THE OCCURRENCE AND DISTRIBUTION OF WATER-SOLUBLE SELENIUM IN SOILS OF THE POWDER RIVER BASIN, WYOMING, USA

Selenium (Se) is a naturally occurring element that can bioaccumulate within organisms. Aquatic life can be adversely affected by chronic exposure to water with Se concentrations of 5 mg/L or greater. The existence of Se in soils of the northwestern Great Plains physiographic province (USA) has been known for decades. However, little is known about the source of the Se or the forms in which it exists. This study examined the distribution of the water-soluble fraction of Se in unsaturated zones across the Powder River Basin of the Great Plains province. Water-soluble Se is highly mobile, and previous studies have shown that changes in land use can result in rapid Se contamination of groundwater. Soil cores were collected from land surface to depths of up to 13 m at 32 locations within the Basin. Soil-water extracts were analyzed for Se and anion concentrations. Soil-water content was also determined. Se was detected in most cores; the maximum concentration was about 1.2 mg/kg of soil, equivalent to about 9.8 mg/L of soil water. More surprisingly, perhaps, are the remarkably similar patterns between Se and chloride (Cl) profiles. In semiarid regions, Cl profiles are commonly used for dating soil pore waters; the profiles consist of 3 distinctive zones: a shallow zone (0-1m depth) where most Cl has been flushed out, a zone of illuviation (1-3 m depth) with high Cl concentrations, and a lower zone (>3 m depth) of lower, somewhat uniform Cl concentrations. Atmospheric deposition is assumed to be the major source of Cl in these soils. The similarity of Se and Cl profiles suggests atmospheric deposition is the main source of Se; however, closer investigation indicates that soil parent material is a more likely source. Cl and water-soluble Se in the unsaturated zones of the Powder River Basin may be derived from different sources, but they are transported below the soil zone by similar mechanisms of water movement. The wide-spread occurrence of water-soluble Se in this region implies that any alteration of land use, such as for agricultural irrigation or energy development, has the potential for contamination of surface water and groundwater.