Selenium (Se) is a potentially toxic element that can be leached to surface and groundwater by irrigation of seleniferous soils derived from Cretaceous shales. Irrigation water containing elevated levels of selenium has been implicated in low reproduction rates, embryo deformities and death in wildlife utilizing these waters. The objectives of this study are to determine current Se concentrations and loads in irrigation source and drain waters and natural streams throughout the Natrona County - Alcova Irrigation District in Wyoming, and compare them with a previous study from 1988. A second phase of the project is to investigate adsorption and biological remediation methods. Both natural streams and irrigation drains were sampled monthly and analyzed for total Se. Discharge was also measured and Se loads were calculated. Selenium levels did not exceed 8 ug g-1 in the irrigation source water, but rose to 354 ug g-1 in some of the drain waters. Selenium concentrations increased with distance downslope in the drainages. Concentrations of Se were highest in waters on the seleniferous soils inside the irrigation district, and lowest outside the district off of the seleniferous soils. Se loads were quite variable and no trends were apparent. Phase 2 of this study has been initiated to examine the effects of some treatments to reduce Se in irrigation return flows. Adsorption onto cupric oxide was not effective due to interference by sulfate. A rotating biological contactor is being constructed to determine the effectiveness of a biological treatment.