Microbiological, Geochemical and Hydrologic Processes Controlling Uranium Mobility: An
Integrated Field-Scale Subsurface Research Challenge Site at Rifle, Colorado
EMSL Project ID
25608
Abstract
The U.S. DOE faces the challenge of cleaning up and/or monitoring large, dilute plumes contaminated by metals, such as U, Tc and Cr, whose mobility and solubility change with redox status. At the Uranium Mill Tailings Site in Rifle, CO, field-scale experiments with acetate as the electron donor have stimulated metal reducing bacteria to effectively remove U(VI) from groundwater. The shallow depth to groundwater (3-4 m), thin saturated zone (~2.5 m), and well-defined groundwater flow system at the Rifle site facilitated the monitoring of microbial and geochemical processes which led to two important findings: the transition from iron reduction to sulfate reduction significantly decreased the U(VI) bioreduction rate, and U(VI) removal from groundwater continued for 18 months, actually increasing after acetate amendment was terminated. Understanding these behaviors in the context of site-specific hydrologic, geochemical, and biological processes and conditions is critical to the design of optimal biostimulation strategies for prolonging uranium bioremediation.The objective of the research planned for the Rifle site is to gain a comprehensive and mechanistic understanding of the microbial factors and associated geochemistry controlling uranium mobility so that DOE can confidently remediate uranium plumes as well as support long-term stewardship of uranium-contaminated sites. Specifically, we propose to test four hypotheses that address knowledge gaps in the following areas: 1) geochemical and microbial controls on stimulated U(VI) bioreduction by iron-reducers, 2) U(VI) sorption under Fe-reducing conditions, 3) post-biostimulation U(VI) stability and removal, and 4) rates of natural bioreduction of U(VI). The approach specifically targets new knowledge that can be translated into scientifically defensible flow and reactive transport process models of microbially mediated and abiotic reactions, taking a major step toward ERSP's long-term goal to "…incorporate coupled biological, chemical and physical processes into decision making for environmental remediation."
The purpose of this proposal is to primarily investigate geochemical factors controlling U mobility at the site, that fall under hypotheses 1, 2, and 4. Proposals that were geared towards hypothesis 4 and an element of hypothesis 1 were being submitted as separate EMSL user proposals by other Co-PIs of the Rifle team.
Most of the proposed work will be focused on utilizing 57Fe-Mössbauer. The focus of this task is to characterize reactivates' of various pools of Fe under different conditions. The interplay between Fe-mineralogy and microbial communities is complex and unresolved but is essential to develop robust reactive transport models that can be applied at any DOE site. We hope to provide some insights by addressing few geochemical issues detailed below. For example, a) characterize various pools of Fe (Fe-oxides and Fe-silicates) , b) determine long-term changes in Fe-mineralogy up on biostimulation [with (VI) in the feed], c) changes in Fe-mineralogy and remobilization of U by different oxidants (dissolved oxygen, nitrate), d) prolong Fe-reduction by spiking with 57Fe-ferrihydrite, and e) Sorb 57Fe(II) onto background sediment as a function of pH, Fe(II) concentration, bicarbonate concentration, and sediment/water ratio to gain insights into sorbed Fe(II) reactivity with U(VI).
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-06-06
End Date
2008-06-08
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Komlos J, Jr., A Peacock, RK Kukkadapu, and PR Jaffe. 2008. "Long-Term Dynamics of Uranium Reduction/Reoxidation under Low Sulfate Conditions." Geochimica et Cosmochimica Acta 72(15):3603-3615. doi:10.1016/j.gca.2008.05.040