Attenuation and Transport Mechanisms of Depleted Uranium in Groundwater at Lawrence Livermore National Laboratory Site 300

Abstract:

Lawrence Livermore National Laboratory (LLNL) Site 300 was established in 1955 to support weapons research and development. Depleted uranium was used as a proxy for fissile uranium-235 (²³⁵U) in open-air explosives tests conducted at Building 812. As a result, oxidized depleted uranium was deposited on the ground, eventually migrating to the underlying sandstone aquifer. Uranium (U) groundwater concentrations exceed the California and Federal Maximum Contaminant Level of 20 pCi L^-1 (30 ug L^-1). However, the groundwater plume appears to attenuate within 60 m of the source, beyond which no depleted U is detected. This study will determine the relative contribution of physical (e.g. dilution), chemical (e.g. surface adsorption, mineral precipitation), and biological (e.g. biotransformation) processes that contribute to the apparent attenuation of U, which exists as uranyl (UO₂²⁺) complexes, at the site. Methods of investigation include evaluating 15 yr of hydrogeologic and chemical data, creating a site conceptual model, and applying equilibrium (e.g. aqueous species complexation, mineral saturation indices) and reactive transport models using Geochemist’s Workbench^TM. Reactive transport results are constrained by direct field observations, including U major ion, and dissolved O₂ concentrations, pH, and others, under varying chemical and hydraulic conditions. Aqueous speciation calculations indicate that U primarily exists as anionic CaUO₂(CO₃)₃^2- or neutral Ca₂UO₂(CO₃)₃⁰ species. Additionally, nucleation and growth of Ca/Mg uranyl carbonate solids are predicted to affect attenuation. Initial reactive transport results suggest surface adsorption (e.g. ion exchange, surface complexation) to layer silicate clays is limited under the aqueous geochemical conditions of the site. Current and future work includes XRD analysis of aquifer solids to constrain iron and aluminum (oxy)hydroxides, and coupling advective-dispersive transport with the chemical and physical processes.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-675707.