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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />HYDROGEOLOGIC CHARACTERIZATION <br />This section describes the geologic and hydrologic framework of the Study Area. It integrates results <br />from the earlier investigations with new analysis. <br />STUDY AREA GEOGRAPHIC SETTING <br />The Study Area is in rural Weld County, Colorado, in the South Platte River valley (Figure 1). The <br />Study Area consists of approximately 78 square miles and lies south and southwest of Greeley <br />along US Highway 85. The roughly triangular shaped Study Area includes the towns of Gilcrest and <br />LaSalle and is bounded on the northwest by the South Platte River. The Study Area includes the <br />South Platte River alluvial valley, consisting of the adjacent floodplain and upper terraces, and the <br />lower reach of Beebe Draw. Study Area topography varies from flat to gently rolling terrain with a low <br />northeast trending ridge separating the South Platte alluvial valley from Beebe Draw (Figure 2). <br />The South Platte alluvial aquifer underlies most of the Study Area and varies from zero to more than <br />100 feet thick. The alluvial aquifer fills a channel incised into bedrock of the Upper Cretaceous <br />Laramie Formation shale and underlying Fox Hills Sandstone. Bedrock outcrops in several locations <br />along the ridge separating the South Platte alluvial valley from Beebe Draw. The Study Area lies at <br />the northern portion of the Denver Basin and on the south flank of the Greeley Arch. <br />Irrigated agriculture plays an important role in the Study Area's water balance. The Study Area <br />includes Milton, Behrens, Chestnut, and Lower Latham Reservoirs, the Speer Canal, Platte Valley <br />Canal, Evans No. 2 Ditch, Farmers Independent Ditch, Western Mutual Ditch, Union Ditch, Latham <br />Ditch, Godfrey Ditch, and the, Gilmore Ditch as shown in Appendix G. In addition to surface water <br />diversions, agricultural groundwater pumping from the highly productive alluvial aquifer plays an <br />important role in the Study Area hydrology, with many production wells capable of producing more <br />than 1,000 gallons per minute (gpm). <br />Several seepage canals such as the Lower Latham Drain traverse the low-lying flood plain to <br />provide drainage in areas of historic high water table conditions near the South Platte River. In <br />addition to irrigation and seepage from reservoirs and ditches, which percolates into the aquifer, <br />since 2002 many ponds for augmentation recharge have been constructed within the Study Area, <br />increasing from approximately 35 ponds in 2002 to over 70 ponds in 2012 as indicated by aerial <br />photograph analysis using historical imagery. The evaluation method did not indicate whether ponds <br />were installed specifically for augmentation recharge, however, for the purpose of this report it will be <br />assumed that most, if not all, of the ponds mapped are used to recharge the aquifer for South Platte <br />River stream flow augmentation or otherwise result in aquifer recharge. Surface water used for <br />stream flow augmentation is diverted from the South Platte River at times of low demand and <br />5 <br />