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~= ~..~ TUTTLE APPLEGATE RINDAHL, INC. '~ Consultants for Land and Resource Development Mr^. Sohn Dner^Per• Mired Land Reclamation Divisio+•~+ Scaly GG, 1988 Page 4 The lowering of the gr•~+_~ndwater table will have a min~~r• impact on the pr•i~+andwat er table upstream of the n7l ni no site. Once nrater•ial has been removed, the aquifer will attempt to reach a state of eb~.iiiibr•ium. This will mean that theta gr•o+.+ndwater^ table in the immediate vicinity of the mine will Iower• and discfiar•ge ^r•o+Indwat er at a ^r•eat er• volume tfian il: pr•ev iously has f.~r• a short period of time. however, this drawdown effect will not extend up the fail Length of then valley. Since the mining ~~aer•at it~n will take tw~~ t~~ three years for- tfie full impact of removing the sand to be felt i+•i any ~~ne area, this lower-ing 1s n.~t s~~nreth ing that is going to happen overnight. Mast mining oper•at i~~ns along the S~~+atfi Platte River- that- use wet mining techniques gener•aI Iy Lower- the +apper side of the; water table by S to B feet. T`n is creates a drawdown on the +xpstr•eam side that generally runs ~~+at within ?,O+D feet ~~f thea mining area. The Snrpacts of this project wiil be greater because ~~f the iar•ger• drop in the water table. It i~~t~ expected that the water- table will reach eq+ailibr•1+_~m within 1@+D+D feet of the minin^ site at Coai Creek. Rougf+ caiculat ions using She equ1116r i+_im equation for drawdown in an uncinf fined aeuifer• sfiow that the read i+as of infl+aence irr appr~~xintately equal t+~ the radius c+f the mining area if projected into an equivalent cir•c fie. Hggradation at tfie l~~wer• end .,f the mining site again is not a significant Pactor• in this mining plan. We are dealinc) witfi ari ephemeral stream that does not m~~ve mucfi material. Eedload is very iow since the flows ar•e generally not great; enough t+~ over•c~~me tr^active forces. Suspended load canrn~t be Large because +~f the simble Lack of total voi+ime of flooa avai labia to carry a s+_isoended l~~ad. The ~~niy time y.,u wi i I see a significant movement of material in tfiis area is:> d+a r^ing low frequency storm events. In this instance, it d~~es not matter- whether- there is a mining oper•at ion in thra area or• not, material is g~~ing to be moved. Eased on the flows we have calculated, aggravation is not g~~ing to be ai problem with this pr~~,7ect. Channel degradat iun at tfie cost ream end .~f the gr•o,]ect will be handled 6y constr+~ctinp a cfiannel drop and r•+and+~wn irate:? the mining area. This str•uct+are will be designed acc~~r•ding to Ur^ban Drainage st andar•ds. In thla manner^, we will. pr^eciude er•os i+~n and headcutt ing +_~ostr•eam •7f the mining site.