fuel oil. The changes that will take place in the oil include the loss 

 of light components by evaporation and the loss of constituents by 

 dissolution or dispersion. Chemical changes caused by oxidation or 

 photodegradation can produce oxygenated compounds, volatile products, 

 and condensation products; and additional chemical changes can result 

 from biodegradation. The presence of polar products and heavier components 

 may produce emulsification, and heavier components combined with 

 foreign matter may produce settling. Spreading will produce thinner 

 films in which all the above processes may be speeded up. 



Chemical decomposition of distillate fuels, including decomposition 

 by microorganisms, is a comparatively slow process. Although chemical 

 or biological processes may be effective in removing final traces 

 of dispersed fuels, they are not expected to contribute materially 

 to the removal of the bulk of the oils. Dissolution is also expected 

 to be a minor factor in oil removal because of the very low solubilities 

 of hydrocarbons in water. 



The chief method of loss of oil over a relatively short time 

 span, of perhaps a week or less, is expected to be by evaporation. 

 The rate of evaporation will depend on the vapor pressure of the 

 oil, on the thickness of the oil, on the temperature, and on the 

 wind speed. 



Vapor Pressure. The vapor pressure of fuel oil depends on the 

 components that are present. The hydrocarbons with the lowest number 

 of carbon atoms have the lowest boiling points and the highest vapor 

 pressures. As the numbers of carbon atoms of the hydrocarbons increase, 

 the boiling points increase and the vapor pressures decrease. A fuel 

 with larger amounts of lower hydrocarbons will have a higher vapor 

 pressure and will evaporate faster. The lower boiling hydrocarbons 

 will tend to evaporate first, and the vapor pressure and the rate 

 of evaporation will then decrease. 



All distillate fuels have at one time been evaporated during 

 the distillation process. The vapor pressures of all the components 

 are thus sufficiently high that essentially all spilled distillate 

 fuel that is not otherwise removed would eventually be lost by evapora- 

 tion. Residual fuels, such as NSFO, consist primarily of components 

 having negligible evaporation rates. Therefore, a comparatively small 

 amount of the NSFO would be lost by evaporation. 



Thickness . The thickness of an oil slick is an important factor 

 in its evaporation rate because the evaporation takes place at the 

 surface of the film. At a given moment , the amount of oil evaporated 

 is not affected by the depth of the oil, and the percentage loss 

 is therefore much greater for a thin film with a high surface-to-volume 

 ratio. Over a period of time, the composition at the surface will change 

 to less volatile components and the more volatile components will migrate 

 to the surface from the interior of the oil layer. This process is more 

 rapid in a thinner layer or in agitated oil. 



