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Model Design. Two models were developed. The model of the overburden was designed <br />• to examine long-term, far field effects due to mining, and does not consider the shallow <br />overburden materials (except as a recharge source), or Tuttle and Calamity Draws. The <br />model for the coal represents only the coal aquifer. <br />The hydrogeol ogic system was represented in each model as a confined, one-layer, <br />homogeneous unit. The generalized ground water flow direction was along the secondary <br />model axis from northeast to southwest. The San Miguel River was defined as a <br />constant-head boundary along the downgradient edge (Figure 17-1). The three other model <br />boundaries represented no-flow conditions. The mine pit was represented as a rectangular <br />set of grid cells 2400 ft x 150 ft (360,000 ft2) in the center of the model grid. These <br />dimensions were based on the approximate pit width, and the greatest length of pit <br />expected during the five-year period simulated. Figure 17-2, a model grid overlay, is <br />provided to orient the reader to the actual scale locations of the Nucla East mining area <br />and the San Miguel River. <br />The drain option in MODFLOW was used to simulate face seepage and dewatering of the mine <br />pit. The ten model cells defining the pit were designated as drain cells. The DRAIN <br />package was used to simulate the pit for two reasons. First, it allowed the pit to be <br />modeled as a constant-head boundary for discharge of water, but did not allow the pit to <br />become a constant-head recharge boundary. Second, it allowed for separate accounting of <br />the water discharged into the pit. The drains maintained predetermined water elevations <br />for the duration of a given stress period (length, 1 year). The elevations of the drains, <br />and hence the drawdown in the pit, was changed on a yearly basis, based on projections of <br />the depth of the pit beneath the existing water table. <br />Water was added to the model using two methods: the RECHARGE package and the General Head <br />Boundary package. The recharge value used represents the long-term recharge to the unit <br />being modeled, and is not indicative of the recharge to the shallow ground water system <br />during the irrigation season. This shallow recharge appears to drain rapidly into Tuttle <br />and Calamity Draws. The model thus represents the deeper overburden, and may <br />underestimate the amount of discharge into the pit during the irrigation season. <br />The General Head Boundary package was used to simulate the increase in recharge caused by <br />• drawdown during the drawdown calculations. This package calculates leakage on the basis <br />of the difference in head in the aquifer and in the beds providing recharge, and the <br />17-10 Revised 04/11/88 <br />