Stable Isotopes in Precipitation and Ground Water in the Yucca Mountain
University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
USGS Staff -- ubP lished Research US Geological Survey
1989 STABLE ISOTOPES IN PRECIPITATION AND GROUND WATER IN THE YUCCA MOUNTAIN REGION, SOUTHERN NEVADA: PALEOCLIMATIC IMPLICATIONS Larry Benson University of Colorado at Boulder, [email protected]
Harold Klieforth Desert Research Institute
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Benson, Larry and Klieforth, Harold, "STABLE ISOTOPES IN PRECIPITATION AND GROUND WATER IN THE YUCCA MOUNTAIN REGION, SOUTHERN NEVADA: PALEOCLIMATIC IMPLICATIONS" (1989). USGS Staff -- Published Research. 789. http://digitalcommons.unl.edu/usgsstaffpub/789
This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- ubP lished Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
Geophysical Monograph 55
STABLE ISOTOPES IN PRECIPITATION AND GROUND WATER IN THE YUCCA MOUNTAIN REGION, SOUTHERN NEVADA: PALEOCLIMATIC IMPLICATIONS
Larry Benson
InterdisciplinaryClimate SystemsGroup Room 135, National Center of AtmosphericResearch, Boulder, CO 80307
Harold Klieforth
AtmosphericSciences Center Desert Research Institute, Reno, NV 89506
Abstract. Unadjustedages of ground-watersamples indicate that most recordof lake-levelchange in the Lahontanbasin was principallya func- rechargein the Yucca Mountain and AmargosaDesert areasof southern tion of the size and extent of the continental ice sheet. When the ice sheet Nevadamay have occurred between 18,500 and 9000 yearsbefore present. was large (Figure 1), the polar-front jetstream was forced over the Comparisonof thestable-isotope ½180-•5D)concentrations in this water southwesternUnited Statesand provided sufficienteffective moisture,in with stable-isotopeconcentrations in precipitationin the southernNevada terms of increasedprecipitation and cloudiness,for the formationof high- area indicatesthat ground-waterrecharge occurred by infiltrationof cold- standlakes [Bensonand Thompson, 1987a,b; Lao and Benson, 1988]. seasonprecipitation. Infiltration of snowmeltalong the bottom of Fortymile Bensonand Thompson [ 1987b]observed that the maximumextent of Late Canyonseems to be the mostlikely rechargemechanism for YuccaMoun- Wisconsinglacation in the northernpart of the Great Basin precededthe tain groundwater. Ground-water recharge in southernNevada was nearly higheststand of Lake Lahontan[Dorn et al., 1987]. This indicatesincreased synchronouswith the last lake cycle in the Lahontanbasin in northern moistureavailability at a time when glaciersin the upper reachesof water Nevada. This regional-scalechange in the hydrologicbalance occurred shedswere retreating.To explainthis observationBenson and Thompson duringthe transitionfrom maximum-glacialto interglacialconditions and [1987a] hypothesizedthat the seasonalityof precipitationshifted to warmer wasassociated with a changein thelocation of theair-parcel moisture-source regionin the easternPacific Ocean and with a warmingof the climatebe- tween 18,500 and 9000 years before present. In the study area the temperatureof condensationof precipitationthat infiltratedto the watertable a•80planktonic • increasedby about5 øC. The warmingtrend also contributedto the re- foraminifera/• cessionof mountainglaciers in northernNevada. Recent atmospheric global (AtlanticOcean)/ / climatemodel simulations provide an integratingtheory that accountsfor the near synchroneityof ice sheet,lake, and ground-watercycles. In the modelthe positionand persistence of thejetstream, which is associatedwith oll the progressionof maximumeffective precipitation, is substantiallyaffected by the size and shapeof the continentalice sheet. 0 /•---•%%_ Alternate Introduction • • • • '% lakelevels
In a recentpaper Benson and Thompson [1987a] suggestedthat the timing foraminifera of the lasthighstand lake in the Lahontanbasin was associatedwith change in the mean positionof the jetstreamduring the Late Wisconsin.Recent (AtlanticOcean) atmosphericglobal climate model (AGCM) results[Kutzbach and Wright, 1985; Manabe and Broccoli, 1985; Kutzbachand Guetter, 1986; Kutzbach, 1987; andRind, 1987] supportthis concept. These studies have shown that between 18,000 and 15,000 years before present(yr B.P.) significant changesoccurred in the glacial age circulation at both the surfaceand jetstreamlevels that primarily were the resultof glacialage lower boundary conditions(ice-sheet size, sea level, sea-iceextent, sea-surface temperature, 0 5 10 15 20 25 30 35 and landalbedo). This widespreadchange indicates the possibilitythat the 103 Year B. P.
Fig. 1. Graphcomparing the Lahontanlake-level record and the •5180 This paper is not subjectto U.S. copyright. Publishedin 1989 by recordsfor Atlantic Oceanplanktonic and benthicForaminifera (seetext the American Geophysical Union. for data sources).
41 Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
42 STABLE ISOTOPES SOUTHERN NEVADA
16ø45' 116ø30' monthsor, alternatively,winters became warmer. It is also possiblethat 37ø00'
the climatebecame generally warmer, but evaporationover lake basinswas NEVADA reducedby persistentcloud cover. This paperaddresses the question:What typeof climaticchange occurred CarsonCity in the Great Basin between 18,500 and 9000 yr B.P.? It does so by ex- aminingthe Great Basin Late-Wisconsinclimatic and hydrologicchange (lake level, groundwater and glacial cycles)in terms of the isotope(&D and• 80) concentrationsin ground water recharged in theYucca Moun- Approximateboundary tain areaof southernNevada since about 18,500 yr B.P. (Figure 2). Yucca of Yucca Mountain Mountain, which lies about350 km southof the geographiccenter of Lake exploratoryblock Lahontan, is the nearest area where a reasonable amount of data exist on the timing and isotopicchemistry of ground-waterrecharge during the Late Wisconsin.The climaticconditions existing during recharge were inferred usingthe light-isotopicconcentration of present-dayprecipitation known Crater Fiat as a function of four interrelatedvariables (season,air temperature,air- parcel trajectory and altitude). ©VH-1
36ø45' 1350 I I I I I I • I Amargosa • 1300- '-"' • , ', Lake-levelcurve • x forthe western
, ahont, / 5m, // • 1200- _•'½/ / I l•0km 1150-
-- -6 Fig. 3. Map showinglocation of selectedtest wells in the Yucca Moun- !• Distributionofgroundwater ages forthe Yucca Mountain and z tain area. - sa Desedareas - 5
- -4½ a maximumvalue. In their calculationsWhite andChuma [1987] usedpure water to representthe compositionof rechargewater and allowedcalcium - -3 carbonate(containing 100 percentdead carbon) to dissolveuntil the com- positionof the simulatedground water was the sameas the composition _ _2 • of the actualground water. The quantityof deadcarbon input to the water wasthen used to correct the •4C age of thesample. However, distribution- - -1• of-speciesand saturation-statecalculations using EQ3NR indicate that present-dayfloodwaters (thought to representthe rechargesource for J-12, 0 5 10 15 20 25 30 35 4•¸ J-13 and UE-29a # 2 groundwater [Claassen,1985]) in FortymileWash 10 3 Year B. P. are saturatedwith calciumcarbonate and contain 100 percentmodern car- bon. Input of floodwaterchemical compositions to EQ3NR/EQ6 simula- FiB. 2. Graphcomparin8 the L•ontan lake-levelrecord with the distribu- tionsof the compositionof YuccaMountain ground water, therefore,are tion of unadjusted8round-water abes Jn the •ucca Mountainand Amar8osa expectedto resultin relativelysmall •4C corrections. Desert areas. From August1983 through August 1986, 429 precipitationsamples were collectedat 12 southernNevada stations (Figures 4 and 5). The latitude, longitude,and altitudeof eachstation are listedin Table 2. The sampling Methods intervalof precipitationsamples was halted within 24 hoursafter the cessa- tion of precipitationin order to isolateindividual precipitation events and Ground water from several wells located on or near Yucca Mountain, minimizethe effect of evaporationon the isotopiccontent of the sample. Nevada, (Figure 3) were sampledand analyzedfrom 1971 to 1984 by The dateand approximatetime thatprecipitation began and ended and the membersof the U.S. GeologicalSurvey. Drilling, samplingand analytical volumeand the type of precipitation(rain, snow, or hail) were recorded. proceduresare thosedescribed in Bensonet al. [ 1983]. The isotopiccom- In June1985 CampbellScientific 21X Microloggers(use of brand, firm, positionand unadjustedradiocarbon ages of the water samplesare listed or trade namesin this report is for identificationpurposes only and does in Table la and lb [Bensonand McKinley, 1985]. not constituteendorsement by the U.S. GeologicalSurvey) were installed Age adjustmentof threeground-water samples (J-12, J-13, UE29-a #2) at sevenprecipitation collection stations (Figure 5). Eachmicrologger was have been attemptedpreviously [White and Chuma, 1987] using the connectedto a temperatureprobe and to a tipping-bucketrain gage. The EQ3NR/EQ6software package [Wolery 1979, 1985]. Corrected •4C ages microloggerrecorded Julian date, time, temperature,and rain-gage bucket of eachof the ground-watersamples were about2000 yearsless than unad- tips at 15-minuteintervals. Prior to the installationof microloggers,data justedages. Studies of the chemicalcomposition of recentfloodwater that from two meteorologicaltowers at YuccaMountain (H.W. Church, Sandia occurredin the FortymileWash area nearYucca Mountain(Benson et al., National Laboratory, written comm., 1985) were usedto estimatetime and in preparation)indicate that the minus2000-year correction factor represents temperatureof precipitationat five stations(Sandy, Dianne, Marge, Carolyn Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 43
V V mm I V V V • V V V V
I I I
.,.., Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
44 STABLE ISOTOPES SOUTHERN NEVADA
v v v Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 45
11 TOO' 116ø30' 116ø00' 115ø30' 38ø30'
Site
37ø00'
ß Joanne
37ø30'
Andy G•a•l Stu'••
36ø00 ,1•0 , 2•0 3•0 4,0m• 10 20 3'0 4'0km Contour Interval 1000 feet
Fig. 4. Map showinglocation of precipitation-collectionstations in southernNevada.
