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RULE 4 PERFORMANCE STANDARDS <br />Collom – Rule 4, Page 21 Revision Date: 4/28/22 <br />Revision No.: TR-154 <br />From a surface mining perspective, the Collom Pit delineates the maximum recoverable coal resources <br />permitted by this mining permit for Colowyo to mine. Highwall mining in the Collom Pit will allow for <br />for maximum recovery in accordance with Rule 4.23.2(1). Please refer to Sections 2.06.9 in Volume 1 <br />for additional discussion regarding the removal of coal using highwall mining methods. <br />4.23.2 Performance Standards <br />4.23.2(1) Undisturbed Areas of Coal Shall be Left in Unmined Sections <br />As for the rules requirements [Rules 4.23.2(1)(a)-(c)] for leaving undisturbed areas of coal in unmined <br />sections, Colowyo requests a variance from the requirements of this rule for the Collom Pit. Colowyo’s <br />highly successful highwall mining methods that will be used in the Collom Pit, will maximize production <br />and ensure no subsidence occurs. Using this particular method of highwall mining by leaving pillars and <br />barriers allows the seams to be mined below each other and still ensures geologic stability once all seams <br />have been mined out. Please see Exhibit 27, Item 6 and Item 7 in Volume 20 for further discussion on <br />the geotechnical design and operational considerations implemented highwall mining the Collom Pit. <br />4.23.2(2) Abandoned or Active Underground Mine Workings <br />No abandoned or active underground mine workings have ever existed or currently exist anywhere near <br />the Collom Pit. Therefore, in accordance with Rule 4.23.2(2), no highwall mining activities will occur <br />closer than 500 feet in horizontal distance of any underground mining workings. <br />4.23.2(3) Contemporaneous Surface and Auger (Highwall) Mining <br />Highwall mining will follow the surface coal mining activities in a top-down sequence in the Collom Pit. <br />Highwall mining has to occur as contemporaneous as possible behind surface mining to ensure further <br />advancement of the Collom box cut is not hindered by highwall mining operations. When surface <br />mining opens up a large enough area in the Collom Pit, highwall mining will commence shortly <br />thereafter. As the Collom Pit is advanced, highwall mining will follow as soon as is practical, and will be <br />timed appropriately with continued surface mining operations. <br />4.23.2(4) Prevention of Pollution of Surface and Ground Water and Fire Hazards <br />Each highwall mining hole will be plugged within 30 days following coal extraction to prevent discharge <br />of water from the hole or access of air to the coal seam inside the highwall miner hole in accordance with <br />Rule 4.23.2(4)(b). Typically, highwall miner holes will be sealed much sooner than 30 days, but to <br />ensure operational flexibly and compliancd with Rule 4.23.4(4) a hole may need to remain open up to 30 <br />days. <br />Groundwater flow regimes and the negligible impact that Colowyo’s surface mining activities have on <br />ground water as a result of mining these target coal seams are detailed extensively in Section 2.04.7(1) <br />and 2.05.6(3)(b)(i & ii). From this body of data and from experiences to date with past highwall mining <br />activities, no toxic forming or acid forming water discharge is anticipated from any of the highwall <br />openings. Should toxic forming or acid forming water discharges be encountered, the opening exhibiting <br />the discharge will be backfilled within 72 hours of completion.