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• 2.06.8 SURFACE COAL MINING AND RECLAMATION OPERATIONS ON AREAS, OR ADJACENT TO AREAS INCLUDING ALLUVIAL VALLEY FLOORS The Keenesburg Mine is located on wind-blown sand deposits. Within the life-of--mine boundary there are no streams of any kind. The permeability of the blow sand is evidently sufficient to prevent overland runoff except possibly for very intense rainfall events. There is no evidence of channels or even rills that would be indicative of surface runoff. The absence of any streams or channels within the life-of--mine boundary is taken to mean that no lands within that boundary qualify as alluvial valley floors. The extent of drawdown of the subsurface water system in the vicinity of the Keenesburg Mine as a result of mining is presented in Appendix I-2. The map shows non-uniformity of the piezometric surface (gradient) as a result of mining and the variable geology and hydrology in the area. As stated in section 2.04.7, a vast majority of subsurface water flow in the overburden is restricted in the transition zone between the overlying sands and underlying clay and clay-shale bedrock. The presence of a very steep hydrologic gradient in the south and east portions of the present mine area, along with the observations of minimal to non-existent pit inflows indicate: 1. Low permeability of overburden materials • 2. Lower saturated thickness of overburden materials than originally estimated 3. Low storage capacity of these materials 4. Isolation of mining operations from the Ennis Draw subsurface water system. The parameter that significantly controls the amount of groundwater originally anticipated to enter the pit is the saturated thickness of the overburden materials. Prior to mining, saturated thickness of the overburden materials was estimated to be 65 feet. Observations within the active pit show that the actual saturated thickness does not exceed 10 feet in isolated locations and within the remainder of the active mine, saturated thickness is zero. Thus, a substantial decrease in saturated thickness would result in a relatively large reduction in the predicted pit inflows. As stated in the report of hydrologic investigations, drawdown of the piezometric surface in the Laramie formation probably will not extend beyond about 0.5 miles from the pit during mining. The transient drawdown created during mining will eventually dissipate and a more-or-less steady post mining flow pattern will develop. If the transmissivity of the backfilled mine area should be identical to the pre-mining values, the post mining flow pattern and ground water discharge would be unchanged from the pre-mining situation. In this case the maximum extent of the affected area would be on the order of 0.5 miles from the life-of--mine boundary. C~ 130