干旱区生态环境知识资源中心

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 groundwater 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 below the former mill site and downgradient of it were declining steadily; however, local flooding in 2010 mobilized stored contaminants in the downgradient floodplain and resulted in an increase in groundwater contaminant concentrations. These stored contaminants or persistent secondary contaminant sources were not considered in the original conceptual site model and groundwater modeling predictions with natural flushing.

Additional data collection was performed on the floodplain in 2014 and 2015 to assess the possibility that secondary contaminant sources (specifically uranium) have been retained at the surface and in the subsurface through various processes. Data collection activities included surficial sediment sampling, trenching for direct subsurface observations, sonic drilling for core collection and multilevel well installations, and hand augering for shallow core collection and the installation of pore-water samplers.

Results indicate that uranium is being concentrated and stored in surface and subsurface evaporites and naturally reduced zones (NRZs) with high organic carbon and sulfide content. The evaporites and NRZs contain up to 66 and 140 mg/kg uranium, respectively, with resulting pore-water concentrations up to 2.68 mg/L uranium in the NRZs. These values compare to <1.3 mg/kg uranium in the typical underlying sand and gravel aquifer solids and up to 1.7 mg/L uranium in the current groundwater. It appears likely that the NRZs provide a mechanism to concentrate uranium under reducing conditions. Subsequent oxidation of the NRZs provides a release mechanism for uranium to the overlying unsaturated zone pore waters and the underlying groundwater. Due to a shallow water table and arid conditions that produce high evapotranspiration rates, the uranium-rich pore waters provide a mechanism to form overlying uranium- rich evaporites. The presence and understanding of these persistent secondary contaminant sources can be used for more informed management of the Riverton site.