Wastewater effluent contains a complex mixture of biologically active chemicals, in-cludingpharmaceuticals. Many pharmaceuticals have demonstrated deleterious effects to aquatic organisms including endocrine disruption, causing intersex characteristics,and reduced fecundity. These biological impacts can be magnified under low-flow conditions where wastewater effluent substantially contributes to the streamflow. Effluent-dominated streamflow conditions are becom-ing more common in temperate regionssuch as Iowa and are expected to increase due to climate change and population shifts.Current understanding of pharmaceuticals in the aquatic environ-ment does not adequately account for how pharmaceutical mixtures evolve spatiotemporally, or how pharmaceutical mixture composition relates to biological effects. Natural attenuation pro-cesses can either reduce or increase the toxicity of pharmaceuticals towards the aquatic species. Thereisa critical needtounderstand the underlying attenuation mechanismsincluding sorption and biotransformation to better predict the fate, transformation,and associated biological impacts of pharmaceutical mixtures. This research will help protect ecosystem health in freshwater re-sources in Iowa and inform stakeholder decisions such as wastewater treatment plant design, op-eration,and upgrade.