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account for riverbed depth. Head-stage was taken from the South Platte River at Platteville, CO(PLAPLACO) <br /> monitoring station which was approximately 4'during the period of interest. See Appendix A for the PLAPLACO <br /> station discharge graph. <br /> ➢ River conductance is a model parameter in the river package which simulates the riverbed's ability to transfer <br /> water to the neighboring cell. The river conductance parameter is calculated by taking vertical hydraulic <br /> conductivity divided by bed thickness multiplied by river width. This value is then multiplied by GMS for the <br /> length of the river arc to which it is applied. Since the riverbed hydraulic conductivity is unknown for this reach <br /> of the South Platte River,it was necessary to calibrate this parameter to groundwater conditions at the Site.An <br /> initial conductance of 36 ft/day was determined from well test reports that occurred upstream of the Site north of <br /> Fort Lupton. Since the South Platte River acts as a drain throughout the reach, conductance values less than <br /> 150 ft/day caused unrealistic head values and flooded cells.Values higher than 160 ft/day cause head values to <br /> drop and dry cells through the area.The river conductance was modeled to increase from 150 ft/day where the <br /> river enters the model to 160 ft/day where the river exits the model. The rise in conductance was implemented <br /> to account for an increase in river width after the addition of the Saint Wain River and groundwater inflows. <br /> A summary of the boundary conditions is presented in Table 6 below. <br /> Table 1 <br /> 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, EVANS2CO river diversion <br /> monitoring stations(See Appendix A). The 111312-1278 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 /> Groundwater Model Report—Section 20 Gravel Mine Site -5- August 2022 <br />