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 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.

- (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.

- (1) a continued fraction approach for mass balance equations, and
- (2) a modified Newton-Raphson technique for all other equations.

PHREEQE is a USGS computer program designed to model geochemical reactions. 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 multiple-phase boundaries.

PHREEQE utilizes extended memory.

Based on an ion pairing aqueous model, PHREEQE can calculate the following quantities during the reaction simulation:

- 1. The pH.
- 2. Redox Potential.
- 3. The total concentration of elements.
- 4. The amounts of minerals (or other phases) transferred into or out of the aqueous phase.
- 5. The distribution of aqueous species.
- 6. The saturation state of the aqueous phase with respect to specified mineral phases.

PHREEQE solves a set of non-linear equations using a combination of two techniques:

- (1) a continued fraction approach is used for mass balance equations, and
- (2) a modified Newton-Raphson technique is used for all other equations.

PHREEQE accesses an external Thermodynamic data base file (included) which has been extensively updated and extended.

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

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