2012-01-24_PERMIT FILE - M2011054 (4)

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Mr. Alex Schatz December 12. 2008 Page 7 of 11 accurately determine the location of these wells and assess the nature of the predicted impacts on well pumping capabilities. Drawdown Sensitivity Runs Six additional model runs with the parameters shown in Table 1 were run to determine the sensitivity of the modeled drawdown to changes in the input parameters. Figures 9 and 10 present the predicted drawdowns under the scenario where the alluvial thickness was increased to 60 feet and the scenario where the hydraulic conductivity was increased to 200 ft /day respectively. The drawdown in these model runs is presented to demonstrate the maximum drawdown that is predicted by the model in the sensitivity analysis. Summary & Conclusions The modeling analyses carried out have indicated that the dewatering of the Debeque mining site may affect up to 10 wells listed in the SEO database. Simulated drawdowns at these wells, listed in Table 2, range from 0 to 8.5 feet. At three wells, the minimum predicted drawdown in the drawdown sensitivity analysis model runs is greater than 4 4 feet. While it is noted that the actual locations of these wells are somewhat in question, it is likely that drawdown levels exceeding. 5 feet may impact one or more of these wells. Because the listed uses for these wells are stock, domestic, or for household use only, it is possible that these wells could continue to pump at their permitted maximum rate of 15 gpm. However, there could be impairment of the ability of these wells to continue to pump as they have historically once the dewatering activities at the mine are initiated. The modeling work carried out for this analysis is based on very limited data with respect to the hydrogeologic nature and saturated extent of the alluvial materials, the depth of the alluvial deposits, the nature and configuration of the bedrock surface underlying the alluvial aquifer and the nature of the interaction (hydraulic connection) between the Colorado River and the alluvial aquifer. As such, the modeling analysis has required that a number of significant assumptions be made pertaining to these factors and which may have a large bearing on the overall results. This analysis and the results obtained regarding the nature of the drawdown caused by the dewatering are thus very preliminary in nature and should . not be considered as definitive. While it is felt that the sensitivity analyses carried out indicate that only minor differences in the drawdown configuration and magnitude would result from changes in the model input data, if more reliable and conclusive predictions are desired or required, then collection of site - specific data will be necessary. This would include obtaining data on the depth to bedrock, the water table configuration in the DAV, aquifer testing to derive hydraulic characteristics within the DAV. field locating of the existing wells, determination of the existing wells V - _,r _ .. _d (;_3 suilant_ 1'._.