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<br />Only three constituent concentrations in the Uncompahgre River exceeded <br />Colorado water-quality standards for the classified uses of the river <br />(table 11). The mercury concentrations in samples collected in March at <br />sites 4 and 9 (table 16) exceeded the State standards (0.05 ~g/L). The <br />sulfate concentration for site 4 for January exceeded State standards. The <br />sulfate concentrations for site 9 exceeded the drinking-water standard of <br />250 mg/L (U.S. Environmental Protection Agency, 1986), however, the <br />Uncompahgre River at site 9 is not classified by the State for use as a <br />domestic water supply. <br /> <br />Selenium concentrations in the Uncompahgre River at site 9 did not exceed <br />the Colorado standard of 35 ~g/L for the lower Uncomphagre River. However, <br />the selenium concentrations in the samples collected in November 1987 <br />(33 ~g/L), January 1988 (34 ~g/L), and in July 1988 (18 ~g/L) exceeded the <br />chronic criterion for protection of aquatic life (5 ~g/L), and the concen- <br />trations in the samples collected in November and January also exceeded the <br />acute criterion for protection of aquatic life (20 ~g/L). <br /> <br />The hydrographs for the Uncompahgre River shown in figures 5 and 6 <br />indicate that the samples collected in November and January represent average <br />hydrologic conditions, but that the samples collected in July were collected <br />at stream discharges considerably less than the long-term stream discharge. A <br />number of constituent concentrations for site 9 were considerably less in <br />March than in the samples collected in November, January, and July at this <br />site (figs. 10 through 12 and table 16). The sample collected in March at <br />site 9 was during a runoff period (fig. 6), and probably most of the increase <br />in stream discharge between site 4 and site 9 in late March was snowmelt from <br />tributaries (such as Spring and Dry Creeks). Dissolved-solids and major-ion <br />concentrations had less seasonal variation than would be expected at site 4 <br />because of the effect of Ridgway Reservoir. <br /> <br />The parathion concentration of 0.33 ~g/L in the sample collected in July <br />at site 9 substantially exceeded the criterion of 0.04 ~g/L for protection of <br />aquatic life. Methyl parathion also was detected in that sample. <br /> <br />Sweitzer Lake <br /> <br />Concentrations of dissolved solids (fig. 13), major ions, particularly <br />sodium, sulfate, magnesium, and nitrite plus nitrate, and selenium (fig. 14) <br />increased in Sweitzer Lake (site 7) during the nonirrigation season (November <br />through March) based on the samples collected in November, January, and Ilarch <br />(table 16). The concentrations shown in figures 13 through 15 for Sweitzer <br />Lake for November and July are averages of the two samples collected during <br />those months, and the concentrations shown for March are the averages of the <br />concentrations in the samples collected at depths of 1 and 19 ft (table 16). <br />Concentrations of boron (table 16) and uranium (fig. 15) did not increase <br />substantially during the nonirrigation season. Constituent concentrations <br />increased in Sweitzer Lake during the nonirrigation season because much of the <br />inflow into the lake probably was irrigation-drainage water. The maximum <br />constituent concentrations for the four sets of samples collected for this <br />study were in March. Specific-conductance measurements of Sweitzer Lake <br />on May 10 (range from 2,830 to 2,900 ~S/cm) were slightly less than the <br /> <br />42 <br />