Distribution and mobilization of dissolved selenium in ground water of the irrigated grand and Uncompahgre Valleys, Western Colorado

Distribution of dissolved selenium in ground water of the irrigated Grand and Uncompahgre Valleys is affected by the aqueous geochemical environment of the shallow ground-water system composed of alluvium and shale residuum. Selenium concentrations are as high as 1,300 micrograms per liter in water from shallow wells. The highest concentrations of dissolved selenium were in water from wells completed in alluvium overlying the Mancos Shale of Cretaceous Age, and the lowest concentrations were in water from wells completed in terrace deposits on the western side of the Uncompahgre Valley and in water from wells completed in Mancos Shale residuum. Factors controlling the mobilization of dissolved selenium in the Grand and Uncompahgre Valleys could include oxidation/reduction, adsorption/desorption, and(or) ion exchange. Infiltration of irrigation water provides oxidizing conditions for mobilization of selenium from alluvium and shale residuum and transport to streams and irrigation drains tributary to the Uncompahgre, Gunnison and Colorado Rivers.