2014-11-20_REVISION - M1977036 (3)

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The model was set up with two vertical layers to allow for the presence of zero - gradient boundaries within the reservoirs at various locations in the Poudre valley. The vertical division allows underflow in the lower layer (layer 2) relative to unimpeded flow across the reservoir. Groundwater elevations in the model were defined by head boundaries estimated based on regional topography, reported groundwater levels where measured, and the artificially lowered water table within the mine pit. Head boundaries assigned to Layers 1 and 2 are shown on Figures 4 and 5, respectively. • Regional Boundaries: Head boundaries at the model edges were assigned such that the water table matches regional topographic slope, with localized flow converging toward the Poudre River. Elevations are expressed as feet relative to NAVD. Topographic elevations across the model area range from 4700 in the north and west, to 4680 in the east. Head boundaries of 4685 and 4660 were assigned to the west and east model boundaries, respectively, to approximate a water table occurring at about 15 -20 feet below grade at these locations. The head boundaries along the model edges were assigned to both layers. • River Boundary: River boundary cells were assigned to match approximate groundwater elevations observed nearby onsite, and to match approximate topography further from the site. The Poudre River head boundary ranged from 4680 to 4665 from west to east. It can be seen that a portion of the river boundary is assigned to Layer 1 (Figure 4) and the eastern portion to Layer 2 (Figure 5). This split occurs where the assigned river head drops below the break between Layer 1 and Layer 2, at elevation 4658. • Mine Pit Head Boundary: Head boundary cells were assigned within the pit at elevations 4645 and 4650, as defined based on pit bottom elevation and observed groundwater elevations (Figure 1). The pit head boundaries are assigned in Layer 2 only. A soil hydraulic conductivity value of 10-2 cm/s (28 ft. /day) was assigned to the model domain. This value is typical for sand and gravel with little or no fines, as was documented in the slurry wall exploration logs. A hydraulic conductivity value of 10,000 ft. /day was assigned to the reservoirs along the Poudre, within Layer 1, to represent a zero - gradient boundary. Recharge from rainfall was assigned at 3.5 inches /year, and evaporative losses of 71 inches /year were assigned to the reservoirs. Computed baseline water table elevations were compared with measurements available for the Greeley 35th Avenue Mine property. Comparing the contours on Figure 6 with those on Figure 2, similar water table elevations and inward gradients are apparent. A general easterly gradient with convergence toward the river and 35th Avenue Mine can be seen on Figures 4 and 5.