Nitrate-Stimulated Release of Naturally Occurring Sedimentary Uranium.

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.

Dissolution of Mn-bearing dolomite drives elevated Cr(VI) occurrence in a Permian redbed aquifer.

Municipalities in central Oklahoma, U.S.A. increasingly rely on water drawn from the Central Oklahoma Aquifer (COA) as surface water resources have not grown in proportion to population and current water demands. However, water drawn from certain regions of the COA frequently contains elevated levels of naturally occurring hexavalent chromium. Rock samples from the Norman Arsenic Test Hole Core (NATHC) were investigated to identify the mineralogic host(s) of Cr and mechanisms of Cr(VI) release via bulk mineralogy and chemistry measurements, selective chemical extractions, and microscale elemental analyses. Results demonstrate most COA Cr is contained in Fe oxides and clays as isomorphic substitutions for Fe(III). Analyses of regional groundwater data, including hierarchical clustering methods and GIS, demonstrate the most intense Cr(VI) occurrence is linked to cation exchange with Na-clays at depth. Cation exchange allows dissolution of Mn-bearing dolomite, which in turn produces Mn oxides in otherwise dolomite-saturated groundwaters. Mn oxides in turn are known to oxidize Cr(III) to Cr(VI). In general, co-occurrence of Mn-bearing carbonates and exchangeable clays in any aquifer, particularly those with Cr(III) present in iron oxide cements, serve as ingredients for groundwater occurrences of oxidizable trace metals.

Occurrence of arsenite in surface and groundwater associated with a perennial stream located in Western Nebraska, USA.

Dissolved arsenic typically results from chemical weathering of arsenic rich sediments and is most often found in oxidized forms in surface water. The mobility of arsenic is controlled by its valence state and also by its association with iron oxides minerals, the forms of which are both influenced by abiotic and biotic processes in aqueous environment. In this study, speciation methods were used to measure and confirm the presence of reduced arsenic species in the surface water of Frenchman creek, a gaining stream that crosses the Colorado-Nebraska border. Selective extraction analysis of aquifer and stream bed sediments shows that the bulk of the arsenic occurs with labile iron-rich oxy(hydroxide) minerals. Total dissolved arsenic in surface and groundwater ranged from ~3-18 µg L-1, and reduced arsenic species comprise about 41% of the total dissolved arsenic (16.0 µg L-1) in Frenchman creek. Leachable arsenic in the aquifer sediment samples ranged up to 1553 µg kg-1, while samples from Frenchman creek bed sediments contained 4218 µg kg-1. Dynamic surface and groundwater interaction sustains arsenite in iron-rich surface headwaters, and the implied toxicity of reduced arsenic in this hydrogeological setting, which can be important in surface water environments around the globe.