Rising and Falling Contaminant Concentrations After Flooding at the Riverton, Wyoming Uranium Mill Tailings Site
The Riverton mill site is located on an alluvial terrace between the upgradient Wind River and the downgradient Little Wind River and both rivers periodically flood. In June 2010, melting snowpack and rainfall resulted in the mean monthly discharge of the Little Wind River equaling 5829 cubic feet per second which was more than 2.5 times higher than the mean discharge for the same month measured since 1941 and caused the highest recorded flood, 4 feet above the National Weather Service flood stage.
The groundwater plume has migrated about one mile downgradient with the highest solute concentrations found near the Little Wind River based on over 20 years of monitoring. After the 2010 flood, dramatic increases in contaminant concentrations including molybdenum and uranium were observed in four groundwater monitoring wells. For example, uranium increased at one well measured two weeks after the peak flooding from about 0.8 mg/L to 2.7 mg/L. The EPA maximum concentration limit in groundwater for uranium is 0.044 mg/L. Subsequently, molybdenum and uranium concentrations have returned to pre-flood levels.
As a result of changing groundwater concentrations from flood events, an enhanced characterization program is evaluating additional sources of contaminants in the unsaturated and saturated zone soils and sediments and re-examining hydrologic and geochemical mechanisms controlling contaminant transport. Field, laboratory, and numerical modeling results indicate that the natural attenuation compliance strategy may not be achievable as previously thought. To protect public health, DOE works with local groups to restrict access and provides a potable water supply.
