Resolution of the ongoing challenge of estimating nonpoint source

Feb 13, 2019 - Resolution of the ongoing challenge of estimating nonpoint source neonicotinoid pollution in the Yangtze River Basin using a modified m...
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Resolution of the Ongoing Challenge of Estimating Nonpoint Source Neonicotinoid Pollution in the Yangtze River Basin Using a Modified Mass Balance Approach Yuanchen Chen,† Lu Zang,† Guofeng Shen,‡ Maodian Liu,‡ Wei Du,‡ Jie Fei,† Liyang Yang,† Long Chen,§ Xuejun Wang,‡ Weiping Liu,∥ and Meirong Zhao*,†

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†

Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Research Center of Environmental Science, Zhejiang University of Technology, Hangzhou 310032, China ‡ Ministry of Education Laboratory of Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China § Key Laboratory of Geographic Information Science (Ministry of Education), School of Geographic Sciences, East China Normal University, Shanghai 200241, China ∥ College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China S Supporting Information *

ABSTRACT: Neonicotinoid insecticides have been widely consumed worldwide, particularly in China. There is a growing interest in the environmental research community about the occurrence, fates, sources, and risks of neonicotinoids. Nine neonicotinoids in river/lake water were measured at 12 sites along the Yangtze River Basin during the dry and wet seasons in 2016, and nonpoint sources were also identified based on a modified mass balance method. A significantly higher concentration of neonicotinoids was found during the dry season probably due to the dilution effect and insecticide consumption. The high pollution levels are due to posing high ecological risks compared with the recommended thresholds. In 2016, 1190 (95% confidence interval (CI) = 822−1690) tons of neonicotinoids were transferred into the adjacent sea. Nonpoint source pollution (1700 (CI = 1200− 2370) tons) was the major contributor (91.3%) to the total input of neonicotinoids into the system. Composition profiles identifying specific neonicotinoid sources indicated some changes in usage patterns from old to new types of neonicotinoids. This spatial and seasonal field study and source identification is expected to fill the data gap regarding the limited information on neonicotinoid use patterns and to inform further effective policy-making and intervention programs in China that should be urgently promoted in the near future.

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INTRODUCTION

studies measured the neonicotinoid residuals in surface water in the past decade and detected high concentrations of neonicotinoids.9,11−15 As a large traditional agricultural country, China is the largest producer, consumer, and exporter of neonicotinoids globally, and the numbers have continued to increase gradually in recent years.16 The level of neonicotinoid pollution in surface/river water in China has gained more attention in recent years. However, none of the studies have reported neonicotinoid pollution in any large-scale aquatic environment in China, and none of them have focused on the source identification of neonicotinoids.

While most organochlorine and organophosphorus insecticides have been banned,1 neonicotinoids have become the most widely consumed class of insecticides around the world in the past decades.2,3 A number of previous studies have revealed the ecological risks of neonicotinoids to insects, mammals, and birds.2−6 Declines of bee and bird populations were demonstrated to be related to high contamination of neonicotinoids in farmland.3,4 In addition, long-term exposure to neonicotinoids can pose adverse health effects to human beings.5,6 Impairment of intellectual development for children and pulmonary dysfunction for adults has been demonstrated to be associated with neonicotinoid exposure.7,8 Owing to the water solubility of neonicotinoids, they are easily transferred from agricultural activities to aquatic environments, such as rivers and lakes.9,10 Many previous © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 29, 2018 January 31, 2019 February 12, 2019 February 13, 2019 DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Map illustrating the 12 sampling sites in the upper and middle−lower Yangtze River. Twelve sites were the Minjiang River, Yibin, Jialing River, Chongqing, Wujiang River, Front of the Three Gorges Dam, Back of the Gorges Dam, Dongting Lake, Hanjiang River, Wuhan River, Poyang Lake, and Nantong River.

