Identification of antibiotic occurrence patterns in a human-influenced river system through dissolved organic matter analysis.

Recognizing the patterns of antibiotic occurrence in river systems is of vital importance for effectively managing antibiotic pollution and reducing risks to aquatic environments. This study investigates the occurrence patterns, sources, and environmental risks of 45 antibiotics belonging to four major classes: quinolones, macrolides, sulfonamides, and tetracyclines in an urban river system (the Xiaoqing River), including its mainstream, tributaries, and downstream sewage treatment plants (STPs). Fluorescence indices of dissolved organic matter (DOM)-humification index (HIX), ultraviolet/visible fulvic acid ratio (r(C2,C1)), and protein/fulvic acid ratio (r(C3,C2))-were analyzed to trace antibiotic sources and dynamics. Results show spatial gradients in diversity of antibiotic compounds and concentrations: STPs effluents discharged the highest antibiotic compounds (covering all major categories detected in the mainstream) and concentrations (STPs outfall > tributaries > mainstream). DOM across all waters exhibited substantial humic substances and autochthonous contributions, indicating watershed-scale homogeneity, while tributary DOM displayed stronger biogenic signals linked to anthropogenic pollution. In the mainstream, the parameter (r (C2, C1)) representing photochemical stability of DOM strongly correlated with principal antibiotic concentrations, suggesting environmental filtering enhances antibiotic persistence. Tributary HIX and elevated r (C3, C2) ratios positively associated with dominant antibiotics, reflecting dual terrestrial sources and unregulated anthropogenic discharges. The findings highlight DOM fluorescence signatures (HIX, r (C2, C1), r (C3, C2)) as critical indicators for tracing antibiotic transport and stabilization mechanisms in human-impacted rivers. This study compellingly emphasizes the crucial role of DOM in signaling the patterns of antibiotics in human-influenced river systems.