[Berg, 1986; Stoddard, 1987]. Acidification of lake .... Melac'k and Stoddard, 1991 ]. Tonnessen [this ...... Dave Clew, Rick Kattelmann, and Kelly. Elder provided ...
WATER RESOURCES RESEARCH, VOL. 27, NO. 7, PAGES 1575-1588,JULY 1991
Solute Chemistry of Snowmelt and Runoff in an Alpine Basin, Sierra Nevada MARK W. WILLIAMS
AND JOHN M. MELACK
CenterJbr Remote Sensingand EnvironmentalOptics, Marine SciencesInstitute, and Departmentof BiologicalSciences, University of' CalO'brnia,Santa Barbara
Snowpack runoff contributionsto the hydrochemistryof an alpine catchmentin the Sierra Nevada were evaluated in 1986 and 1987 by analyzing snowpack, meltwater, and stream water samplesfor major inorganicions, conductance,acid neutralizingcapacity(ANC), and silicate.An ionic pulsein meltwater with initial concentrations twofold to twelvefold greater than the snowpack average, varying with site and with ion, was measuredin lysimeters placed at the base of the snowpack. Maximum concentrations of ions in meltwater were inversely related to the rate of snowmelt; melt-freeze cycles increasedthe concentrationof solutesin meltwater. Hydrogen concentrationin meltwater was buff'eredby ANC producedin part by the dissolutionof particulates.The anionicpulse in meltwater was observed in stream waters during the first 30 days of snowpackrunoff, with NO•concentrationsin stream waters at this time about 1.6-fold greaterthan the averageNO•' concentra-
tionfor the timeperiodof snowpack runoff,CI- about1.5-foldgreaterandSO42-about1.3-fold greater.MaximumH '• concentration duringsnowpack runoff(increase of 170%overwinterconcentrations) occurrednear maximumdischarge.ANC minima occurredat maximumdischargeas a result of dilution, with a decreasefrom winter concentrationsof 70% in 1986and 60% in 1987. Interactions
betweensnowpackrunoffandsoilswereimportantto the chemistryof streamwater.Eightyto ninety
percent oftheH * storedin thesnowpack wasconsumed beforeit reached thebaseofthecatchment. Soilswerea sinkfor NH4t' fromsnowpack meltwater, withlessthanI% of theNHd• released from snowpack storage exported i¾om thebasinasNH,i• . Sulfate concentrations in stream waterswereless
variable thanN(•).• • or CI- concentrations' sorption processes in soilswerea likelycauseforthe regulation of S(),•....... concentrations. Here we report snowpackrunoff contributionsto the hydrochemistry of a headwateralpine catchmentin the
INTRODUCTION
Fieldandlaboratoryexperimentshave demonstrated that Sierra Nevada in 1986 and 1987 and discussfour questions. initialstagesof snowmeltoften have ionic concentrations First, we determine whether the dilute snowpack of the manytimeshigherthanaveragesfor the wholesnowpack,an Sierra Nevada does produce an ionic pulse. Second, we ionicpulse[e.g., Johannessen and Henriksen,1978;Co!investigate the chemicalandphysicalprocesses that deterbeck,1981].Physicalandchemicalprocesses that determine minethe magnitudeandextentof an ionicpulsein a natural theoccurrence,magnitude,and extent of an ionic pulseare snowpack.Third, we determineif the maineffectof snownot sufficientlyunderstoodto predict the ionic concentrapackrunoffisto dilutetheionicconcentrations of streamand tionsof snowpack meltwaterat a pointin time. Researchto lake waters. Fourth, we discussthe hypothesisthat interacdateon the chemistryof snow in the Sierra Nevada has tionsbetweensnowpackrunoffand soilscanbe neglected in shown no evidenceof an ionicpulsein snowpackmeltwater alpinebasins characterized by poorlydeveloped soils. [Berg,1986; Stoddard, 1987]. Acidification of lake and stream waters during spring
