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Groundwater levels for monitoring wells in the Town of Gilcrest were also utilized in calibration of the model. The BBA
<br /> report contained well readings from multiple wells in the Town of Gilcrest measured in 2014. It was assumed that these
<br /> readings were similar to current head elevations and four(4)monitoring wells were included to get a representative
<br /> sample of the Town. Of the four(4)wells chosen, all of them were within one (1)foot of observed.
<br /> Groundwater measurements from the Drouhard Site, located north of Weld County Road 40 east of Weld Count Road 25
<br /> and west of Weld County Road 25.5,were used to calibrate the model southwest of the site (see Figure 3 for monitoring
<br /> well locations). The Drouhard Site has five(5)monitoring wells which have been monitored since 2018(see Appendix
<br /> A). Recorded measurements for May through July were averaged to correspond measurements used at the Section 20
<br /> site. Of the five(5)wells at the Drouhard Site,four(4)of them were within one(1)foot of observed. DROU-MW-01 was
<br /> modeled 1.7 feet above the observed condition. There are four(4)water wells located northwest of DROU-MW-01,
<br /> which may drawdown the water table in that area and may account for the difference in modeled and observed.
<br /> Table 2 below has the comparison between the measured and modeled groundwater levels at the Site,the Town,and
<br /> the Drouhard Site. The head contours for the baseline condition are shown on Figure 4.
<br /> Table 2
<br /> Measured and Modeled Site Groundwater Levels
<br /> Well Observed Modeled Difference Cell IJK
<br /> MW-1 4,725.40 4727.3 1.90 146,111,1
<br /> MW-2 4,718.17 4718.2 0.03 136,119,1
<br /> MW-3 4,725.07 4725.7 0.63 142,136,1
<br /> MW-4 4,732.07 4733 1 0.93 153,136,1
<br /> MW-5 4,736.43 4736.8 0.37 162,133,1
<br /> MW-6 4,738.43 4737.8 -0.63 162,115,1
<br /> MW-7 4,728.92 4729.2 0.28 148,122,1
<br /> BBA-MW4 4741 4740.8 -0.20 148,122,2
<br /> BBA-MW11 4737 4737.9 0.90 148,122,3
<br /> BBA-MW18 4738 4738.4 0.40 148,122,4
<br /> BBA-EC 4734 4734 0.00 148,122,5
<br /> DROU-MW-01 4741.5 4743.2 1.70 177,86,1
<br /> DROU-MW-02 4743 4743.8 0.80 1 176,94,1
<br /> DROU-MW-03 4747.1 4747.3 0.20 182,93,1
<br /> DROU-MW-04 4749.1 4748.5 -0.60 187,93,1
<br /> DROU-MW-05 1 4746.6 4745.7 -0.90 183,84,1
<br /> 6.2 Slurry Wall Installation Simulation
<br /> The Site is planned for a sequence of soil bentonite slurry wall lined cells for the majority of the Site.The only area not
<br /> planned to be slurry wall lined is the siltation pond in the northwest corner of the Site.The slurry walls will be constructed
<br /> in stages starting in the northeast and progressing south then west(see Figure 7 Mine Plan for phasing). In order to
<br /> model for the worst-case scenario,the model includes a simulated slurry wall around the whole section, minus the
<br /> Siltation Pond area. To make the Site a no flow area,the horizontal hydraulic conductivity of the Site was set to 1x101-7
<br /> feet/day.The area where the Western Mutual Ditch runs through the Site remained at the 820 feet/day horizontal
<br /> hydraulic conductivity.
<br /> Groundwater Model Report-Section 20 Gravel Mine Site -7- August 2022
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