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The flatter areas surrounding the site where irrigation water has more time to percolate into the ground were assigned a <br />50% surface runoff and a 50% deep percolation of irrigation excess. In the areas east of the Site, where the steeper <br />slopes are underlain by eolian deposits, approximately 70% of the water not consumed by the crops was attributed to <br />surface runoff and 30% to deep percolation recharging the aquifer. <br />Recharge rates of 1.25 inches per year were modeled on the steeper slopes and 2.2 inches per year on flatter slopes. <br />The recharge over the Piney Creek Alluvium was modeled with a rate of 2.2 inches per year, approximately 17 percent of <br />the rain recharge. This rate is higher because the Piney Creek Alluvium is a coarser grained geological unit resulting in <br />more precipitation percolating into the aquifer and lies on more flat slopes as well as being located in irrigated areas as <br />observed during site visits. See Figure 5 for the distribution of irrigation recharge. <br />Table 5 <br />Recharge by Geological Unit <br />Area b Geolo ical Unit Bechar et (incney earl Notes <br />Ran a 0.65 - 2.6 Brendle, 2002 <br />Eolian 1.25 Irri ated Area, Stee er Slo es <br /> <br />Broadwa <br />2.2 Healy Irrigated Area, Flatter <br />Slo es <br /> <br />Pine CreelR <br />2.2 Partially Irrigated Area, Flatter <br />Slo es, Coarser Grained Material <br />Notes: <br />1. Values for irrigated areas from agricultural practices and discussions with irrigators. <br />2. Piney Creek recharge approximately 17% of annual precipital'ron due to the higher hydraulic conductivity of the Piney Creek <br />geological Unit, <br />3.9 Boundary Conditions <br />Boundary conditions in a groundwater model are one tool used to approximate existing conditions. The boundary <br />conditions are set sufficiently far away from the area of interest to allow the aquifer to respond to local stresses and are <br />based on hydraulic, hydrogeologic, hydrologic, geological and geographical assumptions. <br />This model extends 2.5 miles north and south of the Site and constant head nodes were placed on the southeastern <br />flank at water surface elevations ten feet below the ground. A ridge is located approximately 3 miles west of the Site and <br />presents the definable western limit of the model, The ridge represents a no flow boundary in the model. Refer to fhe <br />geological mapping of the Site on Figure 3. The western limit of the alluvial aquifer was confirmed by researching the <br />states database of permitted wells in the area. Figure 5a & 5b presents the approximate location of the shallow and <br />deep aquifer divide which is approximately the location of the Brighton Ditch. <br />The South Platte River forms a natural hydrologic boundary. The South Platte River has an average slope of <br />approximately 0.1 percent along this western boundary of the model. The river stage was modeled to decrease as the <br />river flows north reflecting observed conditions. A summary of the boundary conditions is presented in Table 6 below. <br />Groundwater Model Report -Hall-Irwin Parker-Dersham Site - 5 - April 2006 <br />