2015-01-12_REVISION - M1985112 (9)

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Mr. J.C. York July 13, 2015 Table 2 - Steady State Mass Balance Prior io Loloff Pit . Parameter Inflows (cfs) Outflows (cfs) In - Out (cfs) River 2.2 3.6 -1.4 Aquifer Subflow 5.6 4.2 1.4 Tota I 7.8 7.8 0.0 Page 5 The net river gain of approximately 0.7 cfs per mile appears reasonable. We do not believe that the pit slurry wall would significantly change the overall timing and location of historical river gains. Model Runs We conducted two model runs to evaluate the hydrologic effects of installing a slurry wall around the Loloff Pit. Run Lol SS6 was meant to simulate the water table before the water table was disturbed by pit excavation. Figure 1 shows the resulting water table gradient where it crosses the Poudre River and through the outline of the future pit area and slurry wall. In run Lol_SS7, the model cells within the pit area are turned off to not allow aquifer flow through the pit which simulates the effect of proposed slurry wall. Figure 2 is the contoured difference between the Post -Pit Run (Lol_SS7) — the Pre -Pit Run (Lol SS6). Positive values on the left side of the pit reflect mounding and negative values on the right side reflect lower water levels in the "shadow" of the pit. The maximum change in water levels is +1- 2 feet. Model Sensitivity The model results are insensitive to differences in hydraulic conductivities (K) since mound height is inversely proportional to K and aquifer inflow is directly proportional to K. Therefore, since the aquifer gradient and thickness are constant, an increase in K will cause a proportional increase in model inflows which would increase mound height proportionally, but this does not occur because the higher K causes a proportional decline in mound build-up. Model results are insensitive to streambed leakance since the streambed is very permeable so large amounts of water can move in and out of the stream in response to up -gradient mounding and the down -gradient shadow effect. The model is sensitive to the area of the slurry wall, but this is a fixed parameter. Therefore, the model is relatively insensitive to input parameters and boundary conditions. We believe however that the model results in Table 1 are sensitive to: 1) the relative location of wells and vulnerable structures such as basements to the up -gradient side of the pit; 2) to the accuracy of reported predevelopment water level depths; and 3) to the timing, location, and magnitude of various types of recharge such as precipitation and canal recharge. Model Results, Uncertainty, Mitigation, and Recommendations Results, uncertainty, mitigation, and recommendations are discussed earlier in the report.