and heavy chemical industries [30]. DEP was the predominant PAE monomer used in cosmetic and personal care products [43]. In sediments, DBP mainly came from heavy chemical industries [30]. In addition, DEHP is the major PAE that leaches from household wastes [44] and is also a major component in atmospheric particles [45]. Recently, PAEs have been found in PM2.5 and PM10 in the ambient air of the Wuhan, Xiangyang, and Xiantao cities [46][47][48][49]. Moreover, atmospheric transport and deposition are also important sources of organic contaminants in the sediment environment (Mi et al., 2019). Therefore, long-range atmospheric transport deposition might also be a source of DEHP in the sediments of the middle-lower Hanjiang River, which requires further and ongoing research in the future. On the whole, domestic waste and heavy chemical industries might be the input sources for PAEs in sediments of the middle-lower Hanjiang River, but this still requires follow-up systematic research.

Impact of Suspended Sediment on the Behavior of PAEs
Owing to the hydrophobicity of organic toxic pollutants such as PAEs, they are easily adsorbed on suspended sediment (SS) in water bodies; therefore, the distribution dynamics of PAEs in different environmental media are not only related to pollution source discharge and sediment adsorption, but also related to SS concentrations and their transport and transformation characteristics [32,50,51].
The South-to-North Water Diversion Project diverts water from the Danjiangkou Reservoir and transports water to Beijing and Tianjin. The middle route of the South-to-North Water Diversion Project is divided into two phases: in the first phase, the average annual water transfer capacity is 9.50 billion m 3 , and the water transfer target of the second phase is 13.0 billion m 3 . The first phase of the middle route of the South-to-North Water Diversion Project was completed in December 2014 and began to supply water to the north ( Figure 1). After the full operation of the first phase of the middle route of the South-to-North Water Diversion Project, the runoff from the Danjiangkou to Xinglong reaches of the Hanjiang River was reduced by 18.0-25.0%, and the annual average discharge was reduced by about 300 m 3 /s, resulting in the significant reduction in the runoff of the Danjiangkou to Xinglong reaches [24]. Additionally, with the development of cascade power stations in the main stream of the Hanjiang River (including Xinji Dam, Yakou Dam, and Nianpanshan Dam), the flow velocity in the reservoir will decrease after the operation of the reservoir, which will greatly reduce the water environmental capacity of the section from the Danjiangkou to Xinglong reaches of the Hanjiang River [52,53]. Due to the implementation of the Yangtze-Hanjiang Water Diversion Project (Figure 1), the river basins below the Xinglong reservoir area of the Hanjiang River can basically meet the needs of ecological flow [52,53]. Due to the decrease in runoff from the upper reaches and the slowing down of the flow velocity, the water environment in the Danjiangkou to Xinglong reaches of the Hanjiang River is worthy of attention. Meanwhile, the changes of suspended sediment (SS) also need to be explained.
Taking the Huangzhuang Hydrological Station from the Danjiangkou to Xinglong reaches of the Hanjiang River as an example, the runoff and SS concentration decreased from 475 × 10 8 m 3 and 1.00 kg/m 3 in 1950-2010 to 380 × 10 8 m 3 and 0.0520 kg/m 3 in 2018 ( Table 2). The annual average grain size of the SS in the middle and lower reaches of the Yangtze River has tended to decrease [24], which may be due to the fact that the finegrained sediments in the riverbed of the Danjiangkou to Xinglong reaches of the Hanjiang River were washed away. When the riverbed of the Danjiangkou to Xinglong reaches of the Hanjiang River was eroded, the PAEs in the surface sediments of the riverbed would have also been released, which might have led to a gradual decline in the concentration of PAEs in the surface sediments. However, the dynamic characteristics of PAEs in water bodies were not consistent with those of PAEs in sediments. Due to the low runoff, the decrease in SS concentration and thus the decrease in the adsorption capacity of PAEs in sediment, the PAE concentration of the water body in the Danjiangkou to Xinglong reaches of the Hanjiang River may increase in the future. Table 2. Annual average hydrological parameters at the Huangzhuang hydrological stations in the middle-lower Hanjiang River.

Period
Runoff ( With the development of cascade hydropower stations in the middle-lower Hanjiang River, it is difficult to reach an equilibrium in a short period of time in the depositionerosion of the reservoirs, and the coarsening of riverbed sediment caused by dam retention and fine particle scouring will continue to increase in future, especially in the Danjiangkou to Xinglong reaches of the Hanjiang River. Therefore, with the changes of runoff, suspended sediment content and particle size, the effect of the concentration fluctuations of PAEs and other organic toxic pollutants in the middle-lower Hanjiang River will continue to exist and need to be further studied systematically.

Conclusions
Through field survey, monitoring, and comparisons with historical data and risk assessments, the distribution characteristics and trends of PAEs were studied in surface water and sediment in the middle-lower Hanjiang River, and the following main conclusions were reached.
The patterns of different distributions of PAEs varied substantially along the river without clear trends. PAE monomers were lower than their corresponding water quality threshold values (GB 3838-2002). However, DEHP concentrations in the surface water exceeded the value of the human health AWQC set by the USEPA at some sampling sites, revealing potential pollution risks, as was the case in the Laohekou, Qianjiang, and Hanchuan sections. DBP and DEHP concentrations at some sampling sites in the sediment exceeded their ERLs, which were relatively high in the Qianjiang and Wuhan sections. Compared with other regions at home and abroad, the pollution levels of surface water and sediments in the middle-lower Hanjiang River were relatively low. Higher concentrations and more PAE monomers were detected in the dry season than in the wet season in surface water and sediments. With the reduction in runoff from the upper reaches, the slowing down of the flow velocity, the decrease in SS concentration, and the decrease in PAEs' adsorption capacity in sediment, the PAE concentration of the water body in the Danjiangkou to Xinglong reaches of the Hanjiang River may increase in the future.
Given that DEHP and DBP are toxic, bioaccumulative, and detected by high frequency and high concentrations, more studies to investigate their levels in organisms and the corresponding toxicity evaluation in biota are also needed. With the increase in microplastics in the environment on the surface, toxic organic pollutants (such as PAEs) in microplastics will also be released into environmental water bodies with changes in environmental conditions. Therefore, the ways in which different environmental factors affect the release of PAEs in microplastics need to be further studied. In addition, long-range atmospheric transport deposition may also be one of the PAE sources in the middle-lower Hanjiang River, which requires further studies to evaluate the impact of this on PAE sources in the Hanjiang River basin.
Author Contributions: L.D., investigation, methodology, formal analysis, and writing. L.L., reviewing and editing, project administration, and funding acquisition. X.P., investigation and validation. S.Z., data curation. Z.L. and C.M., investigation and data analysis. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.
Appendix A Table A1. Geographical information and water quality parameters at the sampling sites from the middle-lower Hanjiang River.

Station
Sampling       Figure A1. Concentration distribution trend of the water quality index (WQI) in the wet season fro the middle-lower Hanjiang River.