Collaborative Research: Enhanced Biogeochemical Flushing of Uranium in Groundwater

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Approximately 115 million people in the United States use groundwater as source of drinking water. Many groundwater aquifers are contaminated with uranium from both natural and anthropogenic sources. Prior to its usage as source of drinking water, uranium contaminated groundwater needs to be treated to achieve a target concentration below the EPA MCL (Maximum Concentration Limit) of 30 micrograms per liter (µg/L). In-situ immobilization is currently the leading remediation technology for uranium-contaminated aquifers. This approach relies on the manipulation of the redox biogeochemistry of uranium to reduce and convert mobile uranyl [U(VI)] ions to the mineral uraninite (UO2) which is generally considered immobile in uranium-contaminated aquifers. However, uraninite is highly susceptible to re-oxidation and re-mobilization by oxidants such as oxygen, thus limiting the efficacy of in-situ immobilization as a remediation technology. The goal of this project is to investigate the enhanced biogeochemical mobilization and flushing of uranium from contaminated aquifers as an alternative remediation technology. To advance this goal, the Principal Investigators (PIs) propose to carry out an integrated research program with field, laboratory, and modeling studies to characterize and unravel the biogeochemical reactions and microbial processes that control uranium mobilization in contaminated aquifers. The successful completion of this research will benefit society through the generation of new fundamental knowledge, data and validated models to advance the design and implementation of more efficient and cost-effective remediation technologies for uranium contaminated aquifers. Additional benefits to society will be accomplished through education and outreach including the mentoring of one graduate student and one post-doctoral scholar at the University of Wisconsin-Milwaukee and one graduate student at the University of Wisconsin-Madison.The remediation of uranium-contaminated aquifers has been largely focused on immobilizing uranium by stimulating the abiotic and biotic processes and reactions that control the reduction and conversion of soluble and mobile uranyl [U(VI)] ions to stable and immobile uraninite minerals. However, in situ immobilization has limited effectiveness and the estimated times to naturally flush uranium to below the EPA MCL often exceed hundreds of years. The overarching goal of this research is to understand how to better tune and manipulate the biogeochemical processes and reactions that control the mobility of uranium in contaminated aquifers with the aim of enhancing uranium mobilization and flushing from these aquifers through the injection of oxygen and bicarbonate enriched surface water. The specific objectives of the research are to: 1) Characterize and unravel the biogeochemical mechanisms of uranium mobilization using geochemical modeling and available thermodynamic and kinetic reaction data, 2) Conduct laboratory experiments to investigate the effect of microbial activity on the mobility of uranium under oxygen- and carbonate-rich conditions, and 3) Determine flow and reactive transport parameters to support the implementation and validation of a uranium transport/mobilization model using finite difference numerical modeling and available uranium mobilization data from single-well tracer experiments. The successful completion of this project has the potential for transformative impact through the generation of fundamental knowledge and modeling tools to advance the design and implementation of in-situ enhanced mobilization and flushing as an alternative remediation technology for uranium contaminated aquifers. To implement the educational and outreach goals of this project, the Principal Investigators (PIs) propose to integrate the findings from this research into an existing course at the University of Wisconsin-Madison. In addition, the PIs plan to leverage existing programs at their respective institutions to design and build an exhibition panel on metal contamination in groundwater and aquifers in Wisconsin that will be displayed at the Northern Great Lakes Visitor Center for viewing by Summer 2024.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.