Geochemical Modeling WithGeochemical Modeling With PHREEQC

Outline and Objectivesj

• Introduction to geochemical modelingg gconcepts and PHREEQC

• Overview of the PHREEQC for Windows• Overview of the PHREEQC for Windowsinterface

• Learn how to create PHREEQC input files• Learn how to create PHREEQC input files

What geochemical processes determine  the chemistry of natural waters?

A i i• Aqueous speciation• Mineral dissolution and precipitation• Ion exchange and sorption• (Bio‐)geochemical redox reactions(Bio )geochemical redox reactions• (Mixing)

Groundwater Chemical Evolution

from: Custodio, 1987

Geochemical ProcessesGeochemical Processes

What Information Can Be Extracted From a Water Analysis?

How to make a sensible interpretation of water quality?

Geochemical Models ‐ QuantitativeGeochemical Models  Quantitative Tools to Improve Understanding

Quantitative models force the investigator to validate or invalidate ideas by putting realvalidate or invalidate ideas by putting real numbers into an often vague hypothesis, …

from: Lichtner et al 1996from: Lichtner et al., 1996

What Is a Geochemical Model?What Is a Geochemical Model?

Input Solution Composition

Ca NaSO4Mg

p p

FeCl HCO3

Inverse modeling 

Reaction modeling Saturation Speciation calculation

(mass balance)

g(mass transfer)Indices Distribution

‐of‐SpeciesReactive transportReactive transport 

modeling

Concentration Units in PHREEQCConcentration Units in PHREEQC

• Internally, PHREEQC uses moles and y, Qmoles/kgwater

• Can define concentrations in mole/kg• Can define concentrations in mole/kg, mole/L, ppm, mg/L in input file, program converts to moles/kgconverts to moles/kgw

• mg/L can lead to confusion (NO3 vs NO3‐N)ll d f d l• all reactions are defined in moles

• PHT3D requires use of moles exclusively 

Law of Mass Action

dDcCbBaA

ba

dc

BADC

K][][][][

22

BA ][][

224

224

)(22

gypsumOHSOCaOHCaSO

60.42

224

2

10]2[]][][[

OHCaSOOHSOCa

K24 ]2[ OHCaSO

Concentration and ActivityConcentration and Activity

• Ac vity ≠ concentra ony• Why?

Ions interact Ions interact Ion‐solvent (water) interactions ‐electrostatic shielding

Ion‐ion interactions ‐ aqueous complexes and ion pairs 

Electrostatic shieldingg

22][ 2 mCa 22][

CaCamCa

:HuckelDebye

2 3050850log

:

II

z

HuckelDebye

21

3.01

5085.0log ii

zmI

II

z

21

ii zmI

Pretty straightforward, can be done by hand...

Activity Coefficients• Effect of a single ion in solution cannot be measured directly (charge balance)y ( g )

• Mean Ion Activity Coefficient – determined for a salt (e.g. KCl, MgSO4):4

ClKKCl

mmK 2

• MacInnes Convention:

ClKKClsp mmK

ClKKCl MacInnes Convention: • Measure other salts in KCl electrolyte and substitute ±KCl in for one ion to measure the other ion w r t ±KCl

ClKKCl

±KCl in for one ion to measure the other ion w.r.t. ±KCl

and ±salt

Debye‐Hückel

IAzii

log

2

zi is charge of ion

IBaii 1

og

zi is charge of ion ai is effective diameter of ion (in angstroms) A and B are “constants”: A=0 5085 B=0 3281 forA and B are  constants : A 0.5085, B 0.3281 for water at 25° C.

• Assumes ions interact coulombically, ion size doesAssumes ions interact coulombically, ion size does not vary with ionic strength, and ions of same sign do not interact

• Accurate up to I = 0.1 M (most fresh waters)

Higher Ionic StrengthsHigher Ionic Strengths

• Activity coefficients decrease to minimal values• Activity coefficients decrease to minimal values around 1 ‐ 10 M, then increase again the fraction of water molecules surrounding ions inthe fraction of water molecules surrounding ions in hydration spheres becomes significant

Activity of water decreases  in a 5 M NaClct ty o ate dec eases a 5 aCsolution, ~1/2 of the H2O is complexed, decreasing the activity to 0.8

