WATER
RESOURCES
RESEARCH,
VOL. 31, NO. 12, PAGES 3171-3182, DECEMBER
1995
Chemical evolution of shallow groundwater along the northeast
shore of Mono Lake, California
Tracy L. Connell
U.S. GeologicalSurvey,Menlo Park, California
ShirleyJ. Dreiss1
Earth SciencesDepartment, Universityof California, Santa Cruz
Abstract. We attemptedto quantitativelydiscriminatebetween hypothesizedsourcesand
geochemicalprocesses
responsiblefor the chemicalevolutionof shallowgroundwater
alongthe northeastshoreof Mono Lake, an alkalinesalinelake locatedin a hydrologically
closedbasinin east central California. Shallowgroundwatersamplesfrom 17 sites
perpendicularand 11 sitesparallel to the lakeshorewere analyzedfor major ions. The
shallowgroundwatercontainsremnant solutesfrom higher lake stands,which are
transportedtoward the lake by lateral flow and surfacerunoff. The flow systemappearsto
be segregatedinto two differentregions:a concentrated,highly salinegroundwater
underlyingmuch of the northeastshore and pocketsof more localizedlower-salinity
groundwater.The saturationstate of the groundwaterwith respectto certain mineralswas
determined,and simulationsfor both evaporativeconcentrationof inflow and mixingof
lake water with inflow coupledwith mineral precipitationwere performed. Solute trends
in the shallowgroundwaterresult primarily from the degreeof mixingwith historicallake
water; however,evaporativeconcentrationand redissolutioncyclesalongwith various
chemicalfractionationmechanismsincludingCa, Mg, and Na carbonateprecipitation,
sulfatereduction,ion exchange,and potentiallyMg silicateformation are alsoimportant
controls.
Introduction
Groundwaterplaysan importantrole in the chemicalevolution of waters in hydrologicallyclosed basins. Likewise,
chemical and physicalprocessesoccurringin closedbasins
significantly
influencegroundwaterevolution[Jones,1966].For
example, in arid regionswith terminal lakes, fluctuationsin
lake level and consequentalteration of lake chemistrycan
affect the chemicalcompositionof surroundinggroundwater
[Langbein,1961]. Near-surfacegroundwatersin exposedlake
beds typically experienceevaporativeconcentrationresulting
in formation of a seasonalsalt crust [Joneset al., 1969]. Understandingthe processesaffectingthe groundwatergeochemistryin theseregionsis essentialto definethe formationmechanismof salinegroundwaterand related salt crust.
centrationwith equilibrium to the atmosphereand only considered mineral precipitation.Eugsterand Jones [1979] discussed several other fractionation mechanisms, including
dissolutionand ion exchange.Using the principlesof closed
basin brine evolution by Hardie and Eugster[1970] and the
aqueoussolutionmodelof Harvieand Weare[1980],Spenceret
al. [1985b] discussedthe geochemicalevolutionof the Great
Salt Lake, Utah, in which waterswere evaporatedto examine
the effectsof mineral precipitation on the residual solutions
and the sequenceof phasesto be expected.In addition, Spencer et al. [1985a]provideda more quantitativeaccountof the
historicalhydrochemistryof the Great Salt Lake usingmixing
and evaporationcurveswith mineral precipitation and dissolution.
Rogerset al. [1992] characterizedthe groundwatersystem
aroundMono Lake as comprisingdifferentregionsdepending
istryand evolutionof groundwaterand surfacewater in hydrologicallyclosedbasins[Jones,1965;Eugster,1970;Phillipsand on recharge,evaporation,responseto lake levelvariation,geoVan Denburgh, 1971; Smith and Drever, 1976; Yuretichand morphology,and subsurfacelithology.They documentedthat
Cerling,1983; Spenceret al., 1985a, b; Thomaset al., 1989]. the arid northeast shore has the lowest recharge in the basin
owing to less rainfall [Vorster,1985], low-permeabilitysediModels for the evolution of closed basin brines were initiated
ments,
and low water table elevationand that the groundwater
by Garrelsand Mackenzie[1967],who quantitativelyevaluated
beneath
the shore is saline. They attempted to quantify the
the evolution of one inflowing water compositionin Mono
Basin, California. Hardie and Eugster[1970] used computer processesresponsiblefor the distribution of total dissolved
solids(TDS) betweenshorelinesediments,lake, and groundsimulations to determine
the fate of several inflow waters.
Thesemodels,however,were basedsolelyon evaporativecon- water through a groundwaterflow and solutetransportmodel
incorporatingfield investigationresults of water table measurements,seasonallake level change,and electrical conduc•DeceasedDecember14, 1993.
tance(EC). However,theyusedEC as a proxyin the absence
This paper is not subjectto U.S. copyright.Publishedin 1995 by the of chemicalanalysis,and they did not evaluatethe behaviorof
American GeophysicalUnion.
individualsolutes.Rogers[1992a]indicatedthat knowledgeof
Paper number 95WR02566.
groundwaterchemistrywould provide further informationon
Numerous
studies have examined
the controls on the chem-
3171
3172
CONNELL
AND
DREISS'
CHEMICAL
EVOLUTION
the sourceof salinewater and processes
the water undergoes.
In addition,in a studyon the sourcesof springwater in Mono
BasinusingmajorsolutesandSr isotopes,
Neumann[1993]was
unableto distinguish
rechargeend-membersthat influencethe
northeastshoregroundwaterchemistry.
This studyis concernedwith identifyingthe possiblecontrols
on the chemicalevolutionof shallowgroundwateralong the
northeastshoreof Mono Lake. These includeevaporationof
recharge,mixing of historicallake water with recharge,and
solute fractionationmechanisms.Becausethis portion of the
shore was lake bed before the Los Angeles Department of
Water and Power (LADWP) diversionsin 1941, the shallow
groundwaterin the area may containremnant solutesfrom
higher lake levels.The specificgoal of this researchwas to
quantitativelydiscriminatebetweenevaporationand mixingas
processescontrollingthe groundwaterchemistry.These processes,in turn, are affected by a variety of factors such as
recharge,surfaceconditions,stratigraphy,
topography,groundwater gradients,and distanceto the modern lake. Thus we
examined the groundwater chemistryvariations along two
transects:oneperpendicularto the lake andanotherparallelto
the lakeshore.The one perpendicularto the lake, called the
Ten Mile Road transect,providestrendsin soluteconcentration with depth and distanceto the modernlake. The transect
alongthe lakeshore,calledthe northeastshorestudy,permits
comparisonof the differencesin hydrochemistryarisingfrom
varying recharge, surface conditions, and stratigraphy.
Groundwatersampleswere analyzedfor the majorsolutes,
p H,
EC, and temperature.The saturationstateof the groundwaters
with respectto certain mineralswas determined,and factors
influencingsolutefractionationwere evaluatedto help determine the processes
responsible.
Saturationdatawere alsoused
as a guideto help establishassumptions
and phaseboundaries
in the geochemicalmodeling,in which we attemptedto discriminatebetweenevaporationand mixingconceptualmodels.
This was accomplishedwith the aid of geochemicalcomputer
modelingusingthe code PHRQPITZ [Plummete! al., 1988]
and the chemicaldata as input.
