Integrating Aquifer Geochemistry and Sediment Properties into Modeling Subsurface Uranium Retention at the Nuclear Metals, Inc. Superfund Site

Uranium has been used extensively since the mid 20th century for energy, defense-related and industrial applications. As a result, anthropogenically deposited uranium wastes affect many environments across the United States. Understanding and predicting the mobility of U in these environments is difficult, because U has complex speciation at circumneutral pH and in groundwater chemistries typical of oxidized aquifers. In this study, the effects of solution chemistry and sediment characteristics on U(VI) speciation and retention have been evaluated and modeled using natural sediments from the Nuclear Metals, Inc. Superfund site in Concord, MA. In these systems, most uranium is present as U(VI), and adsorption is the primary mechanism of U(VI) retention at U concentrations <50 μM. U(VI) mobility increases dramatically in the presence of dissolved calcium and carbonate, due to the formation of soluble Ca uranyl carbonate complexes. The complex adsorption behavior of uranyl was described using both a generalized composite surface complexation model (SCM), and a multi-site Langmuir isotherm based model. Both models incorporated the effects of solution chemistry (excluding pH) on U adsorption, and both effectively described the variability in U adsorption. Sediment surface area, quartz content, and particle size distribution were important sediment variables in predicting uranium partitioning. Although the SCM is a more mechanistic description of U surface complexation, measured adsorption was slightly better fit by the Langmuir isotherm approach. Future efforts to predict U transport in natural systems would be markedly improved by the incorporation of models such as these.