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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />Platte Decision Support System (SPDSS) project developed by the CWCB during the 2000s (CDM <br />2005, 2006a, 2006b, 2006c). This hydrogeologic characterization builds on previous regional <br />investigation findings by providing greater detail using additional data from multiple sources. <br />Smith et al (1964) developed one of the earliest detailed hydrogeologic descriptions of the Study <br />Area and mapped areas of shallow groundwater using data from as early as 1957. As groundwater <br />use expanded and groundwater's interaction with surface water was recognized, the United States <br />Geological Survey (USGS), in cooperation with CWCB, prepared a series of map sets as open file <br />reports throughout the South Platte River Basin (Hurr and Schneider, 1972). The map sets include <br />contour maps showing bedrock surface configuration, the historic groundwater elevations, aquifer <br />saturated thickness, transmissivity, and estimated stream depletion factor distribution in the aquifer. <br />These digitized maps and other data available from the SPDSS project have provided the foundation <br />for understanding hydrogeologic conditions in this reach of the South Platte River alluvial aquifer. <br />Colton (1978) mapped the regional surficial geology at a 1:100,000 scale as part of a series of maps <br />by USGS that includes depth -to -groundwater mapping and water well yields (Hillier and Schneider <br />1979a and 1979b). Robson, Albert, and Heiny prepared a series of USGS hydrologic atlases <br />(Robson, et al., 2000a, 2000b) for the South Platte alluvial aquifer including the Study Area providing <br />data on alluvial aquifer thickness, bedrock surface, and water table elevation. These atlases were <br />incorporated into the SPDSS study, described below, that makes up the foundation of this <br />hydrogeologic characterization. <br />Colorado's Decision Support System is being developed by CWCB with DWR cooperation for each <br />of Colorado's major water basins. The South Platte Decision Support System (SPDSS) has been in <br />development since approximately 2000 and has entailed collection and compilation of geologic and <br />hydrologic data and collection of new data. The SPDSS project also digitized existing map sets by <br />Hurr and Schneider (1972) and made these data sets publicly available. As part of the SPDSS, all <br />available published data, and a significant amount of data not previously in the public domain, were <br />compiled and synthesized to provide a comprehensive data set for the South Platte River Basin <br />between South Denver and Nebraska. The SPDSS program identified data gaps throughout this <br />region and implemented piezometer installation and high-capacity well aquifer testing (CDM 2005), <br />water level compilation and monitoring (CDM 2006a), alluvial aquifer geometry synthesis (CDM <br />2006b), alluvial aquifer property compilation (CDM 2006c) and regional groundwater model <br />development (CDM Smith 2013). SPDSS GIS data products (CWCB 2013) provide much of the <br />basis for the CGS hydrogeologic characterization documented here. <br />