The Yangtze River is one of the largest rivers in China, covering a wide range of agricultural areas, which consume the largest amount of insecticides in the country.17,18 In this study, water samples were collected at 12 sites along the Yangtze River from the upper reaches (Sichuan province) to the lower reaches (Jiangsu province), including some tributaries and lakes during the dry and wet seasons in 2016. This study aimed to characterize the recently used insecticides by measuring neonicotinoids in river/lake water and identify the fluxes/ sources using a modified mass balance method, the first time this technique was employed for riverine neonicotinoids on a large scale. The results describe the neonicotinoid distribution in waters and their emission sources in different areas and seasons. Consequently, this study can also provide some scientific suggestions for intervention programs and guidelines regarding neonicotinoid pollution and its adverse impacts on the aquatic ecosystem and human health in the near future in China.

thiamethoxam (THIA) (99.0%), imidacloprid (IMI) (99.0%), clothianidin (CLO) (99.9%), flonicamid (FLO) (99.0%), thiacloprid (THI) (98.5%), dinotefuran (DIN) (98.0%), nitenpyram (NIT) (98.6%), and imidaclothiz (IMID) (96%), were purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany). The isotope-labeled standards (IMI-d4, THIA-d3, and CLO-d3, purity 99.9%), which were used as internal standards and recovery surrogates, were purchased from C/D/ N Isotopes Inc. (Quebec, Canada). Dichloromethane, acetonitrile, and formic acid (with purities of 99.9%) were purchased from Fisher Scientific (Leicestershire, UK). All standards were dissolved in acetonitrile during the analysis process. Laboratory Analysis. The liquid−liquid extraction method was employed to extract neonicotinoids from the water phase to organic solvent phase (dichloromethane). Briefly, for each sample, 50 mL of water was added into a separating funnel, and 2 g sodium chloride was then dissolved in the water. Then 200 ng of the mixed isotope-labeled standards (IMI-d4, THIA-d3, and CLO-d3) was added into the water sample as the internal standards and recovery surrogates. Thirty millimeters of dichloromethane was also added into the funnel and vibrated for 4 min. Then the organic phase was collected in a flask. This procedure, which is adding 30 mm of dichloromethane into the funnel, then vibrating for more than 3 min, and collecting the organic phase in a flask, was repeated 3 times. The organic phases collected in the repeated procedures above were put together. The extracts−organic phases were transferred into a column filled with 8 g of anhydrous sodium sulfate for purification. Then 20 mL of dichloromethane was used to elute the column. The elute was concentrated to less than 1 mL (before dryness) using rotary evaporation (BUCHI R-215, Switzerland), and 2 mL of acetonitrile was added. Then the elute with the acetonitrile was concentrated to approximate 1 mL and finally transferred to a brown sample bottle for further analysis. Nine neonicotinoids were measured, including ACE, THIA, IMI, CLO, FLO, THI, DIN, NIT, and IMID (Table S2). The target compounds were measured using ultraperformance liquid chromatography coupled with a triple-quadrupole mass spectrometer Xevo TQ-S (UPLC-MS/MS) (Water Corp., Milford, MA, USA). The individual compounds were separated in a YMC ODS-AQ column (100 mm × 2.1 mm, 3 μm, YMC, Allentown, PA, USA) using a gradient flow of the mobile

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METHODOLOGY Sampling and Sites. Water samples were collected at 12 sites along the Yangtze River Basin during dry (from October to April) and wet seasons (from May to September). The locations of the sites are illustrated in Figure 1. At each collection, mixed water samples, which included surface (50 cm under the surface of water), middle (half depth of the water), and bottom (50 cm above the riverbed or lakebed), were collected and combined. The mixed water samples were collected in triplicate during each season and at each site. Typically, the area from Yibin in Sichuan Province to Yichang in Hubei Province, a length of 1040 km, is considered the upper section, while the area from Yichang to the estuary in Nantong, a length of 1893 km, is the middle−lower section (this is also called the Yangtze Plain). Minjiang River, Jialing River, Wujiang River, and Three Gorges Dam belongs to the upper section, and Hanjiang River, Dongting Lake, and Poyang Lake belong to the middle−lower section. Detailed geographical information on the sites is also listed in Table S1. The water samples were stored in brown glass bottles at −20 °C (freezing) and transported to the laboratory for analysis within 1 month. Chemicals and Reagents. The analytical standards of the nine neonicotinoids, including acetamiprid (ACE) (98.1%), B