snowmelt hasbeenreportedin the United States(e.g., in NewYork by Galloway et al. [1987] and in Michiganby Cadleet al. [1984]), in Canada [JefJ?ieset al., 1979], in Norway [Skartveit and Gjessing, !979], and in Sweden [Dickson,1980].The differentialrelease of ionic solutesin thefirstfractionsof snowpackrunoffis oftenconsidered to be the main cause of the acidification. Interactions between
SITE DESCRIPTION
The Emerald Lake watershed (ELW) is a north-facing
glacialcirque,locatedon the upperMarbleFork of the Kaweah River drainage,in the southernSierraNevada of California(36ø35'49"N,118ø40'30"W). The basinsurfacearea is 120 ha, elevationrangesfrom 2800 m to 3416 m, and
topography is steepandrugged,with a medianslopeof 31ø. Exposedrockscomprise 33%of the surfaceareaof the
snowpackrunoff and soils, which can modify the solute basin,unconsolidated clays,sand,gravels,and talus com-
concentrations of snowpackrunoff,are generallyconsidered priseabout 47%.Bedrock inthebasiniscomposed mainlyof negligible in alpineareasunderlainby graniticbedrock[e.g., graniteandgranodiorite withminormaficintrusions, aplite Dreverand Hurcomb, 1986]. Lakes in the Sierra Nevada dikes,andpegmatite veins[Sisson andMoore,1984;Clow, havethe lowest ionic concentrations in the United States
[Landers et al., 1987],andcatchments in theSierraNevada havea limitedcapacityto neutralizeacids[Melacket aI., 1985; Sickman andMelack, 1989].An important question is howwill snowpack runoffaffectthe hydrochemistry of high-altitude streamand lake watersin the SierraNevada.
Copyright 1991bytheAmerican Geophysical Union. Paper number90WR02774. 0043!397/91/90 WR-02774505.00
1987].Conversion of plagioclase to kaoliniteis thepredominantweatheringreactionin the basin[Clow, 1987].Poorly
developed soilscoverabout20%of thebasin,andtheseare acidicandweaklybuffered[HuntingtonandAkeson,1986; Brownet al., 1990].Theprimaryclaymineralsarekaolinite, illite, and severaltypesof vermicu!ite[Weintraub,1986]. Vegetation is sparse,including only scattered coniferous trees,although lowwoodyshrubs areoftenabundant where soilsoccur [Rundelet al., 1989].
1575
1.576
WII.! IAM%AND MI.[ A( K: (.'!•!F, MI,%IRY ½)F•N()WMI I I AND Rt•N()F!•
3000
3300
EMERALD LAKE BASIN co.•o•. ,..•.•^• • • SEQUOIA NATIONAL PARK
2sM•TERS
•
100200
METERS
I;ig. !. 'l',•>pographic map of the Emerald!.ake watershedand lt'changesin (',, and C•,t0
lO-June
changesin AI ,K during the period of spring runoff'is thrther
Fig, 13. Alkalinity(AI•K = ('t, - Ca), ('t,, andstrongacidanion (Ca) concentrations of inflow2 in 1986.A representative sample water collectedprior to snowpackrunoff'(January7)is comparedto samplescollectedduringe!ewttedanionconcentrations(May 1) and during the dilution period of snowpack runoff' (.lune 101. The decrease in alkalinity on May 1 relative to January 7 was due primarilyto increasesin (', (80%) and secondarily
illustmttcd by comparingAI,K to the ratio of (', to C•, (Figure14).At inflow2 in 1986the decreasein A!.K inearly May was the resultof an increasein the ratk)of 4'. to C•, AI.K
then increased in concentration
due t• a decrease in
the (', to (), ratio. l)uring the remainder ()1'springrunoff, AI..K decreasedas the m•tioof 4', to C), renminedrelatively constantand both (', and ('•, decreasedin concentration. Springprecipitationin 1987resultedin the (', to (), ratio0f P < 0.0001' n = 21) and not significantlycorrelatedwith C,, inflow I increasing through the m•)nth of May as ALK (r2 = 0,10;P = 0.16;n = 21).Dilutionof ANC bysnowpack decreased. The rain events in late May •tnd early June runoff'is consistent with similar findingsby Stoddard [!987] causeda peak in the (', to (), ratio)of ().72, which cc)rre(20%).
at Gem Lake and by i. oran/,er and Brakke [1988] in the Cascades.
