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…
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