2009-10-30_PERMIT FILE - C1982056A (5)

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t = thickness of highly fractured zone (ft) P2 = porosity of highly fractured zone • S = confined storage coefficient (all zones) d = average drawdown in dewatered areas (ft) d = average drawdown in non - dewatered areas (ft) Substituting the assumed values in this equation yields a value for V of 158,100 acre -ft (see following calculation). Using a recharge rate of about 510 acre -ft per year, it can be seen that it would take about 310 years (158,100 acre -ft/510 acre -ft/yr) for the overburden unit to recover to premining levels. V = (13,100 acre * ((100 ft * 0.1) + (200 ft * 0.01) + (0.0001 * (1000 ft — (100 ft + 200 ft)))) + 3,840 acre * 0.0001 * 500 ft) V = 158,100 acre -ft Discharge from the unit will start to occur once gradients toward discharge areas are re- established. This may occur before premining potentiometric levels are achieved. Discharge from the unit will lengthen the time required to restore premining potentiometric levels. Drawdown in the Trout Creek Sandstone will occur if communication is established with mine workings through fault zones. As previously discussed, this is not expected to occur. To date, no measurable impact to the Trout Creek Sandstone has been observed. Impacts to existing wells in the Trout Creek Sandstone should be minimal since these wells are all located at the margins of the Twentymile Park Basin, as shown on Map 15, Alluvial Valley Floors and Water Rights. In the unlikely event that drawdown effects do occur, they are not likely to extend to these areas, given the average recharge rate to the unit calculated at approximately 70 gpm and the storage within the unit calculated at about 360,000 acre -ft (refer to Section 2.04.7). TCC has located all active adjudicated Trout Creek Sandstone wells within the affected area. TCC will determine if monitoring III water levels in these wells is necessary, as TCC is the owner of the TCSs wells shown on Map 15. Diminution of Bedrock Discharge to Alluvium and Surface Streams Ground water inflows from bedrock units to the underground mine will cause drawdown of potentiometric levels in these units and changes in the direction of ground water flow. This may result in a decrease of natural ground water discharge to alluvium and surface streams. This effect may persist long after mining ceases since it may take an appreciable amount of time for potentiometric levels to recover to premining conditions. The Wadge overburden unit is the most likely to be affected by the proposed mining operation. The Twentymile Sandstone is not expected to yield significant inflows to the mine and therefore, no diminution of natural discharge is anticipated. The Trout Creek Sandstone is not expected to be substantially affected by the mining. The Wadge overburden unit has been calculated to discharge approximately 11,000 gpd (12.3 acre - ft/yr.) under premining conditions. The dewatering and potentiometric lowering that will occur in this unit during mining will probably result in most of this discharge being eliminated. During mining, this will be more than offset by direct discharge of mine inflow water to Fish and Foidel Creeks. Much of this water is derived from storage in the unit and considerable extra storage will be created by the mining operation. Consequently, once mining ceases, it will take many years for potentiometric levels to recover. It has been calculated in the previous section that it may take about 360 years before potentiometric levels are re- established to premining conditions. Discharge from the unit may start to occur prior to total re- establishment of potentiometric levels, but this will be minimal until the old mine workings are flooded to at least the elevation of discharge area at around 6,600 • feet. At least 90 percent of the mine workings are below this elevation. Consequently, it is assumed for "worst case" predictions, that the mining operation will cause a diminution of bedrock discharge from the Wadge overburden unit of about 11,000 gpd, equivalent to 0.02 cfs, for at least 360 years. This is equivalent to about 0.5 percent of the average low flow in Fish Creek which is the major creek receiving bedrock discharge. A diminution of this magnitude is not significant to flow conditions in Fish Creek. PRO9 -08 2.05 -141 08/25/09