Stimulating the in situ activity of Geobacter species to remove uranium from the groundwater of a uranium-contaminated aquifer

Abstract

The potential for removing uranium from contaminated groundwater by stimulating the in situ activity of dissimilatory metal-reducing microorganisms was evaluated in a uranium-contaminated aquifer located in Rifle, Colo. Acetate (1 to 3 mM) was injected into the subsurface over a 3-month period via an injection gallery composed of 20 injection wells, which was installed upgradient from a series of 15 monitoring wells. U(VI) concentrations decreased in as little as 9 days after acetate injection was initiated, and within 50 days uranium had declined below the prescribed treatment level of 0.18 micro M in some of the monitoring wells. Analysis of 16S ribosomal DNA (rDNA) sequences and phospholipid fatty acid profiles demonstrated that the initial loss of uranium from the groundwater was associated with an enrichment of Geobacter species in the treatment zone. Fe(II) in the groundwater also increased during this period, suggesting that U(VI) reduction was coincident with Fe(III) reduction. As the acetate injection continued over 50 days there was a loss of sulfate from the groundwater and an accumulation of sulfide and the composition of the microbial community changed. Organisms with 16S rDNA sequences most closely related to those of sulfate reducers became predominant, and Geobacter species became a minor component of the community. This apparent switch from Fe(III) reduction to sulfate reduction as the terminal electron accepting process for the oxidation of the injected acetate was associated with an increase in uranium concentration in the groundwater. These results demonstrate that in situ bioremediation of uranium-contaminated groundwater is feasible but suggest that the strategy should be optimized to better maintain long-term activity of Geobacter species.

FIG. 1.

Concept and layout of the in situ test plot installed at the Old Rifle UMTRA site in Rifle, Colo.

FIG. 2.

Bromide within the monitoring-well field. B-01 to B-03, upgradient control wells; M-01 to M-15, downgradient monitoring wells. Groundwater flow is from left to right, and the injection gallery is positioned between the control wells and the first row of monitoring wells.

FIG. 3.

U(VI) in groundwater samples. The layout is described in the Fig. 2 legend.

FIG. 4.

Fe(II) in groundwater samples. The layout is described in the Fig. 2 legend.

FIG. 5.

Acetate in groundwater samples. The layout is described in the Fig. 2 legend.

FIG. 6.

Sulfate concentrations in groundwater samples. The layout is described in the Fig. 2 legend.

FIG. 7.

Clone library analyses of changes in the groundwater microbial community from a representative downgradient well (M-07) within the second row of monitoring wells compared to data from an upgradient control well (B-02) during the experiment. The results are compiled from sequences of at least 30 clones analyzed for each time point. Calculated diversity values (Shannon-Weaver index) are provided in parentheses at the top of each clone library. Similar results were obtained from a nearby downgradient well.

FIG. 8.

Changes observed in bulk biomass PLFA and signature lipids for Geobacter species detected on Bio-Sep beads in capsules suspended in four selected control and monitoring wells (B-02, M-03, M-08, and M-13) at two different depths for each well within the saturated subsurface (designated sampling points 4 and 13 for each well) from two different time points during the field test.