Processes occurring at environmental interfaces such as coarse-to-fine and unsaturated-to-saturated sediment transitions profoundly influence groundwater quality. Steep chemical gradients and mixing occur at these interfaces, driving redox, sorption, precipitation, and colloid formation processes. Uranium is one of the most important and problematic groundwater-quality solutes around the globe. Our team has investigated the impacts of environmental interfaces on uranium behavior in contaminated aquifers across the upper Colorado River Basin. Spectroscopic, microscopic, and geochemical studies of sediments and model systems show that coarse-to-fine interfaces at the margins of clayey sediment lenses sustain deep reducing conditions when they lie below the water table. Under these conditions, U(IV) is stable and predominates as complexes on surfaces of particulate organic matter. When water table fluctuations driven by seasonal or multi-year drought expose reduced sediment lenses to capillary fringe conditions, they experience sharply intensified biogeochemical redox cycling. Under these conditions, both U(IV) and U(VI) can be released to the aqueous phase, allowing for subsequent transport. At the same time, both U(IV) and U(VI) can precipitate in minerals that subsequently are slow to re-dissolve under prevailing aqueous conditions. Thus, the <i>geochemical</i> stabilization or release of uranium is mediated by <i>hydrological conditions and parameters </i>including sediment moisture and grain size/permeability. These factors help to explain regional uranium behavior, plume persistence, and water quality under the variable hydrologic conditions that typify the upper Colorado River Basin.

Influence of environmental interfaces on molecular processes controlling uranium behavior

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.