2014-11-20_REVISION - M1977036 (3)

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Response to comment 1 — In response to Comments 1 and 2, the groundwater model was revised in part for clarification purposes, with model documentation and predictive results provided herein. Predicted results and overall findings are not significantly different than in the previous submittal. The model was revised both in response to DRMS Comment 2; and as a result of re- interpreting observed groundwater elevations in responding to DRMS Comment 1. The groundwater model (Figure 1) was created based on a conceptual model in which groundwater flow direction is influenced by topography and discharges to the Poudre River. Locally, the water table is also influenced by removal of groundwater by pumping from within the mine. Figure 2 shows groundwater contours representing conditions during a slurry wall exploratory boring program conducted in spring of 2014. GEI interprets an inward gradient toward the pit based on the elevation measurements shown on Figure 1. The inward gradient occurs both from the Poudre River north of the pit, and from the south/southeast, where topography gains elevation. The previous model did not include the pit dewatering, but instead represented an average groundwater elevation as observed in the borings surrounding the pit. The revised model was used to compute groundwater elevations representing spring of 2014 (with pit dewatering active); natural conditions (pre - mining), and predictive post -slurry wall conditions. The model objective was to predict groundwater elevations on both an absolute scale (in units of feet relative to the North American Vertical Datum (NAVD) and on a comparative scale. For the comparative evaluation, predicted post -slurry wall groundwater elevations are compared with computed baseline (pre- mining) conditions. It was necessary to estimate baseline conditions, because comparing slurry wall effects to present -day conditions would show a very large rise in water table elevation, where slurry wall effects would not be readily distinguishable from the effects of recovery from pumping. In consideration of the above, the model was revised to include present -day pumping as a means of validating the conceptual model, while the "natural conditions" and predictive model results are presented for comparison to tolerances. Model Description The numerical model was created using Visual MODFLOW and run in steady state. The horizontal and vertical extents of the model are shown on Figure 3. The horizontal extent is intended to be large enough such that effects of the slurry wall can be computed within the model space. Conceptually, the model bottom is assumed to be impervious and represents top of clayey sandstone bedrock. As a simplifying assumption, bedrock surface was assumed to be at elevation 4640 across the model.