Climate change and urbanization are increasing the influence of municipal wastewater effluent on
receiving waters, making wastewater effluent-dominated streams common worldwide, including
in temperate regions. This phenomenon increases loading of contaminants of emerging concern
(CECs) including pharmaceuticals, personal care products, pesticides, and industrial chemicals
from wastewater treatment plants (WWTPs) to drinking water supplies (i.e., de facto reuse) and
ecological systems. Treated effluent from WWTPs releases CEC mixtures that vary
spatiotemporally, which generate complex exposure conditions for biota and potential for
deleterious interactive effects (e.g., drug-drug interactions). When individual CECs present in
mixtures are removed from the aqueous phase via sorption or degrade from the stream at different
rates (i.e., differential attenuation), exposure conditions for biota change. Attenuation rates of
CECs we measured in the field are much greater than those in batch degradation tests—suggesting
other relevant stream processes. Currently, we do not fully understand the role of groundwater-
surface water exchange on complex mixture evolution. There is a critical need to evaluate the role
of groundwater-surface water exchange as a driver of complex mixture evolution using next-
generation high-resolution non-target analytical approaches to quantify CEC spatiotemporal
dynamics.