Hydrological and Biogeochemical Influences on Contrasting Environmental Pathways of Chlorothalonil and Its Transformation Product in Agricultural Systems.

Agricultural pesticides are major contributors to nonpoint-source pollution, yet the ways in which hydrological and biogeochemical processes influence the environmental behavior of pesticides and their transformation products (TPs) remain poorly understood. Here, we investigated the fate of the fungicide chlorothalonil (CHL) and its primary TP, SDS-3701, in a representative tomato production system in South Florida using Pesticide in Water Calculator (PWC) simulations. A process-oriented sensitivity analysis was then employed to clarify how hydrological and biogeochemical processes influence their environmental pathways. Thirty climate-management scenarios encompassing variability in precipitation, irrigation, and application practices were evaluated to assess how environmental drivers influence pesticide transport and persistence. The simulations reveal contrasting environmental pathways for CHL and SDS-3701. CHL behaves as a short-lived contaminant, with surface-water concentrations exhibiting sharp, event-driven pulses reaching ∼30 μg/L and declining within approximately one week due to rapid degradation. In contrast, SDS-3701 is highly mobile and weakly sorbing, leading to sustained downward migration and progressive groundwater accumulation reaching ∼10 μg/L under intensive application over decadal timescales. Hydrologic variability, particularly changes in water availability from precipitation and subsurface irrigation, alters the proportional contributions of transport, sorption, and degradation to overall exposure variability, exerting a stronger influence on exposure than application season or rate. These findings underscore the importance of explicitly considering TPs when evaluating pesticide exposure and highlight how hydrologic variability influences contamination risks in agricultural landscapes. These results provide a mechanistic basis for interpreting parent-TP behavior under variable hydrologic forcing and can inform improved monitoring and risk-assessment strategies for pesticide-impacted agricultural regions.