charged into the marine environment, the composition begins to change immedi- 

 ately. This change in composition, known as weathering, takes place at a rate deter- 

 mined by the nature of the oil and by environmental factors such as temperature, 

 wind speed, sea state, and nutrient level. Physical, chemical, and biological processes 

 are all continuously operative, but the rates of individual processes vary greatly. 



A spill on marine waters results in rapid formation of a surface slick by the spread- 

 ing of a large portion of the discharged oil. The extent of spreading (thickness) of a 

 slick is determined by the quantity and nature of the oil and environmental condi- 

 tions. Fresh oil with a compositon such as that in Figure 1 spreads rapidly. However, 

 as the lower boiling fraction evaporates, attractive forces among the remaining mole- 

 cules overcome the spreading forces, and the spilled oil ceases to spread (Fay, 1969). 

 Formation of a thin layer of oil at the air-sea interface provides a large surface area 

 contact with both atmosphere and sea water that greatly accelerates the rate of other 

 weathering processes. 



Evaporation is the most important weathering process during the first day or two 

 after an oil spill. A typical crude oil can be expected to lose between 25 and 50 percent 

 of its components from a surface slick owing to evaporation alone. A 75 percent loss 

 could result from the evaporation of a No. 2 fuel oil (National Academy of Science, 

 1975). Results from numerous studies indicate hydrocarbons with less than 15 car- 

 bon atoms are readily removed from oil by evaporation under usual marine condi- 

 tions. Of the hydrocarbons in this molecular weight range «Ci 5 ), saturates are gen- 

 erally lost at a slightly faster rate than aromatics with the same carbon number 

 ( McAuliffe, 1 976a). Harrison et al. found, however, that isopropyl benzene was lost 

 at a faster rate than rc-nonane in three of five small spills of South Louisiana crude 

 ( 1975). Evaporation rates from freshly spilled crude oils, while quite high, decrease 

 exponentially. Loss of the most volatile fraction (<Oo) results in a significant in- 

 crease in the density and viscosity of the remaining oil. As the viscosity of the oil in- 

 creases, molecular diffusion within the oil decreases and evaporation rates are 

 greatly reduced. 



Most petroleum components that are aromatic or contain nitrogen, sulfur, or oxy- 

 gen have seawater solubilities in the milligrams per liter range (ppm). Alkyl benzenes 

 have solubilities of 50 to 500 mg/1 (McAuliffe. 1966) while the seawater solubilities of 

 naphthalene and phenanthrene are about 22 mg/1 and 1 mg/1 respectively 

 (Eganhouse and Calder, 1976). Actual concentrations of these compounds in sea- 

 water equilibrated with crude or fuel oils (an oil to water ratio of about 1:9) never 

 approach solubility values. Concentrations of naphthalene in seawater are reported 

 to be less than 1 mg/ 1 owing to the preferential partitioning of naphthalene into the 

 oil phase (Anderson et al., 1974). Comparisons of the rates of loss by evaporation 

 and solution have been made in several studies. Harrison et al. (1975) reported that 

 the rate of evaporation of isopropyl benzene was about two orders of magnitude 

 faster than the rate of solution. Cyclohexane and benzene have similar vapor pres- 

 sures, but benzene has a much higher solubility. McAuliffe (1976a) found that the 

 two compounds were lost from crude oil slicks at similar rates, showing that loss by 

 solution was minor. 



Emulsification is an extremely important process in many spills. An emulsion is 

 the colloidal suspension of a liquid within a second immiscible liquid. Water-in-oil 

 emulsions are formed by the incorporation of tiny droplets of seawater within an oil. 

 Water-in-oil emulsions often result in formation of a stable gel, brownish in color, 

 that is referred to as chocolate mousse or simply mousse. The seawater content of a 

 mousse is generally between 50 and 80 percent. The tendency of an oil toward mousse 

 formation appears to be largely a function of he concentration of surface active 

 compounds present in the oil. Asphaltenes and NSO compounds have been sug- 

 gested as the probable emulsifying agents (MacKay et al., 1973). Mousse formation 

 generally results in a decrease in the rate of weathering of an oil. The greater density, 

 viscosity, and volume of the mousse results in a thickening of the layer of oil and a 

 decrease in the surface area/volume ratio. Mousse formed from Kuwait crude was 



100 



