Behavior of Oil in the Environment
When oil is spilled, whether on water or land, a number of transformation
processes occur, which are referred to as the “behavior” of the oil. Two types
of transformation processes are discussed in this chapter. The rst is weath-
ering, a series of processes whereby the physical and chemical properties of
the oil change after the spill. The second is a group of processes related to the
movement of oil in the environment. Spill modeling is also included in the
section on oil movement. Weathering and movement processes can overlap,
with weathering strongly inuencing how oil is moved in the environment.
These processes depend very much on the type of oil spilled and the weather
conditions during and after the spill.
The Importance of Behavior and Fate
The specic behavior processes that occur after an oil spill determine
how the oil should be cleaned up and its effect on the environment. For
example, if an oil evaporates rapidly, cleanup is less intense, but the hydro-
carbons in the oil enter the atmosphere and cause air pollution. An oil slick
could be carried by surface currents or winds to a bird colony or to a shore
where seals or sea lions are breeding, and severely affect the wildlife and
their habitat. On the other hand, a slick could be carried out to sea where it
disperses naturally and has little direct effect on the environment.
In fact, the fate and effects of a particular spill are determined by the behav-
ior processes, which are in turn almost entirely determined by the type of
oil and the environmental conditions at the time of the spill. Spill responders
need to know the ultimate fate of the oil in order to take measures to mini-
mize the overall impact of the spill.
An Overview of Weathering
Oil spilled on water undergoes a series of changes in physical and chemical
properties that in combination are termed weathering. Weathering processes
occur at very different rates, but begin immediately after oil is spilled into
44 The Basics of Oil Spill Cleanup
the environment. Weathering rates are not consistent throughout the dura-
tion of an oil spill and are usually highest immediately after the spill.
Both weathering processes and the rates at which they occur depend more
on the type of oil than on environmental conditions. Most weathering pro-
cesses are highly temperature dependent, however, and will often slow to
insignicant rates as temperatures approach zero degrees.
The processes included in weathering are evaporation, emulsication,
natural dispersion, dissolution, photooxidation, sedimentation, adhesion to
materials, interaction with mineral nes, biodegradation, and the formation
of tar balls. These processes are listed in order of importance in terms of
their effect on the percentage of total mass balance, that is, the greatest loss
from the slick in terms of percentage, and what is known about the process.
Evaporation is usually the most important weathering process. It has the
greatest effect on the amount of oil remaining on water or land after a spill.
Over a period of several days, a light fuel such as gasoline evaporates com-
pletely at temperatures above freezing, whereas only a small percentage of a
heavier Bunker C oil evaporates. The evaporation rates of the oils discussed
in this book are shown in Figure4.1.
The rate at which an oil evaporates depends primarily on the oil’s compo-
sition. The more volatile components an oil or fuel contains, the greater the
extent and rate of its evaporation. Many components of heavier oils will not
evaporate at all, even over long periods of time and at high temperatures.
Highly evaporated oil on a beach forming an asphalt pavement along with gravel and oyster
shells. (Photo courtesy of the Canadian Coast Guard.)