2013-10-17_PERMIT FILE - C1982056A (4)

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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 . commun ic ation i. s established with mine workings tliro4 auf oznes-� previously d scuss- e &, tfiis is not ei- ected _ to occurTo date, no medsura5 a 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 , Alruvial_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 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 _ �r� s n _not expected The Trout eek ndstone s not expected ect d to be substantially antia no diminution of Twen mile Sandstone is natural ischa a is antici p tantially 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 minim operation will cause a diminution of bedrock discharge from the Wadge overburden unit of about 11,OO.Q gpd, equivalent to 0.0 c s, or a east 3_60 years. This is equivalent to about 0.5 percent of the average low flow in Fish creek wTiich is the major creek receiving be roc isc ar diminution of tTiisma�nitu a is not significant to flow conditions in FT—Cre- ek: - � PR09 -08 2.05 -141 04/27/09