Water Quality Relationships Between the Colorado River and Alluvial Ground Water at the Moab, Utah, UMTRA Project Site

The U.S. Department of Energy Moab Uranium Mill Tailings Remedial Action Project site is a former uranium-ore processing facility located near Moab, Utah. Site operations began in 1956 and ceased in 1984, generating 16 million tons of mill tailings covering 130 acres with a height of up to 94 feet. Contaminants have leached into underlying alluvium and concentrate at a brine/saline water interface 40 feet below the ground surface. Naturally occurring salinity in the alluvium originates from dissolution of the underlying Paradox Formation. Between 2003 and 2006, 41 remediation wells were installed between the tailings and the Colorado River, which lies 750 feet down gradient from the toe of the tailings pile. These wells extract ground water to mitigate adverse impacts of ammonia and uranium on endangered fish habitat areas adjacent to the riverbank.

 Ground water chemistry and elevation of the brine interface are affected by Colorado River stage. Trilinear diagrams and time-concentration plots were developed from results of ground water and surface water samples collected during seasonal sampling events, including the 2008 abnormally high spring river flow. Preliminary 2008 results indicate that a lens of recharging surface water propagated a minimum of 100 feet inland towards the well field. In years with low to average spring runoff, mixing of surface water and ground water occurs only along the riverbank. Evaluation of the extent and duration of the surface water migration inland, elevation of the brine interface, and the magnitude of constituent concentrations relative to river stage are being used to optimize the contaminant mass removal by the well field and to prevent adverse impacts to riparian channel habitat.