Occurrence of organochlorine pesticides from typical water sources in YiXing City, Taihu Upper-River Basin, East China

Abstract

This study is the first report to describe the occurrence of 18 organochlorine pesticides (OCPs) in the three typical water sources in YiXing City, Taihu Upper-River Basin, East China. The fates of the target OCPs in the Jiubin drinking water treatment plant (JTP) were also analysed. The amount of ∑OCPs in the Hengshan (HS), Youche (YC) and Xijiu (XJ) water sources were relatively moderate, with mean concentrations of 535.6, 426.5 and 537.8 ng L⁻¹, respectively. Hexachlorocyclohexanes (HCHs) dominated the total concentration of the OCPs making up a marked portion of the 18 OCPs. The highest levels of ∑OCPs found in the three water sources during the study period were during the dry season. The occurrence and concentrations of these compounds were spatially dependent, and the mean concentrations of ∑OCPs in HS, YC and XJ increased from the surface layer to the bottom layer with various percentage increases. In addition, the removal efficiencies of the ∑OCPs from the sedimentation, filtration and disinfection in the JTP are limited. Beta-hexachlorocyclohexane (β-HCH) and heptachlor posed a cancer risk for children and adults in YiXing City, while the potential ecosystem risk of the other OCPs was relatively low in the three water sources. However, the risks posed by the OCPs due to ingestion of drinking water may still exist; therefore, special attention should be paid to source control into the JTP and advanced treatment processes for drinking water supplies should be implemented.

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1. Introduction

Organochlorine pesticides (OCPs) are a type of synthetic chemical pesticide composed primarily of carbon, hydrogen, and chlorine, and they are considered to be one of the most toxic and persistent organic pollutant (POP) groups in the environment.^1,2 According to the Stockholm Convention on POPs, until 2010, 14 OCPs were among the 21 POPs in annexes to the Stockholm Convention. More recently, endosulfan was classified as a new POP and was added to the Stockholm Convention in May 2011. OCPs have attracted wide concern for decades because of their persistence, biomagnification, and high toxicity to nontarget organisms.^3–5 Due to the usage and production of these anthropogenic pollutants, previous studies have suggested that some OCPs may affect the normal function of the endocrine and reproductive systems of humans and wildlife.^6–9 Because of their wide distribution and high persistence in the environment, they can still be detected in different environmental media from various regions.^10,11

OCPs can enter the aquatic environment through different pathways including effluent discharge, agricultural runoff, atmospheric deposition, and air/water exchange.^12,13 Although these OCPs have been restricted in several countries, including China, since the 1980s, from the 1950s to 1983, approximately 400 000 tons of DDTs and 4 900 000 tons of hexachlorocyclohexanes (HCHs) were produced in China, which accounted for 20% and 33% of the world's total production, respectively.¹⁴ Despite the ban on technical DDT and technical HCH in 1983, dicofol and lindane have continued to be widely used in agricultural practices in China.^15,16 After 1983, approximately 11 400 tons of lindane were still reportedly being produced and DDTs have been continuously produced for approximately 20 years because of export demands, malaria control and dicofol production.^16,17 Until recently, lindane was still being produced and used in China. Extensive and continuous use of OCPs in China have resulted in ubiquitous OCP pollution in various environmental media, especially in surface water. There is substantial information regarding OCP residues in the surface waters of China.¹⁸ However, official data regarding the presence of these pollutants in the aquatic environment for some cities are not available.

YiXing is located upstream of the Taihu Lake in southern Jiangsu, China. YiXing is the famous pottery capital of China and the origin of purple clay teapots. YiXing is one of the most powerful county-level cities in China and has been rated by Forbes as the best county-level city in mainland China. Rivers converge in YiXing and account for approximately half of the entire water and pollution load into Taihu Lake. Therefore, the control of water pollution in YiXing is of great importance for the environmental protection of Taihu Lake. The Hengshan Reservoir (HS) and Youche Reservoir (YC) have become the most important drinking water resources for YiXing City in recent years. Xijiu Reservoir (XJ) was the original water source, but it is now a backup water source due to serious water pollution. The Jiubin drinking water treatment plant (JTP), with a daily production of 300 000 m³ of drinking water, depends completely on water supplied from HS and YC. Given the potential risk of these compounds to human health, OCPs in different media, particularly drinking water, have attracted much attention in recent years. Therefore, collecting more information on OCP contamination from typical water sources in YiXing City is particularly important.

The objectives of the present study were (1) to determine the concentrations and compositions of the 18 OCPs in the water sources; (2) to ascertain the temporal and spatial variation of OCPs in surface water; (3) to examine the removal efficiencies of the OCPs via the traditional JTP drinking water treatment steps; and (4) to evaluate the potential risk of OCPs on human health. Therefore, OCPs from the three typical water sources were investigated to provide data regarding water contamination in YiXing City.

2. Materials and methods

2.1 Description of sampling area and sample collection

The study region and the details of the sampling sites are presented in Fig. 1 and Table 1, respectively. Generally, a total of 24 sampling points were selected from the three water sources and the surface water samples were collected monthly from July 2014 to June 2015. Among these sites, a total of 6 sampling sites were defined in the HS water source, a total of 5 sampling sites in YC, a total of 7 sampling sites in XJ and a total of 6 sites were located on the tributaries of XJ. For the sampling points denoted S4 of HS, S11 of YC and S12 of XJ, samples were taken from six layers: surface (0.5 m below the water surface), 0.2H, 0.4H, 0.6H, 0.8H and bottom (1.0 m above the bed) which the H represents the water depth during the sampling and corresponded to values of 14 m, 13.5 m and 5.5 m for HS, YC and XJ, respectively. That is, the appropriate depth in HS is 0.5 m, 3.1 m, 5.7 m, 8.3 m, 10.9 m, 13 m, and in YC is 0.5 m, 2.6 m, 4.4 m, 6.6 m, 8.8 m, 11 m, and the XJ is 0.5 m, 1.3 m, 2.1 m, 2.9 m, 3.7 m and 4.5 m. In order to avoid the water mixed between different layers, a specific depth water sampler was used. Moreover, four sets of water samples were collected from the JTP at the raw, sediment, filtration and finished water sampling points twice each month. At each sampling point, duplicate water samples were collected and kept in 2.0 L polyethylene bottles. All of the samples were stored at 4 °C within 2 days of collection prior to extraction. All of the containers that were in contact with the samples were sequentially washed with deionized water and river water prior to sampling to eliminate disturbances due to other organic materials.