and Robin). Prior to January26, 1985, datawere not collectedfor snow sourcesof informationwere: Daily WeatherMaps [NOAA, 1983-1986a]; depthor equivalentmoisture. These data are available for someprecipita- ClimatologicalData for Californiaand Nevada[NOAA, 1983-1986b]; tionsamples collected after January 26, 1985.A detaileddiscussion of on- HourlyPrecipitation Data for Nevadaand California [NOAA, 1983-1986c]; siteand laboratory procedures and a listingof basicdata are includedin StormData [NOAA, 1983-1986d];and Weekly Weather and Crop Bulletins Milne et al. [1987]. [NOAA, 1983-1986e]. The locationsof stormmoisture sources were estimated for eachprecipita- The 500-mb level (5500-5800 m) wasused because it is a representative tion episodeusing synoptic weather charts, precipitation measurements, indicatorof troposphericmotion. The airflow in a Pacificstorm is com- satellitephotographs, and other meteorological observations. The principal plex.While the storm itself may approach from the northwest, the surface
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I TABLE 2. Locationand Altitudeof Precipitation-CollectionStations * Gait * Indicatessequential rain sampler * Sandy Stationname Latitude Longitude Altitude Kathy_ o © ; - (meters) Gait -- ß O X • - Lucky Carolyn 36ø49'06" N. 116ø27'56" W. 1,480 Ki - . © ; - Marge 36ø50'13" N. 116027'56" W. 1,500 Joanne - • ; - Dianne 36ø51'21" N. 116072'56" W. 1,475 Jennifer - % - Sandy 36ø52'13" N. 116027'56" W. 1,480 Robin - • S - Andy _ .• - Andy 36o16'05" N. 116ø10'28"W. 760 Sandy Robin 36048'55" N. 116023'42" W. 1,020 Dianne ; ß Collectioncyhnder •nstalled - JenniEr 36053'00" N. 116040'24" W. 1,425 0 M•crologgerunit installed Marge ; . Tipping-bucketassembly •nstalled - Joanne 36037'59" N. 116020'34" W. 855 Carolyn • , Stabondecommssioned - IIII1•111•11 IIIII1•111111111111111111• Ki 37016'05" N. 116029'00" W. 2,145 July January July January July January July January 1983 1984 1984 1985 1985 1986 1986 1987 Lucky 36001'05" N. 115034'00"W. 1,395 Gail 36015'44" N. 115036'44" W. 2,145 Fig. 5. Graphshowing time linesfor stationinstallation, equipment in- Kathy 36ø40'15" N. 115005'50" W. 1,900 stallation,and stationdecommissioning. Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
46 STABLE ISOTOPES SOUTHERN NEVADA
flow may be southerlyand the cloudsand precipitation cells may propagate Desert [Claassen,1985, and Figure 7]. Carbon-14data for wells having toward the east. Most stormsover the Great Basinextend to the tropopause a probablecarbonate lithology were not used.The combineddistribution and, therefore,involve stratosphericair. Major winter stormscommonly of ground-waterages for the YuccaMountain and Amargosa Desert areas extend above 9000 m and summer convective storms often extend above is similar to the lake-level records for western subbasins in the Lake Lahon- 12,000 m. Thereforethe 500-mb flow patternprovides an approximatetra- tan system(Figure 2). Note that rechargemay have continuedfor a few jectory of the mean-horizontalflow of moist air into a storm and is thus thousandyears after the main recessionof Lake Lahontan13,000 yr B.P. an approximateindicator of the moisturesource region. In usingthis method Currey [1988] demonstratedthat the final shallow-lakeoscillation occured of trajectoryanalysis it is alsorecognized that the principal moisture source about11,000 yr B.P. in the CarsonDesert subbasin of the Lahontanbasin region for a particularstorm is not easyto locate;primarily becauseit is anddata for Lake Bonneville[Currey and Oviatt, 1985] indicatethe presence difficult to determinewhere along its trajectory the storm took on water. of a moderatesized lake in the Bonnevillebasin at 10,500 yr B.P. In addi- tion, SearlesLake may have overflowed during this time [Smith, 1968].
Results and Discussion Adjustmentof the ground-waterages to accountfor acquisitionof dead carbonduring rechargeand transportmay further shift the distributionto youngerages (see above), but to a first approximation,the timing of the In this sectionthe following questionsare addressed: last major cycle of ground-waterrecharge in the Yucca Mountain and (1) When did the last cycle of ground-waterrecharge occur in southern AmargosaDesert areasseems to be nearly synchronouswith the last lake Nevadaand was it synchronouswith the lastlake cyclein northernNevada? cyclein northernNevada. Ground-water samples older than 19,000 yr B.P. (2) Did ground-waterrecharge occur by infiltrationof floodwateralong have not been reported,and only one sampleyounger than 9000 yr B.P. the bottom of wash systemsor did rechargeoccur in upland areas? (well UE-293#2, Table la)has been reported. Ground-waterrecharge (3) What was the climate during the last rechargecycle and what type seemsto have startedat the time of maximumglaciation and endedduring of climatechange caused mountain glaciers to recedebefore the LakeLahon- the early stagesof deglaciation(compare Figures 1 and 2). tan achieved its maximum extent? (4) How do theresults of recentAGCM experimentsaid in understanding Reliabilityof GroundWater •4C Ages thechanges in GreatBasin synoptic climate that occurred in thelast 18,000 years? Three processesmay have alteredthe temporaldistribution of ground- Synchroneityof Lake and Ground Water Recharge Cycles water ages:(1) mixing of differentage groundwater, (2) contamination of groundwater by theintroduction of drillingfluid, and(3) contamination of groundwater by exchangeof carbondioxide across the air-waterinter- Theisotopic concentration and unadjusted •4C ages of groundwater from volcanic tuffs in the Yucca Mountain area are listed in Table 1 a. The unad- face. Resultsof borehole-flowsurveys Figures 4-6 in Bensonet al. [1983] indicatethat fluid productionwas usuallyfrom a few discretepermeable justed agesrange from about 3800 yr B.P. to about 18,500 yr B.P. Most intervalsin the fracturedvolcanic aquifer. If water from theseintervals con- of the ages are in the 12,000 to 18,500 yr B.P. range (Figure 6). The taindiffering amounts of 14C,the pumping process would have produced distributionof YuccaMountain ground-water ages is similarto thedistribu- anintegrated or mixedwater sample having a •4C concentrationin direct tion of ages(Table 3) of shallowground-water samples collected from ir- proportionto therelative amounts and •4C activitiesof waterfrom each rigationwells located 15 to 25 km southof YuccaMountain in the Amargosa discretefracture zone. Samplesobtained by pumpingisolated intervals in each of three test wells (wells UE-25b # 1, USW H-l, USW H-6, Table la) indicatevariability in their 14Cages in excessof thatresulting from Test wells within 1 kilometerof countingerror. This indicatesthat mixing of ground-watersamples having YuccaMountain (except USW VH-1) slightlydifferent 14C activities occurred when the entire length of a Yucca
t- ß Mountainborehole was pumped. The mixingof "old" and"young" ground water results in a ground-watersample having an intermediateage. Thereforea reductionin the variability of the distributionof ground-water .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:...... '*, :':':':':':':':':':':':':':':':':':':'...... -.-...... , 4- ,...... ,:':':':':' i:i:i:i:!:i:i:i:i:!:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:i:i:i:!:!:!:i:i:i:i:i:i:i:i:i:i:c• :::::::::::::::::::::::::::::::::::::: > :::::::::: ageshas occurredbut only to a limited extent. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::• i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:!:!:i:• :::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::co .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: co :.:.:.:.:. Duringdrilling of the testwells, fluid (usuallypumped from well J-13) :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::• :::::::::::::::::::::::::::::::::::::: • :::::::::: was introducedinto the volcanicaquifer. In orderto minimizecontamina- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::..... ::::::::,'.... ¾:::::: '-'-'-'-'. tion from the introductionof drilling fluid, wells in the Yucca Mountain ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: area were pumpedfor 2 to 13 daysbefore water sampleswere takenfor :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::"• isotopicanalysis. To detectthe presenceof drilling-fluidcontamination :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::during pumping, ground water was monitored for temperature,specific con- ductance,pH, andlithium (after 1982 a knownconcentration (20 mg L- •) ::::::::: :.:.:.:.: of lithiumchloride or lithiumbromide tracer was added to the drilling fluid)...... :i:i:i:i: ::::::::: :::::::::::::::::::::::::::::::::::::: :::::::::: ::::::::: Lithium was chosenas a tracer becauseits backgroundconcentration in ,:.:.:.:,:.:.:.:.: m• .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.::-:-:.:.:.:.:-:.:.:.:.:.:.:.:.:.:-:-:. :.:.:.:.:.:.:.:-:, YuccaMountain ground water is small,approximately 50 ttg L-1 [Ben- ;';';';'; • ':':':':':':':';':':':':';':':':':':': o0 co ;;;;;1:;; ß:.:.:.:. o• i:i:i:i:!:!:!:i:i:i:i:i:i:i:i:i:i:!:!:• '7 .:.:.:.:. son et al., 1983]. '"'"'":.:.:.:.; 1.1_1 ' .:.:.:.:.:.:.:.;.;.:.:.;.:.:-:.:.:.:.: "'"'"'"'"'"'"'"'"'"'"'"'" • • ...... , 1':';';'; CO CO LI_I LI_I:.:.:.:.: CO CO Samplessubmitted for 14Cdetermination were also analyzed for tritium (3H).Only two samples, both from well UE-29a # 2, contain3H in amounts :.;.;.:.: ::::::::::::::::::::::::::::::::::::: ::::;:::; 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.5 significantlyabove the level of detection(approximately 2 pCi L- 1 in this case).However, the lithiumconcentration in samplesfrom well UE-29a # 2 Unadjusted14CAge (10 3 YearB. P.) is only abouttwice the backgroundconcentration (100-110 ttg L-•) in- dicatinga very smalldegree of contamination(approximately 0.2 partsper Fig. 6. Graph showinghistogram of unadjustedground-water ages from thousand(ø/oo)) by drilling fluid (Figure 12 and Appendixin Bensonet the Yucca Mountain area. al. [1983]). Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 47
TABLE 3. Isotopic Compositionand UnadjustedRadiocarbon Ages of Selected Ground-Water Samplesin the AmargosaDesert [Claassen, 1985] and Pahute Mesa [White and Chuma, 1987] Areas
[AD = AmargosaDesert' PM = PahuteMesa' m = meters:SMOW = standardmean oceanwater: % o =parts per mil' yr B.P. =years betbrepresent (1950): -- = No data]
Well Approx. Approx. 8D b180 14C designation well depth depthto % o % o unadjusted (m) water (m) (SMOW) (SMOW) (yr B.P.)
