MOBILIZATION OF NATURALLY OCCURRING URANIUM IN RESPONSE TO THE INFLUX OF NITRATE INPUTS INTO SUBSURFACE SEDIMENTS

Groundwater uranium (U) concentrations have been measured above the US EPA maximum contaminant level (30ppb) in many US aquifers, including in areas not associated with anthropogenic contamination by milling or mining. We previously demonstrated a correlation between elevated nitrate and uranium groundwater concentrations in two major aquifers suggesting that nitrate plays a role in the oxidative dissolution of U(IV) minerals. To test this hypothesis, shallow subsurface sediments were collected from an alluvial aquifer in the Platte River Floodplain from a region where groundwater U concentrations have been measured over the EPA MCL. Groundwater collected from the borehole revealed a reduced environment (~ -300 mV and dissolved oxygen <0.1ppm) and the presence of sedimentary U(IV) (50% total U) was confirmed by XANES. Up-flow column reactors packed with natural sediment and 50:70 mesh sand (50% mass/mass) under an anoxic atmosphere (Ar:CO2:H2; 80:15:5) were amended with a continuous flow of pH 7.2 bicarbonate buffered artificial groundwater. A flow rate of 12 cm/day of artificial groundwater medium (pH 7.2) was maintained over the course of the experiment. All columns underwent a pre-equilibration period where nitrate was omitted from the groundwater medium. During column pre-equilibration, adsorbed uranium was removed from sediments and measured in the column effluent. When U concentrations consistently were measured below 1ppb, nitrate was added to the influent. Columns in which nitrate was omitted or azide was added served as negative controls. Following the pre-equilibration period, more U was present in the effluent of columns with nitrate compared to unamended columns or columns with azide. Nitrate was reduced to nitrite during this period. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is the mechanism driving U mobilization from natural sediments. Overall, this research indicates that microbial reduction of nitrate to nitrite results in the mobilization of naturally occurring U(IV) minerals.