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Alluvial Sand & Gravel, Broadway Alluvium Qb: The alluvium is classified as sand and gravel deposited by the South <br />Platte River and its tributaries. Locally, the unit includes some gravel of Louviers age. The deposit ranges in thickness <br />on the Site between approximately 30 to 50 feet, bui averages approximately 40 feet. The deposits extend well above <br />the groundwater table to typically within three feet of the ground surface, creating an unconfined aquifer of relatively <br />homogeneous material (Colton, 1978 & Trimble & Machette,1979). <br />Eolian Qe: Eolian deposits (wind blown) of clay and silty sand loess bound the models eastern flanks. These deposits <br />are classified as light brown to reddish brown to olive gray deposits of windblown clay, silt, sand, sand granules mainly <br />as sand dunes. The loess deposit ranges in thickness from 15 feet thick to less than 3 feet thick (Colton, 1978 & Trimble <br />& Machette,1979). <br />3.5 Site Hvdraulic Conductivities <br />Hydraulic conductivity is a measure of the soils ability to transmit water within the aquifer. Some factors affecting <br />hydraulic conductivity are: pore size distribution, grain size distribution, void ratio, roughness of mineral particles, and <br />degree of soil saturation (Das, 1998). The alluvium, underlying the Site, is generally a clean sandy soil, as described <br />above with an approximate depth of 35 feet. Models performed on similar sites overlying Piney Creek Alluvium have <br />typically used hydraulic conductivities of 250 feet per day to 750 feet per day. <br />Table 1 <br />Site Hydraulic Conductivity <br />Geological Unit Hydraulic Conductivity (rudayl Average Geological Unit <br />Thickness h <br />Pine Creek 250 35 <br />Cla stone 0.003 565 <br />Slur Wall 0.0045 NA <br />3.8 Regional Hvdraulic Conductivities <br />Literature sources were used to evaluate the appropriate hydraulic conductivities of materials outside of the Site <br />boundaries. In northeastern Colorado, hydraulic conductivity values for alluvium generally range from 20 to 750 feet per <br />day (Hershey and Schneider, 1972). The Piney Creek and Broadway deposits should vary between the above values, <br />whereas the wind blown finer grained deposits would tend towards the lower end of the reported range. See Figure 4 for <br />the locations of the assigned hydraulic conductivities. <br />Table 2 <br />Regional Hydraulic Conductivity <br />Geological Unif Hydraulic Conductivity (fUday) Average Geological Unit <br />Thickness ft <br />Ran a of Values 20 - 750 NA <br />Eolian 50 7 <br />Broadwa 165 20-50 <br />Pine Creek 250 20 - 35 <br />3.7 Recharge Due to Precipitation <br />The climate in the Ft. Lupton and Brighton area is generally arid (evaporation rates are approximately three feet per <br />year) with temperatures and precipitation as shown below. <br />Groundwater Model Repod-Hall-Irwin Parker-DershamSite -3- Apd12006 <br />