Question about Saturation Index

[Jeonghwan Hwang]

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
=====================================================

[dlparkhurst]

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.

[Jeonghwan Hwang]

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

[dlparkhurst]

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.

[Jeonghwan Hwang]

It helped me a lot in understanding the saturation index.
Thank you for reply

Sincerely,

Jeonghwan Hwang