AQUEOUS GEOCHEMISTRY OF URANIUM, PAJARITO PLATEAU AND SURROUNDING AREAS, NEW MEXICO

Uranium is an actinide of considerable interest in environmental geochemistry studies conducted at Los Alamos National Laboratory (Laboratory), located west of the Rio Grande on the Pajarito Plateau, and in surrounding areas of the Española Basin, New Mexico. The regional aquifer beneath the Pajarito Plateau contains background concentrations of total dissolved uranium ranging from 1.3 to 10.5 nanomolar (nM), and measurable dissolved oxygen (0.06 to 0.19 millimolar, mM), nitrate(N) (<0.11 mM) and sulfate (<0.07 mM). The regional aquifer varies from a calcium-sodium-bicarbonate to a sodium-calcium-bicarbonate solution with bicarbonate concentrations exceeding 1.1 mM. Uranyl carbonato complexes are predicted to dominate in groundwater. Concentrations of natural reductants including hydrogen sulfide and dissolved organic carbon are not sufficient to enhance stability of uranium(IV) complexes and solid phases. Background distributions of total dissolved uranium are believed to be controlled by chemical alteration of volcanic glass, 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 soddyite and is oversaturated with haiweeite depending on calcium and/or silica activity. Anthropogenic uranium has been released from Laboratory outfalls to surface water since 1943. Surface water on the Pajarito Plateau provides recharge to alluvial and perched-intermediate depth groundwater and the regional aquifer. Elevated above-background concentrations of uranium (maximum of 50 nM) occur in the regional aquifer downgradient from Laboratory outfalls. Nitrate, perchlorate, and/or tritium coreleased with uranium(VI) have migrated to the regional aquifer at depths of 183 m or greater within 40 years since 1943. These chemicals are used to determine the source of uranium in addition to uranium isotopes. Laboratory uranium contamination in groundwater is limited to west of the Rio Grande. East of the Rio Grande between Santa Fe and Española, background concentrations of total dissolved uranium up to 7.6 micromolar have been detected. This area contains numerous volcanic glass deposits in various stages of alteration and roll-front uranium(VI) ore bodies.