AD-3 150 78 - 102.0 -12.8 14,900 AD-4 90 21 - 103.0 -13.2 13,200 AD-5 60 45 -99.5 -13.2 12,400 AD-8 60 49 - 103.0 -13.4 12,200 AD-9 110 33 -102 0 -126 10,100 AD-10 -- -97 5 -132 11,200 AD- 11 90 30 - 101 0 -131 12,600 AD-13 50 26 -102 0 -130 13,200 AD-15 60 16 -104 0 -130 13,600 AD-17 -- -- -105 0 -128 13,400 AD-18 100 15 -102 0 -130 10,300 AD-21 -- -- -99 0 -132 10,400 AD-23 50 29 - 103.0 -13.4 14,200 AD-25 -- - 102.0 -13.4 14,900
PM UE- 19gs -113.5 -14.5 -- PM UE-191 -109.5 -14.0 -- PM UE-20a2 -114.0 -14.8 15,000
Relation Of Ground Water Stable-IsotopeConcentrations to the Meteoric Water Line (MWL) :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::,,-- .:.:.:.:.:.:.:,:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:...... ::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::• ::::::::::::::::::::::::::::::::::::::::::::::: The OasisValley andFortymile Wash ground-water basin shown in Figure :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 8 wasformally defined by Winogradand Thordarson [1975]. Physiograph- :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ically, thisbasin is dominatedby uplandvolcanic areas to the north(Pahute ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Mesa andTimber Mountain,Figure 8). Blankennageland Weir [ 1973] sug- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::: ..... ß...... • :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. gestedthat water underlying Pahute Mesa moves southwestwardand :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::'7 :::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::c• :::::::::::::::::::::::::::::::::::::::::::::::: southwardtoward the AmargosaDesert through Oasis Valley, Crater Flat,
...... ,...... 0...... 0...... and westernJackass Flats (Fortymile Canyon area). White and Chuma ...... ,...... :.:.:.•.:.:.:.:.:.:.:.:.:.•.:.:.:.:.:.:.:.:.:.:.•.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.N.:.:.:.:.:.• ,:.:.•.:-•-:-:':.:.:.:.:':-:':':':':':.:':':':-: [1987] haveconfirmed Blankennagel and Weir's [ 1973] suggestionfor the •i:i:!:i:!:i:i:i:i:!:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:i:i:i:!..... i:i:i:!:...... ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::¾ :::::::::::::::::::::::::::: westernpart of the ground-waterbasin by determiningthat isotopically light :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::: ß...... •.:.:.: .,• ...... v...... v...... groundwater from PahuteMesa discharges from springsin the headwaters :':':':':':':':':':':':':':':':':':':':':':':':':':':':':':':':':':'::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::•, :iiiiiiii• •, • ':':':':':':':':':':':':':'::::::::::::::::::::::::::::: of the OasisValley surface-drainagearea. Using ground-waterquality data :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::iiiiiiii= = = = ...... -.....-.-.-.-.....-.-...-. from shallow wells in the AmargosaDesert south of Yucca Mountain, Claassen[1985] determinedthat Amargosa Desert ground water was ...... :.:.:.:.: :::::::::::::::::::::::::: :•ii•ii•iiiiii•ii•iii•i!iii!i•!!iiiiiiiiiiii•i•i•i!i•i•i• ::::::::::::::::::::::::::::recharged to valley fill primarily by overlandflow in, or near, present-day ..-.v.v...-...... -...... -...... stream channels...... v...... -.-.-...... -...... -...... , • 0 • :i:i:i:i:i:!:i:i:!:!:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:i:i• • • • • • ':':':':':':':':':':':':':': Stable-isotopeconcentrations in groundwater from PahuteMesa andthe :.:.:':':-:.:.:.:.:-:':':-:':':.:.:-:.:-:-:':':':.:.:-:-:':':':':':.• • • • • • :i:!:!:i:i:i:i:!:i:!:i:i:i:!:i:i:i:i:!:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i• • • • • • :::::::::::::::::::::::::::: AmargosaDesert (Table 3), Crater Flat (includesdata from testwells USW H-6 and USW VH-1), Yucca Mountain, and Fortymile Canyon(Table la ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: •:•:}:}:i•:i:•:}:•:i:i:!:i:• 02•5 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.5 and lb) wereused to determinethe atmosphericsource (air-parcel trajectory) of water pumpedfrom test wells locatedwithin 1 km of Yucca Mountain (Figure 6). The isotopic concentrationin most ground-waterplots is Unadjusted14CAge (10 3 YearB. P.) "underneath" the present-daymean meteoricwater line (MWL); the fid excessvalue (d) for thesewaters is lessthan 10 (Figure 9). Ground water Fig. 7. Graphshowing histogram of unadjustedground-water ages from from PahuteMesa hasthe mostdepleted isotopic values. The isotopically the AmargosaDesert. Data are for samplestaken from aquifersnot con- lightestsample (UE-20a2) is about 15,000 yearsold [White and Chuma, taining calcium carbonate. 1987]. Ground-water samples from the Fortymile Canyon area are Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
48 STABLE ISOTOPES SOUTHERN NEVADA
116ø30' 116ø00' 37ø30' humidityof air over the moisture-sourcearea [Merlivat and Jouzel, 1979; Sonntaget al., 1979], or (4) evaporationof water in the vadosezone. During the Late Wisconsin,several large lakes were locatedless than 100 km to the southwestof Yucca Mountain (Figure 10). Evaporatedlake watermay have shifted the isotopiccomposition of precipitationin theYucca Mountainarea; however, the magnitudeof this processremains unknown. Part of the scatterin bD-b180and the largevariation in d valuesfor AmargosaDesert groundwater is probably due to evaporationduring or after infiltration of the relatively shallowground water [Claassen,1985]. However, in the caseof Yucca Mountain groundwater, a changein the relative humidity of air over the moisture-sourcearea bestaccounts for the departureof Yucca Mountainground water from the MWL. Ground-water samplesfrom five testwells plot on or within 2 ø/oo bD (the measurement precision)of the MWL (Figure 9). Althoughwe cannotbe completelycer- 37ø00' tain, we contendthis indicates that the evaporationprocess has had a minimal effect on the isotopiccomposition of groundwater; i.e., groundwater at the well USW H-3 sitewas recharged18,000 yr B.P., but its isotopiccon- tent doesnot indicatethe effectsof evaporation.If this contentionis valid, theb180-bD concentration of the 4000 year old UE-29a # 2 groundwater thatplots on the present-dayMWL indicatesthat the humidityof thePacific CraterFlat ½?' "::? '"'::...... Oceanmoisture-source region had probably reached present-day values by about4000 yr B.P. In supportof this argumentwe notethat groundwater in the YuccaMountain area is overlainby a thick (500 to 750 m) unsaturated AmargosaDese• ...... ::.;•?•?.•,.;: zone havinga humidityof nearly 100 percent(E. Weeks, U.S. Geological Survey, written comm., 1988). Thus there exists little or no potentialfor evaporationof Yucca Mountain ground water. Fig. 8. Map showinggeneralized ground-water flow field in the Yucca The data of Figure 9 indicatethat Yucca Mountain groundwater may Mountain region. representa mixtureof AmargosaDesert and Crater Flat groundwater. This impliesthat Yucca Mountainground water was derived from a mixture of overlandflow alongFortymile Canyon and ground-water flow from upland isotopicallythe heaviest,and two samplesfrom the Fortymile area (test areasto the north (PahuteMesa). To the extentthat this argumentis cor- well UE-29a//2) also are the youngest(approximately 4000 years old). rect, the isotopicconcentrations in, and radiocarbonactivities of, Yucca Water from the AmargosaDesert displaysthe largestvariation in d. Mountainground water representmixtures of isotopesfrom two recharge For groundwater unaffectedby hydrothermalprocesses, a distribution sources.In addition, if the humidity model of isotopiccorrection is cor- of •3D-•3•sO lyingbelow the MWL hasusually been attributed to oneof rect, andonly minorcorrections need be madefor evaporation,the oxygen- four processes:(1) incorporationof water reevaporatedfrom overcontinents isotopeconcentration in precipitationreaching the water tablehas increased or lakes [Fontesand Gonfiantini, 1970], (2) fractionationduring evapora- systematicallyby about1.7 % o from 18,500to 9000 yr B.P. (Figure11). tion of rain or snow [Gat and Tzur, 1967], (3) decreasein the relative The scatterin the datais in part attributedto ground-watermixing that oc- curred during pumping, to mixing of ground-waterfrom two recharge sources,and to differencesin the seasonality(temperature and air-parcel trajectory)of recharge.Regression of the b • 80 contentof YuccaMountain groundwater against 14C age (of samples greater than 9000 yr B.P.)results -90, ' ' [ I ' [ [ [ I T[ , I [ [ ' [ ] I [ '/• [ [ ] ' in an R2 of 0.71 indicatinga significantdegree of dependenceof b180 on /•UE29a#2 age.However, regression of the• •80 contentof AmargosaDesert ground -95 wateragainst 14C indicates essentially no correlation (R 2 --0.10). Webelieve the poor correlationis due to evaporationof shallow Amargosaground water.
