Regional Circulation and Fate of Typical Antibiotic Discharges in the Yangtze River Estuarine Region

The discharge of antibiotics from riverine sources into estuaries and adjacent coastal seas is an emerging environmental concern. In this study, we employ seasonal averages derived from a five-year, high-resolution, three-dimensional ocean circulation model to investigate the transport and degradation of a representative antibiotic tracer with a half-life of 31 days, released from the Yangtze River and the Qiantang River into the East China Sea. The model incorporates realistic tides, climatological winds, and seasonal runoff, enabling an examination of typical seasonal conditions. The simulated tracer remains concentrated near the estuarine outlets, with dispersion shaped by the seasonal circulation and stratification. Particle-tracking experiments show distinct pathways: Yangtze-sourced material is rapidly exported southward along the 30 m isobath, traveling about 100 km within 5–10 days, while Qiantang-sourced material exhibits much longer residence times (>30 days) within Hangzhou Bay. Vertical distributions also vary seasonally, with summer stratification confining the tracer to the surface layer and winter mixing dispersing it to deeper waters offshore. These results highlight the contrasting transport behaviors of the two river sources and illustrate how hydrodynamic conditions regulate antibiotic fate in estuarine–coastal environments.