Effects of Textural-Scale Heterogeneity On the Release of Uranium From Contaminated Sediments.

The vadose zone at the Hanford, Washington, 300-Area is contaminated with uranium (U) from historical disposal in settling ponds, and persistent contamination of the groundwater is observed. Release of U from the partially-saturated contaminated sediments will be a function of (1) the degree of connection between major flowpaths and fine-grained pockets of sorbed U, and (2) differential rates of desorption which are a function of the mineralogy active at a particular moisture content. The goal of this study is to perform column leaching experiments at variable moisture regimes with two textural scales, i.e. fine-texture (<2 mm) and field-texture (<31.5 mm). The objectives of this study are (1) to determine the dependence of mass transfer and to provide parameters to describe flow and transport as a function of water content and (2) to discover and quantify the factors controlling the exchange of U between different pore classes.

Contaminated vadose zone sediment was obtained from the “smear zone,” located between the vadose zone and groundwater. The sediment was fractionated into different particle sizes that ranged from <0.053 – 12.5 mm. Chemical extractions, viz. dithionate-citrate-bicarbonate and formate, were performed to quantify total U and dominant reactive mineral fractions. Moreover, the labile form of U, i.e. expected total available U for desorption, was quantified using bicarbonate and synthetic groundwater extractions in batch systems. About 41% of the total U in the finer fractions existed as labile U. The boundary conditions and hydraulic parameters for unsaturated flow experiments were obtained using Hanford analog sediment that was composited to the mean smear zone particle size distribution. The saturated hydraulic conductivity in the field-textured column was about an order of magnitude higher when compared to the fine-textured column. A saturated column leaching experiment was performed using fine-textured contaminated sediment wherein bromide and pentafluorobenzoic acid were used as non-reactive tracers. Both the tracers confirmed the absence of physical nonequilibrium during stop-flow scenarios. Ongoing experiments include a saturated column experiment wherein the sediments are packed to field-textured particle size distribution. We expect that the field-textured column will exhibit physical nonequilibrium limiting the rate of release of U from the reactive fine-grained materials, and that the degree of rate-limited mass transfer will change as a function of moisture content. Currently, efforts are underway to evaluate the existing smear zone model to describe our batch and column results. This research will enable improved field-scale predictions of groundwater U mobility by capturing the coupled hydraulic and geochemical controls in the smear zone.