2022-11-07_PERMIT FILE - M2022001 (11)

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reached. It was assumed that seepage from the ditches reduced over distance as water was consumed. See Figure 1 for inputted parameters. The Western Mutual Ditch was the main factor in determining head values at the Section 20 Site and small changes in head elevation and conductance were used to calibrate the river input to reach observed head measurements throughout the Site. ➢ Drains—A series of drains were set at the base of three sloughs along the South Platte River.These drains were necessary to pull head elevations down to the slough elevation and not flood out the cells in the floodway. The drain inputs are bottom elevation and conductance.The bottom elevation was estimated from the USGS Quadrangle Map and were assumed to beat least 2 feet deep. The conductance input was set high enough to account for all water that was above the drain bottom to model the slough action of removing groundwater and setting groundwater head levels at the base of the terrace. ➢ Recharge—A number of upgradient recharge Sites were listed in the BBA report and were added to this model. The recharge values and location are located on Figure 2. It was assumed that pivot and flood irrigation contributed 0.001 feet per day to the groundwater table over the whole model area during irrigation season. 6.0 MODEL SIMULATIONS 6.1 Existing Conditions Steady-state modeling of average baseline conditions of April through July were used to calibrate the system equilibrium to the measured water levels. Monthly water level readings and other reported data can be seen in Appendix A. The aquifer was modeled as consisting of alluvium from the existing ground surface to the top of bedrock. Civil Resources evaluated the claystone bedrock as an aquiclude,thereby forming the bottom of the model. The observed water levels used to calibrate the steady-state model are shown on Figure 1. Calibration is the process of refining input parameters and boundary conditions,within reason,so the model reflects observed water levels. Parameters including: 1) hydraulic conductivity, 2)river conductance, 3)river head,4)river bottom elevations, and 5)drain conductance were adjusted during calibration. The groundwater levels to the northeast were estimated from the level of the Lower Latham Reservoir. Groundwater levels to the southwest were estimated from monitoring wells in the area.The South Platte River, ditches and drain bottom elevations were estimated utilizing the USGS Quadrangle map and water surface elevations were estimated using stream gage head information from the DWR website(See Appendix A). Figure 5 shows the modeled groundwater contours and Figure 5A shows the modeled groundwater contours zoomed in on the Site and the Town. Table 2 below reports the steady-state modeled water levels and the measured water levels at the Site. Overall,the model matched the existing groundwater gradient to a good degree of accuracy and 6 out of the 7 Site monitoring wells were within 0.5 feet of observed. MW-1 was 2'above the observed head elevation. ➢ MW-1 was drilled approximately 50 feet from a residence at the property and a search of well permits on the DWR Site shows that there are multiple wells, 3 owned by Red Tierra Equities LLC.,are located within a 400 foot radius of the monitoring well. There is also a domestic well approximately 500 from MW-1 across Highway 60. These wells may locally influence the groundwater resulting in lower water levels observed in MW-1 than modeled, especially given that the gradient matches across the rest of the Site. Groundwater Model Report—Section 20 Gravel Mine Site -6- August 2022