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CNAPTERFIVE o~rait Eis Beu~slons The Parachute Creek station is the most downstream station in the drainage basin and is located neaz the town of Pazachute. The station neaz Bead-Salle} Parachute is 0.6 nvle downstream from the confluence of West and East Forks Pazachute Creek neaz Gardner Gulch. Water Quality In general, the water quality in the Parachute Creek drainage basin is typical of surface waters from semi-arid areas of western Colorado. The water is slightly alkaline, with mean pH values ranging from 8.0 to 8.5. The major anion in these waters is bicarbonate, with some carbonate present when the pH exceeds 8.3. The concentrations of trace metals in the waters aze generally low and are well below established water quality standards. Concentrations of chemical constituents are generally lowest in the upstream reaches of the Parachute Creek drainage. Farther down the Parachute Creek drainage, soluble salts accumulate from artural runoff and irrigation return flows (COE 1985). Table 3.3-2 presents a summary of the available water qualiri information for stations 09093000 (near Parachute) and 09093500 (at Parachute). The complete raw data from the USGS records are provided in Appendix J of the Final EIS. Comparison of the available water gualiri information to the Colorado Ambient Water Oualiri Standards indicates that the maximum concentrations recorded for cadmium. lead, and selenium occasionally exceed the chronic aquatic life standards. The exceedances were observed in less than 15 percent of the data. All other pazameters aze within the applicable standazds for both sampling stations over the period of record. Page 3-16: Groundwater Flow Groundwater elevations in the Upper Aquifer range from approximately 7,4(10 feet msl in the southern part of the basin to 5,800 feet msl in the northern portion of the basin neaz the confluences of Piceance Creek and Yellow Creek with the White River. In the Project Area, groundwater generally flows from the southwest to the northeast. Robson at~d Saulnier (1980) estimated horizontal hydraulic gradients ranging from 20 to as much as 120 :feet per mile within the basin, with a gradient of approximately 35 feet per mile evident in the Project Area (Robson and Saulnier 1980). Potentiometric surface maps constmcted for the Upper and Lower Aquifers Robson and Saulnier aze presented in Figures 3 4-1 and 3 4-2 respectively Paee 3-17 The potentiometric map for the Lower Aquifer fFieure 3.4-21 shows groundwater elevations and potentiometric contour configurations sunilaz to those of the Upper Aquifer. Horizontal hydraulic gradients in the Lower Aquifer range from 20 to 130 feet per mile; and the flow direction is generally from the southwest to the northeast. The horizontal gradient in the Project Area is estimated to be approximately 25 feet per mile toward the northeast. An assessment of differences in potentiometric head between the Upper and Lower Aquifers indicates that a downwazd vertical eradient exists in the hieher topographic •rreas in the project viciniri and an upwazd vertical eradient ispresent in topographically lower •ueas alone Piceance Creek (Robson and Saulnier 19801 Vertical eradients indicate the potential for eroundwater movement from the Upper Aquifer downwazd throueh the Mahoeanv Zone into the Lower Aquifer or the potential for azoundwater movement upwazd from the Lower Aquifer across the Maho¢anv Zone into the Upper Aquifer. with the rate of movement controlled by the vertical