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Figures 4 and 5 of Appendix E show the net anticipated drawdown in the aquifer due to the <br />hypothetical maximum potential rate of dewatering at the site. The calculated drawdown ranges <br />from 13 feet near the assumed dewatering well locations to about 3 feet at a distance of <br />approximately 3,000 feet from the pit center. <br />The drawdown at the existing well located north of the Arkansas River, well no. 186110, was <br />computed to be less than half a foot. This assumes, however, that the well actually collects water <br />from the glacial outwash aquifer in question, as opposed to the alluvial aquifer of the Arkansas <br />River, and that the initial water level in the well is above the bedrock underlying the alluvial <br />aquifer. Refer to Summary Tables 2 and 3 contained within Appendix D for additional information <br />regarding the modeled drawdown at several locations in the vicinity of the sand and gravel pit, <br />including well no. 186110. <br />SENSITIVITY ANALYSIS <br />The factor known with the least degree of certainty in the analysis is the homogeneous, isotropic <br />nature of the aquifer with respect to its hydraulic conductivity. Therefore a sensitivity analysis was <br />completed on that parameter. <br />For the sensitivity analysis the hydraulic conductivity was varied by +/- 20% at locations of 1,000 <br />and 2,000 feet from the pit. Shown below are the results of the sensitivity analysis (shown as net <br />drawdown in feet): <br />Distance from Center Well (ft) 1,000 2,000 <br />Base Drawdown (1600 ft/day) 4.3 3.9 <br />5% Increase (1680 May) 4.3 4.0 <br />10% Increase (1760 ft/day) 4.4 4.0 <br />15% Increase (1840 ft/day) 4.4 4.1 <br />20% Increase (1920 ft/day) 4.5 4.1 <br />5% Decrease (1520 ft/day) 4.2 3.9 <br />10% Decrease (1440 ft/day) 4.2 3.8 <br />15% Decrease (1360 May) 4.1 3.8 <br />20% Decrease (1280 ft/day) 4.1 3.7 <br />Table 1- Hydraulic Conductivity Sensitivity Analysis <br />ESTIMATING EFFECTS SOUTH OF THE ARKANSAS RIVER <br />r In general, effects of dewatering operations on ground water flow on the opposite side of a major <br />hydrologic feature such as a river are difficult to explicitly model. These models typically impute a <br />constant head boundary condition to surface water features. That type of boundary condition <br />limits the effects of dewatering to one side of the feature because the head at or opposite the <br />feature are set constant. <br />' Parkdale Aggregate Mine I Estimating Effects South of the Arkansas River