Fig. 1 Map of the sampling sites among the three water sources of YiXing City, Taihu Upper-River Basin, East China.

Table 1 Detailed description of the sampling locations

No.	Sampling site	Latitude	Longitude
1	Hengshan Reservoir	E119°32′24′′	N31°13′33′′
2	Hengshan Reservoir	E119°33′05′′	N31°13′37′′
3	Hengshan Reservoir	E119°34′15′′	N31°13′18′′
4	Hengshan Reservoir	E119°33′55′′	N31°13′46′′
5	Hengshan Reservoir	E119°33′44′′	N31°14′18′′
6	Hengshan Reservoir	E119°34′27′′	N31°14′14′′
7	Youche Reservoir	E119°45′19′′	N31°13′10′′
8	Youche Reservoir	E119°44′23′′	N31°12′38′′
9	Youche Reservoir	E119°45′00′′	N31°12′37′′
10	Youche Reservoir	E119°45′33′′	N31°12′47′′
11	Youche Reservoir	E119°45′08′′	N31°12′55′′
12	Xijiu Reservoir	E119°46′59′′	N31°22′36′′
13	Xijiu Reservoir	E119°44′31′′	N31°23′31′′
14	Xijiu Reservoir	E119°43′59′′	N31°23′18′′
15	Xijiu Reservoir	E119°43′08′′	N31°23′20′′
16	Xijiu Reservoir	E119°42′33′′	N31°23′25′′
17	Xijiu Reservoir	E119°43′47′′	N31°23′48′′
18	Xijiu Reservoir	E119°43′37′′	N31°24′10′′
19	Guijing Bridge	E119°42′07′′	N31°22′46′′
20	Zhongzhang Bridge	E119°40′26′′	N31°23′02′′
21	Wending Bridge	E119°37′58′′	N31°23′36′′
22	Yifeng Bridge	E119°42′05′′	N31°24′34′′
23	Niujia Bridge	E119°42′06′′	N31°26′12′′
24	Wuqing Bridge	E119°40′47′′	N31°26′44′′

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2.2 Chemicals

A mixture of 18 standard OCPs, including α-HCH, β-HCH, lindane (γ-HCH), delta-hexachlorocyclohexane (δ-HCH), heptachlor (Hep), aldrin, heptachlor epoxide (Hep-ep), γ-chlordane, endosulfan, α-chlordane, dieldrin, endosulfan II (ENDOII), endrin aldehyde (End-alde), endosulfan-sulphate (end-sul), endrin ketone, p,p′-DDE (DDE), p,p′-DDD (DDD), and p,p′-DDT (DDT) at 500 μg mL⁻¹ each as well as the internal standards of pentachloronitrobenzene and a surrogate of 4,4′-dichlorobiphenyl at 100 μg mL⁻¹ each, were all purchased from Aldrich Co. (Aldrich Co., USA). All of the solvents (acetone, hexane, and dichloromethane) that were used were HPLC-grade and were purchased from J. T. Baker Co. (USA). Anhydrous sodium sulphate (Tianjin Chengguang Chemical Reagent Co., China) was cleaned at 600 °C for 6 h and then kept in a desiccator before use.

2.3 Sample pretreatment and chemical analysis

Water samples were filtered under a vacuum through glass fibre filters (0.7 μm pore sizes). Prior to extraction, each sample was spiked with 50 ng L⁻¹ surrogate standard. The water samples were extracted based on the classic liquid phase extraction method (USEPA, method 8061) with slight modifications. Briefly, the 1 L water sample was placed in a 2 L separatory funnel and spiked with 50 ng L⁻¹ surrogate. Then, the sample was extracted three times with 120 mL of DCM and then was filtered over sodium sulphate (approximately 20 g), and the organic extracts were concentrated using a rotary evaporator. Finally, the extracts were reduced to 0.5 mL under a gentle nitrogen flow. The internal standard was added to the sample prior to instrumental analysis.

Quantification of the OCPs was performed on an Agilent 6890N GC and an Agilent 5975 mass spectrometer detector (GC-MSD), which operated in the electron impact mode (70 eV). Separation was carried out using a 30 m × 0.25 mm × 0.25 μm HP-5 MS capillary column (Agilent Co., USA). The instrumental conditions were as follows: injector temperature, 280 °C; and ion source temperature, 230 °C. The temperature program was as follows: the column started initially at 100 °C and was held for 3 min, increased to 160 °C at a rate of 20 °C min⁻¹, and then increased to 250 °C at a rate of 5 °C min⁻¹ and was held for 5 min. The carrier gas was helium at a constant flow rate of 1.0 mL min⁻¹. The sample (1 μL) was injected in splitless mode. The mass range, m/z, of 50–500 was used for quantitative determinations. Data acquisition and processing were controlled by HP ChemStation software. Chromatographic peaks of the samples were identified by mass spectra and by comparison with the standards.

