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0113112093 17:09 2799186 DOTY F ASSOCIATES PAGE 03 <br />Mr. Christopher C. LaRue <br />Page 3 <br />January 28,_2003 Doty & Associates <br />HYDRAULIC EVALUATIONS <br />The impact of the proposed mining water control measures on the well was evaluated <br />by predicting drawdowns In response to pumping with and without the presence of the <br />slurry wall. The hydraulic calculations were performed using the Theis equation for <br />unsteady radial flow to a well. It must be stated at the outset that several of the <br />assumptions in the Theis derivation are not met in the present problem. The deviations <br />from the assumptions and the effect of those deviations on the analytical results are <br />as follows. <br />1. The well is not of infinitesimal diameter. Clearly, the Off Partnership well is of <br />significant diameter 14 feet in the saturated zone). When the pump is first <br />activated in a large diameter well, the bulk of the flow comes from wellbore <br />storage and only a fraction of the flow actually comes from the formation. <br />However, this effect dissipates relatively rapidly (according to Lohman, 1979, <br />it is of no practical significance for periods of pumping longer than a few <br />minutes in wells of finite diameter). At a pumping rate of 500 gallons per <br />minute, the volume in wellbore storage 1750 gallons) is removed in 1.5 minutes. <br />The net result is that early time responses (drawdowns) to pumping are less for <br />large diameter wells than for the assumed geometry and, as a result, the early <br />time drawdown predictions made using the method described in this letter are <br />unreliable (predicted drawdowns are too large). <br />2. The well is not fully penetrating. The subject well extends to 20 feet below <br />ground and has approximately 8 feet of saturation. The total alluvial thickness <br />is probably on the order of 40 feet, somewhere between the apparent TH-4 <br />alluvial thickness and that shown in Hurr et al. (1972). Thus, the total <br />saturated thickness is probably approximately 28 feet and the well penetrates <br />only 30 percent of that thickness. Partial penetration causes flow to move <br />upward toward the well, at a higher velocity than would be the case for the <br />horizontal flow assumed in the derivation, with an extra loss of head near the <br />well. According to Kruseman and de Ridder (1991), this effect is most <br />pronounced near the well and Is negligible at distances of more than twice the <br />saturated thickness (on the order of 60 feet from the well In our easel. Thus, <br />the drawdown predictions made using the method described in this letter are <br />unreliable (predicted drawdowns are too small) for points close to the well <br />(within about 60 feet). <br />3. The aquifer is not confined. A significant portion of the water produced by a <br />well completed in an unconfined aquifer comes from storage (drainage from the <br />pore space in response to lowered water levels), again producing a vertical <br />component of flow. The actual drainage from storage occurs over a finite <br />period (as opposed to the essentially instantaneous elastic release in a confined <br />aquifer), resulting in a reduced rate of drawdown that is often called delayed