Summary of Results from Field Research

UMTRA Project sites provide research opportunities with significant benefits to both UMTRA Project sites and DOE cleanup activities in general. The Shiprock, Gunnison and Rifle UMTRA sites were selected for field research because they represent a range of uranium, sulfate, and nitrate concentrations and field conditions that are amenable to sampling. Results from sampling at these sites have produced significant results published in peer-reviewed journals and presented at professional society meetings (see publication list). Selected summaries and examples of these results follow.

Geomicrobiology of the Shiprock, New Mexico UMTRA Site
Uranium mill tailings represent a significant environmental threat in part because of human health and ecological risk associated with U-contaminated groundwater plumes. One such plume at Shiprock, NM provided an opportunity to integrate information on the groundwater flow system, sediment characteristics, geochemistry, and microbiology.

The geohydrology of the Shiprock site consists mainly of a permeable alluvial aquifer (5 to 7 meters of sandy gravel) overlying relatively impermeable Mancos Shale. Groundwater flows from a terrace, where U ore was milled, across a flood plain and discharges to the San Juan River. The groundwater on the terrace is contaminated with sulfate, nitrate, and U and was largely emplaced during milling operations. Groundwater crossing the flood plain mixes with San Juan River water and with water also flowing onto the floodplain from a deep flowing well resulting in a relatively narrow plume with U concentrations up to 4 mg/l. Varying levels of nitrate, sulfate, and dissolved oxygen (as low as 0.47 mg/l) are associated with the U plume.

Based on data from both groundwater and sediment samples, the subsurface microbial community in and adjacent to the plume is active and diverse. Evidence includes:

• Laboratory experiments on sediment samples show p-nitrophenol production rates from microbial utilization of substrate that are consistent with moderate levels of in situ microbial activity
• Depending on location, between 50 to 2000 picomoles/g phospholipid fatty acids (PLFA) including evidence of unusually high proportions of metal and/or sulfate reducing bacteria (M/SRB). PLFA biomarkers indicative of M/SRB showed a positive correlation with groundwater sulfate concentrations.
• High numbers of distinct aerobic colony types suggesting a diverse aerobic population
• Enrichments demonstrating the presence of viable anaerobic bacteria including nitrate reducing bacteria (NRB), M/SRB, and methanogens; bacteria capable of using uranyl (UO22+) as a terminal electron acceptor have also been identified
• Denaturing gradient gel electrophoresis (DGGE) results (Fig. 1) indicating that dominant sediment bacterial communities are fairly simple and consist mostly of bacterial groups associated with metal metabolism or resistance (including the genera Bacillus, Vibrio, Geobacter, Shewanella, Marinomonas, Pseudomonas and Pedomicrobium). The dominant bacterial populations of groundwater samples were commonly only distantly related to any cultured organisms, but organisms related to the genera Sphingomonas, Pedomicrobium and Geobacter, organisms similar to d-subgroup proteobacterial sulphate reducers, actinomycetes, cyanobacteria and e-subgroup proteobacteria were detected.

In contrast, DGGE results for background subsurface sediments show Arthrobacter sp. as the single dominant microbial group, suggesting that the microbial community within the plume reflects a response to various components of the contamination including electron acceptors (e.g. nitrate and sulfate), electron donors (e.g. dissolved organic carbon [DOC] mobilized from the Mancos Shale), and perhaps metals such as U and V. The Shiprock alluvial plain aquifer also exhibits changes in microbial communities and electron accepting processes as a function of depth in the aquifer, possibly as the result of decreasing DO and increasing DOC with depth.

Enrichments and microcosm studies demonstrate that microbial communities in the subsurface are capable of reducing U(VI) to insoluble U(IV) (Finneran et al. 2002). Where nitrate is present, its removal is a prerequisite to sulfate and U reduction but this is rapidly accomplished under laboratory conditions by addition of dilute solutions of electron donor (e.g. acetate). These results suggest that, given appropriate consideration of lateral and vertical variations of terminal electron accepting processes, it may be possible to decrease concentrations of dissolved U, nitrate, and sulfate at U-mill sites by stimulating growth of subsurface microbial communities capable of metal reduction.

Initial Results from push-pull tests, Gunnison Colorado
Push-pull tests from single wells within the uranium-contaminated aquifer at Gunnison, CO demonstrated a potential for stimulating in situ removal of soluble U(VI) upon the injection of acetate into the subsurface. U(VI) concentrations decreased approximately 60% after injection of an acetate solution relative to a control well receiving no acetate (Anderson et al. 2002). Observed loss of U(VI) coincided with the production of Fe(II). Loss of sulfate or production of sulfide was not observed indicating in situ removal of U(VI) occurred under stimulated Fe(III)-reducing conditions. Observed loss of U(VI) within a second well was not observed due to rapid dilution of the injected acetate. The results demonstrate the ability to stimulate the removal of soluble U(VI) under stimulated Fe(III)-reducing conditions within the subsurface of uranium-contaminated aquifers.