2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 31 • 11.2 Stock Watering Ponds and U.S. Forest Service Water Resources The stock watering ponds in the South of Divide and Dry Fork mining areas are located in debris flows or colluvium derived from the debris flows (Dunrud 1989). A total of 26 ponds have been mapped in the South of Divide mining area. Ten of these ponds are also classified as U.S. Forest Service water resources. Eight stock ponds are located in the Dry Fork mining area. The debris flows consist of a heterogeneous mixture of clay derived from the Wasatch Formation and boulders and gravels derived primarily from the Mount Gunnison intrusive (granodiorites and quartz monzanites). Based on observations made during geologic mapping in the area, these debris flows are even less likely to be affected by longwall mining than the alluvium. The debris flows have a very low permeability and, because the clay matrix is armored by the interstitial gravel and boulders, are resistant to erosion (the Deep Creek Ditch locally flows in this material at steep gradients). Based on the above - mentioned observations, no effects are expected when ponds in the South of Divide and Dry Fork mining areas are undermined. The clay -rich material that lines these ponds is expected to provide a seal against subsidence effects. 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 B- seam 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 P23 -4) 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 area above the eastern end of mined B -seam longwall panel 22. 11.3 Streams and Ditches A greater potential exists for longwall mine subsidence to impact streams in the South of Divide and Dry Fork mining areas than in the Apache Rocks and Box Canyon mining areas. This is primarily because a) the overburden depth is less than in the Apache Rocks and Box Canyon mining area, particularly in the northwestern part of the area, and b) the coal extraction thickness is projected to be greater for the E -seam than was mined in the B -seam (14 feet versus 12 feet). Although no subsidence cracks are expected to affect streamflow in the South of Divide and Dry Fork mining areas, based on prior observations in the West Elk and Somerset mine areas, it is recommended that detailed subsidence and surface flow measurements be done in order to verify this concept. It is recommended that measurements be conducted particularly in the northwestern part of the South of Divide mining area, where coal extraction thickness is projected to be a maximum of 14 feet and overburden depth ranges from 375 to 400 feet. 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 streams in the Dry Fork mining area are the upper regions of Dry Fork of Minnesota Creek and Deep Creek. A primary source of water to the Dry Fork and 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 in Section 11.2, this debris flow material is not expected to be impacted by longwall 831-032.810 Wright Water Engineers, Inc.