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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 continue to be operated in areas around the backfilled <br />Gatling pit and in the N pit area. Yields from these wells have been minimal (typically 2 - 4 gpm) and <br />typically the wells have been operated in advance of pit development. <br /> <br />Historically, the northern end of the Ashmore Pit produced the most water, and during peak water <br />production periods was normally pumped 16-20 hours after it had been open for approximately one week. <br />A large pump was used to remove the water and the quantity of water pumped indicated the average yield <br />from the aquifer was in the range of 70 gpm. Yield from the East Ashmore pit aquifers was considerably <br />less. <br /> <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 in <br />this pit. <br /> <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 in <br />unconfined conditions near the pit and therefore, mainly the unconfined properties govern drainage to the <br />pit and drawdowns from that drainage. Drawdowns near the Ashmore Pit are predicted to be a few tens of <br />feet at 1,000 feet from the pit wall after a year or more. Drawdowns of perhaps a few feet are predicted at <br />one mile beyond the pit walls approximately four years after mining. The following calculations illustrate <br />these predictions. <br /> <br />Drawdown calculation near Ashmore Pit <br /> <br />For: Drawdown estimate at one year and 1,000 feet from pit. <br /> <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 /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />4-225