2008-03-21_PERMIT FILE - C1980007A (46)

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West Elk Mine • The greater the overburden thickness, the smaller the probability that fractures from the mined area will reach the recharge source for any given colluvial spring. In the section entitled "Potential for Hydraulic Connection Between Mine Workings and Surface," a detailed analysis concludes that no surface water feature should be interconnected to the mine workings in overburden thicknesses of at least 253 feet. Within the South of Divide permit revision area, the overburden above the E Seam is at least 375 feet, thus all but eliminating the probability of mining-induced impacts on colluvial springs. Roughly two thirds of the springs in the current permit and Box Canyon permit revision areas are colluvial in nature. There is considerable spring monitoring data for the West Elk Mine and the majority of the relevant data are for colluvial springs. As noted above, the relevant data appear to demonstrate that F and B-Seam mining has not affected the monitored springs. This is not surprising in light of the large overburden at the mine and the fact that surface cracks are so infrequently observed. The greater the overburden thickness, the smaller the probability that fractures from the mined area will reach the recharge source for any given colluvial spring. For example, in the case of the Apache Rocks permit revision area, there will be a combination of single-seam mining (B-Seam only) in Sections 26 and 27 and, two-seam mining in descending order will occur in the B and E- Seams in Sections 28, 29, and 30. Within the Box Canyon permit revision area, the overburden thickness above the B-Seam is large, thus all but eliminating the probability of mining-induced impacts on colluvial springs. Because of the interacting fracture zones, two-seam mining impacts • on colluvial springs must be looked at in a somewhat different manner. Conceptually, it is useful to visualize the following subsurface disturbance pattern for two-seam mining. The B-Seam typically averages 1,000 feet to 1,200 feet under the ground surface in the Apache Rocks mining area, while the E-Seam typically averages 800 to 1,000 feet. This indicates that the typical separation distance between the B and E-Seams is 200 feet. The seams are to be mined in descending order. Consequently, as the E-Seam is mined, a caved zone 30 feet above the top of the seam will develop, along with a fractured zone extending up another 200 feet (assuming an E-Seam mining thickness of 11 feet). This will place the combined caved/fractured zone 250 feet above the E-Seam as illustrated in Figure 21. A total thickness of subsurface disturbance of approximately 450 feet can be calculated by adding the height of caved/fractured zone above the E-Seam to the interburden distance between the B- Seam and E-Seam. For this calculation, the interburden distance was used instead of the caved/fractured zone above the B-Seam, because the caved/fractured zone above the B-Seam overlaps with the caved/fracture zone above the E-Seam. Any groundwater Nvithi.?n this 450 foot thickness may be disrupted, and if such groundwaters feed spring or seeps, the spri, igs or seeps may also be adversely affected. With respect to surface flow unpacts, however, the minimum separation thickness between the top of the E-Seam fractured zone and the ground surface is 150 feet, which means that surface flows (including springs) are insulated from the caved and iiactareu zones by a minimuin of 150 feet in the Apache Rocks permit revision area. • Within the current permit area, including the Box Canyon revision area, greater minimum distances between the B Seam mining and surface flows are maintained with the exception of the Gribble 2.05-202 Revised June 2005 PRIO: Rev. March 2006: Rev. May 2006 PRIO:Sep . 2007 PRI 2; Feb 2008 PR12