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Selenium has attracted a tremendous amount of attention in the past few decades due to it's toxic effects on wildlife. High selenium concentrations in agricultural drainage water at Restersen National Wildlife Refuge in California brought this clearly to the attention of land managers. It should be understood that selenium is not a rare earth element. It is found in virtually all materials on earth. It also is an essential element for anima]. nutrition. It is abundant in many phosphate rocks (concentrations of 1-300 mg/kg) and can be introduced to selenium free areas in the form of phosphate fertilizer. Selenium is also abundant in marine shales, particularly the Pierre Shale on the Great Plains, the Mancos Shale in the intermountain West, and numerous other units. Selenium can be leached from bedrock by agricultural drainage water which can then be transported to rivers, lakes, and wetlands. In rivers it is moved downstream yet certain streams -like the Gunnison River and the Colorado River can have Se concentrations of 5 to 10 ug L'1. Certain lakes and wetlands accumulate Se as water evaporates and can attain concentrations of several hundred ug L'1. Because all study areas that we examined this past summer have some water sources that contain moderate to high concentrations of selenium it is necessary to more fully understand selenium in the aquatic environment. A recent special publication of the Soil Science Society of America (SSSA 1989) reviews the chemistry and biological significant of selenium and hexe we provide a summary of many important points. The goal of our review is not to resolve the selenium issue on these sites, but to bring up some of the more important points that will stimulate further investigation and understanding of this problem in the future. The biogeochemistry of selenium is very complex. In general, selenium occurs as four chemical species, selenide (Se2' ), elemental selenium (Se), selenite (Se2'3}, and selenate (Se2-a)• The species which occurs depends upon pH and redox relationships of its environment. At low redox potentials (Eh <0.0 mv) and neutral to alkaline pH's selenite is the most common species. Elemental selenium occurs at redox potentials between 0.4 and 0.0 my at acid to alkaline pH's. The other selenium species occur at higher redox potentials. selenate is the most mobile species and occurs in oxic (aerobic) environments. It can be leached from shale and other parent materials and is water soluble. selenite is somewhat less soluble but is mobile. Elemental selenium and selenide occur in anoxic (anaerobic} environments and are relatively insoluble, immobile and not biologically available. We have measured redox potentials in Colorado marshes for the past 7 years and we estimate that the Escalante marsh sediments under our aquatic, bulrush and alkali bulrush communities have redox potentials 25