Surveillance and Ecological Risk Evaluation of Common Antibiotics in Key Discharge Points Along the Wei River，Guanzhong Basin

：To address the challenges of complex matrix interference and cumbersome sample process⁃ ing in wastewater analysis，we developed an online solid phase extraction and liquid chromatography-tandem mass spectrometry（SPE/LC-MS/MS）method. Utilizing an isotope dilution internal standard approach，this technique facilitates automated processing and single-step quantitative detection of 42 common antibiotics，with a linear range of 1-500 ng/L and r² values greater than 0. 97 for most antibi⁃ otics. Using this innovative method，we conducted a comprehensive monitoring and risk assessment of 42 antibiotics at 24 key discharge points along the Wei River Basin from Xianyang to Xi'an across wet，normal，and dry water periods. Our findings revealed the presence of 42 antibiotics across six major classes， with total concentrations ranging from 5 110. 3 to 21 378. 1 ng/L. Clarithromycin， ofloxacin， and sulfanethoxazole emerged as significant contaminants， exhibiting high detection fre⁃ quencies and concentrations. Seasonal variations in total antibiotic concentrations were noted，with the highest levels recorded in January（dry period），followed by May（normal period），and the low⁃ est in September （wet period）. Using the risk entropy method for assessment， we identified that ofloxacin， azithromycin， and sulfanethoxazole reached high ecological risk levels at multiple moni⁃ toring sites. This study not only provides a highly sensitive and efficient detection method for monitor⁃ ing antibiotics in surface water and wastewater but also offers a scientific basis for investigating the sta⁃ tus of antibiotic pollution and developing effective remediation strategies in the Guanzhong section of the Wei River Basin.