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vertical hydraulic conductivity and thickness of intervening beds. In these simulations, <br />• the drawdown caused by pit dewatering increases the recharge to the aquifer. <br />The overburden and coal layers were simulated in separate models. The model design was <br />the same in both models but the transmissivity and storage values were different. For the <br />overburden, a transmissivity of 26 ft2/day and storage value of 0.0001 were used. The <br />coal layer simulations assumed a transmissivity equal to 2.6 ft2/day and a storage value <br />of 0.00001 (Table 17-3). <br />Model Calibration. The model was calibrated by adjusting the recharge rate over the <br />model area, until there was good agreement between water-table elevations between the San <br />Miguel River and mine area (Figure 17-3). The General Head Boundary package was not used <br />for the steady-state calibration. The final recharge rate for the overburden is 0.000027 <br />ft/day and for the coal is 0.0000027 ft/day. <br />These recharge rates are quite low, but are not unreasonable given the general low <br />permeability of shales and the fact that much of the shallow recharge discharges into <br />• Tuttle and Calamity Draws. It should again be noted that this model is representative of <br />the system where it is relatively unaffected by the existence of the streams. <br />Results. For the transient simulation, the General Head Boundary package was <br />introduced. This package requires input data for the leakance parameter (vertical <br />conductivity divided by th iekness of the intervening low permeability layer). The <br />thickness was estimated by examining lithol ogic logs for core holes in the Nucla East <br />area, which indicated that shale was present in thicknesses of 10 ft or more. To avoid <br />underestimating the predicted rate of pit inflow, a thickness of 10 ft was used. The <br />vertical conductivity was estimated to be 10 7 cm/s based on a reasonable permeability for <br />a shale and produces results consistent with the observed discharges from Pond 001 in the <br />Nucla Mine area. Discharge from the pond, which includes surface flow, ranges from <br />approximately zero to 1.5 cf s. Pit pumpage is a part of this discharge. Simulated pit <br />inflow at Nucla East is within this range. <br />Transient simulations were performed for a five-year period, using the maximum drawdown <br />estimates for the overburden and coal. For the overburden, the pit drawdown was 5.0 ft <br />for years 1 and 2, 8.0 ft during year 3, 15.0 ft during year 4, and 30.0 ft during year 5. <br />The resulting overburden drawdowns after each year are shown in Figures 17-4 through 17-8. <br />17-13 Revised 04/11/88 <br />