Weakly hydrophobic antibiotics leaching in an alpine soil of the Tibetan Plateau in responding to macropore flow

In alpine meadow regions, macropore flow is a critical but inadequately understood pathway for antibiotic transport. The complex relationship between macropore structure, flow dynamics, and solute properties presents a significant research gap. Methodological limitations hinder the accurate characterization of solute migration mechanisms due to complex macropore structures. This study investigated sulfadiazine (SD) and florfenicol (FF) migration behaviors in alpine meadow soil using leaching experiments in soil columns with artificially constructed macropores. Findings reveal that while greater macropore flow intensity (> 1.75 cm/h) and connectivity accelerate solute transport, the effect is not linear. A fundamental shift in transport control occurs with changing macropore flow intensity. At lower velocities, solute mobility is primarily controlled by adsorption reactivity, particularly for the strongly reactive SD. Conversely, at high velocities, hydrodynamic dispersion becomes the dominant factor governing the transport. Moderate dispersion (1.7–1.8 cm 2 /h) optimizes retention by increasing adsorption opportunities, while high dispersion (3.06 cm 2 /h) accelerates solute transport and reduces retention. This causes the transport of the strongly reactive SD to become dispersion-driven, significantly narrowing the behavioral differences between it and the weakly reactive FF. This highlights that under certain hydraulic conditions, physical transport processes can override chemical properties in controlling contaminant fate.