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_33_ ® ~ ~U~ii ~~ <br />Following the closure of the mines the inflow waters will have a longer <br />residence time and, therefore, a longer period of time to chemically react <br />with the mine cavity. The applicant predicts that the Eagle No. 5 Mine <br />workings will completely flood in seven years and the Eagle No. 9 Mine in <br />three years. The applicant a]so predicts that the Eagle No. 5 mine may <br />discharge to the surface and the Eagle No. 9 will not discharge (page III-51 <br />and Section 3.4.7 in the application). As the mine cavities fill, the rate of <br />inflows will decrease as the hydraulic heads develop between recharge areas <br />and the mine water levels decrease. Thus, after the mine workings have <br />completely filled, the rate of discharge from mine workings would be <br />considerably less than the inflow rates observed prior to the closure of the <br />mines. This reduced inflow rate would in turn increase the residency time of <br />mine water within the mine cavity, and thus allow for increased chemical <br />reaction time. The end result would be the development of increased total <br />dissolved solids concentrations, and the potential of developing acid, <br />alkaline and toxic mine discharges. As discussed in Section XIX, of this <br />document, the applicant is required to properly seal mine openings to prevent <br />gravity discharges of mine waters. Thus, the degradation of surface waters by <br />mine drainages will be minimized. <br />Waters stagnating in the Eagle No. 5 and No. 4 Mines would have water <br />qualities more closely approximating the water quality of discharges from the <br />Empire No. 1 strip pit. Total dissolved solids (TOS) from this pit range <br />between 830 mg/i and T010 mg/1 and average 914 mg/1 (page III-66c). The pH is <br />slightly alkaline ranging between 8.02 and 8.30 and averaging 8.14. Dominant <br />ions are sodium and bicarbonate, with high levels of calcium, magnesium, and <br />sulfate. <br />Mine waters may contaminate overlying and underlying aquifers. This <br />contamination of ground water in adjacent aquifers requires that; 1) there be <br />a connection between the mine workings and the aquifer; and 2) there be a <br />greater hydrostatic pressure in the mine workings than in the aquifer to drive <br />the mine water into the aquifer. In the saucer-like basinal structure of the <br />Big Bottom and Round Bottom synclines, there is little likelihood of mine <br />waters percolating downward against a greater hydrostatic pressure and <br />contaminating underlying aquifers. Overlying aquifers, however, may be <br />impacted if the hydrostatic pressure in the mine workings exceeds the pressure <br />in the overlying aquifer, and if there is a sufficient hydrologic connection, <br />such as through faults, natural fractures and/or subsidence fractures. <br />The ground water quality of the Trout Creek sandstone will not be impacted by <br />mine waters from the Eagle No. 5 and No. 9 mine workings. This aquifer is <br />separated from the lowest seam to be mined, the "F" seam, by 320 to 360 feet <br />of strata. The interburden acts as an aquiclude between the mine workings and <br />the Trout Creek sandstone. This, in turn, produces artesian conditions within <br />the Trout Creek sandstone, with an upward gradient of flow with depth. The <br />hydraulic gradient would further limit the potential for deep percolation of <br />mine water to the Trout Creek sandstone. The Trout Creek sandstone will have <br />no mine facilities located over its outcrop or subcrop. Thus, there will be <br />no contamination of this aquifer by the mine operation. <br />