NATURAL ATTENUATION OF URANIUM IN AN OXIDIZING ROCK-SOIL-GROUNDWATER SYSTEM: COLES HILL URANIUM DEPOSIT, VIRGINIA

Transport of metal contaminants in hydrologically active regions is a complex function of physical, chemical and biologic processes. Thus, in order to develop rigorous performance assessment criteria for contaminant remediation and containment (including monitored natural attenuation), constraints from natural systems are required. One of the most important contaminants in this regard is the element U. The environmental behavior of U has been studied thoroughly over the past few decades; however, few of these studies have documented processes leading to its natural attenuation within oxidizing, fluid rich natural systems. Recent environmental petrology studies of the weathered zone overlying the Coles Hill U deposit indicate that the natural attenuation of U in oxidizing environments may occur if certain chemical and physical conditions are achieved. For example, at the Coles Hill site, it has been found that U concentrations in ground waters are being buffered to values less than 20 ppb due to the precipitation of low solubility U(VI) phases. The attenuation mechanism acting within this rock-soil system involves the transformation of U(IV) minerals (coffinite) that make up the primary ore to U(VI) phosphate minerals of the meta-autunite group. Above the water table, which occurs within the saprolite interval, meta-autunite group minerals are rare. Within this unsaturated zone U is primarily associated an aluminum phosphate of the crandallite mineral group and with P sorbed to ferric hyrdoxide mineral coatings. Results from field based studies of the major U reservoirs at Coles Hill have been supplemented by focused experimental studies as well as speciation and equilibrium reaction modeling. This work indicates that in addition to Eh and pH the most important parameter controlling the solubility of U within this system is the activity ratio of dissolved carbonate to dissolved phosphate. Alteration experiments using Coles Hill ore indicate that the transformation of U(IV) to U(VI) assemblages may occur in less than two months (given high enough phosphate levels). Model predictions agree with the field observations that for oxidizing systems with relatively low dissolved carbonate/phosphate ratios U may be immobilized by secondary U phosphate precipitation and sorption processes.