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Model Calibration <br /> The numerical model was calibrated by matching to existing hydrogeologic and potentiometric <br /> conditions in the Little Turkey Creek watershed and bounded by water balance considerations <br /> with respect to groundwater recharge to, and discharge from,the watershed. <br /> The calibration of the model confirmed that the fractured rocks south of Little Turkey Creek in <br /> the south ridge area, require a very low hydraulic conductivity to be able to match the observed <br /> potentiometric distribution in the vicinity of the HRR Quarry given that net annual recharge to the <br /> groundwater system cannot exceed a reasonable percentage of the annual precipitation. A low <br /> bulk hydraulic conductivity was separately calculated, as described in Section 3.5 of the main <br /> report text, based on observed stream flow increase in Little Turkey Creek due to groundwater <br /> discharge from the south ridge. <br /> Two separate analyses, one based on a water balance approach focused on recharge constraints, <br /> and the other based on a stream flow survey in a reach that potentiometric data show to be a <br /> groundwater discharge area, both indicate that the bulk transmissivity (and associated bulk <br /> hydraulic conductivity) within the fractured granodiorite rocks of the south ridge is very low. <br /> The calibrated model indicated bulk transmissivity values for the fractured granodiorite in the <br /> flanks of the valley to be in the range of 1.6 to 16 ftz/day (1.8 x 10-5 to 1.8 x 104 ftz/sec), in good <br /> agreement with values calculated from evaluation of stream baseflow described in Section 3.5.1 <br /> of the report main text. <br /> Model calibration is assumed to be achieved once the model reasonably simulates the interpreted <br /> ground water flow conditions within the area of interest using input values that are within the <br /> range of measured or estimated values. The steady-state calibration of modeled potentiometric <br /> heads to observed potentiometric heads at the seven monitoring facilities installed at the HRR <br /> property is shown in Figure E 4. The overall model mass balance error for the model was less <br /> than 1%. The low mass balance error is indicative of numerical stability and adequate <br /> convergence criteria. The convergence criteria were typically in the range of 1 to 2 feet of head. <br /> Use of the Model to Project Mining Impacts to the Hydrogeologic System <br /> The calibrated model was then used to generate projections of groundwater inflow into the mine <br /> over the course of the 40-year mining operation. The calibrated steady-state heads were used as <br /> the "pre-mining" potentiometric condition. Mining was simulated using the "drain" function in <br /> MODFLOW. Drain nodes allow flow to be extracted from the model at a specified head—in this <br /> case the base of mining, during a specified period of time. The timing of drain nodes was based <br /> on the mining plan which occurs in five phases of mining over a 40-year mining period. The <br /> reclaimed, final configuration of the quarry was also simulated with drain nodes <br /> TRANSIT MIX-HITCH RACK RANCH <br /> HYDROGEOLOGY AND IMPACT ANALYSIS <br /> E-9 <br />