LAUSR

ABSTRACT Selenium (Se) is an essential micronutrient for humans and animals and can be toxic when present in high concentrations in soil and water. Many soils in Gunnison and Uncompahgre River Basins in West-Central Colorado are formed from deposits derived mainly from weathered Mancos shale that have high concentration of Se and salts. Elevated concentrations of Se and salts were detected in streams and rivers in this area. The objectives were to determine the amount and distribution of different forms of Se and relationships with physical and chemical properties for soils derived from Se-rich Mancos shale. The information may help to improve land management practices and minimize the environmental impact on natural water resources. A total of 48 topsoil samples and soils from horizons in 9 pedons were investigated. The total and water-soluble Se and other elements as well as physical and chemical properties were determined in soils. The total Se ranged from 330 to 5,673 µg/kg with a mean of 2,224 µg/kg. The water-soluble Se ranged from a minimum of 4.95 µg/kg to a maximum of 2,415 µg/kg and a mean of 338 µg/kg. A highly significant correlation was detected between the total and water-soluble Se in soils. Both the total organic carbon (TOC) and CaCO3% had significant positive correlations with the total Se. A highly significant correlation was also found between the water-soluble Se and electric conductivity (EC), as well as with the water-soluble chloride, sulfate, nitrate, molybdenum, and sodium in soils. For the pedons tested, in general, both the total and water-soluble Se increased with depth. However, the water-soluble Se concentrations were greater in shallow than deep pedons where most Se was removed from soils with drainage water. It could be concluded that Se leaching from topsoils and that dissolving from Se-rich parent materials would be a major source of elevated Se concentrations in streams/rivers for the West-Central Colorado area. To minimize the impact on water quality, an appropriate land/water management practices should be followed to minimize Se leaching from soils derived from Mancos shale.