3.1 Database availability
A total of 43 studies referring to 10 counties (the United States, Australia, Belize, Canada, China, Japan, the Philippines, Romania, South Africa and Vietnam) were selected, and in these studies NEOs were detected in tap water, seawater, lakes, rivers, reservoirs, estuaries, creeks, wetlands, or open ditches and runoff in agricultural regions (Table S1). Most of the studies focus on eastern Asia and North America, which include countries heavily focused on agricultural production (Fig. 1). The main regions exhibiting NEO use in agriculture are Latin America (29.4%), Asia (23%), North America (22%), and Europe (11% of total global use) (Bass et al. 2015; Simon-Delso et al. 2015). The mean concentrations of eight widely used NEOs (ACE, CLO, DIN, IMI, IMZ, NIT, THI and TXM) were collected, and the information on each form of NEO detection is shown in Fig. 2. IMI is the most frequently reported (39/43, 91%), followed by CLO (36/43, 84%), TXM (32/43, 74%), ACE (31/43, 72%), THI (27/43, 63%), DIN (16/43, 37%), NIT (11/43, 26%) and IMZ (4/43, 9%). IMI, the first NEO developed, is the most frequently reported, possibly due to its broad application and usage (Kollmeyer et al. 1999). Only a few studies include IMZ detection, but this should not be ignored. NEO pollution in some underdeveloped areas, such as Africa, should be considered.
3.2 NEO concentrations in water
Table 1 shows the concentrations and numbers of observations for different NEOs. CLO was the most frequently detected in 1056 water samples, followed by IMI (879), TXM (863), ACE (428), THI (295), DIN (122), IMZ (37) and NIT (29). CLO has the highest mean concentrations at 222.320 ± 46.692 ng L−1. The mean concentrations of other NEOs are ordered as follows: IMI (119.542 ± 15.656 ng L−1) > NIT (88.076 ± 27.144 ng L−1) > TXM (59.752 ± 9.068 ng L−1) > DIN (31.086 ± 9.275 ng L−1) > IMZ (24.542 ± 2.906 ng L−1) > ACE (23.360 ± 4.015 ng L−1) > THI (11.493 ± 5.095 ng L−1). Moreover, concentrations were found to range from 0.001 to 45100 ng L−1 for CLO, from 0.004 to 9140 ng L−1 for IMI, from 0.002 to 4315 ng L−1 for TXM, from 0.002 to 3820 ng L−1 for ACE, from 0.003 to 1370 ng L−1 for THI, from 0.11 to 1022.2 ng L−1 for DIN, from 2 to 672.9 ng L−1 for NIT, and from 0.002 to 81.92 ng L−1 for IMZ(Table 1).
Table 1
Summary of the dataset indicating the number of observations for different NEO types (ACE, CLO, DIN, IMI, DIN, IMZ, NIT, THI, TXM), and statistics (Mean ± standard error (SE), lower 95% confidence interval (LCI), upper 95% confidence interval (UCI)) and the ranges of concentrations of each NEO type.
Type | n | Mean (ng L−1) | SE | Range (ng L−1) | LCI | UCI |
ACE | 428 | 23.360 | 4.015 | [0.0025, 1527.6] | 15.469 | 31.252 |
CLO | 1056 | 222.320 | 46.692 | [0.001, 45100] | 130.700 | 313.939 |
DIN | 122 | 31.086 | 9.275 | [0.11, 1022.2] | 12.725 | 49.448 |
IMI | 879 | 119.542 | 15.656 | [0.004, 9140] | 88.813 | 150.270 |
IMZ | 37 | 24.542 | 2.906 | [0.002, 81.92] | 18.648 | 30.436 |
NIT | 29 | 88.076 | 27.144 | [2, 672.9] | 32.475 | 143.678 |
THI | 295 | 11.493 | 5.095 | [0.003, 1370] | 1.466 | 21.520 |
TXM | 863 | 59.752 | 9.068 | [0.002, 3820] | 41.960 | 77.543 |
Figure 3 displays the distributions of the mean concentrations of each NEO type. The concentrations of CLO and IMI were found to be concentrated at 0~1500 ng L−1 and 0~500 ng L−1, respectively. The concentrations of ACE, DIN, IMZ, NIT, THI and TXM were mainly measured at below 250 ng L−1. However, some unreasonably elevated concentrations of certain NEOs are especially found in agricultural regions. NEOs can be used in pest control to protect crops and are mainly applied for seed treatment, chemigation, and soil treatment (Simon-Delso et al. 2015). NEOs may enter through various media into aquatic systems from agricultural fields through processes such as spray drift, atmospheric deposition, soil erosion and runoff. THI monitored at the outlet of the Yarramundi Lagoon in a turf farm was found to reach levels of up to 1370 ng L−1 (Sánchez-Bayo et al. 2014). The highest IMI concentration found in Solomon Creek in the Californian agricultural region was recorded as 9140 ng L−1 (Anderson et al. 2018). Although the province of Ontario of Canada bans the cosmetic use of some pesticides on lawns and gardens, NEOs are used for seed treatment on row crops such as corn, soybeans, cereal grains and canola, which has led to widespread use in Ontario (Octario 2016). CLO, TXM and ACE levels in drain water around maize fields in Canada have reached 45100 and 7200, 4315, and 1527.6 ng L−1, respectively (Schaafsma et al. 2019). China has the highest production of NEOs, which are frequently detected in rivers flowing through urban environments. In addition to those found in agricultural regions, the highest concentrations of DIN, NIT and IMZ have been detected in the Yangtze River in China, reaching levels of 1022.3, 672.9 and 81.92 ng L−1, respectively (Chen et al. 2019). The Yangtze River is the longest river in China, playing a considerable role in agricultural and industrial activities. NEOs in the Yangtze River have become a source of NEOs in seawater. Although NEO concentrations decrease rapidly by dilution, NEOs are detected near shorelines (Pan et al. 2020). IMZ is a novel NEO that has been gradually applied to vegetables, fruits, and crops on a large scale in China because of its excellent insecticidal activity (Tao et al. 2021). It has been reported that IMZ can cause DNA damage in earthworms (Zhang et al. 2017). Due to IMZ’s increasing use and adverse effects on nontargeted organisms, increasing attention should be dedicated to this NEO. Moreover, different NEO concentrations have been detected in different crop planting periods. Concentrations of IMI and TXM increase markedly in the rice planting month. DIN was detected at a concentration of 220 ng L−1 during rice earwig emergence (Yamamoto et al. 2012). A large proportion of pesticides enter environmental media via runoff, leaching and drifting. These pesticides are absorbed by nontarget plants or organisms and present a potential threat to food safety (Li et al. 2018; Tao et al. 2021). Thus, scientists around the world have gradually recognized NEO risks and increased efforts to monitor NEOs in the environment.
