My dissertation aims to determine the occurrence, fate and implications of pharmaceutical mixtures in a temperate-region wastewater effluent-dominated stream. This research demonstrates variable inputs over multiple years and differential in-stream attenuation mechanisms generate evolving complex mixture exposure conditions for biota, with implications for interactive effects. Effluent-dominated streams are becoming common due to population growth and climate change, thus, understanding the dynamics of complex pharmaceutical mixtures and their potential impacts on aquatic organisms is crucial.