Summation of Summation of Biogeochemical Research Biogeochemical Research

of Sierra Nevada of Sierra Nevada catchments catchments

Kate SamelsonKate SamelsonKendra Morliengo-BredlauKendra Morliengo-Bredlau

Ben WestBen WestCorey LawrenceCorey Lawrence

Origin of the Chemical Origin of the Chemical Compositions of Some Compositions of Some Springs and LakesSprings and Lakes

Robert M. GarrelsRobert M. GarrelsFred T. MackenzieFred T. Mackenzie

IntroductionIntroduction

• Analysis of spring water from Sierra Analysis of spring water from Sierra NevadaNevada

-- known analysis of water constituentsknown analysis of water constituents -- primary analysis of igneous rock primary analysis of igneous rock minerals and soil minerals derived from themminerals and soil minerals derived from them

-- the system is closed: little loss or the system is closed: little loss or gain of water or COgain of water or CO2 2

-- the chemical composition of the rocks the chemical composition of the rocks studied are representative of continental studied are representative of continental crust, and a widespread application to rock-crust, and a widespread application to rock-water systems can be used.water systems can be used.

primary igneous rocks primary igneous rocks + soil water high in + soil water high in COCO22

2-2- = soil minerals = soil minerals + spring water+ spring water

Weathering RelationsWeathering Relations

•Parental granite rocks: quartz Parental granite rocks: quartz diorite and microclinediorite and microcline

•Dissolved content of spring Dissolved content of spring water comes from attack of COwater comes from attack of CO22 rich soil water on the silicatesrich soil water on the silicates

•Kaolinite is the main weathering Kaolinite is the main weathering productproduct

Weathering Reaction Weathering Reaction TestTest

1. back-react with kaolinite to determine if 1. back-react with kaolinite to determine if original rock minerals can be formedoriginal rock minerals can be formed

2. subtract cations and anions in snow 2. subtract cations and anions in snow water from spring water to determine water from spring water to determine the minerals derived form the rockthe minerals derived form the rock

*HCO*HCO33 is a is a fudge factorfudge factor

Weathering Reaction Weathering Reaction ResultsResults

1. Reactions balance for the closed system1. Reactions balance for the closed system2. Weathering product is kaolinite2. Weathering product is kaolinite3. Original rock materials were reconstructed3. Original rock materials were reconstructed

*Reactions depend on depth and retention. *Reactions depend on depth and retention. Rock material encountered will affect pH, Rock material encountered will affect pH, rate and weathering productsrate and weathering products

Assume: CO2 rich water + Assume: CO2 rich water + plagioclase plagioclase kaolinite… kaolinite… montmorillitemontmorillite

• If the above If the above reaction occurs, reaction occurs, then:then:

[Ca[Ca2+2+]][SiO[SiO22]]88

K = K = ------------------------------------

[H+][H+]22

•So, a water So, a water undersaturated undersaturated with with montmorillite montmorillite and saturated and saturated with kaolinite with kaolinite should have a should have a value < Kvalue < K

Evaporation Experiment Evaporation Experiment ProcedureProcedure

ConditionsConditions1. The water remains in equilibrium with 1. The water remains in equilibrium with

a CO2 pressure of 10 a CO2 pressure of 10 -3.5-3.5 atm atm2. Temperature remains constant at 25°C2. Temperature remains constant at 25°C3. Pure water (except for a little CO3. Pure water (except for a little CO22) is ) is

continuously removed form the systemcontinuously removed form the system4. Assume that any solids formed remain 4. Assume that any solids formed remain

in equilibriumin equilibrium

ResultsResults•Waters emerge from closed system when Waters emerge from closed system when they have reached compositions similar they have reached compositions similar to general Sierra water, but it will to general Sierra water, but it will continue to gain and lose different continue to gain and lose different constituents.constituents.

•Difficult to deduce complete reactions Difficult to deduce complete reactions using evaporation technique because using evaporation technique because there are reactions that occur in the there are reactions that occur in the presence of the parent rock and presence of the parent rock and processes are asymmetric processes are asymmetric

Geochemical and Geochemical and Hydrologic Controls on Hydrologic Controls on

the Composition of the Composition of Surface Water in a High-Surface Water in a High-elevation Basin, Sierra elevation Basin, Sierra

Nevada, CaliforniaNevada, California

Mark W. WilliamsMark W. WilliamsAaron D. BrownAaron D. BrownJohn M. MelackJohn M. Melack

IntroductionIntroduction•Emerald lake watershed is a high Emerald lake watershed is a high altitude basin in the southern Sierra altitude basin in the southern Sierra NevadasNevadas

•Solute Composition in lake changes due Solute Composition in lake changes due to three distinct periods:to three distinct periods:

-snow pack runoff-snow pack runoff -transition period between runoff -transition period between runoff and summer flowand summer flow

-low flow from late summer into -low flow from late summer into winterwinter

BackgroundBackground• Traditional dogma: composition of surface Traditional dogma: composition of surface water is controlled by solutes in water is controlled by solutes in equilibrium with bedrock weathering equilibrium with bedrock weathering products.products.

if so, then, if groundwater discharge if so, then, if groundwater discharge is major source of stream flow during is major source of stream flow during storm events in granite basins, chemical storm events in granite basins, chemical weathering is the major process that weathering is the major process that neutralizes incoming acidity…but the neutralizes incoming acidity…but the chemical weathering process may be chemical weathering process may be overwhelmed and too slow at buffering in overwhelmed and too slow at buffering in reservoirs with low retention ratesreservoirs with low retention rates

ObjectivesObjectives

• Determine sources of solutes in stream flowDetermine sources of solutes in stream flow -focus on origin of Ca-focus on origin of Ca++, reactive silicate , reactive silicate and HCOand HCO33

