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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />suggest that local mudstone-dominant tributaries fed this area after the river channel migrated to the <br />northwest. <br />The aquifer depositional history impacts two important alluvial aquifer characteristics in the Study <br />Area. First, alluvium in the central portion of the aquifer is more permeable due to the dominance of <br />coarser -grained material, as shown in Figure 5. Second, the aquifer contains laterally discontinuous <br />layers of finer -grained material derived from slopewash and inflow from minor ephemeral tributaries <br />Fine-grained interbedding results in a stratified aquifer system with localized semi -confined <br />conditions in underlying aquifer units. These low -permeability semi -confining units can prevent <br />recharge from infiltrating to the deeper aquifer thus causing perched water table conditions. <br />CGS Revised Alluvial Aauifer Extent <br />Figure 2 shows the CGS revised alluvial aquifer extent. This new alluvial aquifer extent was derived <br />from the intersection of the new bedrock topography raster map described above and a water table <br />surface based on spring 2012 groundwater elevations, when water levels were high. As a <br />conservative measure, 10 feet was added to the 2012 groundwater surface elevations to obtain a <br />reasonable maximum saturated aquifer extent. The revised CGS alluvial aquifer extent provides <br />much higher level of resolution than the regional SPDSS mapping shown in Figure 1 (CDM 2006b) <br />or the active groundwater model extent portrayed by CDM Smith (2013). <br />Hvdraulic Conductivitv. Saecific Yield. and Transmissivitv <br />Figures 11 and 12 present the aquifer hydraulic conductivity and specific yield, and transmissivity <br />(aquifer properties), respectively. Table 1 summarizes aquifer test data in the Study Area vicinity. <br />The 11 hydraulic conductivity values used for the contours shown in Figure 11 were obtained from <br />the SPDSS database (CWCB, 2013) and derived from aquifer tests performed in wells pumping from <br />385 to 1,284 gpm. Permeability data derived from specific capacity tests were not used in this <br />analysis. Hydraulic conductivity values are the basis for the aquifer permeability presented herein. <br />Hydraulic conductivity values in and adjacent to the Study Area range from 134 to 1,270 feet/day. <br />Point data and contouring presented in Figure 11 indicate a linear zone of highest hydraulic <br />conductivity extending through the alluvial valley axis from the Study Area's southwest corner to the <br />northeast corner, with a second linear zone of high hydraulic conductivity extending along the middle <br />of Beebe Draw and joining with the South Platte alluvial valley immediately west of Lower Latham <br />Reservoir. This hydraulic conductivity distribution is consistent with the alluvial aquifer depositional <br />model described above, in which the alluvial valley margins are dominated by fine-grained <br />slopewash and the central portion of the valley consists of more permeable sand and gravel <br />deposited by the ancestral South Platte River. <br />12 <br />