OF SHALLOW
GROUNDWATER
Mono Basin
no
Basin
California'•
Murphy
Mile Road Site
Mono Lake
Lee Vining
Creek
Ten Mile Road Site
0 ....
Quaternary
and volcanicsediments
ash
Pleistocene
glacial
deposits
Quaternary
volcanics
:2• Tertiary
volcanics
and sediments
Pre-tertiary
granite
andmetamorphics
5 ....
km
10
• 5xxxe
gø••
]TenMile
Road
©D5A
1©©D5B.3
I
2©
3©
4©
5©
6©
Map not to scale
Figure 1. Generalizedgeologicmap of Mono Basinshowing
approximatelocationsof Lee Vining Creek, Rush Creek, Murphy Spring,Methane Springs,the Warm Springscomplex,and
the Ten Mile Road site,includingthe approximatelocationsof
wellsand piezometers.Modified from Rogers[1993].
ers, 1992a].Runoff in the winter from snowmeltand rainfall
can redissolve the crust.
In the arid part of the basin,mountainstreamsand springs
infiltratepermeablealluvialfan and delta deposits,recharging
Mono Basin, located in east central California, is a hydro- groundwater[Vorster,1985].This freshrechargeflowslaterally
logicallyclosedbasinwith Mono Lake beinga regionalground- as groundwater underflow downgradient toward the lake
water sink (Figure 1). Evaporationfrom the lake and near- whereit discharges
alongthe shoreor at the lake bottom both
surfacegroundwaterprovidesthe main sourcesof water lossin in springsand as diffuse seepage[Lee, 1969; Sinclair, 1988;
the basin [National ResearchCouncil, 1987]. Snowmeltand Danskinet al., 1991].Both the near-shorewater table and the
rainfall in the adjacentSierra Nevada providealmostall the lake level are high in late fall, winter, and springowing to
bulk inflow to the basin throughrunoff in perennial streams runoff from rainfall and snowmelt and recede in the summer
and by groundwaterseepage[Vorster,1985;National Research and fall owingto evaporation[Rogers,1992b].
Council, 1987]. The Sierra Nevada snowpackaccountsfor
The stratigraphyalong the northern and northeast shore
about 85% of the surface and subsurfaceinflows to the basin, includesa seriesof sandyand gravellybeachand volcanicash
with the majority carried by Lee Vining and Rush Creeks depositsinterlayeredwith fine-grainedlacustrinesediments
[Vorster,1985;NationalResearchCouncil,1987].
[Stine,1987] (Figures2 and 3). The surfaceof the shoreis
Followingthe export of water from streamsfeedingMono coveredwith sandnorth and west of Warm Springs,whereas
Basin,beginningin 1941by LADWP, the level of Mono Lake southeastof Warm Springsthe surfaceconsistsof clay[Rogers,
hasdroppedmore than 12 m [Stine,1987].This declinein lake 1992b].At mostsites,sedimentsdeeperthan about0.3 m were
level has exposedformer lake bottom, creatingan extensive blackin colorand had a strongsulfurousodor [Rogers,1992b].
area of barren playa alongthe northeastshore.Groundwater
beneaththisportionof the shorelineis alkalineandverysaline.
Methods
A shallowwater table along with strongcapillaryforcesand
evaporativeflux causesevaporationof groundwater,whichreAt the Ten Mile Road transect,water sampleswere colsuitsin the developmentof a 1-cm seasonalsalt crustduring lectedfrom 17 sites,including15 piezometers(Figure 2) and
the summer[Lee, 1969;Basham,1988;GroeneveM,1991;Rog- two deeper wells (Figure 1). Fifteen piezometerswere ar-
Site Description
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
GROUNDWATER
3173
Ten Mile Road Shoreline Piezometers
AverageTotal DissolvedSolids(g/kg)as of 3/92
_•,
I
1951--
_.e
_
iiiii!(•A)
2
VE=50
• 1948--
.1:3
1•
.•-
3
Plezometer array
sand,
some
gravel -i:::• black
silt,
silty
sand, __
4
Methane Springs
sandysilt, someclay
Auger s•tes
5
1945_
6
lake level
ß
.•
•
i.u
1942-
•2-:-:-:-.'-..
Warm Springs
'":::: :.:.:.:.:.:.:.:.:v.-.-.
- -.-.:_.-f(•)!-305
Mono
610
Lake
915
Distance (meters)
East Beach
Figure 2. Ten Mile Road transectcrosssectionshowinggeneralized stratigraphyand locationsof the 1991 piezometers.
Depth of each well's screen is indicated. At each site the
shallow,intermediate,and (where present)deep piezometers
are designatedby lettersA, B, and C. Average total dissolved
solids(TDS) (gramsper liter) values(throughMarch 1991)are
postedat the location of the piezometerscreento which they
apply.Modified from Rogers[1992b].
rangedin sixnestsof two to three piezometerseach,at a lateral
spacingof 150 m, in a transectperpendicularto the lakeshore.
All nestshavetwo piezometersat approximatedepthsof 1 and
2 rn (labeledA and B, respectively),and nests1, 3, and 5 have
a third piezometerat a depthof 4.3 m (labeledC). The deeper
wellssampled,labeledD5A and D5B.3, bottom at 12.7 and 5.6
m below groundsurface,respectively.Two setsof 17 samples
SalmonSpring
o
3 KM
•
I
Figure 4. Map of Mono Lake'snortheastshoreshowingapproximatelocationsof May 1992augerholes.Site numbersare
shownbesidethe augerlocation.Shadingrepresentselevation
contours.
ModifiedfromGroeneveld
[1991]andRogers[1992b].
boron (B) by the methodof UppstrOm,
[1968], and silicaand
SO4by U.S. GeologicalSurveytechniques[Fishmanand Freidwere collected from the Ten Mile Road transect, one in Deman, 1989].Exceptfor two samplesthe chargebalanceis within
cember1991and one in March 1992.In May 1992,11 pitswere 5%. A detaileddiscussion
on methodologyand calculations
of
excavatedby shovel, 1.8 m deep, along the northeastshore error and TDS waspresentedby Cormell[1993].
transect(Figures3 and4), andseepagewaterwascollected.All
sampleswere collectedaccordingto the U.S. GeologicalSurvey water qualitysamplingprotocol[Brownet al., 1970;Hem, Geochemical Results and Interpretation
1989], and temperature,pH, and EC were measuredin the
The soluteconcentrations,
TDS content,pH, EC, andtemperfield at the time of collection.
atureof the groundwater
samples
collectedfromthepiezometers
Sampleswere analyzedfor HCO3, CO3, C1,Na, K, Ca, and at the Ten Mile Road site are presentedin Table 1 and from
Mg using the standard methods of Greenberget al. [1985], the northeast shore seepagepits in Table 2. No significant
West
MonoLakeNortheastShorePit Stratigraphy
East
Ten Mile
Road
Methane
_
1945--•6
•
:>• Springs
•55
_
•88•22 104
--
--1945
__
Warm
•07•16 Springs --4Ass10
ss9ss8ss5
:;:!L.23
1943-__
• Gravel
ss6
-•]
Silt
•8 •99•33•11•33
194
3
ss7
_
'-• Clay
x7 Water
leveland
78
EC (mS/cm @ 25øC)
ss3
ss2
ssl
ss12
ss13 --
1941
(elevationin meters)
Figure 3. Northeastshoretransectgeneralizedstratigraphyand approximateelevationsof augersites,along
with groundwaterand EC values.Site numbersare shownbelowstratigraphiccolumn.Modified from Rogers
[1992b].
