Contamination in the unsaturated vadose zone at many industrial and former nuclear production sites presents a long-term threat to groundwater and potential risk to sensitive receptors (Campbell et al. 2012; Pearce et al. 2018; Truex et al. 2017; Wellman et al. 2008). Remedial strategies for the vadose zone typically include targeted elimination of the contaminant source term, and a reliance on attenuation mechanisms to limit contaminant flux to groundwater (Bagwell et al. 2018; Saslow et al. 2018; Szecsody et al. 2010). However, recalcitrant contaminants with long residence times may require stabilization in place to reduce their flux to groundwater. Because preferential flow path, subsurface heterogeneities, bypassed fractures, and areas of low to no permeability often accumulate contaminants, direct solutions for contaminant mass residing in low-permeability regions are needed. These zones are also responsible for rebound in contaminant concentrations after site cleanup is presumed complete (e.g., Thomson et al. 2008).
A perched aquifer sits beneath the B-Complex in the 200 East Area at the Hanford Site, in southeastern Washington state (Oostrom et al. 2013). Contamination of the vadose zone and the perched aquifer is the result of planned releases from infiltration galleries as well as unplanned releases from the overlying tank farms and associated facilities. The perched zone serves as a continuing source for uranium (U), technetium (Tc), and nitrate (NO3-) to the underlying aquifer. Water extraction is the current response action, but the large footprint and low hydraulic conductivity limit the effective removal of contaminants. Therefore, in situ remedial technologies may be needed in combination with flux control measures to properly address the contamination and complexity of this site.
Aqueous transport of redox active radionuclides (e.g., U, Tc) is strongly influenced by redox and pH conditions (Icenhower et al. 2008; Ilton et al. 2008; Liu et al. 2015; Qafoku et al. 2010). As such, subsurface biogeochemical conditions can be intentionally manipulated to promote reductive precipitation or adsorption to decrease the solubility of these radionuclides (Szecsody et al. 2004). However, reduced oxidation states for most radionuclides are prone to rapid re-oxidation as natural oxidative conditions reestablish. For example, groundwater concentrations of uranium were successfully depleted by in-situ biostimulation at the Oak Ridge Field Research Center (TN, USA) and at the Old Rifle Site (CO, USA); however, rapid re-oxidation resulted in remobilization once nutrient additions, and bioreduction, ceased (N’Guessan et al. 2010; Wu et al. 2007). Previous studies have also shown that pertechnetate can be readily bioreduced to Tc(IV)O2×xH2O (Istok et al. 2004; Fredrickson et al. 2009), a precipitate that can be quickly and nearly completely reoxidized. Tc precipitated with sulfide oxidizes more slowly (i.e., months; Lukens et al. 2002, 2005; Ferrier et al. 2013), but still oxidize.
Because these effects are temporary, reductive immobilization of certain metals and radionuclides is only part of a long-term remedial strategy for contaminated sites. Stabilization measures are required to meaningfully retard or prevent eventual re-oxidation and re-mobilization of radionuclides such as U and Tc. Temporarily reduced Tc(IV) precipitate has been shown by X-ray absorption near edge structure (XANES) surface phase analysis to be stable under oxic conditions due to coating by aluminosilicates in water-saturated sediments (Szecsody et al. 2014), and is also hypothesized to occur at low water saturation for pertechnetate-laden sediment treated with H2S and NH3 gasses (Szecsody et al. 2015). Ammonia gas treatment of unsaturated sediments also results in U(VI) reduction and precipitation and aluminosilicate coatings (Di Pietro et al. 2020; Szecsody et al. 2011).