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1r7/98 1528:7 FROM Todd Gilmsr TO Mdcft Albert 's ur > that passes beneaUt the Project area. During the Laramide. the region and Project area were also cut • by several faults, as shown on the USGS geologic map. High to very high hydraulic conductivities (400 to 2000 feet day) characterize the outwash aquifer. based on an interpretation of sieve analyses of the sands and gravels, percolation tests, and water production rates measured during the drilling of hvo monitoring wells. Saturated thickness of the outwash aquifer ranges from a feather edge along the western, northern and eastern margins to more than 25 feet along paleochannels cut into the Carlisle, Graneros and Monson Formations. Along the Arkansas River and lower Tallahassee Creek, springs mark a series of seepage areas where the outwash aquifer discharges to surface streams. Ground water discharged from the springs and seeps eviderrtly has as its major source surface water from Tallahassee Creek as evidenced by TDS measuremrnts, water chemistry and water table elevations. Water chemistry analyses for major ions indicate that surface water is a calcium bicarbonate-type having TDS ranging from about 160 mg/1 for the Arkansas River to 280 milligrams per liter (mg/I) for Tallahassee Creek, and 380 mg/1 for Currant Creek. Ground water in the outwash aquifer has TDS ranging from about 350 to 400 mg/I, and is also calcium bicarbonate type. On a Piper diagram (Figure 1-2), the outwash water has noticeably more sodium (shifted rightwazd in the crntral plotting diamond) relative to River water. Increased TDS concrntrations result from the dissolution of minerals in soils and the outwash aquifer as water moves from recharge to discharge areas. Increased sodium may occur as the result of cation exchange, or from mixing with more sodium-rich water. Tallahassee Creek recharges the outw•ash aquifer dvectly along the upper part of its course, and indirectly through deep percolation of irrigation water applied in excess of demand on about 60 acres • of native hay fields. The outwash ground water flows southerly and south-southeasterly brneath the Project area, driven by a hydraulic gradiem of about 40 feet per 1000 feet. Ground water flow vela;ities in the outwash aquifer beneath the Project computed from the hydraulic gradient (see Figure 1-3), hydraulic conductivity estimates given above, and an assumed effective porosity of 20%range from 80 to 400 feet per day. Hydrologically, direct impacts of the Project to the outtvash aquifer will be minimal. No "cone of depression" associated with a water supply well or wells will occur or develop, as the water source for the Project will be water pumped from the Arkansas River. htstead, increased recharge neaz the Plant area will cause the water table to rise 2 to 5 fee[ immediately beneath that area, with lesser rises (0.5 to 1 foot) away from the reclutrge area, based on the results of an analytical model of ground water mounding. The limits ofthe outwash aquifer itself and the Arkansas River ad as barriers to the passage of hydraulic effects m the outwash aquifer further to the south. The River also acts as a constant head boundary where bedrock aquifers aze cut by it, and where outwash/alluvium are cut by the River. Bedrock (Dakota Group) is cut by the River opposite the boat takeout at the State Park; several springs emanate from this outcrop. Further to the west, just west of the present site access bridge, Niobrara Formation outcrops in the valley walls cut by the River. A possible impact to the outwash aquifer would result from more stringent administration by the Division of Water Resources of the Tallahassee Creek and Currant Creek junior water rights used to irrigate the hav fields in the Project area. This administration change would likely reduce or • Hydrogeologv Report Parkdale Projee7 F°ge 2 oj4