2.4 Quality assurance and quality control

For each batch of 10 field samples, a procedural blank, a spiked blank, a spiked matrix sample, and a sample duplicate were processed. The instruments were calibrated daily with calibration standards. In addition, the mean surrogate recoveries in the water samples were 76.7 ± 10.5%. The OCP recovery studies were performed using the spiked sample to demonstrate the efficiency of the method. Recoveries of the 18 OCPs ranged from 64.8 to 120.6% in the spiked water. The method detection limits (MDLs) of OCPs were described as 3 : 1 signal versus noise value (S/N), that is the blanks and quantified as the mean field blanks plus three times the standard deviation (3σ) of the field blanks. And the results of (MDLs) of OCPs was shown in the ESI.† All of the results were corrected for the blanks and recoveries.

3. Results and discussion

3.1 OCP levels in the water resources

Residues from HCHs (α-, β-, γ- and δ-HCH), DDTs (DDE, DDD and DDT), heptachlor, heptachlor epoxide, endosulfan-sulfate and another eight organochlorine pesticides were detected in surface water from typical water sources in YiXing City, Taihu Upper-River Basin, East China. Table 2 and Fig. 2 show the results of the water sample analyses. In the investigation from July 2014 to June 2015, the average value of ∑OCPs in the 24 water samples was 513.6 ± 1.38 ng L⁻¹. The ∑₁₈OCPs concentrations at HS ranged from 366.3 to 735.6 ng L⁻¹, with a geometric mean value of 535.6 ng L⁻¹; the concentrations at YC ranged from 303.4 to 516.7 ng L⁻¹, with a geometric mean value of 426.5 ng L⁻¹; and the concentrations at XJ ranged from 337.1 to 753.5 ng L⁻¹, with a geometric mean value of 537.8 ng L⁻¹. The OCP compounds that occurred frequently in the surface water samples in YiXing City were HCHs, heptachlor, heptachlor epoxide, endosulfan-sulphate, and DDTs, with average concentrations of 388.6, 20.3, 56.1, 10.4 and 33.4 ng L⁻¹, respectively. As shown in Fig. 2, HCHs were abundant in the water samples with contributions ranging from 70.9 to 85.4% for HS, 71.1 to 81.9% for YC and 66.9 to 82.3% for XJ. In the three water sources, β-HCH was the dominant HCH, with a mean value of 290.8 ng L⁻¹, followed by γ-HCH and δ-HCH. The next dominant OCP was heptachlor, which ranged from 3.4 to 16.1%, followed by endosulfan-sulphate and heptachlor epoxide, which ranged from 3.8 to 11.1% and 0.1 to 4.6%, respectively. DDTs in the surface water samples in YiXing City had a high detection frequency with relatively low concentrations that ranged from 10.0 to 30.7 ng L⁻¹. The low levels of DDTs found in the water samples may be caused by the large octanol/water partition coefficient for these chemicals (log K_ow ∼ 6.5).¹⁹ The concentrations and detectable frequencies of aldrin, γ-chlordane, endosulfan, α-chlordane, dieldrin, endosulfan II, endrin aldehyde, and endrin ketone were substantially low, illustrating their low pollution levels in the three water sources in YiXing City. The mean concentration values of ∑HCHs, ∑DDTs, ∑heptachlor and endosulfan-sulfate in the HS water source were 422.4, 19.8, 61.4 and 28.3 ng L⁻¹, respectively. For YC and XJ, the mean concentration values were 332.6, 13.8, 52.9, and 26.7 and 470.3, 24.5, 101.8, and 50.1 ng L⁻¹, respectively. In addition, lower levels of ∑HCHs, ∑DDTs, ∑heptachlor and endosulfan-sulphate were found in the tributaries of XJ, with mean values of 241.3, 23.4, 41.9 and 28.2 ng L⁻¹, respectively. Although agricultural use of most OCPs has been banned in China for more than 30 years, the OCP residues in YiXing City are very still serious, indicating the persistence of these OCPs in this area.

Table 2 Concentration of OCPs at different sampling sites in three typical water sources in YiXing City, East China

