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irrigation wells aze all located in the Kuner terrace above the flood plain with a depth-to-water <br />ranging from 15 to 20 feet. <br />The capacity of the valley fill deposits to convey ground water underlying the site is high. The <br />transmissivity of the alluvial deposits underlying the proposed Nissen Gravel Pit is 200,000 <br />gallons per day per foot. The average saturated thickness underlying the proposed gravel pit <br />ranges from 30 to 60 feetz. <br />Ground Water Impact Analysis: <br />In order to predict the impact from the construction of an impervious bentonite slurry wall <br />around the perimeter of the proposed Nissen Gravel Pit, a simple MODFLOW analysis was <br />conducted assuming steady state groundwater conditions. The model was calibrated to the <br />regional groundwater gradients as defined by the Hutr and Schneider USGS 1972 open file <br />report. The model was set up using a grid consisting of 50 rows and 50 columns each having a <br />spacing of ] 58 feet. The grid study azea is shown in the enclosed Figure 1. The head conditions <br />of the South Platte River were input to the model assuming a river boundary condition. General <br />head boundaries were established on the east and south grid boundaries and used to calibrate the <br />model to reflect the steady state groundwater gradient as defined by the USGS using the a <br />hydraulic conductivity of 668 feet per day (calculated from the transmissivity of 200,000 gpd/R <br />and a saturated thickness of 40 feet). Figure 2 presents the model results of the calibrated <br />groundwater gradient represented in feet mean sea level (msl). <br />The location o£ the slurry wall was then imported into the model as a no-flow boundary with the <br />same general tread boundaries and conductivity use in the steady state calibration. The model <br />results from this future condition are shown in Figure 3. The model predicts that the installation <br />of the of bentonite slurry wall will increase the water levels south of the proposed wall 1 feet <br />with a distance of 1,200 feet south of the wall and 1 to 0.5 feet from 1, 200 feet to approximately <br />3,000 feet south of the proposed slurry wall. While this increase is notable, the depth to water in <br />Fhis area of the alluvial is 15 to 20 feet, resulting in a decrease in the depth to water to 14 to 19 <br />feet. - - <br />The impact to the alluvium north and east, or downstream of the slurry wall, is a one foot <br />decrease and is confined to the flood plain. <br />Conclusions: <br />The construction of the proposed benonite slurry wall around the perimeter of the Nissen Gravel <br />Pit will not cause injury to local wells or property given the maximum 1 foot rise in groundwater <br />1Htur, R.T., Schneider, P.A. (1972), Hydrologeologic Characteristics ofthe Valley-Fill Aquifer in the Greeley each <br />of the South Platte River Vallev. Colorado, Open File Report, United States Department of the Interior <br />Geoloeica] Survev. Colorado District. Lakewood. Colorado. <br />Water Rights • Hydrology • Hydraulics • Water Quality <br />B:V..eat\HatltGavd PitstSouth PlatteWissrn Pit Aquifer Impact Analysis.doc <br />2 <br />