2018-02-05_PERMIT FILE - C1981019

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October 19, 2017 Page 4 2.0 COLLOM PIT DEWATERING EVALUATION 2.1 Previous Dewatering Study A dewatering study has been performed previously for the Collom Pit by WMC (2005). After the project kickoff meeting at the start of the current study and prior to the Collom Pit field visit, AAI performed a detailed review of the abovementioned study report, including its conclusions and recommendations. In addition, AAI reviewed the geotechnical highwall stability analysis for the Collom Pit, performed by Call and Nicholas, Inc. (CNI 2005), which had utilized the strata pore pressure scenarios developed during the course of WMC's dewatering study. 2.1.1 Collom Project Pre feasibility Hydrology Report The WMC Collom Project Pre feasibility Hydrology study (WMC 2005) was performed over the 2004-2005 time period and included multiple surface and groundwater investigations. The WMC study provided a baseline survey of surface streams, springs, and seeps within and in the vicinity of the proposed Collom Pit, which had a larger areal expanse than the current mine plan. Several groundwater monitoring wells were established in the Collom area to study seasonal changes in groundwater. The study also aggregated historical surface and groundwater data relevant to the Collom project. Several hydraulic tests were performed on-site, which included slug tests, single -well pump tests, and a multi -well pump test. The most critical element of this study was the large-scale, long-term pumping test conducted along the north perimeter of the Collom Pit during the spring of 2005. This test involved pumping at nearly constant rates (approximately 12 gallons per minute fgpm]) from a pilot dewatering well, with drawdowns monitored in multiple observation wells at varying distances from the pilot well and in different geological units. The hydrogeologic characterization data developed from the pilot test and other field tests were used to perform a subsequent evaluation of depressurization of the pit slopes using the program SEEP/W (Geo -Slope 2016). The pore pressures generated from the slope depressurization models were subsequently used in the slope stability models developed by CNI (2005). 2.1.2 Collom Project Geotechnical Study Report The CNI Collom Project Geotechnical Study (Call and Nicholas 2005) was performed to evaluate the global stability of the Collom Pit slopes in light of the slope depressurization study performed by WMC (2005). AAI relied heavily on the stratigraphic information gathered in and around the Collom Pit from the CNI study in order to identify strata critical for the current dewatering study and the highwall slope stability study to follow. This study also highlighted the similarities in strength values between sandstone, siltstone, and mudstone rock types in the Collom area. 2.1.3 Summary of AAI Review of Previous Studies Overall, the data gathered from the WMC and Hatch studies were considered adequate for the purposes of the current study, even if the operational applicability of those studies remains suspect. In order to establish pre -mining and pre -pumping baseline groundwater Agapito Associates, Inc.