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L.G. Everist <br /> West Farm Groundwater Modeling <br /> The simulated drain parameters are more conservative in terms of dimensions(width, length, slope) and hydraulic <br /> conductivity. Thus, the mitigation (reduction of groundwater mounding) provided by an actual drain should be <br /> more than shown in the model. The results of the simulation with the drain are presented on Figure 4. This map <br /> is the same as Figure 3,showing depth to groundwater, except it shows the mitigated groundwater mounding, <br /> and the location of the proposed drain. The mitigated groundwater mounding upgradient of the property is less <br /> than one foot, an improvement of about two feet in the most critical area. The drained mound is limited by the <br /> downgradient elevation of the drain, which roughly reestablishes the pre-slurry wall groundwater condition. <br /> CONCLUSIONS <br /> This groundwater impact analysis was performed to evaluate the mounding and shadowing effect the construction <br /> of slurry walls has on the local groundwater table. It also was used to evaluate a potential mitigation alternative <br /> for draining the groundwater mound up-gradient of the property line. The model accurately replicated the <br /> conditions of the South Platte River alluvial aquifer based on data available from recent geotechnical <br /> investigations. Model construction was facilitated by using an extensive GIS to inventory, analyze and present <br /> the data. Three constant head boundaries were used at the southern, and northern ends of the model domain <br /> and a third one at the South Platte River. The no-flow boundary is the hydraulic divided between the South Platte <br /> basin and the Saint Vrain Basin. The mass balance had a 0.001 percent error. <br /> The model simulated the hydrologic changes caused by construction of the slurry walls. The predictive simulation <br /> showed that the magnitude of the maximum groundwater mounding upgradient of the southern slurry wall is about <br /> 3 feet, and a minimum depth to groundwater of about 3 feet. The simulation also indicates that the maximum <br /> shadowing effect caused by the reclamation activities is about 2 feet downgradient of the proposed slurry wall. <br /> The model also predicted the effect of a groundwater underdrain installed up-gradient of the slurry walls to lower <br /> the groundwater mound. This simulation results in a maximum groundwater mound up-gradient of the property <br /> line of less than one foot, a more than two-foot decrease in mounded groundwater heads. Installation of a drain, <br /> if necessary,would result in a groundwater table that roughly approximates the pre-slurry wall heads. <br /> Additional wells are planned, including one between MW-6 and MW 7. The site wells will be monitored <br /> approximately monthly. If the depth to groundwater, following the construction of the slurry wall(s) in any exterior <br /> well approaches 2 feet, we recommend that a drain be installed." <br /> The results of the groundwater modeling and conclusions drawn from them represent an approximation and are <br /> based on the best available data. Conservative assumptions were made during the calibration process so that <br /> groundwater mounding was not under-predicted. Given the unknown heterogeneity of the aquifer in the field and <br /> variations in ground surface from the topographic data used, the groundwater mounding and/or drainage <br /> mitigation may deviate from the model simulation. There is a possibility that mounding may be higher than <br /> predicted, although the conservative assumptions of this work make the deviation toward a lower mound in the <br /> field a more likely possibility. <br /> G\2020U ongmont\20C26026 02 West Farts\03_SE_Products\03-Reports\01-Draft\Groundwater Model Memo Docx <br /> Project 20C26026.02/May 19,2023 Page 6 Deere&Ault,a Schnabel Engineering Company <br />