From stormwater to surface water: An overlooked pesticide transport pathway in urban watersheds.

The concentration profiles of various pesticides in the context of urban stormwater drainage systems were investigated in this study. Results showed that the average concentrations of four common pesticides reaching surface water during wet weather was 1.3-1.7 times that in dry weather. The concentrations of N,N‑diethyl-3-methylbenzamide (DEET), 2,4-dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-chlorophenoxyacetic acid (MCPA) and 2‑chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine (atrazine) in samples from stormwater drainage were in the range of 188.39-7047.57 ng/L, 1.22-3.49 ng/L, 0.48-17.97 ng/L, 17.00-59.24 ng/L, respectively. Herbicides mainly enter surface water through stormwater runoff via rainfall-induced scouring of plant surfaces. However, wastewater passing directly into surface waters, caused by illicit connections between stormwater and sewers, is the main source of insect repellent in surface water due to its widespread use in household scenarios. Notably, the mass loads of pesticides discharged through stormwater were 1.6-10.8 times higher than those in wastewater discharge. Evaluation of the ecological risk of the pesticides demonstrated that elevated concentrations of DEET and atrazine in surface waters may pose the highest levels of chronic toxicity to aquatic organisms. In addition, pesticides entering surface water serve as precursors of disinfection of by-products (DBPs) in downstream drinking water treatment plants, posing a threat to drinking water quality and potentially affecting human health.