Spatiotemporal distribution of neonicotinoids in urban rivers: implications for rainfall-driven pollution in...

Neonicotinoids (NEOs), insecticides prevalent in urban river environment, contribute to dynamic pollution through their persistence and rainfall-driven dispersion into aquatic systems. However, the fluctuations in NEO concentrations during transient rainfall events and across rainy seasons remain poorly understood. This study examines the spatiotemporal distribution of five NEOs-imidacloprid, acetamiprid, clothianidin, thiamethoxam, and thiacloprid-under rainfall-driven dispersal in urban rivers of Guangzhou, China. Monitoring data and a Level IV fugacity model integrated with hydrodynamic simulations reveal the critical role of rainfall-driven surface runoff in influencing NEOs pollution levels. Concentrations of NEOs were highest in tributaries near urban villages and residential areas, averaging 169.46 ng/L, with thiamethoxam exhibiting the highest levels and thiacloprid the lowest. Peak concentrations were observed during the rainfall period for all river sections, while NEOs concentrations in upstream returned to their original levels after 24 h of rainfall. Rainfall events increased NEOs flux by an average of 4.7 times, with downstream sections experiencing higher and prolonged concentration and flux due to tidal influences and pollutant advection. Model validation showed good predictive ability, with Nash coefficients above 0.9 for flows and acceptable differences for predicted and observed concentrations. These findings underscore the driving effect of rainfall washoff on urban NEOs pollution, facilitating the development of more targeted pesticide management and pollution control measures in urban river system.