3174
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
GROUNDWATER
Table 1. Averagep H, ElectricalConductance(EC), Temperature,SpeciesConcentration,and Total DissolvedSolids
(TDS) of the GroundwaterSamplesCollectedin December1991 and March 1992From the Ten Mile Road Site
Site
D5A
D5B.3
' 1A
lB
1C
2A
2B
3A
3B
3C
4A
4B
5A
5B
5C
6A
6B
pH
9.17
9.83
EC,
TempermS/cm
ature,
at 25øC
øC
1.42
Ca,
Mg,
K,
Na,
C1,
Alkalinity,
mg/kg mg/kg mg/kg mg/kg mg/kg
mg/kg
340
180
590
Sulfate, Boron,
mg/kg mg/kg
50
2
Silica, TDS,
mg/kg mg/kg Density
10.6
1.1
0.25
24
9.86
9.84
9.91
9.98
9.87
37.8
4.8
5.3
21.1
8.7
7.9
9.3
8.3
8.7
8.8
5.6
5.3
1.7
1.5
1.4
1.4
1.6
1.1
0.55
0.86
1.08
1.34
1.21
0.88
560
76
75
300
100
100
10,400
1,200
1,200
5,800
2,200
1,800
6,400
500
56(1
3,200
1,000
980
14,100
1,900
2,000
7,600
3,300
2,900
3,600
310
370
1,800
520
470
168
19
16
100
32
25
14
13
13
25,600
2,700
2,900
13,400
4,900
4,300
870
1.00
1.03
1.00
1.00
1.01
1.00
1.00
9.80
9.72
15.6
19.2
5.2
5.6
1.3
1.0
0.51
0.65
3,600
5,000
2,400
2,800
4,700
6,900
22.2
36
32.3
36.2
38.8
20.1
48.8
48.4
7.0
5.0
6.0
5.8
4.9
5.8
5.3
5.9
4.8
1.5
1.8
4.9
1.6
1.5
2.3
1.8
1.08
1.54
1.66
2.58
0.68
0.91
2.86
0.95
5,600
10,000
9,100
9,300
11,200
5,100
13,800
14,000
3,000
7,500
6,100
5,900
7,500
4,000
10,200
9,300
7,000
12,000
13,100
13,200
14,400
7,500
17,300
17,300
910
1,500
1,100
2,300
2,000
1,800
2,200
70
2,900
3,400
42
104
80
36
34
16
142
152
60
23
69
32
8,400
111600
11,500
24,400
22,300
22,100
23,700
12,200
33,300
33,300
1.01
1.01
9.92
9.50
9.49
9.45
9.31
9.32
9.36
9.40
170
260
300
580
500
470
520
330
740
740
1.01
1.02
1.02
1.02
1.02
1.01
1.03
1.03
Alkalinityis expressed
asbicarbonate.
Boronand silicaconcentrations
are for the March 1992samplesonly.
seasonalchangein EC readings[Rogers,1992b]or soluteconcentration[Connell,1993]wasobservedduringthe monitoring
period. The major differencesin the chemistryof the watersof
Mono Basinare illustratedusingPiperplots(Figures5 and6).
Surfacewater in Mono Basin varies from a dilute Ca-HCO3
type presentin creeksand a Mg-Na-Ca-HCO3 type in mountain springsto a salineNa-C1-HCO3type presentin the lake
(Figure 5). At the Ten Mile Road transect,shallowgroundwater samplesevolve from a concentratedNa-HCO3 water farthestfrom the lake to a very salineNa-C1-HCO3brine near the
lake (Figure 6). The seepagewater collectedfrom pits along
the northeastshore also is a very concentratedNa-C1-HCO3
brine (Figure 6). An analysisof possiblesolutefractionation
mechanismsand spatialtrendsin solutebehaviorperpendicular to the lake and parallel to shore is presentedin the next
section.The soluteconcentrationsare then used to quantita-
tivelycompareresultsfrom evaporation
an.•mixingsimulations.
Fractionation During Brine Evolution
Chemicalfractionationof solutesmay significantly
influence
the evolutionof water compositionand the behaviorof individualsolutes.And althoughwe cannotquantifyall the various
types of fractionation, we neverthelessevaluated the factors
influencingsolutefractionationto help determinesomeof the
processesresponsible for the chemical evolution of the
groundwater.Groundwaterand lake water samplesare undersaturatedwith respectto halite (Figure 7). Thus,in this study,
C1is usedas a conservative
referenceof concentrationduring
evolution [Eugsterand Jones, 1979], and behavior of other
solutescan be evaluatedrelative to that of C1.Figure 8 shows
Table 2. Thep H, EC, Temperature,SpeciesConcentration,
and Total DissolvedSolidsof the GroundwaterSamples
Collectedin May 1992From PitsExcavatedAlong the NortheastShoreand SamplesFromNeumann[1993]IncludingMono
Lake (MLK), MurphySpring(MUR), RushCreek(RCR), and Lee Vining Creek (LCR)
Sample pH
EC,
TempermS/cm
ature,
at 25øC
øC
SS1
SS2
SS3
SS5
SS6
SS7
SS8
SS9
SS10
SS12
SS13
MLK
MUR
RCR
LCR
63
39
78
116
107
104
62
38
45
61
83
98
0.107
0.065
0.057
9.75
9.70
9.65
9.60
8.60
9.70
9.70
9.60
9.70
9.70
9.50
9.80
8.20
6.40
6.60
Ca,
Mg,
K,
Na,
C1,
Alkalinity, Sulfate, Boron, Silica, TDS,
mg/kg mg/kg mg/kg mg/kg mg/kg
mg/kg
mg/kg mg/kg mg/kg mg/kg Density
11.5
13.0
10.0
14.0
12.0
13.0
9.0
9.0
9.0
11.0
11.0
12.8
7.0
3.0
1.0
Alkalinity expressedas bicarbonate.
2.7
1.4
2.3
2.1
1.1
1.6
1.9
2.5
2.2
2.3
1.6
3.7
8.6
9.4
5.6
2.80
950
1.53
510
2.08 1,300
3.48 2,100
2.53 1,700
4.49 1,900
1.02 1,100
1.01
630
1.81
990
1.22
960
1.68 1,250
37.10 1,600
4.70
2.00
1.20
2.3
1.2
0.8
18,500
10,400
23,200
38,000
32,800
34,100
19,700
13,300
19,500
19,100
23,400
31,900
9
3
2.1
20,800
10,800
25,600
39,200
35,300
32,900
19,400
14,400
1,700
19,300
26,300
20,500
5.3
8.2
5.3
24,600
1,500
29,600
46,900
37,600
44,700
24,300
16,000
20,900
23,600
27,200
25,300
599
33.4
24.6
6,100
5,800
7,600
11,300
10,800
12,400
6,300
6,700
5,300
6,400
7,400
12,500
9.1
24
4.8
167
40
557
26
323
23
328
379
24
12
0.12
0.02
0.03
37
6.2
9.3
58,400
35,300
72,300
114,300
99,100
103,200
58,800
42,900
53,200
57,400
72,000
81,600
98
60
44
1.05
1.02
1.06
1.09
1.07
1.08
1.04
1.03
1.03
1.04
1.05
1.07
1.00
1.00
1.00
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
that B alsobehavesconservativelyin the groundwatersystem,
however, the other ions do not.