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Therefore, the model can be simplified to the following form after removing the negligible amounts of L Sewage In , LSediment Release, and LPrecipitation

phases (100% acetonitrile and Milli-Q water with 0.01% formic acid, Table S3). An electrospray ionization (ESI) source in the positive ion mode with multiple reaction monitoring and two transition ions (quantitation and verification) was utilized. Twenty microliters of extracted water sample was injected in a single run, and the run time for a single sample was approximately 6 min. The flow rate of gradient elution was 0.3 mL/min. Additional information on the quantification method and UPLC-MS/MS conditions is provided in the Supporting Information. Quality Assurance and Quality Control. In every batch of samples one blank procedure sample was also measured simultaneously. Anhydrous sodium sulfate and sodium chloride were baked at 650 °C for more than 6 h. The recoveries of the three surrogates were in the ranges of 79.6− 108.3% (IMI-d4), 80.2−99.3% (THIA-d3), and 84.5−97.1% (CLO-d3). The average spiked recoveries for the individual compounds ranged from 85.5% to 95.3% (Table S4). All of the recoveries suggest that the procedure and condition of the instrument were acceptable. The method detection limits (MDL) of the individual neonicotinoids ranged from 0.0045 to 0.045 ng/L, and the limits of quantification (LOQ) were from 0.0135 to 0.135 ng/L for the nine neonicotinoids (Table S4). Detailed analysis and calculation processes of the surrogate recoveries, spiked recoveries, MDL, and LOQ are also described in the Supporting Information. Estimation of Riverine Neonicotinoid Fluxes. The riverine neonicotinoid fluxes along the Yangtze River are calculated based on a modified mass balance method, which follows similar principles for riverine nutrient fluxes in the literature studies.19−21 The model assumes that targeted pollutant inputs (“In”) into the river equal the losses (“Out”). In the present study, the neonicotinoid fluxes were established separately for the upper and midlower Yangtze River as follows (in units of tons)

L Upstream In + L Tributary In + L Nonpoint In = L Downstream Out + L Water Use Out + L Photolysis + L Evaporation Out

The input of the upstream and tributaries/lakes and the output of the downstream are calculated by multiplying average concentrations of neonicotinoids (CUpstream In, CTributary In, and CDownstream Out) with the water flow volumes (VUpstream In, VTributary In, and VDownstream Out) in different sites accordingly. The water flow volumes at different sites are provided in Table S5.25 LWater Use Out is calculated by multiplying neonicotinoid concentrations (CWater Use Out) with water withdrawal volumes (VWater Use Out) for use. The volumes of annual water use in the upper section and middle−lower section were 454.97 × 108 and 1583.65 × 108 m3, respectively.25 Neonicotinoids can be degraded through the photolysis process. A previous study reported that the half-lives of the neonicotinoids were 0.2−1.5 days, and photolysis of neonicotinoids in water with depths higher than 8 cm was negligible.26 Therefore, we assume that the neonicotinoids in the surface water (8 cm in depth) were all photodegraded within 1 year. Thus, LPhotolysis is calculated by multiplying neonicotinoid concentrations (CPhotolysis) and the volumes of surface water at 8 cm in depth (VPhotolysis) in the two sections. The average width of the Yangtze River is assumed to be approximately 1 km.19 In addition, LEvaporation Out is derived from the multiplication of neonicotinoid concentrations (CEvaporation Out) and water volumes of evaporation (VEvaporation Out). On the basis of a previous study, the annual evaporation of the Yangtze River was estimated to be 421 mm in the dry season and 818 mm in the wet season.27 Consequently, eq 2 can be converted into the following form (where “∑” represents the summation of the two seasons)

L Upstream In + L Tributary In + L Nonpoint In + LSewage In + LSediment Relaease + L Precipitation In

∑ C Upstream In × VUpstream In + ∑ CTributary In × VTributary In

= L Downstream Out + L Water Use Out + L Photolysis + L Evaporation Out

(2)

+ L Nonpoint In

(1)

