Oxidation and mobilization of selenium by nitrate in irrigation drainage

Selenium (Se) can be oxidized by nitrate (NO3-) from irrigation on Cretaceo us marine shale in western Colorado. Dissolved Se concentrations are positi vely correlated with dissolved NO3- concentrations in surface water and gro und water samples from irrigated areas. Redox conditions dominate in the mo bilization of Se in marine shale hydrogeologic settings; dissolved Se conce ntrations increase with increasing platinum-electrode potentials. Theoretic al calculations for the oxidation of Se by NO3- and oxygen show favorable G ibbs free energies for the oxidation of Se by NO3-, indicating NO3- can act as an electron acceptor for the oxidation of Se. Laboratory batch experime nts were performed by adding Mancos Shale samples to zero-dissolved-oxygen water containing 0, 5, 50, and 100 mg/L NO3- as N (mg Nn). Samples,were inc ubated in airtight bottles at 25 degrees C for 188 d; samples collected fro m the batch experiment bottles show increased Sr! concentrations over time with increased NO3- concentrations. Pseudo first order rate constants for N O3- oxidation of Se ranged from 0.0007 to 0.0048/d for 0 to 100 mg Nn NO3- concentrations, respectively. Management of N fertilizer applications in Cr etaceous shale settings might help to control the oxidation and mobilizatio n of Se and other trace constituents into the environment.