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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />eventually discharge to the South Platte River, a significant groundwater flow component is <br />downstream and subparallel to the river. Groundwater flows preferentially through the aquifer <br />via the zone of highest transmissivity underlying the center of the alluvial valley, generally <br />along the US Highway 85 alignment (Figure E9). Groundwater leaves the Study Area as <br />downgradient underflow, water well pumping consumptive use, evapotranspiration, and <br />discharge to the river. Annual groundwater underflow through the Study Area is estimated at <br />10,500 acre-feet and storage is estimated at 320,000 acre feet. <br />The frequency of groundwater level data collection has been very irregular since the <br />measurement record began in 1929 (Table 2). The most significant temporal data gaps are <br />prior to 1965, 1980 through 1994, and 2005 through 2006. Despite a large number and wide <br />distribution of wells that have been measured (Figure E4), none of the wells monitored have <br />a continuous measurement record and almost all wells monitored before 1994 have at least <br />one decade-long interval during which no measurements are available. Additionally, many <br />wells shown in Figure E4 may provide a short period of record from long ago, but are <br />currently unavailable for future measurement, have not been measured for decades, or <br />outside of areas relevant to evaluating high groundwater conditions. Areas where <br />groundwater level data are lacking include: 1) in the northeast part of the Study Area, 2) <br />south and southwest of LaSalle, 3) the vicinity of Lower Latham Reservoir, and 4) a three- <br />mile long band north of Gilcrest (Figure E10). Aquifer property data are available from 11 <br />aquifer tests in or adjacent to the Study Area, however a full three-dimensional <br />understanding of aquifer properties is needed to understand the lateral and vertical <br />heterogeneity of aquifer hydraulic conductivity and specific yield. <br />E3 <br />