Incorporating Transformation Products for an Integrated Assessment of Antibiotic Pollution and Risks in Surface Water

The widespread presence of antibiotics in aquatic ecosystems is a global challenge, yet the occurrence and risks associated with their transformation products (TPs) remain poorly understood. This study investigated the occurrence and potential risks of antibiotics and their TPs in water along the Chaobai River in Beijing. We used high-resolution mass spectrometry and an integrated target, suspect, and nontarget screening approach to identify 21 parent antibiotics and 78 TPs among 90 water samples, with the majority from macrolides and sulfonamides. Notably, target quantification and machine-learning-assisted semiquantification revealed that the cumulative concentrations of TPs were higher than the cumulative concentrations of parent compounds, with average contributions of TPs ranging between 50.7 and 63.7%. Most downstream water samples were largely influenced by domestic sewage, as indicated by the significantly higher concentrations and proportions of TPs, as well as the greater diversity in their composition profiles compared to upstream and reservoir samples. Moreover, of the 78 TPs, 26.9, 67.9, and 6.4% exhibited greater persistence, mobility, or toxicity than their parent antibiotics, respectively. Sixteen macrolide TPs presented both greater ecological risks to aquatic organisms and higher resistance selection risks than their parent antibiotics. TPs contributed substantially to the overall antibiotic-related risks by an average of between 31.2 and 54.1%. This study highlights the occurrence of antibiotic TPs in river water, underscoring the need to consider TPs in comprehensive risk assessments of antibiotics.