depleted in its more volatile components, the rate of evaporation will de- 

 crease as evaporation proceeds. For a heavy residual fuel oil, such as No. 

 6, evaporation plays only a minor role, and is limited to its effect on the 

 light "cutter stock" often added to straight-run No. 6 to improve its hand- 

 ling characteristics. 



3.1.3 Dissolution 



True thermodynamic dissolution of petroleum hydrocarbons is not a sig- 

 nificant contribution to the mass transfer of oil from the surface into the 

 water column. The most volatile portions of crude oils and light distillate 

 fuels are also the most soluble, but rarely would one expect to find a sig- 

 nificant downward flux of oil due to dissolution. In terms of toxicity, 

 however, the dissolved light hydrocarbons present a great environmental 

 hazard. Making the distinction between true "dissolved" and colloidal or 

 supracolloidal hydrocarbon "droplets" in the water column is a difficult 

 problem even in the best of laboratory situations. Suffice it to say, for 

 the moment, that it is not unreasonable to lump the true dissolved fraction 

 of oil with the oil-in-water emulsified oil, discussed in the next section, 

 and the other forms of oil that are worked into the water column by one 

 mechanism or another. 



3.1.4 Emulsif ication 



Oil can form either micellar oil-in-water (o/w) emulsions, or water-in- 

 oil (w/o) emulsions. Micelles are colloidal aggregates of the lipoidal (oil) 

 phase suspended in the aqueous phase. Micelle formation is "good" from a 

 dissipation point of view, since the micelles are microscopic in size and are 

 readily distributed through the water colum. Their microscopic size, incor- 

 porating perhaps only a hundred hydrocarbon molecules per micelle, provides 

 much more surface area than could be available at the underside of a con- 

 tinuous oil slick, promoting degradation of o/w emulsions by microbial 

 oxidation. The w/o emulsions, known as "mousse," are a very different 

 problem, as they float and agglomerate into large masses. Such w/o emulsions 

 can cause fluid oils to become viscous, as well as cause viscous oils to 

 become fluid. Neither result is beneficial, as the product is an emulsion 

 with the consistency of melted Hershey's chocolate, whether one begins with 

 gasoline or with asphalt. "Mousse" is fluid. enough to coat shorelines 

 thoroughly, yet viscous enough to substantially retard evaporation. In the 

 1967 Torrey Canyon accident, the crude oil formed "mousse" before reaching 

 shore. In the 1975 Key West oil spill, a solvent-based detergent was added 

 to the oil before it was pumped over the side, producing a 60/40 w/o emul- 

 sion. During the Arrow spill in Chedabucto Bay, and in the 1969 West 

 Falmouth spill, mousse did not form, although in the latter two accidents oil 

 was mechanically driven into the water column and, in the West Falmouth spill 

 at least, into the sediments. 



3.1.5 Oxidation 



Oxidation of an oil slick enhances the dissolution rate of the oil and 

 produces surfactant molecules that will promote emulsif ication. There are 



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