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effect of other ions have not been obtained at many field locations suggesting that solution and sediment composition may be a factor in some of the seemingly contradictory evidence regarding the concentration of selenium that causes toxicity. Indeed, the effect of other toxic ions on selenium toxicity is one of the more controversial issues. Unlike the Belew's Lake circumstances, elevated selenium in the Upper Colorado Basin and other western locations is accompanied by high concentrations of other ions. For example, Stephens et al. (1992), in a study of Stewart Lake and Ouray National Wildlife Refuge in Utah, found that boron and zinc concentrations, as well as selenium, were frequently greater than aquatic protection standards. Selenium concentrations ranged to 140 gg/L, with most samples having between 50 and 100 gg/L. Uranium was also elevated in some samples. Whether, and how much, the other potentially toxic ions affect selenium toxicity is unknown. Stephens and Waddell (1998) report boron concentrations up to 700 ug/g in aquatic plants in a study on the Uintah and Ouray Indian Reservations. In addition, 54% of the water samples exceeded 750 gg/L, which is the Utah standard for protection of agricultural crops. Some locations exceeded 5000 gg/L-1000 times the current United States EPA criterion of 5 gg/L. Dissolved oxygen fluctuated widely, temperatures were high, and the total dissolved solids (TDS) exceeded 1200 mg/L in 40% of the samples. Vanadium was also elevated. Determining the role of selenium under such conditions is difficult, although elevated concentrations were reported for fish (8.3 ug/g) and invertebrates (-10 ug/g). Definite conclusions regarding selenium accumulation are also difficult to draw from data from Western Colorado's Uncompahgre Project area. Soils on the cretaceous Mancos formation had high total- and water-extractable selenium, but only 5 of 128 samples of alfalfa grown on these soils had selenium concentrations that exceeded a recommended dietary limit for livestock. Of the constituents evaluated in the program, selenium caused the highest proportion of samples to exceed guidelines for fish, but Cu, Zn, B, and V concentrations also frequently exceeded aquatic protection limits (Butler et al. 1996). These investigators stated that irrigation may account for 75% of the selenium in the Colorado River near the Colorado/Utah state line. Nonetheless, toxicity tests on irrigation-drainage water from five streams indicated no significant toxicity differences between control and test samples. 3.1 Toxicity to Fish Selenium's toxicity to fish is well established from the episode at Belew's Lake and a similar unpublished occurrence at Roger's Quarry in Oak Ridge, Tennessee (Southworth, G.R., Oak Ridge National Laboratory, personal communication, September 1999). At both locations, selenium leaching from fly ash disposal resulted in the appearance of deformed fish. Adaptation of data from these incidents to other sites, particularly those in the western United States, remains controversial. Symptoms of selenium deficiency in fish include reduced growth, anemia, exudative diathesis, muscular dystrophy, and increased mortality (Lemly 1998). With respect to specific uptake mechanisms, Lemly (1982) reports that the "maior uptake pathway appears to be erythrocyte transport mediated by active oxo-groups on the selenium anion, with subsequent exchange at preferential sites. High apportionment into the spleen and heart may be influenced by differential formation of selenoproteins."