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Memorandum to Greg Roush• • <br />March 23, 2001 <br />Page 5 <br />MITIGATING FACTORS <br />As shown on Tables 1-3, we expect the actual drawdown to be aoound half that calculated using <br />the Modified Theis Equation. This subjective determination is based on the way we applied the <br />equation, and the mitigating effects of aquifer rechazge and subflow. <br />Conservative Assumptions <br />We believe the Modified Theis Equation applied in the manner discussed overestimates <br />drawdown for the following reasons: <br />• The actual dewatering time would be considerably less (approximately 3 months) due to the <br />fact that dewatering would not actually begin until after the upper 10 feet of unsaturated <br />material was mined away; We therefore expect the actual dewatering time to be less than <br />one year. <br />• The actual ma)cimum drawdown in the pit would equal the pit depth (which would <br />progressively deepen) instead of a water level far beneath the pit bottom in the theoretical <br />well pumping at a constant rate. We therefore expect actual dewatering rates to be below <br />that shown in Tables 1 - 3. <br />Aquifer Recharge and Subtlow <br />The Modified Theis Equation assumes a flat aquifer with no recharge sources. In reality, a <br />drawdown cone will extend only as far as it takes to intercept recharge that equals the pumping <br />rate. At that point in time, the drawdown cone will stabilize. Rechazge sources that are expected <br />to maintain water levels include precipitation, agricultural recharge, and surface water recharge <br />including rivers and canals. <br />The S&H Mine site is surrounded by river and canal rechazge sources. We made preliminary <br />maximum rechazge estimates for these sources to compare against the estimated pit dewatering <br />rates. <br />Stream RechazQe <br />St. Vrain Creek and the South Platte River aze within 1000 feet from the proposed pit site. The <br />amount of stream recharge from these sources is a function of the gradient between the river and <br />the aquifer, the permeability of the river bottom, and the aquifer transmissivity. We estimated <br />the maximum amount of stream recharge from only the South Platte River based on the <br />following assumptions: river length within 1000 feet of the entire proposed pit azea is <br />approximately 4 miles; average river width approximately 20 feet, average aquifer transmissivity <br />(117,000 gpd/ft); average aquifer thickness (37 feet); a vertical gradient between the stream and <br />aquifer; and a vertical permeability 100 times less than the horizontal (Kh/Kv = 100). Under <br />these assumptions, the maximum amount of potential stream recharge is around 8,700 gpm. This <br />LEONARD RICE CONSULTING WATER ENGINEERS, INC. <br />2000 CLAY STREET, SUITC 300 DENVER, COLORADO 802 1 1-5 1 1 9 • PNONe (303) 455-9589 • Fwx (303) 455-01 15 <br />