PERMFILE49068 - Laserfiche WebLink

Home Browse Search

Mr. Alex Schatz March 27, 2006 Page 2 of 6 the vicinity include drillers' geologic (lithologic) logs that were used by us to interpolate the geology of the alluvium near the mining area. The alluvium of the Colorado River consists of gravels and cobbles intermixed with sands and clays and ranges in thickness from 20 feet up to 95 feet thick and is underlain by shales and sandstones of the Wasatch formation. The lithologic logs we reviewed sometimes indicate significant clay layers and/or lenses within the alluvium. Hydrogeologic data for the Colorado River alluvium neaz Silt were not available, but typical hydrogeologic data for sands and gravels have been published. Values for hydraulic conductivity range from 100 to 10,000 feet per day. Values of specific yield range from 23-28 percent. Static water levels reported in nearby wells show the water level to within a couple feet of the ground surface elevation. MODFLOW MODEL To allow for estimation of the potential impact on the local water table and the Colorado River in response to dewatering of the pits, a numerical ground water flow model was constructed. The model code utilized was the United States Geological Survey "MODFLOW" model ("Modulaz Finite Difference Ground Water Flow Model"). The public domain pre- and post-processing softwaze, Processing MODFLOW for Windows ("PMWIN"), was used to develop the model files, to assist in model grid layout, and to graphically present the water table and drawdown configurations following each run. We note that MODFLOW is a widely used and accepted model that has become a standard in the industry. Discretization The model domain was bounded by no-flow cells both to the north just beyond the Colorado River and to the south by the plateau along the edge of the flood plain. The model was bounded to the east and west about one mile from the edge of the proposed gravel pits by constant head cells. Model cells near in and around the gravel pits were discretized to be 50-foot squazes. The cells sizes were stepped up to 500-foot squazes at not more than one and a half times the previous cell dimension. This discretization and model domain is presented in Figure 2. The model was created as aone-layer model representing the alluvial layer in which the gravel pits will be mined. The depth of the alluvium was determined from the above-mentioned geologic logs and subtracted from the ground surface elevation for each well. The elevations of the bottom of the alluvium were then contoured using the Kriging method to develop the topography of the base of the alluvium. The top of the alluvium was modeled as a flat surface at the highest elevation of the model domain. Marlin and Wood Water Consultants, Inc.