-100 USWH 3 •///• • • •5•80and Sea-Surface Temperature in the Eastern Pacific Moisture Source Region
-lO5 4½,• • • To understandthe climatic significance of a 1.7 ø/oo increasein the•80 of groundwater, the b• 80 historyof theoceanic source region must also •f • • • YuccaMountaingroundwater be known.The glacial-interglacial • 80 transitionis recordedin thetests -110 • • FodymlleCanyonground water of Foraminiferapreserved in deep-seasediments. Mix andRuddiman [ 1985] combined• • 80 datasets from Atlantic-sediment cores to forma composite • PahuteMesagroundwater recordof isotopicvariation for 30,000 to 2000 yr B.P. The compositerecord • • •Crater Amargosa FlatDesertgroundgroundwaterwater• indicates a 1.1 ø/oo decreasein •80 in thetests of planktonicForaminifera -15.0 -14.5 -14 0 -13.5 -13.0 -12.5 -12 0 from 14,000 to 9000 yr B.P. Data from the Pacific Ocean, the sourcearea b180 %o of mostmajor stormsystems reaching Nevada, are sparse.Broecker [ 1986] summarizedglacial-interglacial •80 changesthat were measuredin Fig. 9. Graphshowing bD-b• 80 concentrationin ground water from the planktonicand benthicForaminifera for the Atlantic and PacificOceans. YuccaMountain region and its relationto the present-daymean meteoric He reportedthat the decrease in b• 80 for planktonicForaminifera between water line (MWL). 18,000 and 0 yr B.P. was about 1.2 ø/oo in the Pacific Ocean and about Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 49
121ø 119ø 117ø 115ø 113ø 111ø
43 ø
--,,•, _ r':,,i• •!t ,., ./:• •[ \, , 0 rse"on , Idaho t.•,, lh-,,:,:.••//•/tOft.,.•.::.:}•_._.-..--•
41' .--'""'• w;,,,.,...:..., "• '"::::::::::::::::::- -:::.. • • La•'•' :•" :.:.:-:.:.x-:-:. 'x. • '• • ::::::::::::::::::::::, 75"' Lake,, • •
39 ø k, N •WalkerLake
MonoLake'• • • .:•
37 ø
Owen s l..(&•'
...... ,' '••% ,•% • % , k / Physiographicboundaryof
350 % "• / 0 Formerpleistocene lakes 0 • • 100 200m•
o 3OO' 4Okm I I I I I I Fig.10. Map showing location oflakes of assumed Late-Pleistocene agein the Great Basin [after Spaulding etal., 1983].
-12.0 i i i I I I I I I _ I i i i i I i i i i
_
_ 1.8 ø/oo in the AtlanticOcean. Bioturbation of low sedimentationrate cores
_
_
_
_ (sedimentationrateaverages about 2.5 cm/103yr) fromboth oceans may
-12.5 _ have"smoothed" the •80 record;therefore the observed decreases in •80 _ mayrepresent minimum values [Broecker, 1986]. In calculationsthat follow, _ a 1.1% o decreasein •80 of planktonicForaminifera from the Pacific _ -13.0 _ Oceanwill be assumed tohave occurred between 18,500 and 9000 yr B.P. Sea-surfacetemperature (SST) reconstructions by CLIMAP [ 1981]of the CentralPacific Ocean (10 ø to 40ø N. latitude)indicate surface water was 0 -13.5 about1 øCwarmer 18,000 years ago. Fractionation of •80 betweencalcite andwater decreases by 0.25 % o for eachdegree of heating[Friedman andO'Neil, 1977]. Thereforethe •80 of PacificOcean surface water de- -14.0 creasedabout 1.3 % obetween 18,500 and 9000 yr B.P.During the time thatprecipitation in the Yucca Mountain area was becoming isotopically
YuccaMountain ground water _ heavier(plus 1.7 o / oo), the moisture-source region (Eastern Pacific Ocean) -14.5 AmargosaDesert ground water wasbecoming isotopically lighter (minus 1.3 % o). This indicatesthat _
_ precipitationin the Yucca Mountainarea underwenta 3.0 % o increase
_
_ in•80 relativetothe moisture-source areabetween 18,500 and 9000 yr -15.0 B.P. 0 2 4 6 8 10 12 14 1 18 2o Tocalculate the change intemperature associated with the change in• •80 Unadjusted146 Age (10 3 year B. P.) between18,500 and 9000 yr B.P.,the following theoretical relation, derived by Van der Straatenand Mook [1983], wasused: Fig. 11. Unadjustedb•80 of YuccaMountain and Amargosa Desert groundwater plotted as a functionof unadjusted•4C age. b•80=0.56T(øC)- 12.7 (1) Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
50 STABLE ISOTOPES SOUTHERN NEVADA
The calculationindicates that the temperatureof condensationwas about e.g.,6180 ranges from minus 22.5 to plus7.5 ø/ooand a significantdegree minus 5 øC about 18,500 yr B.P. and increasedto about0 øC by 9000 of departurefrom the MWL. On the scaleof this plot the area occupied yr B.P. The temperatureof condensationof the 4000 year-old-waterfrom by YuccaMountain, Crater Flat, andFortymile Canyon ground water (Table UE-29a # 2 alsowas 0 øC, whichindicates that the temperatureof conden- 1) is relativelysmall. In addition,departure from the MWL by groundwater sationof precipitationinfiltrating to the water table did not changeap- is smallrelative to departurefrom the MWL for manyprecipitation events. preciably between 9000 and 4000 yr B.P. Much of the variability in precipitationalong, and normalto, the MWL The 5 øC depressionof air temperatureat high altitudes18,500 yr B.P. is attributableto smallamounts of summerrainfall that partially evaporated is similar to the resultsof Dohrenwend's[1984] studyof nivationland- as it fell. A 6D-6180plot of all rainstormevents is shownin Figure13. formsin the high mountainsof the Great Basin.He concludedthat an ap- Linear regressionof thesedata resultsin the best-fit equation: proximate7 øC full-glacialmean-annual temperature depression occurred throughoutthe Great Basin.The absolutevalues of the temperatureof con- 6D =6.6 6180-7.2 (2) densationat 18,500 and 9000 yr B.P. are quite low when comparedto present-daymean-annual air temperatures(8 to 10 øC) recordedat Pahute However,when data for rain that fell in quantitiesless than 0.25 cm are Mesa (Jan-Hwa Chu, National Weather Service, Nuclear SupportOffice, excludedfrom the data set (thereby eliminatingpart of the bias due to NOAA, written comm., 1987). In fact the presentaverage cold-season evaporation),variability along, and normalto, the bestfit regressionline (Novemberthrough February) temperature for PahuteMesa is about0 øC. is significantlydecreased and the regressionequation, This indicatesthat condensationtemperatures of rechargethat occurredbe- tween 18,500 and9000 yr B.P. were significantlylower thantoday's cold- 6D =7.4 6180+3.8 (3) seasonmean-monthly air temperatures. In the following sectionsthe stable-isotopeconcentration in precipitation beginsto resemblethe MWL. If datafor precipitationin the form of snow will be comparedwith time, temperature,altitude, and air-parcel trajec- (Figure 14) are analyzedseparately (thereby eliminating most of the ef- tories of present-daystorm systemsin order to determinethe changein fectsof evaporation),the bestfit regressionequation becomes almost iden- synopticclimate that caused the 3.0 ø/oo shiftin 6• 80. Thechange in synop- tical to the MWL, tic climateis then usedto suggesta hypothesisthat accountsfor the asyn- chroneityof lake and mountain-glaciercycles. 6D=8.0 6•80+8.9 (4)
Of the severalprecipitation-collection sites, the Ki and Sandysites are Relationof Stable-IsotopeConcentrations in Precipitationto the Present- onesmost relevant to an understandingof the origin of Yucca Mountain Day MWL groundwater. Thesesites are situatedin areasthat are mostrepresentative of sitesthat in the past contributedto the rechargeof groundwater now A plot of isotopicdata for snowand rain collectedduring this studyis found in the Yucca Mountain area. The Ki site is located on Pahute Mesa shownin Figure 12. Precipitationsamples were notanalyzed for 6D after at an altitudeof 2145 m, and the Sandysite is locatedon Yucca Mountain March 10, 1986. The data indicatesubstantial variability along the MWL; at an altitude of 1480 m (Figure 4).
