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h2 = distance between head in workings (ft) <br />D = distance betty-een spoil and workings (ft) <br />• L = length perpendicular to flow over which above conditions prevail (ft) <br />The head differential behveen the spoils and the proposed underground workings is approximately 300 ft (hl -h2). <br />The distance between these two areas in the southeastent pan of the permit area is about 1000 feet (D) and the <br />length aver which these conditions prevail is about 5000 feet (L). Further [o 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 3000 feet (D) and the length over which these conditions <br />prevail is about 15,000 feet (L). Using an average pemteability for the intervening unntined material of 0.1 ft/day <br />and an average thickness of 75 fi yields a transmissivity of 7.5 ft2/day (T). Substituting the values for the two <br />areas into the above equation yields a total recharge rate from the spoil areas of about 170,000 gpd (23,000 <br />ft3/day) or 190 acre-ft per year (see following calculation). An additional 200 gpm (320 acre-ft per year) will <br />come from the 6 Right area. Since this area is higher than most of the mine workings it is expected to flow at [hat <br />rate for most of the time the mine is refilling. Thus, the total average recharge rate after mining is expected to be <br />510 acre-fr per year. <br />Q = ((7.5 ft2/day * 300 ft * 5000 ft)/] 000 fr) +~((7.5 fr2/day * 300 ft * 15000 ft)/ 3000 ft) <br />Q = 23,000 ft3/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 (tl) feet thick and an <br />overlying highly fractured zone about 200 feet thick (t2). The area of the proposed mining operation is about 24 <br />square miles (]5,360 acres) (A1). 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) (A2). For this <br />relatively simple calculation, the following parameters are estimated: <br />• Average drawdown in mined areas <br />Average drawdown in unmined areas <br />Porosity of rubblized zone <br />Porosity of highly fractured zone <br />Confined storage coefficient (all zones) <br />] 000 feet (d 1) <br />500 feet(d2) <br />0.1 (P1) <br />0.0] (P2) <br />0.0001 (S) <br />The total volume of water required to restore potentiometric levels to pre-mining conditions is given by: <br />where: <br />V = Al {tl P] +t2P2 + S (d] - (tl + t2))} + A2 Sd2 <br />V = total volume of water (acre-ft) <br />A I = area dewatered (acres) <br />A2 = area of potentiometric lowering but no dewatering (acres) <br />tl = thickness of rubblized zone(ft) <br />pl = porosity of rubblized zone <br />t2 = thickness of highly fractured zone (fr) <br />p2 = porosity of highly fractured zone <br />S = confined storage coefficient (all zones) <br />d 1 = average drawdown in dewatered azeas (ft) <br />d2 = average drawdown in non-dewatered areas (fr) <br />ubstituting the assumed values in this equation yields a value for V of 186,000 acre fi (see following calculation). <br />sing a recharge rate of about 510 acre-ft per year it can be seen that it would take about 360 years (186,000 acre- <br />ft/510 acre-fl/yr) for the overburden unit to recover to pre-mining levels. <br />TR 99-32 <br />2.05-140 ~p~DDVED FEB 0 8 2000 1/3/00 <br />