2007-01-31_REVISION - M1986015 (2)

Home Browse Search

overflow from the river or breaching due to undercutting along the river bank. The pit wall has already been breached once and if a sediment basin had been at that location it is unlikely the river would have broken all the way into the pit itself. Second, this helps to seal the outside of the pit such that less groundwater flows into the pit from [he river. That, in turn, reduces the amount of pumping needed to maintain a more or less dry bottom in areas where mining is still occurring. And third, because only the inside of the sediment basin is sealed, water in the sediment basin can flow out into the groundwater beyond the edge of the pit. It has already been shown that with the existing sediment basin water levels in monitoring wells east of Phase 1 that were initially depressed rose back to normal levels after the sediment basin was completed. Thus, by having the sediment basins open on at least one side, the sediment basin, being filled with very fine material, acts as aslow-release groundwater reservoir. A possible arrangement of sediment basin cells is shown in Exhibit C-4A for Phase 1. Although many other arrangements are possible and the one shown on the map is not necessarily the arrangement that will be used, the principle is the same for all arrangements. That principle is that each cell must have at least one permeable side that connects to adjacent unmined land. I[ should also be apparent [hat once installed, cells [hat are no[ active sediment basins can be used to take the small amount of seepage water pumped out of Phase 2 so that water can return to [he surrounding system as well as benefit the reclamation plan for Phase 1. This is an option [o simply discharging directly to the river. Thus, Phase 1 and Phase 2 can be integrated in a multitude of pathways. Once Phase 2 is completed and becomes a reservoir [his seepage is no longer a factor to be considered in the dewatering/discharge/sediment basin circuits. At this point in time it does not appear [hat there will be an excess of the overburden for backfilling, when combined with processing fines. The volume of [he overburden is more or less a fixed and definable value. The variable is the amount of fines generated during the processing. That volume is exceedingly difficult to calculate accurately. But volume modeling shows that there should be enough material to complete Phase 1 backfilling with existing overburden, soil, and wash fines. As [he processing plant for this operation may also be used to process gravel from other sites, additional fines will be added. The issue of backfilling and the sources of backfill is more completely addressed in Exhibit E- Reclama[ion Plan. Requirement: It is expected that [he overburden in [he plan[ site will not be removed at the end of the operation, but if additional fill is needed the plant site can provide an additional source for a large amount of fill to complete backfilling. DISCUSSION: Much of the overburden present in the initial mining area in Phase 1 was consumed in the construction of the plant site. To create a plant site that was high enough above the Pueblo East Pit Amendment 3 (2007) Exhibit D Page 29 of 34