Content area
Full Text
Introduction
Now try to solve the problem using the geochemical model PHREEQC. The model calculates that 2.3 X 10^sup -2^ M of calcium will dissolve. This is a factor of 3.6 greater than the "back-of-the-envelope calculation." Why is there a discrepancy? First, the hand calculation did not consider other calcium species that can form in solution, such as CaOH^sup +^ or CaSO^sub 4^^sup 0^. To do this, you must simultaneously solve equations for all the species and their complexes, including possible mineral precipitation. Second, the simplified calculation did not account for the activity coefficient, which corrects for how the ionic strength influences ion interactions. Note that information on other species present in solution is needed to calculate ionic strength. The advantage of using a geochemical modeling program is that the simultaneous solution to equations for all of the species present can be calculated.
This review will discuss PHREEQC (Parkhurst and Appelo 1999), the public domain geochemical modeling software available from the U.S. Geological Survey (USGS). The interactive version, PHREEQCI, was developed by Charlton and Parkhurst (2002).
Overview of Features
PHREEQC
The acronym PHREEQC stands for PH (pH), RE (redox), EQ (equilibrium), C (program written in Q. The interactive version ends with 1. It is one of the most comprehensive geochemical models available (Zhu and Anderson 2002). Geochemical problems are of two general types: forward and inverse. Forward modeling involves taking a solution composition and determining what minerals are in or near equilibrium with the solution. It includes reaction path modeling, which tracks the evolution of water in response to chemical reaction with minerals, surfaces, or mixing.
Inverse modeling involves at least two solution compositions and calculates geochemical reactions that account for the observed changes in chemical composition of water along a flowpath. PHREEQC adds to this combination the ability to describe kinetic parameters and onedimensional advection and dispersion.
With this dizzying array of options, a model can become too complex. PHREEQC handles this complexity by using keywords that turn on the various options. If the keyword is absent, the option is ignored. The user merely has to input data for the selected keywords (Figure 1). Every problem has a title, solution (giving the initial composition of the solution to be modeled), and end for keywords....