Environmental Remediation Sciences Program

Field Scale Research

Microbiological, Geochemical and Hydrologic Processes Controlling Uranium Mobility: An Integrated Field-Scale Subsurface Research Challenge Site at Rifle, Colorado

 

Principal Investigator:
Philip E. Long
Pacific Northwest National Laboratory
Field Site Manager and co-Manager:
Richard Dayvault and Stan Morrison
SM Stoller – DOE Grand Junction Office

Co-Principal Investigators:
Jill Banfield (UC/Berkeley), Darrell Chandler (Akonni Biosystems), Jim Davis (USGS), Bob Hettich (ORNL), Peter Jaffe (Princeton U.), Lee Kerkhof (Rutgers U.), Ravi Kukkadapu (PNNL), Mary Lipton (PNNL), Aaron Peacock (UTK), Nathan VerBerkmoes (ORNL), Kenneth H. Williams (LBNL),
Steve Yabusaki (PNNL)

ABSTRACT
The U.S. Department of Energy faces the challenge of cleaning up and/or monitoring large, dilute plumes contaminated by metals, such as U 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 uranium [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 proposed 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 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). Hypotheses will be tested with a focused set of field and lab experiments that use recently developed sciences of proteogenomics and stable isotope probing to track microbial metabolic status and specific organisms responding to acetate amendment. We will directly relate this information to changes in Fe redox status and sulfide minerals, with field-scale changes detected by non-invasive hydrogeophysics, including 3-D complex resistivity tomography. 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.”

Experimental Test Plot at the Old Rifle UMTRA site in Rifle, CO

(A) Location of the proposed Rifle Field Sie in Colorado. (B) Map of the site showing estimated U(VI) concentration in groundwater as of approximately 1998 and location of existing experimental plots on the site. (C) Photograph of the site looking east.
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