NITRATE-STIMULATED MOBILIZATION OF NATURALLY OCCURRING URANIUM IN GROUNDWATER

Groundwater U concentrations in aquifers within the United States (US) have been measured in excess of the US EPA MCL (30ug/L) in regions with no known direct anthropogenic U contaminant sources implicating a natural U source of contamination. We previously demonstrated a link between groundwater nitrate and uranium contamination. While tetravalent U (U(IV)) phases are usually stable in reduced environments, nitrate input could stimulate oxidative dissolution. Here we evaluate the potential of nitrate-stimulated uranium mobilization directly from subsurface sediments collected from an alluvial aquifer in the Platte River Floodplain, near Alda, NE (US). Sediments and groundwater were collected from a region exhibiting fluctuating redox conditions. The reduction potential indicated a reduced environment (Eh range -361 to -516 mV) and the presence of U(IV) (50% of total U) was confirmed by X-Ray Adsorption Near Edge Spectroscopy (XANES). Upflow meso-scale column reactors were packed with sediment and sterile sand (50% mass/mass) with bicarbonate buffered (pH 7.1) artificial groundwater serving as the influent. Following pre-incubation, the addition of nitrate in the influent stimulated U release from the sediments relative to controls in which nitrate was omitted. Concentrations of U in the effluent periodically exceeded the MCL. Nitrate was reduced to nitrite. Pseudomonas spp. capable of nitrate reduction to nitrite have been successfully stimulated and isolated from this aquifer. Nitrite production may be the mechanism driving the mobilization of U by oxidizing U(IV) minerals and generating U(VI) which would be mobile in groundwater. Alternatively, nitrate reduction reactions may be catalyzing mineral weathering leading to dissolution reactions. Reactive intermediates produced via nitrate reduction reactions could catalyze mineral dissolution leading to the release of naturally occurring U from sediments and increases in groundwater U concentrations above the MCL.