Wednesday, April 14, 2010: 4:20 p.m.
Continental A (Westin Tabor Center, Denver)
Uranium is an actinide of considerable interest in environmental geochemistry studies conducted within the Espaņola Basin, New Mexico. The regional aquifer within the Espaņola Basin contains highly variable background concentrations of total dissolved uranium ranging from 0.002 to 7.6 micromolar. Numerous volcanic glass deposits in various stages of alteration contain roll-front uranium(VI) ore bodies east of the Rio Grande between the cities of Santa Fe and Espaņola. The regional aquifer typically is oxidizing with respect to uranium, sulfur, and nitrogen, and contains measurable concentrations of dissolved oxygen greater than 0.06 millimolar. The regional aquifer consists of calcium-sodium-bicarbonate, sodium-calcium-bicarbonate, and mixed major ion compositions with bicarbonate concentrations commonly exceeding 3.3 millimolar. Uranyl carbonato complexes are predicted to dominate in regional aquifer groundwater. Concentrations of natural reductants including hydrogen sulfide and dissolved organic carbon are not sufficient to enhance stability of uranium(IV) aqueous complexes and solid phases in the regional aquifer. Background distributions of total dissolved uranium are believed to be controlled by precipitation of uranium(VI) minerals, specific adsorption of uranium(VI) complexes onto hydrous ferric oxide, and cation exchange of uranyl cation with calcium in smectite. The regional aquifer shows variable saturation with respect to soddyite and approaches saturation with respect to haiweeite depending on pH and the activities of calcium and silica. West of the Rio Grande, surface water and alluvial and perched intermediate-depth groundwater provides recharge to the regional aquifer beneath the Pajarito Plateau west of the Rio Grande. Anthropogenic uranium has been released from Los Alamos National Laboratory outfalls to surface water since 1943. Localized and elevated above-background concentrations of dissolved uranium (maximum of 0.025 micromolar) occur in the regional aquifer downgradient from Laboratory outfalls.