42-7 Measured and Modeled Contaminant Mobility in Uranium Mill Tailings with Column Tests

Abstract:

Cores were collected during the installation of monitoring wells at Rifle Disposal Site in Rifle, Colorado, where wells were implemented to support tailings dewatering efforts. Column studies were conducted on the core samples under oxic and anoxic conditions to quantify and model the mobility of contaminants including uranium and vanadium, within mill tailings. Major cations, anions, and metals were analyzed using Ion Chromatography and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Geochemical processes controlling contaminant release, including mineral dissolution, mineral precipitation, and metal sorption-desorption reactions, were modeled using the geochemical modeling software PHREEQC. Measured results indicate sulfate, uranium, and vanadium were the major contaminants released during leaching, with uranium exhibiting greater release under oxic conditions. Geochemical modeling identified gypsum dissolution as the primary control on sulfate concentrations, and uranium and vanadium release was governed by carnotite dissolution coupled with desorption processes. Model predictions were sensitive to differences in thermodynamic data among available geochemical databases, introducing uncertainty into the interpretation of controlling reactions.

These findings provide a basis for improving dewatering system operations and maintenance by limiting oxygen ingress. Geochemical amendments (such as hydroxyapatite) can reduce uranium mobility and are being tested on the same tailings’ materials. These hydroxyapatite experiments will test the long-term stability of uranium sequestration and are applicable to any site with a goal of permanently fixing uranium to the solid phase.

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