2008-04-15_REVISION - M1980244 (325)

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Grassy Valley hydrologic conditions 37 Table 4.5 Base case water balance flow results summary Current Conditi ons Mining Conditi ons Post-mining Con ditions Inches of precip Inches of precip to Inches of precip Flowpath to Grassy Valley original Grassy to original Grassy catchment gpm Valley catchment gpm Valley catchment gpm Precipitation onto areas underlain by granite 8.4 580 7.7 507 7.7 507 ET from areas underlain by granite 6.9 453 6.0 397 6.0 397 Infiltration to granite 1.3 87 1.2 76 1.2 76 Precipitation onto areas underlain by diatreme 9.7 669 6.4 421 6.4 421 ET from areas underlain by diatreme 7.1 468 4.5 294 4.5 294 Infiltration to diatreme 3.0 201 1.9 126 1.9 126 Precipitation onto riparian areas 0.8 51 0.8 51 0.8 51 Evaporation from riparian areas 0.8 51 0.8 51 0.8 51 Precipitation onto ECOSA n/a n/a 2.7 177 2.7 177 ET from ECOSA n/a n/a n/a n/a 2.1 137 Infiltration to ECOSA n/a n/a n/a n/a 0.3 17 Runoff from ECOSA n/a n/a n/a n/a 0.4 24 Flow through alluvium at GV-03 <0.1 2 <0.1 2 <0.1 2 Surface water flow at GV-03 0.6 37 0.5 31 0.9 57 4.4.7 Predictions for mining conditions The input data and calibration values presented in the preceding sections were used in the model to simulate flows in Grassy Valley drainage during mining. The model results indicate that reductions in flow to Grassy Creek are expected as a result of the drainage area reduction (WHEX) and ECOSA construction during mining. The simulated average annual flow in Grassy Creek at GV-03 during miniing is estimated to be about 31 gpm, which represents a reduction of approximately 9 gpm relative to current conditions. Simulated average monthly flow rates are illustrated in Figure 41.12, together with current average monthly flow rates (and post-mining flow rates). The largest reduction in flow is expected during the summer months. Flow reductions are expected to occur relatively uniformly along the stream length because of the distribution of miining features and drainage patterns. Base case model results for the mining conditions are summarized in Table 4.5. Proposed mitigation activities by CC&V, as described in Section 5.4, will be managed so as to maintain the riparian areas in Grassy Valley during the mining period. Flow rates in Grassy Creek and piezometer water levels in the riparian areas will be moinitored during the proposed activities. 4.4.8 Predictions for post-mining conditions The input data and calibration values presented above were used in the rnodel to simulate post-closure conditions. Post-closure, the WHEX will be backfilled and graded to drain 2736 Cripple Creek & Victor Gold Mining Company Wafer Management Consultants