1993-06-08_GENERAL DOCUMENTS - C1981041

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The two other refuse piles , CRDA Nos. 1 and 2 , located across the Colorado River from the facility area, have more hydrologic control due to geographic and structural features . All leachate , should it occur, is routed to sediment ponds . CRDA No. 1 , in addition, has a series of rock underdrains that would channel the leachate to the sediment pond directly. Since both CRDA No. 1 and No. 2 are not underlain by alluvium, the area provides a more stable environment for leachate detection and minimization. The proposed CRDA No. 3, like its counterpart, CRDA No. 2, will also be constructed without underdrains due to the surface geology being much drier and lower in permeability than CRDA No. 1 . Peripheral drainage control systems will divert upland runoff from contacting and affecting the surface of the pile. CRDA No. 3 will not be constructed during this permit term. Ground Water Quantity Possible hydrologic impacts on ground water quantity by the underground mining operation include: 1 . The effect of subsidence and the related dewatering of the area, and 2 . The effect of additional water added to the Colorado alluvial system from mine inflow being discharged through the mine and to the surface. Ground Water Quantity: Effect due to Subsidence Ground water quantity will initially be effected due to subsidence activity in the Rapid Creek basin. The effect will be temporary in nature and provide no material damage to downstream users . To quote from the Brooks study of 1986 on the operation' s potential impact to the area, "Mining coal in the Mesaverde Group temporarily will dewater the rock adjacent to the underground mine. This rock will become rubblized and increase the permeability of the rock thus increasing the flow of ground water into the area. " Inflow in the mine is expected to increase as a result with a secondary effect of lowering the potentiometric surface temporarily. Water levels should return to pre-mining levels after the underground mine is sealed, with saturation occurring after several years . The mine is utilizing limited extraction methods in the Cottonwood and Rapid Creek areas . Past monitoring of the alluvium of Rapid and Cottonwood Creeks has not detected any significant depletion of alluvial water in either creek system. With continued limited coal extraction and water monitoring, no depletion of alluvial water is anticipated during the next permit term. There is no beneficial use of the ground water in the mine permit area or vicinity. Mine inflow quantity is documented in the Annual Hydrology Report. It has been demonstrated that most of the mine inflow waters originate in the new faces of virgin coal seams . As 28