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01/31/2003 17:09 2799186 DOTY 8: ASSOCIATES PAGE 05 <br />Mr. Christopher C. LaRue <br />Page 5 <br />January 28. 2003 Doty & Associates <br />conductivity (k) is assumed equal to the Hurr et al. (1972) value (see Figure 1). On <br />the other hand, drawdowns are on the order of 5 feet using the Robson (1989) <br />hydraulic conductivity value. Therefore, the Robson (1989) value of 0.31 centimeters <br />per second (cm/s) will be used in the calculations that follow. In addition, it can be <br />seen that the results are relatively insensitive to the storage coefficient (S). Therefore, <br />the middle value of 0.2 will be used for future evaluations. <br />Drawdowns at the Property Line <br />Drawdowns were predicted at the property line (139 feet from well) with and without <br />the presence of the proposed slurry wall (Figure 2). The slurry wall is modelled as an <br />image well located 368 feet east of the pumping well (twice the distance to the slurry <br />wall); the image well is also pumping at 500 gallons per minute. Drawdowns at the <br />property line are currently approximately 2.4 feet after 180 days of pumping. If the <br />slurry wall is constructed, drawdowns at the property line will nearly double, to <br />approximately 4.6 feet. <br />Drawdowns in the Well with the Slurry Will <br />Drawdowns at the pumping well are likely to increase from approximately 5.1 feet to <br />6.9 feet if the slurry wall is constructed (Figure 3). This is nearly equivalent to the <br />total saturated thickness penetrated by the well 18 feet). Remembering that the Theis <br />expression is likely to under-predict drawdowns at locations near the well because of <br />partial penetration and aquifer thinning, it is very likely that the well will be pumped <br />dry at this pumping rate if the wall is installed. In addition, if the hydraulic <br />conductivity decreases in the downstream direction (as suggested in Robson, 1989), <br />the actual hydraulic conductivity may be somewhat less than the 0.31 cm/s value <br />used in the calculations and the drawdown in the well will be even larger. <br />Effect of French Drain <br />Although not a part of the application, it appears that the applicant is considering the <br />installation of a french drain outside the slurry wall to reduce the impacts of the pit <br />and slurry wall on the hydrologic system. In the absence of design details, we assume <br />that the drain will be constructed at least 5 feet below the water table. This implies <br />a depth of approximately 20 feet below ground If the drawndown water table is <br />considered. We developed an estimate of the flow that such a drain could deliver to <br />the Off Partnership well using the one-sided bank storage expression presented in <br />McWhorter and Sunada (1977). The one-sided expression is appropriate, at least for <br />an evaluation of an assumed design, because there will be limited and possibly <br />negligible flow from the slurry wall side of the drain. We predict a flow to the drain <br />of approximately 0.077 gallons per minute per foot of drain, using an hydraulic <br />conductivity of 0.31 cm/s, a storage coefficient of 0.2, and a time of ISO days. For <br />purposes of this analysis, we also assume that only that portion of the drain along the