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<br />Because the hydrograph at site AR34.5 showed
<br />a slight increase above the base flow from
<br />February 1,1995, through March 31, 1995, this
<br />period was defined as the early spring snowmelt
<br />period upstream from the reservoir. Downstream
<br />from Terrace Reservoir, the entire period from
<br />October 1,1994, through March 31,1995, was
<br />defined as the base-flow period. These flow
<br />periods are used in describing the temporal
<br />variations in metal chemistry and metal loads.
<br />A comparison of the streamflow hydrographs
<br />indicates that, from April I through June 8, 1994
<br />(the early spring snowmelt, pre-peak snowmelt,
<br />and peak snowmelt periods), the volume of water
<br />released from Terrace Reservoir was within
<br />5 percent of the volume of water that entered the
<br />reservoir. During the post-peak: snowmelt period
<br />from June 9 through July 14, 1994, 14 percent more
<br />water was released from the reservoir than entered
<br />the reservoir due to downstream irrigation demand.
<br />During the summer period from July 15 through
<br />September 30, 1994, 31 percent more water was
<br />released from the reservoir than entered the reservoir
<br />due to downstream irrigation demand. As irrigation
<br />demand for water decreased and the reservoir outlet
<br />was closed in early November, water was stored
<br />in the reservoir during fall and winter. During the
<br />base-flow and early spring snowmelt periods from
<br />October I, 1994, through March 31, 1995,46 percent
<br />more water entered the reservoir than was released
<br />from the reservoir. Over the entire study period, there
<br />was only a minor difference between the volume
<br />of water that entered the reservoir and the volume
<br />of water that was released from the reservoir.
<br />Streamflow varies from year to year, depending
<br />on precipitation and temperature conditions in the
<br />upper basin. However, historical discharge data
<br />indicate that peak flow during the study period was
<br />consistent with amount and timing of streamflow in
<br />previous years.
<br />Stogner and Edelmann (1996) indicated
<br />that reservoir residence times during periods of
<br />stratification generally were shortened by 40 to
<br />75 percent of the theoretical (well-mixed) residence
<br />time. The shortest estimated residence times of 3 to
<br />5 days occurred during late May to early June, a
<br />period corresponding to peak snowmelt runoff and
<br />strong thermal stratification. The short residence
<br />
<br />8
<br />
<br />times during late May to early June are important in
<br />that they decrease the amount of time for physical or
<br />chemical processes to occur that might decrease
<br />concentrations of metals before water is released
<br />from the reservoir.
<br />During the study period (April 1994 through
<br />March 1995), pH was measured concurrently
<br />with the collection of water-quality samples.
<br />The instantaneous pH measured at site AR34.5
<br />(upstream from Terrace Reservoir) (fig. 1) ranged
<br />from 4.3 to 7.8 (fig. 3). The highest pH occurred
<br />during the pre-peak snowmelt period; the lowest
<br />pH occurred during storm runoff during the summer.
<br />At site AR31.0 (downstream from Terrace Reservoir)
<br />(fig. 1), pH ranged from 4.6 to 7.6. The highest
<br />pH occurred during the pre-peak snowmelt period,
<br />and the lowest pH occurred during summer in
<br />mid-July (fig. 3).
<br />
<br />METAL CHEMISTRY
<br />
<br />Preliminary ecological and human-health risk
<br />assessments indicated tbat concentrations of dissolved
<br />and total aluminum, cadmium, copper, iron,
<br />manganese, and zinc were large enough to be of
<br />environmental concern (Morrison and Knudsen
<br />Corporation and ICF Keiser Engineers, 1994).
<br />Therefore, the discussion of metal chemistry in this
<br />report is directed toward describing the variations
<br />that were measured at sites AR34.5 and AR31.0
<br />for these metals. Water samples also were analyzed
<br />for dissolved and total arsenic, barium, chromium,
<br />cobalt, lead, mercury, nickel, selenium, silver, and
<br />vanadium. More than 90 percent of the samples
<br />analyzed for dissolved arsenic, lead, and vanadium
<br />and dissolved and total chromium, selenium, and
<br />silver had concentrations less than the analytical
<br />reporting level. Concentrations of total arsenic,
<br />dissolved and total barium, dissolved and total cobalt,
<br />total lead, dissolved and total nickel, and total
<br />vanadium measured in water samples collected at
<br />site AR34.5 generally were greater than the analytical
<br />reporting level (table 2), but were not considered to
<br />be of environmental concern. Therefore, only limited
<br />discussion of these data are included in this report.
<br />Samples collected for analysis of dissolved metals and
<br />trace elements were filtered through a 0.45-j.1m filter.
<br />
<br />Assessment of Metal Trlnlport Into Ind Out 01 Terrace Reservofr, Conejol County, Colorado,
<br />April 1994 Through Merch 1995
<br />
<br />Ol'3163
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