3.3 Effect of physicochemical properties on NEO concentration
Figure 4 and Table 2 present the relationship between NEO concentrations and nine physical and chemical properties. Different properties show different responses to NEO concentrations in water. NEO concentrations increase with temperature, ORP and the percentage of cultivated crops (Line regression, Temperature: adjusted R2 = 0.0811, p < 0.0001; ORP: adjusted R2 = 0.0931, p = 0.0029; Cultivated crop: adjusted R2 = 0.0307, p = 0.0008) (Fig. 4d, f, i). When summer arrives, pest damage increases with increasing temperature, and insecticide use is increased to decrease crop losses. Rainfall is a key factor in increasing NEO residues in water. NEOs can enter water via surface and underground runoff, creating higher insecticide concentrations in water. For instance, in the province of Guangdong located in the subtropical zone of South China, the climate is warm and humid for most of the year. Thus, large quantities of pesticides are used for pest control, and Guangdong Province has the highest pesticide application dosage (Li et al. 2014). Only one paper presents the value of ORP, and the representativeness of the relation needs to be further confirmed (Yi et al. 2019). Concentrations of NEOs generally increase as the percentage of cultivated crops increases. High NEO concentrations are detected in surface water around areas of agricultural activity when the planting season arrives. According to a study conducted in the USA, streams show higher NEO concentrations in the planting season than in other seasons (Hladik and Kolpin 2016). Another study from Canada shows that one side of the Two Mile Creek watershed includes over 50% orchards, and an IMI concentration of 816 ng L−1 was detected in this creek (Struger et al. 2017). A positive relationship between cultivated crops and NEO concentrations has been observed in other studies (Hladik et al. 2014; Iancu et al. 2019).
Table 2
Description of the models that explain the relationships between mean concentrations of NEOs and stream discharge, turbidity, pH, temperature, dissolved oxygen, ORP, precipitation, conductivity and the percentage of cultivated crops.
Model | R2 | Adjusted R2 | F value | p | n |
Mean Concentration = 10.545 - 0.000368*Stream discharge | 0.0510 | 0.0433 | F1,125=6.658 | 0.011 | 126 |
Mean Concentration = 141.816 - 0.0639*Turbidity | 0.000187 | -0.00781 | F1,126=0.0234 | 0.879 | 127 |
Mean Concentration = 607.822 – 73.932*pH | 0.0248 | 0.0225 | F1,429=10.872 | 0.0011 | 430 |
Mean Concentration = -53.602 + 3.708*Temperature | 0.0839 | 0.0811 | F1,339=30.954 | <0.0001 | 340 |
Mean Concentration = 124.006 – 12.910*DO | 0.0906 | 0.0794 | F1,82=8.0743 | 0.0057 | 83 |
Mean Concentration = 77.593 + 0.817*ORP | 0.104 | 0.0931 | F1,82=9.421 | 0.0029 | 83 |
Mean Concentration = 10.796 - 0.0497*Precipitation | 0.0236 | 0.0223 | F1,734=17.736 | <0.0001 | 735 |
Mean Concentration = 52.817 - 0.024*Conductivity | 0.0104 | 0.00456 | F1,170=1.778 | 0.184 | 171 |
Mean Concentration = 7.237 + 0.314*Cultivated crops (%) | 0.0336 | 0.0307 | F1,331=11.480 | 0.0008 | 332 |
NEO concentrations decrease with stream discharge, pH, DO and precipitation (Line regression, Stream discharge: adjusted R2=0.0433, p=0.011; pH: adjusted R2 = 0.0225, p = 0.0011; DO: adjusted R2 = 0.0794, p = 0.0057; Precipitation: adjusted R2 = 0.0223, p < 0.0001) (Fig. 4a, c, e, g). The negative relation between NEO concentrations and stream discharge or precipitation may be caused by the dilution of NEOs when strong precipitation occurs (Struger et al. 2017). The pH value is an important factor that affects NEO solubility in water. NEOs have longer term residuals under acidic or neutral conditions than under less alkaline conditions (Yi et al. 2019). It was reported that NEOs hardly degrade at pH 4.0~7.0, while NEOs hydrolyze readily with a high pH value (pH = 10). (Todey et al. 2018). The self-purifying ability of a water body has a great effect on the degradation of NEOs, following higher DO concentrations.
The NEO concentrations show no significant correlations with turbidity and conductivity (p > 0.05) (turbidity: adjusted R2 = -0.00781, p = 0.879; conductivity: adjusted R2 = 0.00456, p = 0.184) (Fig. 4b, h). NEOs are more likely to dissolve than combine with particulate or colloidal matter (Sánchez-Bayo and Hyne 2014). However, these relationships need further confirmation.