4o i i i i i i i i 40 "_ 20 ' 20 I I I I I I I
ßß -20 $D=88180+ 10--. -20
-40 -40 ß ooød• ß ßo ß .. - -60 •.• *** ß ß oo -80 ß ** -80 -
ß o
-IO0 YuccaMountain ground water -100 -
o -120, ß -120 - ß o ß ß Rain -140 o Snow -140 - ß ß ß Rain
-160 o -160 -
-180 i I I i i -180 I I I I I I I I I -24 -21 -18 -15 -12 -9 -6 -3 0 3 6 9 -24 -18 -15 -12 -9 -6 -3 0 3 6 9 ,•180 %o õ180 %0
Fig. 12. 6D-6•80 of all precipitationsamples collected in southern Fig. 13. 6D-6180of all rainsamples collected in southernNevada from Nevadafrom aboutJuly 1983 throughMarch 1986. about July 1983 through March 1986. Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH $1
40 i i i i i i i i i i 40 I I I I I I I I I I
20 20
-20 -20 ß
ß o
-40 -40 oo/•-SD: 8.08180 + 8.9 o• -60- • o ß I•1 i• o ø ,• -80- • oo ,,• -80 - -100 - -100- nground water Ki Site -120 - ß Rain > 0.25 cm ß Rain < 0.25 cm o Snow -140 o SROW
-160
-180 o • • i • • • • • • • -24 -21 -18 -15 -12 -9 -6 -3 0 3 6 -21 -18 -15 -12 -9 -6 -3 0 3 6 9 8180%o 8180%o Fig. 14. fiD-fi•SOof all snowsamples collected at the Sandysite. Fig. 15. •D-•80 of rainand snow samples collected atthe Ki sitelocated on Pahute Mesa. The isotopiccompositions of rain and snowcollected at both sitesare plottedin Figures15 and16. The dataindicate that Ki, at thehighest altitude, receivesa greaterproportion of its precipitationin the form of snow. To comparestable-isotope concentrations in precipitationcollected at thevarious siteswith the isotopiccomposition of groundwater currently present in the 4O I I I I I I I I Yucca Mountainarea, the meanisotopic concentrations in snowand rain plus snowwere calculatedfor eachprecipitation collection site. The use 20 of weightedmeans is inappropriatebecause the seasonaldistribution of modernprecipitation cannot be assumedto be the sameas duringthe last o ground-waterrecharge cycle. The dataindicate that the mean isotopic com- positionof present-daysnow plus rain is significantlyheavier (2 to 7 ø/oo -2o •lSO) thanthe isotopiccomposition of YuccaMountain ground water (Figures17 and 18). However,the mean isotopic composition of snowalone -4o is more similarto the isotopicconcentration in YuccaMountain ground water(differs by 0 to4 ø/oo •lSO). This indicates that ground-water recharge -6o in the Yucca Mountainarea may have occurredin the form of snowmelt runoff. -80
-lOO Effect of Altitude and Temperatureon the Isotopic Compositionof SandySite - Precipitation -12o - / _.".. / ß Rain> 0.25cm - ,/' ' / ß Rain<0.25cm The isotopiccomposition of precipitationdiffers from siteto siteas shown -14o in Figures17 and 18. Part of this differenceprobably is due to differences in recordlength; e.g., threeof four sites(Sandy, Marge, Carolyn)on Yucca Mountain were decommissionedafter 12 to 18 monthsof operation.The -16o fourth site (Dianne) remainedin operationfor more than 3 years (Figure -18o 5). Theeffect of altitudeon theisotopic composition of precipitationis shown -24 -21 -18 -15 -12 -9 -6 -3 0 3 6 9 in Figure19. Regressionof mean•80 datafor eachstation indicates a dependenceof •80 onaltitude of aboutminus 2.0 % o per 1000m. A 8180 %0 correlationbetween • •80 andair temperature of individual rainstorms was not found. Temperaturedata for snowstormsis not available becausethe tipping-bucketrain gagesused to record the time and amount of rainfall Fig. 16. •D-•80 of rainand snow samples collected at the Sandysite were unheatedand, therefore,were coveredduring the winter months.In located on Yucca Mountain. Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
$2 STABLE ISOTOPES SOUTHERN NEVADA
40 I I I I I I I I I I I I I I I I I I I I I I I I I I I
__ __
20
__ __ -20 8180 = -2 00 x 10 3AIt -6 41 f Joanne • • (R2 = 0 59) - ß , . bandy -40 -•- ..... LUCKy.[(ß J - / ...... -T-"½_ ..... _•..... /Kathy Kathy_ Ma,rge 1'---- i Robin• \ a, ef JoanneDianne _ Andy•Robin 'r•ennlfer ---"•- _ Sandy•------..C•ß i4'•.-.._.. Andy -12 Carolyn-'-'"le % Lucky -80 - JenmferGall K• / -16
-100 - YuccaMountain(• groundwater -20 Mean$18 0 ofprecipitation -120 -
-140 - ß Precipitation-Collection site, 800 1000 1200 1400 1600 1800 2000 2200 2400 meanvalue, snow and rain
-160 - Altitude(m)
-180, -24 -21 -18 -15 -12 -9 -6 -3 0 3 6 9 Fig. 19. Mean6180 values in southernNevada precipitation plotted as a function of site altitude. 8180
Fig. 17. Mean6D-6•80 valuesof precipitationat 12 collection sites in southernNevada compared to the6D-6 • 80 concentrationin Yucca Moun- anycase the effect of altitudeon the isotopic composition of precipitation tain ground water. isnot considered tohave had any effect on the isotopic composition ofground waterrecharged between 18,500 and 9000 yr B.P. in thatthe altitudeof the siteof rechargeis not consideredto havechanged during that time.
40 I I I I I I I I I Air-Parcel Trajectoriesand the Isotopic Compositionof Precipitation
20 In his investigationof precipitationcharacteristics of the Great Basin, Houghton[ 1969] identifiedthree principal components (Pacific, Gulf, Con- tinental)all of which occurthroughout the Great Basin,but eachof which -20 is dominantin certainregions of the Great Basin(Figure 20). The Pacific component,with a winter precipitationmaximum, predominatesin the
-40 western,northern, and southern regions. The Continentalcomponent, with a springprecipitation maximum, predominatesin the centraland eastern regions. The Gulf component,with a summer precipitationmaximum, Andy predominatesin the southeasternregion. Eachof thesecomponents can be ß -- Lucky Kathy• Carolyn___ß•,'•Robin identifiedwith recognizablecirculation patterns and air-parceltrajectories. -80 - Dlanne--•_•__%• Joanne Marge____.•-•1' Jennifer The conceptof air-parcel classificationoriginated with Scandinavian meteorologistsin the 1920's,and the principalair parcelsaffecting the Great -100 - Basinare designatedas maritime polar (mP), maritimetropical (mT), con- YuccaMountain ground water tinental tropical (cT), continental polar (cP), and arctic (A) [Byers, -120 - 1959]. Each of theseair massesmay generallybe identifiedwith source region, seasonaloccurrence, and air-parcel trajectory (Figure 21). A -140 - ß Precipitation-Collection s•te, meanvalue, snow subclassificationderived by Bergeron[1930] includes mP k andcP k polar air parcelsthat are colderthan the the surfaceover whichthey flow. They -160 - are, thus,subject to rapidmodification leading to thermalinstability which often resultsin convectiveshowers. When an air parcelpasses through a -180 region on the boundaryof two sourceareas, or when the air parcel en- -24 -21 -18 -15 -12 -9 -6 -3 0 3 6 9 traines moisturefrom two source regions (convergentflow), a mixed 180 %0 classificationis used;e.g., mT/mP. In thisand following sections reference will be made to the jetstream.We will use this term to mean a quasi- Fig. 18. Mean6-6180 values of snowsamples from 12 collectionsites horizontal zone of maximum winds embedded in the mid-latitude westerlies in southernNevada compared to the6-6180 concentration in Yucca Moun- andconcentrated in the high troposphere. Two jetstreams are distinguished. tain ground water. Thepredominant one, the polar front jetstream, is associatedwith the polar Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 53
40 Oregon I I I I I I
20
0
-20
-40
PacificComponent -"• ":• GulfComponent -60 '•'"""'Mean •D-•80 value ofair-parcel typefor all collection sites
-80 - mPk/mT.....
