Antibiotic transport requires a renewed focus on baseflow as a critical non-point source pathway

Current research predominantly assumes that riverine antibiotic loads primarily derive from elevated surface runoff. However, the significance of baseflow is largely overlooked due to a lack of quantitative estimation and mechanistic understanding. This study analyzed the role of baseflow on transporting tetracyclines (TCs) from non-point sources in an agricultural catchment. We found that baseflow load accounted for ∼40% (39.8% annually and 45.8% monthly) of the riverine load of TCs. The threshold effect of baseflow index indicates that baseflow dominates the low-level but ongoing loading of TCs for 94.8% of the time in a year. Baseflow yield of TCs decreased with increasing drainage area size but showed no clear pattern across source input gradients, suggesting that baseflow loading of TCs is primarily transport-limited. Export regimes of riverine and baseflow TCs registered chemodynamic pattern. Baseflow exhibited a stronger flushing pattern for tetracycline and chlorotetracycline compared to quickflow due to pool mass, hydrological transport, and biogeochemical processes in the subsurface environment. Our results highlight that baseflow is a chronic pathway that constantly transports considerable TCs to receiving rivers and significantly influence TCs export behaviors. Management and control of antibiotic pollution at the catchment scale require mediating surface and subsurface transport mechanisms and limiting sources.