2018-05-18_PERMIT FILE - C2009087A (3)

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RULE 2 - PERMITS In addition, the potential for drawdown to affect the gaining reach of Grassy Creek (see Robson and Stewart, 1990, Table 3, Grassy Creek Stations shown on p. 38), where the creek crosses the outcrops of the Trout Creek Sandstone and Williams Fork Formation at the southwestern perimeter of the PSCM permit area, and springs in that area, is minimal. As explained in more detail in the paragraph below, the Twentymile and Trout Creek aquifers are hydrologically isolated from the Wadge coal and so will not be affected by mining- related drawdown. Consequently, there would be no change in streamflow gains or losses where the stream crosses the sandstone outcrops and no potential for springs related to either sandstone to be affected. The subcrop of the Wadge coal beneath the gaining reach of Grassy Creek is outside of the predicted 5 -foot drawdown contour and so will not be affected by mine - related drawdown during the permit period. In the scenarios where drawdown was calculated using the expected and high values of transmissivity and hydraulic conductivity, the 5 -foot drawdown contour could reach the area where the Wadge coal subcrops beneath the alluvium after 12 to 19 years. In the scenario where drawdown was calculated using the low values, the 5 -foot drawdown contour would not reach that area. The springs in the vicinity of the southwestern perimeter of the PSCM permit area are generally not associated with bedrock aquifers and are most commonly associated with perched groundwater, where discontinuous units of higher permeability are underlain by beds with lower permeability. Springs are generally not affected by underground mining except where they lie within potential subsidence zones. The springs in the area near the southwestern perimeter of the permit area are not within potential subsidence zones. In addition, the springs lie on the margins of the Twentymile Park structural basin, and recharge areas are upgradient of the springs and the proposed mining activities, so it is unlikely that recharge to the springs will be impacted. • The vertical extent of potentiometric head changes in the bedrock units will be limited by the low permeability and thickness of shale beds above and below the Wadge coal. Previous studies, as well as monitoring at the nearby Foidel Creek Mine, indicate that the major regional sandstone aquifers (the Twentymile and Trout Creek aquifers), which occur, respectively, approximately 600 feet above and 235 feet below the Wadge coal, are hydrologically isolated from the Wadge coal and the small section of the overburden unit to be disturbed by mining at the PSCM. They are therefore unlikely to be affected by the proposed mining. The isolation is created by the very small hydraulic conductivities of the intervening shale sequences above and below the Wadge coal and its overburden and underburden. The interval separating the Wadge coal from the overlying Twentymile aquifer includes approximately 540 feet of marine shale. Shale samples from this area tested for vertical permeability, as reported by Williams and Clark (1994), gave values less than 10 ft/d. The interval separating the Wadge coal from underlying Trout Creek aquifer includes shale beds and a hydrologically - significant, regionally- extensive bed of altered volcanic ash (kaolinitic claystone, or "tonstein ") referred to as the Yampa Bed by Brownfield and Johnson (2008). Where tested in other areas (Water Management Consultants (WMC), 2005), the Yampa Bed exhibited a vertical hydraulic conductivity of 2.8x 10 -7 ft/d, about the same as or less than that of the shales reported by Williams and Clark (1994). The spoils aquifer in the Seneca II spoils adjacent to the proposed underground mining activities is upgradient of and in hydrologic connection with the units to be disturbed by the mining. However, as described below and, in greater detail, by Williams and Clark (1994), it is unlikely to be detestably impacted by the proposed PSCM activities. The spoils aquifer receives about ten times the recharge as adjacent undisturbed bedrock areas (approximately 3 inches per year, versus 0.31 • inch per year in other areas, according to Williams and Clark, 1994). The spoils aquifer also has a hydraulic conductivity hundreds of times greater than that of the undisturbed bedrock. Because of these factors, the spoils aquifer is saturated to the elevation of the down -dip low wall of the spoils - filled reclaimed mine pit, and "overflow" exits the spoils aquifer as springs at the low points in the low wall adjacent to Little Grassy Creek. PSCM Permit App. 2.05 -77.3 Revision 03/05/10