=

where L Upstream In , L T r i but ary I n , L No n po i nt I n , L Se wag e In , LSediment Release, and LPrecipitation are the inputs from upstream, tributaries/lakes, nonpoint sources, sewage discharges, sediment release and precipitation, respectively, and LDownstream Out, LWater Use Out, LPhotolysis, and LEvaporation Out are the losses from downstream, water use, photolysis process, and evaporation, respectively. When solving the equation, the following assumptions were made: (a) compared with runoff from agricultural areas into the rivers (nonpoint source), the sewage discharge from industrial production (point source) into the rivers is considered small for neonicotinoids;9,22 (b) because neonicotinoids are a type of hydrophilic substance, it is difficult for them to absorb into sediment, which results in low concentrations in the sediments,9,22,23 and thus, the release of neonicotinoids from sediments is expected to be very low; and (c) the exchange of neonicotinoids between water and air consists of precipitation and evaporation. According to previous studies, the concentration of neonicotinoid in the air was in the range from 1.0 to 4.0 pg/m3,22,24 6−7 orders of magnitude lower than that in the water found in this study.

∑ CDownstreamOut × VDownstream Out + C Water Use Out × VWater Use Out + C Photolysis × VPhotolysis +

∑ CEvaporation Out × VEvaporation Out

(3)

All of the components in the model can be directly calculated based on reference values and the measured neonicotinoid concentrations, except LNonpoint In. LNonpoint In for the two sections of the Yangtze River is calculated by solving eq 3. Data, Sensitivity, and Uncertainty Analysis. In this study, all of the neonicotinoid concentrations were obtained from laboratory measurements of field samples, and the means and standard deviations (SDs) were calculated from the replicates of different sites and seasons. However, when calculating LWater Use Out, LPhotolysis, and LEvaporation Out, the corresponding three reference values (VWater Use Out, VPhotolysis, and VEvaporation Out) are used. Therefore, the sensitivity analysis was conducted to reveal the sensitivity for the calculation of LNonpoint In to the three reference values. The reference values varied by 50%, 75%, 150%, and 200% of the initial values and C

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 2. (A) Total concentrations of neonicotinoids in the 12 sites along the Yangtze River, and (B) concentrations of individual neonicotinoids during dry and wet seasons (ng/L). Twelve sites were Minjiang River, Yibin, Jialing River, Chongqing, Wujiang River, Front of the Three Gorges Dam, Back of Three Gorges Dam, Dongting Lake, Hanjiang River, Wuhan River, Poyang Lake, and Nantong River. Nine individual neonicotinoids were acetamiprid (ACE), thiamethoxam (THIA), imidacloprid (IMI), clothianidin (CLO), flonicamid (FLO), thiacloprid (THI), dinotefuran (DIN), nitenpyram (NIT), and imidaclothiz (IMID). Error bars represent the standard deviations (SD) are also shown. Means and SDs in the left panel were calculated from three replicates in different sites and seasons, and those in the right panel were calculated from 36 water samples (12 sites with 3 replicates) in dry or wet season.

during the dry and wet seasons, respectively. In comparison, DIN concentrations were the highest, with levels of 470 ± 640 ng/L during the dry season and 190 ± 320 ng/L during the wet season. The second highest concentration was found for NIT (dry season = 430 ± 380 ng/L; wet season = 150 ± 240 ng/L). FLO was only detected in Nantong (estuary of the Yangtze River) during the dry season with a concentration of 1.06 ± 0.230 ng/L. Comparison with Previous Studies and Potential Ecological Risk Thresholds. Neonicotinoid pollution in surface water was studied in a few previous studies in countries such as Japan and the United States (Table S7).11,13,15,28−32 For ACE, CLO, DIN, IMI, and THIA, the mean concentrations in this study (ranging from 8.40 to 470 ng/L) were much higher in this study than those reported in river water in Japan and the United States (ranging from 1.40 to 19.0 ng/L). The contamination levels of neonicotinoids in this study (i.e., IMI = 0.292−49.9 ng/L) were much lower than those measured from agricultural water in the United States, Brazil, and Australia (i.e., IMI = 50.0−1930 ng/L). The neonicotinoid contamination in surface water of agricultural land was believed to be more serious than that of rivers due to direct use and contamination from agricultural activities. The neonicotinoid pollution in surface water is expected to pose ecological risks for the aquatic ecosystem. To evaluate the ecological risks, the concentrations of neonicotinoids in water of the Yangtze River were compared with a previous study that reported the acute and chronic ecological risk thresholds for aquatic invertebrates (acute = 200 ng/L; chronic = 35.0 ng/L) based on the species-sensitive distribution model from 46 acute and 18 chronic toxicity studies.12 The concentrations of neonicotinoids were normalized to IMI based on the molecular weights of individual compounds when compared with the thresholds. It was found that 75% of the sampling sites (except

