Uranium Mill Tailings Remedial Action Research Articles

Microbial Functional Gene Diversity with a Shift of Subsurface Redox Conditions during In Situ Uranium Reduction

To better understand the microbial functional diversity changes with subsurface redox conditions during in situ uranium bioremediation, key functional genes were studied with GeoChip, a comprehensive functional gene microarray, in field experiments at a uranium mill tailings remedial action (UMTRA) site (Rifle, CO). The results indicated that functional microbial communities altered with a shift in the dominant metabolic process, as documented by hierarchical cluster and ordination analyses of all detected functional genes. The abundance of dsrAB genes (dissimilatory sulfite reductase genes) and methane generation-related mcr genes (methyl coenzyme M reductase coding genes) increased when redox conditions shifted from Fe-reducing to sulfate-reducing conditions. The cytochrome genes detected were primarily from Geobacter sp. and decreased with lower subsurface redox conditions. Statistical analysis of environmental parameters and functional genes indicated that acetate, U(VI), and redox potential (E(h)) were the most significant geochemical variables linked to microbial functional gene structures, and changes in microbial functional diversity were strongly related to the dominant terminal electron-accepting process following acetate addition. The study indicates that the microbial functional genes clearly reflect the in situ redox conditions and the dominant microbial processes, which in turn influence uranium bioreduction. Microbial functional genes thus could be very useful for tracking microbial community structure and dynamics during bioremediation.

In‐well sediment incubators to evaluate microbial community stability and dynamics following bioimmobilization of uranium

AbstractAn in‐well sediment incubator (ISI) was developed to investigate the stability and dynamics of sediment‐associated microbial communities to prevailing subsurface oxidizing or reducing conditions. Herein we describe the use of these devices at the Old Rifle Uranium Mill Tailings Remedial Action (UMTRA) site. During a seven‐month period in which oxidized Rifle Aquifer background sediment (RABS) were deployed in previously biostimulated wells under iron‐reducing conditions, cell densities of known iron‐reducing bacteria, including Geobacteraceae, increased significantly, showing the microbial community response to local subsurface conditions. Phospholipid fatty acid (PLFA) profiles of RABS following in situ deployment were strikingly similar to those of adjacent sediment cores, suggesting ISI results could be extrapolated to the native material of the test plots. Results for ISI deployment with laboratory‐reduced sediments showed only slight changes in community composition and pointed toward the ability of the ISI to monitor microbial community stability and response to subsurface conditions. © 2009 Wiley Periodicals, Inc.

Multicomponent reactive transport modeling of uranium bioremediation field experiments

A reaction network integrating abiotic and microbially mediated reactions has been developed to simulate biostimulation field experiments at a former Uranium Mill Tailings Remedial Action (UMTRA) site in Rifle, Colorado. The reaction network was calibrated using data from the 2002 field experiment, after which it was applied without additional calibration to field experiments performed in 2003 and 2007. The robustness of the model specification is significant in that (1) the 2003 biostimulation field experiment was performed with 3 times higher acetate concentrations than the previous biostimulation in the same field plot (i.e., the 2002 experiment), and (2) the 2007 field experiment was performed in a new unperturbed plot on the same site. The biogeochemical reactive transport simulations accounted for four terminal electron-accepting processes (TEAPs), two distinct functional microbial populations, two pools of bioavailable Fe(III) minerals (iron oxides and phyllosilicate iron), uranium aqueous and surface complexation, mineral precipitation and dissolution. The conceptual model for bioavailable iron reflects recent laboratory studies with sediments from the UMTRA site that demonstrated that the bulk (∼90%) of initial Fe(III) bioreduction is associated with phyllosilicate rather than oxide forms of iron. The uranium reaction network includes a U(VI) surface complexation model based on laboratory studies with Rifle site sediments and aqueous complexation reactions that include ternary complexes (e.g., calcium–uranyl–carbonate). The bioreduced U(IV), Fe(II), and sulfide components produced during the experiments are strongly associated with the solid phases and may play an important role in long-term uranium immobilization.

Protective Layer Design in Landfill Covers based on Frost Penetration

We develop a design procedure to determine the minimum thickness of a protective soil layer over a barrier layer in a landfill cover design. The procedure equates the minimum thickness of the overlying protective layer to the predicted extreme freezing-events depth that may occur in the landfill design life. This procedure is applicable to Resource Conservation and Recovery Act Subtitle C hazardous waste landfills and any landfill designed to encapsulate hazardous waste from the environment for long periods of time. This procedure was formulated by the Uranium Mill Tailings Remedial Action Project’s Technical Assistance Contractor to protect radon and infiltration barriers from degradation over a 200-year minimum design life. The procedure uses common soil parameters, a personal computer version of the modified Berggren equation, and site-specific daily maximum-minimum temperature data compiled for a minimum of 30 years to compute frost depths. The computed frost penetration magnitudes are then used with a Gumbel extreme-value probability distribution to predict frost depths for recurrence intervals corresponding to a design life that exceeds the record of observed data.

