Laserfiche WebLink
Model Results <br />The model was first run to establish baseline conditions for comparison to the slurry wall <br />predictive model. Baseline conditions represent an estimate of the pre - mining static water table. <br />To compute the baseline condition water table, the head boundary representing mine pit <br />dewatering was removed from the model. The computed baseline water table is shown on Figure <br />7. <br />The slurry wall was simulated by assigning a no -flow boundary to the entirety of the 35`h Avenue <br />Mine. The no -flow condition is based on the assumption that the slurry wall will isolate <br />surrounding groundwater from anything occurring within the slurry wall. Figure 8 shows the <br />location of the no -flow boundary and the steady state water table predicted to form after slurry <br />wall completion. Figure 9 shows drawdown contours, representing the predicted post -slurry wall <br />water table elevation minus the baseline elevation. <br />Response to comment 2 — <br />GEI agrees with Comment 42. The zero - gradient boundaries were approximated in the May <br />2014 submittal, as cells in the numeric grid with high assigned values of hydraulic conductivity <br />(1,000 ft. /day). Conductivities used in the May 2014 submittal produced a relatively flat <br />gradient, but not completely flat (zero gradient) as the Division points out. High hydraulic <br />conductivities can represent behavior across open water, provided the hydraulic conductivities <br />assigned to the zero - gradient boundary are significantly higher than the values assigned to <br />surrounding soil. GEI re -ran the model with a much higher hydraulic conductivity (10,000 <br />ft. /day) assigned to represent the open water. The revised figures represent the revised model <br />results, where it can be seen that there is no computed head drop across these boundaries. With <br />the revised zero - gradient boundary, the revised (November 2014) groundwater head predictions <br />and drawdown trends remain similar to those submitted in May 2014. <br />Response to comment 3 — <br />The predicted drawdown contours were calculated by subtracting the slurry wall conditions from <br />the baseline conditions. We interpret the MODFLOW results to be consistent with conceptual <br />flow, where the mine is naturally upgradient from regional flow converging northeasterly toward <br />the Poudre River. The drawdown contours south of the enclosure have negative values, <br />indicating that the predicted water table is higher (mounded) relative to baseline conditions. <br />Shadowing occurs east of the enclosure due to partial cutoff of an easterly flow component of <br />regional flow. The model predicts mounding of up to 1.5 feet along the southern portion of the <br />West Pit border, as this section is up gradient from the from the Poudre River. As can be <br />inferred from the predicted contours in Figure 8, regional groundwater is predicted generally <br />