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<br />.CONCLUBIONB 127
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<br />:will'provide the!greiLtest ,long-term- benefits for all. water users and
<br />. prevent future controversies. ':Thererore, it should;not ba.iniarred :from
<br />. .this report tha.t the report anML can: so;,fely, economicBlly, and legaliy
<br />:: support a large additionalnnmber of luge-capacity wslls.
<br />- Colitinued collection and study cif data is needed to determine the
<br />safe peren:nial yield of the .aquifers.in the reporh area, a.nd to assist
<br />orderly and effi.oient. development. of .the ground wa.ter of the .vu.U8Y~
<br />.fill cleposits. Such a.n...in.vestigll.tion should include (1). IEU:cintenance
<br />lof: complete- 'and accurate records on .additional large-clLpa.oity :wells
<br />: that are installed in the area; . (2) continued measurement of water
<br />:levels in'a network of..observntion.weUs; (3) system.ntic colleotion of
<br />.. discharge. and POwel'-oonsumption records for large...(lfLplLCity wel~
<br />;.a.nd.from.those'records,.computn.tion of the.total yeo.rIy volume of
<br />With&& wILls; . (4:) . eva.lua.tion of ;all. sources or recharge to n.nd. dis.-
<br />Chal'ge.:from the groundp'Water. reservoir;.. (5) qun.utitative studies of
<br />the 'relation of streo..m:llow. to ground water; (6) collection and 6ValU-
<br />lIrtion otadditionnJ. datIL on the ul1emiool quality of the water.
<br />, . The coUeeted'do.ta shoUld: be evalua.ted pe.riodienlly in order to detect
<br />current or impending overdevelopmant of the ground-water I'esources~
<br />Such dat8i and periodic review mIl make water problems easisr to solve,
<br />. and: they .may.indeed' provide the.. basis. for developn1snt of an, intew
<br />g.ra.ted . suclo.ce-. and. grouad-water. -irrigation systB6 involving the
<br />utilizu,tion of artifi.eial recharge lll!ld diseha.rge as 0. .meDJis of balancing
<br />..t.he:efIects of wet lmd dry years.
<br />. Wo.tar fnom the, va1ley;..:flll deposits in .most. of .the: report. 11.1:80. has
<br />.111 ~peciHcconductDJlce of J...,OOOr.:.l,BOOmicromhos'pe1." om and is.of either
<br />.the. co.lmum -bico.rbonate or the'caleium. sulfate type.. The water is
<br />. suito.ble:ior ir.riglLtion in most'placeS but:hn.s a.bigh or very high sa.lin-
<br />. ity. hazard,~d in some places hns:a' high lea.ah.i.ng requirement. It is
<br />of:poor,qumity :Iol".publlQ;.supply~and domestio uae.beoouse it is very
<br />,hn:r& .In. 0.' few.section.!l in. T..g S.; R.. 6'1 ;W., near. Derby" the. water
<br />'.has u. speoific eonductlLllOO of more than. 4tQOO micromhos per em .lmd
<br />.is of the calciwn.chlomde 'type, a..type found.nowhere.~elee in the
<br />report area. The' high. specific oondootance 'l1Jld the water typa in-
<br />. dictate contamination of the~ground. water 'by.. waste from the .D.t'SGD.o..l.
<br />. In: the downvalley. parts of the .South .Pl-a.tt.e River valley:.nnd of most
<br />of ' the tributary. vaEeys, the water has a, specific. conductance of about
<br />2,250 mim:omhos'per cm l1Jld is of the calcium sulfate or sodium.suHl1.te
<br />.type; Near Denver ground water in the Sand Creek and.South Platte
<br />. River valleys is"conmminated by petrole:e.m. wastes, but the severity
<br />nnd. nreahment .of the contamina.tion were not. determined.
<br />. The'quality.of the wn.t.er.in the valley-:..fill deposits in the xeport.aren.
<br />is similar. to the'quality of thBwuter in the surface streams and is a:f-
<br />. .meted: by irrigation pmctices. . U Be of the. wa.ter. for'irriga.tion tends
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<br />126 GROUND W ATERI SOUTH PLA.TI'E RIVEa 'BASIN
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<br />mus~ be less th!1J1 ground-water discharge because not 0.11 the natural
<br />discharge co.n be intercepted, and beca.use of limiting hydrologic and
<br />legal' fa.ctors.
<br />The average amount of water than ca.n be pumped from the valley-
<br />fill deposits over a long period without exoeeding the rlLte of recharge
<br />and permanently depleting the supply in storage n.nd thus progres-
<br />sively lowering the water table should be del:ermined separately for
<br />eaoh district. This amount is the safe perennial yiald. Its determino.~
<br />tion Nquires records for several years of pumpo.ge, water-level
<br />changes, precipitation, runoff, and lLD. lLIlnlytico.l appraisal of these
<br />data. The hydrologic :features of the report area are: complext o.nd
<br />the Elfi,fa yield can be determined only by further st.udy and systematic
<br />observations while ILdditional de-velopmen.t proceeds. .
<br />As new w.ells are brought :into production, Wo.teT levels will decline,
<br />but such 'lIr decline alone is not necessa.rily proof of overdevelopment.
<br />Some 'Water-level decline is .inevitable if Il. IlLrge amount of 'WELter.
<br />is wen from the ground-water reservoir. The criticn1 point, howw
<br />ever, is whether or not the Witter levels recover n.fter the pumping
<br />seo.son, or -o.fter se;eral pumping seasons dUl'ing 0. series of dry yeal'S,
<br />In a natura.l ground-water regiment discharge is ffilltinly equal to re.-
<br />charge; that is, the system. is in dynamic equilibrium. In an o.rti-
<br />ficiaUy altered regimen, such iLS the report area, ilischo.rge' will conw
<br />tinue to equal rooharge only so long as the safe perennial yield is
<br />not ex:caeded. .
<br />:Much of the ground warer discharged iTom 'Wells in a.reos wllere
<br />the water table is at or near the land surface is snlvaged water, be~
<br />cause lowering the water table reduces natural dischs,rga by eva.po-
<br />transpiration. Also, a.rtificia.l lowering of the water table crao.tes
<br />space for storo.ge of additional racho.rge. The ada, pel'ennio.1 yield
<br />is Dot being exceeded in such iLreaS. In other areas, however, large-
<br />sonJa pumping mo.y deplete. ground wo.terin storage. and reduce the
<br />natural dischn.rge into streams to such an extent that surfn.ce-watel'
<br />rights a.re infringed upon and wo.ter levels in wells drop below the
<br />intake of pumps. Mosi; of the lo.rge,~Cftpacity wells in the project
<br />o.reo. reach, or nearly reach bedrock; therefore, the toto.! depth of the
<br />wall is the ma.ximum depth to which the WlLter table and the intake
<br />pipe ca.n be lowered.
<br />Two importllJlt factors a.re involved in tJ.t6 concept of SIl.Ie perannial
<br />yield, one being the hydrologic factors just disnussed and the other
<br />being the legal :fn.ctor. .Although, strictly speaking, the maximum
<br />possible withdrawal of ground water from IL district is limited' by hy~
<br />drologic factors n.lone, the maximum aetuol withdrawal may be limited
<br />by legnJ fnetors. It may be decided by competent legal authority to
<br />restrict ground-water development to 11 yen.rly withdrawal rn.te that
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