Laserfiche WebLink
L.G. Everist <br /> West Farm Groundwater Modeling <br /> The preconditioned conjugate-gradient(PCG2) solver package of MODFLOW 2005 was used to solve the <br /> groundwater flow equations for the model. This package defines the number of outer and inner solver iterations, <br /> as well as criteria for both maximum head and residual change between iterations before allowing convergence. <br /> Tolerances for the maximum change in head and flow residual between iterations were specified as 1x10-3 feet <br /> and 100 cubic feet per day (cfd), respectively. These tolerances result in a mass balance of less than 0.001 <br /> indicting model convergence and solution accuracy. <br /> MODEL CONSTRUCTION <br /> The West Farm groundwater model was developed using a Geographic Information System(GIS) database and <br /> GIS data analysis techniques. (ESRI, 2023). The model was constructed by importing shapefiles representing <br /> aquifer parameters and boundary conditions into Groundwater Vistas. The model domain is a rectangular area <br /> 8,550 feet wide by 8,850 feet long (Figure 1). The domain was divided into a grid of cells measuring 150 feet <br /> each side. Active cells contain values representing the following parameters: <br /> 1. The elevation of the top of the aquifer <br /> 2. The elevation of the bottom of the aquifer <br /> 3. The hydraulic conductivity of the aquifer <br /> 4. The initial groundwater head within the aquifer <br /> 5. The boundary conditions for the model <br /> The following sections discusses the general procedure used for determining hydraulic parameters, boundary <br /> conditions, delineation of calibration targets, and goals of model calibration as related to the conceptual model. <br /> Hydraulic Parameters <br /> The maximum top of the alluvial aquifer is represented by the topography of the ground surface. Topographic <br /> data used for this model input are from a 10-meter digital elevation model (DEM) obtained from the USGS <br /> National Elevation Data set contoured at 5-foot intervals. Because of this 5-foot contour interval there is some <br /> variance between model elevations and actual ground elevations. <br /> The low permeability Laramie Formation bedrock forms the bottom of the alluvial aquifer. Therefore, the model <br /> contains an elevation map of the bedrock surface. To create this surface, bedrock elevation data was obtained <br /> from the geotechnical investigations described above. The bedrock elevations were contoured in AutoCAD. <br /> The resulting bedrock elevation contour map was imported into Groundwater Vistas as the bottom of the aquifer. <br /> Overall, the spatial reliability of the bedrock data is considered good and deemed appropriate for the scope of <br /> this groundwater model. <br /> The hydraulic conductivity (K) of the alluvial aquifer used in the model was input as 500 feet per day (fpd). This <br /> value is based on average values from the Colorado's Decision Support Systems GIS map and our experience <br /> in the area. We assumed an anisotropy ratio of 0.1 (Kv/Kr), meaning that the value in the vertical direction (Kv) <br /> is one order of magnitude lower than the value in the radial direction (Kr). <br /> A groundwater elevation contour map for the alluvial aquifer provided the starting heads for the finite difference <br /> solution and was used to define constant head boundary values. This surface was developed using the <br /> groundwater level data collected from the on-site monitoring wells. The pre-slurry wall heads or initial conditions <br /> depth to groundwater is shown on Figure 2. <br /> Project 20C26026.02/May 19,2023 Page 3 Deere&Ault,a Schnabel Engineering Company <br />