Laserfiche WebLink
Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />In the Study Area the South Platte alluvial aquifer is a heterogeneous geologic unit <br />composed of interbedded gravel, sand, silt, and clay filling an irregularly shaped paleovalley <br />incised into low -permeability bedrock (Figure E3). Highly permeable coarse-grained material <br />dominates the central portion of the aquifer and is interbedded with lenses of less permeable <br />fine-grained material. On the aquifer flanks, sheetwash deposits derived from the fine- <br />grained Laramie Formation or loess form low -permeability deposits locally overlain by sand <br />and loess. Aquifer thickness exceeds 100 feet in the deepest portions of the aquifer; <br />however the majority of the alluvial sediments are 45 to 85 feet thick. Using spring 2012 <br />water levels, the groundwater volume stored in the alluvial aquifer in the Study Area was <br />estimated at 320,000 acre feet. <br />Hydraulic conductivity values from nine aquifer tests range from 385 to 1,270 feet/day (Table <br />1). Specific yield values from five of the aquifer tests have a bimodal distribution averaging <br />0.04 and 0.15, indicating localized semi -confined and unconfined conditions, respectively. <br />Transmissivity values from the same locations range from 40,000 to 350,000 gallons per day <br />perfoot <br />Groundwater level measurements were compiled from 136 wells (Figure E4) with a <br />combined period of record from 1929 to 2013 (Table 2). In the central and northeast portions <br />of the Study Area historic groundwater level data indicate a relatively stable trend over the <br />period of record (Figures E5 and E6, respectively). In the western portion of the Study Area <br />groundwater levels have declined from the 1940s until the early 2000s, when levels began to <br />rise, however groundwater levels are still 5 to 8 feet below the 1940s levels (Figure E7). <br />As shown in Figure E8, shallow groundwater occurs along three distinct alignments in the <br />Study Area: the floodplain immediately adjacent to the South Platte River, Beebe Draw from <br />Milton Reservoir to the vicinity of Lower Latham Reservoir, and the upper terraced portion of <br />the alluvial valley flank below the Platte Valley Canal and the Evans Ditch. All of these areas <br />have historic shallow groundwater conditions as indicated by features such as drainage <br />canals, mapped wetlands and depth to groundwater maps from previous investigations using <br />data from the 1950s and 1970s (Smith 1964, Colton 1978). <br />Groundwater enters the Study Area as underflow at the hydraulically upgradient (southwest) <br />end of the Study Area, underflow from the south in the Beebe Draw alluvial aquifer, and as <br />recharge from infiltration of precipitation and irrigation, seepage from recharge ponds and <br />irrigation ditches, and other surface water sources. Groundwater flow in the central portion of <br />the Study area is generally subparallel to the South Platte River, and has a strong northward <br />flow component throughout the entire study area. This indicates that while groundwater will <br />E2 <br />