Nutrient–Pesticide Coupling Enables Proxy-Based Detection of Neonicotinoids in Complex Water Systems

Agricultural intensification and urban expansion are driving the overloading of freshwater systems with pesticides, antibiotic/pharmaceutical and personal care products, yet predictive frameworks for their occurrence remain limited. Here we demonstrated that nutrient-pesticide coupling provides a mechanistic basis for proxy detection of neonicotinoids across complex water systems. Using an integrated evaluation across rivers, wastewater, and groundwater in Mexico, we quantified pesticides and antibiotic/pharmaceutical and personal care products (APPCPs) alongside hydrochemical parameters. Neonicotinoids and their transformation products co-occurred consistently with fertilizer-derived nutrients (nitrate, nitrite, sulfate) and trace metals (Fe, Cu), revealing strong hydrochemical coupling driven by shared application regimes and transport pathways. Multivariate and correlation analyses show that nutrient signatures can reliably predict neonicotinoid occurrence and intensity, particularly in agro-urban interfaces where irrigation return flows and wastewater inputs converge. Despite generally low acute ecological risks (RQ < 0.1), the persistent co-occurrence of pesticides and antibiotics indicates complex mixture exposures with potential chronic and antimicrobial resistance implications. These findings establish nutrient-based proxy indicators as a scalable, cost-effective framework for detecting pesticide pollution, advancing integrated monitoring strategies for emerging contaminants in data-scarce regions.