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<br />pH. Values of pH ranged from 5.9 to 10.0 stan-
<br />dard units. Only 23 of more than 1,000 pH values were
<br />outside the range of the recommended stream standard
<br />for pH (6.5-9.0). Ofthose values, 18 were measured at
<br />site 29 between 1969 and 1997. There was no discern-
<br />ible long-term or seasonal pattern to the measurements
<br />that were outside the recommended range for pH.
<br />Eighty percent of the pH values were between 7.5
<br />and 8.6 within the Gore Creek watershed. The toxicity
<br />of dissolved ammonia increases with increasing pH,
<br />but ammonia concentrations were low at site 29,
<br />precluding any possible concern for ammonia toxicity.
<br />Water temperature. Water temperatures within
<br />the Gore Creek watershed typically were low. More
<br />than 90 percent of the temperature measurements
<br />were less than 110 C. During an August 1996 sampling
<br />of 13 sites on the main stem of Gore Creek, Wynn
<br />and Spahr (1998) found little variability in water
<br />temperatures (80-160C). Temperature can influence
<br />the composition of fish communities. Relatively low
<br />water temperatures such as those in the Gore Creek
<br />watershed provide favorable habitat for trout and
<br />sculpin (Deacon and Mize, 1997).
<br />
<br />Inorganic Constituents
<br />
<br />Inorganic constituent data for major ions, trace
<br />elements, and nutrients are useful for describing water-
<br />quality conditions. Major-ion data are needed to deter-
<br />mine the relative significance of various sources of
<br />dissolved constituents in the water column such as
<br />ground-water discharge or precipitation runoff.
<br />Trace-element data provide information about the
<br />natural and human sources of contaminants such as
<br />zinc or cadmium, which can be harmful to aquatic
<br />life. Nutrient concentrations can have a large effect
<br />on stream biota. Seasonal differences and changes
<br />in nutrient concentrations can be a primary factor for
<br />changes observed in the algal and macroinvertebrate
<br />communities.
<br />Major ions. Concentrations of major ions
<br />(calcium, magnesium, sodium, potassium, silica,
<br />chloride, sulfate, and bicarbonate) in Gore Creek
<br />vary with streamflow and season. Data collected peri-
<br />odically from 1995 to 1997 (46 samples) at site 29,
<br />at the mouth of Gore Creek, are plotted on a trilinear
<br />diagram in figure 14. Trilinear diagrams are useful for
<br />examining the relative percentages of major ions for
<br />multiple samples so that trends or patterns in relative
<br />major-ion chemistry may be determined (Freeze and
<br />Cherry, 1979).
<br />
<br />The data were grouped in 3-month time periods in
<br />which the samples were collected (October-December,
<br />January-March, April-June, July-September) so that
<br />seasonal (and associated streamflow) patterns could
<br />be examined. Percentages of major cations indicated
<br />little variation. Calcium was the dominant cation,
<br />accounting for more than 60 percent of cations in all
<br />samples (fig. 14). The relative percentages of anions
<br />indicated seasonal variability. Bicarbonate accounted for
<br />50-60 percent of anions during January-March but as
<br />much as 90 percent during high flow (April-June) when
<br />most streamflow originates from snowmelt. Sulfate and
<br />chloride ions were prevalent, accounting for as much as
<br />50 percent of all major ions, during the annual recession
<br />of the hydrograph (October-December) and during
<br />winter low-flow (January-March) conditions when a
<br />large part of the streamflow consists of base flow rather
<br />than snowmelt (fig. 14).
<br />Trace elements. Historically, concentrations
<br />of several trace elements have occasionally (cadmium,
<br />copper, iron, and silver) or frequently (manganese, in
<br />Black Gore Creek) exceeded aquatic-life stream stan-
<br />dards in the Gore Creek watershed (Northwest Colorado
<br />Council of Governments, 1995). Presence of all of these
<br />trace elements except silver is at least partially attribut-
<br />able to bedrock geology (Northwest Colorado Council of
<br />Governments, 1995; Steele and others, 1991; Wynn and
<br />Spahr, 1998). However, studies in other locations have
<br />linked increased copper and manganese concentrations
<br />to highway runoff and traction sand (Kobringer, 1984;
<br />Novotny and Olem, 1994). Silver concentrations have
<br />been associated with treated wastewater effluent
<br />(Northwest Colorado Council of Governments, 1993).
<br />Land disturbance near Black Gore Creek during
<br />construction of the Interstate 70 roadway in the 1970's
<br />probably caused some of the documented increases
<br />in trace-element concentrations. A study of temporal
<br />trends of trace-element concentrations in the watershed
<br />indicated that cadmium, copper, manganese, and zinc
<br />concentrations have decreased over time (Northwest
<br />Colorado Council of Governments, 1995). Part of the
<br />decrease in trace-element concentrations may be related
<br />to the restabilization of the hill slopes that were disturbed
<br />during highway construction. The stream standard for
<br />manganese, 50 j1gIL, was exceeded only one time among
<br />11 samples collected at 4 sites in Black Gore Creek in
<br />1996 (Crowfoot and others, 1996; Lorch, 1998). That
<br />sample, collected near Vail Pass where the creek flows
<br />into Black Lake (site 30), contained a manganese concen-
<br />tration of 530 j1gIL. The range in manganese concentra-
<br />tions for the 10 samples collected downstream from the
<br />Black Lakes in 1996 was 2-36 j1gIL.
<br />
<br />SURFACE WATER 29
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