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-:~1- purposes. The down dip limit of mining will not encroach upon the alluvium; however, the alluvium on the East side of Trout Creek. will receive seepage from the post- mining spoils, and mining could potentially lower the water table in the alluvium. The applicant has projected the magnitude of TDS loading which the alluvium will n, receive. In the West Ridge area, TDS levels in the discharge leaving the area are v,(t ,~~' expected to increase from 2122 mg/1 to 2828 mg/1. The 2122 mg/1 represents discharge v'{ under presently disturbed conditions. In the Moffat area, where mining has not yet occurred, TDS levels in the discharye will increase from 570 mg/1 to 2290 mg/1. The discharges leaving the Aloffat area and tJe st Ridge will enter the East bank. alluvium, prior to discharginy to Trout Creek.. After enteriny the Trout Creek channel, the discharges will become tremendously diluted. This has been adequately documented by the applicant, since the combined ground :eater and surface water contributions to Trout Creek from the affected areas are less than 1 percent or` the average flow in Trout Creek above the permit area. The instream TD.S levels of Trout Creek will, there- fore, not be significantly impacted. For further discussions of Trout Creek impacts, ~~ see Surface Water and the AIluvial Valley Floor sections of this document (p. ,~95 5~ The water supply users of the Trout Creef: alluvium in the adjacent area arc all ti '' located on the western bank, with the exception of one well upstream from the site. Due to the nature of the spoil water discharge to Trout Creek, the suoplies should not be adversely affected by increases in TDS levels. The Trout Creek alluvium may also be potentially c7ewatererl to a slight degree as a result of mining. This impact is not expected to be significant in Corms of ground water use in the area; however, it may have effects on the alluvial •,•a11ey floor. Fora further discussion of the potential AvF impacts and mitigation, see the AVF section of this document, The actual rock units which will be disturbed at the Edna mine do not have sufficient water yield characteristics to be considered as aquifers. Several of the units do, however, contain water which will be intercepted duriny the mining sequence. None of the units to be disturbed are being utilized as a water supply in the area. The applicant has presented sufficient quantification on the amount of water which will be intercepted during mininy. The applicant has committed to mutiny the inflow waters pumped from the oils through the sedimentation ponds. Sufficient capacity has been included in the pond design to h~u:dle the estimated pit inflow. In this respect, the application is in compliance. The major identifiable impact which the mining activities o.~ill have on the ground water regime will involve the replacement of low-permeability undisturbed rock with higher permeability spoil. This will promote vertical infiltration with the resultant formation of a perched water table aquifer along the buried pit bottoms ayainst the high wall. This con di ti en has been documented in the existing disturbed areas at the Edna mine site. The resulting impact involves the generation of spoil water high in TDS. This water can then enter into the Trout Creek alluvium. !lowe ve r, as discussed earlier under Trout Creel: alluvial impacts, the amount of yround water contributions from the affected area is small (less than 1~) relative to the total discharge of Trout Creek above the permit area. The impact, therefore, has limited significance. The duration of this impact, however, ;pill br_ for many years, as it will take many 4ears for the dissolvable solids to be fully leached from the spoils. This impact will be addressed in the review of the alluvial valley floor monitoring program.