MOBILIZATION OF NATURALLY OCCURRING URANIUM FOLLOWING THE INFLUX OF NITRATE INTO AQUIFER SEDIMENTS

Nitrate has been proposed to stimulate the mobilization of naturally occurring uranium from sediments into groundwater. The direct role of nitrate stimulated U mobility was tested in a series of upflow column bioreactors. Shallow subsurface sediments were collected from an alluvial aquifer in the Platte River Floodplain. Groundwater collected from the borehole revealed a reduced environment (~ -300 mV and dissolved oxygen <0.1ppm). The presence of reduced uranium species in sediments, U(IV), (50% total U) was confirmed by XANES. Up-flow column reactors were packed with natural sediment and 50:70 mesh quartz sand (50% mass/mass) under an anoxic atmosphere (Ar:CO₂:H₂; 80:15:5) and amended with a continuous flow of anoxic (Ar:CO_2; 80:20) bicarbonate buffered artificial groundwater (pH 7.2) at a flow rate of 12 cm/day. All columns underwent a pre-equilibration period where nitrate was omitted from the groundwater medium. During column pre-equilibration, uranium was measured in the column effluent indicating U desorption. When U concentrations were consistently measured below 1ppb, nitrate was added to the influent in duplicate reactors. Duplicate reactors in which nitrate was omitted served as a control for medium-uranium interactions and duplicate reactors amended with nitrate and azide served as a control for microbial mediated interactions. Following the nitrate amendment, U effluent concentrations increased after 8 days reaching a maximum of 10 ppb. Concurrent with increases in U effluent concentration, nitrate was reduced. During this period, the concentration of nitrate in the effluent decreased and nitrite was measured (up to 0.4mM). No significant loss of uranium was observed (<1 ppb) in the reactors in which nitrate was omitted or azide was added. No significant nitrate reduction was observed in reactors in which azide was added, indicating microbial nitrate reduction was suppressed. Together these results indicate that microbial activity, specifically nitrate reduction, is one mechanism driving U mobilization from natural sediments. We also observed the mobilization of U from sediments following desorption of U from the mineral surfaces demonstrating that, in addition to nitrate mediated U mobilization, desorption also plays a role in the mobilization of naturally occurring U.