Laserfiche WebLink
1 <br /> <br /> <br />On-site drill logs were completed throughout the North Taft Hill Expansion site showing <br />groundwater in the range of 4 %: to 8 %z feet below the ground surface. The average saturated <br />thickness presented in these borings is approximately 13 feet. <br />Other well borings were also completed to the east and south of the subject site. <br />' MODELING PARAMETERS <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br />1 <br /> <br /> <br />1 <br />1 <br /> <br />Earth Engineering Consultants, Inc. (EEC) performed calculations to estimate the hydraulic <br />conductivity at the North Taft Hill Expansion site. The site to the south is currently being <br />dewatered. Based on measured groundwater level data at various observation points around the <br />site and the steady state pumping rate, EEC estimated the hydraulic conductivity to be 400 <br />ft/day. This value is in the middle of the range for hydraulic conductivities for clean sand and <br />gravel alluvium, as reported in Groundwater by Freeze and Cherry. Therefore, the field-tested <br />value appeared to be reasonable for the project location. <br />A sensitivity analysis was completed using hydraulic conductivities of 200 ft/day and 800 ft/day <br />in the model. It was detet~rtined from this analysis that the results from using the new hydraulic <br />conductivities did not vary significantly from what is presented in this report. <br />Outer modeling parameters include a uniform aquifer porosity of 0.35 and specific yield of 0.3, <br />which are typical values for an alluvial aquifer. <br />METHODS AND APPROACH <br />The proposed dewatering operation has the potential of adversely affecting wells in the <br />surrounding areas. In the groundwater modeling efforts with GMS, the goal was to establish two <br />model runs: 1) a model run showing groundwater levels with the existing slurry wall to the north <br />of the North Taft Hill Expansion site as the baseline situation and 2) a model run showing <br />groundwater levels with the existing slurry wall and the proposed dewatering operation. The <br />difference in the groundwater levels from the two model tuns would show the potential impacts <br />from the proposed dewatering operation. <br />Initially, the extents of the model boundary were defined. The river was determined to be the <br />southern boundary of the model. The west, north, and east boundaries were set at a distance such <br />that the MODFLOW cells defining the boundaries would not become dry during the simulation. <br />Groundwater level information from the monitoring programs and drill logs were used to <br />establish the initial groundwater level contours. The model was calibrated to match the existing <br />groundwater levels observed in the well data. From these groundwater levels, the direction of <br />groundwater flow was determined. The results indicated a flow gradient not only parallel to the <br />river but also toward the river, which is expected for this reach of the Poudre River. Figure No. <br />2 shows the original groundwater contours and flow lines. <br />' Freeze and Cherry, Groundwater, Prentice-Hall, lnc., New Jersey, pp 29. <br /> <br />