2008-04-15_REVISION - M1980244 (309)

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• Valley that are screened in shallow, alluvial/colluvial materials. These wells range in depth from 30 to 105 feet bgs, and the thickness of the alluvial/colluvial materials (interpreted as the bottom of the screened interval in the shallow wells) ranged from 10 to 25 feet. Water levels in the alluvial wells do not exhibit significant seasonal variation. There are 22 wells in Grassy Valley that are screened in deep bedrock: 6 wells are completed in diatreme bedrock, and 16 in the surrounding Precambrian granite. The depths of these wells range from 185 to 820 feet bgs (9,547 - 10,066 ft amsl), and screened intervals range from 20 to 110 feet. Water levels in the diatreme show some seasonal variation indicating that some recharge is occurring in this bedrock. Seasonal variations in water levels are much more minor in granite wells, indicating less recharge in the granite than in the rocks within the diatreme. Bedrock water levels in Grassy Valley range from approximately 9,728 ft amsl (in the granite) to 10,148 ft amsl (in the granite). The Grassy Valley feeder of the diatreme is separated from the main diatreme body by granite zones and northwest-trending, sub-vertical faults. The Grassy Valley feeder of the diatreme is believed to be less transmissive than the main diatreme, likely due to less fracturing and faulting of the rock mass during and after emplacement. Figure 4.8 of Appendix 2 illustrates the location of the Grassy Valley feeder of the diatreme and granite zones between the feeder and the main diatreme lobe relative to the Grassy Valley catchment boundary. Water quality was summarized for seven wells in the Grassy Valley area. The oldest well was installed in 1994 and the most recent well was installed in 1997. Overall, the ground water in Grassy Valley is characterized as calcium - magnesium sulfate-bicarbonate type waters Average ground water field pH measurements range between 5.9 and 7.7, with the slightly acidic pH readings in the deeper bedrock wells GVMW-2A and GVMW-5A. Average sulfate concentrations in the ground water samples ranged from 9 to 311 mg/1. Average nitrate concentrations ranged from 0.1 to 10.7 mg/1, with the higher concentrations detected in the deep bedrock wells GVMW-2A and GVMW-5A, and the shallow well GVMW-2B. 4.6.3 Summary Due to the volcanic diatreme in the District and the historic drainage tunnels, the area is unsaturated and does not contain significant quantities of ground water. Present mining activity by CC&V has not encountered ground water other than local perched water tables that tend to contain a limited amount of water. Deep • Cripple Creek & Victor Gold Mining Company Cresson Project Mine Life Extension 4-15