made the other reference values constant. Finally, the corresponding model outputs−inputs of nonpoint source were all recorded. In addition, the uncertainties of the analytical measurements on the concentrations were reported by means and SDs along with the sample sizes. The uncertainties of the fluxes were calculated with 100 000 times for every calculation by the Monte Carlo simulation method using MATLAB software (MathWorks, USA), resulting in the means, SDs, and confidence intervals (CIs, 95%). Waters MassLynx software V4.1 was utilized to perform the instrument control, data acquisition, and data analysis during UPLC-MS/MS analysis. R Studio Software (version 1.0.153) was employed to perform the statistical analysis. The significance level was set at the level of 0.05 in all statistical analyses.

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RESULTS Contamination Levels. The total concentration of neonicotinoids (∑neonicotinoids) was 990 ± 490 ng/L during the dry season and 390 ± 360 ng/L during the wet season for all of the sites along the Yangtze River Basin. The coefficients of variation (CVs) were 0.495 and 0.923 for the dry and wet seasons, respectively, indicating large variations in this study. ∑neonicotinoids in various sites during two seasons are compared in Figure 2A. For most sites, significantly higher levels of ∑neonicotinoids were found during the dry season; in particular, ∑neonicotinoids for Yibin and Minjiang River during the dry season were 9.1 and 6.8 times higher than that during the wet season, respectively. The highest contamination level was found in Wuhan, and relatively lower concentrations were found in Poyang Lake, Jialing River, and Wujiang River. Figure 2B shows the concentrations of nine individual neonicotinoids during two seasons (data in Table S6). The concentrations of IMI were 23.6 ± 21.4 and 21.5 ± 13.1 ng/L D

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 3. Annual means of total neonicotinoid fluxes in the Yangtze River Basin (tons). Yangtze River Basin is divided into two sections (upper section and middle−lower sections). In addition to these sections, the inputs of upstream, tributary/lake, and nonpoint sources and the outputs of downstream, photolysis processes, evaporation, and water use are taken into consideration.

during the dry season, LNonpoint In deceased by 6.5% and 29% for the upper and middle−lower sections and then increased by 13% and 59% for the upper and middle−lower sections, respectively. When VWater Use Out was halved and then doubled during the wet season, LNonpoint In deceased by 3.0% and 30% for the upper and middle−lower sections and then increased by 6.1% and 60% for the upper and middle−lower sections, respectively. The fluctuations in LNonpoint In caused by the halving and doubling of VPhotolysis were in the range from 0.006% to 0.06% in different sections and seasons, while the percent of change in LNonpoint In caused by the halving and doubling of VEvaporation Out was less than 0.3%. VWater Use Out, VPhotolysis, and VEvaporation Out had stronger impacts on LNonpoint In in the middle−lower section during the dry season compared with the upper section during the wet season. Therefore, other than the concentrations of neonicotinoids in the river/lake water, the accuracy of VWater Use Out is the key influencing factor on the accuracy of the estimation of LNonpoint In.

