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• Effect of Mining on Ground Water <br />Any possible effect of mining on the subsurface waters of Ennis Draw will be <br />significantly reduced, as the area of proposed mining in B -Pit will no longer be <br />adjacent to Ennis Draw. <br />Instantaneous pit inflows from the Laramie Formation had been expected to range <br />from 20t to 130f gpm, depending upon the number of pits open and duration <br />since opening. (McWhorter, 1978). It was also estimated that piezometric <br />drawdowns in the Laramie formation due to presence of the mine will be extended <br />no further than 0 5 miles. The quality of pit inflows is expected to be comparable <br />to water resident in the Laramie shales and coals - no appreciable degradation of <br />water quality is anticipated as Laramie ground water passes through spoils in route <br />to the active pit (McWhorter, 1978). <br />As stated in the mine permit, mine operations are to remain 100 feet west of Ennis <br />Draw. This was based on recommendations presented in the hydrological report <br />relating the ability of ground water to move through the overburden. Pit inflows <br />were estimated to be between 20 - 130 gallons per minute. <br />During the duration of mining in A -Pit since the opening of the mine in 1980, <br />minimal to non-existent pit inflows have been encountered. The high estimates for <br />• pit inflow had been based upon information at the time, prior to mining. There <br />estimates were based on: <br />l . A high assumed saturated thickness of the overburden <br />2. The ability of the overburden to transmit water (clays) <br />3. The overall extent/presence of water in the overburden (clays) <br />As mining progressed, pit inflows estimated before ntining were not encountered. <br />This suggests that the three factors listed above would be down graded, primarily <br />because saturated thickness of the overburden was estimated to be 65 feet, when <br />reality is about 5 ft. -10 ft. maximum, and in some areas, zero. <br />Subsurface water exists in the lower portion of the coal seam presently being <br />mined and as isolated, perched conditions in the overburden. Groundwater in <br />neither the coal nor overburden is known to provide a water supply for any <br />purpose in the study area (McWorther 1980). This may be partially due to the <br />overall inability of the coal or overburden aquifer to discharge water to wells <br />completed in them. <br />In the transition zone between the contact of eolian sands and weathered-bedrock <br />clays, lateral movement of this water is generally toward the north-northeast. The <br />• saturated thickness of these silty-clayey fine grained sands immediately above the <br />clay bedrock is from 0-10 feet, with subsurface water occurrence controlled largely <br />51 <br />