Occurrence, distribution, and source track of antibiotics and antibiotic resistance genes in the main rivers of Chongqing city, southwest China

In this study, the occurrence of 14 antibiotics, four corresponding antibiotic resistance genes (ARGs) and two microbial source tracker (MST) indicators were analyzed in two rivers of Chongqing city, southwest China. The results showed that 13 antibiotics were detected in all 12 sites and their detection frequencies were much higher in September, but concentrations were lower than that in March. Of them, erythromycin (ETM) and ofloxacin (OFL) were the predominant antibiotics in both seasons. The remarkably higher concentration of antibiotics in sediments of these rivers than those in other rivers were found. Environmental risk assessment found that four antibiotics posed high risk toward some sensitive algae. For ARGs, their relative abundances were higher in waters than those in sediments, higher in March than in September. Correlation analysis showed that antibiotics were not the exclusive selective pressure of ARGs; many environmental factors like dry matter contents on a mass basis, organic matter, total organic carbon, dissolved organic carbon, temperature, oxidation reduction potential and nitrite could affect the occurrence of ARGs. MST indicators analysis demonstrated that this river basin was largely polluted by human and pig feces, and human feces might be one main source of the four ARGs and five antibiotics.