the Jialing River, Wujiang River, and Poyang Lake from the 12 sites) exceeded the acute risk threshold, and all of the sampling sites exceeded the chronic risk threshold, suggesting severe river water pollution after wide and intense use of neonicotinoids during the past decades. Riverine Neonicotinoid Fluxes. The annual riverine neonicotinoid fluxes for the upper and middle−lower sections are illustrated in Figure 3. From Yibin to Yichang and finally to Nantong, the flux of neonicotinoids increased by approximately 44 times from the upper site (Yibin) to the final site (Nantong), and 1190 (CI = 822−1690) tons of neonicotinoids were eventually discharged into the China East Sea. It was found that the nonpoint discharge from agricultural land into the river was the dominant influencing factor on riverine neonicotinoid balance. For the entire river, the upstream, tributaries, lakes, and nonpoint discharges contributed to 1.5%, 2.5%, 4.7%, and 91.3% of the neonicotinoids in the Yangtze River, respectively. LNonpoint In was 692 (CI = 544−893) tons and 1010 (CI = 658−1480) tons in the upper and middle− lower sections, respectively. In comparison, the sources of tributaries/lakes were only 41.12 (CI = 35.4−48.9) and 93.0 (CI = 58.6−140) tons in the upper and middle−lower sections. For the output fluxes, water use was dominant, with levels of 71.1 (CI = 56.8−90.5) and 595 (CI = 438−810) tons for the upper and middle−lower sections, respectively. The losses from photolysis and evaporation processes were considerably smaller compared to that of water use practice by 2 orders of magnitude and thus had a limited impact on riverine neonicotinoid balance. Sensitivity Analysis of the Modified Mass Balance Model. The sensitivity analysis reveals that the selected parameters (VWater Use Out, VPhotolysis, and VEvaporation Out) had limited impacts on the result of nonpoint source estimation (Figure S1). VWater Use Out was the most influential parameter when compared with VPhotolysis and VEvaporation Out. When VWater Use Out was halved (50%) and then doubled (200%)

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DISCUSSION Spatial and Seasonal Variations of Contamination Levels. Large variations were found for the concentrations. When comparing the two seasons, ∑neonicotinoids during the dry season was much higher than that during the wet season. This was probably attributable to the dilution effect33,34 or smaller consumption of neonicotinoids during the wet season. As seen in Figure 2A, ∑neonicotinoids in the sites of mainstream, such as Yibin, Three Gorges Dam, Wuhan, and Nantong, were significantly higher than those in the tributaries (such as Jialing River, Wujiang River, and Hanjiang River) and lakes (including the Poyang and Dongting lakes) (p < 0.05), suggesting that the mainstream of the Yangtze River is the sink of the neonicotinoids emitted into the Yangtze River Basin. According to the variance analysis, season and sampling site contributed 5.0% and 95%, respectively, to the variations of ∑neonicotinoids, indicating that the sampling site was the E

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 4. Relative contributions of individual neonicotinoids at different sites during dry and wet seasons (%). Nine individual neonicotinoids were acetamiprid (ACE), thiamethoxam (THIA), imidacloprid (IMI), clothianidin (CLO), flonicamid (FLO), thiacloprid (THI), dinotefuran (DIN), nitenpyram (NIT), and imidaclothiz (IMID).

wet season (1.2). When comparing the nonpoint source levels in the same section during different seasons, larger amounts were determined during the dry season than during the wet season, and the ratio of the dry to wet season in the middle− lower section was 2.1, which was higher than that in the upper section (1.5). From a previous report, it was found that the larger agricultural production (including cereal, fruit, and vegetable yields) in the middle−lower section compared with that in the upper section was supposed to posing the larger nonpoint discharges in the middle−lower section of the Yangtze River Basin.35 Figure 5 shows the composition profiles of nine individual neonicotinoids of the nonpoint sources in the two sections during two seasons. NIT and DIN were the dominant components among the nine compounds, with a total relative contribution of more than 84% involving the two sections and seasons. It has been demonstrated that these two newly commercialized types of neonicotinoids are now widely and intensely used in the Yangtze River Basin and runoff into the river through the nonpoint discharge pathway compared with some old types (i.e., IMI and ACE). For the season comparison, the difference of the nonpoint source pattern between the dry and the wet seasons for the middle−lower section was obviously larger than that for the upper section. For instance, the relative contributions of NIT and IMI were higher and that of DIN was lower during the wet season. For the section comparison, the difference between the upper and the middle−lower sections for the wet season was larger than that for the dry season. Thus, it can be concluded that the difference in neonicotinoid consumption was affected by the factors of season and region. All of the differences in nonpoint discharge levels and composition profiles could reflect the agricultural conventions of neonicotinoid use in different areas and seasons in the Yangtze River Basin because neonicotinoids transferred into the river water mainly from nonpoint sources.

