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Hazeltine site consist of gravel, sand and silt of the Post-Piney Creek and Piney Creek Alluvium (Qp). <br />The deposits that are found to the southeast are associated with an alluvial terrace. The terrace deposits <br />contain gravel, sand, and silt of the Broadway Alluvium (Qb). At the east corner of the model lies a <br />relatively small portion of wind-blown deposits (Qes) that consist of fine to medium sand and silt. <br />USGS mapping indicates that the bedrock underlying the alluvium is most likely claystone and <br />sandstone of the Arapahoe Formation. Detailed mapping based on drill logs (VanSlyke, et al., 1988) <br />also indicates that the underlying bedrock is the Arapahoe Formation. <br />The boring logs indicate that the general subsurface profile consists of approximately one to 14 feet <br />(four feet typical) of silty sand overburden overlying up to 50 feet (25 feet typical) of sand and gravel on <br />top of claystone bedrock. The total thickness of the alluvial deposit encountered in the borings ranges <br />from approximately nine to 70 feet (32 feet typical). In many locations, the boring logs showed a mud <br />lens (a sandy clay layer) within the sand and gravel. <br />Information on the aquifer properties for the alluvial aquifer in the vicinity of the Hazeltine site was <br />obtained from Wilson (1965), which contains a compendium of aquifer properties from aquifer tests <br />throughout Colorado. Three published aquifer tests have been conducted near the Hazeltine site and the <br />hydraulic conductivity values from the tests were used as guidance when assigning values in the model. <br />The following table summarizes the information from the aquifer tests. <br />Aquifer Tests Conducted Near Hazeltine Mine (Wilson, 1965) <br /> <br />Well Location <br />Geologic Unit Well Depth Pumping Rate Hydraulic <br />Conductivity <br /> ~~ (gPm) da <br />T1S, R66W, Sec. 30 Valley Fill <br />Two miles northeast of Deposits (Qp) 35 140 334 <br />Henderson <br />T1S, R67W, Sec. 35 Valley Fill <br />One mile east of Deposits (Qp) 39 360 307 <br />Henderson <br />T2S, R67W, Sec. 22 Valley Fill <br />Three miles southwest Deposits (Qp) 47 300 1,030 <br />of Henderson <br />No information was available in Wilson (1965) for the hydraulic conductivity of terrace deposits (Qb) <br />and the wind-blown deposits (Qes) east of the reservoir. However, other literature sources report that <br />hydraulic conductivity values of alluvium in northeastern Colorado may range from 20 to 750 feet per <br />day (Hershey and Schneider, 1972). Therefore, the range of hydraulic conductivities for the three <br />geologic units in the azea of the reservoir are expected to be from 20 to 1,030 feet per day. The <br />hydraulic conductivity of the terrace deposits is expected to be similar to the valley fill deposits, as the <br />percentage of gravel, sand and silt are similar. The wind-blown deposits are expected to have a lower <br />hydraulic conductivity because they consist primarily of fine to medium sand. <br />Based on experience and literature values, the Specific Yield of the alluvial aquifer was assumed to be <br />0.2 throughout the domain. <br />- 3 - July 2004 <br />10919_018\GW ModeNtcport\HazcRinc_Rp 9 D.doc <br />