Occurrence and Spatiotemporal Dynamics of Pharmaceuticals in a Temperate-region Wastewater Effluent-dominated Stream: Variable Inputs and Differential Attenuation Yield Evolving Complex Exposure Mixtures.

Effluent-dominated streams are becoming increasingly common in temperate regions and generate complex pharmaceutical mixture exposure conditions that may impact aquatic organisms via drug-drug interactions. Here, we quantified spatiotemporal pharmaceutical exposure concentrations and composition mixture dynamics during baseflow conditions at four sites in a temperate-region effluent-dominated stream (upstream, at, and progressively downstream from effluent discharge). Samples were analyzed monthly for 1 year for 109 pharmaceuticals/degradates using a comprehensive U.S. Geological Survey analytical method and biweekly for 2 years focused on 14 most-common pharmaceuticals/degradates. We observed a strong chemical gradient with pharmaceuticals only sporadically detected upstream from the effluent. Seventy-four individual pharmaceuticals/degradates were detected, spanning five orders of magnitude from 0.28 to 13,500 ng/L, with 38 compounds detected in >50% of samples. "Biweekly" compounds represented 77±8% of overall pharmaceutical concentration. The antidiabetic-drug metformin consistently had the highest concentration with limited in-stream attenuation. The antihistamine-drug fexofenadine inputs were greater during warm than cool-season conditions, but also attenuated faster. Differential attenuation of individual pharmaceuticals (i.e., high=citalopram; low=metformin) contributed to complex mixture evolution along the stream reach. This research demonstrates variable inputs over multiple years and differential in-stream attenuation of individual compounds generate evolving complex mixture exposure conditions for biota, with implications for interactive effects.