Occurrence, spatial variation, seasonal difference, and risk assessment of neonicotinoid insecticides, selected agriculture fungicides, and their transformation products in the Yangtze River, China: From the upper to lower reaches.

Neonicotinoid insecticides (NNIs) and agricultural fungicides (including strobilurin, azole, and morpholine fungicides) are widely used, while data on their contamination in the Yangtze River of China and the risks posed by them are limited. The occurrence and distribution of ten NNIs, twenty-one transformation products (TPs) of them, seventeen agricultural fungicides, and six TPs of them were investigated in the main stream of the Yangtze River. Surface water samples (n = 144) were obtained from 72 sampling points in dry season and wet season. Among the NNIs, the detection frequencies (DFs) of acetamiprid (ACE), clothianidin, dinotefuran, flonicamid, imidacloprid (IMI), thiacloprid (THCP), and thiamethoxam (THM) were higher than 85%, with the median concentrations ranged from 0.06 ng/L (THCP) to 3.63 ng/L (IMI). The DFs of the TPs descyano-acetamiprid, desmethyl-acetamiprid (DM-ACE), N-[(6-Chloropyridin-3-yl) methyl] methylamine, desnitro-clothianidin, desnitro-imidacloprid, desnitro-thiamethoxam, imidacloprid-urea, and thiamethoxam-urea (THM-urea) were higher than 80%, with the median concentrations ranged from 0.25 ng/L for DM-ACE to 2.41 ng/L for THM-urea. Some agricultural parent fungicides, including azoxystrobin (AZS), carbendazim (CBDZ), difenoconazole, dimethomorph, propiconazole, pyraclostrobin, and tebuconazole (TBCZ), were detected in all the samples; others were also detected in more than 80% of the samples except for fluoxastrobin (12.5%). The median concentrations of the frequently detected fungicides ranged from 0.02 ng/L (trifloxystrobin) to 26.8 ng/L (CBDZ). The DFs of the fungicide TPs azoxystrobin acid (AZS acid), difenoconazole-alcohol, tebuconazole-tert-butylhydroxy (TBCZ-OH), and 5-hydroxymethyl-tricyclazole were higher than 75%, with the median concentrations ranged from 0.09 ng/L (TBCZ-OH) to 1.80 ng/L (AZS acid). The summed concentrations of the NNIs and their TPs at the sampling points varied between 0.23 and 418 ng/L, and the summed concentrations of the selected fungicides and their TPs varied from 0.29 to 1160 ng/L. The spatial distribution of most target analytes revealed an increasing trend in their concentrations from the upstream to downstream Yangtze River (250 times increase in their cumulative concentration). Most target pesticides in this study had significantly higher concentrations during wet season than those during dry season. Furthermore, ecological risk assessment suggested that ACE, IMI, THM, CBDZ, TBCZ, and thifluzamide in some samples (n = 1, 11, 1, 1, 1, and 6, respectively) posed high risks to the ecosystem (risk quotient > 1). Priority attention should be paid to the ecological risk posed by these pesticides. Thirty-seven samples had concentrations of individual target analytes over 100 ng/L and four samples had cumulative concentrations of the target analytes over 500 ng/L, exceeding the European Commission guideline values. Taken together, our findings demonstrate a widespread occurrence of the NNIs, agricultural fungicides, and their TPs in the mainstream of the Yangtze River and potential ecological risks posed by some of them.