In-situ immobilization of heavy metals, such as uranium, through biological reduction is a promising means for stabilizing contaminants within subsurface sediments. Species of U(VI) are highly mobile in groundwater systems while those of U(IV) are only sparingly soluble. Stimulation of biological uranium reduction at the field scale presents several challenges, including heterogeneous sediment mineralogy, a complex and evolving community of bacteria, and the presence of multiple electron donors and acceptors. The NABIR Field Research Center (FRC) at Oak Ridge National Laboratory is additionally complex owing to U concentrations of 1,000 g/kg, pH values less than 3.4, and exceedingly high concentrations of nitrate (8 – 10 g/L) and aluminum (0.5 g/L). Here we present spectroscopic and geochemical evidence confirming field-scale in-situ biological uranium reduction at the NABIR FRC. However, even after long-term stimulation of biological activity, approximately 50% of uranium within the solid phase remained oxidized. We show that aqueous and solid phase uranium(VI) speciation may be limiting microbial uranium reduction. Furthermore, uranium(IV) was rapidly oxidized after the cessation of electron donor and exposure to air or other oxidants (nitrate); however, Fe(III)-oxides do not appear to be the dominant uranium(IV) oxidants. These factors complicate the long-term immobilization of uranium through in-situ stimulation of biological activity.