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Mountain Coal Company, L.L.C. Exhibit 57 West Elk Mine Lower Refuse Pile multiple flow events, typically ranging in thickness from 1 to 3 feet. Individual events are often marked by • multiple organic horizons. Materials within the fan are highly stratified and somewhat variable ranging from relatively coarse. clean sand and gravel to silty or sandy clay. However, the predominant material present within the fan. particularly in the toe area of the pile, appears to consist of a silty sand and gravel material with abundant boulder and cobble-sized fragments of sandstone. The back (southwest) portion of the pile lies above colluvial materials associated with the steeper slopes in the area. Although still somewhat variable in character, a typically hieher percentage of silt and clay-sized fines are present in the sand and gravel materials. Also, zones of nested cobbles or boulders are common. Some distance to the north of [he pile ,the geological environment begins to chanee as you move down the terraces to the relatively clean eravel. cobble. and boulder alluvium terraces in the transitional environment. It is anticipated that an inte~neering of the debris flow materials and the coarse, somewhat cleaner alluvium would be present, resulting in a "hybrid" material having intermediatecharacteristics. The largest single even[ observed in the test pits had a maximum thickness of three fee[. This would indicate that the fans present were most likely constructed by many small events and were not the result of a few large events. Despite this and the evidence of long inactivity on the fan, it is assumed that it is possible during the life of the pile for another debris flow even[ to occur, blocking the existine drainage within Sylvester Gulch. The reconstruction of Sylvester Gulch Road (see Section 7.1) in the area provides for a drainage channel sufficient to convey the 100-year flood event safely past the pile. even in the event that the natural channel now existing in Sylvester Gulch is fully blocked by debris along its full lenb~th adjacent to the pile. The relocated Sylvester Gulch Road provides a buffer between any flooding or debris flow events associated with SylvesterGulch and the waste pile construction. Past evidence indicates that the majority of debris flow activity in Sylvester Gulch has ceased, or at least E remained inactive for a long period of time. Further, the evidence indicates that the magnitude of such events is small and that the provisions made for these events in the design should be adequate to prevent loss of refuse into the gulch for storm events up to the 100-year event. However, it is not possible to quantify the probability of such a debris flow event occurring within Sylvester Gulch since, unlike hydrologic events, a succession of debris flow events are not statistically independent. The movement and redistribution of loose material or debris during an event has an effect on the probability associated with subsequent events. The quantitative information available on the likelihood and the character of future debris flow events in SylvesterGulch represents the best information available. 5.~ Subsurface Hydrology 5.x.1 Permeability Data In-situ falling head permeability tests were performed in one piezometer and all of the monitoring wells. Measured values of horizontal permeability were found to range from ?.~ x 10~ cm/second to 3.6 x 105 cm/second (see Table 6). These values represent horizontal permeability values and in the highly stratified soils observed at the site, vertical permeability is expected to be even lower by at least one order of magnitude. Previous investigators have reported the permeabiliryof the waste material to be approximately I.'_ x 10~ cm/second. This permeability contrast of more than one order of magnitude will cause most of the leachate water to perch within the waste and above native soils. Using conservative estimates of infiltration (to be discussed in the following section), it is estimated that the maximum height of mound development within the pile will be approximately''7 feet. The water will migrate laterally and ultimately be intercepted in a system of underdrains where it will be routed to the existing sedimentation ponds. ~,.. 13