Sodium and potassium. Na generallybehavesconservativelyin the groundwatersat the Ten Mile Road Site, demonstratingonly slight depletionrelative to C1 with proximityto
the lake (Figure 8). In northeastshoreseepagewater, Na/C1
decreasesevenmore comparedto the Ten Mile Road groundwaters and lake water. These samples,along with the lake
water sample,approachsaturationwith respectto gaylussite
(Na2Ca(CO3)2'5H20)(Figure7), whereasthe Ten Mile Road
samplesare undersaturated;therefore mineral precipitation
can accountfor the lossof Na in only the most concentrated
waters. Gaylussiteis a commonmineral in recent muds of
alkalinesalinelakes [Eugster,1980].Bischoffet al. [1991] observedgaylussitecrystalson hard substratesbelow the lake
surface,
andBischoffet
al. [1993]suggested
thatgaylussite
can
form from ikaite. In addition,cyclicdeflationof alkali carbonate crustsand dissolutionand infiltrationresultingin C1recyclingcan removeNa relative to C1.
K is depeletedrelative to C1 in the groundwaterand lake
water (Figure 8). Groundwaterscollectedfrom the Ten Mile
Road Site show a slight enrichmentof Na over K as they
becomemore concentrated(Figure 8). K is probablyremoved
from thesewatersby ion exchange[Joneset al., 1969;Eugster
andJones,1979],in whichK displacesCa on the exchangesites
of lacustrineclaysor volcanicglassesin ash layerswhen exposedto salinewater [Eugster,1980].
Calciumand the carbonatespecies. Ca is depletedrelative
to C1in groundwaterand lake water, and althoughthe carbonate speciesincreasein concentrationas water evolvesfrom
dilute inflow to concentratedgroundwaterand lake water,
thesespeciesare alsoremovedfrom solutionin 'moreconcentrated groundwatersand lake water (Figure 8). Groundwater
and lake watersamples
are supersaturated
witti respectto
calcite
andapproach
saturation
withrespect
to'•gaylussite
(Fig-
OF SHALLOW
GROUNDWATER
3175
PiperPlot showingthe chemicalcompositionof
shallowgroundwateralongthe NortheastShore
Cl
+SO
4_20•0
Ca + Mg
AIk.
/
80
'4•Na
+K
4
.
20 40 60 80 O/o•nits
20 40
Ca'
' Na+ K
AIk.•
x Ten Mile Road
o piezometers
(12/91)
and (3/92)
_
.
60
80
'
a NortheastShore
auger
holes
(5/92)
Figure 6. Piper plot showingthe chemicalnature of groundwater alongthe northeastshoreof Mono Basin.Symbolsrepresent individual samplingsites.
calcite,andsamples
from the northeastshorerangefrom 4- to
9-fold supersaturation.
Recent studiesby Bischoffet al. [1993] have shown that
althoughcalcite is supersaturatedin Mono Lake water, precipitationof this mineral is stronglyinhibitedby high concentrationsof orthophosphate.They suggestedthat duringwinter
months, ikaite (CaCO3.6H20) precipitatesas spring water
mixeswith lake water becauseof its low solubilityat low temperaturesand becauseof the inhibitoryeffectof high concentrations of orthophosphatein the lake. During warmer seasons,ikaite is either transformedto anhydrousCaCO3, which
is taken up into shorelinetufa, or dispersedthroughoutthe
ure 7). Thusthe precipitationof calciteis a probablereasonfor
the Ca depletion[Lee, 1969;Eugster,1980].The precipitation lake by wind to form gaylussite[Bischoffet al., 1993]. Similar
of calcium carbonate minerals, such as calcite, is well docu- processesmay be occurringin the groundwateralong the
mentedfor Mono Basin[Russell,1889],and thesemineralsare northeast shore in which calcite precipitatessecondarily.In
currentlythe primary precipitate [Cloud and Lajoie, 1980]. addition,cyclicwettingand dryingof playa sedimentsfollowed
Groundwatersamplesfrom the Ten Mile Road site increase by subsequentremovalof alkaline earth carbonatesby deflalakewardfrom 1.7- to 5.9-fold supersaturation
with respectto tion [Eugster,1980;Drever, 1988] may accountin part for the
lossof Ca and the carbonatespecies.
Magnesium and silica. The behavior of Mg and silica in
waters of Mono Basin, as well as other hydrologicallyclosed
PiperPlot showingthe chemicalcomposition
of waters in Mono Basin
basins,is complexand not well understood.Both are depleted
relative to C1 in the groundwaterand lake water (Figure 8;
/•
[]Creeks
Tables 1 and 2). Thesesolutesmay be removedfrom solution
el+SO
427 x•oCa+MgoMt.
Springs by
incorporationin authigeniclayered Mg-silicates[Spencer,
7 4••
•6o•,
+Shore
Springs
//
'
i
\ \
x Warm Springs
1985b] suchas tri-ocathedralsmectite[Yuretichand Cerling,
AIk / •o,
,40•
a
, •{• /• •0
MonoLake 1983;Jones,1986]. Another likely sink for Mg includesbiogenic or chemical precipitation of Mg carbonate minerals
/ +.0 +
•
[Eugster,1980;Sackset al., 1992].Silicamayalsobe removedin
lake water by diatomuptake [NationalResearchCouncil,1987].
Sulfate. SO4, in general, is concentratedby evaporation
duringbrine evolution.Groundwaterand lake water showalmost no fractionation,with the exceptionof one of the Ten
Mile Road deeppiezometers(5C) (Figures8 and 2). Groundwater and lake water samplesare undersaturatedwith respect
2• x4060/
80
20' 40 60
Na
+ %bnits
K
AIk.
• O80
to mirabilite(Na2SO4.10H20)(Figure 7); they are alsounderFigure 5. Piper plot showingthe chemicalnature of surface saturatedwith respect to gypsum[Connell, 1993], primarily
watersin Mono Basin.Symbolsrepresentindividualsampling becauseof the low concentrationsof Ca. The mostlikely mechanismfor SO4 lossin the deeper groundwaterat 5C is microsites.Modified from Neumann [1993].
3176
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
GROUNDWATER
1.2
Ten
Mile
Road
Site
I ß• ß ß
1
Northeast Shore
X
Supersaturated
-
Mana
Lake
Water
0.8
ss5
Saturated --
Undersaturated
O
0 (• 0 0 O0 ß
0.6
0.4
0
0
$
%
o
0
0.2
Supersatitrated
Saturated
-0
-0.2
10(
1,"000
10,000
100
Northeast Shore
X Mana
Lake
Water
-7
100
1,000
10,000
100
concentration
of C1(mg/kg)
concentrationof C1 (mg/kg)
1
1
Supersaturated
.Saturated.
