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October 19, 2017 Page ES -2 <br />conclusions and recommendations of the study. In addition, AAI reviewed the geotechnical <br />highwall stability analysis for the Collom Pit, performed by Call and Nicholas Inc. (CNI 2005). <br />In AAI's opinion, hydraulic parameter data from the initial WMC Collom Project Pre feasibility <br />Hydrology study (WMC 2005) were a useful starting point for calibration of AAI's numerical <br />model to the 2005 pumping test data. Similarly, AAI relied heavily on the stratigraphic <br />information gathered in and around the Collom Pit as documented in the CNI Collom Project <br />Geotechnical Study (Call and Nicholas 2005) in order to identify strata critical for the current <br />dewatering study and the highwall slope stability study to follow. <br />At the beginning of the project, the AAI Project Team performed a 2 -day tour of the <br />existing surface pits at the Colowyo Mine and of the Collom Pit site. AAI visited the currently <br />operating South Taylor Pit and the West Pit to observe strata conditions and assess them from a <br />hydrogeologic perspective because similar ground conditions are anticipated in the Collom Pit. <br />The Collom Pit area was the primary focus of the tour, which included visits to the pilot <br />dewatering test location, Little Collom Gulch, Jubb Creek, Collom Gulch, and a few <br />seeps/springs. <br />AAI used the three-dimensional (313) fmite-difference numerical modeling code Visual <br />MODFLOW to develop and evaluate the effect of a dewatering program for the Collom Pit. The <br />dewatering evaluation was carried out in two distinct stages. The first stage involved calibrating <br />hydraulic parameters of the Collom area strata, which were based primarily on the ground <br />response to the pilot dewatering test as well as other in-situ test results. Subsequently, the <br />calibrated strata properties were used in the predictive analyses to assess and optimize the effects <br />of variable dewatering well locations, for variable well depths and screen depths, and variable <br />dewatering time periods. <br />Based on the results of the predictive analysis, two alternative layouts (scenarios) for <br />dewatering and monitoring wells were developed, as summarized in Figure 8 and Tables 6 and 7 <br />in the report. For each scenario, piping to a storage tank was designed. Other results of the <br />dewatering study are summarized as follows: <br />• For either of the two dewatering well scenarios, the dewatering can be started at the end <br />of the third year of mining, assuming currently projected box -cut development rates. <br />AAI believes that Scenario 2, which positions the in -pit pumping wells two cuts upslope <br />of the box cut, is more useful as it will provide dewatering for a longer period within the <br />highwall prior to the wells being mined through. <br />• Overall, five monitoring wells are recommended with multiple piezometers within each <br />well to allow for ground-truthing of the model predictions. Monitoring data collected in <br />the first year of dewatering, from a larger area within the Collom Pit, could be used to <br />refine the predictive models and allow consideration of additional pumping wells if <br />necessary. <br />• Angled drilling of the pumping wells was considered; however, AAI did not find <br />conclusive evidence that the fracture network in the Collom area will provide preferential <br />paths to water movement that can be exploited by angled pumping wells. Hence, vertical <br />pumping wells are recommended. <br />Agapito Associates, Inc. <br />