2008-12-08_PERMIT FILE - C1982056A (4)

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Substituting the assumed values in this equation yields a value for V of 186,000 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 360 years (186,000 acre- •ft/5IOacre-ft/yr) for the overburden unit to recover to pre-mining levels. V=(15360 acre'((IOOft'0.1)+(200ft'0.01)+(0.0001 •(1000ft- (! 00 ft + 200 ft))))) + 3840 acre " 0.0001 •500 ft) V = 186,000 acre-ft Discharge from the unit will start to occur once gradients toward discharge areas are reestablished. This may occur before pre-mining potentiometric levels are achieved. Discharge from the unit will lengthen the time required to restore pre-mining potentiometric levels. Drawdown in the Trout Creek Sandstone will occur if communication is established with mine workings through fault zones. As discussed above this is not expected to occur. In fourteen years of mining, no 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 I5, Alluvial Valley Floors and Water Rights. Tn 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 (page 2.04-28). TCC has located all active adjudicated Trout Creek Sandstone wells within the affected area. TCC will detennine if monitoring water levels in these wells is necessary, as TCC is the owner of the TCSs wells shown on Map l5. Diminution of Bedrock Discharee to Alluvium and Surface Streams .Ground water inflotivs from bedrock units to the underground mine all cause drawdorr~n 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 pre-mining 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 pre- mising 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 pre-mining conditions. Discharge from the unit may start to occur prior to total reestablishment 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 6600 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 front 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. TR99-32 2.05-141 '4~~~®v~~ FEB u d 2000 1/2800