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Oxidation and Mobilization of Selenium by Nitrate in Irrigation Drainage

U.S. Geological Survey, Water Resources Division, P.O. Box 3367, Durango, CO 81301.

^* Corresponding author (wgwright{at}usgs.gov).

ABSTRACT

Selenium (Se) can be oxidized by nitrate (NO^–₃) from irrigation on Cretaceous marine shale in western Colorado. Dissolved Se concentrations are positively correlated with dissolved NO^–₃ concentrations in surface water and ground water samples from irrigated areas. Redox conditions dominate in the mobilization of Se in marine shale hydrogeologic settings; dissolved Se concentrations increase with increasing platinum-electrode potentials. Theoretical calculations for the oxidation of Se by NO^–₃ and oxygen show favorable Gibbs free energies for the oxidation of Se by NO^–₃, indicating NO^–₃ can act as an electron acceptor for the oxidation of Se. Laboratory batch experiments were performed by adding Mancos Shale samples to zero-dissolved-oxygen water containing 0, 5, 50, and 100 mg/L NO^–₃ as N (mg N/L). Samples were incubated in airtight bottles at 25°C for 188 d; samples collected from the batch experiment bottles show increased Se concentrations over time with increased NO^–₃ concentrations. Pseudo first-order rate constants for NO^–₃ oxidation of Se ranged from 0.0007 to 0.0048/d for 0 to 100 mg N/L NO^–₃ concentrations, respectively. Management of N fertilizer applications in Cretaceous shale settings might help to control the oxidation and mobilization of Se and other trace constituents into the environment.