PNNL

Abstract

Multiple strategies and scenarios were evaluated to advance Operable Unit (OU) III of the Monticello Mill Tailings Site (MMTS) towards closure. MMTS is a former mill site that was used to process uranium and vanadium ore in southeastern Utah until 1960. Uranium is the primary contaminant of concern in OU III, which includes surface water in Montezuma Creek and groundwater present in a shallow alluvial aquifer along the creek valley for several miles downgradient of the former mill site. Surface remedial actions were completed in 2000. In 2004, the selected final remedy was monitored natural attenuation (MNA) and institutional controls, and a pump-and-treat system installed in the highest-concentration area as a contingency remedy. The pump-and-treat system was expanded in 2015, and average uranium concentrations have declined locally over time. However, monitoring data and column studies suggest that the system effectiveness will continue to decrease over time. Three closure strategies were considered: restoration of MNA; alternate concentration limits; and a technical impracticability (TI) waiver. The preferred closure strategy is to restore MNA and maintain institutional controls, without active remediation. The strategy includes demonstrating criteria for MNA acceptance, pump-and-treat termination, long-term monitoring, and eventual site delisting. Several potential scenarios for transitioning to the preferred closure strategy were identified. Recommended actions over the next few years include updating the conceptual site model, conducting numerical modeling to predict remedial performance and time frames, geochemical analyses to assess uranium transport, and summarizing the lines of evidence and basis for MNA.