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West Elk Mine <br />additional surface flow. Furthermore, sediment carried by these ephemeral channels will fill the <br />cracks hi over time. This was observed by Tieman and Rauch (1992) in a study of de -watering <br />effects at a longwall mine in northern West Virginia. They stated, "Streams over panels mined out <br />more than two years ago do not exhibit abnormally low stream flow. This implies that subsidence <br />fractures over such older panels have closed or filled with sediment so that streamflow could have <br />recovered from any previous mining impacts." <br />To quantify the probability that any given channel in the Apache Rocks or Box Canyon permit <br />revision area will encounter a surface crack, there are two basic methods that have been used to <br />calculate crack occurrence frequency. First, the total perimeter (length) of the mined longwall <br />panels, as of January 1997, (i.e., panels 1NW through 8NW) is 75,000 feet and the total observed <br />cracked length over the longwall panels (again, exclusively over the pillars) is 115 feet. This <br />translates into a probability of occurrence of 0.2 percent. The second approach is to recognize that <br />the total surface area of the development mining and chainibarrier pillars is approximately 344 <br />acres, while the cracked surface area (104 square feet) is approximately 0.002 acres. This translates <br />into a probability of occurrence of 0.001 percent. While the crack probability was higher in the <br />Apache Rocks mining area due to the topography and rock outcrops, the South of Divide <br />mining area will be more analogous to the northwest panels. <br />These calculated risk factors are important because they enable a determination of the frequency <br />with which any given channel is likely to experience a crack. For example, there are eight stream <br />channels in the Apache Rocks mining permit area representing 18 crossings of the chain/barrier <br />pillars. At any given crossing, the probability of encountering a crack is 0.2 percent. Therefore, <br />throughout the entire Apache Rocks mining area, the combined probability of encountering a <br />surface crack is only 3.6 percent. Within the Box Canyon mining area, there are seven stream <br />channels representing four stream systems (i.e., Sylvester Gulch, Box Canyon, the unnamed <br />tributary west of Box Canyon, and an unnamed tributary to Raven Gulch). These seven stream <br />channels cross chain/barrier pillars seventeen tunes. The resulting combined probability of <br />encountering a surface crack in this area is 3.4 percent. <br />There are eleven stream channels in the South of Divide mining area representing <br />approximatly 24 crossings of the chain/barrier pillars. At any given crossing, the probability of <br />encountering a crack is 0.2 percent. Therefore, throughout the entire South of Divide mining area, <br />the combined probability of encountering a surface crack is only 4.8 percent. These risk <br />factors help to put the loss of surface water due to surface cracking into perspective. Based on his <br />annual observations at the mine, Mr. Dunrud finds it highly unlikely that a surface crack will <br />develap within the alluvium and/or colluvium of the stream channels in the South of Divide <br />permit revision area. <br />In the unlikely event that a channel does encounter a surface crack. the next questions are: (1) How <br />much water will be lost?, and (2) what is the significance of the water "loss"? As for question (1), <br />it is..coinservative to assume that a Crack over a pillar v,;1111 UC i\JV icct i 30 feet deep and <br />conservatively, an average Gf 0.25 feet wide, for a total volume of 0.02 acre-feet. A loss of this <br />magnitude, even if it were to occur a number of times per year, is not significant. Furthennore, and <br />of perhaps greater importance, surface flows lost to cracks will still eventually return to the North <br />2.05-191 Revised June 200.1 PR10, Januar, 2006, March 2006; Rev. May 2006 PRIG, Nov. 2006 TR107;Sep. 2007 PR12;Feb 2008 PR12 <br />