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West Elk Mine <br />overburden thickness ranges from 600 to 1,200 feet. Within the Box Canyon mining area, the <br />overburden thickness for the B -Seam ranges from 500 to 2,200 feet. <br />Springs (and their sources) can be effected by subsidence in two different ways. First, fractures can <br />extend upward from the mine seam and intersect the spring or spring source. As explained in <br />Sections 2.05.6 (6)(e)(i)(C&D), Subsidence Zone Description, the height of the caved/fractured <br />zone extending upward from the mine seam is conservatively estimated at 270 feet. Secondly, there <br />is a small probability of surface cracks developing in association with the mining (see Section <br />2.05.6 (3)(b)(iii & Viii) Streams for estimate of surface crack probability) and the typical maximum <br />depth of such cracks in the Apache Rocks and Box Canyon mining areas is conservatively estimated <br />at 100 to 200 feet. <br />These two distances indicate that, for a spring to avoid any potential impacts, the source of the <br />spring must be at least 270 to 370 feet above the uppermost seam that is being mined and more than <br />100 to 200 feet below the ground surface. From a practical standpoint, because every spring (by <br />defmition) "daylights" at the ground surface, there is an extremely small risk that any given spring <br />will encounter a surface crack. However, it is essential to recognize the following: <br />• Site specific evidence demonstrates that surface cracks are most likely to occur over chain <br />pillars, barriers and mine boundaries. Consequently, if a spring and its source are not <br />located in these areas, they will be unaffected by surface cracks. This is true for most of <br />the springs in the Apache Rocks and Box Canyon mining areas. <br />• The spring source could easily be unaffected by a surface crack, even if the spring itself is <br />affected (at the point where it surfaces). In this case, the spring would re-emerge at <br />another location. <br />It is feasible to define which of the springs (and sources) in the permit area are at least 270 feet <br />above the uppermost coal seam that is being mined, which will leave them essentially immune to <br />mine fracture impacts. This analysis has been conducted, and all of the springs in the Apache Rocks <br />and Box Canyon reining areas are at least 270 feet above the B -Seam (in those areas where only the <br />B- Seam is being mined) and at least 250 feet above the E -Seam (where both B and E -Seam mining <br />is being conducted. Above the LONE and 1 1N longwall panels, all springs are at least 270 feet <br />above the B -Seam. Map 37 depicts these springs. <br />The probable implications to springs caused by subsidence vary between colluvial/alluvial springs <br />and bedrock springs. Colluvial and alluvial springs are much more likely to be impacted by surface <br />cracks than bedrock springs are, while bedrock springs are much more likely to be impacted by the <br />fractures extending upward from the mining area than all-a,-ial and colluvial springs. These impacts <br />are discussed in the context of the schematic cross-section nrov ided in Figure 20. Figure 20 depicts <br />the mechanisms via which colluvial/alluvial and bedrock springs can be impacted by longwall <br />reining, as a fariction of the four different subsidence zones. < <; C,1{`,'ed zone (2) Fractured zone (3) <br />Continuous deforination zone, and (4) Zone of surface cracking. <br />2.05-197 Revised June 200.1 PRIG, Janumy 2006, March 2006; Rev. .Allay 2006 PRIO. Nov. 2006 TRI07;Sep. 1007 PR12;Feb 2008 PR12 <br />