Groundwater uranium (U) concentrations
have been measured above
the U.S. EPA maximum contaminant level (30 μg/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.
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