Pilot-Scale in Situ Bioremediation of Uranium in a Highly Contaminated Aquifer. 1. Conditioning of a Treatment Zone WEI-MIN WU, † JACK CARLEY, ‡ MICHAEL FIENEN, † TONIA MEHLHORN, ‡ KENNETH LOWE, ‡ JENNIFER NYMAN, † JIAN LUO, † MARGARET E. GENTILE, † RAJ RAJAN, § DANIEL WAGNER, § ROBERT F. HICKEY, § BAOHUA GU, ‡ DAVID WATSON, ‡ OLAF A. CIRPKA, | PETER K. KITANIDIS, † PHILIP M. JARDINE, ‡ AND CRAIG S. CRIDDLE* ,† Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, Ecovation Inc., Victor, New York 14564, and Swiss Federal Institute of Aquatic Science and Technology (EAWAG), P.O. Box 611, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland To evaluate the potential for in situ bioremediation of U(VI) to sparingly soluble U(IV), we constructed a pilot test facility at Area 3 of the U.S. Department of Energy Natural and Accelerated Bioremediation Research (NABIR) Field Research Center (FRC) in Oak Ridge, TN. The facility is adjacent to the former S-3 Ponds which received trillions of liters of acidic plating wastes. High levels of uranium are present, with up to 800 mg kg -1 in the soil and 84-210 μM in the groundwater. Ambient groundwater has a highly buffered pH of ∼3.4 and high levels of aluminum (12-13 mM), calcium (22-25 mM), and nitrate (80-160 mM). Adjusting the pH of groundwater to ∼5 within the aquifer would deposit extensive aluminum hydroxide precipitate. Calcium is present in the groundwater at levels that inhibit U(VI) reduction, but its removal by injection of a high pH solution would generate clogging precipitate. Nitrate also inhibits U(VI) reduction and is present at such high concentrations that its removal by in situ denitrification would generate large amounts of N 2 gas and biomass. To establish and maintain hydraulic control, we installed a four well recirculation system parallel to geologic strike, with an inner loop nested within an outer loop. For monitoring, we drilled three boreholes perpen- dicular to strike across the inner loop and installed multilevel sampling tubes within them. A tracer pulse with clean water established travel times and connectivity between wells and enabled the assessment of contaminant release from the soil matrix. Subsequently, a highly conductive region of the subsurface was prepared for biostimulation by removing clogging agents and inhibitors and increasing pH. For 2 months, groundwater was pumped from the hydraulically conductive zone; treated to remove aluminum, calcium, and nitrate, and supplemented with tap water; adjusted to pH 4.3-4.5; then returned to the hydraulically conductive zone. This protocol removed most of the aqueous aluminum and calcium. The pH of the injected treated water was then increased to 6.0-6.3. With additional flushing, the pH of the extracted water gradually increased to 5.5-6.0, and nitrate concentrations fell to 0.5-1.0 mM. These conditions were judged suitable for biostimulation. In a companion paper (Wu et al., Environ. Sci. Technol. 2006, 40, 3978-3987), we describe the effects of ethanol addition on in situ denitrification and U(VI) reduction and immobilization. Introduction Uranium is a major groundwater contaminant at U.S. Department of Energy (DOE) sites, and poses risks for liver damage and cancer. For 31 years, trillions of liters of acidic plating wastes containing high levels of uranium and nitric acid were generated at the Y-12 Facility, Oak Ridge, TN and were discarded in the unlined S-3 Ponds. In 1983, attempts were made to neutralize and denitrify the waste, and the area was capped and converted to a parking lot. Despite these measures, contamination continued to migrate from the source along geologic strike and dip to greater depths and to surface discharge points. To assess the potential for immobilization of uranium and other radionuclides and metals, the DOE established a Field Research Center (FRC) at the Y-12 Facility as part of the DOE Natural and Accelerated Bioremediation Research (NABIR) program. Since 2001, we have performed experiments in FRC Area 3, an area im- mediately adjacent to and downgradient from the S-3 Ponds along strike. To enable hydraulically controlled experiments, we developed a recirculation system and facilities for above- ground removal of clogging agents and inhibitors and used a stepwise procedure to condition the site prior to bio- stimulation (Figure 1). As shown in Table 1, ambient groundwater is acidic (pH 3.4-3.5) and buffered by high levels of aluminum (up to 20 mM). It also contains high levels of nitrate, originally introduced as nitric acid (up to ∼160 mM), sulfate introduced as sulfuric acid (up to ∼10 mM), calcium (25 mM), metals, and organics (2, 3). Solid-phase analyses of a core sample from well FW104 gave total carbonate extractable U, NO3 - , SO4 2- , and PO4 3- of 450, 780, 900, and 940 mg kg -1 , respectively (4). Zones with the highest U concentrations were dominated by clay- and silt-sized particles consisting primarily of illite. Fe(III) and Al(III) (hydr)oxides (>30 000 mg kg -1 ), are abundant, typically as coatings on clay minerals (2). Soluble uranium is present at highly toxic levels of 84-210 μM, exceeding the U.S. Environmental Protection Agency drinking water standard (0.126 μM or 0.03 mg L -1 ) by one thousand times. Most of this uranium migrates along strike within a narrow region of preferential flowsmost likely a fracture zone ∼1 m thick at a depth of 10.9-12 m where there is a high specific discharge of 0.1-1.0 m d -1 . Analyses of soil within this zone revealed high levels of sorbed or precipitated U(VI) (ranging up to 800 mg kg -1 ) and phosphate (∼1000 mg kg -1 ) in close association with iron oxides. Thus, even though sorption or precipitation of U(VI) provides a natural mech- anism of U(VI) immobilization, levels of U(VI) in the water * Corresponding author e-mail: criddle@stanford.edu; phone: 650-723-9032; fax: 650-725-3164. † Stanford University. ‡ Oak Ridge National Laboratory. § Ecovation Inc. | Swiss Federal Institute of Aquatical Science & Technology (EAWAG). Environ. Sci. Technol. 2006, 40, 3978-3985 3978 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006 10.1021/es051954y CCC: $33.50  2006 American Chemical Society Published on Web 05/13/2006