Applications and Case Studies > Soil profile geochemistry

Question about Saturation Index

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**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: ---Equilibrium_phases 1

quartz 0 10

gypsum 0 10

--- End code ---

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

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