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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 />I'.U919 019\TS GW M9dcpT5 Rcport\Tuuon SouJi_ftpI Drzfl.doc <br />