From Fields to Rivers: Tracking Neonicotinoid-Dominated Pesticide Contamination in Agro-Urban Waters of Northeastern India

India contributes approximately 3.75% of global pesticide consumption and applies comparatively low amounts per unit area (≈0.5 kg ha⁻¹); however, pesticide usage is strongly insecticide-dominated, with insecticides accounting for nearly 80% of total application. This study examines the occurrence, distribution, and transformation of pesticides in surface water and groundwater across agro-urban catchments of Guwahati, Assam, a rapidly urbanizing region influenced by intensive agriculture. Pesticides were widely detected in surface waters, reflecting combined inputs from agricultural runoff, irrigation return flows, and stormwater, whereas groundwater generally contained only trace concentrations (<1 ng L⁻¹), indicating effective subsurface attenuation. Surface water concentrations ranged from 0.01 to 92 µg L⁻¹, with the highest cumulative loads (>100 ng L⁻¹) observed in agro-urban catchments directly receiving agricultural drainage. Neonicotinoid insecticides dominated the chemical profiles, contributing more than 50% of total pesticide mass, with thiamethoxam, imidacloprid, clothianidin, and acetamiprid detected in over 70% of samples at typical concentrations of 10¹–10³ ng L⁻¹ and maxima approaching ~90 µg L⁻¹ at agriculturally influenced sites. Several transformation products, including thiamethoxam-urea and desnitro-imidacloprid, were consistently detected and in some cases exceeded parent compounds, accounting for 20–40% of the total neonicotinoid signal and indicating slow degradation and sustained environmental release. In contrast, herbicides such as 2,4-D, diuron, and metolachlor occurred less frequently (<40%) and at lower concentrations (<5 µg L⁻¹). Strong positive correlations among neonicotinoids and their metabolites (r > 0.7) suggest shared sources and transport pathways, while upstream sites showed lower pesticide diversity, underscoring the dominant influence of agro-urban activities on surface water contamination.