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> Claystone bedrock—acting as an aquiclude forming the bottom of the aquifer. <br /> The shallow alluvial aquifer was modeled as a single aquifer(sand and small gravel),with zoned hydraulic conductivities <br /> based on historical well pumping tests, underlain by impervious claystone bedrock. The model parameters and <br /> boundary conditions,such as Site topography, bedrock elevations,and hydraulic conductivities are further discussed in <br /> the following sections. <br /> 5.1 Regional Topography <br /> Surface topography was obtained from USGS digital quadrangle(the Milliken 7.5 minute quadrangle). The digital map <br /> was converted into a Civil 3d surface and imported to GMS and interpolated to the top elevations of the aquifer layer. <br /> 5.2 Bedrock Surface <br /> The bedrock surface for the Site was obtained from borings completed by the Operator at the Site, borings completed by <br /> Civil Resources at the Section 20 Site,and 177 wells logs retrieved from the Colorado Division of Water Resources <br /> website. These points were entered into Civil 3d to create a surface and smoothed using natural neighbor interpolation <br /> with a 20-foot by 20-foot interval. The locations of the bedrock points are shown on Figure 1. Bedrock is exposed to the <br /> west of the South Platte River, indicating that the alluvial aquifer pinches out to the west. A feature line was used to map <br /> out the bedrock outcrop to bring the bedrock surface to meet the ground surface west of the river. Corrections were <br /> made to this surface to lower the bedrock in areas in order to maintain usable cells along the river corridor and bedrock <br /> is likely shallower than the model input in this region. <br /> 5.3 Hydraulic Conductivity <br /> Hydraulic conductivity is a measure of the soils ability to transmit water within the aquifer. Some factors affecting <br /> hydraulic conductivity are: pore size distribution, grain size distribution,void ratio,roughness of mineral particles, and <br /> 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 B4-66-11adc well 3 miles north of Gilcrest and the Lorenz Well approximately'A 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 1/2 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. A map with the horizontal hydraulic conductivity values utilized in the model is shown <br /> on Figure 3. <br /> Groundwater Model Report—Monarch DENM Gravel Mine Site -4- January 2023 <br />