Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 281

• Water vapor. Humidity plays a significant role in the water cycle by affecting air
movement and by reacting with contaminants in the gas phase and contami-
nants released into the atmosphere attached or sorbed onto particulate matter.
Contamination goes along for the ride when precipitation formed around contam-
inated condensation nuclei is transported from the atmosphere to the lithosphere
and then entrained by surface runoff.
• Fog. Fog can be an effective agent for the transfer of acid rain by transferring it to
vegetation or other surface materials through direct contact.
• Flood events and hurricanes. Due to their catastrophic nature and magnitudes,
floods and hurricanes may not only increase contaminant migration but can also
cause significant releases. During the 1993 floods of the Mississippi River, numer-
ous barrels containing hazardous waste were swept away and deposited in the
Gulf of Mexico.
• Solar energy. Sunlight breaks down some contaminants through a process called
photolysis. In toxic microorganisms, ultraviolet light passes easily through cell
walls, cytoplasm, and nuclear membranes and prevents DNA replication.
Specific vegetative factors that affect migration of contaminants in soil include (USGS
2006a)
• Roots. These pathways within shallow subsurface geological materials can
enhance the migration of contaminants (Chapter 3). Certain plants can uptake
contaminants. Many types of plants and trees have the capability with their root
systems to assist in the removal of contaminants from shallow subsurface soil.
Contaminants may be stored in plant tissues or are transformed through biologic
processes.
• Microorganisms. Microorganisms in the soil often biodegrade many different
types of contaminants by using the contaminants themselves as a source of food.
8.3.2 Basic Contaminant Degradation Concepts
The degradation of specific compounds in the environment is expressed in terms their
half-life. Half-life is the average amount of time required to degrade half or 50% of a
specific contaminant population (USEPA 1996a). Contaminants degrade through biotic or
abiotic processes, and the processes controlling their rate of decay depend upon the fol-
lowing factors (USEPA 1996a; USGS 2006a):
• The nature of the release. This group of factors includes
• Media receiving the release. Was it into the atmosphere, surface or subsurface
soil, surface water, ocean, directly to groundwater, or a combination of media?
• Amount (volume or mass) of the release
• Number of contaminants released. Was it a single contaminant—or a mixture
of several contaminants?
• Physical state of the release (i.e., liquid, solid, or gas)
• Time duration of the release