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RULE 2 - PERMITS <br />area, and the storage coefficient of the area that experiences drawdown. The volume of water <br />• required for filling the void space from mining and the cone of depression in the overburden is: <br />V = mine void space + (overburden drawdown • area • storage coefficient) <br />The mine area during the first 26 months will encompass about 1075 acres. That area will include <br />approximately 272,400 feet of mine openings averaging 8 feet high by 20 feet wide. Thus, the mine <br />void space is about 43.6 million cubic feet, or 1,000 acre -feet. The unmined coal between the mine <br />openings covers about 596 acres at an average thickness of 9 feet, and is expected to be completely <br />dewatered. Thus, assuming an average coal porosity of 10 percent, the dewatered coal void volume <br />would be 536 acre -feet. Therefore, the total void space within the mine perimeter, including the <br />mine voids and the dewatered coal pore space, is about 1,536 acre -feet. The average drawdown in <br />the overburden within the mined area will be about 700 feet and within thecalculated 5 -foot <br />drqwdown contour (see Figures 2.05.6 -F4, 205.6 -F4.1 and 2.05.6 -F4.2) the average drawdown will <br />be about 230 to 260 feet. from the mine perimeter out to the 5 -foot drawdown contour covers from <br />about 2,000 to about 3,000 acres. Assuming a storage coefficient of 10 for the overburden, the <br />volume of water required for recovery of the potentiometric surface would bein the following range: <br />V = 1,536 ac -ft + (700 ft • 1596 ac • 10 + ( 230 ft • 2,000 ac • 10 " 4 ) <br />= 1,694 ac -ft <br />to <br />V = 1,536 ac -ft + (700 ft • 1,596 ac • 104)+ (260 ft • 3,000 ac • 10 -4 ) <br />= 1,726 ac -ft <br />The time to refill the void space and the cone of depression is the above volume divided by the <br />recharge rate for the affected area: <br />t = V / (recharge rate • area) <br />The calculated time would be in the following range: <br />to <br />= 1,694 ac -ft / [0.0258 ft/yr • (1,075 ac + 2,000 ac)] <br />= 21.4 years <br />t = 1,726 ac -ft / [0.0258 ft/yr • (1,075 ac + 3,000 ac)] <br />= 16.4 years <br />Calculations of residual drawdown and groundwater inflow to the mine workings after dewatering <br />ceases (see Figures 2.05.6 -F4, 2.05.6 -F4.1 and 2.05.6 -F4.2) suggest that more than 30 years may be <br />required for complete recovery in the Wadge coal. When hydraulic gradients through the mined <br />area (rather than toward the mined area from all sides) are re- established, groundwater flow through <br />affected units in the mined area toward the areas downgradient will resume. If this occurs before <br />the potentiometric surface has recovered to pre- mining levels, the recovery time would be <br />lengthened. <br />Changes in groundwater quality in the area around the mine will be very limited during mining, as <br />the mine will be dewatered continuously and all groundwater flow in the immediate vicinity of the <br />PSCM Permit App. 2.05 -79 Revision 03/05/10 <br />