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RULE 2 - PERMITS <br />Groundwater flow from the bedrock into the mine workings was also estimated by analytical <br />• methods. The rate of inflow to the mine workings from the overburden and underburden can be <br />estimated using the following equation (Freeze and Cherry, 1979), <br />Q= K•A•(h, —h,)lL <br />where: Q = volumetric flow rate <br />K = vertical hydraulic conductivity <br />A = cross - sectional area of flow (area of mine floor) <br />h2 = head in underburden <br />h head in mine (elevation of mine floor) <br />L = length of flow path from the underburden to the mine <br />The inflow rates from the bedrock (overburden and underburden) were calculated for the 26 <br />months of mining and for the post - mining period after dewatering ceases and the mine openings fill <br />with groundwater. The vertical hydraulic conductivity of the underburden is estimated to be 10 <br />ft/d, from data presented by Williams and Clark (1994). The mine floor area will increase as mining <br />progresses and is estimated to be approximately 354 acres or 15,403,250 ft at the end of 26 months <br />of mining. The head difference, h -h depends on the difference between the underburden head and <br />the mine floor elevations and will increase as mining progresses, ranging from approximately 50 <br />feet initially to approximately 1,200 feet at the end of 26 months of mining. The length of the flow <br />path, L, was set at 50 feet. The calculated inflow from the overburden andunderburden ranges from <br />well below 1 gpm initially to approximately 22 gpm at the end of 26 months of mining. <br />• The results of the inflow calculations are illustrated graphically on Figures 2.05.6 42 and 2.05.6 - <br />F2.1.. The graphs show the expected high and low inflow rates calculated from the range of <br />hydraulic conductivities and transmissivities obtained within the PSCM area. The expected inflow <br />rates reach an initial peak at about 18 gpm (range = 11 to 22 gpm) after 3 months ,of mining, <br />decrease to 3 gpm (range = 2.2 to 3.4 gpm) after 4 months, then gradually increase to about 45 gpm <br />(range = 35 to 51 gpm) after 26 months. After dewatering activities cease, inflow will continue as <br />the mine voids fill with groundwater. During this period, the inflow rate is predicted to decrease <br />rapidly at first and then more gradually. Under the expected scenario, the calculated inflow rate is <br />26 gpm (range = 23 to 28 gpm) at the end of the permit period (60 months after mining started) and <br />4 gpm (range = 3.7 to 5.0 gpm) 30 years after the end of the permit period. <br />The inflow rate on a unit basis of gpm per acre of mined area is also shown on the graphs in <br />Figures 2.05.642 and 2.05.6 —F2.1 (values are on the right vertical axis). The unit inflow rate will <br />decline rapidly as the immediate vicinity of the mine is dewatered and the potentiometric surface <br />declines, from about 7 to 15 gpm per acre when mining first starts to a low of 0.05 to 0.08 gpm per <br />acre within four months. After the initial openings are constructed and the mining advances more <br />rapidly, the unit inflow rate will increase to 0.11 to 0.16 gpm per acre by 26 months, then gradually <br />decrease when active dewatering ceases, reaching 0.06 to 0.07 gpm per acre at the end of the permit <br />term and 0.01 gpm per acre after 420 months (30 years after the end of the permit period.. For <br />comparison, the average unit inflow rate at the nearby Foidel Creek Mine as observed over years of <br />mining activity is reported to be approximately 0.027 gpm per acre. <br />PSCM Permit App. 2.05 -73 Revision 03/05/10 <br />