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and flow residual between iterations were specified as l x 10-5 feet and 100 cubic feet per day <br />cfd), respectively. These values of tolerance resulted in mass balance less than 0.0023%, <br />providing an accurate solution upon convergence. <br />MODEL CONSTRUCTION <br />The Pueblo East Pit groundwater model was developed using a Geographic Information System <br />GIS) database and GIS data analysis techniques. (ArcGIS, 2016). The model was constructed <br />by importing ArcView shapefiles representing aquifer parameters and boundary conditions into <br />Groundwater Vistas. The model domain is a rectangular area 9,200 feet wide by 12,850 feet <br />long (Figure E-1). The domain was divided into a grid of cells measuring 50 feet square. Each <br />active cell contains a value representing the following aquifer parameters: <br />The elevation of the top of the aquifer <br />The elevation of the bottom of the aquifer <br />The hydraulic conductivity of the aquifer <br />The initial groundwater head within the aquifer <br />The boundary conditions for the model <br />This section discusses the general procedure used for determining hydraulic parameters, <br />boundary conditions, delineation of calibration targets, and goals of model calibration as related <br />to the conceptual model. <br />Hydraulic Parameters <br />The maximum top of the alluvial aquifer is represented by the topography of the ground surface. <br />Topographic data used for this model input are from a 10-meter digital elevation model (DEM) <br />obtained from the USGS National Elevation Data set contoured at 5-foot intervals. <br />The low permeability Pierre Shale bedrock forms the bottom of the alluvial aquifer. Therefore, <br />the model also contains an elevation map of the bedrock surface. To create this surface, bedrock <br />elevation data was obtained from several sources. The primary data sources include geotechnical <br />investigations for gravel pit reservoir feasibility and design projects, including for the Southwest <br />Reservoir, the Rich Pit, and the Central Reservoir. Additionally, well data from the Colorado <br />State Engineer's Office (SEO) were used in conjunction with the USGS digital elevation model <br />DEM) to obtain groundwater and bedrock elevations. The bedrock elevation data were <br />contoured in the GIS using the natural neighbor interpolation method within the ArcGIS 3D <br />Analyst extension (ArcGIS, 2016). Then the contours were adjusted using geologic judgment. <br />The result is a bedrock elevation contour map imported into Groundwater Vistas as the bottom of <br />the aquifer. Well construction reports for the SEO wells were reviewed and if bedrock depth <br />was different from well depth then a change was made. The locations of the SEO wells are <br />relatively accurate because all the wells were located by distances from section lines. Overall, <br />the spatial reliability of the bedrock data is considered good and deemed appropriate for the <br />scope of this groundwater model. <br />E-3