Fate Of An Oil Spill

The dispersal of an oil spill is illustrated in Figure 6-7. When oil is spilled, the lower boiling fractions evaporate and dissolve rapidly, with the result that 25-50% of a crude oil spill is soon lost from the surface of the sea. The extent to which these processes occur is determined by the temperature of water and the intensity of wind and wave action. The residue of higher molecular weight hydrocarbons is slowly degraded by microorganisms. This process may be slow because the other nutrients needed for growth, especially nitrogen, are quite limited in the ocean. In addition, the insoluble petroleum contains molecular species that are too large to be assimilated through the cell walls of microorganisms and will undergo biodegradation only after the petroleum has been broken down into smaller molecules by a combination of photochemical and oxidative processes. As a general rule, microorganisms cannot assimilate

FIGURE 6-7 Fate of spilled oil. Redrawn with permission from R. Burwood and G. C. Speers, Estuarine and Coastal Marine Science, Vol. 2, pp. 117-135. Copyright © 1974. Used by permission of Academic Press.

organic molecules of molecular weight greater than 500 Da. Consequently the remaining higher molecular weight material is whipped up into a "mousse" of hydrocarbons and water by waves and winds, and this mousse slowly breaks up into tar balls. These float unless mixed with enough sand to give them a density that exceeds the density of seawater in which case they sink to the bottom. Continued action of light, oxygen, and waves breaks the material up into smaller and smaller particles and to molecules that can eventually be degraded by microorganisms.

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