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groundwater from the model simulating return flow back to the river.Table 5 below shows the groundwater head
<br /> elevations at the monitoring well locations for this model iteration.
<br /> Table 5
<br /> Base Model and Modeled Site Groundwater Level After Cell 3 4 SW with Dewatering Wells
<br /> Well Base Modeled Cell 3 and 4 Difference Cell IJK
<br /> MW-1 4,727.1 4,727.5 0.4 146,111,1
<br /> MW-2 4,717.9 4,716.8 -1.1 136,119,1
<br /> MW-3 4,724.9 4,726.0 1.1 142,136,1
<br /> MW-4 4,732.5 4,733.6 1.1 153,136,1
<br /> MW-5 4,736.7 4,736.0 -0.7 162,133,1
<br /> MW-6 4,737.7 4,738.2 0.5 162,115,1
<br /> MW-7 4,728.9 4,730.7 1.8 148,122,1
<br /> BBA-MW4 4,740.3 4,740.5 0.2 148,122,1
<br /> BBA-MW11 4,736.9 4,737.0 0.1 148,122,1
<br /> BBA-MW17 4,734.4 4,734.5 0.1 166,164,1
<br /> BBA-MW18 4,737.3 4,737.4 0.1 148,122,1
<br /> The quantity of water required to be pumped or gravity drained away from the southeastern corner increases as
<br /> more slurry walls are constructed. After Cell 5N is constructed,the modeled pumping rate was increased to 2.5
<br /> CFS spread over three(3)wells to maintain pre-slurry wall groundwater elevations south of the Site(see Figure
<br /> 10). The modeled pumping rate was further increased to 3.25 CFS with the addition of Cell 5S.
<br /> 6.3.1.2 Phase 2
<br /> The mitigation in Phase 2 was modeled to be completed concurrently with Cell 6. This included an addition of
<br /> an underdrain in the southwest corner of the site between the Western Mutual Ditch and the Cell 6 slurry wall.
<br /> The pipe invert elevation for the underdrain was set at 4,735 feet in the east and 4,733 feet in the west.This
<br /> section of pipe was modeled to remove water from the model to simulate piped gravity flow to the river.The
<br /> modeled quantity of water that was removed by the drain was approximately 3.3 CFS (287,344 cubic feet per
<br /> day)at full buildout of both sites.
<br /> An underdrain was included at the Monarch DENM site between the Cell 3 Slurry Wall and Weld County Road
<br /> 42 to account for MMMA's contribution to mounding in the area.The pipe invert elevations were set at 4,733
<br /> feet in the east and 4,732.9 feet in the west and removed approximately three(3) CFS from the model.
<br /> Table 6 depicts the groundwater levels for the surrounding properties at the full buildout of the site and the site
<br /> layout and phasing is shown on Figure 6 and 6A.
<br /> Groundwater Model Report—Section 20 Gravel Mine Site -11- August 2022
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