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5.4 Specific Yield <br /> Specific yield for the aquifer was set at 20%which corresponds to the value determined by BBA and in support of <br /> numerous water court decrees. <br /> 5.5 Boundary Conditions <br /> Boundary conditions were set based on geologic features on the east and west sides of the model. The constant head <br /> boundaries on the south and north sides were varied to match observed groundwater data sets. <br /> ➢ Boundary conditions in a groundwater model are one tool used to approximate existing conditions at the outer <br /> extent of the model. The boundary conditions are set sufficiently far away from the area of interest to allow the <br /> aquifer to respond to local stresses and are based on hydraulic, hydrogeologic, hydrologic,geological and <br /> geographical assumptions. <br /> ➢ The model extends 3.3 miles southwest where constant head nodes were placed approximately 14 feet below <br /> the ground surface to represent existing groundwater levels taken in the area. <br /> ➢ Another constant head boundary was placed approximately 3.7 miles northeast Site and constant head nodes <br /> were placed on the southeastern flank at water surface elevations approximately ten feet below the ground. <br /> ➢ To the southeast of the model surface topography and bedrock slope up in elevation creating a natural barrier <br /> to flow. The eastern limit of the alluvial aquifer was confirmed by researching the states database of permitted <br /> wells in the area and mapping where the permitted aquifer switches from the alluvial aquifer to a deep confined <br /> aquifer. The approximate location of the shallow and deep aquifer divide is approximately 1/4 mile east the <br /> location of the Platte Valley Ditch. <br /> ➢ The South Platte River forms a natural hydrologic boundary to the west of the model. Model inputs for the <br /> MODFLOW river package are bottom elevation, head-stage and conductivity. The bed elevation was estimated <br /> using the USGS topographic quadrangle and Google Earth top of bed elevation and subtracting 6 feet to <br /> 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 Vrain River and groundwater inflows. <br /> A summary of the boundary conditions is presented in Table 1 below. <br /> Groundwater Model Report—Monarch DENM Gravel Mine Site -5- January 2023 <br />