About the Site
- Former uranium and vanadium milling site
- Uranium Mill Tailings Remedial Action (UMTRA) Cleanup: 1992 - 1996, certified by the Nuclear Regulatory Commission January 1998
- $119.17M to move 4,967,451 dry tons of tailings to Estes Gulch disposal cell
- Contaminants of Concern: uranium, vanadium, and selenium
- Current Owner/Area: City of Rifle, 22 acres
- Status: Managed under the DOE Long-Term Surveillance and Monitoring Program. Remedial action is Monitored natural attenuation.
Site Issues
The site was expected to naturally flush its uranium contamination within 100 years during which time institutional control must be maintained. While the Old Rifle Site is still expected to meet this goal, the rate of uranium loss from groundwater is lower than expected. Further, the role of microbial processes and the range of processes responsible for uranium removal are not well understood, limiting our ability to predict long-term fate and transport of uranium in such systems. Behavior of the uranium plume at the site over the last several years suggests that a remaining source of uranium in the vadose zone source may be important in maintaining plume concentrations.
Hydrology
The site has an unconfined alluvial aquifer The surface of the aquifer (i.e., the water table) lies approximately 3.5m below the ground surface. Groundwater flows through the aquifer in an arc with recharge up gradient from surrounding highlands or the Colorado River. The aquifer is heterogeneous but typical hydraulic conductivities are 7.5 m/day (geometric mean of 16 recent measurements).
Previous Scientific Experiments
- Previous field experiments at the Old Rifle site demonstrated that the introduction of acetate stimulated metal reduction in the subsurface resulting in a loss of soluble uranium from the groundwater.
- Loss of soluble uranium correlated with a substantial shift in the subsurface microbial community towards members of the Geobacter family during stimulated metal reduction.
- Geobacteraceae are known to couple acetate oxidation to the reduction of Fe(III)oxides and U(VI); loss of soluble uranium was attributed to the stimulation of Geoabacteraceae and enzymatic reduction of soluble U(VI) to insoluble U(IV).<
- Upon continued acetate addition, the subsurface microbial community shifted towards sulfate reduction and a microbial community dominated by sulfate reducing organisms.
- At this site, loss of U(VI) from groundwater under sulfate-reducing conditions was less pronounced than under metal-reducing conditions and the conclusion was that metal-reducing bacteria were required for continued U(VI) reduction.
- The general correlation of stimulated loss of soluble U(VI) and enrichment of Geobacteraceae in the subsurface at the Rifle site has been replicated in separate experimental plots multiple times.