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What is PhreeqcUsers and why was it set up?



PHREEQC Users is a new initiative aiming to provide a much needed discussion space for geochemists of all experience levels to share ideas and experience of using the PHREEQC code for a whole range of geochemical simulations.

What is PHREEQC?

PHREEQC stands for PH REdox EQuilibrium (in C language) and is a widely used public-domain geochemical modelling software available from the USGS. It is available to download for free with versions for PC and Mac. A graphical interface version is only available for PC. PHREEQC can be downloaded from the USGS website here.

What does it do?

PHREEQC is used for simulating a variety of reactions and processes in natural waters or lab experiments. PHREEQC was developed for hydrogeochemistry projects and is capable of simulating a huge range of geochemical scenarios. It can be used to calculate speciation, saturation indices of minerals, mixing of solutions, reaction path modelling, inverse modelling and much more.

Calculations are made using designated thermodynamic databases which include a wide range of data for mineral phases and compounds. PHREEQC makes calculations using an input script in which the problem is specified via KEYWORDS and associated datablocks. A BASIC code user punch block can also be included to make user-defined calculations tailored exactly for the problem.

An example

A hydrogeologist takes groundwater samples from a borehole array and sends the water samples to a lab for specific element (e.g. Pb, Fe, Cu...) or species (e.g. SO4-2) analysis. The concentration data is reported back for the dissolved species in the water samples. The data for all the water samples can then be written as an input script in PHREEQC as a SOLUTION_SPREAD and used to calculate the mineral phases that are oversaturated or undersaturated at equilibrium with the waters, simultaneously for all the water samples. Or PHREEQC could be used for inverse modelling, to assess the minerals in the aquifer which have given the chemical signature to the waters.

Why is this forum needed?

There are many specialist geochemists around the world using PHREEQC for numerous practical purposes such as assessing treatment of acid rock drainage or irrigation water studies. This site is a dedicated place where they can share their experience and knowledge, collaborate, discuss input scripts or modelling strategies. Before the inception of this forum, there were only limited options available for online discussion and fostering user interaction.

"Unknown unknowns"

The options for this code are huge. The user must make sure they first have a solid grasp of how to use the code, understand the limitations and assumptions, and then make sure that the simulation is representing the real life situation as closely as possible. The results also need careful interpretation. The huge array of options can appear overwhelming at first sight and might deter beginners.

One of the biggest issues is the "unknown unknowns" - i.e. not knowing what can be successfully simulated with PHREEQC. In reality most low temperature aqueous geochemists, geologists and hydrogeologists can find a use for it - if only they knew how! We hope that the contributors to will go some way to showing what is possible.

How is PhreeqcUsers funded?

Costs associated with the website arising from web and domain hosting. The website is non-profit and is funded solely by donations. We have set up a funding campaign on GoFundMe: and welcome any contribution. Thank you!

Feedback is constantly developing. We welcome any feedback - please leave feedback in the Feedback board: is a non-commercial independent enterprise with no government or non-governmental affiliations.


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