Laserfiche WebLink
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 and <br />additional dewatering wells installed downgradient of pit areas have not been utilized since. Several small <br />dewatering wells have been installed and operated in areas upgradient of the open cuts in Gattling and <br />East Flintlock pits in an effort to relieve pore pressures in these areas and maximize highwall stabilities. <br />Yields from these wells have been minimal (typically 2 - 5 gpm) and the wells have been operated far only <br />brief periods immediately in advance of pit development. <br />Continuous dewatering wells may be installed in East Ashmore, Flintlock, Gattling and East Flintlock pits if <br />necessary during permit term 2003-2007. Historically, the northern end of the Ashmore Pit has produced <br />the most water, and during peak water production periods was normally pumped 16-20 hours after it had <br />been open for approximately one week. A large pump is used to remove the water and the quantity of <br />water pumped indicated the average yield from the aquifer was in the range of 70 gpm. Over the past five <br />years, from 1997 through 2001, yield from the East Ashmore pit aquifers has diminished considerably. <br />Average yields as calculated from pit pumping records during this period are in the range of 10 gpm. <br />Derringer Pit remained dry in the southern portion but normally yielded water at the northern end. <br />Pumpage from Derringer pit over the previous five years averaged around 30 gpm and fell to 0 as mining <br />operations were completed in this pit. Derringer and Enfield pits have merged and it is anticipated the <br />pumping requirements in this location will either cease entirely or be minimal during the current permit <br />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 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. The rate of mine movement reduces the quantity <br />of time an area will be impacted by the mine drainage. 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 />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 />4-225 t=.: _ r'-. ~ ~; <br /> <br />P!L-S <br />~~ <br />