Introduction

Study Area

The Amargosa Desert  is in the southern portion of Nye County in south central Nevada, within the GreatBasin, and is part of the Death Valley ground water basin. The ephemeral Amargosa River   begins in theasis Valley, turns southeast to cross through the length of the !margosa Desert, then continues south until it

bends west, and finally enters Death Valley from the southeast.  Yucca Mountain, north of the !margosaDesert, is a group of north"trending bloc#"faulted ridges of volcanic roc#s $ash"flow and ash"fall tuffs% [2].

&ucca 'ountain has been chosen as the site of a high"level nuclear waste repository e(pected to holdappro(imately )),*** metric tons of radioactive waste. Fortymile Wash  is an ephemeral drainage thatoriginates in the uplands between Timber 'ountain and +hoshone 'ountain, it flows southward along the east

of &ucca 'ountain, and fans out in the northern part of the !margosa Desert before reaching the !margosaiver. Highay !" fault is named so as it lies below and appro(imately along the highway. -urthermore, a

dee# car$onate a%ui&er  that is locally up to /** meters $m% thic# ['] and composed mainly of 0aleo1oiclimestones and dolomites [(], underlies most of the tuff volcanic roc#s and alluvial fill ["]. The #resent climate 

in the !margosa Desert region is considered arid to semiarid, with average annual precipitation ranging fromless than 23* millimeters $mm% at lower elevations to more than 45* mm at higher elevations [)]. 6n contrast,the climate at the end of the Tioga glacial ma(imum of 7isconsin glaciation in North !merica, at appro(imately

22,8** years before present $yr B0%, was wetter and colder than the present [*+,]. Contemporarypotentiometric water levels of the region are presented on Figure -.

Data Set

Ground water chemistry ma9or ion data were obtained from the Nye County Nuclear 7aste epository 0ro9ect

ffice $N70% website as of 'arch 4**3 [!] and a :os !lamos National :aboratory report [-.]. Data werecompiled into a single database, covering the !margosa Desert region. +ampling locations are mostly wells,

some of which have multiple screened depths, while the remaining are fresh springs; springs containing highlevels of evaporites were e(cluded from analysis.

Multivariate Statistical Analysis o& /roundater 0hemistry !rturo 7oocay $awoocay<miners.utep.edu% and =ohn C. 7alton $walton<utep.edu%

>nvironmental +cience and >ngineering

The ?niversity of Te(as at >l 0aso, 8** 7. ?niversity !ve. >l 0aso, Te(as )@@/5

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Re&erences2. BuEo, T., 0ersonalcommunicationby email.buqo@aol.com, withattachment F'aster

egional 7ater :evels 4**.(ls. !ugust 4), 4**.

4. el#ar +., 0.Tseng, T.'iller, . 0awar, !. 'ei9er,B. obinson, G. Hyvolos#i, >.wic#lis, !.!. >ddebbarh, andB. !rnold. 4**3. +iteIsubsitescalesaturated"1one flow"transport models for &ucca'ountain. IHLRWM2003, :as Vegas, NVJ22"242.

3. +tet1enbach.=., V.-.Kodge, C. Guoa, 6.'. -arnhama, .K. =ohannessonc. 4**2.

Geochemicaland statisticalevidenceof deep carbonateground water withinoverlyingvolcanic roc# aEuifersIaEuitards of southernNevada. Journal of Hydrology  43J 48"4)2.

. -lint !.:., :.>. -lint, >.'. wic#lis, G.+. Bodvarsson, and=.T. -abry#a"'artin. 4**2.Kydrologyof &ucca'ountain, Nevada. Reviewof eo!"y#ic#$ 3@, no. J )")*.

8. wic#lis >.'., !. 'ei9er,and =.T.-abry#a"'artin. 4**3. Geochemicalinverse modelsof ground water mi(ingand chemical evolutionin the&ucca 'ountainarea. IHLRWM 

2003, :as Vegas, NVJ345"33@.

/. -lint !.:., :.>. -lint, G.+. Bodvarsson, >.'. wic#lis, and=.T. -abry#a"'artin. 4**2.

>volutionof the conceptualmodel of unsaturated1one hydrologyat &ucca'ountain,Nevada.Journal of Hydrology , 4)J 2"3*.

). Benson, :., '. ashgarian, . ye, +. :und, -.0aillet, =. +moot, C. ester, +.'ensing, D. 'e#o, and +. :indstrLm. 4**4. Kolocene multidecadal and

multicentennial droughts affecting northern California and Nevada. %ua&ernary'cienceReview# 2(J /8@"/54.

