2008-03-21_PERMIT FILE - C1980007A (5)

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West Elk Mine No stock watering ponds in the Apache Rocks and Box Canyon mining areas have-been noticeably affected when longwall mining occurred beneath them. The ponds above mined longwall panels 13-A, 15, 16, and 22 in Horse Gulch, Sylvester Gulch (stock ponds P27-2 and P27-3), and West Flatiron (stock pond P234) respectively, showed no noticeable water level change when compared to pre-mining water levels (after taking seasonal precipitation levels into account). These ponds, which are located in clay-rich (Wasatch clay) material in order to minimize leakage, occur in overburden that ranges in depth from about 800 feet in Horse Gulch to 2,200 feet in the stock pond P23-4 above the eastern end of mined longwall panel 22. Streams and Ditches The primary streams in the South of Divide mining area are Dry Fork of Minnesota Creek, Deer Creek, Poison Creek, and Lick Creek. The primary source of water to Minnesota Reservoir comes from the Deep Creek Ditch, wherein water is diverted from the upper drainage of Deep Creek and transmitted to Dry Fork. The Deep Creek ditch was constructed in debris flows or colluvium and alluvium derived from the debris flow, as described above, this debris flow material is not expected to be impacted by longwall mining. The end of Deep Creek Ditch, where it transitions into Dry Fork, lies above an area that will be undermined by panel E5 gate entries(see Sketch A). Maximum horizontal strain over panel E5 in this area is predicted to be 1.0 percent (Em of 0.010) as shown in Table 3 of Exhibit 60E. Figure 2 of Exhibit 60E shows the conceptual model for surface cracking that applies to panel E5. The predicted maximum depth of surface cracks over panel E5 is 5 to 10 feet, as shown in Table 3 for slopes less than or equal to 30 percent. Surface cracks may develop within the transition area of Deep Creek Ditch into Dry Fork wherever bedrock is exposed. This may occur since the transition area is located above gate pillars, where tension cracks can develop. If cracks would develop that run across the ditch and cause water loss the cracks would be sealed by Mountain Coal Company (MCC) to mitigate this problem. Accelerated erosion may occur where subsidence results in a steepening of the gradient of the ditch. This is most likely to occur in the transition area where approximately 250 feet of ditch could have a steeper gradient due to subsidence. The existing pre-mining gradient in this area is approximately 1.6 percent and could steepen to a maximum post-mining gradient of 2.6 to 3.5 percent. The area of the Deep Creek Ditch that is affected primarily lies on bedrock and thus accelerated erosion should not be a major issue. A small amount of heaving may occur in areas of exposed bedrock that lie in front of the subsidence trough but since the ditch is located above gate pillars heaving probably would not occur. Since the gradient of the ditch is expected to increase and not decrease at any location above panel E5 there should not be any ponding within the ditch from subsidence. Mine subsidence may cause minor vibration at ground surface. The intensity of the vibration can not be estimated based on the current state of practice. According to I?T10 a mining induced seismic event occurred in Utah had a magnitude 4.2. Tetra Tech understands that this occurrence caused some rocks to become dislodged and tumble downhill. These occurrences are infrequent and are theorized to happen in areas of deep cover that has a very hard rock, such as sandstone, in the upper portion of the overburden. If the hard rock does not immediately fail (resulting in breakage to land surface) but remains in place until a large area of hard rock fails at once, the resulting event could be considered seismic in magnitude. These events have not been noted above the West Elk Mine and are considered not likely to occur. Based on this information mining induced seismicity triggering landslides in the area above the Deep Creek Ditch is considered to be a low probability. There are, however, pre-mining landslides in the area being mined. The change in the gradient in this area from the mining could increase the potential for 2.05-151 Revised June 200.1 PRIG, Rev. JVarch 2006.• Afay 2006PR10, Nov. 2006TR707,Aprd 2007TR108;Sep. 2007 PR12; Feb. 2008 PR-12