Updated: December 10, 2001.
Copyright © 2001 by Walt W McNab, Concord, CA, U.S.A.
All Rights Reserved.

Computer-Mediated Distance Learning

Course on

 

MULTISPECIES REACTIVE

TRANSPORT IN GROUNDWATER

 
 

INSTRUCTOR:

WALT W McNAB

Senior Scientist
Exponent-Failure Analysis Associates
Oakland, California, USA.

TOPIC B: MODELING APPROACHES

 Lecture 3: Introduction to Geochemical Speciation Modeling with PHREEQC:

An Example Application

 Introduction

For the remainder of this course, we will be using PHREEQC as a tool for solving problems in aquifer geochemistry. Beginning in the next lecture, we will see how the geochemical and transport equations may be coupled in the solution of reactive transport problems. In the meantime, in this current lecture, we will take a look at some more of the details involved in setting up a geochemical problem in PHREEQC.

 An example: oxidation of toluene in an (initially) aerobic aquifer

In previous lectures, weíve occasionally used the example of fuel hydrocarbon oxidation in the context of understanding redox processes in groundwater environments. Letís return to this example in some detail here, focusing on how to get PHREEQC to simulate the problem for us.

To begin, letís consider the following system. Suppose we have an aquifer with a groundwater geochemical composition that has been analyzed for major cations and anions as shown on Table 1. Now, letís make some assumptions about some of the factors that could influence the aquifer geochemistry under ambient conditions that might play a role in determining the (bio)geochemical laboratory and field studies to oxidize into CO2 in a variety of different redox regimes (i.e., using different electron acceptors) via biotransformation

Table 1. Background aquifer chemistry.

Parameter

Conc. (ppm)
Alkalinity

300
Ca2+

66
Cl-

76
K+

2
Mg2+

24
NO3-

20
Na+

80
SO42-

50
pH

7.6

Biotransformation reactions involving toluene may include:

In addition to these primary reactions, changes in water chemistry can induce other secondary reactions that may be heterogeneous in nature. For example, the reactions given by

Other reactions could include the dissolution or precipitation of calcite in response to changes in pH and concentration of CO32-, and the evolution of CO2 and/or CH4 gas from solution.

Letís take a look at how we can set up an input file for PHREEQC that captures all of these features.

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Here are some of the results, extracted directly from the "select.out" file, for you to examine:

 Your assignment

If you are able, try conducting a brief informal literature search on the topics of biotransformation, biodegradation, or natural attenuation of fuel hydrocarbons in groundwater. You will find that all of the phenomena predicted in our simple example are regularly observed in the field. The remarkable thing about this is that we did not have to explicitly instruct the model: "Dear bacteria, please oxidize toluene using oxygen first, then nitrate, followed by iron, sulfate, removing calcite as a pH buffer, or closing the system so that CO2 and CH4 canít evolve? Experiment!

You are now ready to continue to

TOPIC B: MODELING APPROACHES.

LECTURE 4: Coupling  Geochemical and Transport Equations.

 

You may e-mail me questions and comments.

Walt W. McNab
E-mail address: Walt McNab <WaltMcNab@prodigy.net>

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