2007-11-16_GENERAL DOCUMENTS - C1981041 (3)

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The fourth water bearing zone is the Rollins sandstone, which is stratigraphically below the coal seam to be mined. This unit is comprised of a clean, tan-to-cream sandstone that ranges from 89 to 114 feet thick throughout the permit area. Due to the extreme depth (from 50 to 1900 feet below surface) and location of the mining operation, it is believed that no damaging effect to the hydrologic properties of the Rollins will occur. To date, there has been no significant impact on this unit. Additional issues that affect the hydrology of the site include waste pile impact on groundwater supplies, in particular leachate generation. The effect of leachate from the Roadside coal processing waste pile (RSRDA) on water quality is negligible. Using the worst-case scenario, where all precipitation enters the waste pile, an average of 0.01 cfs would enter the Colorado River via alluvium percolation. This would have the effect of raising the total dissolved solids content of the Colorado River by 0.01 percent. There is no evidence that leachate from the pile has ever reached or affected the Colorado River. 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 neither CRDA-1 nor CRDA-2 are underlain by alluvium, the area provides a more stable environment for leachate detection and minimization. During the period of active use, minor intermittent seepage was observed from the rock underdrains beneath CRDA-1, and from widely scattered seep areas on the CRDA-1 and CRDA-2 bench outslopes. Groundwater Quantity Possible hydrologic impacts on groundwater 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. Groundwater Quantity: Effect due to Subsidence Groundwater quantity will initially be affected 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 groundwater 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 utilized 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, and no significant depletion is Permit Renewal No. 5 33 November 16, 2007