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West Elk Mine <br />• Conceptually, it is useful to visualize the following subsurface disturbance pattern for two-seam <br />mining. The B-Seam typically averages 1,000 feet to 1,200 feet under the ground surface in the <br />Apache Rocks mining area, while the E-Seam typically averages 800 to 1,000 feet. This indicates <br />that the typical separation distance between the B and E-Seams is 200 feet. The seams aze to be <br />mined in descending order. Consequently, as the E-Seam is mined, a caved zone 30 feet above the <br />top of the seam will develop, along with a fractured zone extending up another 200 feet (assuming <br />an E-Seam mining thickness of 11 feet). This will place the combined caved/fractured zone 250 <br />feet above the E-Seam as illustrated in Figure 21. <br />Following E-Seam mining, the B-Seam will be mined. In general, similar heights for the caved <br />zone and fractured zone above the B-Seam will be observed, which means that the top of the B- <br />Seam fracture zone will be above the previously mined E-Seam. This secondary mining of the B- <br />Seam should not increase the combined caved/fractured zone of the E-Seam. Therefore, a total <br />thickness of subsurface disturbance of approximately 450 feet can be calculated by adding the <br />height of caved/fractured zone above the E-Seam to the interburden distance between the B-Seam <br />and E-Seam. For this calculation, the interburden distance was used instead of the caved/fractured <br />zone above the B-Seam, because the caved/fractured zone above the B-Seam overlaps with the <br />caved/fracture zone above the E-Seam. Any groundwater within this 450 foot thickness may be <br />disrupted, and if such groundwaters feed spring or seeps, the springs or seeps may also be adversely <br />affected. With respect to surface flow impacts, however, the minimum separation thickness <br />between the top of the E-Seam fractured zone and the ground surface is 150 feet, which means that <br />surface flows (including springs) aze insulated from the caved and fractured zones by a minimum of <br />150 feet in the Apache Rocks permit revision area <br />Within the current permit area, including the Box Canyon and West Flatiron revision area, greater <br />minimum distances between the B-Seam mining and surface flows aze maintained with the <br />exception of the Gtibble Gulch azea, west of the 7NW longwall panel. In this azea, the minimum <br />300 feet of B-Seam overburden provides little buffer between the combined caved/fractured zone <br />height and the known springs. Detailed monitoring of this azea during mining will allow MCC to <br />assess potential impacts to well springs in the upper fractured zone and to support or refine <br />predicted effects. <br />There is a higher probability that colluvial and alluvial springs will be impacted by surface cracks, <br />although this risk is also quite small, for the many reasons that are presented in Section 2.05.6 <br />(3)(b)(iii & viii) Surface Water Quantity Effects, Streams. Consider the following factors: <br />1. It is not feasible to quantify the probability that a spring will encounter a surface crack. <br />However, as with stream channels and stock ponds, the probability that a spring in the <br />mining azea will encounter a crack is quite small. <br />Z. Spring flows that enter a crack will continue to move downgradient, leading to spring <br />relocation rather than spring loss. <br />• <br />1.05-163 November 1004 PRI l <br />