2008-03-05_REVISION - M1997054 (24)

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7r2Jm 15:20:47 FROM Todd Gihnar TO Mrich Nbori that passes beneath the Project area. During the Laramide. the region and Project area were also out 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 oftwo monitoring wells. Saturated thickness ofthe 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 Morrison 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 evidently has as its major source surface water from Tallahassee Creek as evidenced by TDS measurements, 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/l for the Arkansas River to 280 milligrams per liter (mgn) for Tallahassee Creek, and 380 mg/l for Currant Creek. Ground water in the outwash aquifer has TDS ranging from about 350 to 400 mg/l, and is also calcium bicarbonate type. On a Piper diagram (Figure 1-2), the outwash water has noticeably more sodium (shifted rightward in the central plotting diamond) relative to River water. Increased TDS concentrations result from the dissolution of minerals in soils and the outwasb 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 outwash aquifer directly 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 beneath the Project area, driven by a hydraulic gradient of about 40 feet per 1000 feet. Ground water flow velocities in the outwash aquifer beneath the Project computed from the hydraulic gradient (see Figure 1-3), bydraulic 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 outwash 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. Instead, increased recharge near the Plant area will cause the water table to rise 2 to 5 feet immediately beneath that area, with lesser rises (0.5 to 1 foot) away from the recharge area, based on the results of an analytical model of ground water mounding. The limits of the outwash aquifer itself and the Arkansas River act as barriers to the passage of hydraulic effects in the outwash aquifer further to the south. The River also acts as a constant head boundan- where bedrock aquifers are cut by it, and where outwash/alluvium are out by the River. Bedrock (Dakota Group) is out by the River opposite the boat take-out 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 out by the River. 1 1 1 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 hay fields in the Project area. This administration change would likely reduce or Rydrogeologv Report PanUale Project Page 2 of 4 sur:.