2022-12-21_PERMIT FILE - M2022001

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MW-5 and MW-6,coincides with an approximate one (1)foot rise in town. To mitigate against possible groundwater elevation rise in town, a two(2)foot cut off was used to implement mitigation at the southern site boundary. 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 comer of the Site which will also mitigate some of the shadow affect. Mitigation for the shadow affect is not anticipated to be necessary since the saturated thickness of the alluvial deposit is eighteen (18)to twenty(20)feet a full buildout of both sites. Even though mitigation is not anticipated to be necessary for the shadow affect to the north, it is recommended that an infiltration pond be installed north of Cell 7 to receive the rerouted groundwater from the south of the site. This will enable all the mitigation infrastructure to be constructed within the Division of Reclamation, Mining, and Safety's Affected Area Boundary. Two(2)mitigation scenarios were modeled to provide optionality for the method of return of the groundwater to the hydrologic system;either directly to the river or through infiltration. Each scenario was modelled in two(2)phases coinciding with the construction schedule of the Site and the Monarch DENIM slung walls.The first phase is an underdrain or series of dewatering wells along the eastern half of the site parallel to Weld County Road 42. The second phase is the construction of an underdrain between Cell 6 and the Western Mutual Ditch west of where the ditch crosses on to the property to approximately 200 feet east of Highway 60. 6.3.1 Mitigation Scenario 1—Direct Discharge to the South Platte This mitigation scenario utilized the Well and Drain Package to remove water from the model. Water is not returned to the model to simulate surface discharge or direct discharge to the South Platte. This option would require an Amendment to the DRMS Reclamation Permit Application or Permit to add the underdrain discharge pipe to the Affected Area. The exact route for the offsite underdrain discharge pipe for this Scenario is not discussed in this report. 6.3.1.1 Phase 1 Mounding at the southern end of the Site boundary and in Town is not expected to rise more than two(2)feet before the construction of the Cell 4 slurry wall so mitigation is not warranted until the Cell 4 slurry wall is constructed or Site monitoring observes a higher than anticipated groundwater elevation at the southern monitoring wells(MW-5 and MW-6). See Table 4 below and Figures 8 and 9. Table 4 Base Model and Modeled Site Groundwater Level After Cell 3 Slurry Wall Modeled Cell 3,4 and 5N Well Base Slurry Wall Difference Cell IJK MW-1 4,726.9 4,726.4 -0.5 146,111,1 MW-2 4,717.8 4,716.0 -1.8 136119,1 MW-3 4,725.0 4,725.8 0.8 142136,1 MW-4 4,732.5 4,734.1 1.6 153,136,1 MW-5 4,736.7 4,736.8 0.1 162,1331 MW-6 4,738.4 4,737.2 -1.2 162,115,1 MW-7 4,729.1 4,728.2 -0.9 148,1221 BBA-MW4 4741.3 4,740.9 -0.4 148,122,2 BBA-MW11 4,737.1 4,737.5 0.4 148,122,3 BBA-MW18 4,7386 4,738.9 0.3 148,122,4 BBA-EC 4,732.5 4,732.9 0.4 148,122,5 A series of wells were modelled at the southeastern comer of the site after the addition of the Cell 4 slurry wall and the Monarch DENM Cell 1 slurry wall. These wells were set to pump 2.25 cubic feet per second(CFS)and removed the Groundwater Model Report—Section 20 Gravel Mine Site -10- August 2022