Laserfiche WebLink
Mr. Alex Schatz <br />December 12, 2008 <br />Page 4 of 11 <br />representation of the alluvium over the entire DAV based on the reported depths of the <br />wells in the SEO database in the DAV area presented in Figure 3. <br />Physical Properties, Model Calibration and Pre- Mining Results <br />Though the material of the alluvial aquifer varies throughout the model domain, it <br />was modeled as a homogeneous, isotropic aquifer. The alluvial sediments were modeled <br />as homogeneous and isotropic because there is no data to suggest that these properties <br />differ greatly within the DAV. A more thorough analysis of the hydrologic properties of <br />the alluvium may indicate zones of higher and lower hydraulic conductivity within the <br />alluvium; however this analysis was beyond the scope of this study. <br />The model was bounded on the western edge by the Colorado River. The <br />Colorado River was modeled as a general head boundary. The elevation of the general <br />head boundary was set to the elevation of the lowest point in the land - surface according <br />to the USGS National Elevation Dataset 10 meter Digital Elevation Model (DEM) of the <br />DAV. No -flow boundaries were used to represent all other model boundaries and <br />reflecting the limited extent of the alluvial deposits as they contact the bedrock formation <br />at the lateral boundaries of the DAV. Figure 5 presents the location of the general head <br />boundary cells in the ground water flow model. <br />The lake located directly west of the mining site was modeled as a high - hydraulic <br />conductivity zone with a specific yield of LO. This allows the simulated lake to act in the <br />model essentially in the same manner as the real lake interacts with the alluvial aquifer. <br />The lake will buffer the effect of dewatering- generated drawdowns west of the mine area. <br />Figure 6 presents the location of the lake adjacent to the Debeque Mine. <br />A "base- case" model with the most reasonable estimates for alluvial thickness, <br />areal recharge, hydraulic conductivity (k) and model boundary conditions was <br />constructed. The model parameter values are as follows: alluvial thickness = 50 feet, <br />areal recharge = 0 inches per year, and hydraulic conductivity = 100 ft/day. Due to the <br />arid climate in the DAV area, it was assumed that little recharge would be expected. The <br />sensitivity of the model to this conservative assumption is described later. <br />Figure 6 presents the modeled heads under the base -case model scenario. In the <br />base -case, the predicted heads generally decrease from northeast to southwest and show a <br />direction of flow parallel to the Colorado River. At the river, the predicted hydraulic <br />head is in equilibrium with the lowest point in the MODFLOW cell representing the <br />river. Due to the high- hydraulic conductivity of this alluvial system, the predicted <br />hydraulic heads elsewhere in the model area are close to the elevation of the nearest <br />general head boundary cell representing the river. On the eastern boundary of the model, <br />the model predicted a large area of dry cells. Cells in this area became dry because the <br />bottoms of these model cells were generally above the elevation of the nearest general <br />head boundary cells representing the Colorado River. At this time, it is unknown if the <br />alluvium is unsaturated on the eastern border of the model as predicted by the dry cells <br />