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Mountain Coal Company <br />West Elk Mine <br />Exhibit 69 <br />lvester Gulch Facilities <br />The existing Pond 017 will be used as the primary aeration cell in the revised treatment system. <br />To enhance aeration, air diffusers will be installed on or near the bottom of the existing pond <br />with appropriately sized compressors on the banks to provide air flow. The system will be <br />designed so that aeration can be continuous, except during maintenance activities. For mine <br />water inflows containing higher iron concentrations, a float-mounted surface aerator system <br />designed for high-volume water movement may be used to further enhance oxygenation. <br />The Pond 017 volume at the existing operating water surface elevation (El.) of 6822.8 feet above <br />mean sea level (ft MSL) is approximately 200,000 gallons. This provides a normal detention <br />time of approximately 8.3 hours at the design flow rate of 400 gpm. To provide a larger pond <br />operating volume and prevent unintended discharges from the existing Pond 017, the existing <br />outlet structure will have a sealed steel plate attached with a top El. of 6826.5 ft MSL. The total <br />pond volume is anticipated to be approximately 325,000 gallons at El. 6825.0 ft MSL, the normal <br />maximum operating water surface, as shown on the area-capacity curve (Drawing Sheet Q. <br />The minimum freeboard in the existing Pond 017 will be 1.5 feet with the maximum emergency <br />spillway water surface at El. 6826.5 ft MSL and the pond crest at El. 6828.0 ft MSL, which <br />exceeds what is required by Rule 4.05.9(7)(d). <br />Design of Second Pond <br />• To provide for settling after aeration, a Second Pond is planned for construction. The pond is <br />designed to be in compliance with applicable portions of Rules 4.05.6 and 4.05.9. The Second <br />Pond will cover a total area of approximately 1.1 acres from the edge of the excavation east of <br />Pond 017 to the toe of the embankment fill east of the Second Pond. <br />The depth to bedrock in the pond vicinity is approximately 25 to 36 feet below existing ground <br />surface (ft bgs). Prior to excavation, topsoil, which is present at a thickness of approximately 6 <br />to 12 inches in the Second Pond area, will be stripped and hauled to a nearby topsoil stockpile for <br />later use on external embankment slopes and adjacent disturbed areas. During excavation, pond <br />embankments will be constructed using select compacted material from the excavation. These <br />embankments will be located on the north, east and south sides of the pond. The embankments <br />will have a crest width of 12 feet and 2.5h:1v external side slopes. The maximum height of these <br />embankments will be approximately 9.5 feet above existing ground surface on the embankment <br />centerline. Based on the existing pond slopes and geotechnical data reviewed for the project, the <br />interior pond excavation slopes are designed to be 2h:ly and the maximum excavation depth will <br />be approximately 17 feet. <br />The combined upstream and downstream side slopes of the settled embankment are designed to <br />be 4.5h:1v, as opposed to the minimum 5h:ly stipulated per Rule 4.05.9(7)(e). However, as <br />allowed by the same Rule, slope stability analyses have been performed (Attachment 1) to <br />support the design as being stable, with a worst-case factor of safety of 1.36 assuming very <br />conservative soil strength parameters and a design seismic load (more than the minimum safety <br />factor of 1.3 for static conditions per Rule 4.05.9(8)(b) and more than the minimum seismic <br />4 <br />Rev. 10197-TR83, Rev. 04109-TRI16