Laserfiche WebLink
Mr. Alex Schatz <br />December 12, 2008 <br />Page 5 of 11 <br />present in the model. A more thorough field investigation would be necessary to <br />determine the degree to which the alluvium is saturated in this area. There are four wells <br />in the SEO database that are drilled in the area that is simulated as unsaturated. Two of <br />these wells (listed under permit numbers 85129 and 44784), have depths of greater than <br />60 feet which would place at least part of their perforated intervals in the Wasatch <br />formation. The other two wells (listed under Permit Numbers 93289 and 92767), have no <br />depth given. All four of these wells are listed as stock or domestic wells and therefore <br />the Wasatch formation may provide enough water for their intended uses. <br />At the mining site, the depth to water under the base -case scenario ranged from 9 <br />feet to 16 feet and increased from west to east. This range of depths to water is consistent <br />with the 5 feet to 11 feet reported by Banks and Gesso at the mining site. <br />This water table configuration presented in Figure 7 was used as the starting point <br />for the predictive runs incorporating the dewatering of the pits described in the section <br />"Drawdown Simulation Results." <br />Pre - mining Results and Sensitivity Analysis <br />Under each of the model runs listed in Table 1, the ground water flow model <br />simulated a reasonable water -table elevation at the proposed mining site. Due to the <br />scarcity of data on water table elevations this was the only data to which the predicted <br />heads could be compared. However, the pattern of declining water table elevations from <br />north to south and the predicted direction in the water table gradient are reasonable for <br />this alluvial system. <br />Six additional model runs were conducted with varying values for the input <br />parameters. The input parameter values for each model run are presented in Table 1. <br />The predicted heads in the model were generally insensitive to the input <br />parameter values, with the exception of the parameter for recharge applied to the model. <br />The predicted heads in the model runs with varying hydraulic conductivities and model <br />thickness varied by less than 1 foot from the base -case. The predicted heads in the <br />scenario where recharge was set to 1 inch per year were generally about 1 foot higher <br />than the base -case model run. The predicted heads in the scenario where recharge was <br />set to 3 in/year were generally about 3 feet higher than the base -case on the eastern side <br />of the model area. In this scenario, the number of dry cells on the eastern edge of the <br />model area was considerably reduced. <br />The small changes in the predicted heads in each model run indicate that the <br />simulated heads are most sensitive to the value of the recharge parameter and are <br />generally controlled by the elevation of the general head boundary representing the <br />Colorado River. <br />