TABLE 2-1 Processes That Move Petroleum Hydrocarbons Away from Point of Origin

Input Type

Persistence

Evaporation

Emulsification

Dissolution

Oxidation

Horizontal Transport or Movement

Vertical Transport or Movement

Sedimentation

Shoreline Stranding

Tarballs

Seeps

years

H

M

M

M

H

M

M

H

H

Spills

Gasoline

days

H

NR

M

L

L

L

NR

NR

NR

Light Distillates

days

M

L / L

H

L

M

H

L

L

NR

Crudes

months

M

M

M

M

M

M

M

H

M

Heavy Distillates

years

L

M

L

L

H

L

H

H

H

Produced water

days

M

NR

M

M

L

L

L

L

NR

Vessel operational

months

M

L

M

L

M

L

L

L

M

Two-stroke engines

days

H

NR

M

L

L

L

NR

NR

NR

Atmospheric

days

H

NR

M

M

H

NR / NR

NR

NR

NR

Land based

U

M

L

L

L

M

M

M

NR

U

NOTE: H = high; L = low; M = moderate; NR = not relevant; U = unknown

directly assess environmental damage from petroleum hydrocarbon mass loading rates. As discussed in Chapters 4 and 5 to a very large degree, loading rates reflect the intensity and location of societal use of petroleum, whereas effects tend to reflect the amount of toxic hydrocarbon compounds reaching a marine organism and the differing susceptibility of various organisms, populations, and ecosystems to the effects of these hydrocarbons. The reader is therefore strongly cautioned against inferring impacts from the mass loading rates. For instance, one might be tempted to calculate the “Exxon Valdez-equivalence” by comparing the quantity of petroleum released from a specific source to that released during the Exxon Valdez spill and then concluding that the impact of the petroleum release will be a corresponding multiple of the Exxon Valdez impact. This is a flawed analysis. Ecotoxicological responses are driven by the dose of petroleum hydrocarbons available to an organism, not the amount of petroleum released into the environment. Because of the complex environmental processes acting on the released petroleum, dose is rarely directly proportional to the amount released. In addition, one must consider the type of petroleum released and the susceptibility of the target organisms. Complex geochemical and pharmacokinetic models are required to translate petroleum release rates into environmental exposures. Even once these factors are accounted for, it is often difficult to reach consensus on the magnitude and duration of environmental effects (Box 2-1).

The amount of petroleum made available to an organism through various environmental processes (whether for ingestion or absorption) is referred to as being biologically available, or simply “bioavailable.” Just as combustion during smoking makes nicotine in tobacco bioavailable to the smoker, physical, chemical, and even biological processes determine how bioavailable toxic compounds in oil and other petroleum products will be to marine organisms. It is understandable, therefore, that the release of equal amounts of the same substance at different times or locations may have dramatically different environmental impacts.

Broadly speaking, the term “bioavailability” can therefore be used to describe the net result of physical, chemical, and biological processes that moderate the transport of hydrocarbon compounds from their release points to the target organisms. As the spill moves from the release point to the marine organism, these processes alter the chemical composition of the petroleum mixture, which in turn likely alters the toxicity by selectively enriching or depleting the toxic components (Bartha and Atlas, 1987). Physical weathering processes (Table 2-1) may encapsulate some or all of the petroleum in forms that are less available to organisms (e.g., tarballs). Various physiological and behavior processes moderate the movement of petroleum from the surrounding environment into marine organisms. Individual petroleum components pass into organisms at different rates, depending on their physical and chemical properties. Organisms respond to hydrocarbons in their surroundings and moderate or accentuate exposure. Incidental ingestion of oil by preening birds enhances exposure, while short-term cessation of filter feeding by bivalves in response to hydrocarbons in the water limits exposure. Once the hydrocarbons are in the organisms, there is a wide variation in the types and magnitudes of physiological responses. Many organisms readily metabolize and excrete hydrocarbons, although these pathways may create more toxic intermediates. In short, the processes of bioavailability, including petroleum fate and transport in the coastal ocean and disposition within marine organisms, are the most complex and least understood aspects of oil in the sea. Although there is a reasonable understanding of the amount of petroleum hydrocarbons released to the coastal ocean, and one can estimate the impact of spilled petroleum under previously studied conditions, generalizing these findings to predict hydrocarbon impacts from all sources on North American coastal waters is currently not possible.



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