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RULE 4 PERFORMANCE STANDARDS <br />27 plus years of mining activities in the West, East, and Section 16 pits, and are so limited in <br />thickness and extent that it is not practical to recover the remaining coal reserves by either surface <br />or underground mining methods. At Colowyo, future surface mining will entail moving to <br />separate, distinct, and in some cases distant pits while future underground mining will entail <br />mining totally different seams in different locations. <br />4.23.5 Undisturbed Areas of Coal Shall Be Left in Unmined Sections <br />As for the CDRMS Rules (Rules) requirement for leaving undisturbed areas of coal in unmined <br />sections, Colowyo contends that this application of the Rules does not apply since the seams to be <br />highwall mined are being accessed from active surface pits that by this Permit and other applicable <br />sections of the Rules are required to be backfilled and fully reclaimed. Hence should undisturbed <br />barrier areas of coal be left for some future access, these potential portal areas would be <br />inaccessible for future generations because they would be buried under the pit backfill. <br />Additionally and importantly, as discussed above, due to the many geological reasons, there is not <br />economical coal to be recovered from "behind" the areas slated to be highwall mined. <br />4.23.6 Abandoned or Active Underground Mine Workings <br />To Colowyo's knowledge, no abandoned or active underground mine workings have ever existed <br />or currently exist in any of the coal seams in the areas proposed to be highwall mined. No highwall <br />mining will be allowed to take place within 500 feet of any abandoned or active underground <br />mining operation. <br />4.23.7 Surface Mining Activities and Highwall Mining <br />The highwall mining shall follow the surface coal mining activities in a contemporaneous manner <br />consistent with the applicable requirements of CDRMS Rule 4. Due to active pit progressions and <br />sequencing of mining (in addition to meeting the Permit requirements for contemporaneous <br />reclamation), it is required that highwall mining occurs timely if not immediately following <br />conclusion of pit mining activities. Also, as described more fully in 2.06.9(2), the need to backfill, <br />i.e., contemporaneously reclaim the pits, is mandatory for Colowyo in order to build the pit floor <br />from which to work from to mine the successively higher (in the geologic column) coal seam. <br />Hence successful highwall mining is in part dependent upon timely and successful <br />contemporaneous reclamation of the pits. <br />4.23.8 Prevent Pollution of Surface and Groundwater and to Reduce Fire Hazards <br />Ground water in the pit or highwall mining holes will not be problematic being that the Colowyo <br />pits are essentially dry (minor perched aquifers with limited seasonal flows) and are located above <br />the first regional aquifer (Trout Creek) by a substantial distance. Ground water flow regimes and <br />the negligible impact that Colowyo's surface mining activities have on ground water as a result of <br />mining these target coal seams/rock interburdens are detailed extensively in Permit Section <br />2.04.7(1). From this extensive body of data and from experiences to date with mining activities, <br />no toxic forming or acid forming water discharge is anticipated from any of the highwall openings. <br />Rule 4 Performance Standards 4-87 Revision Date: 11/27/18 <br />Revision No.: TR -129 <br />