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The proposed Sage Creek/Wolf Creek mine plan indicates that the mining area will be <br />immediately down dip of the outcrop. This location will limit the up-gradient area that can <br />contribute to pit inflow. The mining panels will be generally excavated perpendicular to the <br />seam strike. The minimum volume of pit inflow contributed by the Sage Creek and Wolf Creek <br />ground water systems is predicted to occur in 2002 (see Table 17-31a1. This volume is based <br />on excavation of only the box cut and the first two panels. Successive pit advance is planned <br />perpendicular to seam strike and will average 300 feet per year. Maximum pit inflows should <br />occur in 2010. The estimated pit inflow volume ranges from 84,637 gal/yr (232 gal/day) in <br />2002 to 502,264 gal/yr (1,376 gallday) in 2010. <br />Drawdown from Pit Pumpage and Extent of Water Level Declines <br />The following analyses will show aworst-case drawdown scenario for each aquifer by artificially <br />determining Pumpage rates so that highwall drawdowns will not exceed saturated thicknesses of <br />the water-bearing units before the end of a given time period. For this exercise, the radius of <br />influence will include areas of one-foot and five-foot water-level declines. Equations, variable <br />definitions, and assumptions inherent in the Theis method, used to estimate the radius of <br />influence from the pits, are discussed in Section 17-1 of the existing permit. Results of the <br />Theis analysis are summarized in Table 17-32, Pit Pumpage Rates and Estimated Drawdown <br />Distances for Seneca II-W South Area Overburden and Coal Ground Water Systems. <br />Drawdown distances were calculated for durations of one year as well as for the life of the mine <br />(through 2011). For the Wadge ground water system, the largest estimated radial distance will <br />occur in the overburden aquifer, where aone-foot drawdown is predicted to occur approximately <br />151 feet from the mined area after one year of pit inflow and approximately 285 feet from the <br />area after four years of inflow. For the Sage Creek/Wolf Creek systems, the estimated <br />drawdown in the overburden is predicted to occur approximately 333 feet from the mined area <br />after one year of pit inflow and approximately 1,441 feet from the area in year 2011. Since the <br />overburden and coal aquifers crop out to the east, there will be minimal drawdown as a result of <br />pit inflows in this direction. The drawdown areas are presented on Exhibit 17-1A. <br />It is anticipated that the extent of piY-inflow-induced drawdown will be less than predicted by <br />the above exercise. The calculations are based on artificial, constant pumping rates that cause <br />maximum drawdown in order to present worst-case scenarios from mining impacts. The Theis <br />equation does not account for aquifer recharge, which will reduce drawdown. In addition, the <br />overburden aquifer is unconfined. The Theis method, which is designed for confined aquifers, <br />will predict larger drawdowns when applied to an unconfined aquifer. <br />93 Revised 1/02 <br />