Environmental Aqueous Geochemistry of Uranium in Aquifer Systems, Pajarito Plateau, New Mexico

Operated by the Los Alamos National Security, LLC for the DOE/NNSA Abstract Los Alamos National Laboratory (LANL) has conducted multidisciplinary research on uranium since the mid-1940s. Treated and non-treated industrial aqueous discharges, mainly containing isotopically natural uranium with a 238U/235U atom ratio = 137.8813, have been discharged to Acid, Pueblo, Los Alamos, and Mortandad Canyons. These discharges provide recharge to shallow alluvial and perched-intermediate depth groundwater and the regional aquifer at typical depths of <24 meters (m), 183 m, and 296 m, respectively. Background water chemistry in the regional aquifer varies from an oxidizing (median Eh = 332 mV), calcium-sodium-bicarbonate to a sodium-calcium-bicarbonate composition. Small amounts of enriched uranium, containing a 238U/235U atom ratio <137.8813, have been locally measured in alluvial groundwater within Mortandad Canyon. The Environmental Protection Agency drinking water standard for total uranium and the New Mexico Water Quality Control Commission standard for dissolved uranium is 0.126 μM (0.030 mg/L). Upper tolerance limits have been calculated by the New Mexico Environment Department and LANL for numerous solutes naturally present in the regional aquifer. These include total dissolved uranium (5.336 nM, 0.00127 mg/L), dissolved oxygen (0.253 mM, 8.10 mg/L), nitrate(N) (0.056 mM, 0.78 mg/L), chloride (0.084 mM, 2.98 mg/L), perchlorate (4.324 nM, 0.00043 mg/L), and sulfate (0.061 mM, 5.84 mg/L). Background concentrations of dissolved uranium in the regional aquifer increase with average groundwater age, especially east of the Rio Grande. Concentrations of dissolved uranium(VI) elevated above background, with a maximum value of 0.0504 μM (0.012 mg/L), have been detected in several regional aquifer monitoring wells installed in Pueblo, Los Alamos, and Mortandad Canyons. Concentrations of nitrate, perchlorate, and/or tritium coreleased with aqueous uranium(VI) species are elevated above background in the regional aquifer at several monitoring wells. Thermochemical calculations suggest that uranyl carbonate-carbonato complexes, including UO2CO3 0, UO2(CO3)2 2-, Ca(UO2)2(CO3)3 0, and UO2(CO3)3 4-, dominate in the regional aquifer. These uranyl complexes are mobile under oxidizing and circumneutral pH conditions characteristic of the regional aquifer at Los Alamos. Concentrations of natural reductants, including dissolved hydrogen sulfide, dissolved ferrous iron, dissolved organic carbon, and solid organic matter are not sufficient to enhance reduction of uranium(VI) to uranium(IV) aqueous complexes (UOH3+ and U(OH)4 0). The regional aquifer is undersaturated with respect to amorphous UO2, uraninite, and coffinite. Background concentrations of dissolved uranium(VI) in upper sections of the regional aquifer are initially controlled by partial dissolution of soluble volcanic glass followed by specific adsorption of uranium(VI) complexes onto hydrous ferric oxide and cation exchange of uranyl cation with calcium on smectite surfaces. Upper sections of the regional aquifer beneath the Pajarito Plateau are enriched in silica and groundwater shows variable saturation with respect to uranophane (Ca(H3O)2(UO2)2(SiO4)2 3H2O) and are oversaturated with respect to haiweeite (Ca(UO2)2Si6O15·5H2O) depending on pH and calcium and silica activities. Purpose and Scope