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t 2 <br />= thickness of highly fractured zone (ft) <br />P2 <br />= porosity of highly fractured zone <br />• S <br />= confined storage coefficient (all zones) <br />d <br />= average drawdown in dewatered areas (ft) <br />d 2 <br />= average drawdown in non - dewatered areas (ft) <br />Substituting the assumed values in this equation yields a value for V of 158,100 acre -ft (see following <br />calculation). Using a recharge rate of about 510 acre -ft per year, it can be seen that it would take about 310 <br />years (158,100 acre- ft/510 acre- ft/yr) for the overburden unit to recover to premining levels. <br />V = (13,100 acre * ((100 ft * 0.1) + (200 ft * 0.01) + (0.0001 * (1000 ft — (100 ft + 200 ft)))) + 3,840 <br />acre * 0.0001 * 500 ft) <br />V = 158,100 acre -ft <br />Discharge from the unit will start to occur once gradients toward discharge areas are re- established. This may <br />occur before premining potentiometric levels are achieved. Discharge from the unit will lengthen the time <br />required to restore premining potentiometric levels. <br />Drawdown in the Trout Creek Sandstone will occur if communication is established with mine workings <br />through fault zones. As previously discussed, this is not expected to occur. To date, no measurable impact to <br />the Trout Creek Sandstone has been observed. Impacts to existing wells in the Trout Creek Sandstone should be <br />minimal since these wells are all located at the margins of the Twentymile Park Basin, as shown on Map 15, <br />Alluvial Valley Floors and Water Rights. In the unlikely event that drawdown effects do occur, they are not <br />likely to extend to these areas, given the average recharge rate to the unit calculated at approximately 70 gpm <br />and the storage within the unit calculated at about 360,000 acre -ft (refer to Section 2.04.7). TCC has located all <br />active adjudicated Trout Creek Sandstone wells within the affected area. TCC will determine if monitoring <br />• water levels in these wells is necessary, as TCC is the owner of the TCSs wells shown on Map 15. <br />Diminution of Bedrock Discharge to Alluvium and Surface Streams <br />Ground water inflows from bedrock units to the underground mine will cause drawdown of potentiometric <br />levels in these units and changes in the direction of ground water flow. This may result in a decrease of natural <br />ground water discharge to alluvium and surface streams. This effect may persist long after mining ceases since <br />it may take an appreciable amount of time for potentiometric levels to recover to premining conditions. <br />The Wadge overburden unit is the most likely to be affected by the proposed mining operation. The <br />Twentymile Sandstone is not expected to yield significant inflows to the mine and therefore, no diminution of <br />natural discharge is anticipated. The Trout Creek Sandstone is not expected to be substantially affected by the <br />mining. <br />The Wadge overburden unit has been calculated to discharge approximately 11,000 gpd (12.3 acre - ft/yr.) under <br />premining conditions. The dewatering and potentiometric lowering that will occur in this unit during mining <br />will probably result in most of this discharge being eliminated. During mining, this will be more than offset by <br />direct discharge of mine inflow water to Fish and Foidel Creeks. Much of this water is derived from storage in <br />the unit and considerable extra storage will be created by the mining operation. Consequently, once mining <br />ceases, it will take many years for potentiometric levels to recover. It has been calculated in the previous <br />section that it may take about 360 years before potentiometric levels are re- established to premining conditions. <br />Discharge from the unit may start to occur prior to total re- establishment of potentiometric levels, but this will <br />be minimal until the old mine workings are flooded to at least the elevation of discharge area at around 6,600 <br />• feet. At least 90 percent of the mine workings are below this elevation. Consequently, it is assumed for "worst <br />case" predictions, that the mining operation will cause a diminution of bedrock discharge from the Wadge <br />overburden unit of about 11,000 gpd, equivalent to 0.02 cfs, for at least 360 years. This is equivalent to about <br />0.5 percent of the average low flow in Fish Creek which is the major creek receiving bedrock discharge. A <br />diminution of this magnitude is not significant to flow conditions in Fish Creek. <br />PR09 -08 2.05 -141 08/25/09 <br />