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Joe Lamanna <br /> December 21, 2022 <br /> Page 2 <br /> MODFLOW Model <br /> A MODFLOW model was previously prepared to analyze water level changes and impacts to the <br /> alluvial ground water system and neighboring wells as a result of the Stage 6A and 6B gravel pit <br /> dewatering and lining. This previous model has been modified to analyze the existing gravel pits <br /> and the potential impact of the lining of Stage 7A and 7B on water levels in the South Platte <br /> River alluvium and the interaction of the aquifer with the river. <br /> A single layer MODFLOW model was utilized to represent the alluvial aquifer bounded by <br /> lower permeability bedrock present at the site. <br /> The model domain includes approximately 32 square miles, discretized to 100-foot square cells. <br /> Top and bottom elevations of the model cells were based on SPDSS ground surface elevation <br /> mapping, SPDSS bedrock elevation mapping, SPDSS alluvial boundary mapping and test <br /> drilling data provided by the client. SPDSS water level elevation information was used to define <br /> the initial heads in the model and to compare modeled water level elevation conditions to <br /> mapped conditions. <br /> The model includes several boundary conditions to represent the alluvial aquifer, the South Platte <br /> River and its tributaries, the bedrock below the alluvium and the lined and unlined pits. <br /> • The alluvial aquifer was modeled using head dependent cells. <br /> • The bedrock was modeled using no-flow cells bounding the head dependent cells. <br /> • Alluvial underflow was modeled using constant head boundaries at the upstream and <br /> downstream extents of the model as well as at the location of tributaries (Big Dry Creek <br /> and Little Dry Creek). <br /> • The South Platte River, Big Dry Creek and Little Dry Creek were modeled using the <br /> MODFLOW river package to allow water to infiltrate into the alluvial aquifer. The South <br /> Platte River was modeled as a 100-foot wide channel, with a river bottom 5 feet beneath <br /> the river stage. Big Dry Creek was modeled as a 15-foot wide channel, with a river <br /> bottom 2 to 3 feet beneath the river stage. Little Dry Creek was modeled as a 10-foot <br /> wide channel, with a river bottom 2 feet below the river stage. Each of the rivers was <br /> modeled with a riverbed thickness of 1 foot and a hydraulic conductivity of 5 feet per <br /> day. The river stage for each river cell was approximately equal to the surface elevation <br /> for that cell. <br /> • Pit lining was simulated using no flow cells. <br /> • Ponds were simulated by increasing the hydraulic conductivity in cells at the location of <br /> the ponds. <br /> • Recharge was not simulated in the MODFLOW model. <br /> A hydraulic conductivity of 770 feet per day was used for the model for the alluvial material <br /> based on SPDSS data points located within two miles of the Morton Lakes boundary. A specific <br /> yield of 0.20 was used for the model based on our experience with alluvial systems. <br /> .r' <br />