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Question about Saturation Index
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Topic: Question about Saturation Index (Read 1477 times)
Jeonghwan Hwang
Top Contributor
Posts: 77
Question about Saturation Index
«
on:
August 08, 2022, 09:07:03 AM »
Hi, I have a question about saturation index.
I made a groundwater-mineral reaction model by referring to previous study.
This model considers mineral reactions, cation exchange, and surface complexation reactions.
I tried to make the results of previous studies similar.
When the saturation index of Gypsum and quartz is set to 0, the concentration of Si and SO4 is calculated very differently.
As a result, when the saturation index of quartz and gypsum was modified,
it was confirmed that the concentration values calculated in previous studies were similar to my model.
But I wonder if this is the right approach.
Does it reliable to change the Saturation index of minerals in model?
Thank you for reading
Sincerely,
Jeonghwan Hwang
========================Model==========================
PHASES
Calcite
CaCO3 = CO3-2 + Ca+2
-log_k -8.48
-delta_h 0
Quartz
SiO2 + 2H2O = H4SiO4
-log_k -4.00
-delta_h 0
Gypsum
CaSO4:2H2O = Ca+2 + SO4-2 + 2H2O
-log_k -4.85
-delta_h 0
Siderite
FeCO3 = Fe+2 + CO3-2
-log_k -10.80
-delta_h 0
Dolomite
CaMg(CO3)2 = Ca+2 + Mg+2 + 2 CO3-2
-log_k -17.90
-delta_h 0
FeS(ppt)
FeS + H+ = Fe+2 + HS-
-log_k -3.92
-delta_h 0
Fe(OH)3(a)
Fe(OH)3 + 3 H+ = Fe+3 + 3 H2O
-log_k -4.89
-delta_h 0
EXCHANGE_MASTER_SPECIES
Z Z-
EXCHANGE_SPECIES #SKB TR-06-16 Input data
Z- = Z-
log_k 0.0
Z- + Na+ = NaZ
log_K 0.0
Z- + K+ = KZ
log_k 0.6
2Z- + Ca+2 = CaZ2
log_k 0.41
2Z- + Mg+2 = MgZ2
log_k 0.34
SURFACE_MASTER_SPECIES
Mont_s Mont_sOH
Mont_w Mont_wOH
SURFACE_SPECIES
Mont_sOH = Mont_sOH
log_k 0
Mont_sOH + H+ = Mont_sOH2+
log_k 4.5
Mont_sOH = Mont_sO- + H+
log_k -7.9
Mont_wOH = Mont_wOH
log_k 0
Mont_wOH + H+ = Mont_wOH2+
log_k 6.0
Mont_wOH = Mont_wO- + H+
log_k -10.5
sELECTED_OUTPUT
-reset false
-file Forsmark_GW.txt
-solution true
-pH true
-pe true
SOLUTION 1
units mol/L
pH 7.2
pe -2.42
Temp 15
C 2.20E-03 as HCO3
Ca 2.33E-02
Cl 1.53E-01
Fe 3.31E-05
K 8.75E-04
Mg 9.30E-03
Na 8.88E-02
S 6.80E-03 as SO4
Si 1.85E-04
-water 0.43
END
USE Solution 1
EQUILIBRIUM_PHASES 1
Quartz -0.15 1.306
Gypsum 0.58 0.081
SAVE Solution 2
END
USE Solution 2
EXCHANGE 1
NaZ 0.846
CaZ2 0.106
KZ 0.024
MgZ2 0.047
SURFACE 1
-sites_units absolute
Mont_sOH 0.0627
Mont_wOH 0.0627
no_edl
END
=====================================================
===============Calculated results in previous study================
mol/L
pH 7.08
pe -2.19
Temp 15
HCO3 2.14E-03
Ca 9.97E-03
Cl 1.53E-01
Fe 3.31E-05
K 1.14E-03
Mg 4.97E-03
Na 1.69E-01
SO4 2.94E-02
Si 6.60E-05
=====================================================
Logged
dlparkhurst
Top Contributor
Posts: 3712
Re: Question about Saturation Index
«
Reply #1 on:
August 08, 2022, 04:03:52 PM »
You can run the concentrations from the previous paper with SOLUTION (using Alkalinity for HCO3 and S(6) for SO4). Depending on the database Quartz is close to saturation and Gypsum has an SI of about -0.25.
You can contact the authors to find out the details of what they did. Normally, I don't change target saturation indices unless there is a good reason. If gypsum is present, I expect that it would react fairly quickly to equilibrium, so maybe gypsum is absent.
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Jeonghwan Hwang
Top Contributor
Posts: 77
Re: Question about Saturation Index
«
Reply #2 on:
August 09, 2022, 01:42:58 AM »
Thank you for answer.
I have one more question.
If I want to use Equalibrium_Phases to create a reaction between water and minerals, I need to put those minerals in.
Equilibrium_phases 1
quartz 0 10
gypsum 0 10
In this case, it is known that the precipitation/dissolution reaction occurs until the Saturation index of quartz and gypsum becomes 0 for the concentration condition of the solution.
Can the saturation index of the mineral (quartz, gypsum) initially set after this calculation be changed?
According to the report I referenced, they reacted by adding quartz and gypsum to the initial groundwater.
Running PHREEQC with the calculated groundwater conditions gives SI of 0.01 for gypsum (reliable) but -0.15 for quartz (cannot understand).
I wonder that this is possible.
Thank you for reading
Sincerely,
Jeonghwan Hwang
Logged
dlparkhurst
Top Contributor
Posts: 3712
Re: Question about Saturation Index
«
Reply #3 on:
August 09, 2022, 02:30:35 AM »
In this code snippet, the zeros are the target saturation index. If you set them to other values, you can obtain those saturation index values in the reacted water.
Code:
[Select]
Equilibrium_phases 1
quartz 0 10
gypsum 0 10
As I said in my previous email, the published final water gives SI(qtz) ~ 0, SI(gyp) ~ -0.25.
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Jeonghwan Hwang
Top Contributor
Posts: 77
Re: Question about Saturation Index
«
Reply #4 on:
August 10, 2022, 01:06:55 AM »
It helped me a lot in understanding the saturation index.
Thank you for reply
Sincerely,
Jeonghwan Hwang
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