Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 276

8.3 Principles of Contaminant Fate and Transport
Interaction with the environment begins immediately after a contaminant has been
released. Once released, a contaminant can do three things (Manahan 1993; Hemond and
Fechner-Levy 2000):

• Stay put
• Migrate in soil, water, air, or a combination of media
• Degrade, transform, or get destroyed

The factors controlling these three outcomes depend upon the physical chemistry of the
contaminant and the characteristics of the receiving environment (USGS 2006a; Rogers
et al. 2007). For example, some contaminants may change form, as when many of the chem-
ical compounds in gasoline immediately evaporate after their exposure to the atmosphere.
Other contaminants may degrade in a matter of minutes after being released, while some
may last for thousands of years or sometimes longer, as with certain radioactive com-
pounds. Therefore, computing a mass balance should be the first action when assessing
any particular release, as this will ensure that the mass or volume of contaminant released
is accurately measured. Conducting a mass balance also serves to validate or refute our
current understanding of the behavior of the different environmental elements influenc-
ing the contaminant once it has been released.
A simple mass balance is expressed as

Amount released amount re = cov ered amount lost to theenvironment + (air water + + soil )

(8.1)

Once in the environment, contaminants can and often move between soil, surface water,
groundwater, and the atmosphere, and they can also degrade. Many contaminants
degrade quickly if conditions are favorable, yet others persist and last for years or decades
depending upon the conditions present (USGS 2006a). Factors degrading contaminants
fall into two broad categories: biotic degradation and abiotic degradation. Biotic degrada-
tion involves microorganisms or fungi and occurs when an organism, such as a bacterium,
uses a contaminant as a source of food and either degrades or transforms the contaminant
(USGS 2006a). Abiotic degradation involves other processes not including microorgan-
isms. Examples of abiotic degradation include photolysis—the degradation as a result of
exposure to sunlight (USGS 2006a)—and hydrolysis. As described in Chapter 3, hydrolysis
involves cleaving a molecule into two parts by the addition of a molecule of water.
When examining contaminant degradation in the environment, it is important not
to confuse dilution with degradation. If given enough time, contaminants may become
diluted, and this process results in a decrease of the contaminant concentrations per unit
volume of the media being measured. However, dilution is not degradation, since it does
not involve a chemical transformation of the contaminant. We will discuss degradation in
more detail in Section 8.3.2.

8.3.1 Basic Contaminant Transport Concepts

Contaminant transport in the environment is dominated by three physical transport
mechanisms (USEPA 1996a; USGS 2006a): advection/convection, molecular diffusion, and