Nitrate-Stimulated Release of Naturally Occurring Sedimentary Uranium

Groundwater uranium (U) concentrations have been measured above the U.S. EPA maximum contaminant level (30 mu g/L) in many U.S. aquifers, including in areas not associated with anthropogenic contamination by milling or mining. In addition to carbonate, nitrate has been correlated to uranium groundwater concentrations in two major U.S. aquifers. However, to date, direct evidence that nitrate mobilizes naturally occurring U from aquifer sediments has not been presented. Here, we demonstrate that the influx of high-nitrate porewater through High Plains alluvial aquifer silt sediments bearing naturally occurring U(IV) can stimulate a nitrate-reducing microbial community capable of catalyzing the oxidation and mobilization of U into the porewater. Microbial reduction of nitrate yielded nitrite, a reactive intermediate, which was further demonstrated to abiotically mobilize U from the reduced alluvial aquifer sediments. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is one mechanism driving U mobilization from aquifer sediments in addition to previously described bicarbonate-driven desorption from mineral surfaces, such as Fe(III) oxides.

Automated summary

Groundwater uranium concentrations above the EPA maximum contaminant level have been found in many US aquifers, including those not associated with anthropogenic contamination. In this study, it was demonstrated that high-nitrate porewater can stimulate a microbial community that can mobilize naturally occurring uranium from sediments. Microbial reduction of nitrate produced nitrite, which abiotically mobilized uranium from aquifer sediments.