,OregonJ Idah •:.,: OregonI Idaho ,• YuccaMountain _/•)0 -100 - groundwater--"-"L;•7 cP .... • D- •180data for air parcel ."•""•"•.•i ,'...... -120 - typeat Sandy collection site - 0 mT, MaritimeTropical -140 - ß mP, MaritimePolar _ I o mPk,Maritime Polar, modified ß c P , ContinentalPolar -160 - ,, A , Arctic _ h •..•'..X.P,G /'- '/' Utah ß mT/mP "':":,.' "'"' • mT/CT -180 - ß mPk/mT _ of the three -u/,,).'"• ,.J (C) -•'/x•.,•.'•X•'- Relative i I ContinentalComponent',')•..•,-'k• componentsimportance i -200 I i I iI I I I ComponentPercent Component -25 -20 -15 -10 -5 0 5 10 ofAnnual Precipitation of precipitation 0-10[---']30-40 :•p•'] 60-70_.•.., C Primary 180%o •0-20:':-:..:....:• ,o-so...=:..-'• 70-90 B c Se•o,d•ry 20-30:'.-.":..."• so-so • Fig. 22. (5-(5180of individualstorms at theSandy site classified by air- parceltrajectory. Mean values of (5and (5180 for trajectorytype for all Fig. 20. Climatic gradientsin the Great Basin[after Houghton,1969]. sites are also shown.
front of middle and upper-middlelatitudes. The subtropicaljetstream is 120" found between latitudes 20 ø and 30 ø N. [Huschke, 1959]. 110 ø The stable-isotopeconcentration of precipitation at the Sandy site, classifiedby sourceregion (Figure 22), indicatesthat air-parceltrajectories can also be classifiedby isotopicconcentration. Mean isotopicvalues of end-membertrajectories for all southernNevada sites(listed in Table 4) areindicated in Figure22 bydashes along the (5180 and fid axes.The (5180 of mP stormsaverages about 2 ø/oolighter than mT storms,the (5180 of mPk stormsaverages about 4 ø/oolighter than mT storms,and the (5180 of cP stormsaverages about 7 ø/oo lighter than mT storms.Arctic trajec- tories have extremelylight isotopicconcentrations but data are available for only one storm.
Utah Comparisonof air-parcelisotope concentrations (Table 5) andthe isotope concentrationin Yucca Mountain ground water (Table la) indicatesthat California\,,•, I isotopicdistributions of cP, mP, and mPk air-parceltrajectories bracket the isotopicrange of Yucca Mountainground water. This indicatesthat Yuc- ca Mountainground water wasnot derived from monsoonal(mT) air parcels. Instead,it appearsthat the oceanicmoisture-source region for Pacificfrontal systemsmoved southwardbetween 18,500 and 9000 yr B.P.
AGCM Experimentsand the SynopticClimate of the Great Basin
Recentatmospheric global climate model (AGCM) simulationsby Kutz- 200' , 400 6100mi " bachand Wright [ 1985],Kutzbach and Guetter [1986], andKutzbach [1987] provideinsight into the mechanismand magnitudeof synopticclimatic
400 800km : ..ß L changethat occurred in the southwesternUnited States during the past 18,000 years.AGCM experimentsat 3000-yearintervals for the past18,000 years Fig. 21. Major air-flow patternsand air-parceltrajectories affecting were done by Kutzbachand Guetter [1986] to estimatethe magnitude, California and the Great Basin [after Holmboe and Klieforth, 1957]. timing,and pattern of the climaticresponse to changesof orbitalparameters Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
54 STABLE ISOTOPES SOUTHERN NEVADA
TABLE 4. Storm Summary
[A= Arctic'fid = deldeuterium; -- = nodata; CF = coldfront; h 180 = deloxygen 18; % o= partsper mil' cm = centimeter;mb = millibar'PF = polar front'cP k = continentalpolar modified' mP = maritimepolar; cP = continentalpolar; mP k = maritimepolar modified; cT = continentaltropical; mT = maritimetropical' R = rain; S = snow'SMOW = standardmean ocean water] Precipitation amount (cm) Sandy site Flowtype Air-parcel Precip. Sandy Gail b• 80 fid Date (500-mblevel) type type site site ø/oo ø/oo (SMOW) (SMOW)
1983
-11 Aug 9-10' Summer monsoon mT 0.83 -3.6-1.2 -20 Aug 15-19' Summer monsoon mT 5.30 -16.4 -119 -81 Sep 25-26 Low-latitude trough mT 1.19 -12.112.2 -91 -58 Sep 30-Oct 1 Cyclone mPk--/mT 1.07 -2.8-8.3 -8 Nov 24-25 Mid-latitude trough, CF mT/mP 0.67 -11.3 -76 Dec 03-26* Mid-latitude trough, CF mP -9.1 -60
1984 Feb 10 Mid-latitude trough, CF mT/mP S 0.00 -- Feb 13-16 Mid-latitude trough, CF mP S -- -- - 10.0 Mar 14 Mid-latitude trough, PF mP R/S 0.00 0.50 -- Mar 29 Cold cyclone cP S 0.00 0.02 -- Mar 30-Apr 1 Cold cyclone cP S 0.04 0.02 -22.2 - 179 Apr 6 Mid-latitude trough/cyclone mPk-,/mT R/S 0.13 0.00 - 9.6 -70 Apr 19 High-latitudetrough/cyclone, CF mPk R/S 0.00 0.00 Jun 14 Cyclone cPk--/mT R 0.00 0.17 Jun 23 Summer monsoon cT/mT R 0.00 0.17 -- Jun 30 Summer monsoon mT R 0.00 0.58 -42 Jul 2 Summer monsoon mT R 0.21 0.12 -2.8 Jul 12-15 Summer monsoon cT/mT R 0.64 6.10 - 3.2 -28 Jul 16-18 Summer monsoon cT/mT R 0.00 1.55 Jul 18-20 Summer monsoon 11.8'* -82 Jul 21-22 Summer monsoon cT/mTmT R I [ 0.180.99 - 11.8-- Jul 25 Summer monsoon cT/mT R 0.00 1.17 -36 Jul 27-28 Summer monsoon mT R 1.35 11.28 - 5.6 Jul 29-31 Summer monsoon 0.90 -8.9 -60 Jul 31-Aug 1 Summer monsoon mT R ] [6.640.21 +27 Aug 10-11 Summer monsoon cT/mT R 0.03 0.00 + 7.0 -55 Aug 14-15 Summer monsoon mT R 5.93 4.22 - 8.7 -72 Aug 15-20 Summer monsoon mT R 3.30 3.73 - 10.4 Nov 22-25 Cyclone mT/mPk R/S 2.20 -- -- - 102 Nov 26-Dec 11 Low-latitudetrough/cyclone mPk/mT R/S 0.61 -- - 14.4 Dec 18-20 High-latitudetrough/cyclone 2.99 - 103 Dec 25-27 Low-latitude cyclone mPk/mTmT R/SS 1 [ -- - 14.7 --
1985 Jan 7-8 Mid-latitude cyclone mPk R/S 1.13 -- - 17.1 - 125 Jan 26-27 Cyclone cP R/S 0.12 -- - 14.2 - 103 Feb 2-3 High-latitudetrough, CF A R/S 0.04 -- - 18.2 - 142 Feb 20 High-latitude trough cP S 0.00 -- Mar 11 Cyclone mPk R/S 0.00 0.60 -- Mar 14 Cyclone cP/mPk R/S 0.00 0.00 -- Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
BENSON AND KLIEFORTH 55
TABLE 4. Storm Summary--Continued
[A= Arctic;bD = deldeuterium; -- = nodata; CF = coldfront; b• 80 = deloxygen 18; % o= partsper mil; cm = centimeter;mb= millibar;PF = polar front;cP k-- continentalpolar modified; mP = maritimepolar; cP = continentalpolar; mP k = maritimepolar modified; cT-- continentaltropical; mT = maritime tropical; R-- rain; S = snow; SMOW - standardmean ocean water] Precipitation amount (cm) Sandy site Flow type Air-parcel Precip. Sandy Gail • • 80 bD Date (500-mb level) type type site site o/ oo o/ oo (SMOW) (SMOW) Mar 18-19 Cyclone mPk R/S 0.21 -- -9.7 -85 Mar 27-29 High-latitude trough, CF mP R/S 0.11 -- - 10.5 -70 Apr 17-18 Cyclone mPk R/S 0.00 0.89 -- May 9-10 High-latitudetrough/cyclone mPk R 0.65 1.13 - 16.9 - 119 Jun 2-3 Cyclone mPk R 0.00 0.06 -- Jun 24-25 High-latitutetrough mPk R 0.01 0.11 -4.3 -46 Jul 8-9 Summer monsoon mT R 0.00 0.09 -- -- Jul 10-11 Summer monsoon mT R 0.00 0.38 -- -- Jul 18-21 Summer monsoon mT R 0.89 7.35 +0.3 +2.9 Jul 25 Summer monsoon mT R 0.00 0.00 -- -- Jul 28-29 Summer monsoon mT R 0.00 0.00 -- -- Aug 26 Summer monsoon mT R 0.00 0.00 -- -- Sept5-8 Cyclone mT/mPk R 0.00 0.03 -- -- Sept 18-19 Trough/cyclone mPk/mT R 1.65 3.54 - 11.8 -83 Sept 27-28 Cyclone mT R 0.01 0.63 -5.5 -52 Sept 30-Oct I Cyclone/trough cP/mPk R 0.00 0.05 -- Oct 6-11 High-latitudetrough/cyclone mPk R/S -- - 5.1 - 40 Oct 21-22 High-latitude trough, CF mP R/S ------Oct 28 Mid-latitude trough, CF mP R ------Nov 9-13 Cyclone mPk S -- -- - 12.8 - 83 Nov 23-25 High-latitudetrough/cyclone mPk R/S ------Nov 27-Dec 2 High-/mid-latitude trough mP R/S ------Dec 16 Anticyclone cP S ------Dec 30 Cyclone mP S ------
