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CALIBRATION <br />The model is simple and homogeneous, containing heads that are well constrained by measured <br />values for boundary conditions as well as a reasonable estimate of hydraulic conductivity. The <br />calibration targets used for the pre-mining condition steady state model illustrate that the input <br />groundwater heads are generally within three feet of measured heads. Although the simulated <br />heads are mostly higher than field observations, Well THR-10 showed a residual of less than a <br />foot. <br />The mass balance reported by MODFLOW for the steady state pre-mining model is as follows: <br />Inflows 332,980.89 cfd <br />Outflows = 332,986.86 cfd <br />Difference = -5.97 cfd (-0.0018 percent) <br />This illustrates that the initial steady state model is accurately solved. <br />Because the pre-mining groundwater table represents data from 1966 and 2017, and the mass <br />balance is accurate, this suggests the model is sufficiently calibrated to be used for predicting <br />water levels relative to pre-mining conditions after emplacement of the slurry walls and <br />backfilled pit. <br />PREDICTIVE SIMULATIONS <br />Using the steady state model for pre-mining conditions as the base model, predictive simulations <br />were performed for the following conditions: <br />1. Unmitigated groundwater mounding for the reclaimed condition. <br />2. Mitigated groundwater mounding with an underdrain along the property boundary. <br />Predicted Unmitigated Groundwater Mounding <br />Groundwater mounding was predicted for the reclaimed condition representing the proposed the <br />slurry wall and the pit backfilled with wash fines. To understand the magnitude and extent of <br />potential groundwater mounding upgradient of the slurry wall and backfilled pit, a steady state <br />simulation including these reclamation conditions was performed. The pre-mining model was <br />changed by: <br />1. Inputting the slurry wall as a no-flow boundary. <br />2. Inputting a zone with a hydraulic conductivity value K=1x10-6 cm/s (3.0x10-3 fpd) for the <br />pit backfilled with wash fines. An anisotropy ratio of 0.1 (Kv/Kr) was used for this zone <br />of hydraulic conductivity. <br />All other aquifer parameters and boundary conditions were unchanged. Initial heads were the <br />model simulated heads from the pre-mining steady state model so that mounding would be <br />E-5