0
Ten
Mile
Road
Site
I
O
Undersaturated
..........
Su'•;•a•it'•i'
o
Undersaturated
,Saturated.
Undersaturated
Ten
Mile
Road
Site
I
x
•
Northeast Shore
Mana
Lake
Water
o
•O O
5c
Northeast
ShoreSite
2 Ten
Mile
Road
X
Mana Lake Water
-6
100
1,000
10,000
100 ooo
concentration
of C1(mg/kg)
100
1,000
10,000
100
concentratioh
of C1(mg/kg)
Figure 7. Saturationindices(SI) of calcite,gaylussite,
halite,and mirabiliteversusC1concentration
for the
Ten Mile Road and northeastshoregroundwaterand Mono Lake water samples.
bially mediatedsulfatereduction.During this process,hydrogen sulfidegasis released[Drever,1988],and thisgashasbeen
found duringwell installationand samplecollection[Rogers,
1992b].In addition,black sedimentswere encountered0.3 m
belowsurfaceat mostsites[Rogers,1992b],andDomagalskiet
al. [1990] have documentedamorphousFeS precipitationin
Mono
Lake bottom
sediments.
Spatial Trends in Hydrochemistry
the winterredissolves
the crust,and thisrunofffrom snowmelt
andrainfallhascreatedchannelsthat trenddownslope
toward
the lake. The winter runoff transportssolutesdowngradient
towardthelake,increasing
soluteconcentration
withproximity
to the lake. In addition,discharging
springsand seepscan
redissolve the crust at discrete locations. The dissolution of the
crustby the dilute runoff causesfractionation,in which soluble
constituents,like Na and C1, dissolvefirst and the least soluble
silica and alkaline earth carbonates remain in the crust. Runoff
Ten Mile Road transect. Along the transectperpendicular
to the lake the TDS and concentrationsof C1, B, Na, K, and
alkalinity increasetoward the lake for the shallowerhorizons
and are uniformwith depth(Tables1 and2; Figure2). Values
in the deeper C horizon showlittle spatialvariation.These
and mixeswith existinggroundwater,therebyaddingmore
solubleconstituents
to downshore
groundwaters.
Thisfractionation due to seasonalwettingand dryingcyclesincreasescon-
deeper(C) wellsbottomin a sandysilt layer(Figure3), and
centrations of more conservative solutes toward the lake in the
accordingto Rogers[1992a]thesewellshad the longestrecovery ratesafter purging.The recoveryratessuggest
that thisunit
has low permeabilityand isolatesdeeper groundwaterfrom
shallower
groundwaters,
andin partaccounts
for theuniformly
flowsdownslope,undergoesfurther evaporation,becoming
even more concentratedwith solubleconstituentsdownshore,
low concentrations
of Ca, Mg, and silica.
Northeast shore transect. Along the transectparallel to
shallower waters above.
the northeastshore,the TDS and concentrations
of C1,B, Na,
Significantly,the SO4, Ca, Mg, and silica concentrations K, SO4, and alkalinityvary considerably
from relativelylow
differ from these trends.Although the concentrationof SO4 valuesnearTen Mile Road (ss9and ss10)andWarm Springs
increases
towardthe lakein the A andB horizons,it decreases (ss2)to highervaluessoutheast
of MethaneSprings
(ss5,ss6,
lakewardin the deeperhorizon.The concentrations
of Ca and andss7)that generallyexceedthosein Mono Lake (Table2;
Mg are extremelylow and do not changespatially.The con- Figure4). Ca and Mg concentrations
are extremelylow in all
centrationof silicain the groundwaterand lake water remains the samples,and silica concentrationsremain lower than 40
fairly low at generallylessthan 40 mg/kg. The lack of corre- mg/kg. These results indicate that the same solute fractionspondenceof these soluteswith the trends mentioned above ationmechanisms
discussed
earlieroccurall alongthe northandtheextremely
lowconcentrations
of Ca andMg areexplained east shore.
by the solutefractionationmechanismsdiscussedearlier.
Mixing of freshwaterappearsto be locallyimportantin exThe shorelineat the Ten Mile Road site slopesgentlyto- plaining the waters with low concentrationand TDS values.
ward the lake, and the near-surfaceshallowgroundwaterflows The CottonwoodCreek delta,near ss9and ss10,and springs
in the samedirection[Rogers
et al., 1992].The lackof recharge associated
with the Warm Springscomplex,nearsitess2,may
in the late summerand fall, alongwith a shallowwater table providethefreshinflowintothegroundwater
system
(Figure4;
and fine-grainedsediments,enhancescapillaryevaporation. Table 2). A springmoundnear site ss2,locatednorthwestof
Seasonalsurfaceconditionsindicatethat exceptfor winter, the Warm Springs,had an EC of 2.8-1.08 mS/cm [Groeneveld,
northeast shore is covered with an effiorescent crust. Runoff in
1991;Rogers,1992b].
ß
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
GROUNDWATER
B versus C1
3177
Na/C1 vs C1
lOO
100
ß .......
I
........
I
i
........
I
i
;
;
!
[] i
i
........
; I
i
•
0.01 .............
['• .......!..............
•"
• •
•
0.001
•
•
•
•
,
•,![]1t]
.............
..'
.............
'..
..............
!..............
i.............
?.............
I
......
i I [] Shore
Springs
;
!
........
4' Creeks
;
[
!
I
!
i
i [
10 ...............:..............:.............4....[
+ /6
I
i I o Mt.Spring
[]
+
WarmSprings
X
MonoLakeWater
0 TMRGroundwater
ß NSGroundwater
o •';•
•
|
1 .............. ,;----o.....................................
.
•.............. • .............
;
m suo••
0 TMR•o•dwater •
X Mono•e WaterI•
.............'•.............
•..............
•'" • NS•o•dwat•
O.1
0.01
1
10
100
1,•
:
0.1
........•? .......• ........[ ........• ........• .....
0.0001
........ '
0.01
10,0•
........ '
0.1
Na/K
K/C1 versus C1
........
]
'Om
........
]
........
[]
,
........
t
Mt. Springs
4'
Creeks
[]
+
ShoreSprings
Warm Springs
0
ß
X
TMR Groundwater
NS Groundwater
Mono Lake Water
........ ;
100
.......
1,000
10,000
versus C1
lOO
.....
O
........ l
10
Cl (mmol/kg)
C1 (mmol/kg)
10 , ß ....... ]
........ '
1
|
,
.
_.
i
o
!
'
10..............
' ..............................
+++
+ i;
[]i•o +++
+++
•
.
,
.
,
,
[]i, o•
1
o
1
O.Ol
10
100
1,000
10,000
........
0.01
[]
4'
[]
O0
0
TMR Groundwater
ß
X
NS Groundwater
Mono Lake Water
i
........
O.1
+
i
Ca versus C1
•
•
••
+ -t•
1 ..............
; ....... O,.... : ..............
•.-.
i
........
10
ShoreSprings
Warm Springs
i
........
100
!
........
1,000
10,000
Cl (mmol/kg)
Alkalinity/C1 versusC1
........ , ........ , ........ , ........ • ........ • .......
