Bridging single-species research and mixture reality: Emerging contaminants fate and transport in vadose zones.

Vadose zones serve as interfaces controlling contaminant transport from the land surface to underlying aquifers. While traditional research has largely focused on individual contaminant species, real-world contamination often involves complex mixtures containing dozens to thousands of chemical species. This disconnect creates considerable challenges for predicting contaminant behavior, assessing environmental risks, and developing effective remediation strategies. This review examines complex contaminant mixtures in vadose zones, with emphasis on per- and polyfluoroalkyl substances, pharmaceuticals and personal care products, and hydraulic fracturing fluid additives and contaminants. We synthesize recent advances in understanding mixture behavior, including competitive sorption at air-water interfaces and solid surfaces, co-facilitated transport mechanisms, and transformation dynamics under co-contaminant conditions. Field observations from contaminated sites reveal that mixture effects alter transport rates, retention capacities, and degradation pathways relative to single-species predictions. While vadose zones function as persistent secondary sources, transient saturation conditions can enhance contaminant transport by an order of magnitude relative to constant-flow predictions. Current modeling frameworks remain limited in their capability to account for complex physicochemical interactions in contaminant mixtures, particularly under transient flow and heterogeneous environments. We identify and describe six priority areas for new or enhanced research to bridge current knowledge gaps: mixture sorption and competitive transport, transformation products and reaction pathways, field-scale validation studies, multi-mechanism remediation technologies, integrated mixture toxicity assessment, and climate change and evolving land use impacts.