Selenium in Soils and Plants from Native and Irrigated Lands at the Kendrick Reclamation Project Area, Wyoming

In response to earlier findings or elevated levels of selenium in the Kendrick area, two detailed geochemical surveys were conducted in 1988 to stud y the distribution of selenium in soils (0-1 m depth), and new growth of associated big sagebrush (Artemisia tridentata Nutt.) and alfalfa (Medicago sativa L.). A survey of the native rangeland focused on specific geologic units as sources of selenium; whereas, a gridded survey of the irrigated lands assessed the extent of its mobilization, transport, and concentration. Only three of the approximately 200 soil samples contained total selenium slightly greater than the 3.3 ppm maximum baseline established for soils from the northern Great Plains. In contrast, selenium concentrations in about one-fifth of the big sagebrush samples exceeded the 1.1 ppm maximum baseline established for this species from the West. Selenium tended to be elevated, but not uniformly so, in both soils and sagebrush collected from areas underlain by the Cody Shale of Cretaceous age. Alfalfa from about 15% of the irrigated fields contained selenium in excess of about 4 ppm, levels reported to be potentially hazardous to livestock if fed over prolonged periods. Most of these samples were concentrated in an area of 11 contiguous sections where selenium-enriched surface and drain waters also occurred. The agricultural soils just to the north of this seleniferous area had slightly higher levels of selenium compared to those elsewhere in the irrigated lands. At present, the cause for this displaced anomaly is unclear. Followup sampling in 1989 of two fields where selenium levels in alfalfa collected in 1988 were 25 and 15 ppm yielded samples that contained only 0.2 and 0.7 ppm, respectively. This dramatic and puzzling temporal disparity may be explained by marked differences in weather patterns and irrigation practices for the 2 years. Such a disparity underscores the need to monitor a potentially seleniferous area over an extended period.