New Mexico Geological Society Annual Spring Meeting — Abstracts

Uranium is an actinide of considerable environmental interest present in aquifer systems worldwide. Dissolved concentrations of natural (background) uranium vary from less than 8.40e-09 M (2.0 µg/L) to 7.65e-06 M (1.82 mg/L) in groundwater east of the Rio Grande within the Española Basin, New Mexico. Total dissolved concentrations of natural uranium range from 5.04e-10 M to 5.76e-09 M (0.12 µg/L to 1.37 µg/L) in the regional aquifer beneath the Pajarito Plateau west of the Rio Grande. Dominant uranyl aqueous complexes consisting of UO₂(CO₃)₂^2-, Ca(UO₂)₂(CO₃)₃⁰, and UO₂(CO₃)₃^4- are mobile under circumneutral pH and oxidizing conditions characteristic of the Santa Fe Group (Tesuque Formation). Oxidative dissolution of uranium(IV) minerals and hydrolysis of uranium(IV)-bearing silicates and oxides associated with Proterozoic granitic rocks in the Sangre de Cristo Mountains enhanced mobilization of uranium from source material. Hydrolysis of soluble uranium-bearing volcanic ash and granitic detritus present in the Tesuque Formation also contribute to highly variable uranium concentrations occurring in Santa Fe Group groundwater. Uranium(IV, VI) is associated with clay galls, opal, chert, fossil bone, carbonaceous material, smectite, and ferric (oxy)hydroxide in the San Jose mining district (Arroyo Seco and Oxide Butte) exposed as outcrops in the present-day vadose zone. The dominant uranium(VI) minerals identified in this mining district include carnotite (K₂(UO₂)₂V₂O₈·3H₂O), metaautunite (Ca(UO₂)₂(PO₄)₂·2-6H₂O), and schröckingerite (NaCa₃(UO₂)(CO₃)₃(SO₄)F·10H₂O). Evapoconcentration of porewater in the Santa Fe Group likely was a critical process that enhanced solute saturation leading to precipitation of uranium(VI) minerals that have higher aqueous solubilities compared to uranium(IV) minerals, including uraninite and coffinite. 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 dissolved uranium range from 3.21 to 52.21 µg/g and from 8.48 to 107.8 µg/g, respectively, using a ratio of 150 mL DI and acid to 100 g solid. Based on X-ray diffraction analyses, smectite varies from 45 to 68 weight percent in two samples collected from Oxide Butte, whereas this clay mineral is only present up to 1-2 weight percent in two samples collected from Arroyo Seco. Lower leachate concentrations of dissolved uranium suggest that this metal is strongly adsorbed and/or precipitated as uranyl phases on smectite surfaces abundant in the Oxide Butte samples. Higher concentrations of uranium were leached from the smectite-poor Arroyo Seco samples, suggesting weak adsorption of this actinide onto mineral surfaces. Desorption/dissolution coefficients for uranium based on ratios of 3050 digestion acid- to DI-leach results range from 3.10 to 5.74 mL/g and from 3.16 to 3.96 mL/g, for the Arroyo Seco and Oxide Butte samples, respectively. Oxyanions of arsenic, selenium, and vanadium represent competing adsorbates that may limit uranium(VI) adsorption onto smectite and ferric (oxy)hydroxide present at Oxide Butte. Higher concentrations of dissolved uranium(VI) occurring in oxidizing groundwater in other areas of the Española Basin are associated with increasing concentrations of dissolved sodium and decreasing concentrations of dissolved calcium. This suggests that exchange reactions result in adsorption of Ca²⁺ onto exchange sites and release of Na⁺ and UO₂²⁺ to groundwater.