GENERAL47522 - Laserfiche WebLink

Home Browse Search

Affected Environment and Environmental Consequences Chapter 3 BLM estimates that the coal in the Dry Fork tract averages about of 11,700 BTUs, has an average sulfur content of about 0.5 percent, and an average ash content of about seven percent (BLM 2004). These coals are desirable because they are considered "compliance" coals under standards of the Clean Air Act (CAA). Geologic hazards are present in the LBA tract in the form of rock falls, landslides, and slumps associated with unstable moderate to steep slopes. Areas of instability are typically associated with steep slopes and saturated soil conditions. Slope aspect relative to geologic structure also affects stability, where dipping structure daylights on exposed slopes. Projections from the West Elk Mine indicate that two fault zones may be encountered in the LBA tract. The inferred Gunnison and Deep Creek fault zones are shown on Figure 3-2. The faults have not been confirnied; their presence is inferred from spatial location, some seismic data, and geometric analysis. There are no discernible features of these faults on the lands surface, suggesting that they are buried features if they exist at all (Koontz 2005). Soils Soils in the project area have developed from a combination of residual, colluvial, and alluvial materials derived from local bedrock. The soil survey identified and described six map units within the LBA tract. The map unit name, percentage coverage within the LBA tract, dominant soil series and attendant percent map unit composition, relative depth, hazard classifications (water erosion, shrink swell, and mass movement), and considerations as described in the soil survey are shown in Table 3-1. Soils in the project area are generally deep, fine- textured and well suited for vegetative production with steep slopes being the primary limitation on use. Erosion and mass movement are potential hazards associated with most soils in the area, due to fine textures. Soils on steeper slopes have slower infiltration rates, resulting in more surface flow and erosion. Mass movement on steep slopes is also a potential hazard, with Wetopa and Wesdy soil types having the highest potential hazard rating within the LBA tract (see Table 3-1). Fine textures and high activity clays result in a moderate to high shrink- swell hazard ratings for most soil types. At the leasing stage, the locations of surface uses are not known; therefore more detailed soil information at the sites proposed for disturbance, potential salvage depths, and volumes cannot be estimated. Location-specific soil resource data will be reviewed in more detail following if and when post-leasing surface use is proposed. These uses will be evaluated in subsequent NEPA analysis. 3.2.4 Environmental Consequences 3.1.4.1 Direct and Indirect Effects Alternative A Topography and Geology The tract would not be leased and no mining would occur; therefore, all topographic and geologic conditions within [hc Dry Fork LBA tract would remain in their current state. Ongoing natural processes, including erosion and mass wasting, would continue. Sails The tract would not be leased and no mining would occur; therefore, soil conditions would exist in their current state without effect. Ongoing natural processes would continue. Alternative B The surface expression of underground coal mining-related subsidence is the development of broad elliptical troughs or basins overlying the mined-out area. This area is defined laterally on the land surface by the angle of draw, overburden type, and thickness (i.e., the kinds and thickness of rocks between the coal seam and the land surface), the width of the longwall panels, and the height of the coal mined. The angle of draw is the angle formed by the vertical line drawn from the edge of the longwall panel or gob area to the point of zero subsidence on ground surface. There are four subsidence zones that develop in [he overburden above mining area (Peng 1992). A generalized schematic of the subsidence zones are shown in Figure 3-4. 3-8 Dry Fork Lease-By-Application FENS