OCPs	HS						YC					XJ
	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	S13	S14	S15	S16	S17	S18	S19	S20	S21	S22	S23	S24
α-HCH	5.2	5.2	5.4	11.1	8.5	5.3	10.0	6.8	5.3	7.1	5.4	14.5	9.6	10.5	9.8	9.6	17.6	10.3	9.7	9.9	5.3	5.6	5.8	7.6
γ-HCH	24.0	21.9	26.7	89.9	38.0	70.5	64.2	59.1	59.1	60.4	31.9	50.2	57.8	52.4	45.5	52.3	61.5	48.0	39.0	26.0	31.3	25.4	33.1	24.9
β-HCH	226.4	200.0	362.4	432.0	381.0	343.2	212.7	308.8	194.7	288.7	166.0	277.1	347.3	360.0	361.8	253.5	365.8	449.7	353.3	215.7	181.9	241.4	226.6	229.0
δ-HCH	23.9	22.5	39.9	95.1	51.9	44.5	36.3	40.5	33.7	42.2	30.0	65.5	50.1	41.4	74.9	66.8	71.9	56.7	16.5	30.3	20.5	16.5	25.5	57.1
Hep	31.2	44.3	68.6	25.5	55.4	66.5	73.0	59.9	38.0	40.6	37.7	73.0	83.8	89.7	77.5	80.5	85.2	104.4	31.5	42.7	39.9	12.9	38.5	46.4
Aldrin	nd	nd	0.2	1.6	2.6	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	0.9	0.9	0.9	nd
Hep-ep	11.7	9.3	15.2	21.7	6.9	11.9	9.4	1.1	2.1	2.1	0.9	0.8	18.0	15.6	26.9	25.0	20.9	10.9	6.6	6.2	10.9	2.3	5.2	8.1
γ-Chlordane	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd
Endosulfan	0.6	nd	nd	11.1	nd	nd	nd	nd	nd	1.1	0.9	2.1	nd	nd	nd	nd	nd	nd	0.9	1.1	0.9	2.3	2.4	nd
α-Chlordane	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd
Dieldrin	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	6.8	1.1	2.5	4.1	nd	nd	nd	nd	nd	nd	2.1	1.6
Endosulfan II	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd
Endrin aldehyde	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd	nd
end-sul	21.2	27.4	23.4	28.4	40.6	28.4	30.8	29.6	20.5	31.7	20.8	33.0	46.8	54.9	49.6	47.3	52.5	46.3	21.3	22.2	29.9	30.5	45.9	19.5
Endrin ketone	0.9	1.1	nd	nd	nd	nd	nd	nd	2.9	0.6	0.2	nd	nd	nd	nd	nd	6.1	5.9	1.8	nd	nd	nd	nd	3.3
p,p′-DDE	7.8	7.1	3.8	5.3	8.5	7.4	6.1	4.1	5.1	3.9	3.4	5.1	9.2	9.8	10.8	8.8	6.9	7.2	8.7	8.9	5.8	7.1	8.3	9.1
p,p′-DDD	3.4	3.1	2.9	3.1	4.5	4.2	3.9	1.9	3.1	2.9	1.8	2.1	4.6	6.3	5.9	4.4	2.5	4.3	5.9	5.1	3.2	3.6	5.8	6.1
p,p′-DDT	10.2	9.9	4.8	6.7	12.6	11.2	8.3	4.9	6.1	5.4	4.6	6.8	13.1	14.6	12.5	10.5	8.5	9.7	12.9	11.5	6.9	8.9	11.8	12.1

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Fig. 2 Relative contributions of the 18 OCPs congeners in the three water sources.

Little data regarding the levels of these 18 OCPs in the study area have been published. Therefore, to understand the status of OCPs, only levels of HCHs and DDTs in water sources from YiXing City were compared to those reported for other rivers or lakes in the world, as shown in Table 3. The total concentrations of HCHs and DDTs in the present study were lower than in the concentrations in other water sources around the world, e.g., Tonghui River (70.12–992.6 ng L⁻¹ and 18.79–663.3 ng L⁻¹), Qiantang River (0.7–543.1 ng L⁻¹ and nd–86.39 ng L⁻¹), Taihu Lake in China (<10–36 100 ng L⁻¹ and <100–9300 ng L⁻¹) and Gomti River in India (0.02–4846 ng L⁻¹ and 0.20–4578 ng L⁻¹), while the concentrations were higher than reported in others, e.g., Wuhan reach, Yangtze River (0.55–28.07 ng L⁻¹ and nd–16.71 ng L⁻¹), Pearl River in China (0.21–3.12 ng L⁻¹ and 0.23–3.28 ng L⁻¹) and Ebro River in Spain (3.38 ng L⁻¹ and 3.4 ng L⁻¹). Compared to other regions worldwide, the levels of HCHs in the water sources in YiXing City were comparable to those in the Northern rivers in Russia (1–176 ng L⁻¹) and the Kucuk Menderes River in Turkey (187–337 ng L⁻¹), but were significantly higher than the Atoya River in Nicaragua (nd–19.0 ng L⁻¹). In addition, the levels of DDTs in this area were lower than those in the Peacock River in China (nd–176 ng L⁻¹) and the Nestos River in Greece (nd–64 ng L⁻¹). As a whole, the concentrations of OCPs in the surface water in YiXing City were moderate compared to those reported for other rivers around the world. This indicates that the residue of these OCPs have significantly decreased in the environment because of the usage ban for HCHs and DDTs in China for the last 30 years.

Table 3 Total concentrations of OCPs in surface waters around the world (μg L⁻¹)

Locations	Sampling time	∑HCHs	∑DDTs	Reference
Tonghui River, China	2002	70.12–992.6	18.79–663.3	21
Qiantang River, China	2005–2006	0.7–543.1	nd–86.39	22
Wuhan reach, Yangtze River	2005	0.55–28.07	nd–16.71	23
Taihu Lake, China	1999–2001	<10–36 100	<100–9300	24
Pearl River, China	2002–2003	0.21–3.12	0.23–3.28	25
Peacock River, China	2006	nd–19	nd–176	19
Northern Rivers, Russian	1990–1996	1–176	nd–38	26
Atoya River, Nicaragua	1993	nd–19.0	nd–73.2	27
Gomti River, India	1996–1999	0.02–4846	0.20–4578	28
Nestos River, Greece	1996–1998	nd–68	nd–64	29
Ebro River, Spain	1995–1996	3.38	3.4	30
Kucuk Menderes River, Turkey	2002–2003	187–337	72–120	31
Hengshan Reservoir	2014–2015	41.1–432.9	nd–24.9	This study
Youche Reservoir	2014–2015	36.7–358.9	nd–20.1	This study
Xijiu Reservoir	2014–2015	75.3–534.9	nd–32.1	This study

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3.2 Temporal and spatial variations of the OCPs

The 18 OCPs in the surface water in YiXing City were studied, and the concentrations of ten compounds, including HCHs, DDTs, heptachlor, heptachlor epoxide, and endosulfan-sulphate, are shown in Fig. 3. The other eight target compounds concentrations of aldrin, γ-chlordane, endosulfan, α-chlordane, dieldrin, endosulfan II, endrin aldehyde, and endrin ketone were substantially low and ranged from below the detection limit to 6.8 ng L⁻¹, as shown in Table 2. The weather in YiXing City is humid and abundant in rainfall throughout the year. The water levels of the water sources in YiXing City were higher during the autumn rain season and were lowest during the spring dry season. The OCP levels of the three water sources showed a similar variable tendency in the time dimension, and there was a marked variation of OCP concentrations during the study period. Generally, the concentrations at most sampling sites were highest during the dry season (autumn and winter). The lowest values always appeared during the flood season (spring and summer). During the flood season, the average precipitation was 1246.3 mm and the dam water level prompted to 34.9 m, while during the dry season the average precipitation was 335.3 mm and the dam water level dropped to 30.5 m. OCPs are lipophilic, have higher volatility and water solubility, could be transferred via surface runoff, and, due to their persistence, the rainfall in this area has a direct effect on the OCPs concentration variation.²⁰

Fig. 3 Temporal variations in the OCPs for the three water sources from July 2014 to June 2015.

