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(Anderson and others, 1961). Two soil samples collected by the U. S. Bureau of Reclamation in 1978 at Sweitzer Lake had 0.12 and 0.24 ~g/g selenium reported as soluble selenate. Table 4.--Summary of selected water-quality data for samples collected from the Mancos Shale in west-central Colorado [Analyses by U.S. Geological Survey; concentrations are in micrograms per liter, except for dissolved solids and nitrate nitrogen, which are in milligrams per liter; all constituents are dissolved unless noted; NO, not detected; <, less than; --, not determined. Data-collection period is 1974-80] Constituent Number of Median Maximum Minimum samples Dissolved solids 13 1,810 8,220 306 Nitrate as nitrogen 12 2.0 8.4 .03 Arsenic 8 2 5 Arsenic, total 2 2 1 Boron 9 330 1,000 <20 Cadmium 6 <2 3 NO Cobalt 6 NO NO Copper 6 3 43 NO Iron 13 50 2,100 <10 Lead 6 5 10 NO Mercury 6 <.1 <.1 <.1 Nickel 6 2.5 6 <2 Selenium 8 8 100 <1 Selenium, total 2 16 1 Zinc 6 30 4,200 <20 The only sites where bottom-material samples from streams were collected were at the two sites on Spring Creek. The U.S. Geological Survey collected three samples at each site during 1977-81. All samples had selenium concen- trations reported as 0 ~g/g (reporting limit probably was 1 ~g/g). The mean concentrations of selected trace elements for the three samples collected at the site near the mouth of Spring Creek (downstream from irrigation drainage) were: arsenic, 5 ~g/g; copper, 2 ~g/g; lead, 10 ~g/g; mercury, 0.01 ~g/g; vanadium, 2 ~g/g; and zinc, 20 ~g/g. The Colorado Division of Wildlife reported 305 ~g/g of selenium in a bottom-sediment sample collected from Sweitzer Lake in 1974 (Colorado Division of Wildlife, written commun., 1987). Biological Data The Colorado Division of Wildlife collected data in 1974 and 1977 from Sweitzer Lake (Colorado Division of Wildlife, written commun., 1987). The lake was sampled because of suspected problems related to selenium in water 26