1986
Jan 4-5 Mid-latitude trough mP/mT R/S - 14.7 - 108 Jan 15 Low-latitude trough mT R/S -- -- Jan 30-Feb I Low-latitude trough mT R/S -- -- - 10.5 - 74 Feb 3-4 Mid-latitude trough, CF mT/mP R/S 0.00 0.16 -- -- Feb 5 High-latitude trough, CF mP/cP S 0.00 0.46 -- Feb 13-16 Low-latitude trough, CF mT R/S 1.56 -- -4.7 -31 - 10.7 -69 Feb 17-19 Low-latitude trough, CF mT R/S 0.00 1.58 -- Mar 2 Low-latitude trough, CF mT R 0.00 0.21 -- Mar 8-10 Low-latitudetrough, CF mT/mPk R/S 2.18 -- -13.2 -97 Mar 12 mT/mPk R/S 0.08 -- - 10.7 -81 Mar 14-16 Mid-latitudetrough, CF mPk R/S 0.08 -- -16.7 -12.2 Mar 29 Cyclone mT R 0.00 -- -- Apr 6 Cyclone mT/mPk R/S 0.26 -- - 5.7 Apr 16 High-latitude trough, CF mP R 0.00 -- -- May 4-6 High-latitudetrough/cyclone, CF mPk/cPk R/S 0.31 -- - 11.4 -- -11.7 -- May 31-Jun 1 Anticyclone mT/cP R 0.10 -- +3.0 Jul 14-15 Summer monsoon mT R 0.14 -- -6.9 -- Jul 21-23 Summer monsoon mT R 1.07 -- - 8.6 -- - 10.0 Aug 9 Summermonsoon cT/mT R 0.38 -- +1.2 -- Aug 18 Summer monsoon cT/mT R 0.43 -- +0.7 -- Aug 26-27 Summermonsoon cT/mT R 0.85 -- -3.2 -- * Exact timining of precipitationnot known for this period which was prior to installationof tipping-bucketrain gage and micrologger. ** Data from nearby station (Dianne). Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
56 STABLE ISOTOPES SOUTHERN NEVADA
TABLE 5. Air-Parcel Stable-IsotopeStatistics for SelectedSouthern Nevada PrecipitationCollection Sites [n = number of samples;x = mean; o = standarddeviation; -- = insufficientdate]
Air-parcel All Sites Ki Sandy source • 180(o/ oo ) fiD(ø/ oo ) • 180(o/o o ) fiD(ø/ oo ) • 180(o/ oo ) fiD(ø/ oo)
A 5 - 17.3 2.2 - 131 20 1 - 19.4 -- - 150 -- 1 - 18.2 -- - 142 -- cP 15 - 14.5 3.9 - 112 29 3 - 14.1 2.2 - 100 21 2 - 18.2 5.7 - 141 54 mP 34 -9.9 3.1 -72 20 5 - 11.0 5.5 -90 25 4 - 10.6 1.5 -73 11 mPk 68 - 11.7 4.7 -87 34 14 - 13.4 4.4 -95 33 9 - 11.7 4.8 -83 32 mP/mT 13 - 10.6 1.5 -77 8 2 - 13.2 3.8 -96 37 2 - 13.0 2.4 -92 23 mPk/mT 37 - 10.5 4.0 -75 30 6 - 11.5 2.4 - 86 12 8 -9.6 3.9 -70 31 mT 121 -7.7 4.4 -54 27 16 -7.9 3.8 -59 25 21 -8.1 4.4 -57 32 mT/cT 28 -5.4 6.0 -49 36 3 -7.2 3.4 -54 18 5 +0.5 4.2 -0.5 39
and glacial age boundaryconditions. The resultsof thoseAGCM simula- at 15,000yr B.P. withalmost no change. By 12,000yr B.P. two important tions for perpetualJanuary and July climates,pertinent to this study,are changeshad occurred.First, the northernbranch of thejet movedsouth, shownin Figure 23. over andalong the southernflank of the ice sheet,and merged with the Kutzbach[ 1987]summarizes the temporal behavior of theNorth American southernbranch over the northeasternUSA. This largechange must have jetstreamsfor the monthsof July andJanuary as follows:"... In July, the beenrelated to the reducedsize and height of the ice sheet.... Thejet at splitflow aroundthe North Americanice sheetat 18,000 yr B.P. persisted 9000 yr B.P. followedabout the sametrack as at 12,000yr B.P., but with weakenedintensity. At 6000 yr B.P., andthereafter, only a singlejet core was simulatedover Alaska and northernCanada, and windswere weak com- 90380ø 150ø 120ø 90ø 60ø 30ø paredto earliertimes. In summarythe modem single jet coreof Julyfollows I I I I generallythe sametrack as the northern branch of thesplit jet duringJuly at the time of the glacialminimum. The southernbranch of the splitjet duringglacial summers has no moderncounterpart in Julybut resembles somewhata modernJanuary feature. The 12,000to 9000yr B.P. patterns
60 ø are transitionalbetween glacial maximumand modernpatterns .... In .• -•. >•- •..- January,the split flow aroundthe North American ice sheetand the in- tenseNorth American/North Atlantic jet coreat 18,000yr B.P. persisted at 15,000yr B.P. with almostno change.By 12,000yr B.P. the flow had Nevada adjustedto a singlecore of maximumwinds that followedthe "west-coast
30 o ridge,east-coast trough" pattern of today;i.e., thepatterns of 12,000,9000, 9,12 kyrB P 6000, 3000, and 0 yr B.P. all havea singleflow maximumrather than the _ O,3,6kyr BP LL• _ split flow. At 12,000 yr B.P., however,the jet maximumwas almostas 15,15kyrBP L• strongas at 18,000to 12,000yr B.P. (exceptin the southernUSA); whereas, by 9000yr B.P., thejet corestrength was significantly weakened and similar , , , t to that of today." Since1948 severalauthors [Riehl et al., 1954;Horn andBryson, 1960; Sabbaghand Bryson, 1962; Pyke, 1972]have suggested that the progres- 90ol800 150I ø 120I ø 90I ø 60I ø 30ø sionof maximumprecipitation along the westerncoast of North America is relatedto the mean positionof the jetstream.Therefore in terms of January ' 'J --L_- L_J precipitation,glacial age (18,000 to 15,000 yr B.P.) summerclimate was similarto present-daywinter climatein the Great Basin[Antevs, 1952]. Asdiscussed above, the change in • 80 concentrationof water recharged 60 ø in theYucca Mountain area from 18,500to 9000 yr B.P. is thoughtto in- dicatea progressivewarming of climateand a transitionfrom cP and North Pacific mP air-parceltrajectories to centralor even southernPacific mP trajectories.We considerthis time of transitionto be relatedto changes
30 o in positionand strength of thejetstream [Kutzbach and Guetter, 1986]. The recessionof montaneglaciers prior to theLake Lahontan highstand 14,000 to 12,500yr B.P. is interpretedto haveresulted in partfrom a progressive 15, 18 kyrB P increasein the cold-seasontemperature. As pointedout by one of the reviewers(Fred Phillips,written comm., 1988), a smallincrease in winter temperaturewould probably not have a largeeffect on glacier mass balance. Glaciersare sensitiveto two variables:winter precipitationand summer Fig. 23. Schematiclocation of core of maximum winds at 5.5 km at temperature.Therefore we suggestthat glacierrecession was due to an in- 3000-year intervals, 18,000 to 0 yr B.P., July and January [after Kutz- creasein the temperaturesof bothcold andwarm seasons.The rise in Lake bach, 1987]. Lahontanbetween about 15,000 and 14,000yr B.P. mayhave been caused Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
/ BENSON AND KLIEFORTH 57
by reductionof lake-surfaceevaporation rate and not by increasedprecipita- (11) The prehighstand-lakerecession of northernNevada glaciers is at- tion in the watershedarea [Benson, 1986]. We associatethis effectively tributed, in part, to a year-roundwarming trend that occurredbetween wetter conditionwith the year-aroundproximity of the meanposition of 18,500 and 9000 yr B.P. thejetstream. The near synchroneity of thenorthern Nevada lake-level cycle (12) AGCM simulationsby Kutzbachand Guetter [1986] and Kutzbach and the southernNevada rechargecycle suggeststhat frontal systems [1987] provide an integratingmechanism that accountsfor the near syn- associatedwith the jetstreamspanned a large area. chroneityof continentalice sheet,lake, and ground-watercycles; i.e., the positionand persistence of thejetstream (which is associatedwith the pro- gressionof precipitationand cloudiness) is substantiallyaffected by the size Conclusions and shapeof the continentalice sheet.