Fig. 3 illustrated the fluctuations of OCPs in the three water sources over time. HCHs and heptachlor had a significant fluctuation during the study period in the HS water source, especially β-HCH, with an average concentration of 292.5 ng L⁻¹ during the dry season and 101.5 ng L⁻¹ during the flood season, followed by γ-HCH, with a mean concentration of 122.7 ng L⁻¹ during the dry season and 29.5 ng L⁻¹ during the flood season. The α- and δ-HCH had no significant fluctuations during the summer and winter, while the concentration decreased rapidly in autumn to three to five times lower than in spring. The variation in the heptachlor concentration ranged from 77.6 ng L⁻¹ in the spring to 40.1 ng L⁻¹ in the winter. For DDTs, heptachlor epoxide, and endosulfan-sulphate, the concentrations had no apparent variation during the study period. The OCPs in the YC water source had a similar temporal variation to the HS water source. The mean concentration of ∑HCHs ranged from 437.9 ng L⁻¹ in the spring to 195.5 ng L⁻¹ in the winter. The average concentration of β-HCH was 312.7 ng L⁻¹ during the dry season and 150.5 ng L⁻¹ during the flood season and reached the lowest concentration of 119.2 ng L⁻¹ in the winter. The average concentration of γ-HCH was 84.2 ng L⁻¹ during the dry season and 40.7 ng L⁻¹ during the flood season. There were no obvious fluctuations in the α-HCH, δ-HCH, DDTs and heptachlor epoxide concentrations, while the heptachlor and endosulfan-sulphate concentrations fluctuated. For the XJ water source, the OCP concentrations obviously changed, except for those of α-HCH and DDTs. The average concentration of β-HCH was 401.5 ng L⁻¹ during the dry season and 263.1 ng L⁻¹ during the flood season. The average concentration of γ-HCH ranged from 122.7 ng L⁻¹ to 29.5 ng L⁻¹ from spring to winter. The concentrations of δ-HCH, heptachlor, endosulfan-sulphate and heptachlor epoxide varied slightly. Generally, the highest concentration of OCPs was in the XJ water source followed by the HS and YC water sources.

The spatial variations of the annual concentrations of individual OCPs in the three water sources in YiXing City are shown in Fig. 4. The ten compounds (HCHs, DDTs, heptachlor, heptachlor epoxide, and endosulfan-sulphate) were the main OCP contaminants in the three water sources. The present study focused on the spatial variations of these ten OCP contaminants as individual OCP studies. The average concentrations of HCHs, heptachlor, and endosulfan-sulphate, which are the major component of OCPs, especially β-HCH, indicated serious pollution in the YiXing area. Heptachlor epoxide and DDTs had comparatively high concentrations with mean concentrations of 10.4 and 20.2 ng L⁻¹, respectively, among all of the sampling sites. The other eight OCPs were the minor components among the OCPs with low detection frequencies. Moreover, β-HCH was high in XJ reaching 490.3 ng L⁻¹ and low in YC. Rainfall and pollution source distributions may be the main causes for the spatial variance. HS and YC are located far from urban areas and are mainly surrounded by farmland. The drug protocol for crops could cause some OCP pollution. The regions that were studied in XJ included important river transportation channels, so the high OCP concentrations in XJ might be due to the re-emission of these substances from suspended sediments because of turbulence in the river channel and wastewater from human activities. High bioactivities of aquatic organisms from this region could increase the re-suspension of OCPs that have accumulated in the sediment, which could accelerate the release of previously adsorbed pollutants from suspended particles into water. In general, the results showed that the spatial distributions of OCPs in the three water sources were site-specific.

Fig. 4 Spatial variations in the annual individual OCP concentrations in the three water sources.

