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

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Groundwater levels for monitoring wells in the Town of Gilcrest were also utilized in calibration of the model. The BBA report contained well readings from multiple wells in the Town of Gilcrest measured in 2014. It was assumed that these readings were similar to current head elevations and four(4)monitoring wells were included to get a representative sample of the Town. Of the four(4)wells chosen,three (3)of them were within 0.5 feet of observed. BBA-EC, northeast of town on the eastern side of Highway 85 was 1.5 feet lower than the observed elevation head. The monitoring wells included in the BBA report were recorded in 1 foot intervals,which could account for the 1.5 feet difference.Table 2 below has the comparison between the measured and modeled groundwater levels at the Site. The head contours for the baseline condition are shown on Figure 4. Table 2 Measured and Modeled Site Groundwater Levels Well Observed Modeled Difference Cell IJK MW-1 4,725.40 4727.1 1.70 146,111,1 MW-2 4,718.17 4717.9 -0.27 136,119,1 MW-3 4,725.07 4724.9 -0.17 142,136,1 MW-4 4,732.07 4732.5 1 0.43 153,136,1 MW-5 4,736.43 4736.7 0.27 162,133,1 MW-6 4,738.43 4737.7 -0.73 162,115,1 MW-7 4,728.92 4728.9 -0.02 148,122,1 BBA-MW4 4741 4740.3 -0.70 148,122,1 BBA-MW11 4737 4736.9 -0.10 148,122,1 BBA-MW17 4735 4734.4 -0.60 166,164,1 BBA-MW18 4738 4737.25 -0.75 148,122,1 DROU-MW-01 4741.5 4743.4 1.90 177,86,1 DROU-MW-02 4743 4744 1.00 176,94,1 DROU-MW-03 4747.1 4747.3 0.20 182,93,1 DROU-MW-04 4749.1 4750 0.90 187,93,1 DROU-MW-05 4746.6 4746.8 0.20 1 183,84,1 6.2 Slurry Wall Installation Simulation The Site is planned for a sequence of soil bentonite slung wall lined cells encompassing the majority of the Site.The only area not planned to be slung wall lined is the siltation pond in the northwest corner of the Site and a potential recharge pond north of Cell 7.The slurry walls will be constructed in stages starting in the northeast and progressing south then west(see attached Mine Plan for phasing). In order to model for the worst-case scenario,the model includes a simulated slurry wall around the whole section, minus the Siltation Pond area. To make the Site a no flow area,the horizontal hydraulic conductivity of the Site was set to zero(0)feet/day.The area where the Western Mutual Ditch runs through the Site remained at the 805 feet/day horizontal hydraulic conductivity. The mounding and shadowing for the surrounding properties and wells is described in Table 3 below: Groundwater Model Report-Section 20 Gravel Mine Site -7- August 2022