U.S. Navy Ship Salvage Manual Volume 6 - Oil Spill Response

S0300-A6-MAN-060tions. Because light compounds are more likely to evaporate than dissolve in water, very little ofthe total spill volume is lost to dissolution. Dissolution starts immediately and continues as longas oil is in the water because oxidation and microbial action constantly produce water-solublepolar compounds.2-3.1.4 Emulsification. Emulsification is the process by which one liquid is dispersed intoanother immiscible liquid in droplets of optically measurable size. Light and medium crude oils(those having asphaltene contents greater than 0.5 percent) absorb water to form stable water-inoilemulsions. Because of their typical dark color and foamy texture, water-in-oil emulsions arecalled chocolate mousse or simply mousse.The rate at which emulsions form is a function of oil viscosity and sea state. Emulsions of lowviscosityoils, such as diesel oils and JP-5, form very easily and may occur in just two or threehours during winds above Beaufort Force 3, absorbing 60 to 80 percent water, by volume in theprocess. More viscous oils, under the same conditions, may absorb only 10 percent water, by volume,in 10 hours. Emulsions seldom occur in calm water. The stability of emulsions variesdirectly with the amount of asphaltenes contained in the oil. Some emulsions separate whenheated by sunlight during calm weather or when stranded on shorelines. Water-in-oil emulsionsbreak down readily in hot climates and persist for long periods of time in cold environments.Color varies from black for low water content, large-droplet emulsions through brown and reddishbrown to orange for high water content, small-droplet emulsions.2-3.1.5 Oxidation. Hydrocarbon molecules react with oxygen to water-soluble (polar) compoundsand persistent tars. The polar compounds-keytones, alcohols, aldehydes and carboxyolicacids–dissolve in water or act as detergents and emulsifiers. Although enhanced by sunlight(photo-oxidation), oxidation is a slow process. Thin films will break down under strong sunlightby no more than 0.1 percent per day.Oxidation of thick oils sometimes creates tar balls and tar mats. This phenomenon occurs whenthe lighter fractions oxidize, leaving a persistent skin that isolates the unaffected oil from furtheroxidation and other weathering processes. Oil in this form weathers slowly and remains in themarine environment for a long time.2-3.1.6 Biodegradation. Hydrocarbons and their oxidation products are food to some naturallyoccurring bacteria and other microbes. Most of the microorganisms which consume hydrocarbonsrequire oxygen in either the free or dissolved form. There is usually sufficient oxygen near thesurface to allow the maximum biological degradation to occur. However, in benthic regions andbelow the photic zone, the supply of oxygen is severely limited. Degradation by microbes is not asignificant removal mechanism in these areas, although there are a few organisms that willdecompose hydrocarbons when little or no dissolved or free oxygen is present.The initial effects of biodegradation on oil spills are generally minimal and are limited by thequantities of the nutrients nitrogen (N) and phosphorus (P), which the microbes need to surviveand grow. If the limiting nutrients nitrogen and phosphorous are present in sufficient quantities,temperature controls the rate of microbial degradation. As microbes can only attack that part of2-11