1988-04-11_PERMIT FILE - C1981008A (15)

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