3.3 Vertical distribution of the OCPs in the water resources

The vertical distributions of the mean concentrations of the 18 total OCPs (∑OCPs), ∑HCHs and ∑DDTs in the three water sources are shown in Fig. 5. For the ∑OCPs in HS (Fig. 5a), the mean concentrations increased from 435.6 to 536.8 ng L⁻¹ from the surface layer to the bottom layer with an arc change trend. The ∑HCHs concentration increased from 203.1 to 299.9 ng L⁻¹ from the surface to the bottom layer. In addition, the mean ∑OCPs and ∑HCHs concentrations increased by 47.6% and 27.1%, respectively, from the bottom layer to the surface layer. The ∑DDTs concentration increased from 19.1 to 25.9 ng L⁻¹ from the surface to the bottom layer. For the OCPs at YC (Fig. 5b), the mean ∑OCPs concentration increased from the surface layer to the bottom layer with fluctuating changes in the mid layers ranging from 426.6 to 489.8 ng L⁻¹, respectively, and decreased at the 0.2H and 0.6H layers. The mean ∑HCHs and ∑DDTs concentrations exhibited a similar vertical distribution that was steady from the surface layer to the bottom layer. For the OCPs in the XJ (Fig. 5c), the mean ∑OCPs and ∑HCHs concentrations exhibited a similar trend, decreasing from the surface layer to the 0.4H layer, but increasing from the 0.4H layer to the bottom layer. In addition, the mean ∑OCPs and ∑HCHs concentrations increased from 537.6 to 556.4 ng L⁻¹ and from 394.7 to 400.3 ng L⁻¹, respectively, from the surface layer to the bottom layer. The mean ∑OCPs and ∑HCHs concentrations increased by 1.5% and 3.6% from the bottom layer to the surface layer. The mean ∑DDTs concentration increased from the surface layer to the bottom layer, ranging from 23.6 to 30.8 ng L⁻¹, respectively, and decreased at the 0.8H layers. The observation that the ∑OCPs, ∑HCHs and ∑DDTs concentrations were higher in the bottom layers than in the surface layers may be due to the re-emission of these substances from suspension and sediment into the water. OCPs can transport and spread among the different environmental media to form the cross contamination and suspended particulate matter (SPM) is an important medium for OCPs on the transportation in aquatic environment. OCPs may be removed from the water to adsorb on the SPM due to their high affinity for organic matter, and finally accumulated in sediment. On the other hand, OCPs in sediment can transport back to water via re-suspension, in addition, the transformation of OCPs was also influenced by aquatic environment.^32,33 And the potential factors such as soil property may influence the distribution patterns and fate of HCHs and DDTs on a large spatial scale.³⁴ In general, the mean ∑OCPs, ∑HCHs and ∑DDTs concentrations were lower in the surface layer and increased at the bottom layers, but fluctuated in the middle layers for the three water sources.

Fig. 5 Vertical distributions of ∑OCPs, ∑HCHs and ∑DDTs in (a) HS, (b) YC and (c) XJ.

Fig. 6 The composition of (a) OCPs, (b) HCHs, and (c) DDTs in the three water sources in YiXing City.

3.4 OCPs profiles and sources

The composition differences of the HCH isomers or DDT congeners in the environment could reveal different pollution sources.^22,35 The relative composition characteristics of OCPs in the water from YiXing City are indicated in Fig. 6. Fig. 6a shows that HCHs, DDTs, heptachlor, heptachlor epoxide, and endosulfan-sulfate were the dominant OCPs in the water sources from YiXing City, and they accounted for 76.6, 5.5, 8.4, 2.5, and 6.9% in HS, 81.8, 4.9, 6.7, 0.6, and 6.0% in YC, and 76.8, 6.1, 7.9, 2.1 and 7.2% in XJ, respectively.

The direct application of technical HCHs may introduce HCHs into the environment, and it is well known that technical HCH primarily contains α-HCH (60–70%), β-HCH (5–12%), γ-HCH (10–15%), δ-HCH (6–10%) and other minor isomers, while linden consists of 99% γ-HCH.³⁶ These HCH isomers have different physicochemical properties. α-HCH and γ-HCH can easily break away from sediment, while β-HCH with a low-water solubility and vapour pressure is the most stable and is relatively resistant to microbial degradation and α-HCH and γ-HCH could be converted to β-HCH in the environment; therefore, β-HCH would be predominant in the environment if there are no fresh inputs of technical HCH.^20,37 The compositions of the HCH isomers in this study are plotted in Fig. 6b. The average percentage of HCH isomers measured in HS were 1.6% α-HCH, 10.7% γ-HCH, 76.7% β-HCH, and 11.0% δ-HCH; in YC were 2.1% α-HCH, 16.5% γ-HCH, 70.4% β-HCH, and 11.0% δ-HCH; and in XJ were 2.5% α-HCH, 10.7% γ-HCH, 75.4% β-HCH, 11.5% δ-HCH. As seen in Fig. 6b, β-HCH was the predominant HCH isomer and the ratio of α-HCH/γ-HCH varied from 0.13 to 0.23 (mean 0.17) in the three water sources. The α-HCH/γ-HCH ratio is 3 : 7 in technical HCH, but tends to increase over time due to the faster degradation of γ-HCH compared to α-HCH. However, continued usage of lindane would lead to a decline in the α-HCH/γ-HCH ratio; therefore, when the ratio of α-HCH/γ-HCH is lower than 1, there was a recent input of lindane.³⁸ According to the different proportion of α-HCH and γ-HCH, these observations indicated that the sources of the HCHs were mainly from the historical use of HCHs and there was a new input of lindane in the three water sources.

Technical DDT and dicofol containing high DDT-related compound impurities were the main sources of DDT pollution in China, and technical DDT is typically composed of 77.1% p,p′-DDT, 14.9% o,p′-DDT, 4% p,p′-DDE and some other trace impurities.³⁹ DDTs could be biodegraded into DDE under aerobic conditions and to DDD under anaerobic conditions; therefore, if there was no new technical DDT input, the compositional percentage of p,p′-DDT would decrease and the DDE + DDD metabolites would increase. The ratio of p,p′-(DDE + DDD)/DDT is useful to trace the decomposition and to identify a new input of DDT.²⁰ The compositions of DDTs in the three water sources are plotted in Fig. 6c. As shown in Fig. 6c, the average compositions of the detected DDT congeners followed the order of p,p′-DDT > p,p′-DDE > p,p′-DDD, with p,p′-DDT dominating the concentration, and the ratios of (DDE + DDD)/DDT were more than 1.0 in the three water sources. The high ratio of (DDE + DDD)/DDT suggested that a historical application of technical DDT is the main source in the three water sources. These finding indicates that much of the degradation occurred after the official ban of DDT in 1983 and that the DDT compounds in YiXing may be mainly derived from DDT-treated aged and weathered agricultural sources.

