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6.0 MODEL SIMULATIONS <br /> Model simulations were completed to calibrate the model to reasonably represent the existing conditions and <br /> subsequently estimate the proposed change in hydrologic conditions. <br /> 6.1 Existing Conditions <br /> Steady-state modeling of average baseline conditions of April through July were used to calibrate the system equilibrium <br /> to the measured water levels. Monthly water level readings and other reported data can be seen in Appendix A. The <br /> aquifer was modeled as consisting of alluvium from the existing ground surface to the top of bedrock. Civil Resources <br /> evaluated the claystone bedrock as an aquiclude,thereby forming the bottom of the model. The observed water levels <br /> used to calibrate the steady-state model are shown on Figure 1. <br /> Calibration is the process of refining input parameters and boundary conditions,within reason, so the model reflects <br /> observed water levels. Parameters including: 1)hydraulic conductivity, 2)river conductance, 3)river head,4)river <br /> bottom elevations, and 5)drain conductance were adjusted during calibration. The groundwater levels to the northeast <br /> were estimated from the level of the Lower Latham Reservoir. Groundwater levels to the southwest were estimated from <br /> monitoring wells in the area.The South Platte River, ditches and drain bottom elevations were estimated utilizing the <br /> USGS Quadrangle map and water surface elevations were estimated using stream gage head information from the DWR <br /> website(See Appendix A). <br /> Figure 5 shows the modeled groundwater contours and Figure 5A shows the modeled groundwater contours zoomed in <br /> on the Site and the Section 20 Site. Table 2 below reports the steady-state modeled water levels and the measured <br /> water levels at the Site. Overall,the model matched the existing groundwater gradient to a good degree of accuracy and <br /> 6 out of the 7 Site monitoring wells were within 1.0 feet of observed with one well, PZ-09,which was modeled at 1.46 <br /> feet above the observed head elevation. <br /> Groundwater levels for monitoring wells in the Town of Gilcrest were also utilized in calibration of the model. The BBA <br /> report contained well readings from multiple wells in the Town of Gilcrest measured in 2014. It was assumed that these <br /> readings were similar to current head elevations and four(4)monitoring wells were included to get a representative <br /> sample of the Town. Of the four(4)wells chosen,three(3)of them were within 0.5 feet of observed. BBA-EC, northeast <br /> of town on the eastern side of Highway 85 was 1.5 feet lower than the observed elevation head. The monitoring wells <br /> included in the BBA report were recorded in 1-foot intervals,which could account for the 1.5 feet difference.Table 2 <br /> below has the comparison between the measured and modeled groundwater levels at the Site. The head contours for <br /> the baseline condition are shown on Figure 4. <br /> Groundwater Model Report—Monarch DENM Gravel Mine Site -7- January 2023 <br />