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auvi u~ ni vnvtii Z ZZ 3~ ~ -r~•JO mi f Z~Z~7 •73!V fCD6P+ ~a L/ O <br />MEMQRANDUM <br />Date: February 21, 1993 <br />To: Darrel Buffington - SHB Denver <br />Fram: Tod Monks - SHH Albuquerque <br />Subject: Revision to Original Dowe Flats Geohydrology Evaluation <br />This memorandum addresses data requirements for the groundwater <br />augmentation plan at Dowe Flats. In particular, it addresses the <br />determination of the following items included in your memorandum to <br />me, dated September 2, 1993. <br />1. Determine pumping rates required for mine development <br />that will cause a drawdown to a level of the pit bottom <br />at years 5, 10, 15, 20, and 25. <br />2. Determine the time required for dewatering to <br />significahtly affect groundwater discharge to the St. <br />Vrain River. <br />3. Determine the steady state water table surface for the <br />completed pit. <br />APPROACH <br />To provide answers to the above items, I used a combination of <br />analytical methods and preliminary simplified numerical analysis. <br />Although these methods do not provide a great deal of precision due <br />to the lack of existing data, if assumptions are relatively <br />accurate they can provide reasonable estimates. <br />ANTICIPATED PUMPING RRTES REQUIRED FOR PHASED MINE DEVELOPMENT <br />using the provided post mining bedrock surface map and included <br />phased mining approach, I calculated a range of anticipated pumping <br />rates at the end of 5, 10, 15, 20, and 25 years. A Darcian <br />approach was used in the calculations and the following assumptions <br />were made. <br />• All pits were excavated to a depth of 125 feet. The <br />lower 100 feet of material was assumed to be saturated. <br />• The hydraulic conductivity of bedrock in the vicinity of <br />