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Effects of Temperature on the Oxidation of Selenium by Nitrate

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Effects of Temperature on the Oxidation of Selenium by Nitrate

Study Area: Lower Gunnison and Colorado River Basins, west-central Colorado.
Period of Project: March 1997-September 2001
Project Number: CO441
Project Chief: Kirby Wynn
Cooperator: Bureau of Reclamation, Colorado River Water Conservation District, Department of the Interior.

BACKGROUND:

High selenium concentrations have been identified in the lower Gunnison River basin and in the Uncompahgre River. Based on studies by the National Irrigation Water Quality Program (NIWQP) of the Department of the Interior, the primary selenium source is irrigation drainage from soils derived from the Mancos Shale. Elevated selenium concentrations in irrigation drainage could be exacerbated by the application of nitrate fertilizers to soils derived from the Mancos Shale. Reduction of nitrate fertilizers could prevent the oxidation and mobilization of selenium into the hydrologic environment. Laboratory experiments done on Mancos Shale soils at 25 degrees C indicate that higher nitrate concentrations yield higher selenium concentrations. Because ground-water temperatures in western Colorado are less than 25 degrees C, additional experiments are being done at 5 and 12 degrees C to test whether temperature has an effect on the oxidation of selenium by nitrate. In addition, experiments are being done to test the oxidation of selenium by dissolved oxygen compared to the oxidation of selenium by nitrate.

OBJECTIVES:

1. To describe the effects of different nitrate concentrations on the rates of selenium oxidation.
2. To describe the effects of different temperatures on the rates of selenium oxidation.
3. To describe the relative effects of selenium oxidation by nitrate compared to selenium oxidation by dissolved oxygen.