Unraveling transport mechanisms and rainfall event-driven controls on non-point source antibiotic pollution by high-resolution process-based modeling.

In agro-ecosystems, heterogeneous sources and variable pathways of non-point source (NPS) pollution complicate the understanding of antibiotic behavior at the watershed scale. A quantitative understanding of the mechanisms governing source-transport-fate processes remain limited, particularly under varying rainfall conditions. This study used a distributed and process-based model to simulate daily fluxes of four tetracyclines (TCs) in an agricultural watershed and to identify key transport mechanisms and rainfall-driven controls. Partial least squares path modeling revealed that transport processes significantly influence TCs fate, with riverine processes outweighing terrestrial transport and source input. Rivers dissipated 74.3 % of terrestrial TCs, significantly related to cumulative riverine transport distance. Riverbed sediment acted as a source for 92.3 % of the year via diffusion, resuspension, and deposition, and high-flow conditions converted it from source to sink. Extreme rainfall events, heavy rainfall events, and prolonged rainfall events were identified as the three patterns driving antibiotic transport and fate from event-based results. Prolonged rainfall events, often overlooked, pose chronic risks through groundwater discharge and sediment diffusion. These findings underscore the critical role of in-stream processes and three distinct rainfall event patterns in governing antibiotic pollution, highlighting the necessity of integrating riverine management with rainfall-driven strategies in watershed-scale pollution control.