TABLE 4-7 Processes that Move Petroleum Hydrocarbons Away from Point of Origin

 

Weathering

 

Input Type

Petroleum 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

2-stroke engines (gasoline & light distillates)

days

H

NR

M

L

L

L

L/NR

NR

NR

Atmospheric

days

H

NR

M

M

H

NR / NR

L

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

driven by the persistence of the release on the water surface as well as the areal extent of the input. Thus, even though atmospheric inputs are expected to have low persistence because they volatilize quickly, they are introduced over large areas compared to point sources such as spills. Heavy oils and seeps form residues that can persist at sea for long periods.

Vertical transport and movement include the processes of vertical dispersion, entrainment, Langmuir circulation, sinking, and overwashing. Rankings reflect the potential for mixing into the water column by natural processes, given that very light oils evaporate quickly and heavy oils are too viscous to disperse naturally. Sedimentation rankings consider the suspended sediment concentrations at the release sites and the potential for the oil to adhere to sediments after stranding onshore or by mixing with suspended sediments.

Shoreline stranding is ranked highest for those petroleum hydrocarbons that persist on the water surface and are likely to be released close to shore, increasing the potential for a significant amount of the released oil to strand. Tarball formation, like shoreline stranding, is ranked highest for releases of crudes and heavy oils that form persistent residues.

Seeps

Crude oil released into the marine environment through natural seepage undergoes most of the same physical and chemical process as crude oil released into the ocean at the seafloor. One main difference is in the rate of addition of oil to the environment. With natural oil seeps, the leakage rate is relatively low and chronic. On the other hand, oil spills result in a release that is a sudden, one-time event. The same basic processes act to degrade and remove oil with time, although in the case of seeps, crude oil is replenished as long as the seeps remain active. Thus, the persistence of natural oil seeps is reckoned in terms of years.

Ranking the fate processes that move petroleum hydrocarbons away from natural oil seeps is difficult because these seeps occur worldwide in numerous geographic settings. In the four major weathering processes of evaporation, emulsification, dissolution, and oxidation, they are ranked “medium.” The overall ranking is similar to that of crude oil spills. Also, comparison between natural seeps and crude oil spills show the similarities in rankings between the processes of transport, sedimentation, shoreline stranding, and tarball formation.

Spills

Spills range widely in oil type, spill size, location, and environmental conditions during the release.

Gasoline

With a very low viscosity, gasoline spills spread rapidly as thin sheens. Gasoline is also light, with a specific gravity of about 0.8, so it causes a slick on the sea surface, or rapidly re-floats if dispersed into the water column. Evaporation-volatilization is the dominant process affecting spills on the water surface and may eliminate nearly all of the spilled gasoline within a few hours to a day. As a result, gasoline spills in marine waters have low persistence. There is little potential for spreading, mixing into the water column, sedimentation, or stranding, and no risk of forming emulsions or



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