3.5 OCPs levels in the drinking water treatment plant

The operation of the JTP includes coagulation, sedimentation and filtration treatment processes, which are typical treatments for drinking water. The JTP draws water from HS and YC and was investigated to assess the fate of OCPs during the drinking water treatment process. Five groups are taken from over twenty sets of results during the study periods from the storage tank of the JTP and the measured concentrations from the raw, sedimentation, filtration and final water samples taken from the JTP are shown in Fig. 7a. As shown in Fig. 7a, the removal efficiencies of the ∑OCPs from the sedimentation, filtration and disinfection in the JTP are limited. According to the results, the removal rate of ∑OCPs is less than 10% after flocculation–sedimentation, filtration has an 8% removal rate for ∑OCPs and the disinfection process plays a limited role in the removal of the ∑OCPs. As shown in Fig. 7b, the removal rates of α-HCH, γ-HCH, β-HCH and δ-HCH were 14.5, 12.5, 13.5 and 14.2%, respectively, by the conventional process in the JTP. The most important compound detected in the finished water was β-HCH, with a mean concentration of 200.8 ng L⁻¹, suggesting that β-HCH makes up the highest relative composition of total OCP concentrations in the drinking water. In addition, the standard in China concerning analytical controls on drinking waters specially identify OCPs such as HCHs, DDT and lindane as organic pollutant indexes by the new drinking water standard in 2007 (Standard for drinking water quality; GB5749-2006), which has set the guideline for HCHs, DDT and lindane in drinking water was 0.005, 0.001 and 0.002 mg L⁻¹. The DDTs that were detected at extremely low levels in the drinking water had a very limited removal rate of approximately 4%. The mean concentrations of heptachlor, heptachlor epoxide, and endosulfan-sulphate in the finished water were 22.5, 14.3 and 21.2 ng L⁻¹, respectively, with removals of 31.8, 29.9 and 23.5%, respectively.

Fig. 7 OCPs (a) removed and (b) detected in the water samples from the JTP.

Because the bulk organic parameter in water generally reflects the content of many organic pollutants, the calculated removal values of the parameters were measured. As shown in Table 4, the removal of TOC, UV₂₅₄, and COD_Mn were 34.7, 56.6, and 54.5%, respectively, for the raw HS water source and 25.6, 45.7, and 57.1%, respectively, for the raw YC water source. However, the removal efficiency of the total OCPs was lower than the efficiency of the bulk organic parameters, indicating that traditional drinking water treatment does not work well to eliminate these micro-pollutants, regardless of the water source. Traditional drinking water treatment focuses on the particles and colloids in terms of physical processes. Previous studies have shown that oxidation is the principal mechanisms for removing OCPs from aquatic systems.⁴⁰ Therefore, the treatment process should include the key techniques to remove OCPs from water, and further research on advanced drinking water treatments for OCP removal is required.

Table 4 Water quality of the raw and drinking water samples from the JTP drinking water treatment plant

Parameter	HS	YC	XJ	Drinking water
pH	7.46 ± 0.3	7.36 ± 0.3	7.50 ± 0.3	7.2 ± 0.2
Turbidity (NTU)	3.9 ± 0.5	5.6 ± 1.0	39 ± 4.0	0.2 ± 0.02
Total organic carbon (mg L^−1)	2.3 ± 0.5	2.0 ± 0.5	7.5 ± 0.5	1.5 ± 0.5
UV absorbance at 254 nm (1 cm^−1)	0.057 ± 0.018	0.046 ± 0.015	0.107 ± 0.018	0.025 ± 0.005
CODMn	3.35 ± 0.5	3.50 ± 0.5	5.04 ± 0.5	1.50 ± 0.22

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3.6 Toxicological risk of OCPs

The results from this study show that the concentrations of OCPs in the three water sources were below the environmental quality standard for the aqueous phase (GB3838-2002), which was established by the State Environment Protection Agency of China (SEPA) (<1000 ng L⁻¹ for lindane and <2000 ng L⁻¹ for ∑DDT). According to the guideline recommended by SEPA, the criteria maximum concentration (CMC) of γ-HCH in the aqueous phase should be less than 950 ng L⁻¹ and the concentration of γ-HCH in all of the samples in YiXing City was within the required range of the guideline, but was slightly higher than the guideline of 20 ng L⁻¹ for HCHs that was established by the European Union. In addition, the concentration of DDTs in the three water sources was within the required range of the quality standard (25 ng L⁻¹) that was applicable to surface water established by the European Union. The results indicate that HCHs in YiXing City might have an ecological risk and that DDT residues are less of an ecological hazard.

The health risk assessment is the process to estimate the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future. Because ten out of the 18 OCPs made up most of the OCP contamination and the other eight OCPs were below the limit of detection in the water sources in YiXing City, the health risk assessment focused on the ten OCP contaminants, including α-HCH, γ-HCH, β-HCH, δ-HCH, p,p′-DDT, p,p′-DDE, p,p′-DDD, heptachlor, heptachlor epoxide and endosulfan-sulphate.

