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~OUHTAIN COAL <br />~ COMPANYLLC. <br />A Subsidiary ofArch Western Resources, r i r <br />MCC Response: The text was modified or <br />2.OS.ti(3)(a)(i) - "Protection of Hydrologi <br />mitigation measures that will be taken as <br />geometry changes due to subsidence. <br />West Elk Mine <br />P O Box 59l <br />5174 Highway 133 <br />Somerset, CO 81434 <br />(970)929-5015 <br />Fax (970)929-5595 <br />added to in the first several paragraphs of section <br />c Balance" to include descriptions of prescriptive <br />needed to correct stream channel morphology and <br />~~ 97. Original Comment: Conclusion #2 in Exhibit tiOB says predicted vertical displacement, tilt, and <br />horizontal strain predicted for the SOD area are likely to be conservative values (I.e. greater than what <br />will actually occur). Yet in the same report, Figure 6, observed maximum subsidence over panels INW, <br />2NW, and 3NW was greater than predicted in two out of three panels, contradicting conclusion #2. <br />Please explain and correct as necessary. <br />MCC Response: Figure 6 in Exhibit 60B relates to calibration of the model and not prediction, so <br />there is no contradiction. <br />New Comment: The original question stands. This issue requires further explanation. <br />MCC Response: The sentence is no longer included in Exhibit 60B. Figure 6 in Exhibit tiOB relates <br />to calibration of the subsidence computer model from the initial longwall panels, where the <br />subsidence data had to be filtered to remove effects ojthe F Seam mining. The best fit was <br />. determined for the maximum subsidence outside of the influence of the F Seam workings over <br />longwall Panel 3NW. However, the original statement referred to Mr. Dunrud's conceptual model, <br />which tends to predict maximum subsidence that are greater than or equal to the computer <br />predictions. For example, the maximum subsidence predicted by the computer model over the E <br />Seampanels is approximately 9.6 (Figure 7 ojExhibit 60B), whereas Mr. Dunrud's conceptual <br />model predicts a maximum subsidence ojup to 11.2 feet (Table 2 ojExhibit tiOB). While the <br />computer mode[ may give the most likely value, the conceptual model value is considered to be <br />conservative. <br />\L~ 98. Original Comment: The DMG would not concur with conclusion #3 of Exhibit 60B. Based on mining <br />to-date at the West Elk mine, subsidence of a perennial stream and its associated alluvium has occurred <br />only in extremely limited areas due to B-seam mining with relatively thick overburden. Mining of 14 <br />feet of the E-seam coal in the northwest corner of section 33, T13S, R90W along the Dry Fork is <br />predicted to result in 6-11 feet of subsidence along the Dry Fork alluvium in areas of 400-500 feet of <br />overburden. This location will also bound un-mined coal, where maximum perpetual tension could <br />result in massive, long-term cracks in and around the alluvium. Relatively few studies have measured <br />the hydrologic impact from longwall undermining. However, one important study, Water Resources <br />Investigations Report 95-4025, Hydrology of the North Fork of the Right Fork of Miller Creek, Carbon <br />Count Utah, Before, During, and After Underground Coal Mining, 1995 found that subsidence in 300- <br />500 feet of overburden caused the stream to enter a fracture, drying up the stream bed below. Water <br />quality was also dramatically impacted. Temporary loss of stream flow is also documented in Colorado <br />(Foidel Creek mine). The potential for significant impacts to the Dry Fork above Minnesota Reservoir <br />above Panels E2 and E3 cannot be understated. Please reconsider conclusion #3. <br />MCC Response: Exhibit 60B was revised. <br />New Comment: It did not appeaz that Exhibit 60B was revised to a significant degree. The original question <br />stands and will need further discussion. <br />13 <br />