User's guide to PHREEQC (Version 2)-a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations

PHREEQC version 2 is a program written in C for performing a wide variety of low-temperature aqueous geochemical calculations, built on an ion-association aqueous model. Its capabilities span speciation and saturation-index calculations; batch-reaction and one-dimensional transport calculations involving reversible reactions (aqueous, mineral, gas, solid-solution, surface-complexation, and ion-exchange equilibria) and irreversible reactions (specified mole transfers, kinetically controlled reactions, mixing, and temperature changes); and inverse modeling that finds sets of mineral and gas mole transfers accounting for compositional differences between waters within specified uncertainty limits.

Relative to version 1, version 2 adds many features, including simulating dispersion or diffusion and stagnant zones in one-dimensional transport, modeling kinetic reactions with user-defined rate expressions, ideal and non-ideal solid solutions, fixed-volume as well as fixed-pressure gas phases, surface or exchange sites that vary as minerals dissolve or precipitate, isotope mole balances in inverse modeling, multiple convergence-parameter sets, and spreadsheet-compatible input and output. The report presents the governing equations for these calculations, describes the program input, and provides examples demonstrating most of its capabilities.