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➢ Alluvial wells: Other than the seven monitoring wells drilled at the Site for monitoring groundwater <br /> levels, there are also nineteen pumping wells permitted within 600 feet of the mine property.There are <br /> seven wells north of the Site, two wells east of the Site, nine wells south of the Site and one well west <br /> of the Site. If pumping, groundwater will be drawn to these wells. <br /> 5.0 GROUNDWATER MODEL <br /> The MODFLOW program utilizes a finite-difference mathematical model to simulate groundwater flow developed by the <br /> United States Geological Survey(USGS). Groundwater Modeling System (GMS V 10.4.10)provided the graphical user <br /> interface for entering and reviewing the MODLFOW data. The model has a total area of approximately 42 square miles, <br /> extending from the South Platte River to the west,the Town of Platteville to the south,the Evans#2 Ditch to the east and <br /> the Town of La Salle to the north (refer to Figure 1). The grid size is 200 feet by 200 feet squares. <br /> The following boundary conditions were applied to the model as presented in Figure 2: <br /> ➢ South Platte River to the northwest(River Package within MODFLOW); <br /> ➢ A constant head boundary to the south; <br /> ➢ A no flow boundary to the east to simulate the rise in bedrock east of Highway 85; <br /> ➢ A constant head boundary to the north; <br /> ➢ Claystone bedrock—acting as an aquitard 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 Civil Resources at the Site as well as 177 <br /> wells logs retrieved from the Colorado Division of Water Resources webSite. These points were entered into Civil 3d to <br /> create a surface and smoothed using natural neighbor interpolation with a 20-foot by 20-foot interval. Bedrock is <br /> exposed to the west of the South Platte River, indicating that the alluvial aquifer pinches out to the west.A feature line <br /> was used to map out the bedrock outcrop to bring the bedrock surface to meet the ground surface west of the river. <br /> Corrections were made to this surface to lower the bedrock in areas in order to maintain usable cells along the river <br /> corridor and bedrock 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 /> Groundwater Model Report—Section 20 Gravel Mine Site -3- August 2022 <br />