2022-10-27_PERMIT FILE - M2022001

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Freshwater and Siltation ponds would be filled in with overburden at the site which would lower their horizontal hydraulic conductivity to be similar to the Eolian deposits in the area. The horizontal hydraulic conductivity in these areas was set to 65 feet per day.The additional impacts from adding this site to the model are presented in Table 3a below and a layout of the site is shown on Figure 5 and 5A. Table 3a Mi ing Groundwater Level Table With Monarch DENM Site(Slurry Walls In-Place After Slurry Surface Basement Wall Difference(FT Groundwater Depth Elevation Location /n Baseline Construction Rise or Fall) Below Surface Goo le Earth Cell IJK Patrid a House n 47363 4745.5 92 185 4764 161,1351 Cline House n 4736.8 47477 10.9 18.3 4766 162,132,1 Hardinq House Y 47366 47488 122 162 4765 161 1291 Wiedeman House 13434 n 47382 47508 12.6 17.2 4768 163,121,1 Wiedman House 13218 n 47385 47512 127 108 4762 163 1151 Share House y 47384 47509 12.5 91 4760 163,111,1 HS Land and Cattle n 47326 4744.7 121 103 4755 1551071 Karbowski Brave House n 47255 47315 6.0 20.5 4752 1451081 Schmidt Well NA 4714.4 47104 -4 0 226 4733 1321121 United Well NA 47120 47070 -5 0 410 4748 132,126,1 Owens Well NA 47142 47117 1 -2 5 343 4746 1321331 Wright House Y 4717.4 4712.8 -4 6 392 4752 1351321 Schmidt House n 47184 47177 -0 7 30.3 4748 1351351 Smit House y 4718.8 47192 04 288 4748 135,136,1 Kissler House y 47295 4734.8 5.3 202 4755 148 1371 Woolverton House y 47308 4736.5 5.7 255 4762 1501361 Shimon House y 47330 47397 6.7 193 4759 154 1361 Kern House y 4733.8 47409 71 19.1 4760 156,1371 Rainbolt House y 47359 47444 85 206 4765 161,1371 Gilcrest Valley HS NA 4737.0 47425 55 35 4746 170 1551 The addition of the full build out of the Monarch DENM Site increases mounding on the southern side of the Section 20 Site by limiting groundwater flow to the west. Slurry Walls along Highway 60 and the addition of the Monarch DENM Siltation and Freshwater ponds creates a groundwater corridor along Highway 60 north to the South Platte River,which causes the groundwater at the HS Land and Cattle building to rise approximately twelve (12)feet from baseline conditions. 6.3 Mitigation This section describes potential mitigation actions to decrease the mounding affect of slurry wall construction on the surrounding properties and the Town. The Monarch DENM Gravel Mine timeline submitted to the DRMS was utilized in conjunction with the Section 20 expected timeline to determine the phasing of mitigation construction. These timelines are approximate and are subject to change which may necessitate early construction of mitigation structures depending on measured groundwater conditions at the Site. The shadow affect to the north will likely be minimized by the proximity to the unnamed slough and the infiltration ponds to the northeast of the Site. During mining at the Site, an unlined siltation pond will be present in the northwest corner of the Site which will also mitigate some of the shadow affect. At this time no mitigation is recommended to address shadowing to the north unless groundwater monitoring demonstrates that the drop in groundwater head elevations is larger than anticipated. Mitigation would be warranted if any of the three well owners to the north complain about Groundwater Model Report—Section 20 Gravel Mine Site -9- August 2022