Ion pairing also increases, further increasing the activity effects (e.g. NaCl0)

Davies EquationDavies Equation

I

II

Azii 3.01

log 2

• No ion size parameter – only really accurate for

• No ion size parameter – only really accurate for monovalent ions (zi=±1)

• Often used for seawater (working range up to 0 7 M)• Often used for seawater (working range up to 0.7 M)

Extended Debye‐Hückel (WATEQ)

IbIAz

Az ii

ilog2

2

IbIBa

Az ii

i 1log

1.0

• Adds a correction term to account for

0.8

0

ent ()

term to account for increase of i at higher ionic strength 0.4

0.6

y C

oeffi

cie

higher ionic strength

0 0

0.2

Act

ivity

z = 1

z = 2

0.0

Ionic Strength I (M)

Pitzer Activity Coefficients

• Accurate for high ionic strength (>1 M)

a c a

McaaMccMaMaaMM MmZCBmFz2 )()2(ln

a a c a

caacmmaaaa Cmmzmm'

''

ma is concentration of anionmc is concentration of cationC B i ifi tC, B, ,  are ion‐specific parametersf is a function of I, molalities of cations and anions

Specific Ion Interaction Theory (SIT) A ti it C ffi i tActivity Coefficients

• Ion and electrolyte‐specific approach for activity y p pp ycoefficients

• Limited data, assumes no interaction with neutral species

I l 2k

kikii m

IBI

Az

1ln 2

ik interaction parametermk concentration of ion kk

A = 0.51, B = 1.5 at 25 C

Setchenow EquationSetchenow Equation

IKl IKii log

• For neutral (uncharged) species such as dissolved gases, weak acids, and organic species

• Ki is generally <0.2 

• i > 1, meaning that if a reaction is at equilibrium, mii imust decrease with increasing I (“salting out” effect)

Aqueous Complexes and Ion PairsAqueous Complexes and Ion Pairs

04

03

2 mmmmmCaSOCaCOCaOHCaCa

...CaF4

mmCaPO

Cannot be calculated by hand… Enter aqueous geochemical models!Enter aqueous geochemical models!

Aqueous Speciation CalculationsAqueous Speciation Calculations

Ca NaSO4Mg

FeCl HCOFeCl HCO3

Inverse modeling (mass balance)

Reaction modeling (mass transfer)

Saturation Indices

Speciation calculation

Distribution

( )

(mass transfer)Indices Distribution‐of‐Species

Reactive transport d limodeling

How Do Yo Do It? 

• Solve simultaneous equations for:equations for:  mass action 

mass balance mass balance charge balanceN i ll l d b• Numerically solved by iteration (until convergence is reached)is reached)

Aqueous Models

Ion association  (D‐H, Davies) Pros

Data available for most elements (Al, Si) Redox species Relatively easy to add new elements

Cons Ionic strength < 1 Best suited for Na‐Cl medium Inconsistent thermodynamic data Temperature dependence

25

Aqueous Models

Pitzer Pros

Accurate to high ionic strengthThermodynamic consistency for mixtures of electrolytes

ConsLimited number of elementsVery limited redox

ff l dd lDifficult to add new elementsLimited data on temperature dependence

Aqueous Models

SIT Pros

Better option than ion association for higher ionic strength

Fewer parameters than PitzerRedoxActinides

Consd dTemperature dependence

Consistency?

Saturation statesSaturation states

]][[ 24

2 SOCaKgypsumSolubility Product:

]][[ 24

2

4

SOCaIAPgypsum

gypsumy

Ion Activity Product:

gypsumgypsum IAPK At equilibrium:

0log

gypsum

gypsum KIAP

SI

0: gypsumSIatedSupersatur

gypsumK

0: gypsumSIatedUndersatur

Seawater SpeciationSeawater Speciation

Seawater SpeciationSeawater Speciation

Result SIcalcite = 0.4

PHREEQC: What Can It Do?

