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HAHN WATER RESOURCES, LLC <br /> Dewatering System <br /> Dewatering of the Rifle Pit No. 1 will be accomplished with a perimeter trench running along the <br /> northern and western edges of the pit. The trench will be excavated approximately 4 feet into the <br /> underlying bedrock. Water accumulating in the trench will be pumped out of the trench and <br /> conveyed offset, ultimately discharging to the Colorado River. The dewatering system was <br /> represented using the "drain" package. The drain representation requires that an elevation be <br /> specified for the drain invert. The model then compares the elevation of the adjacent <br /> groundwater to the invert elevation. If the groundwater elevation exceeds the invert elevation, <br /> the model causes groundwater to move to the drain (where it is subsequently removed from the <br /> groundwater system). The dewatering system is modeled as being fully installed and fully <br /> operational at the start of the simulation. <br /> Model Stress Periods <br /> The model was set up as a transient, 3-year simulation, consisting of 99 stress periods of equal <br /> length of 10 days, and a final stress period of 105 days. <br /> RESULTS <br /> The model predicts a rapid response of groundwater levels to the dewatering. The following <br /> figure (Figure 1) shows the predicted groundwater level decline as a function of time <br /> (hydrograph) at a location near the western edge of the pit. The water level hydrograph indicates <br /> that most of the groundwater level response will occur within the first 60 days of the initiation of <br /> dewatering. The hydrograph also indicates that groundwater level response will reach a near- <br /> equilibrium condition within about 150 days of the start of dewatering. Dewatering will most <br /> likely be developed in stages, over time, as the pit is advanced. Accordingly, water level <br /> response times could differ significantly from those predicted in response to a fully developed <br /> dewatering system that is fully operational from the start of mining. <br /> Predicted Water Level Decline vs.Time <br /> Western Edge of Pit <br /> a ----- — <br /> 3 <br /> V <br /> 2 <br /> o <br /> 0 so 100 ISO 200 250 300 <br /> ,nme(om) <br /> Figure 1. Predicted Groundwater Level Decline vs. Time in Response to Dewatering <br /> 3 <br /> 6589 Elaine Road -Evergreen,Colorado 80439- office 720.242.8639 - cell: 303.870.5757 <br /> hahnwaterresources@gmail.com - www.hahnwaterresources.com <br />