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Table 5
<br /> Base Model and Modeled Site Groundwater Level After Cell 3 4 5N and 6 Slurry Walls
<br /> Modeled Cell 3,4,5N and
<br /> Well Base 6 Difference Cell IJK
<br /> MW-1 4,727.3 4,721.6 -5.7 146,111,1
<br /> MW-2 4,718.2 4,712.4 -5.8 136,119,1
<br /> MW-3 4,725.7 4,727.8 2.1 142,136,1
<br /> MW-4 4,733.0 4,737.3 4.3 153,136,1
<br /> MW-5 4,736.8 4,740.5 3.7 162,133,1
<br /> MW-6 4,737.8 4,743.4 5.6 162,115,1
<br /> MW-7 4,729.2 4,717.9 -11.3 148,122,1
<br /> BBA-MW4 4,740.8 4,743.1 2.3 148,122,2
<br /> BBA-MW11 4,737.9 4,740.0 2.1 148,122,3
<br /> BBA-MW18 4,738.4 4,740.5 2.1 148,122,4
<br /> BBA-EC 4,734.0 4,735.7 1.7 148,122,5
<br /> To mitigate the groundwater mounding effects south of the Site and in Town, an underdrain is recommended to be
<br /> installed after the installation of the Cell 5N slurry wall. The mitigation scenario was modeled as a drain south of the Site
<br /> running from the eastern comer of the Site to the southern midsection line east of where the Western Mutual Ditch
<br /> crosses on to the property. The drain was set approximately 3 feet below the base groundwater elevation at an elevation
<br /> of 4734 feet in the west and 4732.4 feet at the eastern terminus. The amount of water that would need to be diverted to
<br /> keep the groundwater head elevations in the Town the same as base conditions is dependent on whether Cell 5S is
<br /> slurry wall lined or not.
<br /> 6.3.1 Cell 5S Slurry Wall Mitigation Scenarios
<br /> Cell 5S is located in the southeast corner of the Site and is between the Western Mutual Ditch to the north and the
<br /> Farmers Independent Ditch to the south. The cell is situated on a higher terrace than the area to the north of the
<br /> Western Mutual Ditch. Bedrock for most of the Site is approximately 50 feet below surface except for the Cell 5S area
<br /> where bedrock drops to approximately 95 feet below surface in the southeast corner of the cell. In order to keep
<br /> groundwater head elevations in Town at baseline conditions,the amount of water diverted by the underdrain or
<br /> dewatering wells almost doubles if the cell is modeled as a slurry wall lined cell as shown in Table 6 below. In both
<br /> conditions, a deep underdrain or a series of wells was able to effectively mitigate the mounding at the Site and the Town.
<br /> Table 6
<br /> Water Diversion N cessary With or Without Cell 5S Slurry Wall
<br /> Modeled MODFLOW Flow Budget Water Diverted Through Difference From Baseline Head at
<br /> Condition ft^3/d Underdrain/Wells CFS Gilcrest HS
<br /> With Cell 5S
<br /> Slurry Wall 495,997 5.7 CFS 0.0'
<br /> Without Cell 5S
<br /> Slurry Wall 255,798 3.0 CFS 0.0'
<br /> Groundwater Model Report—Section 20 Gravel Mine Site -10- August 2022
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