From river to groundwater: Antibiotics pollution, resistance prevalence, and source tracking.

The extensive use of antibiotics has led to their frequent detection as residues in the environment. However, monitoring of their levels in groundwater and the associated ecological and health risks remains limited, and the impact of river pollution on groundwater is still unclear. This study focused on the highly urbanized Maozhou River and its groundwater. Forty-five antibiotics and microbial community composition were analyzed by high-resolution LC-MS/MS and 16S rRNA gene sequencing, respectively. These endpoints were measured in sediment, surface- and groundwater sampled during wet and dry seasons, while isolation and resistance profiling of Escherichia coli was performed in groundwater. This study aimed to assess the ecological and health risks posed by antibiotics in the Maozhou River and its groundwater, to assess the prevalence and type of antimicrobial resistance in Escherichia coli, and to trace the sources of antibiotic resistance genes (ARGs) in groundwater. Multiple antibiotics detected in the river and sediment were predicted to pose high risks to algae growth and bacterial resistance selection. In groundwater, the antibiotics erythromycin and norfloxacin were predicted to pose a medium risk to algae and a low risk towards bacterial resistance. Furthermore, significant positive correlations were observed between several predominant bacterial phyla in the river and groundwater and the detected antibiotics, suggesting a possible effect of local antibiotic residues on bacterial community composition. Antimicrobial susceptibility testing of 76 Escherichia coli isolates revealed 74 % exhibited resistance to at least one tested antibiotic and 7.9 % exhibiting multidrug resistance, which was confirmed by ARG-targeted PCR analysis. SourceTracker analysis of ARGs in groundwater indicated that ARG contamination in shallow groundwater was primarily from river sediments, while contamination in deeper groundwater originated mainly from river water. The results emphasize the need to address river pollution, as it directly impacts groundwater quality, particularly in areas with severe antibiotic contamination.