The chronic daily intake (CDI) is used to estimate human exposure to contaminants and is calculated using the following formula: , where C = chemical concentration in water (mg L⁻¹); IR = water ingestion rate (l day⁻¹) (for children: IR = 1.0; for adults: IR = 2.0); EF = the exposure frequency (350 days per year); ED = exposure duration (year) (for children: ED = 6; for adults: ED = 70); BW = bodyweight (kg) (for children: BW = 14; for adults: BW = 60); and AT = average lifespan (days) (for children: AT = 2190; for adults: AT = 25 550). The carcinogenic risk (R) is calculated as follows: R = CDI × SF, where CDI is the chronic daily intake from the oral exposure route (mg kg⁻¹ per day) and SF is the slope factor of the contaminant via the oral exposure route [(mg kg⁻¹ per day)⁻¹]. The non-carcinogenic risk, hazard quotient (HQ), is calculated using the following equation: HQ = CDI/R_fD, where R_fD (mg kg⁻¹ per day) is the reference dose of the contaminant via the oral exposure route. The values of the slope factor and reference dose (Table 5) for the OCPs were obtained from the US EPA Integrated Risk Information System.³⁹

Table 5 Toxicological parameters of the ten organochlorine pesticides^a

Parameter	SF (mg kg^−1 per day)^−1	RfD (mg kg^−1 per day)
a —: no given parameter value.
α-HCH	6.3	5.00 × 10^−4
β-HCH	1.8	2.00 × 10^−4
γ-HCH	1.3	3.00 × 10^−4
δ-HCH	—	—
p,p-DDE	0.34	—
p,p-DDD	0.24	2.00 × 10^−3
p,p-DDT	0.34	5.00 × 10^−4
Heptachlor	4.5	5.00 × 10^−4
Heptachlor epoxide	9.1	1.30 × 10^−5
Endosulfan-sulfate	—	5.70 × 10^−3

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Because of the ten OCPs concentrations in the three water sources in YiXing City had a wide range, especially, the ten OCPs concentrations could be detected in drinking water, while the other eight OCPs were not detected, therefore, the health risk levels caused by the ten OCP groups, whether for adults or for children, was focused on the ten OCPs. A summary of the statistics of the health risks caused by the OCP groups for adults and children is presented in Table 6. The health risks for adults and children exposed to OCPs in YiXing City were calculated according to the health risk assessment model (US EPA). The average cancer risk for children was approximately two times that of adults for the ten OCPs. The cancer risk from YiXing City followed the order of β-HCH > heptachlor > heptachlor epoxide > γ-HCH > α-HCH > p,p′-DDT > p,p′-DDE > p,p′-DDD > endosulfan-sulphate > δ-HCH. As shown in Table 6, the cancer risk caused by β-HCH and heptachlor for children and adults was greater than the acceptable risk level (1 × 10⁻⁶) that is recommended by the US EPA for carcinogens, which indicates that β-HCH and heptachlor were the main cancer risk factors in YiXing City and may pose a cancer risk to the local population, especially to children. Therefore, measures should be put into place to protect water sources and control the damage from OCPs to aquatic organisms and human health in YiXing City. The HQs of the ten OCPs for individuals were also calculated according to the assessment model (US EPA). According to the standards, when the ratio exceeds 1, there is an adverse human health effect. The non-carcinogenic risks from the ten OCPs for adults and children were much less than 1, suggesting that these OCPs in YiXing were considered unlikely to pose any non-carcinogenic effects to individuals.

Table 6 Summary statistics for the health risks of the ten OCPs for adults and children in the three water sources of YiXing City^a

Pesticide	Cancer risk (10^−6)				Hazard quotient (10^−2)
	Range		Mean		Range		Mean
	Adults	Children	Adults	Children	Adults	Children	Adults	Children
a —: no given reference dose of the contaminant via oral exposure route parameter value.
α-HCH	1.05–3.54	2.24–7.59	1.69	3.62	0.03–0.11	0.07–0.24	0.05	0.11
β-HCH	9.55–25.83	20.47–55.36	16.73	35.85	2.65–7.81	5.68–15.38	4.65	9.96
γ-HCH	1.95–2.93	1.95–6.28	1.89	4.06	0.50–0.75	0.50–1.61	0.49	1.04
δ-HCH	—	—	—	—	—	—	—	—
p,p-DDE	0.04–0.12	0.08–0.25	0.08	0.16	—	—	—	—
p,p-DDD	0.02–0.05	0.04–0.10	0.03	0.06	0–0.01	0.01–0.02	0.01	0.01
p,p-DDT	0.05–0.16	0.11–0.35	0.10	0.22	0.03–0.10	0.07–0.20	0.06	0.13
Heptachlor	1.86–15.02	3.98–32.18	8.07	17.29	0.14–1.11	0.29	0.60	1.28
Heptachlor epoxide	0.23–7.82	0.50–16.77	3.03	6.48	0.20–6.61	2.38	2.56	5.48
Endosulfan-sulfate	—	—	—	—	0.01–0.03	0.02–0.07	0.02	0.04

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4. Conclusions

This study provides the first detailed data on the contamination status of 18 OCPs from the three typical water sources in YiXing City, Taihu Upper-River Basin, East China. The results showed that the levels of OCPs in the surface water of YiXing City were moderate compared to that of the other rivers in the world. The total amount of OCP contamination for the three water sources decreased in the following order: XJ > HS > YC. The ∑OCPs concentrations with mean values in HS, YC and XJ were 535.6, 426.5 and 537.8 ng L⁻¹, respectively. Ten OCPs were detected frequently, including HCHs, DDTs, heptachlor, heptachlor epoxide, and endosulfan-sulphate. HCHs are the most abundant OCPs in the water sources, especially β-HCH. The occurrence and distribution of OCPs in the water sources greatly varied, suggesting that the spatial distribution of OCPs was site-specific. Moreover, the mean concentrations of ∑OCPs increased from the surface layer to the bottom layer, with a fluctuation change in the middle layers. The monitoring of OCPs in JTP revealed that OCPs were present in the drinking water and that the ecological and health effects of these substances consumed from drinking water, even at such low concentrations, requires notice. Hence, source control and advanced treatment processes in drinking water treatment plants should be a focus of future studies.

Acknowledgements

This study was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (No. 2014ZX07405002) and the National Natural Science Foundation of China (No. 51608148).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24345b

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