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Memorandum to Greg Roush• • <br />Mazch 23, 2001 <br />Page 2 <br />We defined these variables as follows: <br />s =pit drawdown (R) <br />Q = pit dewatering rate (gpm) <br />T = aquifer transmissivity (gpd/ft) <br />t =time (days) <br />r =distance from the pit center (R) <br />S =aquifer specific yield <br />The equation can be used to conservatively estimate aquifer drawdowns at distances (r) <br />away from the pit for specific times (t) using representative aquifer properties (T and S). <br />AQUIFER PROPERTIES <br />The S&H mine pit site is situated immediately east of the confluence of the South Platte River <br />and St Vrain Creek. The aquifer consists of high permeability sand and gravel deposits. The <br />aquifer is approximately 5 miles wide in the azea and the proposed mine is approximately'/< of a <br />mile wide (East-West) and located in the western half of the aquifer. Several canals divert water <br />from the South Platte and provide irrigation water to farms within the floodplain. Mostly <br />agricultural wells penetrate the aquifer but some domestic, stock, and commercial wells are <br />present. <br />The USGS (Harr, 1972) chazacterized the aquifer beneath the site (Open File Report 73-124). <br />The report contains maps of well locations, bedrock elevations, water table elevation, and aquifer <br />transmissivity and saturated thickness. Aquifer transmissivity in the azeas of the proposed mine <br />range from 50,000 to 200,000 gpd/ft and the saturated thickness ranges from 20 to 65 feet. <br />Based on our pumping test and computer modeling experience in the azea, we believe the aquifer <br />specific yield is azound 20 percent. <br />CONCEPTUALIZATION <br />We used the Modified Theis Equation to predict drawdowns caused by a theoretical well located <br />at pit center for 365 days of continuous pumping. Figure 1 shows a conceptualized cross-section <br />of the pit with a hypothetical dewatering well at its center. The well is assumed to begin <br />pumping at the commencement of mining and continues for one yeaz. The tazget drawdown at <br />the edge of the pit (r = 118 feet) is 13 feet. This allows a 2-foot thick seepage face for water to <br />enter the pit. The dewatering rate (Q) was solved for when drawdown at the edge of the pit (x- <br />118 feet) was 13 feet. <br />LEONARD RICE CONSULTING WATER ENGINEERS, INC. <br />2OOO CLAY STN EET. SUITE 300. DENVEw, COLOR woo 8021 7-S 1 19 • PRONE (303) 455-9589 • FAx (303) 455-01 15 <br />