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36 Grassy Valley hydrologic conditions <br />. Because of the uncertainty associated with the estimated average hydraulic conductivity of <br />the alluvium and the estimated effective area of flow, these parameters were evaluated in the <br />sensitivity analysis (Section 4.4.8). <br />Flvw at GV-03: <br />Flow at GV-03 was calculated as: <br />The total flow toward Grassy Creek -the amount of water calculated to be flowing <br />through the underlying alluvial groundwater system <br />Runoff from ECOSA: <br />Runoff from the ECOSA is calculated as: <br />(rainfall + snowmelt) x area of the ECOSA X 13.5 percent <br />4.4.6 Calibration to current conditions <br />The average annual precipitation onto the Grassy Valley drainage is 18.9 inches, which <br />would be equivalent to an average annual water budget of 2,097 acre-feet or an equivalent <br />flow of approximately 1,300 gpm. The ET and snow melt values used in the calibration are <br />presented in Tables 4.2 and 4.3. The estimated average annual ET based on the calibration <br />is about 15.0 inches per year. This ET rate is used in the subsequent base .case simulations <br />of mining and post-mining conditions. The average annual infiltration rate to bedrock is <br />estimated to be 200 gpm into the diatreme bedrock (30 percent of precipitation), and 87 gpm <br />into granite bedrock (15 percent of precipitation). <br />The base case represents what is believed to be the most likely result based on the most <br />likely parameter estimates. The model was calibrated by simulating average monthly flow at <br />GV-03 by modifying ET and snow melt rate values. Figure 4.11 illustrates the calibration of <br />simulated and observed average monthly flow rates at GV-03. A reasonable calibration to <br />the observed values was achieved. The simulated average annual flow at GV-03 is <br />approximately 37 gpm, which compares well with the measured average annual flow of <br />approximately 40 gpm. Base case model results for current conditions are summarized in <br />Table 4.5. <br />The predictions of mining and post-mining conditions (Sections 4.4.7 and 4.4.8) reflect these <br />base case results. Because there is uncertainty associated with some of the input data in <br />the modeling, some parameters were selected for sensitivity analysis, as presented Section <br />4.4.9. <br />[7 <br />2736 Cripple Creek & Victor Gold Mining Company <br />Water Management Consultants <br />