2007-01-31_REVISION - M1986015 (2)

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M DISCUSSION: For Phase 2, a groundwater barrier wall will be installed prior [o mining. This will serve three main purposes. Firs[, i[ will tremendously reduce dewatering costs which, in Phase 1, have reached extreme levels. As it turned out, the water came into the pi[ much faster than was expected and an extraordinary amount of pumping has had to be done to keep the pit in operation. In the spring when the river is high sometimes no amount of pumping would keep the water out and it would temporarily build up. Furthermore, if there was a power outage for even a short time, the water would increase in [he bottom of the pi[ a[ an alarming rate and this would take additional time to pump out before operations could resume. Second, installing a groundwater barrier wall in Phase 2 near the start of operations will eliminate the effects of dewatering on adjacent wells. Although [he dewatering impacts are reduced, [hose are replaced with other concerns such as, in particular, potentially blocking the groundwater flow from reaching downstream locations. The groundwater barrier wall might act as a dam thereby preventing groundwater from even reaching the 25'" Lane wells and producing basically the same effect as dewatering. This potential effect would occur only if the entire flow to 25'" Lane was blocked. Monitoring wells to the north of where the groundwater barrier wall will be installed show there is around 20 fee[ of permeable sands and gravels under a thick overburden which will allow ground water to flow north of the groundwater barrier wall to supply wells along 25`" Lane. As discussed further in Exhibit G-Water Information, modeling and examination of the gravels through which [he water flows shows that constructing a groundwater barrier wall in the location shown will basically produce an island in [he "aquifer river." That is, water will continue to flow completely around [he blockage (groundwater barrier wall) and [hereby supply water [o the 25'" Lane wells through a groundwater path that goes north of the groundwater barrier wall. None of [he wells on 25'" Lane are high production wells so [he amount of water needed is not great. It is believed, based on the groundwater modeling, that [here will be sufficient flow around the groundwater barrier wall [o supply the needs of these wells. [n consideration of the other possible impacts of the barrier wall, south of the groundwater barrier wall will be the river and the broad flood plain to the south of the river. To [he west of the barrier wall, there may be some increase in groundwater elevation due [o [he groundwater barrier wall. Just as a river can often exhibit a hump immediately upstream from an island the groundwater barrier wall could have a similar effect. But the fact that the groundwater can flow completely around the groundwater barrier wall provides a means to prevent the groundwater from becoming excessively elevated [o the west. Wells further to [he west may actually see some beneSt from the groundwater barrier wall. However, wells to the west are so far from [he barrier wall (greater than 600 feet) any increase in water level will be small and unlikely [o even be noticed. Shadowing to the east is also reduced by installing the groundwater barrier wall as an island rather than a complete blockage to the flow. Pueblo East Pit Amendment 3 (2007) Exhibit D Page 18 of 34