Modeling of density-dependent groundwater flow and transport at the uranium mill tailings site near Moab, Utah

摘要

A vertical section model of density-dependent groundwater flow was developed for a former uranium ore-processing site near Moab, Utah, that is now administered by the U.S. Department of Energy (DOE). The model accounted for simultaneous flow of water and transport of total dissolved solids (TDS) at concentrations that range from 1 kg/m~3 to those for brine (TDS > 35 kg/m~3). The assumed primary source of the brine was dissolution of deep evaporite deposits in the underlying Paradox Formation. Shallower slightly saline water in the alluvial groundwater system was conceptualized as emanating from sources near the west site boundary, subsequently flowing over and mixing with brine fluids at depth and ultimately discharging to the Colorado River east of the site. The model was approximately calibrated by matching currently observed brine surface elevations and water table levels in the shallow groundwater system. The ambient flow system as affected by brine was found to help concentrate the discharge of tailings-related contaminants near the westernmost portion of the river. Transient simulations indicated that the existing location of the brine surface is slowly changing, reflecting the legacy of tailings operations at the site between 1956 and 1984 and a gradual return to pre-milling conditions since that time.