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degree of soil saturation (Das, 1998). The alluvium, underlying the region, is generally a clean to silty sand and gravel, <br /> as described above with an approximate depth of 50 to 95 feet in the upper terrace and shallower than 10 feet along the <br /> South Platte River floodplain. Aquifer properties used in a previous study of the area performed by Bishop-Brogden <br /> Associates, Inc.(BBA)for the Town of Gilcrest were incorporated as this report utilized two long-term aquifer pumping <br /> tests in the vicinity of Gilcrest:the B4-66-11 adc well 3 miles north of Gilcrest and the Lorenz Well approximately'/z mile <br /> to the east of Gilcrest. <br /> ➢ The B4-66-011 adc well approximately 3 miles north of Gilcrest is the nearest controlled long-term aquifer <br /> pumping and showed a hydraulic conductivity of 1,270 ft/day. <br /> ➢ BBA performed an analysis of water level data from observation wells located near the Lorenz Well <br /> approximately'/2 mile to the east of Gilcrest. Their analysis indicated a hydraulic conductivity of approximately <br /> 150 ft/day. <br /> The hydraulic conductivity was set at the lower end of the ranges provided by BBA and were increased until the model <br /> heads converged. The model is sensitive to horizontal hydraulic conductivity and operates without errors in a narrow <br /> range of values at the high end of the ranges provided by BBA. <br /> Horizontal hydraulic conductivity for the calibrated model range from a low of 65 ft/day for the hills to the east and a high <br /> of 805 ft/day underlying the Site. An area of high horizontal hydraulic conductivity. A map with the horizontal hydraulic <br /> conductivity values utilized in the model is shown on Figure 3. <br /> 5.4 Specific Yield <br /> Specific yield for the aquifer was set at 20%which corresponds to the value determined by BBA and in support of <br /> numerous water court decrees. <br /> 5.5 Boundary Conditions <br /> Boundary conditions were set based on geologic features on the east and west sides of the model. The constant head <br /> boundaries on the south and north sides were varied to match observed groundwater data sets. <br /> ➢ Boundary conditions in a groundwater model are one tool used to approximate existing conditions at the outer <br /> extent of the model. The boundary conditions are set sufficiently far away from the area of interest to allow the <br /> aquifer to respond to local stresses and are based on hydraulic, hydrogeologic, hydrologic,geological and <br /> geographical assumptions. <br /> ➢ The model extends 3.3 miles southwest where constant head nodes were placed approximately 14 feet below <br /> the ground surface to represent existing groundwater levels taken in the area. <br /> ➢ Another constant head boundary was place approximately 3.7 miles northeast Site and constant head nodes <br /> were placed on the southeastern flank at water surface elevations approximately ten feet below the ground. <br /> ➢ To the southeast of the model surface topography and bedrock slope up in elevation creating a natural barrier <br /> to flow. The eastern limit of the alluvial aquifer was confirmed by researching the states database of permitted <br /> wells in the area and mapping where the permitted aquifer switches from the alluvial aquifer to a deep confined <br /> aquifer. The approximate location of the shallow and deep aquifer divide is approximately'/4 mile east the <br /> location of the Platte Valley Ditch. <br /> ➢ The South Platte River forms a natural hydrologic boundary to the west of the model. Model inputs for the <br /> MODFLOW river package are bottom elevation, head-stage and conductivity. The bed elevation was estimated <br /> using the USGS topographic quadrangle and Google Earth top of bed elevation and subtracting 6 feet to <br /> Groundwater Model Report—Section 20 Gravel Mine Site -4- August 2022 <br />