• Developed by David Parkhurst and Tony Apello• Calculate the equilibrium composition of a system q p y(speciation)

• Calculate the change in composition in response g p pto reactionsMineral/gas equilibria Ion exchange Surface complexationp Kinetic reactions (time‐dependent)

• Calculate the change in composition in response g p pto 1‐D transport (and reactions)

Thermodynamic Databases in PHREEQCThermodynamic Databases in PHREEQC

• phreeqc dat• phreeqc.dat• wateq4f.dat• llnl dat• llnl.dat• minteq.dat

it d t• pitzer.dat• sit.dati d• iso.dat

PHREEQC Thermodynamic DatabasesPHREEQC Thermodynamic Databases

• SOLUTION MASTER SPECIES—Redox states and _ _gram formula mass

• SOLUTION_SPECIES—Reaction and log K _ g• PHASES—Reaction and log KOptional:p• EXCHANGE_MASTER_SPECIES—Names• EXCHANGE SPECIES—Reaction and log KEXCHANGE_SPECIES Reaction and log K• SURFACE_MASTER_SPECIES—Names• SRFACE SPECIES—Reaction and log K• SRFACE_SPECIES Reaction and log K

Thermodynamic Databasesy

llnl datwateq dat

11

llnl.datwateq.dat

(As) = 10‐7(As) = 10‐7

.5

1

H2AsO4-

H3AsO4(aq)

.5

H3AsO4

H2AsO4-

0

Eh (v

olts

)

A O---

HAsO2(aq)

HAsO4--

0

Eh (v

olts

)

H3AsO3

HAsO4--

–.5AsO2

-

AsO4

As

–.5

AsO4---

H2AsO3-

0 2 4 6 8 10 12 14

pH

AsO2OH--

25°C

dv M on Feb 07 2005

0 2 4 6 8 10 12 14

pH

2 3

25°C

dv Mon Feb 07 2005

Thermodynamic Databasesy

llnl datwateq4f dat

11

llnl.datwateq4f.dat

(As) = 10‐7(S) = 10‐3

(As) = 10‐7(S) = 10‐3

.5

H3AsO4

H2AsO4-

.5H2AsO4

-

H3AsO4(aq)

(S) = 10 3(S) = 10 3

0

Eh (v

olts

)H3AsO3

HAsO4--

Orpiment0

Eh (v

olts

)

A O---

HAsO2(aq)HAsO4

--

O i t

–.5

AsO4---

AsS(OH)(HS)-

H AsO -

Realgar

–.5

AsO2-

AsO4Orpiment

Realgar

0 2 4 6 8 10 12 14

pH

H2AsO3

25°C

dv Mon Feb 07 2005

0 2 4 6 8 10 12 14

pH

AsO2OH--

As25°C

dv M on Feb 07 2005

Where Does the Thermodynamic Data ?Come From?

As2S3 SolubilityAs2S3 Solubility

PhreePlot:PhreePlot:PHREEQC with fitting(www.phreeplot.org)Vlassopoulos et al (2010) WRI‐13

Sorption Reaction Databases• Measured sorption edges, 

isotherms for Cd and Pb for three different soils

• Used PEST to fit universal set of log K’s for ion exchange on clays, sorption of Fe‐oxidesclays, sorption of Fe oxides

• Tested transferability on isotherm data for fourth soil

Serrano et al (2009) GCA

PHREEQC for Windows (PfW)PHREEQC for Windows (PfW)

• Graphic interface developed by Vincent Postp p y• Based on PHREEQC version 2.xx• Includes:Includes:

input editor output editoroutput editor database editorbasic spreadsheet basic spreadsheet

graphical output (USER_GRAPH)

PHREEQC KeywordsPHREEQC Keywords

SOLUTION Data BlockSOLUTION Data BlockSOLUTION 1 Seawater

temp 25temp 25pH 7pe 4redox peunits ppmdensity 1Ca 412.3Mg 1291 8Mg 1291.8Na 10768K 399.1Fe 0.002Alkalinity 141.682 as HCO3Cl 19353S(6) 2712-water 1 # kgwater 1 # kg

SOLUTION SPREADSOLUTION_SPREAD

SOLUTION_SPREAD-units mg/l

pH Cl S(6) Ca Mg Na K Alkalinitymg/l as CaCO3

6.23 5050 498 149 290 2440 98.2 3016.04 5600 516 154 296 2740 105 2146.17 4880 397 154 278 2550 90.9 2995.91 8880 578 157 299 2610 91.2 1386.00 7660 800 159 328 2700 91.5 133

