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West Elk Mine '. • The cracks along the southern edge of Section 17 had influence from both MCC's F-Seam mining and Bear's B and C-Seam mining and the bamer pillars between them. • The cracks located between SNW and 6NW longwall panels were most likely caused by the reinitiation of an old landslide. The cracks above 8NW longwall panel that CDMG had eazliei attributed to subsidence, were in areas of low overburden and within a lazge old landslide mass. Other cracks have been observed within the old landslide mass that aze completely outside the mining area of influence. Therefore, when assessing the potential for surface cracks due to longwall mining subsidence, the presence of mitigating factors must be considered, such as existing landslide activity, adjacent to or overlying mine workings, and very low overburden. Without such factors being present, surface cracking has not been noted over longwall mining in the West Elk mining area. An additional factor in predicting the probability of surface crackirg is the location with respect to the longwall panel layout. With the exception of two of the 8NW longwall panel cracks, all of the cracks were located above chain or barrier pillars, where stresses aie maximized. The only cracks to have formed within the internal portions of the longwall panels were very minor and had begun to heal shortly after mining. These cracks are consistent with those formed by dynamic subsidence as discussed within the section entitled Effects of Mining on Surficial Geologic Features. 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 are 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 Richazd Dunrud have detemvned that, except under precipitous slopes, the maximum depth of surface cracks that will develop in the current pemut azeais 100 feet, and that typical crack depths will razely be greater than 50 feet. hi 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 overburdenisubsides. Tension cracks over chain pillazs 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 pillars, temporary loss of flow may occur to more permeable zones in the overburden. Given the discontinuous nature of these sand units within the Mesaverde Fjormation, 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 1.05-151 November 1004 PRIl