Introduction
PHREEQE is a USGS computer program designed to model geochemical reactions. Based on an ion pairing aqueous model, PHREEQE can calculate pH, redox potential, and mass transfer as a function of reaction process. The composition of solutions in equilibrium with multiple phases can be calculated. The aqueous model including elements, aqueous species, and mineral phases is exterior to the computer code and is completely user definable.
PHREEQE Applications
PHREEQE can simulate several types of reactions including (1) addition of reactants to a solution, (2) mixing of two waters, and (3) titrating one solution with another. In each of these cases, PHREEQE can simultaneously maintain the reacting solution at equilibrium with multiple phase boundaries. PHREEQE calculates the following quantities during the reaction simulation: (1) the pH, (2) the pe, (3) the total concentration of elements, (4) the amounts of minerals transferred into or out of the aqueous phase, (5) the distribution of aqueous species, and (6) the saturation state of the aqueous phase with respect to specified mineral phases. PHREEQE solves a set of nonlinear equations using a combination of two techniques: (1) a continued fraction approach for mass balance equations, and (2) a modified NewtonRaphson technique for all other equations. PHREEQE includes PHRQINPT, a preprocessor for creating/modifying input data files. PHREEQE accesses an external thermodynamic database which has been extensively updated. Both source and executable codes are included with the package.
PHREEQE can simulate several types of reactions including equilibrium speciation for a single solution, addition of reactants to a solution, mixing of two solutions, and titrating one solution with another. In each of these cases PHREEQE can simultaneously maintain the reacting solution at equilibrium with multiplephase boundaries.
PHREEQE utilizes extended memory.
 Based on an ion pairing aqueous model, PHREEQE can calculate the following quantities during the reaction simulation:
 The pH.
 Redox Potential.
 The total concentration of elements.
 The amounts of minerals (or other phases) transferred into or out of the aqueous phase.
 The saturation state of the aqueous phase with respect to specified mineral phases.
The aqueous model, including elements, aqueous species, and mineral phases, is exterior to the computer code and is completely user definable.
PHREEQE solves a set of nonlinear equations using a combination of two techniques: (1) a continued fraction approach is used for mass balance equations, and (2) a modified NewtonRaphson technique is used for all other equations.
PHREEQE Data Files
PHREEQE includes an external interactive program, PHRQINPT, which is used to create/modify input data files for PHREEQE. PHRQINPT prompts for and formats the data. Help is available at each prompt to explain the type of data being requested and the program contains error checking to ensure that all necessary data are included. PHRQINPT reads an external Minerals Thermodynamic database which is included.
PHREEQE accesses an external Thermodynamic database file (included) which has been extensively updated and extended.
PHREEQE Revisions
Revisions to PHREEQE have been made to:
 Streamline the linear equation solver
 Enable the aqueous model to be enlarged
 Simplify the definition of file unit numbers
Check out AquaChem! AquaChem features a graphical user interface to the popular PHREEQC model, the latest version of PHREEQE.
