Date of this Version
Geochimica et Cosmochimica Acta 70 (2006) 1873–1887; doi:10.1016/j.gca.2005.10.037
An alkaline brine containing uranyl (UO22+) leaked to the thick unsaturated zone at the Hanford Site. We examined samples from this zone at microscopic scale to determine the mode of uranium occurrence—microprecipitates of uranyl (UO22+) silicate within lithic-clast microfractures—and constructed a conceptual model for its emplacement, which we tested using a model of reactive diffusion at that scale. The study was driven by the need to understand the heterogeneous distribution of uranium and the chemical processes that controlled it. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1–3 µm precipitates were difficult, but indicated a uranyl silicate. These observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a reactive diffusion model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore–space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During this time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated Vadose Zone.