2025-04-09_PERMIT FILE - C1981019A (4)

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-14 Revision Date: 4/9/25 Revision No.: MR-261 3) The depth and elevation of any impounded waters at the time of the certified report; 4) Existing storage capacity of the impoundment; and 5) Any other aspects of the structure affecting stability, or requiring maintenance. Colowyo will maintain a copy of each certified report at the mine site. Colowyo successfully demonstrated that failure of small impoundments would not create a threat to public health and safety or threaten significant environmental harm. A written safety demonstration completed by a professional engineer is located in Exhibit 7, Item 11, Volume 2B, in accordance with rule 4.05.9(18)(b). None of the small post-mining impoundments act as primary sediment control structure for a particular area; they are all constructed in reclaimed areas of the mine to enhance the approved postmining land use; they are all under two-acre feet. All impoundments will be maintained according to the specifications set forth in this part. Maintenance for impoundments may include (if necessary) mowing and cutting of excess vegetative growth for the purpose of facilitating inspections and repairs including keeping ditches, culverts, spillways, and other outflow structures free of debris. All combustible material, other than mulch or other material needed for erosion control and surface stability (vegetative growth) will be removed. Plans for any modification of any sedimentation impoundments or dams will be submitted to the Division, and no modification will begin until approval of the plans have been granted unless such modification is necessary on an emergency basis for public health, safety or the environment would be endangered. Colowyo will inspect the condition of each pond quarterly (until a waiver for annual inspection is granted) with the reports submitted quarterly as well. None of Colowyo’s post-mining impoundments will meet the size criteria of 30 CFR 77.216(a)(1989). 4.05.10 Underground Mine Entry and Access Discharges Colowyo currently conducts surface coal mining exclusively. 4.05.11 Ground Water Protection There are no aquifers or continuous sources of ground water within the stratigraphic section from which the Colowyo Mine mines coal using surface mining techniques. This is also noted in Section 2.04.7. In addition, there are no continuous aquifers of regional extent within the entire Williams Fork Formation in the location of the Colowyo Mine. Occasionally, a minimal amount of water is found under perched conditions in noncontiguous lenticular sandstones and in fractured coal under the sandstones. No toxic concentrations of acid forming materials have been found in the overburden, reclaimed slopes or surface and ground water system associated with the Colowyo Mine. No adverse effects on groundwater quality are expected to occur due to mine excavations or backfilling.