Attenuation and Transport Mechanisms of Depleted Uranium in Groundwater.

Lawrence Livermore National Laboratory Site 300 was established in 1955 to support weapons research and development. At Building 812, depleted uranium was used as a proxy for fissile uranium-235 in open-air explosives tests. Depleted uranium was deposited on the ground and migrated into the underlying oxic sandstone aquifer. Groundwater uranium concentrations exceed the California and Federal Maximum Contaminant Level of 20 pCi L^-1 (30 ug L^-1). The uranium plume appears to attenuate within 60 meters. Further characterization of the uranium distribution is needed to evaluate potential remedial approaches. The objective of this study is to determine the relative contribution of physical, chemical (e.g. surface adsorption, mineral precipitation), and biological processes that mediate the attenuation of uranium. We are evaluating 15 years of hydrogeologic and chemical data, creating a site conceptual model, and applying equilibrium (e.g. aqueous species complexation, mineral saturation indices), kinetic, and transport models using Geochemist’s Workbench. Reactive transport results are constrained by direct field observations (e.g. U major ion, and dissolved O₂ concentrations, pH) under varying chemical and hydraulic conditions. Aqueous speciation calculations indicated U primarily exists as anionic CaUO₂(CO₃)₃^2- or neutral Ca₂UO₂(CO₃)₃⁰ species. Additionally, formation of Swartzite (CaMg(UO₂)(CO₃)₃ •12(H₂O) solids are predicted. Initial results suggest surface adsorption (e.g. surface complexation) to layer silicate clays is limited under the site’s aqueous geochemical conditions. Current work includes miscible-displacement, surface area, and batch sequential extraction analysis of aquifer solids to constrain iron and aluminum (oxy)hydroxide surfaces.

Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-694815.