Geology Reference
In-Depth Information
basis matches that emanating from natural seepage. Nevertheless, the
impacts can be severe when the subsequent slick impinges on coastal
ecosystems.
Regardless of the source, the resultant oil slicks are essentially surface
phenomena that are affected by several transportation and transforma-
tion processes.
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With respect to transportation, the principal agent for
the movement of slicks is the wind, but length scales are important.
Whereas small weather systems, such as thunderstorms, tend to disperse
the slick, cyclonic systems can move the slick essentially intact. Waves
and currents also affect the advection of an oil slick. To a more limited
extent, diffusion can act to transport the oil.
Transformation of the oil involves phase changes and degradation.
Several physical processes can invoke phase changes. Evaporation of the
more volatile components is a significant loss mechanism, especially for
light crude oil. The oil slick spreads as a buoyant lens under the influence
of gravitational forces, but generally separates into distinctive thick and
thin regions. Such pancake formation is due to the fractionation of the
components within the oil mixture. Sedimentation can play a role in
coastal waters when rough seas bring dispersed oil droplets into contact
with suspended particulate material and the density of the resulting
aggregate exceeds the specific density of seawater. Colloidal suspensions
can consist of either water-in-oil or oil-in-water emulsions, which behave
distinctly differently. Water-in-oil emulsification creates a thick, stable
colloid that can persist at the surface for months. The volume of the slick
increases and it aggregates into large lumps known as ''mousse'', thereby
acting to retard weathering. Conversely, oil-in-water emulsions com-
prise small droplets of oil in seawater. This aids dispersion and increases
the surface area of the slick, which can subsequently accelerate weath-
ering processes.
Chemical transformations of oil are evoked primarily through pho-
tochemical oxidation and microbial biodegradation. Not only is the
latter more important in nature, but strategies can be adopted to
stimulate biological degradation, consequently termed bioremediation.
All marine environments contain microorganisms capable of degrading
crude oil. Furthermore, most of the molecules in crude oils are suscep-
tible to microbial consumption. Oil contains little nitrogen or phospho-
rus and therefore, microbial degradation of oil tends to be nutrient
limited. Bioremediation often depends upon on the controlled and
gradual delivery of these nutrients, while taking care to limit the con-
current stimulation of phytoplankton activity. Approaches that have
been adopted are the utilisation of slow-release fertilisers, oleophilic
nutrients and a urea-foam polymer fertiliser incorporating oil-degrading
