300 Area Treatability Test: Laboratory Development of Polyphosphate Remediation Technology for In Situ Treatment of Uranium Contamination in the Vadose Zone and Capillary Fringe

Abstract

SummaryThe Hanford Site is a former nuclear defense production facility. A groundwater plume containing uranium, originating from a combination of purposeful discharges of wastewater to cribs, trenches, and ponds, along with some accidental leaks and spills related to nuclear fuel fabrication activities, has persisted beneath the Hanford Site 300 Area for many years. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, the remedial action objective to lower the concentration of groundwater uranium to the U. S. Environmental Protection Agency maximum contaminant level concentration of 30 µg/L has not been achieved within the anticipated 10-year time period. Some unknown amount of contamination remains in the vadose zone beneath the lower extent of the excavation activities. Additional contamination also may remain beneath buildings and facilities in the southern portion of the 300 Area, which has not been decontaminated and decommissioned. The use of polyphosphate technology for source treatment in the vadose zone and capillary fringe is expected to accelerate the natural attenuation of uranium to thermodynamically stable uranium-phosphate minerals. This effort will complement the current 300 Area treatability test being conducted within the saturated zone (e.g., 300 Area aquifer) for in situ treatment of uranium-contaminated groundwater.Polyphosphate technology has been demonstrated for in situ precipitation of phosphate phases to control the long-term fate of uranium. A critical component of the development and testing is detailed evaluation to determine if polyphosphate technology could be modified for infiltration from ground surface or some depth of excavation to stabilize source uranium phases. This report presents results from bench-scale treatability studies conducted under site-specific conditions to optimize the polyphosphate amendment for implementation of a field-scale technology demonstration to stabilize uranium within the 300 Area vadose zone and capillary fringe of the Hanford Site. Documented in this report are data related to 1) the retardation of polyphosphate as a function of water content and pore water velocity, 2) the reaction between uranium-bearing solid phases and aqueous polyphosphate remediation technology as a function of polyphosphate composition and concentration, 3) the mechanism of autunite formation via the reaction of solid-phase calcite-bound uranium and aqueous polyphosphate remediation technology, 4) the transformation mechanism and reaction kinetics between uranyl-carbonate and -silicate minerals with the polyphosphate remedy under advective conditions, and 5) the extent and rate of uranium released and immobilized as a function of polyphosphate composition and the infiltration rate of the polyphosphate remedy. Kinetic rate law parameters were determined from single-pass flow-through experiments. Pressurized unsaturated flow tests were used to determine the effect of polyphosphate composition, concentration, and infiltration rate...

Use of Polyphosphate to Decrease Uranium Leaching in Hanford 300 Area Smear Zone Sediments

“…In addition to those noted in previous publications (Wellman et al 2007(Wellman et al , 2008a(Wellman et al , 2011Bovaird et al 2010;Vermeul et al 2009), some limitations of polyphosphate treatment technologies were identified, which impact field scale applicability in different treatment zones. The trend of increased sedimentphosphate contact time resulting in higher phosphate precipitate (and a greater decrease in uranium leaching) implies that polyphosphate injection into groundwater may not deposit sufficient phosphate precipitate due to high groundwater flow (i.e., insufficient contact time).…”

“…It is hypothesized that due to the slowly changing phosphate precipitates that form, uranium leaching will unlikely decrease during initial treatment, but over months to years, lower solubility hydroxyapatite can form coating uranium surface phases (i.e., uranium-carbonates, sodium-boltwoodite) or uranium phosphate phases may also form. Previous research demonstrated that under high bicarbonate conditions, autunite does not form, but uranium phosphate precipitates do form (Wellman et al 2008a Aqueous uranium leached from the phosphate-treated sediment columns over hundreds to thousands of hours generally shows decreased uranium mass leaching out and decrease in uranium release rate from phosphate-treated sediment compared with untreated sediment. During continuous groundwater or river flow for phosphate-treated sediments, column effluent uranium was 1 to 10 µg/L, compared to 5 to 12 µg/L for untreated sediments at exactly the same flow rate.…”

“…Although most contaminated sediment from the highly contaminated ponds was removed in the early 1990s, uranium continues to leach from variably saturated (i.e., smear zone) sediments beneath the original ponds seasonally and is the largest source of uranium in groundwater that eventually advects into the Columbia River (Catalano et al 2008;Bond et al 2008;Liu et al, 2008). The use of phosphate treatment of sediments has been shown to effectively decrease uranium leaching short-term column experiments (Shi et al 2009) and in numerous batch and one-dimensional (1-D) column studies (Wellman et al 2006a(Wellman et al , 2006b(Wellman et al , 2007(Wellman et al , 2008a(Wellman et al , 2008b(Wellman et al , 2011Bovaird et al 2010). However, a field-scale groundwater injection of a phosphate solution in the Hanford 300 Area showed limited effectiveness at decreasing uranium concentration in fast flowing groundwater (Wellman et al 2011;Vermeul et al 2009).…”

“…The comparison between untreated and phosphate-treated column results are used to characterize uranium leaching rate at multiple stop flow events, total leached uranium mass, and changes in uranium surface phases. Phosphate treatment is decreasing uranium leaching by a combination of postulated geochemical and physical processes that include uranium species adsorption onto the phosphate precipitates (Shi et al 2009), non-uranium phosphate precipitates coating uranium surface phases, and the formation of low solubility uranyl phosphate precipitates (Wellman et al 2008a(Wellman et al , 2008b.…”

Remediation of Uranium in the Hanford Vadose Zone Using Gas-Transported Reactants: Laboratory Scale Experiments in Support of the Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau

“…Injection of sodium phosphate is potentially the best reactant (Table 2.1), as the formation of autunite will likely immobilize uranium from further advection. The use of sodium phosphate (with and without tripolyphosphate [Wellman et al 2008a]) has been tested at low water content. Unknowns associated with this technology include the distribution of phosphate mass that would result from 99% gas/1% water injection.…”