dominant contributor to the total variations compared with the season. The composition profiles of neonicotinoids in various sites along the Yangtze River during the two seasons are compared in Figure 4. The results show that no matter the season, dry or wet, DIN and NIT dominated the neonicotinoid total concentrations with a mass percentage of more than 50%. As the most concerning type of neonicotinoids, the relative contributions of IMI were much lower than those of DIN and NIT and even lower than those of IMID, which was recently commercialized. These results indicated that some changes in the neonicotinoid use pattern have occurred. Large differences were also found among different seasons and sampling sites for the composition profiles. For instance, the relative contributions of IMI were 4.0% and 15% and those of NIT were 46% and 27% during the dry and wet seasons, respectively. The relative contributions of IMI and NIT varied from 1.2% to 24% and from 11% to 47%, respectively, among different sampling sites. The differences in neonicotinoids among different seasons and sampling sites can be demonstrated by the differences in the usage pattern of the neonicotinoids during agricultural practices. Characteristics of Nonpoint Sources. For the neonicotinoid flux analysis, the seasonal difference was also taken into consideration (Figure S2). During the dry season, the LNonpoint In of neonicotinoids was 416 (CI = 352−502) and 678 (CI = 470−960) tons in the upper and middle−lower sections, and during the wet season, it was 276 (CI = 192−391) and 330 (CI = 187−524) tons in the corresponding sections. A larger LNonpoint In suggested that the consumption of neonicotinoids during agricultural activities was larger during the dry season than during the wet season. Though a smaller LNonpoint In in the upper section of the Yangtze River than in the middle−lower section was found in both seasons, the ratio of the middle−lower section to the upper section during the dry season was 1.6, which was much higher than that during the F

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 5. Relative contributions of nine individual neonicotinoids to the nonpoint source discharges into the upper and middle−lower sections during dry and wet seasons (%). Nine individual neonicotinoids were acetamiprid (ACE), thiamethoxam (THIA), imidacloprid (IMI), clothianidin (CLO), flonicamid (FLO), thiacloprid (THI), dinotefuran (DIN), nitenpyram (NIT), and imidaclothiz (IMID).

Source Analysis of Riverine Neonicotinoids into the Adjacent Sea. Some differences in the riverine neonicotinoid mass balance between two seasons were found (Figure S2; Table S8). The neonicotinoid discharges into the adjacent sea/ East China Sea were 750 (CI = 537−1040) and 439 (CI = 284−649) tons during the dry and wet seasons, respectively. For the individual compounds, DIN and NIT were the top contributors, with discharges of 412 (CI = 271−603) and 271 (CI = 181−393) tons during the dry season and 176 (CI = 115−258) and 187 (CI = 132−263) tons during the wet season, respectively. IMI, ACE, and THIA were the most commonly used types of neonicotinoids in China.16,22 However, their discharges turned out to be relatively small compared with those of DIN and NIT. The annual discharges of IMI, ACE, and THIA were 35.7 (CI = 28.8−45.2), 8.56 (CI = 6.29−11.6), and 32.0 (CI = 23.4−43.8) tons, respectively. FLO had the smallest discharge, with 0.428 (CI = 0.391− 0.478) tons during the dry season and none during the wet season. The inputs of the neonicotinoids are composed of LUpstream In, LTributary In, and LNonpoint In. Figure 6 clearly depicts that LNonpoint In dominated the inputs of most neonicotinoids

