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West Elk Mine • have formed within the internal portions of the longwall panels were very minor and had begun to heal shortly after mining. These cracks aze consistent with those formed by dynamic subsidence as discussed within the section entitled Effects of Mining on Surficial Geologic Features. Also discussed in that section were Mr. Dunrud's observations of additional cracks in the Apache Rocks mining area (longwall Panels 12, 13, and 13A). Each of these cracks was located on ridges or bluffs in zones of tension, and observed depths were only a few feet. The observed dimensions of these cracks, widths of three to four inches, lengths of 10 to 20 feet, and depths of less than one foot, indicate that the probable hydrologic consequences aze negligible. If one of these cracks were to intercept surface flow during the short time that the crack was open, the volume of water that could be stored is less than 0.0002 acre-feet. This fmding is very important in that it indicates that the potential loss of surface flows via cracks may be a problem only where the channels cross the pillars. It is in these locations where monitoring is most important and, if necessary, mitigation might be needed. WWE and 12ichazd Dunrud have deternilned that, except under precipitous slopes, the maximum depth of surface cracks that will develop in the current permit azea is 50 feet, and that typical crack depths will rarely be greater than 25 to 35 feet. In the locations where surface fracturing does occur, temporary loss of surface flow to more permeable zones in the overburden may be observed. Cracks also become "sealed" with time as sediment transported by water washes into them. Tension or surface cracks, which occur over longwall panels, will tend to "heal" themselves through compression as the longwall progresses and adjacent overburden subsides. Tension cracks over chain pillars may stay open even after adjacent overburden has subsided, but will eventually heal/seal. In those locations where surface flow occurs across tension cracks at chain or barrier pillazs, temporary loss of flow may occur to more permeable zones in the overburden. Given the discontinuous nature of these sand units within the Mesauerde Formation, however, these more permeable units will likely become saturated quickly, thereby reducing the introduction of additional surface flow. Furthermore, sediment carried by these ephemeral channels will fill the cracks in over time. This was observed by Tiernan and Rauch (1992) in a study of de-watering effects at a longwall mine in northern West Virginia. They stated, "Streams over panels mined out more than two yeazs ago do not exhibit abnormally low stream flow. This implies that subsidence fractures over such older panels have closed or filled with sediment so that streamflow could have recovered from any previous mitring impacts." To quantify the probability that any given channel in the Apache Rocks or Box Canyon pemrit revision azea will encounter a surface crack, there aze two basic methods that have been used to calculate crack occurrence frequency. First, the total perimeter (length) of the mined longwall panels, as of January 1997, (i.e., panels 1NW through 8NW) is 75,000 feet and the total observed cracked length over the longwall panels (again, exclusively over the pillars) is 115 feet. This translates into a probability of occurrence of 0.2 percent. The second approach is to recognize that the total surface azea of the development mining and chain/barrier pillazs is approximately 344 acres, while the cracked surface azea (104 square feet) is approximately 0.002 acres. This translates into a probability of occurrence of 0.001 percent. While the crack probability was higher in the C~ 1.05-160 Revised June 1005 PR/0