Long and 17 co-authors reported in 2006 on their results of four prior field experiments (2002 to 2005) and one field experiment in progress (2006 extending into 2007).
Visual evidence of Fe(III) vs. sulfate reduction in cross-well mixing tubes.
Fe reduction (pink color, absence of black ppt.)
Sulfate reduction (black is sulfide, likely FeS (+calcite)
The Rifle, Colorado uranium mill tailings site has a shallow alluvial aquifer with a well-defined porous medium flow system that is ideal for field-scale electron donor amendment for uranium bioreduction. Key results from these experiments are summarized as follows:
- Biostimulation removes U(VI) from groundwater under field conditions in a permeable alluvial aquifer.
- Direct microbial reduction by Geobacter sp. is largely responsible for U(VI) loss during the Fe-reduction phase of field experiments.
- Maintenance of Fe-reducing conditions is critical to ongoing removal of U(VI) during biostimulation.
- However, long-term (~2 years) post-amendment removal of U(VI) occurs when significant sulfate reduction was achieved during amendment.
- Post-amendment removal of U(VI) may be microbially mediated, an hypothesis that is currently being tested in the 2006 field experiment.
- Geophysical techniques, especially complex resistivity and self potential measurements provide effective techniques for minimally invasive monitoring of biostimulation and post-biostimulation in situ processes (see project lead by Susan Hubbard, LBNL).
These results suggest that bioremediation of U(VI) is possible even in high-permeability alluvial aquifers. Sulfate reduction may not contribute to direct U(VI) bioreduction, but may be used to advantage to sustain biosorptive U(VI) removal after ending electron donor amendment. Significant opportunity exists to tailor in situ processes to achieve U(VI) removal and sequestration. For example, it should be possible to add sulfate to low sulfate systems if precipitation of sulfide proves to be a critical factor in long-term control of U(VI) concentration in groundwater.
The experimental approach used at the Rifle site is to introduce the electron donor at low concentrations under natural gradient conditions. Experiments conducted since 2002 have consistently replicated loss of U(VI) from groundwater synchronous with growth of Geobacter sp. after amendment of the subsurface with acetate targeted at in situ concentrations of either ~3 mM or ~10 mM. Electron-donor amendment and associated Geobacter growth dominates the system until Fe reduction consumes bio-available Fe(III) solids, eventually limiting Geobacter growth. Because there is ~ 6 to 10mM sulfate in the groundwater, sulfate reduction dominates the system after bio-available oxidized Fe is consumed, resulting in extensive precipitation of amorphous FeS and calcite, particularly near the point of injection and in the deeper parts of the aquifer.
Typically, U(VI) concentrations rebound with the onset of extensive sulfate reduction, but, remarkably, after acetate amendment is stopped, U(VI) concentrations fall again and can remain at ~20% of background levels for >1 year. Our current hypothesis is that the extent of post-amendment U(VI) removal is correlated with the amount of FeS precipitated. However, recent laboratory studies (N’Guessan et al. 2006.) indicate that FeS04 cannot reduce U abiotically in these sediments suggesting that a microbial community different from either the Fe or sulfate reducers produces biopolymers that preferentially sorb U(VI) or that newly-formed Fe(III) oxides sorb U(VI).
We are currently testing this hypothesis at the field scale by comparing side-by-side experimental plots, one in which sulfate reduction is allowed to continue for approximately two months and another in which sulfate reduction is limited to approximately three weeks. Post-amendment comparison of U(VI) removal is expected to show that the experimental plot with little sulfate reduction will rebound to background U(VI) concentrations within a few weeks, whereas the plot with extensive sulfate reduction will show removal of U(VI) for at least several months after acetate amendment was stopped. Post-amendment groundwater and sediment sampling will enable us to assess both microbial community and mineralogy changes associated with ongoing U(VI) removal.
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