**Department of Geosciences
North Dakota State University
WEB-PHREEQC:
AQUEOUS GEOCHEMICAL MODELLING
**

Input for WEB-PHREEQ is form-based, that is, you enter data into the spaces provided. In some cases, you may need to choose a particular chemical species using drop-menus. For example, you have a choice of entering sulfur data as S, SO_{4}^{2-}, or H_{2}S.

On the first page, you have the following choices:

"Simple" speciation is the default. This input form is meant to be used for quick speciation calculations of a single solution. Note that there are many interesting problems that can be solved using the so-called "simple" page! For example, you can vary a parameter, such as partial pressure of CO

The "advanced" page allows the user to make calculations over a range of variable values. For example, you might wish to calculate silica solubility over a temperature range of 25 to 100 degrees C in 50 steps. The "advanced" page lets you save selected output of these calculations to a compact file that is easily imported to a spreadsheet or plotting program.

**Single solution vs. Mixing of two solutions**

Choose whether you wish to do calculations for a single solution, or whether to mix two solutions using phreeqc's MIX keyword.

**Databases**

There are also three databases to choose from: "phreeqc", which is the default database, "wateq4f", or "minteq."
Each has different choices of input parameters that may be applicable to your problem.

On the second page, by checking the "input" box at the bottom of the input screen, you have the option to view the input file generated by the web interface.

**Notes on input**:

- To input the partial pressure of CO
_{2}, type in the log of the partial pressure. For atmosphere, enter the value "-3.5". - You have a choice of a fixed or a "floating" pH calculation. For the latter, PHREEQC will attempt to calculate the equilibrium pH based on charge balance. In some cases, the calculation may not be possible, and you may receive an error message.
- Simultaneous input of different carbon species, for example, P
_{CO2}along with Alkalinity and a carbonate choice, may lead to an error.

WEB-PHREEQC will:

- read and print your input data,
- check charge balance of your solution,
- correct relevant K's for temperature and pressure if necessary,
- make an initial guess at activities of species,
- iteratively solve the non-linear simultaneous equations using the Newton-Raphson technique,
- determine speciation and solubility indices (SI's),
- output results to a web page (in the "advanced" case, you can choose which variable you wish to have output in table format)

**Acknowledgments**:WEB-PHREEQ is part of an instructional module
for Geochemistry 428/628 at North Dakota State University. Thanks to Sandy
Sprafka for obtaining initial funding for this project, and to Andrew Yahin for
the coding. Version 2 of WEB-PHREEQ was coded by Tim Mooney, though certain
sections of code are largely untouched from the version 1 code.

Many thanks to David Parkhurst and his crew for developing PHREEQC. This web implementation © B. Saini-Eidukat, Andrew Yahin, and Tim Mooney.

B.Saini-Eidukat

Geochemistry

Geology/Chemistry 428/628