--

• Determine whether stream water is in Determine whether stream water is in equilibrium with mineral weathering equilibrium with mineral weathering productsproducts

• Investigate if acidity due to atmospheric Investigate if acidity due to atmospheric deposition is neutralized by weathering of deposition is neutralized by weathering of other processesother processes

• Investigate effects that subsurface routing Investigate effects that subsurface routing has on composition of surface waterhas on composition of surface water

Site InformationSite Information• Soils are strongly acidicSoils are strongly acidic

• Bedrock: granite and granodioriteBedrock: granite and granodiorite• Soils are mainly derived from Soils are mainly derived from

weathering of the bedrockweathering of the bedrock• Snowfall accounted for ~95% of the Snowfall accounted for ~95% of the

precipitation input during study precipitation input during study yearyear

-very vulnerable to acid pulse-very vulnerable to acid pulse

MethodsMethods1. All water samples were analyzed 1. All water samples were analyzed

for major inorganic ions and for major inorganic ions and reactive silicates (Si)reactive silicates (Si)

2. Buffering capacity determined 2. Buffering capacity determined with Gran titration methodwith Gran titration method

way to establish [ ] of way to establish [ ] of HCOHCO33

--

3. Garrels and Mackenzie approach 3. Garrels and Mackenzie approach used to determine whether the used to determine whether the streamwater content was a streamwater content was a product of the basin/catchmentproduct of the basin/catchment

4. Cl4. Cl-- was used as a scaling was used as a scaling factor to determine base factor to determine base cations in snowpack melt cations in snowpack melt waterswaters

5. Reservior residence time was 5. Reservior residence time was determined using a determined using a 66LiBr LiBr tracertracer

6. Ca2+:Na+ ratio was used to help 6. Ca2+:Na+ ratio was used to help explain possible soil explain possible soil buffering processbuffering process

6. Ca2+:Na+ ratio was used to 6. Ca2+:Na+ ratio was used to help explain possible soil help explain possible soil buffering processbuffering process

soil retains Casoil retains Ca2+2+ more than Na more than Na++, , Na leaches out faster than CaNa leaches out faster than Ca

is cation exchange in soils is cation exchange in soils is a major buffering process, is a major buffering process, the ratio of Na:Ca should be the ratio of Na:Ca should be high in snowmelt runoff and high in snowmelt runoff and low in end of melt seasonlow in end of melt season

7. Back reactions are difficult 7. Back reactions are difficult to calculate due to to calculate due to differences in retention differences in retention time, depth (rock time, depth (rock encountered)encountered)

used contributions of Ca2+ used contributions of Ca2+ to calculate products of to calculate products of mineral weatheringmineral weathering

contribution % change with contribution % change with seasonal changes in water seasonal changes in water flowflow

… …hydrologic flow paths hydrologic flow paths changechange

… …relative importance of relative importance of different biogeochemical different biogeochemical processes changeprocesses change

Residence Times:Residence Times:• 4/10/1086 – 8/30/1986 4/10/1086 – 8/30/1986 143 days 143 daysResidence time: 7 – 23 daysResidence time: 7 – 23 days

-Averages in May ’86 and ’87 had -Averages in May ’86 and ’87 had almost daily turnover in talusalmost daily turnover in talus-second method incorporating soil -second method incorporating soil saturation reported soil retention saturation reported soil retention rates of minutes to hours before being rates of minutes to hours before being available to surface flowavailable to surface flow

• Groundwater discharge in low flow was Groundwater discharge in low flow was ordered from months to yearsordered from months to years

ConclusionsConclusions• Mass balance did not work:Mass balance did not work: wrong weathering reactions usedwrong weathering reactions used other processes beyond weathering of other processes beyond weathering of plagioclase contributed to dissolved solutesplagioclase contributed to dissolved solutes

deficit of HCO3- indicates additional deficit of HCO3- indicates additional sources of alkalinity beyond mineral sources of alkalinity beyond mineral weatheringweathering

• Weathering model inaccurate:Weathering model inaccurate: model is good for low flow periods but not model is good for low flow periods but not as well with high flowas well with high flow

unknown and possibly synergistic effects unknown and possibly synergistic effects during high flow during high flow

stream water in ELW are in steady state stream water in ELW are in steady state with weathering products during low flow with weathering products during low flow periodsperiods

ConclusionsConclusions cont…cont…

• Mineral weathering does not seem to be the Mineral weathering does not seem to be the primary process of bufferingprimary process of buffering

buffering of acidic cations occurs too buffering of acidic cations occurs too rapidly to be attributed to silicate weatheringrapidly to be attributed to silicate weathering

cation exchange seems to be the major cation exchange seems to be the major buffering agent because their kinetic rates buffering agent because their kinetic rates match what is needed to buffer during such match what is needed to buffer during such short retention timesshort retention times

• ELW is subject to acid pulses from low pH snow ELW is subject to acid pulses from low pH snow meltmelt

buffering during “high flow” comes from buffering during “high flow” comes from soilssoils

buffering during “low flow” comes from buffering during “low flow” comes from weathering weathering

buffering controls changebuffering controls change

ConclusionsConclusions cont… cont…

• Snow pack runoff in ELW infiltrates soils Snow pack runoff in ELW infiltrates soils and unconsolidated materials, undergoes and unconsolidated materials, undergoes reactions with soil water and soil exchanges reactions with soil water and soil exchanges and is then discharged to stream flowand is then discharged to stream flow

• Granitic basins are sensitive to atmosphic Granitic basins are sensitive to atmosphic deposition of acids due to low residence deposition of acids due to low residence time during melt periodstime during melt periods