SOLUTION or SOLUTION SPREADSOLUTION or SOLUTION_SPREAD

• Distribution of species• Saturation indices• Initial conditions for reaction path calculations• Initial/final compositions for inverse modeling• Initial and boundary conditions for reactive‐t t d litransport modeling

MIX

MIX

MIX 1 0.752 0.25

EQUILIBRIUM PHASESEQUILIBRIUM_PHASES

EQUILIBRIUM PHASES 1EQUILIBRIUM_PHASES 1Calcite 0 0Dolomite 0 1.5CO2(g) -2.0

EXCHANGE 1

EXCHANGE

-equil with solution 1X 1.0

SURFACE

SURFACE 1-equil solution 1equil solution 1

# assume 1/10 of iron is HFOHfo_w 0.07 600. 30.

REACTIONREACTIONREACTION

CH2O 110 mmol in 10 steps

KINETICS

KINETICS 1K-feldspar

-m0 2.16 # 10% K-fsp, 0.1 mm cubes-m 1.94-parms 1 36e4 0 1parms 1.36e4 0.1

RATES

RATESK-feldspar

-start10 dif_temp = 1/TK - 1/29820 pk_H = 12.5 + 3134 * dif_temp30 pk w = 15.3 + 1838 * dif tempp _ _ p40 pk_OH = 14.2 + 3134 * dif_temp50 pk_CO2 = 14.6 + 1677 * dif_temp60 rate = 10^-pk_H * ACT("H+")^0.5 + 10^-pk_w + 10^-pk_OH * ACT("OH-")^0.370 rate = rate + 10^-pk CO2 * (10^SI("CO2(g)"))^0 670 rate = rate + 10 pk_CO2 (10 SI( CO2(g) )) 0.680 moles = parm(1) * parm(2) * rate * (1 - SR("K-feldspar")) * time90 if SR("K-feldspar") > 1 then moles = moles * 0.1100 save moles

end-end

SAVESAVESOLUTION 1REACTION 1

NaCl 11.0 mol

SAVE solution 10END

USE

END

USE

USE solution 10EQUILIBRIUM PHASES 21EQUILIBRIUM_PHASES 21

CO2(g) -3.5SAVE solution 11SAVE equilibrium_phases 11END

SELECTED OUTPUTSELECTED_OUTPUTSELECTED_OUTPUT

-file r11.csv-reset false-reaction-totals O(0) C(4) C(-4) Fe(3) Fe(2) S(6) S(-2)

ilib i h ki it-equilibrium_phases mackinawite

USER PUNCHUSER_PUNCHUSER_PUNCH

-head pH Ca mg/L Mg mg/Lead p Ca_ g/ g_ g/-start

10 PUNCH -la("H+")20 PUNCH TOT("Ca")*40.1*1e330 PUNCH TOT("Mg")*24.3*1e3

-end

USER GRAPHUSER_GRAPH

USER_GRAPHh di ti Si-heading time Si-axis_titles YEARS CONCENTRATION-headings Years-axis scale y axis 0 1 e-4axis_scale y_axis 0 1.e 4-axis_scale x_axis 0 10-start10 graph x total time/3.1536e7 # time in years on x-axisg p _ _ / # y20 graph_y tot("Si") # parameter on y-axis-end

Additional Keyword Data BlocksAdditional Keyword Data BlocksSOLUTION_MASTER_SPECIESSOLUTION_SPECIES

EXCHANGE MASTER SPECIESEXCHANGE_MASTER_SPECIESEXCHANGE_SPECIES

SURFACE_MASTER_SPECIESSURFACE_SPECIES

PHASES

INVERSE_MODELING_INVERSE_MODELING 1

-solutions 10 5-uncertainty 0.1uncertainty 0.1-phases

pyrite Fe(OH)3(a)H2O(g)H2O(g)CalciteOCCaX2KXKXNaX

-balancesAlkalinity 0.05 0.05C 0.05 0.05C 0.05 0.05Ca 0.05 0.05Cl 0.05 0.05Fe 0.05 0.05Mg 0.05 0.05g 0.05 0.05Na 0.05 0.05S 0.05 0.05

PHREEQC Transport Calculations

Advection 1 2 3 4 5 6 nAdvection …

Dispersion/

Diffusion1 2 3 4 5 6 n…

Diffusion

Reaction 1 2 3 4 5 6 n…