5. Karvey,!.'., 0.>. 7igand, and+.G. 7ells. 2@@@. esponseof alluvialfansystems tothelate 0leistoceneto Koloceneclimatic transitionsJ contrasts betweenthemargins of

pluvial:a#es :ahontanand 'o9ave, Nevadaand California, ?+!.)a&ena 3/J 488"452.

7i#lot

 ! biplot is simultaneous bivariate $factor loadings and factor

scores% scatter plots that provides a visual picture of therelationships between and among different ions and samplinglocations, in addition, it shows ob9ective sampling"location

groupings, and thus provides more insight than 0iper diagrams[-"+-)]. >ach factor, with a certain chemical composition, implies a

dominating hydrochemical process, and a clustered group impliesa hydrochemical facies with similar genesis, evolution andIor

composition [-*] indicated by the underlying factors.

0onclusionsThree common trends are observed from contours of the rotated factorsJ -irst, a large trough of moredilute waters follows along the path of -ortymile 7ash and turns to the southeast, where the wash 9oinsthe !margosa iver. +econd, the presence of Fnoise in the contours, apparent along the Kighway @8-ault. Third, a gradual increase in Ca4A, 'g4A and Cl" along the pathway of the !margosa iver comingout of the asis Valley. The geochemical data presented herein suggests that groundwater beneath-ortymile 7ash follows the surface of the wash until it appears to merge and mi( with groundwaterbeneath the !margosa iver. The signature from -ortymile 7ash is believed to represent the relic offocused infiltration of surface runoff along the course of the wash during past pluvial periods, when theclimate was colder and wetter than the present and the amount of runoff in the wash was significantlygreater. The results from the multivariate statistic analyses provide ob9ective grouping of ma9or ions,into hydrochemical processes; and sampling"locations, into hydrochemical facies. +pecifically, evidenceis presented of past"focused recharge around -ortymile 7ash climate"induced changes surroundingthe wash, and some potential interaction of ground water with Kighway @8 fault. The wor# hereindemonstrates how the use multivariate methods of statistical analyses of water chemistry providefurther understanding of ground water flow and evolution in the !margosa Desert region.

Ac8noledgments

This wor# was funded by the Nye County, Nevada, Nuclear 7aste epository0ro9ect ffice through cooperative research grant D>"-C45"*47242/3 from the?.+. Department of >nergy, ffice of Civilian adioactive 7aste 'anagement

Figure 2 The biplots presented here have two scalesJ one for factor

scores of sampling locations $i.e., bottom and left%, and the other for factorloadings of ions $i.e., top and right%. >ach ion vector indicates the directionof increasing ion content in the samples, and their pro9ection onto thefactor a(is is their correlation to that factor. This biplot is a diagramcustomi1ed to the dominant hydrochemical processes $i.e. the factors%,showing the hydrochemical facies and demonstrating the chemicalcomposition of the processes and facies of the system.

Figure - +tatic ground water elevation contours in meters above sea level based on 2,*55 wells

[-], $34 wells within the area are shown% overlaid on a satellite image of the !margosa Desert

region. Contour intervals are reduced from 2** to 4* meters $m% between the 5** and //* m levels.

A$stract

  The multivariate statistical methods of 0rincipal Component -actor !nalysis $0C-!% and #"'eansCluster !nalysis $'C!% are seEuentially used on groundwater chemistry form the !margosa Desert region to

estimate possible hydrochemical processes and facies along with probable groundwater flow paths andevolution in the region. 0C-! is applied to ma9or ion data and results are rotated, thus reducing the number ofvariables describing the system and allowing for better interpretation of the systemMs variation in terms of

hydrochemical processes. 'C! is applied to factor scores derived from the rotated 0C-! to allow thegrouping of sampling locations with similar water chemistries into hydrochemical facies. The resulting rotated

factor loadings and scores are presented as biplots, demonstrating relationships between and among variablesand sampling"locations. Derived factor scores and hydrochemical facies overlaid on a digital elevation map ofthe region provide a visual picture of hydrochemical evolutions, locali1ed recharge centers and potential

groundwater interactions with geologic and topographic features in the region.

@. N70 $Nuclear7asteepository 0ro9ect ffice%. 4**3. Geochemistrydata files. NyeCounty,Nevadawebsite. !ccessed'arch 4**3.httpJIIwww.nyecouty.com%

2*. :!N: $:os !lamos National:aboratory%, 4**3. egional ground water hydrochemicaldatain the&ucca'ountainare usedas direct input to!N:"NB+"***42, evision*2.

