Occurrence of antibiotics in an agricultural watershed in south-central Idaho

The polar organic compound integrative sampler (POCIS) is a tool that has been effectively used to passively sample organic pollutants in water. In this study, POCIS were used to investigate the occurrence of 21 veterinary and human antibiotics and a beta agonist (ractopamine) in irrigation return flows of an intensively managed agricultural watershed in south-central Idaho. Erythromycin-H2O, monensin, oxytetracycline, ractopamine, sulfadimethoxine, sulfamethazine, sulfamethoxazole, trimethoprim, and tylosin were detected at frequencies ranging from 3.1 to 62.5%, with monensin having the highest rate of detection. Based on uptake rates from published literature, aqueous concentrations were estimated to range from 0.003 ng L-1 for tylosin to 249 ng L-1 for oxytetracycline. While monensin had the lowest time-weighted average concentration (0.24 ng L-1), the fact that it was the most frequently detected compound suggests that a portion of the pharmaceuticals were entering the return flows as runoff from animal manure-treated fields during the irrigation season. The same pharmaceuticals (except oxytetracycline, sulfamethazine, and tylosin) were also detected at a background site that consisted of diverted Snake River water that is the source of irrigation water for the watershed. Therefore, antibiotics, as well as ractopamine, are likely being applied to soils and transported with runoff to irrigation return flows. This study provides additional evidence that surface waters within agricultural watersheds contain low-level pharmaceuticals associated with veterinary and human uses