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

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decreased pumping rates or their wells going dry. This is not anticipated because the modeled saturated thickness of the alluvial deposit is still eighteen (18)to twenty(20)feet at full buildout of both sites. The groundwater table to the south of the Site and underlying the Town of Gilcrest is anticipated to rise as a result of the full buildout of the slurry walls at the Site and the Monarch DENM Site. Mitigation for mounding is warranted at the southern end of the Site. The mounding is expected to be greatest towards the southwest comer of the Site and decreases to the east at full buildout. Mounding of more than two(2)feet at the south site perimeter, monitoring wells 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 south site boundary. Mitigation was modelled in two(2)phases coinciding with the construction schedule of the Site and the Monarch DENM slurry 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 Phase 1 Mitigation 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 47269 4,726.4 -0.5 146,111,1 MW-2 4,7178 4,716.0 -1.8 136,119,1 MW-3 4,725.0 4,725.8 0.8 142136,1 MW-4 4,732.5 4,734.1 1.6 153136,1 MW-5 4,736.7 4,736.8 0.1 162133,1 MW-6 4,738.4 4,737.2 -1.2 162115,1 MW-7 4,729.1 4,7282 -0.9 148122,1 BBA-MW4 4,741.3 4,740.9 -0.4 148,122,2 BBA-MW11 4,737.1 4,737.5 0.4 148122,3 BBA-MW18 4,7386 4,7389 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 corner 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 from the model simulating return flow back to the river.Table 5 below shows the groundwater head elevations at the monitoring well locations for this model iteration. Groundwater Model Report—Section 20 Gravel Mine Site -10- August 2022