Abstract describing the most recent data from the Oak Ridge, TN, NABIR Field Research Site
(For the complete slide presentation, click HERE. Links to individual slides are included below).
In situ microbial reduction of mobile U(VI) to sparingly soluble U(IV) was evaluated at the DOE NABIR Field Research Center in Oak Ridge, TN. The site has extremely high levels of solution and solid phase U(VI) and many other contaminants at high concentrations and at low pH. Hydraulic control was established using an outer recirculation loop to protect a nested inner loop from ambient groundwater containing inhibitors of U(VI) reduction and clogging agents. The subsurface was conditioned by flushing with acidified clean water to remove bulk aqueous aluminum, calcium, and nitrate, and the pH of the inner loop was increased from 3.6 to 6. Laboratory microcosms experiments indicated that addition of ethanol to previously conditioned soil would stimulate efficient U(VI) reduction. Accordingly, periodic addition of ethanol was initiated within the inner loop. Biostimulation occurred in two phases: (1) an in-situ denitrification phase, with removal of residual nitrate; and (2) a sulfate- and U(VI)-reduction phase. The results indicate that an in situ reduction zone can be established and maintained long-term, and that groundwater U(VI) concentrations below the US EPA drinking water standard are achievable, despite highly unfavorable initial conditions. Carbonate addition enabled manipulation of U(VI) bioavailability from the solid phase. Well surging enabled control of well clogging and collection of sediment samples (also Slide 7 and Slide 8) for monitoring of U(VI) reduction. To suppress methane fermentation, sulfate depletion was avoided, and pH values were maintained at values less than 6.3. The overall approach and suite of strategies tested are expected to be of value at other radionuclide- and metal-contaminated sites and for highly contaminated sites generally, especially those containing mixed wastes.
Updated 9/12/2005
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