Microgeographic Approach to the Occurrence, Fate, and Risk of Antibiotic Residues in Wastewater Effluent-Impacted Streams across Rural and Suburban Areas of Minnesota

Knowledge gaps persist in our understanding of the presence of antibiotics and associated risks in aquatic systems. In this work, the occurrence, persistence, and risk of different classes of antibiotics in streams in low population density areas of Minnesota, USA, were assessed. Samples from two creeks (low-order streams) and a river (fourth-order stream) impacted by wastewater effluent were collected at five locations: upstream of discharge, the mixing zone, and three downstream sites (maximum 4 km). The highest antibiotic concentrations were detected at the mixing zone (up to 823 ng L^–1), with sulfonamides being the most frequently detected, followed by fluoroquinolones, trimethoprim, and macrolides. The antibiotic levels decreased downstream following a first-order apparent attenuation constant that ranged between 2.3 × 10^–5 and 6.5 × 10^–3 m^–1, corresponding to temporal rate constants of 8.0 × 10^–6 to 1.9 × 10^–3 s^–1. Spatial and temporal in-stream attenuation patterns differed among streams, reflecting differences in hydrological conditions. Antibiotic emissions varied across the sites, with the highest loads for sulfonamides in first-order streams and elevated values for macrolides and fluoroquinolones in the fourth-order stream. A risk assessment showed that, despite overall decreased risks, low-order streams affected by point sources may still pose threats to ecological species and may promote antimicrobial resistance even in areas with low human population density.