Occurrence, spatiotemporal distribution, and ecological risks of antibiotics in a river basin of eastern China.

The widespread use of antibiotics has resulted in their frequent occurrence in aquatic environments, raising ecological concerns; however, conducting precise monitoring and tracing of a broad spectrum of antibiotics concurrently in complex river networks at the watershed scale still faces significant challenges. This study investigated the pollution characteristics of 32 target antibiotics in a major river basin in eastern China. A total of 184 river water samples collected across 2024-2025 were analyzed. Both detection frequencies (DFs, 0-82.86%) and concentrations (0-36,577 ng/L) of the antibiotics exhibited pronounced spatiotemporal and compound-specific variations. The risk quotient (RQ), calculated as the ratio of the detected concentration to the predicted no-effect concentration, ranged from 0 to 18.92. Sulfachloropyridazine (SCP), sulfamethoxazole (SMZ), and sulfathiazole (STZ) were identified as priority antibiotics for routine monitoring due to high DFs (≥ 50%) or high RQs (> 1 in some sampling points). In contrast, 14 antibiotics that were not detected in any samples and 10 others that exhibited consistently low DFs (< 20%) and low RQs (< 0.1) were assigned lower monitoring priority. Spatial analysis revealed two independent pollution hotspots dominated by SCP and by SMZ and florfenicol, likely sourced from livestock farming and urban sewage, respectively. The apparent attenuation coefficient of SCP, derived from the field monitoring data, revealed both seasonal and spatial variations, ranging from 0.28 to 2.24 km-1. These findings provide a high-resolution spatiotemporal dataset and identify key antibiotics for future monitoring and targeted management in similar river systems.