Effect of runoff discharge on the environmental levels of 13 veterinary antibiotics: A case study of Han River and Kyungahn Stream, South Korea

•1236 ng/L of oxytetracycline (OTC), 2093 ng/L of tetracycline (TC), and 793 ng/L of chlorotetracycline (ChTC) were detected.•The levels of antibiotics in Kyungahn Stream were high during and after rainy season.•Runoff discharges from wide watershed could result in lower levels of antibiotics.

In this study, the effect of heavy monsoon rains on the environmental levels of 13 veterinary pharmaceuticals was investigated. Kyungahn Stream has an annual average flow rate (AVF) of 4.2 m3/s and receives runoffs from agricultural areas and livestock farms scattered in the upper stream area, and Han River has an AVF of 845 m3/s with four sewage treatment plants with individual capacities of > 1 million m3/day.Grab samples collected in three different rain seasons, before, during, and after rain, were analyzed. Kyungahn Stream and Han River showed completely different patterns of environmental levels of pharmaceuticals: the former had higher detection rates and concentrations of the pharmaceuticals during and after rainy season, implying direct influence by rainfalls, whereas in the latter, higher detection rates were observed before rain (1236 ng/L of oxytetracycline (OTC), 2093 ng/L of tetracycline (TC), and 793 ng/L of chlorotetracycline (ChTC) as the highest values).