PHREEQE is a geochemical reaction model which is based on an ion pairing aqueous model. PHREEQE can determine pH, redox potential, and mass transfer as a function of reaction progress, and also solution composition in equilibrium with multiple phases. The aqueous model, including elements, aqueous species, and mineral phases, is exterior to the computer code, is completely user definable, and has been regularly updated by the authors since its first release. PHREEQE can simulate several types of reactions including addition of reactants to a solution, mixing of two waters, and titrating one solution with another. In each of these case, PHREEQE can simultaneously maintain the reacting solution at equilibrium with multiple phase boundaries. PHREEQE solves the set of non-linear governing equations using a combination of two techniques: 1) a continued fraction approach for mass balance equations, and 2) a modified Newton-Raphson technique for all other equations; the resulting linear equations are solved using Gaussian elimination. Activity coefficients are calculated from either the Debye-Huckel or Davies formulation.

The data base includes the following elements: allumium, barium, borium, bromide, cadmium, calcium, carbon, chloride, copper, fluoride, hydrogen, iron, lead, lithium, magnesium, manganese, nitrogen, oxygen, phosphorus, potassium, silica, sodium, strontium, sulfur and zinc. It also has a database for arsenic, cesium, iodine, nickel, rubium, selenium, silver, and uranium.

The interactive preprocessor PHRQINPT is included with the PHREEQE package to facilitate formulation and editing of the PHREEQE input data. PHRQINPT explains the meaning and significance of each variable when required, and internally checks the validity of entered values.

PHREEQE and PHRQINPT are public domain programs; the IGWMC version is compiled to run in the DOS environment and includes source code, executable image, and example data sets. The documentation consists of installation instruction and the user's manual for both PHREEQE and PHRQINPT.

SYSTEM REQUIREMENTS

Intel 80i86 based computer, 640 Kb RAM, DOS 3.0 or higher; math coprocessor optional.

Authors: D.L. Parkhurst, D.C. Thorstenson, and L.N. Plummer (U.S. Geological Survey).