10 .............. }.............. •.............. }--.
........
1
el (mmol/kg)
100
o
Mt.
4'
Creeks
O
Mt. Springs
4'
Creeks
[]
ShoreSprings
lOO
4,
•Mt.
Spring
[]i []
+Warm
Springs
Creeks
Shore
Springs
Warm Springs
..............
i..ffi......O....½..+.
.........
•.. O TMRGroundwater
0 TMRGroundwater
ß
NS
Groundwater
X
Mono Lake
:0
Mono
LakeWater
+.
v
i.ø!1
i
o.'o i
0.1 .............. •---ffi....................... •.............. • ..... O" "• ..............
rO
i
.
0.01
..............
[]
oo
:o
o '•.
o
: ...............
ß
.Bt........................................................
,
,
,
o.1
0.01
0.1
1
10
100
1,000
10,000
0.01
0.1
Mg versusC1
10 ...............................................
'•-•'+
.
100
1,000
10 ooo
SO4versusC1
O Mt.
Springs
4'
Creeks
[]
+
ShoreSprings
Warm Springs
0
ß
TMR Groundwater
NS Groundwater
X Mono
Lake
Water
+
•
10
Cl (mmol/kg)
C1 (mmol/kg)
100........• ........] ........,
1
!
o ..............................
X
,'.............................
0.1..............
i'"•.......
T.............
]..............
!"g'2'i't!............
0.01
0.01
i!
........ i
O.1
i
[]:
........ l
i%ø0 O:
ø•(
:
' .......
. . .o..3 ........ ; ........ i
10
100
c] (mmo]/kg)
1,000
10,000
0.01
0.1
1
10
100
1,000
10,000
C1 (n'nnol/kg)
Figure 8. Plots of solutesas a function of C1 concentrationsfor the Ten Mile Road and northeastshore
groundwaterand Mono Lake water samples.Analysesof mountainsprings,creeks,shoresprings,and warm
springsbasedon data from Neumann [1993].
3178
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
GROUNDWATER
At sitesss5,ss6,and ss7,with the mostconcentratedgroundLake Water mixedwith Murphy Spring
water, the lack of proximity to discretefreshwaterdischarge
at25øC;
logPco2
= -3.5'equilibrated
tocalcite
areas,along with evaporativeconcentrationand redissolution
cycles,contributesto the elevatedTDS and soluteconcentration values(Figure 4; Table 2). In addition,the proximityof
1--__-CI-......
g._
....•
..........
-Mg2+
I
concentrateddischargesourcessuchas the Methane Springs
0.1'--'
..............
'• • ,,
'C1complexmay also be important. Samplescollectedby Groeneveld [1991] from seepsat and near Methane Springshad
relativelyhighEC values(11-16 mS/Cm)and B concentrations
(120-350 ppm). Oremlandet al. [1987]and Groeneveld[1991]
observedan increasein the quantity of methane venting at
distinct localesalong a transectfrom Ten Mile Road to the
0.0001 :
Methane Springscomplex.Groeneveld[1991] noted that the
seepage of methane is accompaniedby the occurrence of
highly concentratedwater. The general southwarddip of the
topographyand underlying strata along this portion of the
0.000001
,,,
•,,,
I,,,
•,,,
•,,,
• , , ,
shoremay inducea southwardflow of the surfaceand subsur100
80
60
40
20
0
-20
wt % Lake Water
face water carryingsolutesfrom theseconcentrateddischarge
sourcesthrough the groundwatersystem.
Figure 9. Calculated resultsof Mono Lake water mixed with
Groundwatersfrom the sitessouth of Warm Springs(ssl, Murphy Springwater, equilibratedto calciteprecipitation.
ss12,and ss13)haveintermediateconcentrations.
The surface
•"Na+........
"Ca2+
I
i.........•,.•,,•
SO42-•
"'
•--''-• 'Na+
I
0.001
•'-Mg2+
-CO32I
0.00001
f''Ca
2+'
of the northeast
shore is covered with sand north and west of
Warm Springs,whereas south of Warm Springsthe surface
consistsof clay and an additional clay layer is presentbelow Mixing Simulations
surface(Figures3 and 4). The surficialand deeperclay horiIn the mixingsimulationsthe mixingof inflowingwaterwith
zonsmay prevent mixingof solutesfrom redissolvedsalt crust remnant Mono Lake water was conducted at a constant temand flushingwith fresherrecharge.
perature(25øC)and atmospheric
CO2 pressure(log PCO2 --3.5) usingthe chemicalcompositionof Mono Lake water
presentedby Neumann [1993].The input chemicalcomposiComputer Modeling
tion,p H, and temperatureof the inflowingwater usedin this
We examinedtwo conceptualmodelsfor the hydrochemistry studywere thosefor Murphy Spring and Lee Vining Creek,
usingthe computercode PHRQPITZ [Plummetet al., 1988] alsopresentedby Neumann [1993]. Two sourceswere chosen
and measuredchemicaldata:(1) the mixingof compositionally as representativeof water that when mixed with Mono Lake
differentwaterswith and withoutcalciteprecipitationand (2) compositionwater, would either directlyproduceor indirectly
the concentrationof sourcewaters by evaporationcoupled affectthe chemicalcompositionof the groundwateralongthe
with calciteprecipitation.The first, referred to as the mixing northeastshore.MurphySpringrepresentsupgradientgroundmodel, simulatedthe effectsof mixinginflow waterswith rem- water rechargefrom the Bodie Hills (Figure 1). Lee Vining
nant Mono Lake water in the pores of the playa sediments. Creek is a representativeSierraNevadawater and, alongwith
The second,referred to as the evaporationmodel, simulated Rush Creek, carries the majority of runoff reachingMono
the effectsof evaporatingand therebyconcentratinginflowing Lake (Figure 1) [Vorster,1985].
waters.To comparethe model resultsto the actual measured
The modelingindicatesthat by mixingprogressively
higher
water composition, we utilize a root-mean-square error percentagesof Mono Lake compositionwater with either Mur(RMSE) in the form
phy Springor Lee Vining Creek compositionwater,the trends
RMSE = [(E{(C ....... d-- Ccalculated)/C
....... d)2)/(N -- 1)] ø'5,
observed for some of the absolute concentrations
in the shal-
low groundwatercanbe duplicated(Figure9). Individualmixture ratiosof Lee Vining Creek to Mono Lake water required
to yield the chemicalcompositionof specificgroundwatersamplesare aboutthe sameasthoseobservedfor Murphy Spring,
because the highly concentratedalkaline saline lake water
compositiondominatesthe resultantmixture compositionat
very low percentages
regardlessof the inflow source[Connell,
1993].
