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A numerical groundwater model was developed using Visual Modflow 2.8.2 and <br />expanded using 3.0 (Waterloo Hydrogeologic, Waterloo, Ontario). The model grid is <br />oriented parallel to the South Platte River. The grid consists of 400 columns and 200 <br />rows. The cell size is approximately 21 meters by 21 meters (70 feet by 70 feet). The <br />model is divided into two layers approximating the sand dunes and the river channel <br />deposits. The elevations for the layers were imported from contour surfaces generated <br />with Surfer (Golden Software, Golden, Colorado) using data from the geologic logs and <br />USGS topographic data. <br />The northern (river), eastern, and western, boundaries of the model were simulated as <br />constant head boundaries. The southern boundary was simulated as a no flow boundary. <br />Recharge was added on the eastern edge of the southern boundary to simulate the <br />additional flows from the sand hills into the river channel deposits <br />A time -line of the model's development is provided below: <br />1. Initially, a steady -state run was made based on data from October 26, 2000. <br />2. A transient model (Tamarack Recharge Model) was set up to simulate recharge <br />operations for the entire water year from November 1, 2000 to October 31, 2001. <br />During this transient model run seven production wells were periodically pumped, <br />two recharge ponds were operated, the artificial stream was activated along with the <br />three semi -lined storage ponds, and two wetland areas were periodically recharged. <br />Water level measurements from 24 to 31 wells at 10 different time periods, for a total <br />of 267 calibration points, were used in calibrating the model. <br />3. During operation and calibration of the Tamarack Recharge Model, it was noticed <br />that the recharge operations significantly impacted the western boundary of the <br />Tamarack Model. Upon review, the authors decided to extend the western boundary <br />of the Tamarack Model. The model grid was expanded 3,000 meters (2 miles) to the <br />west of the Crook Bridge. Thereby, the west boundary is more than 5,000 meters (3 <br />miles) away from the closest recharge well or pond. In addition, the time frame of <br />the model was expanded to include all recharge operations. The time span included <br />in the updated model is from February 1, 1997 to October 31, 2002. This re- <br />calibrated model is called the West Expanded Tamarack Model. <br />4. The West Expanded Tamarack Model was used as a baseline to analyze the return <br />of ground water pumped as part of the Tamarack Recharge Project to the South Platte <br />River. The two components of the river recharge analysis include 1) determining the <br />amount and distribution of river recharge and 2) comparing the recharge to Stream <br />Depletion Factors (SDFs) <br />In addition, it was noted that approximately 250 acre -feet of recharge water was lost <br />to additional evapotranspiration (ET) in the model. This occurs because the model is <br />set up to vary the amount of ET based on the depth to ground water. During times of <br />recharge, the depth to groundwater near the river is slightly elevated resulting in <br />Page 4 of 6 <br />