(except CLO), ranging from 59% to 98% of the total inputs, suggesting that most of the neonicotinoids discharged into the adjacent sea were from the nonpoint source along the Yangtze River. Some differences in the inputs of the nonpoint sources were found between two seasons. For most compounds (i.e., DIN, FLO, NIT, and THI), the inputs of the nonpoint sources were larger during the dry season. Since the period of paddy planting is during the wet season in the Yangtze River Basin and ACE and IMI are the major types of neonicotinoids used in paddy planting,36 the inputs of the nonpoint sources of ACE and IMI were larger during the wet season. In addition, the outputs of neonicotinoids along the Yangtze River reduced the discharges into the adjacent sea (Figure 7). The results show that for each individual compound and ∑neonicotinoids, water use practice dominated the outputs and the amounts of photolysis and evaporation processes were considerably small. The relative contributions of water use practice to the discharges into the sea were larger during the dry season compared with those during the wet season. This was attributable to the higher concentrations of neonicotinoids during the dry season. G

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Environmental Science & Technology

Figure 6. Relative contributions of the inputs of the nine individual neonicotinoids into the adjacent sea/East China Sea by source (including nonpoint, tributary/lake, and upstream sources) (%). Mean fluxes with 95% confidence intervals (CIs) of nonpoint sources for individual compounds during dry and wet seasons (tons). During the wet season, FLO was not detected. Nine individual neonicotinoids were acetamiprid (ACE), thiamethoxam (THIA), imidacloprid (IMI), clothianidin (CLO), flonicamid (FLO), thiacloprid (THI), dinotefuran (DIN), nitenpyram (NIT), and imidaclothiz (IMID).

Figure 7. Waterfall plots of the mass balance for some individual neonicotinoids (ACE, DIN, IMI, and NIT) and total neonicotinoids during dry and wet seasons (tons). Input sources include upstream, tributary/lake, and nonpoint sources, and outputs consist of water use, photolysis, and evaporation processes. H

DOI: 10.1021/acs.est.8b06096 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Limitations and Implications. Neonicotinoids are widely consumed during agricultural practice in China. Owing to the high aqueous solubility of neonicotinoids, they can be transported from agricultural lands to rivers and oceans and be relatively stable in the aquatic system, resulting in high ecological risks. The present study is the first wide-scale field investigation of neonicotinoid pollution in river/lake waters in China. In addition, we utilized a modified mass balance approach based on concentration results and some parameters to estimate the nonpoint source levels of riverine neonicotinoids. Since the application information on neonicotinoids in China is very limited, this attempt is expected to fill the data gap regarding neonicotinoid application in the Yangtze River Basin. In addition, the informative illustration of the neonicotinoids’ spatial and seasonal distribution and analyzing the sources is expected to be instructive and meaningful for further informed policy making. The modified mass balance model used here was simplified based on some assumptions. Though these hypotheses may give rise to some biases in the estimation of neonicotinoid fluxes and nonpoint sources, this is still an acceptable method for the newly studied insecticides, neonicotinoids, until now because no study has provided any nonpoint source information on a large scale. It is expected that obtaining more reliable parameters of some transferring processes (i.e., deposition rate from air to water and release rate from sediment to water) will make the model more accurate and rational in the future. Through this investigation we found that the neonicotinoid contamination in the Yangtze River Basin was more severe than in many other developing countries, and the potential ecological risks for aquatic invertebrates mostly exceeded the thresholds. Though some studies have reported the potential risks for human health, the accurate risks are unable to be evaluated here because the residents are not directly exposed to the neonicotinoids from the water collected in the Yangtze River. To conclude, it is urgent to develop strict policy and intervention programs based on source identification to control neonicotinoid use in agricultural practices in China in the near future.

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Long Chen: 0000-0001-9574-7307 Xuejun Wang: 0000-0001-9990-1391 Weiping Liu: 0000-0002-1173-892X Meirong Zhao: 0000-0003-3132-9223 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (41701584 and 21677130).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b06096. Quantification method and quality control; UPLC-MS/ MS method conditions; descriptions of the sites; information about the nine neonicotinoids; mobile phase gradient in chromatograpghy; method detection limits and spiked recoveries; water flow rates of the different sites; concentrations of individual neonicotinoids in different sites and seasons; comparison of neonicotinoid concentrations in previous studies and this study; fluxes of individual neonicotinoids in different sections and seasons; sensitivity analysis results; total neonicotinoid fluxes in the Yangtze River during two seasons (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-571-8832-0265; E-mail: [email protected]. ORCID

Guofeng Shen: 0000-0002-7731-5399 I

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