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<br />4,840 ft, 4,805 ft, 4,776 ft, and 4,768 ft. Water can be released concur-
<br />rently through the south outlets from one or more intake lines. The fish
<br />hatchery outlets, wllich are comprised of multilevel intake lines, can release
<br />reservoir water from elevations of 4,85] ft, 4,8]1 ft, 4,786 ft, or 4,736 ft
<br />(U.S. Bureau of Reclamation, written commun., 1986).
<br />
<br />Retention time is defined as the time necessary for the volume of water
<br />in a reservoir to be replaced by inflowing water or the time necessary for the
<br />volume of water in a reservoir to be drained by outflow. The flow-through
<br />time may not represent the actual residence time of water entering the res-
<br />ervoir because of various mixing and circulation patterns that occur within
<br />the reservoir. During 1985, an average cumulative rate of water released from
<br />the reservoir was about ],320 ft'/s (2,620 acre-ft/d) (U.S. Bureau of Reclam-
<br />ation, written commun., 1986). This outflow rate is equivalent to an average
<br />retention time of 102 days or almost 3.5 months, assuming a reservoir volume
<br />of 267,800 acre-ft (average of the end of the month contents for 1985). The
<br />retention time for May, June, and July 1985 averaged 48 days (streamflow
<br />averaged 2,780 ft'/s and reservoir contents was 264,000 acre-ft). Large
<br />variations in retention time have occurred in Pueblo Reservoir as a result
<br />of large variations in storage and outflow. Variations in retention time at
<br />varying flows and selected capacities are shown in figure 4 and can range from
<br />a few days to several months.
<br />
<br />WATER QUALITY OF THE UPPER ARKANSAS RIVER
<br />
<br />The chemical quality of water in Pueblo Reservoir is greatly affected
<br />by the quality of water in the Arkansas River because the Arkansas River is
<br />the primary inflow to Pueblo Reservoir. The chemical quality of the upper
<br />Arkansas River basin has been studied by Cain (1987), Crouch and others
<br />(1984), Roline and Boehmke (1981), Miles (1977), La Bounty and others (1975),
<br />Moran and Wentz (1974), Wentz (1974), and the Federal Water Pollution Control
<br />Administration (1968). The reader is referred to these reports for a detailed
<br />description of historical water quality of the upper Arkansas River basin.
<br />The following discussion is intended to provide an overview of the quality of
<br />water in the upper basin and to provide a general understanding of the quality
<br />of water entering the reservoir.
<br />
<br />The chemical quality of surface water in the upper Arkansas River basin
<br />is affected by runoff from snowmelt and rainfall, mine drainage, wastewater-
<br />treatment-plant effluents, ground water, and land and water use within the
<br />basin. Water entering the basin near the headwaters of the Arkansas River and
<br />its tributaries is derived mostly from snowmelt and generally is suitable for
<br />most uses (Federal Water Pollution Control Administration, 1968). As the
<br />streams come in contact with mine drainage, the pH of water in the streams
<br />decreases, and dissolved-solids and metal coocentrations increase. The
<br />reaches of stream affected by metal-mine drainage were determined by Wentz
<br />(1974) (fig. 5) and generally are localized except when abnormal quantities of
<br />acid mine drainage are discharged, such as the discharges that occurred during
<br />1983 and 1985. On February 23, 1983, a large quantity of metal-mining sludge
<br />was discharged from the Yak Tunnel in the Leadville area. The effect of the
<br />discharge on metal concentrations was measured 17 mi downstream on the
<br />Arkansas River. During October 1985, another large quantity of metal-mining
<br />
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