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Table 1: Boundary Conditions <br /> Area Boundary Notes <br /> North Model extends 3.7 miles to the See Figure 1 <br /> northeast <br /> South Platte River located River bounds the western <br /> West approximately 4,000 feet west of side of the model,see <br /> Site Figure 1 <br /> South Model extends 3.3 miles to the Constant head nodes,see <br /> _ southwest Figure 1 <br /> East Steeper Slopes to a ridge,wells See Figure 1 <br /> change from shallow to deep <br /> The model boundary conditions are shown on Figure 2 <br /> 5.6 Internal Influences <br /> Surface features and activities that affect the alluvial groundwater include: <br /> ➢ Ditches—There are four(4)ditches that traverse the interior of the model. These ditches were modeled in <br /> GMS as river nodes. As discussed in the South Platte River boundary condition above,the model inputs for the <br /> MODFLOW river package are:bottom elevation, head-stage,and conductance. <br /> The bed thickness for the ditches were all assumed to be 2 feet thick and to have a relatively low vertical <br /> hydraulic conductivity to limit seepage and were used as a calibration tool to match local groundwater <br /> elevations. Head stage was estimated from the FRMDITCO,WESDITCO, EVANS2C0 river diversion <br /> monitoring stations(See Appendix A). The HBI2-1278 Study of the South Platte River Alluvial Aquifer, <br /> performed by Colorado State University, states that calculated ditch seepage in the ranges from 10 to 50 <br /> percent,averaging at 23 percent. In the area of interest, ditch width ranges from approximately 10 feet wide to <br /> 30 feet wide.Assuming the 2-foot bed thickness and starting with 10 percent seepage, ditch conductance was <br /> set at 0.5 ft/day and was raised incrementally until known groundwater head elevations and gradient were <br /> reached. It was assumed that seepage from the ditches reduced over distance as water was consumed. See <br /> Figure 1 for inputted parameters. <br /> The Western Mutual Ditch was the main factor in affecting groundwater levels at the Monarch DENM Site and <br /> small changes in head elevation and conductance were used to calibrate the river input to reach observed head <br /> measurements throughout the Site. <br /> ➢ Drains—A series of drains were set at the base of three sloughs along the South Platte River.These drains <br /> were necessary to pull head elevations down to the slough elevation and not flood out the cells in the floodway. <br /> The drain inputs are bottom elevation and conductance.The bottom elevation was estimated from the USGS <br /> Quadrangle Map and were assumed to be at least 2 feet deep. The conductance input was set high enough to <br /> account for all water that was above the drain bottom to model the slough action of removing groundwater and <br /> setting groundwater head levels at the base of the terrace. <br /> ➢ Recharge—A number of upgradient recharge Sites were listed in the BBA report and were added to this model. <br /> The recharge values and location are located on Figure 2. It was assumed that pivot and flood irrigation <br /> contributed 0.001 feet per day to the groundwater table over the whole model area during irrigation season. <br /> Groundwater Model Report—Monarch DENM Gravel Mine Site -6- January 2023 <br />