For reactions which consider the CO2 gas as part of the system enter the CO2 partial pressure. Atmospheric CO2 pressure is 10^(-3.52).
For more help load the files: "Example 4 CO2 Type 2.geo"and "Example 5 CO2 type 2.geo" to see how to set up a typical case.
The text below was extracted from the Geochem-PC 2.0 User Notes. Thanks to Dave Parker, Bud Norvell, and Rufus Chaney.