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Paper No. 2
Presentation Time: 1:45 PM
GEOCHEMICAL PROCESSES CONTROLLING TRANSPORT AND DEPOSITION OF URANIUM, ESPAÑOLA BASIN, NEW MEXICO
Uranium is a trace metal of considerable interest within aquifer systems worldwide. Concentrations of natural uranium vary from less than 8.40e-09 M (0.002 mg/L) to 7.65e-06 M (1.82 mg/L) in groundwater within the Española Basin, New Mexico. Uranium concentrations exceeding the EPA drinking water standard of 0.030 mg/L are of public health concern. Oxidative dissolution of uranium(IV) minerals, associated with Proterozoic granitic rocks in the Sangre de Cristo Mountains, and hydrolysis of uranium-bearing volcanic ash and granitic and volcanic detritus within the Tesuque Formation, contribute to highly variable uranium concentrations measured in groundwater. Sub-economical grade uranium is associated with clay galls, opal, chert, fossil bone, carbonaceous material, and ferric (oxy)hydroxide within the San Jose mining district (Arroyo Seco and Oxide Butte). Uranium(VI) minerals identified include carnotite (K2(UO2)2V2O8·3H2O), meta-autunite (Ca(UO2)2(PO4)2·2-6H2O), and schröckingerite (NaCa3(UO2)(CO3)3(SO4)F·10H2O). Millimolar concentrations of dissolved uranium are required to precipitate meta-autunite, implying that this phase formed in the unsaturated zone. Groundwater, however, approaches equilibrium with respect to carnotite and haiweeite (Ca(UO2)2(Si2O5)3·5H2O). Haiweeite is a potential hydrolysis product associated with dissolution of soluble uranium-bearing volcanic ash. Higher concentrations of uranium in groundwater typically are associated with increasing concentrations of sodium and decreasing concentrations of calcium. PHREEQC simulations suggest that adsorption of Ca2+ onto ferric (oxy)hydroxide releases Na+ and UO22+ to groundwater. Results of deionized (DI) water leach tests and EPA 3050 partial digestions (pH1) performed on oxidized sediments collected from the San Jose mining district show that concentrations of leached and digested uranium range from 3.21 to 52.21 mg/kg and from 8.48 to 107.8 mg/kg, respectively. Distribution coefficients for the Arroyo Seco and Oxide Butte samples range from 59.4 to 79.11 L/kg and from 24.9 to 35.1 L/kg, respectively, using a 1.5:1 DI water-solid ratio. Ferric (oxy)hydroxide, solid organic matter, and smectite are the dominant adsorbents for uranium(VI) within the study area.