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3.0 MINING CONDITIONS <br />As described in the introduction, mining will dewater the area of excavation by <br />trenching and pumping. For the purpose of an analytical drawdown solution, the <br />gravel pit is considered as a pumping well with a large radius. For the purpose of <br />predicting the drawdown as a function of distance, the analytical solution to <br />confined well hydraulics developed by Theis is applied. Specifically, a <br />spreadsheet developed by the USGS to solve the Theis equation has been used. <br />The spreadsheet outputs for given distances from the pits are given in Appendix <br />B. The pump rate was determined by iteratively increasing the pump rate with <br />the spreadsheet program very near the pit until drawdown approached 3 -4 ft from <br />the pit floor. The height of 3 -4 ft from the pit floor was chosen due to observed <br />seepage face heights at similar gravel pits. The top of the seepage face is the <br />considered the lowest point of drawdown in the saturated media. The pump rate <br />of 650 gpm was selected which produced a drawdown of 16 ft out of 20 ft of <br />saturated thickness. This pump rate is consistent with pump rates observed <br />locally in the valley bottom at the Bokleman Walters Pit to the east and Flying W <br />pit to the west. <br />K= Hydraulic Conductivity of Gravel 700ft/day <br />b= Aquifer thickness = 20ft (based on potentiometric head) <br />S= Storage coefficient = .000512 <br />Pumping Rate =650 GPM <br />The water table drawdown is shown graphically on Sheet 2 Cross Sections, <br />mining condition. Maximum drawdown at gravel pit wall seepage face is <br />predicted to be 16ft. At 1305ft from the pit, drawdown is predicted to be 2.6ft. <br />The drawdown at the south property boundary is predicted to be approximately <br />4.25 ft. The drawdown at the west property boundary is predicted to be 4.8 ft and <br />the drawdown at the east property boundary is predicted to be 7.3 ft <br />The State Engineers office requires notification of any registered wells within 600 <br />ft of a pumping well. There are no registered wells within 600 ft of the Stroh Pit. <br />Water table drawdown due to pumping at the Stroh pit is not predicted to have <br />any effect on man made structures within the area of influence. <br />Stroh Pit <br />Groundwater Analysis Report <br />4of8 <br />