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the spoils in these areas is therefore near the land surface elevation. The head differential across the unmined <br />interval during and following mining is considerably higher than under premining conditions due to the <br />• dewatering of the mined zone and overlying overburden unit. This situation induces recharge at a <br />proportionately higher rate than under baseline conditions. The rate of recharge to the workings from the spoils <br />at any given area is governed by the distance between the two areas, the transmissivity of the unmined material <br />in this interval, the head differential across the interval, and the length over which these conditions prevail. The <br />total recharge rate is given by the standard steady state flow equation. <br />Q=!-(hl-h2 - <br />D <br />where: Q = recharge rate (ft /day) <br />T = Transmissivity of unmined interval (ft /day) <br />h, = potentiometric head in spoils (ft) <br />h = distance between head in workings (ft) <br />D = distance between spoil and workings (ft) <br />L = length perpendicular to flow over which above conditions prevail (ft) <br />The head differential between the spoils and the proposed underground workings is approximately 300 ft (h, — <br />h The distance between these two areas in the southeastern part of the permit area is about 1,000 feet (D), and <br />the length over which these conditions prevail is about 5,000 feet (L). Further to the west, the distance between <br />existing or proposed spoils and the proposed underground workings increases rapidly due to the location of <br />Pinnacle Peak. An average distance in this area is about 3,000 feet (D), and the length over which these <br />conditions prevail is about 15,000 feet (L). Using an average permeability for the intervening unmined material <br />of 0.1 ft/day, and an average thickness of 75 ft yields a transmissivity of 7.5 ft'/day (T). Substituting the values <br />for the two areas into the above equation yields a total recharge rate from the spoil areas of about 170,000 gpd <br />• (23,000 ft /day) or 190 acre -ft per year (see following calculation). An additional 200 gpm (320 acre -ft per <br />year) will come from the 6 -Right area. Since this area is higher than most of the mine workings, it is expected <br />to flow at that rate for most of the time the mine is refilling. Thus, the total average recharge rate after mining is <br />expected to be approximately 510 acre -ft per year. <br />Q = ((7.5 ft /day * 300 ft * 5,000 ft) /1,000 ft) + _ ((7.5 ft /day * 300 ft * 15,000 ft)/ 3,000 ft) <br />Q = 23,000 ft /day = 170,000 gpd = 190 acre - ft/year <br />The overburden unit in mined areas is estimated to consist of a rubblized zone about 100 (t,) feet thick and an <br />overlying highly fractured zone about 200 feet thick (t The area of the proposed mining operation is about 24 <br />square miles (13,100 acres) (A,). Outside of the mined area, the overburden unit will have reasonably similar <br />characteristics to baseline conditions. This area is approximately 6 square miles (3,840 acres) (A For this <br />relatively simple calculation, the following parameters are estimated: <br />Average drawdown in mined areas <br />= 1,000 feet (d,) <br />Average drawdown in unmined areas <br />= 500 feet (d <br />Porosity of rubblized zone <br />= 0.1 (p,) <br />Porosity of highly fractured zone <br />= 0.01 (p <br />Confined storage coefficient (all zones) <br />= 0.0001 (S) <br />The total volume of water required to restore potentiometric levels to pre- mining conditions is given by: <br />V = A, {t,p, + t2p2 + S (d, - (t, + t + A Sd <br />• where: V = total volume of water (acre -ft) <br />A, = area dewatered (acres) <br />A = area of potentiometric lowering but no dewatering (acres) <br />t, = thickness of rubblized zone(ft) <br />p, = porosity of rubblized zone <br />PR09 -08 2.05 -140 08/25/09 <br />