1995-10-02_REVISION - M1977493 (7)

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Tenmile Drainage Ground Water Pathways Tenmile Drainage This technical attachment provides an assessment of the ground water pathways in the Tenmile drainage below Mayflower Tailing Pond. This assessment was conducted to evaluate whether the existing ground water monitoring wells, GWM -1 and GWM -2 are positioned to intercept potential seepage from the Mayflower Tailing Pond. A cross - section through the Tenmile drainage is presented in Figure 1. This figure shows the location of the Tenmile Tailing and Mayflower Tailing Impoundments, the geologic formations and units which underlay the tailing ponds, local structural geology, and the location of monitoring wells GWM -1 and GWM -2. As shown in Figure 1, GWM -1 and GWM -2 are located approximately 2,500 to 6,000 feet, respectively, downgradient of the Mayflower Tailing Impoundment. Both wells were completed in the alluvium. The headwater of Tenmile Creek begins below the Mayflower Tailing Impoundment (5 -Dam), at Outfall 001 after the Climax Treatment discharge combines with surface water from interceptor ditches. Both GWM -1 and GWM -2 are located adjacent to Tenmile Creek. Ground Water Pathways Ground water pathways within the Tenmile drainage will be governed by the hydraulic properties and continuity of the hydrostratigraphic units in which flow is occurring. As shown in Figure 1, the Mayflower Tailing Impoundment is primarily sited on alluvial deposits within the Tenmile drainage. The alluvial materials consist of coarse sands, cobbles, and gravels. A limited portion of the tailing pond is also sited on the Minturn Formation. The Mayflower starter dam was constructed of coarse tails founded directly on the alluvial deposits within the drainage. Toe drains were incorporated in the design to enhance tailing drainage and dam stability. Based on the construction of the Mayflower Tailing Impoundment, ground water which may exit the pond would either directly enter the alluvium and /or the Minturn Formation. Field data of in -situ permeability tests conducted in the alluvium (Appendix A) shows the hydraulic conductivity for these materials ranges from 8.5 x 10 to 2.5 x 10 centimeters per second (cm/sec). Packer pressure tests conducted in the Minturn Formation (Appendix A) indicate a hydraulic conductivity ranging from 5.7 x 10" cm/sec to being impermeable at depth. Therefore, at a minimum, the Minturn Formation is one- to two - orders of magnitude less permeable than the alluvium. At depth, the Minturn Formation is considerably less permeable than the alluvium.