L*030+RR,3(233.00(, 'ay,4**3.

22. +tat+oft, 6nc. 2@5"4**/. '*-I'-I)*)om!u&er !rogram manualfor Window#. Tulsa,

. ernel release 8.8. www.statsoft.com

24. 'ellinger '., 2@5). 'ultivariate data analysisJ its methods. )"emome&ric# and 

In&elligen& Labora&ory 'y#&em#  4J 4@"3/. encher, !.C., Me&"od# of Mul&ivaria&e *naly#i#, +econdedition, 7iley"6nterscience, ?+!, 4**4.

23. encher,!.C., Me&"od# of Mul&ivaria&e*naly#i#, +econd edition, 7iley"6nterscience,?+!, 4**4.

2. Grande =.!., !. Gon1le1, . Beltrn, andD. +nche1"odas. 2@@/. !pplicationoffactor analysis inthe aEuifer system of !yamonte"Kueva$+pain%. roundWa&er  3, no.

2J 288"2/2.

28. GOler,C., G. Thyne, =.>. 'cCray,and !.. Turner.4**4. >valuationof graphicaland

multivariate statistical methods for classificationof water chemistrydata. HydrogeologyJournal  2*J 88P).

2/. Dalton, '.G., and+.B. ?pchurch. 2@)5. 6nterpretationof hydrochemical facies byfactoranalysis.roundWa&er  2/, no. J 445P433.

2). Thyne, G., C. GOler,and>. 0oeter.4**. +eEuentialanalysis of hydrochemicaldataforwatershedcharacteri1ation.roundWa&er  4, no. 8J )22P)43.

25. :awrence -.7., and +.B. ?pchurch. 2@54. 6dentification of rechargeareas usinggeochemical factor analysis. roundWa&er  4*, no. /J /5*"/5).

Factor Analysis

 ! 0rincipal Component -actor !nalysis $0C-!% of the data

was performed ?sing '&a&i#&icaQ ) [--]. The first fourfactors were e(tracted to reduce the number of variablesfrom seven to four and to find relationships among the

original variables [-2]. riginal variables are e(pressed asa linear combination of the underlying common factors,

[-'+-(], a high loading indicates a high degree ofcorrelation.   ! normali1ed varima( rotation of factors isperformed and rotated factor loadings for the ma9or ion

chemistry are generated along with factor scores for eachsampling location. otated factor loadings are presented on

1a$le -5

0luster Analysis

The first four rotated 0C-! scores were sub9ected to a  "

'eans Cluster !nalysis $'C!% to group observationswith a similar genesis into separate hydrochemical facies.The 'C! is nonhierarchical supervised partitioning

method that tries to group data that is similar and separatedata that is not based on >uclidean distances of variables

[-']. -rom previous analysis it was determined to groupdata into seven $#% clusters. !lthough the sevenhydrochemical facies are derived independently of

lithological data, they are found to be in good agreementwith their respective lithology and thus these groups are

validated as hydrochemical facies [-*].

Figure ' a9 Rotated Factor -  is dominated by 'g4A and Ca4A ions, which are typically associated with the

dissolution of carbonates, and is interpreted as an indication of the degree of influence of, or mi(ing with, the

carbonate aEuifer. Kigh values are found at Crater -lat, !margosa -lat and !sh 'eadows, which are downgradient of outcrops of the underlying carbonate aEuifer $Bare 'ountain +pecter ange, +tripped Kills, and+#eleton Kills%.

$9 Rotated Factor 2  is primarily composed of Cl ", NaA, and +4"; high levels of these ions are generally

associated with elevated amounts of the water evaporation that caused their concentration, and is perceived

as a measure of the degree of evolution through evaporation. :ow values form a trough surrounding-ortymile 7ash.

c9 Rotated Factor '   is dominated by al#alinity and NaA, and is most li#ely related to the weathering ofsilicate minerals with the generation of al#alinity and the concomitant release of NaA. These values present aclear separation between groundwater west and east of &ucca 'ountain.

d9 Rotated Factor (   is mostly composed of A and suggests that silicate weathering is significant in thissystem. These values appear to create a faint pathway originating in the asis Valley and following the

 !margosa iver.

Rotated Factor 0ontours on D:Ms

Contour plots of each of the resulting factors were overlaid on a digital elevation model $D>'% of the region inorder to reveal ground water signatures and potential flowpaths. ! contour plot of a factor would beeEuivalent to a contour plot of a hydrochemical process indicating its direction of evolution and delineating

areas influenced by that process [-,].