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Dewatered mines were represented by constant head boundary condition cells. The head elevations <br /> were set to estimated groundwater levels near the mine edge during dewatering. Dewatered mines in the <br /> model include the Challenger Pit, southern portion of the Rogers Pit, and the Baseline Mine(Walker Pit <br /> extension). <br /> 2.3 Aquifer Properties <br /> The model simulated the shallow unconfined aquifer within three geologic units(Figure 3). Information <br /> on the type of units found at the site was obtained from geologic maps prepared by the U.S.Geological <br /> Survey(Trimble and Machette, 1979;and Colton, 1978). The valley fill deposits along the South Platte <br /> River and at the Tucson South site consist of gravel,sand and silt of the Post-Piney Creek(Qpp),Piney <br /> Creek(Qp), and Louviers (Qlo)Alluvium. The deposits that are found to the west are associated with <br /> an alluvial terrace. The terrace deposits contain gravel,sand and silt of the Broadway Alluvium(Qb). <br /> Along the west side of the model lies a relatively small portion of wind-blown deposits(Qes and Ql) <br /> that consist of fine to medium sand and silt. USGS mapping indicates that the bedrock underlying the <br /> alluvium is most likely claystone and sandstone of the Denver or Arapahoe Formation. Detailed <br /> mapping based on drill logs(VanSlyke,et al., 1988)also indicates that the underlying bedrock is the <br /> Arapahoe Formation.The bedrock outcrops along the west side of the model. <br /> The boring logs indicate that the general subsurface profile consists of approximately one to 10 feet <br /> (three feet typical)of silty sand overburden overlying approximately five to 46 feet(23 feet typical)of <br /> sand and gravel on top of claystone bedrock.The total thickness of the alluvial deposit encountered in <br /> the borings ranges from approximately four to 50 feet(30 feet typical). In many locations the boring <br /> logs showed a mud lens(a sandy clay layer)within the sand and gravel. Groundwater depths in the mine <br /> area at the time of drilling ranged from approximately eight to 12 feet below the ground surface,but are <br /> expected to be higher during irrigation and periods of higher runoff. <br /> Information on the aquifer properties for the alluvial aquifer in the vicinity of the Tucson South site was <br /> obtained from Wilson(1965),which contains a compendium of aquifer properties from aquifer tests <br /> throughout Colorado. Seven published aquifer tests have been conducted near the Tucson South site <br /> and the hydraulic conductivity values from the tests were used as guidance when assigning values in the <br /> model. The following table summarizes the information from the aquifer tests. <br /> -3- August 2004 <br /> 1\3919 DIMS GW ModeATS RepwMam South Rpt Iha&doc <br />