Awards

2002 Award Recipients

PROJECT:	Factors Controlling In Situ Uranium and Technetium Bio-Reduction and Reoxidation at the NABIR Field Research Center
PRINCIPAL INVESTIGATOR:	Jack Istok
Field Research Center

Rationale: Ongoing laboratory and field studies by our group have demonstrated that it is possible to stimulate denitrification and microbially-mediated U reduction at FRC Area 1. Microcosm studies showed that (1) NO3- removal is necessary for U reduction, (2) rates of U reduction are limited by the availability of suitable electron donors, (3) rates of U reduction are greatly reduced below pH 5, and (4) NO3- and the products of denitrification can rapidly reoxidize (and therefore remobilize) microbially-reduced U. Although denitrification has been successfully stimulated in field tests (as measured by NO2- production) through the addition of various electron donors (e.g. acetate, glucose, and ethanol), unequivocal evidence for U reduction has yet to be obtained, in part because NO3- levels, although reduced, have remained high following donor addition. Similar processes are believed to control the reduction pertechnetate (Tc(VII)O4-) to Tc(IV)-containing mineral phases, although the limited scope of our current project precludes their examination. Note that although Tc concentrations are monitored during our ongoing field tests, unequivocal evidence for Tc reduction following electron donor addition has not been obtained.

Research Goal and Hypotheses: The overall goal of this project is to better understand factors and processes controlling microbially-mediated reduction and reoxidation of U and Tc in the unconsolidated residuum overlying the Nolichucky shale in FRC Areas 1 and 2. Based on the results of ongoing laboratory and field research conducted by our group in Area 1, we have developed five hypotheses to be tested by the proposed research:

1. The small rates of denitrification and U bio-reduction observed in laboratory incubations of sediments from Area 1 at low pH (< 5) are due to the presence of high concentrations of toxic metals (especially Al and Ni). Rates of Tc reduction will also be small at low pH in the presence of high concentrations of toxic metals.

2. In situ rates of U and perhaps Tc bio-reduction can be increased by increasing system pH and thus precipitating toxic metals from solution.

3. In situ rates of U and Tc bio-reduction can be increased by the addition of humic substances, which complex toxic metals such as Al and Ni, buffer pH, and serve as electron shuttles to facilitate U and Tc reduction.

4. Microbially-reduced U and Tc are rapidly oxidized in the presence of high concentrations of NO3- and the denitrification intermediates NO2-, N2O, and NO.

5. An electron-donor-addition strategy (type and form of donor, with or without pH adjustment and with or without the co-addition of humic substances) can be devised to reduce U and Tc concentrations for an extended period of time in low pH groundwater in the presence of high concentrations of NO3-, Al, and Ni. This strategy operates by removing or complexing these components of FRC groundwater to allow the subsequent reduction of U(VI) and Tc(VII).

Approach: Hypotheses will be addressed by an extensive series of field push-pull tests supported by detailed geochemical analyses and laboratory microcosm experiments. Field tests will be conducted in wells in Areas 1 and 2; laboratory studies will utilize groundwater and sediment from the same locations. Detailed geochemical analyses will be performed to quantify radionuclide and toxic metal speciation, complexation, precipitation, and co-precipitation as a function of pH, electron donor, and humic substances. Microcosm studies will be conducted with live and killed microorganisms in sediments (1) to examine effects of toxic metal concentrations on U and Tc reduction with and without electron donor and humic substances additions and with and without pH adjustment, and (2) to examine the effects of denitrification reactions on the rates of reoxidation of microbially-reduced U and Tc. Field push-pull tests will be designed to parallel laboratory microcosm studies. Changes in U and Tc speciation in sediment and groundwater will be determined in laboratory and field samples following electron donor and/or humic substances addition.

 

PROJECT:	In Situ Immobilization of Uranium in Structured Porous Media via Biomineralization at the Fracture/Matrix Interface
PRINCIPAL INVESTIGATOR:	Tim Scheibe
Field Research Center

We propose a series of interdependent tasks culminating in an in situ field-scale biostimulation experiment at Area 2 of the NABIR Field Research Center (FRC) to evaluate the feasibility of stimulating microbial U(VI) reduction activity in targeted pore fractions of structured porous media. The research plan is designed to evaluate the hypothesis that U(VI) in low-permeability porous regions (micropores) of saprolite at the FRC can be immobilized and isolated from mobile groundwater by stimulating localized in situ microbial U(VI) reduction in hydrologically-accessible fractured zones (meso- and macropores). Such activity will cause precipitation of insoluble UO2 within the mesopore domain, thereby reducing or eliminating a long-term source of groundwater contamination that is otherwise extremely difficult to remediate. Planned research elements include field hydrologic and geophysical characterization, sediment wet chemical analysis and evaluation of microbial metal reduction potential, bench-scale reactive transport experiments using intact sediment blocks, and a field-scale biostimulation experiment. The proposed research will result in improved understanding of complex interactions between biogeochemical transformation and hydrologic flow and transport processes in structured porous media, and will lead to development of a general strategy for controlled bioremediation of metals and radionuclides in such subsurface environments. These results will also enhance our ability to upscale laboratory bioremediation experiments to the field scale.

