Graduation date: 2007
Metal and hydrogen ion acidity and extreme nitrate concentrations typical of
Department of Energy (DOE) legacy waste sites pose formidable challenges to
successful implementation of in situ bio-immobilization. Intermediate-scale (~ 2.5 m),
flow through models of an in situ bio-barrier were constructed to investigate U and Tc
removal from groundwater at a contaminated site in Oak Ridge, TN. In one study,
ethanol additions to pH-neutral contaminated site groundwater, flowing through a
mixture of site sediment and crushed limestone, effectively stimulated iron- and sulfatereducing
conditions and sustained U and Tc removal for 20 months. In a related study,
ethanol additions to nitric acid contaminated site groundwater, flowing through a
mixture of site sediment and crushed limestone, effectively promoted denitrification for
over 20 months. A combined signature lipid and nucleic acid-based approach was used
to spatially characterize microbial communities at relatively small spatial scales (<67
cm) in sediment along the groundwater flow paths in both experimental systems. The
results showed that ethanol additions stimulated growth of a distinct microbial
community in both the pH-neutral and nitric acid-contaminated systems, and that shifts
in community composition were spatially correlated with geochemistry along the
groundwater flow paths. Collectively, these results suggest that an in situ bio-barrier
could be potentially effective for U and Tc removal from nitric-acid contaminated
groundwater at the FRC. Hydraulic conductivity decreased by an order of magnitude in
both experimental systems due primarily to solids deposition. Preferential flow path
formation, due to biomass accumulation or solids deposition, could ultimately reduce
residence time within the treatment zone and decrease U and Tc removal efficiency.