306-12 The Shifting Redox Landscape: Connecting Legacy and Emerging Contaminants in a Changing Water Cycle

Abstract:

Redox conditions are among the most dynamic controls on contaminant behaviour in aquatic environments, yet they are often studied within single contaminant classes rather than across the groundwater–surface water continuum. Drawing on research from contrasting hydrogeochemical settings, this keynote explores how changing redox environments link legacy inorganic contaminants with emerging pollutants. Studies of arsenic and other redox-sensitive elements show that mobility cannot be predicted from concentration alone. Seasonal and spatial shifts in oxidation and reduction alter mineral stability, aqueous speciation and element partitioning. In As–Cr–V–Se systems, pronounced redox decoupling shows that co-occurring elements may respond differently to the same hydrogeochemical transition. Research on mercury further demonstrates how redox state, pH and groundwater evolution combine to create spatially variable contaminant vulnerability.

At groundwater–surface water interfaces, these processes become especially dynamic. Riverbank filtration can attenuate contaminants, but the degree of attenuation depends on residence time, sediment–water interactions, microbial activity and the sequence of biogeochemical conditions encountered along the flow path. In contrast, river–aquifer connectivity can also transport wastewater-derived contaminants and antibiotic-resistant microorganisms from surface water into groundwater. These insights extend to emerging contaminants such as pesticides, pharmaceuticals, PFAS and microplastics, whose fate may be indirectly shaped by redox-driven changes in mineral surfaces, organic matter, microbial processes and contaminant interactions. Their importance grows during floods, droughts, recharge events and other hydrological extremes that rapidly reorganize subsurface redox architecture.

Overall, this work shifts redox potential from a static water-quality parameter to a shifting redox landscape in which hydrology controls redox transitions, redox transitions reorganize biogeochemical processes and these processes govern contaminant mobility, transformation and attenuation. This framework bridges classical hydrogeochemistry with the challenge of multi-contaminant water systems under climate change.

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