Updated: December 10, 2001. Copyright © 2001 by Walt W McNab, Concord, CA, U.S.A. All Rights Reserved.
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MULTISPECIES REACTIVE TRANSPORT IN GROUNDWATER |
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Senior Scientist Exponent-Failure Analysis Associates Oakland, California, USA. |
TOPIC B: MODELING APPROACHES
At this point in the course, we have explored all of the introductory material we will need (with the exception of coupled transport modeling, which will come later) to begin exploring some examples of geochemical modeling applications in earnest. What we need now is a model to work with. As weíve alluded to many times in the prior lectures, we will be using PHREEQC, the latest version of the PHREEQE geochemical models developed by the U.S. Geological Survey for modeling chemical reactions in natural waters. The most recent version of PHREEQC has the capability of modeling solute transport (in one dimension) along with solution geochemistry, a feature we will be exploiting in example problems in upcoming lectures. In the meantime, in this lecture, we will simply learn how to obtain the PHREEQC program and how to define problems for it to solve.
PHREEQC is a free, public-domain program that can be downloaded for use from the Internet. A general version of the program is available as platform-specific executable files for the PC, Mac, and Unix operating environments. This version requires the manual editing of an ASCII input file that defines the problem. The program will run, using the specified input file and selected thermodynamic database (supplied with the program) and produce an ASCII output file for examination. For the PC platform, a graphical user interface (GUI) pre-processor is available that provides assistance in preparing the input file (PHREEQCI). For this course, we will focus our discussion on the actual input files for problem set up, so whether or not you choose to use the GUI (if you are using a PC running Windows 95 or later) is up to you.
However, for this course, you must select PHREEQC Version 2 or later, or PHREEQCI alpha test version 2 or later.
The generic (i.e.,
non-GUI) version of PHREEQC is available for download from the following web
site:
http://water.usgs.gov/software/phreeqc.html
After downloading,
simply follow the installation instructions (provided by the web site) to set
the program up on your computer. A userís manual will be included among the
downloaded files. It is recommended that you print out a copy for yourself to
keep as a quick reference and to follow some of the example problems provided
(in addition to those we will use in this course). If you wish to use the GUI
version instead, it may be found at:
Additional information
on PHREEQC, and an alternate download site for both the generic and GUI
versions, can also be found at
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/index.htm
will look like this:
.gif)
The actual input file
read by the program is
.gif)
After running the
model, the output file will look like this
Input file: C:\WINDOWS\Desktop\Work\Reactive transport class\PHREEQC runs\CalciteEq.in
Output file: C:\WINDOWS\Desktop\Work\Reactive transport class\PHREEQC runs\CalciteEq.out
Database file: C:\PROGRAM FILES\USGS\PHREEQCI2.2\phreeqc.dat
------------------
Reading data base.
------------------
SOLUTION_MASTER_SPECIES
SOLUTION_SPECIES
PHASES
EXCHANGE_MASTER_SPECIES
EXCHANGE_SPECIES
SURFACE_MASTER_SPECIES
SURFACE_SPECIES
RATES
END
------------------------------------
Reading input data for simulation 1.
------------------------------------
SOLUTION 1
temp 25
pH 7
pe 4
units mmol/kgw
redox pe
density 1
water 1 # kg
EQUILIBRIUM_PHASES 1
Calcite 0 10
END
-------------------------------------------
Beginning of initial solution calculations.
-------------------------------------------
Initial solution 1.
-----------------------------Solution composition------------------------------
Elements Molality Moles
Pure water
----------------------------Description of solution----------------------------
pH = 7.000
pe = 4.000
Activity of water = 1.000
Ionic strength = 1.001e-007
Mass of water (kg) = 1.000e+000
Total alkalinity (eq/kg) = 1.082e-010
Total carbon (mol/kg) = 0.000e+000
Total CO2 (mol/kg) = 0.000e+000
Temperature (deg C) = 25.000
Electrical balance (eq) = -1.082e-010
Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.05
Iterations = 0
Total H = 1.110124e+002
Total O = 5.550622e+001
----------------------------Distribution of species----------------------------
.gif)
------------------------------Saturation indices-------------------------------
Phase SI log IAP log KT
H2(g) -22.00 -22.00 0.00 H2
H2O(g) -1.51 0.00 1.51 H2O
O2(g) -39.12 44.00 83.12 O2
-----------------------------------------
Beginning of batch-reaction calculations.
-----------------------------------------
Reaction step 1.
Using solution 1.
Using pure phase assemblage 1.
-------------------------------Phase assemblage--------------------------------
Ionic strength = 3.848e-004
Mass of water (kg) = 1.000e+000
Total alkalinity (eq/kg) = 2.454e-004
Total CO2 (mol/kg) = 1.227e-004
Temperature (deg C) = 25.000
Electrical balance (eq) = -1.082e-010
Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.00
Iterations = 16
Total H = 1.110124e+002
Total O = 5.550658e+001
----------------------------Distribution of species----------------------------
.gif)
H2 1.679e-016 1.679e-016 -15.775 -15.775 0.000
O(0) 0.000e+000
O2 0.000e+000 0.000e+000 -60.830 -60.830 0.000
------------------
End of simulation.
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------------------------------------
Reading input data for simulation 2.
------------------------------------
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End of run.
-----------
Your assignment is to download and install the PHREEQC program and to look through the manual to become familiar with the formulation of simple problems. In our next lecture, we will put PHREEQC to work on some zero-dimensional (i.e., no transport) problems to learn a little about its capabilities. In the final lecture of Topic B, we will employ the transport capabilities of the program to begin solving reactive transport problems.
You are now ready to continue to
TOPIC B: MODELING APPROACHES.
LECTURE 3: Introduction to Geochemical Speciation Modeling with PHREEQC: Example Applications
..
You may e-mail me questions and comments.
Walt W. McNab
E-mail address: Walt McNab <WaltMcNab@prodigy.net>