Figure9 illustratesthe resultsof the Murphy Springmixing
allowingcalciteprecipitationto equilibrium.The Ca concentration decreasesuntil a mixture ratio of approximately10%
lake water and then remainsconstantwith increasingaddition
evaporation line, and concentration factors were estimated of lake water. Calcite precipitationremovesthe availableCa
visuallyfrom the plot [Connell,1993]. The molal concentra- from the inflowingspringwater as it mixeswith the alkaline
tionsof the other nonconservative
ionswere multipliedby the lake water. The resultsof the Murphy Springmixingwithout
constant concentration factor for C1, which allowed for com- calciteprecipitationare the sameas thosewith calciteprecipparison of results of the model simulationsto the measured itation, exceptfor Ca, which decreasesgraduallyas greater
concentrationvaluesof pure groundwatersamples.Selectsam- additionsof lake water are mixedwith the relativelyhigherCa
ples were chosenfor simplification.
contentspringwater [Connell,1993]. Carbonatespeciesare
where C is the concentrationof a singlespecies(moles per
kilogram) and N is the number of samples.The RMSE is
relativeto the measuredspeciesconcentration,whichservesto
normalizethe magnitudeof the error due to differencesin the
absoluteconcentrationsbetween species.
In orderto determinethe modelconcentrationfactor(either
the mixtureratio of remant lake water to inflow or the evaporation concentrationfactor) necessaryto produce the observedchemicalcompositionof a particulargroundwatersample, the actual measured concentration of C1 was
superimposedon the model-simulated C1 mixing curve or
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
OF SHALLOW
unaffectedby calcite precipitation becauseof the relatively
high alkalinity relative to Ca, based on the chemicaldivide
conceptbyHardieandEugster[1970],andbecauseit is an open
EvaporateMurphy Spring
at 25øCandlog Pco2= -3.5
1I
system
(constant
P½o2).
Figure 10 presentsthe comparisonbetween the measured
concentrations
and
the
mixture
simulation
' ".i'""
.''.'."'"__'
'.'.'."'"__'
' ii.....
' ........
' ........
' ' '•
ß Na*
RMSE=0.260
1
•
1
[] K+ RMSE=0.2551
ß B RMSE
= 0.3481
.•-
•/
"
•
:_
0.1
•.0•
9
7
•__•0.01
.-' ' .......K-'
6
$
0.ore
5
4
0.•01
3
0.•1
'%%.
........
1
lected from sites furthest from the modern lakeshore, D5A,
Murphy Springmixtureconcentration
with
calciteprecipitationversusmeasuredconcentration
lO
0.1[ ooO•
,•
from the Murphy Springmixingmodel. RMSE valuesfor the
Lee Vining Creek mixture simulationare nearly the same as
thoseobservedfor the Murphy Springmixingmodel IConnell,
1993].For groundwaterfrom boththe northeastshoreseepage
pits and the Ten Mile Road site piezometers,Na, K, and B
havelow relativeRMSE values.The carbonatespeciesand Ca
are slightlyhigher,while SO4is significantlyhigher,suggesting
that theseobservedsoluteconcentrations
cannotbe explained
by the mixingmodel alone, and they are higher presumably
becauseof fractionatingreactions.The model-predictedCO3
concentrationsfor the least concentratedgroundwaterscol-
10
...........
3179
8
concentrations
1A, and lB, are significantlylower than the observed,indicating the model cannot adequatelysimulatep H effect on car-
GROUNDWATER
•
........
10
Concentration
t%.........
1•
Factor
Figure 11. Calculatedresultsof evaporativeconcentrationof
Murphy Springwater.
bonate speciationfor low lake water additions.The mixing
simulationpredictsthe SO4 concentrationto be higher than
the observedconcentration measured at site 5C, and this dis-
crepancysignificantlyraisesthe RMSE valuefor thissolute.As
discussed
earlier, microbiallymediatedreductionaccountsfor
the relatively lower measuredSO4 concentrationat this site.
Evaporation Simulations
o.ool
o.oool
1 o-s
1:1 correlation
1 0'6 / ........ I ........ I ........ I ........ I ........ I .... ,,,,I , , , ,,,,
1 0'e
10's 0.0001 0.001
0.01
0.1
1
lO
measured molal concentration
Murphy Springmixtureconcentration
with
calciteprecipitationversusmeasuredconcentration
The choiceof the startingwater to be evaporatedwasbased
on the likelihoodthat if allowedto evaporateunderthe model
conditions,the chemicalcompositionof the resultantsolution
alongsomepointof the reaction(path)wouldresemblethat of
the groundwateralongthe northeastshore.If this is valid, the
sourceof the chemicalcompositionof the groundwatermay be
attributedto the evaporationof watersflowinginto the basin.
We selectedMurphy Springand RushCreek runoffasstartingwater, assuminga constantatmosphericCO2 pressure(log
PCO 2 of -3.5), a constanttemperatureof 25øC,and the continuousremovalof water vapor.This model only allowsequilibrationwith precipitatingcalciteand halite; however,the halite phaseboundarywasnot reachedduringthesecalculations.
MurphySpringwaschosento representupgradientgroundwater recharge(Figure 1). Rush Creek, alongwith Lee Vining
Creek, carriesthe majority of runoff from the Sierra Nevada
[Vorster,1985](Figure1). In general,the modelyieldedsimilar
results(RMSE values)for the evaporationsimulations
of Rush
Creek water comparedto thoseof Murphy Springwater, and
thus the following discussionappliesto the Murphy Spring
+ HCO
3'RMSE
=0.415
I
1:1
correlation
0.1
0.01
ß c03:mWSE
=0.562
1
•,•
[] SOn
2'RMSE=6.1581
5cr• r••
modeled
0.001
/•
ß1A,
lB
results.
The resultsindicatethat as evaporationproceeds,the chemistry
of the inflowwater changesfrom a Na-Ca-HCO3-C1water
ß D5A
,
, • ,,,,,i
,
, , ,,,,,i
,
, , ,,,,,i
!
i •,
to an alkalineNa-CO3-C1water (Figure 11). Calcitebeginsto
0.0001
precipitateat a concentrationfactor lessthan 2, and thereafter
0.0001
0.001
0.01
0.1
1
measured molal concentration
Ca is continuouslyremovedfrom solutionby precipitation.
Concentrationsof HCO3 and CO3 andpH rise in the courseof
Figure 10. Comparison between Murphy Spring mixture
evaporation.
Concentrationsof Na, K, CI, and SO4increaseas
simulatedconcentration,equilibratedto calciteprecipitation,
and measuredconcentration(molality) usingselectsamples evaporationproceedsbecausethese solutesdo not form any
from the Ten Mile Road transect(D5A, D5B.3, 1A, lB, 1C, solidproductsin this model.At the Ten Mile Road transect
to producethe
3A, 3B, 3C, 5A, 5B, and 5C) and the northeastshoretransect the evaporativeconcentrationfactorsnecessary
(ss2and ss8).RMSE valuesshownfor eachsolute.
observedchemicalcompositionof the groundwatersamples
3180
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
versus measured concentration
' "•'."'•
'. '.'.'""•
'. •'. .....
ß Na
+rosE=0.444
1
m
K+ RMSE
=
ß B
RMSE
=5.555
0.379
II
X Ca
2+
RMSE
=0.756
•
0.1
•
O.Ol
E
........ 2
concentrationis important. Determinationof bromide (Br)
and evaluationof C1/Brratios might help distinguishevaporation from dissolution,and the volume lost to evaporativeflux
o.ool
1:1 correlation
•
GROUNDWATER
differentinflow waters dominated,not evaporativeconcentration and solutelossduring inflow. Evidence of a seasonalsalt
crustcoveringmost of the northeastshoreduringthe summer,
combinedwith the fact that someof the samplescollectedfrom
shallowpits excavatedalong the northeastshore have higher
TDS and soluteconcentrationvaluesthan the samplecollected
from Mono Lake, stronglysupportsthe idea that evaporative
Murphy Springevaporatedconcentration
10
OF SHALLOW
mightbe indicated
by D/•80 data.
