CONTRIBUTION OF URANIUM-BEARING EVAPORITES TO PLUME PERSISTENCE ISSUES AT A FORMER URANIUM MILL SITE, RIVERTON, WYOMING

Milling activities at a former uranium mill site near Riverton, Wyoming, USA, contaminated the shallow groundwater beneath and downgradient of the site. Uranium mill tailings have been removed, and contaminant transport modeling predicted that natural flushing of the groundwater aquifer to a nearby river would achieve compliance with applicable groundwater protection standards by the year 2097. From 1989 to 2009, contaminant concentrations beneath and downgradient of the former mill site were declining steadily; however, local flooding in 2010 mobilized stored contaminants in the downgradient floodplain which resulted in an increase in groundwater contaminant concentrations.

Due to a shallow water table (<2 m) and arid conditions that produce high evapotranspiration rates, the uranium in the groundwater plume can be stored in overlying evaporites deposited in a silt layer. Data from 2015 indicate the presence of subsurface evaporites with up to 8.8 mg/kg uranium overlying the contaminant plume, compared with up to 1.9 mg/kg uranium at a location outside of the plume. In addition, other constituents such as sulfate and chloride are concentrated in the subsurface evaporites but are not correlated with the location of the contaminant plume. During flooding or high groundwater levels, these evaporite constituents can be dissolved and transported into the underlying groundwater. In 2015, multilevel groundwater monitoring points were installed at a distance of 200–800 m from the river bank. A flood in the spring of 2016 confirmed that the dissolution of subsurface evaporites over the contaminant plume contributed additional uranium, sulfate, and chloride to the underlying groundwater in concentrations up to 1.76, 3160, and 205 mg/L, respectively, above preflood concentrations. Outside of the contaminant plume, uranium, sulfate, and chloride were also flushed into the underlying groundwater, but at lower concentrations, with up to 0.01, 280, and 38 mg/L, respectively, added to preflood concentrations. The amount of uranium in the evaporite deposits that is not mobilized or is reconcentrated after a flooding event is still being investigated. The presence and understanding of these secondary contaminant sources contained in evaporites are being used for a more informed management approach at the Riverton site.