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Mr. Tom Kaldenbach April 5, 2013 Page 8 seeping water at the base of the backfilled area could be blocked as the fill settles, resulting in much of the fill becoming saturated with the seeping water. A seepage rate of 1 gpm in the Glory Hole would be sufficient to saturate the entire 56,000 cubic yards of backfilled alluvial fill in less than 2 years (assuming the fill has 15% porosity). Alluvial material is proposed to be backfilled in the Glory Hole to a height greater than 150 feet. If this material becomes saturated it could exert more than 150 feet of head on the hydraulic seal in the Steve Level at the base of the fill and exceed the seal's designed maximum pressure of 120 feet of head. A three - dimensional drawing showing the flow path of groundwater from the Glory Hole to the mine pool is shown in Figure 3 attached to this document. The Glory Hole was mined out along the Nebraska vein, Colorado vein, Kansas vein, and the Flat vein as shown in Figure 4 attached to this document. Within the Glory Hole, any infiltrating groundwater percolates downward vertically toward the underlying mine pool as shown by the blue arrows in Figure 3. Because flow is driven by gravity, the vertical component of flow is dominant. In some cases, the vertical flow may have an angular component, as water seeps down the excavated face of the underground workings (Figure 3). The Glory Hole is only one potential pathway for vertical infiltration of groundwater to the mine pool. Numerous additional veins were mined on and between the Charlie and Steve Levels (Figure 4) and several airway intake and exhaust holes were constructed between various levels of the mine. The inactive #2 Shaft extends from the Charlie Level to the Steve Level. The #2 Shaft, air holes, drawpoints, stopes, and other conduits would not be filled with the backfill that is proposed for placement in the Glory Hole and will be available as pathways for vertical seepage to groundwater in the mine pool. The volume of backfill proposed for placement into the Glory Hole (32,000 — 54,000 cubic yards ( "cyds ")) is one third to one half of the total calculated volume of underground workings above the Steve Level [95,140 cyds, including the network of drifts, shafts, winzes, and raises on and between Charlie, Minnesota, CV, LBJ, and Upper Levels]. The backfill is not expected to become saturated because these remaining mine voids will allow downward vertical percolation of groundwater, thus preventing head buildup within the backfill. The backfill itself is expected to maintain sufficiently high permeability (hydraulic conductivity) to transmit the small quantities of infiltrating precipitation that occur seasonally. The backfill will be placed in the mine without the possibility of mechanical compaction, and as a result, the backfill density will remain relatively low and permeability will remain relatively high. When excavated and dumped into the Glory Hole, the initial hydraulic conductivity is expected to be greater than the existing in situ hydraulic conductivity (K) of the alluvium and fill (5.0x10 -3 to 1.7x10 -2 cm /sec, or 14 to 48 ft/day). Although the change in K that occurs during settlement and gravity compaction may be difficult to quantify, the characteristics of the waste rock and alluvial fill material (low clay content, blocky, competent, no secondary clay alteration) will prevent significant compaction. If the hydraulic conductivity of the placed backfill decreased to one tenth the lowest measured hydraulic conductivity of the in situ alluvium and fill (from 14 ft /day to 1.4 ft/day), the backfill would be capable of transmitting 34.4 gpm,* which is significantly higher than the rate of precipitation infiltration over the backfill area. [ *Maximum possible seepage rates through the backfill to the local water table (mine pool) can be estimated using a Darcy calculation where Q = KIA. In this equation, K is hydraulic conductivity (1.4 ft/day), I is the gradient (unit vertical gradient of 1.0 ft/ft), and A is the area of the seepage face above the water