2008-04-15_REVISION - M1980244 (325)

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30 Grassy Valley hydrologic conditions 1. The Cresson Project involves disturbing part of the upper Grassy Valley drainage (approximately 11%) as part of the Wildhorse and WHEX portions of the East Cresson surface mine. This will have the effect of reducing the recharge area to the drainage, and may cause a minor, but temporary, reduction in flow in Grassy Creek. 2. The construction of the ECOSA on the southern side of the valley may have two effects on the flow regime: • Some areas of Grassy Valley drainage upgradient of the ECOSA, which currently drain toward GV-03, may have an altered flow ,path. These areas were treated in the model such that interflow from these areas will either report to GV-02 or to the WHEX. During construction of the ECOSA, completed areas of the ECOSA will be regraded and covered with a 3-foot thick, low permeability cover and revegetated. Vegetation will take time to become established (2-5 years); therefore, for the purpose of modeling, it has been assumed that there will be no ET from this facility until the post-closure period. Also, there will be insufficient precipitation during the mining period to raise the moisture content of the stockpiled overburden materials from the starting moisture content to field capacity, so no throughflow is expected. These modeling assumptions represent the most conservative case, as these conditions would result in the maximum potential reduction in surface water flow in Grassy Valley during the mining period. 4.4.4 Conceptual model -post-mining conditions The post-mining conditions water balance model is used to evaluate the potential for Ion 9 term impacts to flows in Grassy Valley after reclamation and closure. Figure 4.10 illustrates the conceptual model for the post-mining period. The surface and groundwater flow systems described under current conditions are expected to function in much the same way after closure. The proposed MLE closure activities include: • The WHEX portion of the East Cresson Mine will be backfilled following mining. The backfill will be graded towards Grassy Valley and will be covered with growth media. The post-closure WHEX footprint will have. a higher infiltration rate to bedrock than the undisturbed WHEX footprint for two reasons. First, the coarse backfill material and placed cover material will lower water storage capabilities and thus less water will be available for ET. Second, the potential for interflow along the alluvium/bedrock interface will be eliminated by the presence of the coarse backfill material which will enhance vertical infiltration and not horizontal flow. Infiltration to the backfilled WHEX will drain toward the main diatreme. Runoff will drain to Grassy Valley. • The closed ECOSA will be constructed with a low permeability cover and revegetated. In the post-closure modeling scenario, it is assumed that the vegetation will be well established and that the low permeability cover will promote runoff and reduce infiltration. This will have the effect of enhancing runoff/interflow from this area, thereby increasing the flow in Grassy Creek relative to current conditions. Infiltration to underlying bedrock will be lower than the pre-mining condition because it will be limited by the low permeability cover. It 2736 Cripple Creek & Victor Gold Mining Company Water Management Consultants