Updated: November 11,
2000.
Copyright © 2000 by Walt McNab, Concord, CA, U.S.A.
All Rights Reserved.
Computer-Mediated Distance Learning
Course on
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MULTISPECIES
REACTIVE TRANSPORT IN GROUNDWATER |
INSTRUCTOR:
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Senior
Scientist |
PREFACE
What is
ěmulti-species reactive transportî?
Multi-species reactive
transport entails the simultaneous transport of dissolved chemical species in
groundwater that chemically interact with one another as well as with the
solid matrix. Reactive transport phenomena play a major role in a variety of
processes occurring in subsurface fluids, including metasomatism, or ore
forming processes, soil formation phenomena, and the fate and transport of
contaminants, both organic and inorganic.
(Comment:
Do not worry if, as you read this PREFACE, you encounter terms with which you
are not familiar. Just continue to read in order to understand what I have in
mind in presenting this course. Iíll come back to explain the meaning of
this term in the course itself.)
Why do we need to model multi-species reactive transport?
Mathematical models of
reactive transport phenomena provide a powerful means for understanding and
predicting the chemical evolution of the subsurface environment in
multi-component, open-system settings. However,
the simulation of reactive transport is inherently complex because a large
number of transport and chemical reaction equations must be solved
simultaneously. These equations
include the partial differential equations that describe the mass balances for
each of many chemical species (or components) present in the system, together
with algebraic equations that describe the chemical reactions that take place.
The reactions themselves may be instantaneous or else time dependent.
Nevertheless, the advent of readily available powerful computational
hardware has permitted the development of models of increasing sophistication
capable of handling ever more complex (and thus more realistic) reactive
transport scenarios. After almost two decades of development, powerful reactive
transport models are now available in the public domain that may be downloaded
via the Internet.
PHREEQC and Course objectives:
This online class will
explore reactive transport modeling via the PHREEQC geochemical speciation
computer program available from the U.S. Geological Survey.
PHREEQC was originally designed to model chemical reactions in natural
waters of given compositions in open or closed batch systems.
However, a newer version of PHREEQC has the capabaility of also solving
the mass balance equations (also referred to as advective-dispersive transport
equation) in one dimensional flow domain for each of the components in the
water composition. The resulting
model is quite powerful and is capable of simulating a large variety of
geochemical problems.
To whom should this course be of interest?
This class will be of
interest to earth scientists and engineers concerned with problems involving
the evolution of the subsurface geochemical environment in response to coupled
chemical and solute transport processes.
This will include contaminant hydrologists interested in the transport
of metals and dissolved organic compounds through an aquifer and how these
substances interact with the local aquifer geochemistry.
Soil scientists interested in problems involving soil genesis will also
benefit from knowledge of the material presented in this course.
Finally, geologists will be able to use the material in this course to
model problems involving such topics as diagenesis and metasomatism.
Course extent and prerequisites:
The class is designed for
geoscientists with a basic understanding of groundwater flow and transport
processes and a working knowledge of mass action and mass balance principles
for modeling chemical reactions in aqueous systems. General familiarity with geochemical concepts applied to
groundwater systems (e.g., pH, Eh, redox processes) is also expected.
The class will entail reviewing the basic transport equations, the
chemical reaction equations, and methods for their solution, using various
examples. We will then move on to
a tutorial on the PHREEQC model, moving from its batch geochemical speciation
capabilities, kinetic models, to its transport capabilities.
We will then examine a number of examples of how the PHREEQC reactive
transport model can be used, in a one-dimensional geometry, to understand a
number of example problems in different contexts, including groundwater
contamination (acid mine drainage, fuel hydrocarbon biotransformation), soil
formation (the formation of spodosols through podzolization), and
ore-generating processes in near-surface environments (copper supergene
deposit formation).
And a more
personal note:
This is a CMDL self-study
course. I have written this course in the same language that you are used to
hear in a frontal lecture in a non-virtual classroom. In fact, because Iíll
never be sure that you have understood what I have been trying to
explain, I made a special effort to explain things in more detail..
Occasionally, and not only at the end of the course, I'll summarize the
presented material.
I have been making use of
the power of hyperlinks. A click will give you
a reference, an additional explanation, or a definition of a term in a
glossary. I'll often ask questions and ask you to write your answer and then
CLICK to see my answer. The course contains many problems and exercises that
you'll be asked to solve. The solutions are presented and can be viewed.
Multiple-choice problems are also included.
Use the advantage of this
teaching technique. Learn whenever and wherever it is convenient to you.
However, I urge you to set special hours, say 2x2 or 3x2 hours per week in
which youíll sit at your computer and study.
Finally, a personal
comment before we start to climb this mountain together. It certainly will not
be an easy task. I would like to tell you in a few words what you are going to
encounter along the way.
Two things should be stressed. First, this is not
another book on MULTISPECIES REACTIVE
TRANSPORT IN GROUND WATER. I
have chosen a very informal way of presenting the subject matter. You should
remember constantly that I am your personal tutor on the subject. This is
something that is quite different from your experience when you read a book,
or when you sit in a lecture hall with many other students. Obviously, this
will happen if I have succeeded in achieving my own goals in authoring this
course.
It
is my hope that you will like this approach and benefit from the enormous
advantages which it offers.
This
is a self-study course. I expect you to study
the material and solve the problems by yourself. I have made an effort to make
your task as easy as possible by writing the lecture, selecting the
explanations, the examples, and the exercises in a way that facilitates
self-study. However, I am always available to respond to email questions and
comments. In fact, I encourage you to use this option of communicating with
me, or with your local instructor.
You can now go to the TABLE OF CONTENTS and start studying the course.
You
may e-mail me questions and comments.
Walt
W. McNab
E-mail address: Walt McNab <WaltMcNab@prodigy.net>
.