0.0001
Differencesbetween the soluteconcentrationspredictedby
the modelsand those obtained from analysesof groundwater
samplessuggestthat many of the solutesmust be affectedby
fractionation processes.The saturation indices indicate that
precipitationof alkaline and alkaline earth carbonates,suchas
calcite and gaylussite,may remove Ca, Mg, Na, and the carbonate speciesfrom solution.In addition, cyclicwetting and
drying of playa sedimentsresultingin formation and redissolution of saltcrustmay alsoremovealkaline and alkaline earth
.,..•
10 '$
......
1 0'6
, .................
1 0-6
10 '5
i
0.0001
........
0.001
measured
i
........
0.01
i
, • i illill
0.1
I , Ill
1
molal concentration
Murphy Springevaporatedconcentration
versus measured concentration
carbonates and concentrate the more soluble salts. However,
10
0.1
mineralogicalfractionalcontrolson the hydrochemicalsystem,
inferred from calculatedsaturationstate data, remain speculative until salt crust samplescan be analyzed. Analysesof
samplesfrom the northeastshoreplaya are in progressto help
determine mineralogicalfractionation controlsand redissolution changeswith mineralogy.The formation of layered Mg
silicatesmay remove Mg and silica from solution.Reduction
processes
may removeSO4from solution,and K may exchange
5Cß
0.01
or adsorb on active surfaces.
[] CO32RMSE=3.602
•
0.001
0.0001
• i:icorrelation
ß SO42
- RMSE
=27.155
II
",
0.0001
,
,,,,,,I
,
0.001
,
,,,,,,I
,
0.01
measured
,
,,,,,,I
,
,
,,,,,,I
0.1
......
1
,,I
10
molal concentration
Figure 12. ComparisonbetweenMurphy Springevaporation
simulatedconcentration,equilibrated to calcite precipitation,
Controlson spatialvariationsin the chemistryof the shallow
groundwaterperpendicularto the lake includemixingof remnant lake water with dilute inflow, seasonalevaporativeconcentrationof runoff coupledwith redissolution,and other solute fractionation
mechanisms.
Trends
in the behavior
of the
individualsolutesand TDS (Table 1; Figure3) are a function
of distance from
the modern
lakeshore
and the recent
occu-
and measuredconcentration(molality) using selectsamples pation on the northeast shore during higher lake levels.The
from the Ten Mile Road transect(D5A, D5B.3, 1A, lB, 1C, decreasein solute concentrationsin the shallowgroundwater
3A, 3B, 3C, 5A, 5B, and 5C) and the northeastshoretransect with distancefrom the lake suggeststhat solutesare trans(ss2, ss8,ss13,and ss5). RMSE valuesare shownfor each ported toward the lake by lateral groundwaterflow and/or
solute.
surfacerunoff. The spatialsimilarityand lower TDS valuesin
the deeperwells indicateat leastpartial chemicaland physical
isolationof the deeper stratigraphicunit and the overall domincreaselakeward, as would be expectedif evaporationis the inance of lateral groundwaterflow versusvertical flow at any
dominant processacting to concentratethe dissolvedsolutes one site.
The spatialpatternsobservedfor the soluteconcentrations
downshore[Cormell,1993].
Figure 12 presentsthe comparisonbetween the measured and TDS in groundwaterparallel to the northeastshoreindiconcentrations
and the evaporationsimulationconcentrations. cate that the systemis segregatedinto two differentregions:a
RMSE valuesfor Na, B, Ca, and HCO 3 are all within about the concentrated,highly saline groundwaterunderlyingmuch of
same range, and SO4, CO3, and K are significantlyhigher, the northeast shore, and pockets of more localized lowersuggestingthat the observedsolute concentrationscannot be salinity groundwater. The differencesin the two areas are
causedby differing inflow quantities and compositions.The
explainedby the evaporationmodel alone.
lower-salinitygroundwatermay be controlledby localizedrecharge through distinct flow systems,such as channelizedor
Discussion
piped freshwaterinflow which dischargesat discretepoints.
The mixingsimulationsprovidea better conceptualmodel of According to Cloud and Lajoie [1980], as fresh intowing
how the systemworksthan the evaporationsimulations,based groundwaterrich in Ca reachsthe highly saline groundwater
on the lower RMSE valuesthan the evaporationmodels.How- near the lakeshore,calcite is precipated into vertical tubular
ever, becauseMono Lake is mainly derived from the evapora- structures.These structuresmay then form discreteflow chantion of Sierra Nevada water, it is difficult to distinguishbe- nels in which the more dilute water risesby densityand hytween these processes.In a studyof the geochemistryof the draulichead differencesand subsequently
dischargesat particGreat Salt Lake, Spenceret al. [1985a] found that mixing of ular locations,typicallyspringmounds.The lessconcentrated
CONNELL
AND
DREISS:
CHEMICAL
EVOLUTION
dischargefrom thesespringmoundsmay rechargethe shallow
groundwatersystemin nearbyareas,or freshgroundwatermay
mix with nearbyconcentratedgroundwater.The mostconcentrated groundwatermay be attributed to a combinationof
factors,includingthe lack of proximityto the discretefreshwater dischargeareas,stratigraphiccontrolson flushingrate,
and evaporativeconcentrationand redissolutioncycles.In addition, the proximityto concentrated,methane-ventingseeps
and springs,may alsobe important.The seepageof methane,
accompaniedby the occurrenceof highlyconcentratedwater
with elevatedB content,mayresultfrom recirculationof older,
deeperbrines.Oremlandet al. [1987] indicatethat the source
of methanefor theseseepsoriginatesfrom a biogenicnatural
gasdepositof Pleistoceneagewhichunderliesthe currentand
OF SHALLOW
GROUNDWATER
3181
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Great BasinUnified Air PollutionControlDistrict,usingpass-through
fundsfrom the Los AngelesDepartmentof Water and Power,and the Jones,B. F., The hydrologyand mineralogyof Deep SpringsLake,
Earth SciencesDepartment of the Universityof California, Santa
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Surv. Bull., 1578, 1986.
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Larry Miller and Ron Oremlandprovided Jones,B. F, A. S. VanDenbergh,A. H. Truesdell, and S. L. Rettig,
equipmentfor someof the chemicalanalyses.
Bob Rosenbauerand
Interstitialbrinesin playasediments,Chem.Geol.,4, 253-262, 1969.
Jim Bischoffassisted
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by Blair Jones,YousifKharaka,Briant
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Kimball,Laura Toran, Greg Dick, and BarbaraBekinshasbeen help- Lee, K., Infrared explorationfor shorelinesprings:A contributionto
ful.
the hydrologyof Mono Basin,California,196pp., Ph.D. dissertation,
Stanford Univ., Stanford, Calif., 1969.
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