2018-02-05_PERMIT FILE - C1981019

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October 19, 2017 Page 10 2.5 Collom Pit Dewatering Predictive Analyses and Results Upon completing calibration of the hydrologic parameters (conductivity and storage coefficients) using Visual MODFLOW with the pumping test well and surrounding monitoring wells, AAI used the same hydrogeologic model to study the effects of dewatering on the Collom Pit. This stage of the study focused on optimizing several critical aspects of dewatering, including the duration of dewatering, well locations, well depths, casing depths, and well spacing. The predictive analysis was performed in stages using hydrologic parameters derived from the calibration effort described in Section 2.4. The model geometry was retained from the calibration phase; the only change being reduction of the model element size within and around the Collom Pit from 500 ft x 500 ft to 100 ft x 100 ft, in order to more accurately capture the pore pressure changes and water level drawdown within the pit perimeter. In all the predictive models, the baseline water elevations were established by combining the current monitoring well water levels close to the pit and historical water level data away from the pit. A summary of water levels throughout the modeled zone is presented in Table 4. As the first step to designing the dewatering system, a single dewatering well was simulated at the location of the pilot well to observe its impact on depressurizing the virgin ground, at various time intervals, ranging from 1 to 10 years. An average pumping rate of 12 gpm per well was assumed throughout the duration of pumping. The 12-gpm pumping rate was a conservative estimate of the average pumping rate based on observations made during the pilot dewatering test and is consistent with the head versus flow characteristic curve for a 15-gpm submersible pump. The analysis results obtained from simulation of a solitary pumping well indicated that the coal layers showed the fastest response to pumping, owing to their higher hydraulic conductivities, whereas non -coal layers (especially layers 4 and 6) lagged behind for the initial 3 to 5 years, until equilibrium was achieved, after which the overall drawdown zone in all layers expanded radially outward at a uniform rate. The sequential growth of the draw -down zone from a single pumping well at the pilot well location over time is demonstrated in Figures 3a -i. The results are plotted in a vertical plane oriented at N22E, passing through the pilot test well location. Each curve represents an equivalent drop in pressure head at a specific location relative to the pumping well, in the respective plots. The 50 -ft drawdown curve is identified in all the figures to demonstrate the growth of the 50 -ft drawdown zone, relative to the pumping well, with continued pumping. Based on preliminary results obtained from the simulation of a single pumping well, the most suitable dewatering hole locations around the pit perimeter were iteratively identified as on the lowwall, sidewall, and highwall sides of the ultimate box cut. (Wells beyond the ultimate pit crest are the preferred solution because they will not be subject to damage from mining activities.) Multiple hydrologic models were developed using Visual MODFLOW that were representative of the Collom Pit geometry through box -cut development and the subsequent strips. AAI analyzed the effect of different dewatering timing scenarios—concurrent and delayed (1, 2, and 3 years and so onto ensure that the water table was depressed below the box cut. Based on the modeling results and the fact that the existing water table is approximately 350 ft below surface, AAI determined that dewatering could start 3 years after the start of box - cut development; essentially meaning dewatering begins at the start of the fourth year of mining in the Collom Pit. Agapito Associates, Inc.