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.' - ,'~. '\ ....'j"'"..; .. Purpose and Scope This report describes the reconnaissance phase of the comprehensive water-quality investigation of Pueblo Reservoir that was done during 1985. The objectives of this report are to: (I) Provide a better understanding of the reservoir and the quality of water entering the reservoir, and (2) make a preliminary assessment of some of the water-quality characteristics of Pueblo Reservoir. Specifically, this report describes the water quality of the upper Arkansas River basin and the quality of water entering the reservoir. The areal, vertical, and seasonal water-quality variations that occurred in Pueblo Reservoir during 1985 are described. Measurements of water temperature, spe- cific conductance, light transparency, dissolved oxygen, and pH are reported for numerous sites in the reservoir. In addition, analyses of major nutrients (nitrogen and phosphorus), major chemical constituents, and trace elements, and densities and relative abundance of phytoplankton and zooplankton from water samples collected from Pueblo Reservoir are included. Finally, infor- mation about potential contaminants to Pueblo Reservoir is provided in the "Supplemental Information" section at the back of the report. The onsite measurements and water samples for chemical and biological analyses were collected during May through December 1985. Methods of Investigation After an initial reconnaissance of Pueblo Reservoir was made during May 1985, seven transects were established from the inflow (transect I) to the dam (transect 7) (fig. 1). Several of the transects are located immediately downstream from tributaries. Three sampling sites were selected along each transect. In addition, station 07097000, Arkansas River at Portland (pl. I) was selected to monitor the quality of the major source of water entering the reservoir. Station 07099400, Arkansas River above Pueblo, was selected to monitor outflow from the reservoir. Water-quality data collected during the reconnaissance study are available upon request from the U.S. Geological Survey, Water Resources Division, Pueblo Subdistrict office in Pueblo, Colo. During 1985, measurements of water temperature, specific conductance, light transparency, dissolved oxygen, and pH were made monthly from June through October and during December at most of the 21 sites in the reservoir. Water temperature, specific conductance, dissolved oxygen, and pH were measured at 3-ft intervals from the reservoir surface to the reservoir bottom with a multiparameter instrument. Water temperature, specific conductance, dissolved oxygen, and pH also were measured at station 07097000, Arkansas River at Portland and at station 07099400, Arkansas River above Pueblo. Light transparency was measured with a Secchi disk--a white, flat, circular disk about 8 in. in diameter. In addition to onsite measurements, water samples were collected monthly from July through October at the middle site of each transect for analyses of chemical and biological constituents. Samples were collected for chemical analyses from near the reservoir surface and from near the reservoir bottom using a 4-L, nonmetallic, 2-ft-long water-sampling bottle. The chemical con- stituents discussed in this report were analyzed by the U.S. Geological Survey Denver Central Laboratory using methods described by Fishman and Friedman 4