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Section 780.21 (c) Continued. <br />Revised 11/20/80 <br />decrease in infiltration rates in estimating the postmining ground <br />water recharge capacity of the coal aquifer. Since the total area <br />to be mined amounts to about 540 acres, and assuming an annual <br />precipitation rate of 11 inches per year and that 18 percent of <br />this precipitation will infiltrate into the substrate, the post- <br />mining recharge rate should be 89.1 acre-feet per year. This <br />increased rate of recharge will enable rapid recovery of the origi- <br />nal storage volume of the undisturbed coal aquifer from the tempo- <br />rary dewatering due to mining. The backfill replaced in the mine <br />pits will serve to recharge the undisturbed coal aquifer down-dip <br />from the active mining area. To illustrate aquifer recharge, a <br />water budget set forth below has been prepared to evaluate the re- <br />covery of water levels in the backfill and the coal aquifer. <br />Inflow to the backfill is estimated as 89.1 ac-ft/yr. Storage <br />within the spoils aquifer is calculated as the sum of total pit <br />volumes times the porosity of the backfill. From Table 74, Pit <br />Volume Calculations, the total volume of the existing pit and Pits <br />111, 112, and 113 is 55.0 million yards. Due to swelling of the <br />overburden the void space of the backfill will probably be about <br />24%, making for 13.2 million yards or 8182 ac-ft. of available <br />storage. The water in the backfill will recharge the undisturbed <br />coal aquifer at a rate controlled by the permeability of the coal <br />aquifer. Average permeability is given in Section 779.15 as 3.8 <br />2 <br />gpd/ft The total postmining recharge area of the coal aquifer in <br />the mine plan area is 23,000 feet long and an average of 50 feet <br />thick, making for 1,150,000 ft2. Recharge to the coal 'aquifer <br />should therefore be about 4,370,000 gpd or 4,900 ac-feet/yr, once <br />the water level in the backfill rises sufficiently to provide <br />recharge water to the undisturbed coal aquifer. <br />In summary, with inflow to the backfill of 89.1 ac-ft/yr, potential <br />outflow to the coal aquifer of 4,900 ac-ft/yr, and storage capacity <br />780-116 as <br />