However, during the first part of snowmelt, runoff alkalinity declineddue to the increasein concentrationel'strong
spondcdwith the AI..K minimum. AI,K then increasedwith time,as the (',, to (.;t, ratio decreasedtoward presnowpack runoff'concentrations. Alkalinity of the outflow tblloweda similar pattern in N•th years; however, the changesin
Inflow 2, 1986 60-
40-0.4
20-0.2
1
Apr
May
I
June
I
Inflow 1, 1987 -0.8
60-
-0.6
40-0.4 20-
-0.2
0
Mar
I
Apr
•
May
I
June
I .......0
Fig. 14. Atimeseries ininflowing waters ofALK (Cb- Ca)andtheratioof CatoC• during theperiodofsnowpack runoff in 1986and1987.At inflow 2 in 1986thedecrease inALK inearlyMaywasprimarily fromincreases in Ca.At inflow1in 1987thelargeincrease in theCato COratioanddecrease in ALK in lateMayandearlyJunewasassociated with spring precipitation.
WII.I,IAMSAN[) M[:I•AC'K: CIIEMISTRYOF SNOWMELT ANDRUNOFF
Co,andalkalinity of theoutflow at alltimeswerelessthan
!587
Interactions between snowpack runoff and soils were
thatof inflowingwaters. The attentuationof theseconcen- importantto the hydrochemistryof the basin. Soils were an trations in theoutflowwithrespectto theinflowsistheresult apparentsinkfor NH•- in snowpackrunoffand in spring of combining inflowingwaters with differentchemistries, precipitation. Oxidationof NH[ in soilsmay havecontribandwith dittOringlake water chemistry,in the hike outflow. uted to the elevatedNO•- concentrationsin streamwaters. Massbalancecalculations andthe low H • concentrations Sulfate concentrationsin stream waters were apparently
instream watersrelativeto H • concentrations in thesnow- regulatedby soil processes.Eighty to ninety percent of the runoffandspringprecipitation wasneutralpack andin springprecipitation indicate thattheH+ in H + in snowpack snowpack runoffwasneutralized bet•re it reachedEmerald ized before it reached Emerald Lake. Lake. Ion exchange, weathering, adsorption, titration of
HC03-, andprotonation of anionsof weakorganic acidsare
allpossible sources ofH • buffering during snowpack runoff. Discriminationof the various processes that may have
neutralized the H -• in snowpackrunoffis beyondthe scope
ofthispaper.The consumption of H * doesindicatethat interactionsof snowpack runoff with physical and chemical
processes in thebasinarean importantfactorin thechemistryof stream waters. SUMMARY ANI) C()NCI.USIONS
Acknowledgments. Kathy Tonnessenprovided encouragement throughout the study. Dave Clew, Rick Kattelmann, and Kelly Elder provided invaluable humor and strength in many a cold snowpit. Jim Sickman, Mike Williams, and Helen Hardenbergh collected and analyzed water samples, Frank Setare developed our QA and QC protocol for snow chemistry analysis, and De!ores Lucero-Sickman and Frank Setare performed the chemistry analyses. Editing comments by John Stoddard and an anonymous reviewer substantially improved the manuscript. Funding was provided by California Air Resources Board contracts A3-103-32, A6-•47-32, and A3-096-32, and an NSF Pre-doctoral Fellowship.
An ionic pulse in meltwater with initial concentrations twofoldto twelvefold greater than the snowpack average,
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I Received February 2, 199()' revised September 1, 1990; accepted December 19, 1990.