A comparisonof stable-isotopeconcentrations in precipitation and ground- water samplesfrom the Yucca Mountainarea of Nevadaindicates that: Appendix. One of the reviewers (Isaac Winograd, U.S. Geological (1) The stable-isotopeconcentrations of rain that falls todayin amounts Survey,written comm., 1987) has suggested that the 3H concentrationin greaterthan 0.25 cm and of snowplot closeto the mean MWL. samplesfrom UE-29a # 2 mayhave resulted in the mixingof old andmodern (2) The b•80 concentrationsin air-parcel trajectories from cP, mP, and groundwater; i.e., thesamples contain modern prenuclear age 3H and •4C. mPk moisture-sourceareas bracket the stable isotopic range of YuccaMoun- If it is assumedthat the old water was rechargedabout 9000 yr B.P. (the tain ground water. age of water from nearbywell J-12), a mass-balancecalculation indicates (3) Precipitationfrom A, cP, andmP moisture-sourceareas occurs during that about57 percentof the UE-29a # 2 sampleis old groundwater, and winter mostly in the form of snow. 43 percentis modernground water. A furthercalculation indicates that the (4) The b•80 of precipitationin southernNevada decreases by about hypotheticalprenuclear age modernwater containsapproximately 86 pCi 2.0 ø/oofor each1000-m increase in ground-surfacealtitude. This is due L- • of 3H. Thisamount of 3H is toolarge by at leasta factorof 4 to have to thefact that snow makes up a largepart of precipitationat higher(colder) originatedin the modernprenuclear age andindicates that mixing of ground altitudes. water has not occurred. (5) YuccaMountain ground water (exceptingsamples from wells USW Test-well data (Table l a) indicatethat the water table is near the land H-3 and UE-29a # 2) has deuteriumexcess values less than 10. The devia- surfaceat the siteof UE-29a# 2. During1958-1968, 3H concentrations tion from the MWL is thoughtto indicatechange in the relativehumidity in the northernhemisphere reached approximately 6500 pCi L-• as the of the moisture source area. result of atmospherictesting. It is suggestedthat contaminationof well (6) Most of the rechargein the Yucca Mountainand AmargosaDesert UE-29a # 2 occurred as the result of diffusion of 3H across the air-water areasof southernNevada occurred between 18,500 and9000 yr B.P. AGCM interfacesince the initiationof atmospherictesting. However, this doesnot experimentsfor thistime period[Kutzbach, 1987] indicate that the source implythat •4C hasdiffused into the aquifer. The partial pressure of CO2 areaof precipitationwas the high- and mid-latitude eastern Pacific Ocean. in the overlyingunsaturated zone is 10-3 (Ed Weeks,U.S. Geological Theseexperiments are consistentwith the resultsof the isotopedata that Survey,written comm., 1988) andthe partialpressure of CO2 in UE-29a # 2 indicate that the moisture sourceregion in the easternPacific moved groundwater is 10-2 (EQ3NRcalculation, this study). This indicates that southwardbetween 18,500 and 9000 yr B.P. the potentialfor carbonmigration is upwardfrom the groundwater to the (7) The b• 80 concentrationof YuccaMountain and Amargosa Desert unsaturatedzone and not in the oppositedirection. groundwater increased by about1.7 % o from 500 to 9000 yr B.P. Dur- Unlikethe mixing hypothesis, the 3H-diffusion hypothesis does not require ingthe same time period, the b• 80 concentrationin Central Pacific surface thatsignificant amounts of ground-waterrecharge take place in the Fortymile waterdecreased by about1.3 ø/oo. Thereforea shiftof about3.0 ø/oooc- Wash areatoday. This is consistentwith the resultsof ongoingneutron-hole curredin theb • 80 concentrationof precipitation infiltrating to theground- studiesof moisturetransport in the Fortymile Wash area that indicatethat watertable, relative to the b•80 concentrationin themoisture-source area. present-dayfloodwater does not penetrate to thewater table [Alan Flint, U.S. (8) Between18,500 and 9000 yr B.P. the temperatureof condensation GeologicalSurvey, written comm., 1987]. Given the aboveobservations, increasedby about5 øC; i.e., from aboutminus 5 øC to 0 øC. The low it would appearthat mixing of groundwater of differing ageshas not absolutevalues of this temperaturerange indicatethat infiltrationof significantlyaltered the distributionof ground-waterages in the Yucca precipitationto theground-water table occurred by snowmeltrunoff. This Mountain area. conclusionis consistentwith the resultsobtained by Claassen[1985] in a studyof the age and sourceof groundwater rechargedto the Amargosa Acknowledgments.The authorswish to expressappreciation to Frederick Desertarea, in a studyby Simpsonet al. [ 1972]of theseasonality of recharge Paillet, Irving Friedman, Robert Thompson,Isaac Winograd of the U.S. in the Tucsonground-water basin, and with the findingsof Winogradand GeologicalSurvey, John Kutzbachof the University of Wisconsin, Fred Riggs[1984] who showedthat rechargeto springsystems in the Spring Phillips of the New Mexico Institute of Mining and Technologyas well Mountainsarea of southernNevada occurs by infiltrationof winter-season as oneanonymous reviewer for their constructivereviews of the manuscript. precipitation. Thanksalso are dueto Wendy Milne andMy-Lien Nguyenboth of the U.S. (9) Most of the groundwater that underliesYucca Mountainprobably GeologicalSurvey for their assistancein preparingthe preliminary graphics. was rechargedby infiltrationof snowmeltalong the bottomof Fortymile Canyon,however, some Yucca Mountainground water may havebeen rechargedby mixturesof upland-and wash-type ground water. Significant References rechargehas not been noted during times of recentflooding of Fortymile Canyon.This suggeststhat times of floodingmust have lasted much longer Antevs, E., Cenozoicclimates of the Great Basin, GeolgischeRundschau, in the 18,500 to 9000 yr B.P. time interval. v. 40, p. 94-108, 1952. (10) Althoughthe potential for ground-watermixing during pumping com- Benson,L. V., The sensitivingof evaporationrate to climatechange; results plicatesthe argument,recharge of groundwater in southernNevada ap- of an energy-balanceapproach, U.S. GeologicalSurvey Water Resources pearsto haveoccurred nearly synchronously with thelast lake cycle in the InvestigationsReport 86-4148, 40 p., 1986. Lahontanbasin of northernNevada. Both the last lake cycle and ground- Benson,L. V., and McKinley, P. W., Chemicalcomposition of ground waterrecharge, events indicating regional-scale change in the hydrologic water in the Yucca Mountain area, Nevada, 197-184, U.S. Geological balance,occurred during the last glacial-interglacialtransition. Surveyopen-file report, 85-484, 10 p., 1985. Geophysical Monograph Series Aspects of Climate Variability in the Pacific and the Western Americas Vol. 55
58 STABLE ISOTOPES SOUTHERN NEVADA
Benson,L. V., Robison,J. H., BlankennagelR. K., andOgard, A. E., Wright,Jr., TheGeology of NorthAmerica, K-3, p. 425-446, Geological Chemicalcomposition of groundwater andthe locationsof permeable SocietyAmerica, 1987. zonesin the YuccaMountain area, Nevada, U. $. GeologicalSurvey open- Kutzbach,J. E. and Guetter, P. J., The influenceof changingorbital file report, 83-85,1, 19 p., 1983. parametersand surfaceboundary conditions on climate simulationsfor Benson,L. V. and Thompson,R. S., Lake-levelvariation in the Lahontan the past 18,000 years, Journal of AtmosphericScience, v. 43, Basinfor the past50,000 years,Quaternary Research, v. 28, p. 69-85, p. 1726-1759, 1986. 1987a. Kutzbach,J. E., andWright, H. E., Jr., Simulationof the climateof 18,000 Benson,L. V. and Thompson,R. 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Van der Straaten, C. M., and Mook, W. G., Stable isotopiccomposition Winograd, I. J. and Riggs, A. C., Rechargeto the Spring Mountains of precipitationand climatic variability, in Paleoclimatesand Paleowaters: Nevada: isotopicevidence, Geological Society of AmericaAbstract with A Collectionof EnvironmentalIsotope Studies, Proceedings Symposium Programs, v. 16, p. 698, 1984. Vienna, I.A.E.A., Vienna, p. 53-64, 1980. Winograd, I. J., and Thordarson,W., Hydrogeologicand hydrochemical White, A. F., andChuma, N. J., Carbonand isotopicmass balance models framework, southcentral Great Basin, Nevada, California with special of OasisValley-Fortymile Canyon groundwater basin, Southern Nevada, referenceto the NevadaTest Site, U.S. GeologicalSurvey Professional Water ResourcesResearch, v. 23, p. 571-582, 1987. Paper, 712C, 126 p., 1975.