PHREEQC is a computer program for speciation, reaction-path, advective transport, and inverse geochemical calculations. Designed to perform a wide variety of aqueous geochemical calculations, PHREEQC is based on an ion-association aqueous model and has capabilities for: (1) speciation and saturation-index calculations; (2) reaction-path and advective-transport calculations involving specified irreversible reactions, mixing of solutions, mineral and gas equilibria, surface-complexation reactions, and ion-exchange reactions, and (3) inverse modeling, which finds sets mineral and gas mole transfers that account for composition differences between waters, within specified compositional uncertainties. PHREEQC essentially combines the capabilities of four earlier codes: PHREEQE, WATEQ4F, PHREEQ-M, and NETPATH, with some added capabilities and improvements.

PHREEQC is derived from the Fortran program PHREEQE, but has been completely rewritten in C with the addition of many new capabilities. New features include capabilities: (1) to use redox couples to distribute redox elements among their valence states in speciation calculations; (2) to model ion-exchange and surface- complexation reactions; (3) to model reactions with a fixed-pressure, multicomponent gas phase (that is, a gas bubble); (4) to calculate the mass of water in the aqueous phase during reaction and transport calculations; (5) to keep track of the moles of minerals present in the solid phases and determine automatically the thermo-dynamically stable phase assemblage; (6) to simulate advective transport in combination with PHREEQC's reaction-modeling capability; and (7) to make inverse modeling calculations that allow for uncertainties in the analytical data.

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 user interface is improved through the use of a simplified approach to redox reactions, which includes the explicit mole-balance equations for hydrogen and oxygen; the use of revised input that is modular and completely free format; and the use of mineral names and standard chemical symbolism rather than index numbers. The use of the C programming language eliminates nearly all limitations on array sizes, including numbers of elements, aqueous species, solutions, phases, and lengths of character strings.

A new equation solver that optimizes a set of equalities subject both to equality and inequality constraints is used to determine the thermo-dynamically stable set of phases in equilibrium with a solution. A more complete Newton- Raphson formulation, master-species switching, and scaling of the algebraic equations reduce the number of failures of the numerical method in PHREEQC relative to PHREEQE.

PHREEQC is a public domain, DOS-based program.

SYSTEM REQUIREMENTS

Intel 80386/80486/Pentium-based computer with activated math coprocessor (DX type), 4 Mb RAM, DOS 5.0 (or higher), and at least 3 Mb hard disk space. The program runs as a DOS application under MS Windows.

Author: David L. Parkhurst, (U.S. Geological Survey).