Concepts and Results in the Uranium Mill Tailings Remedial Action Ground Water Project

The U. S. Dept of Energy is responsible for bringing 22 former uranium millsites into compliance with ground water standards determined by the U. S. Environmental Protection Agency. These standards require that contaminants in the ground water not cause an unacceptable risk to human health and the environment. One of three compliance strategies is selected for each site; no further action, passive (natural flushing) remediation, or active-engineered remediation. Selection is based on site characteristics and conditions. Initially, target strategies are proposed using a limited data set, matching the site conditions to the provisions in the standards. More than one strategy may be targeted based on knowledge of the site. Data gaps are identified, and additional site data are collected. The targeted strategy(ies) is (are) then confirmed or reselected. With this process, additional site characterization activities are focused on a particular remediation strategy(ies), thus avoiding the tendency to keep studying the site. This process also helps to avoid the tendency to focus on just one remedial action alternative too early, prior to having enough characterization data. Lessons learned from applying this process to initial sites include: (1) risks associated with using limited characterization data sets are typically manifested during the remedial construction phase; (2) targeting more than one strategy offsets the risk of a compliance strategy not suceeding; (3) different contaminants may require different compliance strategies; (4) the process requires built-in flexibility because each site is generally unique and parameters vary in degrees of freedom between sites, resulting in unique solutions; and (5) early stakeholder involvement is essential to gain buy-in of the final solution. Lessons learned are continually updated to incorporate experience and efficiencies.

Calculation of the number of cancer deaths prevented by the Uranium Mill Tailings Remedial Action Project.

The Uranium Mill Tailings Remedial Action Project has completed remedial action at 22 uranium mill tailings sites and about 5,000 properties ("vicinity properties") where tailings were used in construction, at a total cost of $1.45 billion. This paper uses existing data from Environmental Impact Statements and Environmental Assessments, and vicinity property calculations, to determine the total number of cancer deaths averted by the Uranium Mill Tailings Remedial Action Project. The cost-effectiveness of remediating each site, the vicinity properties, and the entire project is calculated. The cost per cancer death averted was four orders of magnitude higher at the least cost-effective site than at the most cost-effective site.

Cleanup Protocol for 226Ra-contaminated Cobbly Soil at Umtra Project Sites

The nonuniform distribution of 226Ra and other radiological contamination of cobbly soil encountered on several Uranium Mill Tailings Remedial Action Project sites is presented and discussed, and the concomitant challenges to the intent and implementation of the U.S. Environmental Protection Agency's soil cleanup standards are noted. In response to technical assessments and information presented to the U.S. Nuclear Regulatory Commission by the U.S. Department of Energy, the Nuclear Regulatory Commission has recently resolved the dilemma by concluding that compliance with Environmental Protection Agency soil cleanup standards for cobby soil at Uranium Mill Tailings Remedial Action Project sites would be adequately attained using bulk radionuclide concentrations, instead of requiring that the radionuclide concentration of the finer soil fraction passing a #4 mesh sieve met the standards. A Nuclear Regulatory Commission-approved procedure developed for cobbly soil remediation is outlined and discussed. The site-specific implementation of this procedure at Uranium Mill Tailings Remedial Action Project sites containing cobbly soil is estimated to save millions of dollars.

Reliability of Chemical Analyses of Water Samples — The Experience of the UMTRA Project

AbstractGround water quality investigations require reliable chemical analyses of water samples. Unfortunately, laboratory analytical results are often unreliable. The Uranium Mill Tailings Remedial Action (UMTRA) Project's solution to this problem was to establish a two‐phase quality assurance program for the analysis of water samples. In the first phase, eight laboratories analyzed three solutions of known composition. The analytical accuracy of each laboratory was ranked and three laboratories were awarded contracts. The second phase consists of on‐going monitoring of the reliability of the selected laboratories.The following conclusions are based on two years of experience with the UMTRA Project's Quality Assurance Program:• The reliability of laboratory analyses should not be taken for granted.• Analytical reliability may be independent of the prices charged by laboratories.• Quality assurance programs benefit both the customer and the laboratory.

Experiences with a PFN Uranium Logging System

The Prompt Fission Neutron (PFN) uranium logging system has been used by Sandia National Laboratories for field operations for about six years; during that time, the system has been used at several exploration sites from southern Texas to Wyoming. Some of the improvements, recent changes to the system, and new applications are discussed here. A high-voltage doubler has been designed for the neutron generator which permits operation at pulse rates of up to one kHz. Other improvements in hardware have also been made. The PFN tool has been tested at moderately elevated temperatures (~93°C). The tool has been used in support of the Uranium Mill Tailings Remedial Action Program (UMTRAP) at various mine tailing sites in Colorado and Wyoming. The log interpretation model has now been completed, and a patent on the system has been issued. The tool design will be released to the National Technical Information Service, and is available for anyone to build and use; the software is also available.

RA-226 Radioassay of Soil and Tailings

ABSTRACTStudies of inactive uranium tailings piles have shown that tailings sands containing Ra-226 and other radionuclides may be dispersed by wind and water erosion, causing contamination of adjacent areas. To conduct an effective cleanup operation, it is necessary that boundaries of contamination be well defined. To accomplish this, data from surface gamma-ray surveys made under the Measurement Monitoring Program of the U.S. DOE's Uranium Mill Tailings Remedial Action Project (UMTRAP) are first used to delineate a general outline of the contaminated area. Then, data fram portable scintillometer surveys and fran sealed-can gammaray analyses of soil samples are used to more precisely define the perimeter of Ra-226 contamination. These field measurements are supported by radiochemical analyses of randomly selected samples. Because of its adaptability to the widely varying chemical composition of the material in these samples, the camplexing agent EDTA is used in a complexometric leaching procedure to analyze Ra-226. By this procedure, natural concentrations of Ra-226 in soil (∼1 pCi/g) can be measured routinely. The potential limit of detection is in the 0.1–0.5 pCi/g range. Details of the method, which includes leaching of radium followed by radon de-eranation, are described. Comparative data for various soil and tailings samples are presented.