Inverse Mass-Balance Modeling versus

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Inverse Mass-Balance Modeling versus Forward
Modeling
How much calcite precipitates?
Forward Approach What is the strategy? What
data do you need? What assumptions do you need
to make?
Limestone
Inverse Approach?
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Solid to Solution (dissolution, exchange)
Solution to Solid (precipitation, exchange)
Need to Know Initial Solution Final
Solution Reacting Phases
gases, water
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Inverse modeling is applicable when Waters are
evolutionary!
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No mixing. (initial water may be formed by
mixing two waters)
Well 1
Well 2
OK
OK
No
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How we do the mass balance (the very short
version)
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How much calcite precipitates?
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Sierra Nevada Spring CompositionsGarrels and
Mackenzie (1967)
 
mass transfers (mmol/kg water)
 
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Sierra Nevada Spring CompositionsGarrels and
Mackenzie (1967)
Ephemeral Spring
Perennial Spring
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SOLUTION 1 Ephemeral Springs temp 25
pH 6.2 pe 4 redox pe
units mmol/kgw density 1 Ca
0.078 Cl 0.014 K 0.028
Mg 0.029 Na 0.134 S(6)
0.01 Si 0.273 Alkalinity 0.328
-water 1 kg SOLUTION 2 Perennial Springs
temp 25 pH 6.8 pe 4
redox pe units mmol/kgw density
1 Ca 0.41 Cl 0.03 K
0.04 Mg 0.071 Na
0.259 S(6) 0.025 Si 0.41
Alkalinity 0.895 -water 1 kg
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Always as dissolution
Thermo data
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Plagioclase (An38)
Na0.62Ca0.38Al1.38Si2.62O8 5.52H 2.48H2O
1.38Al3 0.38Ca2 2.62H4SiO4 0.62Na
Biotite ?
KMg3AlSi3O10(OH)2
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SOLOUTION 2 Perennial Spring FINAL Phase
SI log IAP log KT Anhydrite
-3.99 -8.35 -4.36 CaSO4 Aragonite
-1.92 -10.26 -8.34 CaCO3 Calcite
-1.78 -10.26 -8.48 CaCO3 Chalcedony
0.16 -3.39 -3.55 SiO2 Chrysotile
-10.85 21.35 32.20 Mg3Si2O5(OH)4 CO2(g)
-2.05 -20.20 -18.15 CO2 Dolomite
-3.99 -21.08 -17.09 CaMg(CO3)2 Gypsum
-3.77 -8.35 -4.58 CaSO42H2O H2(g)
-21.60 -21.60 0.00 H2 H2O(g)
-1.51 -0.00 1.51 H2O Halite
-9.73 -8.15 1.58 NaCl O2(g)
-39.92 43.20 83.12 O2 Quartz
0.59 -3.39 -3.98 SiO2 Sepiolite
-7.17 8.59 15.76 Mg2Si3O7.5OH3H2O Sepiolit
e(d) -10.07 8.59 18.66
Mg2Si3O7.5OH3H2O SiO2(a) -0.68 -3.39
-2.71 SiO2 Talc -6.82 14.58
21.40 Mg3Si4O10(OH)2
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albite NaAlSi3O8 anorthite CaAl2Si2O8 This
would allow variable plagioclase composition, but
needs to be near An25
biotite dissolution vermiculite
precipitation This would allow for K release by
Fe2 oxidation in the biotite
Evaporative Concentration? Deep Brine?
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Stop
while you can!
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How do you determine the mineralogy?
Thin Section and use an ion probe or a SEM with
EDX
Mineralogy and composition of specific
minerals. Poor job of fine grained secondary
phases such as clays and oxy-hydroxides
X-ray diffraction
Gives mineralogy, including fine grained phases
and clays. Does not give the specific mineral
compositions.
Geologic/Hydrologic Information
A good guess.
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Snowmelt
Ephemeral Spring
Perennial Spring
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24Sierra Nevada Spring CompositionsGarrels and
Mackenzie (1967)
Problem Can the Ephemeral Springs be the
result of weathering in the soil zone?
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SOLUTION 1 Ephemeral Springs temp 25
pH 6.2 pe 4 redox pe
units mmol/kgw density 1 Ca
0.078 Cl 0.014 K 0.028
Mg 0.029 Na 0.134 S(6)
0.01 Si 0.273 Alkalinity 0.328
-water 1 kgSOLUTION 3 Precipitation
temp 25 pH 5.8 pe 4
redox pe units umol/l density
1 Na 0.11 Ca 0.068 Mg
0.022 K 0.02 S(6) 0.01
Cl 0.013 Si 0.27
Alkalinity 0.314 -water 1 kg
INVERSE_MODELING 1 -solutions 3
1 -uncertainty 0.05 0.05 -phases
Kaolinite Ca-Montmorillonite
CO2(g) Plag(An38) SiO2(a)
Gypsum K-feldspar Biotite
-tolerance 1e-010 -mineral_water
truePHASESPlag(An38) Na0.62Ca0.38Al1.38Si2.
62O8 5.52H 2.48H2O 1.38Al3 0.38Ca2
2.62H4SiO4 0.62Na log_k 0Biotite
KMg3AlSi3O10(OH)2 6H 4H2O Al(OH)4-
3H4SiO4 K 3Mg2log_k 0END
No halite or calcite. Add K-feldspar.
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Phase mole transfers Minimum
Maximum Kaolinite -4.796e-005
0.000e000 0.000e000 Al2Si2O5(OH)4 Ca-Montm
orillon -9.511e-005 0.000e000
0.000e000 Ca0.165Al2.33Si3.67O10(OH)2
CO2(g) 7.739e-004 0.000e000
0.000e000 CO2 Plag(An38) 2.098e-004
0.000e000 0.000e000 Na0.62Ca0.38Al1.38Si
2.62O8 SiO2(a) 8.404e-005
0.000e000 0.000e000 SiO2 Gypsum
9.990e-006 0.000e000 0.000e000
CaSO42H2O K-feldspar 1.832e-005
0.000e000 0.000e000 KAlSi3O8
Biotite 9.659e-006 0.000e000
0.000e000 KMg3AlSi3O10(OH)2
No halite or calcite. How would you get these in
the soil? Add K-feldspar. That makes
sense. But.Gypsum? There is something we are
missing with SO4
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Isotopes
-isotopes 13C 34S
PHREEQC treats each isotope as a separate
component. Calcite is no longer CaCO3 it is
now CaC12.999C13.001O3
PHREEQC does NOT handle fractionation
processes. NETPATH handles fractionations, but
does not allow uncertainty in the concentrations
measurements.
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Mixing
Use -Mix to make a starting solution, and use
this as the initial solution.
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solid solutions - use a mixture of the end
members individually
-balances used to force balancing of an
element not in the solid phases For example,
Cl- to quantify evaporation in a soil.
Evaporation - include H2O as a heterogeneous
phase precipitation of H2O evaporation
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No Way
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Suggestions Try to only change one thing at a
time. The solid phases are important. It helps
to look at the solids! Many minerals are messy,
but the variations in composition can be
important. The model results will only be as
reliable as your understanding of the
hydrochemical system.