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• As stated above, mine drainage to the Trapper pits has not been as large as expected. In some <br />instances, inflow to pits has'however been large enough to utilize continuous mine dewatering <br />installations. Continuous dewatering wells were installed in Derringer and Flintlock pits with the intent of <br />gradually pumping pit water and consequently reducing the erosion potential 'in surface drainage <br />downstream. Dewatering wells in both Derringer and Flintlock pits were deactivated in 1998. Several <br />small dewatering wells have been installed and operated in areas around the open cuts in Gaging and <br />East Flintlock pits. Yields from these wells have.been minimal (typically 2 - 8 gpm) and typically the <br />wells have been operated in advance of pit development. <br />Continuous dewatering wells may be installed in East Ashmore, Flintlock, Gatling and East Flintlock pits <br />if necessary during permit term 2008-2012. Historically, the northern end of the Ashmore Pit has <br />produced the most water, and during peak water production periods was normally pumped 16-20 hours <br />after it had been open for approximately one week. A large pump is used to remove the water and the <br />quantity of water pumped indicated the average yield from the aquifer was in the range of 70 gpm. Yield <br />from the East Ashmore pit aquifers has diminished considerably. Since 1997, average yields as <br />calculated from pit pumping records during 2005 and 2006 averaged 14 GPM. <br />Derringer Pit remained dry in the southern portion but normally yielded water at the northern end. <br />Pumping from Derringer pit averaged around 30 gpm and fell to 0 as mining operations were completed <br />in this pit. In Derringer pit it is anticipated the pumping requirements will be minimal during the current <br />permit term. <br />Drawdown in the coal aquifers will be large adjacent to the open pits. The gradient of the piezometric <br />surface should be steep in the coal aquifers, because they contain low transmissivities. A large <br />percentage of the water yield to a pit is from the non-confining portion of the aquifer. The aquifer will be <br />in unconfined conditions near the pit and therefore, mainly the unconfined properties govern drainage to <br />the pit and drawdowns from that drainage. Drawdowns near the Ashmore Pit are predicted to be a few <br />• tens of feet at 1,000 feet from the pit wall after a year or more. Drawdowns of perhaps a few feet are <br />predicted at one mile beyond the pit walls approximately four years after mining. The following <br />calculations illustrate these predictions. <br />Drawdown calculation near Ashmore Pit <br />For: Drawdown estimate at one year and 1,000 feet from pit. <br />Transmissivity (T) = 250 gal/day/ft <br />Specific yield (S) = 0.01 <br />Distance from pit's edge (r) = 1,000 ft <br />Time since start of pumping = 365 days <br />CJ <br />4-225