2008-03-21_PERMIT FILE - C1980007A (8)

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As discussed in Section 5.3.2, the maximum crack depth in the South of Divide and Dry Fork mining areas is estimated to be 15 feet in terrains with slopes less than 30 percent, with depths up to 35 feet occurring locally in steep topography. For the Dry Fork channel near the western edge of panel E2, the maximum projected crack depth is 15 feet. Consequently, the combined maximum heights of the caved and fractured zones and the maximum crack depth is 253 feet. Even at the minimum overburden thickness of 375 feet, an estimated 122 feet of unfractured bedrock will remain in tact. In addition, the presence of soft shales and claystones in the E Seam overburden will increase the probability that the strata will warp rather than fracture during the subsidence process. 11.4 Springs, Water-Bearing Zones, and Ground Water Wells The Mountain Coal Company Spring and Stock Pond Location Map for the South of Divide shows one decreed spring (Spring 21), three springs found flowing at every site visit (in 1975, 1977, 1979, and 1980), thirty-three intermittent springs, and two groundwater wells that are currently monitored. The same map indicated seven perennial streams in the Dry Fork area. Only a few springs in the West Elk Mine area indicate a source from a local bedrock water-bearing zone. Most springs likely have sources from local water-bearing zones in surficial material (debris flows, colluvium, and possibly alluvium). In contrast to surface water containment structures, such as reservoirs, ponds, streams and ditches, springs and water-bearing zones may have water sources that are either in bedrock beneath the blanket of clay-rich surfcial material (debris flows, alluvium, and colluvium), or have a source from within the surficial material. Subsidence may affect a spring or waterbearing zone source located in bedrock, whereas effects may or may not be expected where the spring source is within the surficial material. Tension cracks produced in sandstone bedrock during the subsidence process, for example, may divert water to a lower rock layer and therefore change the flow location. However, local water-bearing zones in permeable zones, which are interlayered with clay-rich zones (Wasatch clays) in the surficial deposit, may yield to tensile stresses without cracking. There is no field documentation known to Dunrud (2006, p. 27) or the current author to either support or refute this statement. Therefore, it is important to monitor all known spring flows for a few years (to account for seasonal variations) prior to any mining in the area. 11.4.1 Springs Decreed Spring 21 is located within the areas of mining influence of un-mined longwall panels E5, E6, and E9 in the NE%4 of Section 5, (Township 14 South, Range 90 West). Maximum tilt and strain is expected to occur in this area, because it is located above the projected haulageway and barrier pillar to the haulageway. The overburden depth at this spring site to the E Seam is about 650 feet. Springs mapped in the South of Divide mining area, which have been found flowing at every site visit, include: 1) a spring located 800 feet west, southwest of the Minnesota Reservoir dam- outside the area of any planned mining influence; 2) a spring located along Dry Fork 700 feet west of the confluence of Poison Creek and Dry Fork, above projected longwall panel E2 in about 650 feet of overburden to the E Seam; 3) a spring located south, and outside of the area of influence of longwall panel E8. Of the springs mapped in the South of Divide area, only the decreed Spring 21 and the spring located along Dry Fork and above longwall panel E2, may be impacted by longwall mining. The source of decreed Spring 21 may be a local water-bearing zone in bedrock of the Mesaverde Formation, whereas, the source of the spring along Dry Fork (above longwall panel E2) is likely to be a local waterbearing zone in colluvium or alluvium derived from debris flows. There are seven springs that are identified as perennial in the Dry Fork mining area. Springs 2-1 and 2-2 are adjacent to each other and located above the bleeder entries in panel E5 where the overburden is about 1,200 feet of overburden. These springs should not be affected by mining. Springs 2-3 and 2-4 are adjacent to each other and located above the center of panel E-4. The Tetra Tech - 0907171P 27