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2000 Award Recipients

PROJECT:	Field-Scale Evaluation of Biostimulation for Remediation of Uranium-Contaminated Groundwater at a Proposed NABIR Field Research Center in Oak Ridge, TN
PRINCIPAL INVESTIGATOR:	Craig Criddle
Field Research Center

Microbial reduction of uranium may prevent its migration to receptor streams. However, application of this technology to field sites is untested, and future site remediation will require improved understanding of basic processes and implementation strategies in heterogeneous environments. The objectives of this study are (1) to develop a predictive capability for the rates and mechanisms controlling microbial reduction of U in heterogeneous field settings, and (2) to develop a system capable of delivering electron donor to a highly heterogeneous subsurface environment enabling spatially uniform in situ immobilization of U in groundwater upon passage through a subsurface biocurtain. To meet these objectives, we propose a 3-phase field study in a near surface aquifer southeast of the S-3 pond in the Bear Creek hydrogeologic regime at Oak Ridge, TN. This aquifer contains gram per liter levels of NO₃^- and part per million levels of U(VI). The NO₃^- must be removed because it prevents reduction of U, and, if the NO₃^- is reduced to N₂ the resulting gas could reduce aquifer permeability. We will critically evaluate three in-situ concepts for nitrate removal: an in-well vacuum stripper; an in-well bioreactor; and ion selective resins. The most effective and least expensive system will be coupled to a system for in-situ uranium removal. By removing the nitrate, we will be able to impose hydrological and geochemical controls on the U source permitting reliable determination of U reduction rates within a downgradient biocurtain for U immobilization. Novel nonreactive and reactive tracers will be used to quantify hydrological and geochemical processes and to develop mass balances. Field-scale and companion bench-scale studies will evaluate hypotheses on dissimilatory metal-reducing activity. Microbial community dynamics will be characterized using molecular methods.

PROJECT:	In Situ Determination of Uranium Reduction Kinetics at the Bear Creek Valley Field Research Center: Feasibility Studies
PRINCIPAL INVESTIGATOR:	Dr. Jonathan Istok
Field Research Center

Objective: The overall objective of this project is to assess the feasibility of deploying the single-well, "push-pull" test to determine in situ kinetics of microbially-mediated uranium reduction at the NABIR Field Research center (FRC). The push-pull test methodology is being developed within the NABIR Assessment Program Element and consists of the injection of a prepared aqueous test solution into the saturated zone, followed by the extraction of the test solution/groundwater mixture from the same location. The injected test solution contains various combinations of tracers, electron acceptors (including uranium), and/or electron donors, depending on the objective of the individual test. By monitoring the changing composition of the injected test solution through time, the kinetics of electron acceptor and electron donor utilization may be quantified.

Approach: Based on a literature review and available site characterization data, we have developed three hypotheses about subsurface microbial activity at the FRC:

1. Indigenous microorganisms with the capability to reduce uranium are present and these organisms are likely SO₄^2- and Fe(III)-reducing bacteria.

2. The intrinsic rate of uranium reduction is limited by the availability of a suitable electron donor. The form and amount of electron donor addition can be controlled to promote uranium reduction.

3. Adding electron donor to stimulate denitrification, Fe(III) reduction, and SO₄^2- reduction will result in enhanced rates of uranium reduction.

In this project, these hypotheses will be tested by performing a series of push-pull tests in a set of monitoring wells completed within the unconsolidated residuum overlying the Nolichucky shale in a field plot to be established west of the former S-3 ponds at the FRC. In these tests, injected test solutions will consist of either tap water or site groundwater amended with tracers and one or more exogeneous electron donors to stimulate the formation of anaerobic and reducing conditions within a small portion of the aquifer surrounding the monitoring well. Although push-pull tests are currently being successfully used to examine a similar series of hypotheses at a number of field sites, it is possible that two issues may complicate deployment of the methodology at the FRC:

1. The addition of sufficient quantities of electron donor may result in the production of large quantities of nitrogen gas (due to microbial reduction of high ambient nitrate concentrations) and/or large increases in biomass (due to microbial growth), both of which could reduce the aquifer’s hydraulic conductivity by "plugging" a portion of the pore space.

2. The complex hydrogeology of the site may result in unacceptably large dilution of injected test solutions with ambient groundwater, preventing the creation of anaerobic and reducing conditions within the aquifer during a test. Preferential flow may result in hydraulic cross-connections between nearby wells.

The feasibility study consists of three phases. During Phase I, a field plot will be established west of the former S-3 ponds and a set of monitoring wells will be installed. Laboratory microcosm studies will be performed using sediment and groundwater collected during monitoring well installation to select electron donors to promote uranium reduction for use in subsequent field manipulation experiments. Laboratory column studies will be performed to evaluate the effects of electron donor addition on nitrogen gas production, microbial growth, and sediment hydraulic conductivity. Short-term (~ hrs) push-pull tracer tests will be conducted in each monitoring well to evaluate the ability to inject and extract test solutions independently from all wells and to determine initial values for aquifer hydraulic conductivity and dispersivity prior to electron donor additions. Long-term (~weeks) tracer tests will also be conducted to evaluate our ability to monitor test solution composition during the course of field manipulation experiments.

During Phase II, field manipulation experiments will be conducted to test the three hypotheses. Injected test solutions will consist of site groundwater amended with a tracer and electron donors identified in microcosm studies. By monitoring the changing composition of the injected test solution over several weeks, the kinetics of electron donor and electron acceptor utilization will be determined. Moreover, the effect of electron donor additions on aqueous uranium concentrations and groundwater geochemistry will be quantified. To serve as a control, injected test solutions in one well will consist of site groundwater amended with only a conservative tracer.

During Phase III, an additional series of short-term push-pull tracer tests will be conducted to determine the effect of electron donor additions (carried out during phase II) on aquifer hydraulic conductivity and dispersivity, retardation factors, and quantities of exchangeable uranium.