Jerden and Sinha reported in 2002 that the unmined Coles Hill Utanium deposit of south central Virginia represents a natural setting where U is stabilized by phosphate mineral precipitation in an oxidizing bedrock aquifer. Drill cores from the shallow portion of the deposit preserve a sharp Fe redox front defined by Fe(III) oxide staining. This front is located near a discontinuity in U mineralogy with U(IV) assemblages (e.g. coffinite, uraninite) on the reducing side, and U(VI) assemblages on the oxidizing side.
The discontinuity in U mineralogy does not, however, represent a major discontinuity in whole rock U concentrations, with sample groups from both oxidized and reduced sides of the front generally ranging from 500 to 1000 ppm. This observation suggests that the volume of shallow bedrock associated with the deposit has not lost significant amounts of U during the oxidation and incipient chemical weathering. The precipitation of Ba uranyl phosphate (Ba meta-autunite) is responsible for U retention within this zone.
Ground waters sampled from the weathered bedrock aquifer associated with the deposit contain less than 15 ug l-1 dissolved U. This suggests that the low solubility of the Ba meta-autunite limits U concentrations to values lower than the US-EPA maximum contaminant level of 30 ug l-1. Ground water speciation and mineral saturation calculations show that, in addition to Eh and pH, the most important factor controlling this U fixation process is the activity ratio of dissolved phosphate to dissolved carbonate.
Experimental results suggest that, at the Coles Hill site, the oxidation of U(IV) to U(VI) and subsequent precipitation of uranyl phosphate occurs rapidly (time scale of weeks) relative to ground water transport (e.g. 20 m/a). Furthermore, based on the rate of downward migration of the redox front, it is estimated that the oldest U(VI) phosphate assemblages associated with the Coles Hill U deposit have been stable for up to 150 ka. These observations have important implications for the design and long term performance assessment of phosphate-based stabilization and reactive barrier techniques.
The authors concluded that geochemical data from the Coles Hill U deposit indicates that there is minimal loss of U from the deposit during oxidation and incipient chemical weathering of the primary U(IV) ore. Petrographic and mineral chemical data (EMP, SEM-EDS, XRD) shows that the dominant process responsible for U retention is the precipitation of low solubility U(VI) phosphate (Ba meta-autunite). The physical characteristics (i.e. grain size, and growth habit) and mineral associations of the Ba meta-autunites depend on the micro-environments in which they form.
The chemistry, mineral saturation state and aqueous speciation of waters from the oxidizing bedrock aquifer indicate that the low solubility of stable U(VI) phosphate minerals can limit U concentrations to less than 15 ug l-1. The phosphate precipitation process is facilitated by high phosphate activities (e.g. 10-5.5) relative to dissolved carbonate, in oxic, slightly acidic (pH of ~6) conditions.
Experimental results suggest that the rates of U(IV) oxidation to U(VI) and subsequent precipitation of uranyl phosphate phases occurs on a time scale of weeks and that these secondary U(VI) phases are stable for tens to hundreds of thousands of years. The Coles Hill U deposit therefore demonstrates the natural long-term attenuation of U within an oxidizing apatite-rich environment, and thus lends confidence to the use of apatitebased technologies for U remediation and containment.
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