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October 19, 2017 <br />EXECUTIVE SUMMARY <br />Page ES -I <br />As requested by Colowyo Coal Company L.P.'s, Colowyo Mine (Colowyo), Agapito <br />Associates Inc. (AAI) performed a detailed dewatering and depressurization study for the <br />Collom Pit. Similar to past and currently active pits at the Colowyo Mine, several coal seams at <br />the Collom Pit will be mined using an updip approach. To aid highwall design and allow for <br />steeper highwalls and less overburden removal, Colowyo expects that deeper portions of the <br />Collom box cut and pit expansion will require depressurization. Several dewatering studies have <br />been performed for this operation previously. The objective of the present study was to assess <br />the usefulness of the previous studies and to develop an optimal dewatering plan for the Collom <br />Pit. <br />Surface coal mining is currently active in the South Taylor Pit and is expected to be <br />completed in approximately 2022. Colowyo intends to transition mining operations to the <br />Collom Pit, approximately 4 miles northwest, during the 2018 to 2022 timeframe. The Collom <br />Pit represents the long-term future of the Colowyo Mine. <br />The mineable seams and host strata within the Collom Pit have variable dips of 4 to <br />14 percent (%), with the maximum pit depth of approximately 750 ft. Subsurface strata in the <br />Collom area are saturated approximately 250 to 300 ft below ground surface. The preliminary <br />box cut will be made on the north side, and mining will advance updip to the south. Box -cut <br />development will take approximately 9 years (2018 to 2027) at current production levels. <br />Following box -cut development, the pit highwall will be advanced upslope at a rate dependent <br />on the required production level at the time the cut is taken. <br />As stipulated in the contract between Colowyo and AAI, the work performed by AAI <br />consisted of the following: <br />1. Review and evaluate data from previous studies. <br />2. Determine if the existing hydrogeologic model is sufficient for completing the scope of <br />work. <br />3. Evaluate the regional hydrology with respect to dewatering the Collom Pit. <br />4. During the course of the study, AAI reviewed groundwater levels data from the Collom <br />Pit area collected by Colowyo, which were contrasted with baseline data established in an <br />earlier study (WMC 2005) from the project area. <br />5. Provide a detailed final design of the dewatering system to include the following: <br />a. Well and horizontal drain spacing and locations. <br />b. Timing of system installation with respect to dewatering the box cut and subsequent <br />life -of -mine (LOM). <br />c. Location and design of monitoring well system with remote well monitor systems. <br />d. Recommendations for any additional drilling, testing, or monitoring requirements. <br />A dewatering study was performed previously for the Collom Pit (WMC 2005), which <br />was reviewed in detail by AAI. AAI performed a detailed review of the study including the <br />Agapito Associates, Inc. <br />