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location of Pinnacle Peak. An average distance in this area is about 3,000 feet (D), and the length over which
<br />these conditions prevail is about 15,000 feet (L). Using an average permeability for the intervening unmined
<br />material of 0.1 ft/day, and an average thickness of 75 ft yields a transmissivity of 7.5 ft2/day (T). Substituting
<br />the values for the two areas into the above equation yields a total recharge rate from the spoil areas of about
<br />170,000 gpd (23,000 ft3/day) or 190 acre -ft per year (see following calculation). An additional 200 gpm (320
<br />acre -ft per year) will come from the 6 -Right area. Since this area is higher than most of the mine workings, it is
<br />expected to flow at that rate for most of the time the mine is refilling. Thus, the total average recharge rate after
<br />mining is expected to be approximately 510 acre -ft per year.
<br />Q = ((7.5 ft2/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 ft3/day = 170,000 gpd = 190 acre-ft/year
<br />The Wadge overburden unit in mined areas is estimated to consist of a rubblized zone about 100 (t,) feet thick
<br />and an overlying highly fractured zone about 200 feet thick (t2). The area of the Wadge Seam mining operations
<br />is about 24 square miles (13,100 acres) (A,). Outside of the mined area, the overburden unit will have
<br />reasonably similar characteristics to baseline conditions. This area is approximately 6 square miles (3,840
<br />acres) (A2). For this 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 (d2)
<br />Porosity of rubblized zone
<br />= 0.1 (p,)
<br />Porosity of highly fractured zone
<br />= 0.01 (p2)
<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 = Al {t,p, + t2p2 + S (d, - (t, + t2))} + A2 Sd2
<br />where: V =
<br />total volume of water (acre -ft)
<br />A, =
<br />area dewatered (acres)
<br />A, =
<br />area of potentiometric lowering but no dewatering (acres)
<br />t, =
<br />thickness of rubblized zone(ft)
<br />p, =
<br />porosity of rubblized zone
<br />t2 =
<br />thickness of highly fractured zone (ft)
<br />P2 =
<br />porosity of highly fractured zone
<br />S =
<br />confined storage coefficient (all zones)
<br />d, =
<br />average drawdown in dewatered areas (ft)
<br />d2 =
<br />average drawdown in non-dewatered areas (ft)
<br />Substituting the assumed values in this equation yields a value for V of 158,100 acre -ft (see following
<br />calculation). Using a recharge rate of about 510 acre -ft per year, it can be seen that it would take about 310
<br />years (158,100 acre-ft/510 acre-ft/yr) for the overburden unit to recover to premining levels.
<br />V = (13,100 acre * ((100 ft * 0.1) + (200 ft * 0.01) + (0.0001 * (1000 ft — (100 ft + 200 ft)))) + 3,840
<br />acre * 0.0001 * 500 ft)
<br />V = 15 8, 100 acre -ft
<br />Assuming that the enhanced permeability of the rubblized zone and highly fractured zone above the Wolf Creek
<br />Seam results in a hydraulic connection between the Wadge and Wolf Creek seams, the additional time required
<br />to fill the volume encompassing the Wolf Creek Seam and Wadge/Wolf Creek Interburden affected by mining
<br />can be calculated by the same method. The area encompassed by the mine (A,) is 4,688 acres, and the average
<br />thickness of the interburden is 167 feet. The rubblized zone (t,) would comprise the lower 100 ft of interburden,
<br />and highly fractured zone 42) would comprise the remaining 67 feet of interburden. Mining of the Wolf Creek
<br />Seam is not expected to significantly increase the area or degree of dewatering area above the Wadge Seam or
<br />the surrounding area experiencing drawdown. Consequently, A2, d, and d2 are zero, and the equation becomes:
<br />PR14-10 2.05-141 12/18/14
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