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I • Section 780.21(c) Continued. Revised 11/20/80 <br />Recharge Capacity <br />The baseline rate of recharge of the coal aquifer in the mine plan <br />area was estimated in Section 779.15 as 4.75 acre-feet per year. <br />As explained in Section 816.51, the proposed method of mining will <br />result in greater postmining permeability and recharge capacity in <br />the backfilled areas. However, since the actual rate of recharge <br />depends on the water available to the aquifer from infiltration, <br />then the postmining infiltration rates and the size of the recharge <br />area are Lhe controlling factors in projecting the postmining rate <br />of ground water recharge. <br />Based on rainfall simulation studies by Lusby and Toy (1976), and <br />Gilley, et. al. (1977) at several mine sites in the Powder River <br />and Great Plains coal ffelds, the infiltration rates of undisturbed <br />and newly reclaimed saturated soils were compared. Infiltration <br />rates for wet soils were found to be consistently 25 to 46 percent <br />lower on reclaimed sites than on undisturbed sites. These results <br />were obtained from high-intensity rainfall rates of over 2 inches <br />per hour for 45 to 60 minutes. Based on extrapolated data from <br />these tests, 18 percent of high-intensity rainfall may be expected <br />to infiltrate through saturated reclaimed topsoil. <br />The recharge area of the undisturbed coal aquifer in the mine plan <br />area is a narrow subcrop of the Sudduth coal seam which is about 50 <br />feet wide where the aquifer is exposed beneath the topsoil. In the <br />undisturbed condition, confining layers of shale overlying the coal <br />aquifer at depth prevent recharge by downward leakance. However, <br />mining will result in an expansion of the postmining recharge area <br />so as to include the entire backfilled area, because the confining <br />layers will be removed and replaced with unconsolidated backfill. <br />Another factor to consider, but difficult to quantify, is that the <br />permeability of the backfill will exceed that of the undisturbed <br />agaffer. Both of These factors balance against the expected <br />780-115 as <br />