Hydrological controls on antibiotics and mixture risks in upland and lowland rivers: a dual-tracer study combining antibiotics and DOM optical properties

Land use and hydrology drive distinct spatiotemporal antibiotic patterns. • Upland urban rivers peak in dry seasons; lowland agricultural rivers in wet. • DOM co-varies with antibiotics and mediates mobility via composition. • Effective control requires season- and region-specific management strategies. Understanding how antibiotics behave in upland and lowland rivers draining mixed urban-agricultural catchments is essential for hydrology-driven water quality management. We investigated 48 antibiotics and Dissolved Organic Matter (DOM) optical properties at 49 sites along 12 rivers spanning upland headwaters influenced by urban land use and lowland reaches draining predominantly agricultural plains during dry (low-flow) and wet (high-flow) seasons. By combining antibiotic profiles with DOM fluorescence and absorbance indices, including PARAFAC components, we applied a dual-tracer framework that uses antibiotics together with DOM optical properties to interpret spatial and seasonal patterns under contrasting hydrological and land-use settings. Upland rivers subject to urban point sources showed pronounced dry-season antibiotic concentration peaks, consistent with wastewater-dominated inputs under limited dilution. In contrast, lowland rivers draining intensively cultivated areas exhibited the highest cumulative mixture‑level ecological risks, driven by cocktails of antibiotics transported from both urban and agricultural sources. Protein‑like DOM signals were associated with human-use antibiotics in urban‑influenced upland reaches, whereas humic-like components aligned with veterinary antibiotic dominance in agricultural lowlands, indicating stronger runoff contributions during wet conditions. Overall, our results show that flow regime and land use jointly control the redistribution of antibiotic concentrations and mixture risks across multiple rivers, and demonstrate the value of a dual-tracer approach that combines antibiotics with DOM optical properties to diagnose point- and diffuse-source contributions in mixed-use river catchments. Download: Download high-res image (187KB) Download: Download full-size image