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INTRODUCTION

PHREEQC version 2 is a computer program for simulating chemical reactions and transport processes in natural or polluted water. The program is based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid solutions, exchangers, and sorption surfaces, but also includes the capability to model kinetic reactions with rate equations that are completely user-specified in the form of Basic statements. Kinetic and equilibrium reactants can be interconnected, for example by linking the number of surface sites to the amount of a kinetic reactant that is consumed (or produced) during the course of a model period. A 1D transport algorithm comprises dispersion, diffusion, and various options for dual porosity media. A powerful inverse modeling capability allows identification of reactions that account for observed water compositions along a flowline or in the time course of an experiment. An extensible chemical data base allows application of the reaction, transport, and inverse-modeling capabilities to almost any chemical reaction that is recognized to influence rain-, soil-, ground- and surface water quality.

PHREEQC is based on the Fortran program PHREEQE (Parkhurst and others, 1980). PHREEQE was capable of simulating a variety of geochemical reactions for a system including:

· Mixing of waters,

· Addition of net irreversible reactions to solution,

· Dissolving and precipitating phases to achieve equilibrium with the aqueous phase, and

· Effects of changing temperature.

PHREEQE calculated concentrations of elements, molalities and activities of aqueous species, pH, pe, saturation indices, and mole transfers of phases to achieve equilibrium as a function of specified reversible and irreversible geochemical reactions.

PHREEQC version 1 (Parkhurst, 1995) was a completely new program written in the C programming language that implemented all of the capabilities of PHREEQE and added many capabilities that were not available in PHREEQE, including:

· Ion-exchange equilibria,

· Surface-complexation equilibria,

· Fixed-pressure gas-phase equilibria, and

· Advective transport.

Other improvements relative to PHREEQE included complete accounting for elements in solids and the aqueous and gas phase, mole balance on hydrogen and oxygen to account for the mass of water in the aqueous phase, identification of the stable phase assemblage from a list of candidate phases, use of redox couples for definition of redox state in speciation calculations, and a more robust non-linear equation solver.

PHREEQC version 2 is a modification of PHREEQC version 1. All of the capabilities and most of the code for version 1 are retained in version 2 and several new capabilities have been added, including:

· Kinetically controlled reactions,

· Solid-solution equilibria,

· Fixed-volume gas-phase equilibria,

· Variation of the number of exchange or surface sites in proportion to a mineral or kinetic reactant,

· Diffusion or dispersion in 1D transport,

· 1D transport coupled with diffusion into stagnant zones, and

· Isotope mole balance in inverse modeling.

The numerical method has been modified to use several sets of convergence parameters in an attempt to avoid convergence problems. User-defined quantities can be written to the primary output file and (or) to a file suitable for importation into a spreadsheet, and solution compositions can be defined in a format that is more compatible with spreadsheet programs.


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