1995-10-02_REVISION - M1977493 (7)

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Downgradient of the Mayflower Tailing Impoundment, all geologic formations (with the exception of the alluvium) are intersected by a large intrusive, the Humbug Stock. Based on lithologic descriptions from coreholes 499, 1201, and 1251, the Humbug Stock is characterized as a quartz monzonite material. The stock is described as being highly fractured, mineralized and intensely argillized. Although ground water data are unavailable from these coreholes, the core log descriptions indicate the majority of the fractures are either healed or argillized and clay filled. Packer tests conducted in the Humbug Formation (Appendix A) indicate the hydraulic conductivity of the stock ranges from 1.7 to 5.2 x 10 " cm/sec. Therefore, the Humbug Stock is at least one - order of magnitude less permeable than the Minturn Formation and over two - orders of magnitude less permeable than the alluvium. In terms of the hydrogeologic environment, the Humbug Stock represents a discontinuity in the hydrostratigraphy and ground water flow. Not only does the stock intersect the other formations at a steep angle, the stock also has a lower permeability. Additionally, drill core data indicates the surrounding sedimentary units have been metamorphosed, thus changing their hydraulic properties. On average, metamorphic rocks have a lower permeability range than their sedimentary counterpart (Freeze and Cherry, 1979), therefore, it is likely that a trend of decreasing permeability is also present as the Humbug Stock is approached. The presence of the Humbug Stock and the Precambrian crystalline basement rock (Figure 1) effectively forms a "low permeability" feature within the Tenmile drainage subsurface, which would tend to force ground water present in the Minturn Formation and associated units to discharge toward the surface. Ground water discharging from deeper units toward the surface would first enter the permeable alluvium which provides a continuous pathway over the Humbug Stock and down the Tenmile drainage. The locations of the monitoring wells GWM -1 and GWM -2 presented in Figure 1 shows that these wells are located near the downstream end of the Humbug Stock within the alluvium. Based on the ground water pathway data previously presented, it is apparent that ground water exiting deeper hydrostratigraphic units and entering the alluvium would be intercepted by these wells. Therefore, GWM -1 and GWM -2 are positioned to adequately monitor ground water emanating from the Mayflower Tailing Impoundment. Conclusions The primary ground water pathway from the Mayflower Tailing Impoundment into the Tenmile drainage is through alluvium and Minturn Formation. The alluvium pathway is a permeable continuous pathway and is likely the primary pathway into the Tenmile drainage. The Minturn Formation pathway, while viable, is not continuous. The Humbug Stock which intersects the Minturn Formation presents a discontinuity in the hydrostratigraphic unit. In addition, the Minturn Formation has been metamorphosed in the areas adjacent to the Humbug Stock, thereby changing the hydraulic