What Is PHREEQC?
PHREEQC (pronounced “freek”) is a geochemical modeling software developed and freely distributed by the United States Geological Survey (USGS). The name stands for pH-REdox-EQuilibrium C, reflecting its core capability of computing pH and redox equilibria in water–rock systems.
Here are some of the things PHREEQC can do:
- Aqueous speciation (species distribution in solution)
- Mineral dissolution and precipitation (
EQUILIBRIUM_PHASES) - Water mixing (
MIX) - Kinetic rate calculations (
KINETICS) - Redox sequences
PHREEQC is used across a wide range of fields—groundwater quality analysis, hydrothermal alteration modeling, CO₂ geological storage (CCS), seawater–freshwater mixing, and more. Another major advantage is that it is completely free and open-source, making it highly accessible for researchers and students alike.
Step 1: Downloading PHREEQC
Go to the Official USGS Page
Open your browser and navigate to the following URL:
PHREEQC Version 3: https://www.usgs.gov/software/phreeqc-version-3
In the Downloads section near the middle of the page, select the installer that matches your operating system. For Windows, download the phreeqci-3.8.6-17100.msi file (as of May 2026).
Because PHREEQC is widely known, unofficial distribution sites do exist. To ensure the integrity of the thermodynamic databases (.dat files), always download directly from the official USGS page.
Running the Installer
Double-click the downloaded phreeqci-3.8.6-17100.msi file and follow the installation wizard. The default settings will work just fine. Once installation is complete, the files will be located in a folder such as:
C:\Program Files (x86)\USGS\Phreeqc Interactive 3.x.x-xxxxx\The executable you need is phreeqci.exe (the GUI version of PHREEQC), found inside the bin folder.
Step 2: Launching PHREEQC and Preparing a Working Folder
Create a Working Folder (Important)
Before you start using PHREEQC, it is strongly recommended to create a dedicated folder on your Desktop.
Example: C:\Users\YourName\Desktop\PHREEQC_work\PHREEQC may not handle non-ASCII paths correctly. Folder names containing Japanese characters, spaces, or special symbols can cause errors when saving or loading files. Use alphanumeric-only names such as PHREEQC_work or phreeqc.
Launch PHREEQC
Double-click phreeqci.exe inside the bin folder. Alternatively, you can create a desktop shortcut for quick access.
Once the PHREEQC window opens, you have successfully launched the program. You will see an icon bar on the left side of the screen—these icons serve as shortcuts to various calculation blocks (SOLUTION, EQUILIBRIUM_PHASES, etc.).
Step 3: Defining Pure Water (SOLUTION)
What Is SOLUTION?
The first thing you do in PHREEQC is define the “water (solution)” you want to analyze. This definition block is called SOLUTION.
Pure water (distilled water) is water with no dissolved solutes. To define pure water in PHREEQC, you can simply use the default values in the SOLUTION block.
GUI Procedure
Click the SOLUTION icon in the icon bar on the left. The SOLUTION settings window will appear.
For pure water, click OK without making any changes. The following code will be auto-generated in the text area on the left:
SOLUTION 1
temp 25
pH 7
pe 4
redox pe
units mmol/kgw
density 1
-water 1 # kgHere is what each parameter means:
| Parameter | Value | Meaning |
|---|---|---|
| temp | 25 | Temperature [°C] |
| pH | 7 | pH (neutral) |
| pe | 4 | Redox potential (pe value) |
| redox | pe | Defines the redox reference as pe |
| units | mmol/kgw | Concentration units (mmol per kg water) |
| density | 1 | Density [g/cm³] |
| -water | 1 | Amount of water [kg] |
pe = 4 corresponds to the typical redox conditions at Earth’s surface (a mildly oxidizing environment). Higher pe values indicate more oxidizing conditions, while lower values indicate more reducing conditions. This will be covered in detail in a future article on redox reactions.
Step 4: Entering END to Close the Calculation Block
In PHREEQC, every calculation block must end with the keyword END.
Click at the end of the text area and type END on a new line:
SOLUTION 1
temp 25
pH 7
pe 4
redox pe
units mmol/kgw
density 1
-water 1 # kg
ENDWithout END, PHREEQC cannot determine where one calculation ends when multiple blocks are present. Make it a habit to always include it.
Step 5: Running the Calculation
Select SOLUTION 1
Click SOLUTION 1 in the tree view on the left side of the screen. The row will be highlighted when selected.
Next, click the dot (・) next to the row. This will automatically append END (if you have already typed it manually, this step is not necessary).
Execute with RUN
Click the Run button in the menu bar or toolbar.
File Save Dialog
When the save dialog appears, navigate to the working folder you created in Step 2 and enter a filename.
Example: pure_water.pqiChoose the Thermodynamic Database
From the Database files dropdown, select phreeqc.dat. This is the standard thermodynamic database for PHREEQC. For geothermal systems involving many minerals and chemical species, or for high-temperature conditions, use llnl.dat instead.
Then click Start to begin the calculation. Once the calculation is complete, click Dismiss to close the dialog.
Step 6: Reading the Output
When the calculation finishes, the Output is displayed on screen. The beginning of the pure water output looks like this:
-----------------------------Solution composition------------------------------
Elements Molality Moles
Pure water
----------------------------Description of solution----------------------------
pH = 7.000
pe = 4.000
Specific Conductance (µS/cm, 25°C) = 0
Density (g/cm³) = 0.99706
Volume (L) = 1.00297
Activity of water = 1.000
Ionic strength (mol/kg) = 1.007e-07
Mass of water (kg)= 1.000
Total alkalinity (eq/kg) = 1.070e-07
Temperature (°C)= 25.000Key Points for Interpretation
- pH = 7.000 — Neutral, exactly as specified in the input.
- Specific Conductance = 0 — Pure water contains no electrolytes, so electrical conductivity is zero.
- Ionic strength = 1.007e-07 — Even in pure water, \(\mathrm{H^+}\) and \(\mathrm{OH^-}\) are present, so ionic strength is not zero but an extremely small value.
The output also includes the species distribution:
----------------------------Distribution of species----------------------------
Log Log Log mole V
Species Molality Activity Molality Activity Gamma cm³/mol
H2O 5.551e+01 1.000e+00 1.744 0.000 0.000 18.073
H+ 1.001e-07 1.000e-07 -7.000 -7.000 0.000 0.00
OH- 1.001e-07 1.000e-07 -7.000 -7.000 0.000 -4.15In pure water, you can confirm that both \(\mathrm{H^+}\) and \(\mathrm{OH^-}\) have concentrations of \(10^{-7}\) mol/kg (= pH 7).
First Calculation Complete
You have now completed the entire workflow—from installing PHREEQC to running a pure water calculation and interpreting the results.
Here is a summary of the key points:
| Step | Key Point |
|---|---|
| Folder Name | Always use English characters (no spaces/symbols). |
| SOLUTION | Use default settings to define pure water. |
| END | Mandatory at the end of every execution block. |
| Output | Check pH, conductivity, and ionic strength for verification. |
In the next article, we will input real seawater data and calculate Speciation (species distribution). We will enter major ions such as Na, Ca, Mg, and SO₄ to examine what chemical species are present in the water.
References
Other articles in this series:
- #1 Installation and Initial Calculation (This article)
- #2 Analyzing Seawater with Speciation
- #3 Mineral Equilibrium and Temperature Effects
- #4 Calcite–CO₂ Interaction (Open vs. Closed Systems)
- #5 Mixing Groundwater and Seawater
- #6 Pyrite Oxidation and AMD Formation
- #7 Solubility Diagrams (Gibbsite)
DeepFlow | Science beneath the surface