Oil Spills

An oil spill can be defined as an accidental or deliberate dumping of oil or petroleum products into the ocean and its coastal waters, bays, and harbors, or onto land, or into rivers or lakes (Holum 1977). Between one and ten million metric tons (one metric ton is 1000 kilograms) of oil are put into the oceans every year. The oil is released, most often, in small yet consistent doses from tankers, industry, or on shore waste disposal (Boesh, Hersher, et al. 1974). Tanker spills cost the United States more than one hundred million dollars every year. Spill frequency increases proportionally with tonnage carried, in a linear manner. Non-tanker spills also increase linearly and account for thirty percent of all spills. The Atlantic area near Europe averages eight spills a year, the American area seven, and the Pacific two. Spills of more than ten thousand metric tons account for about two and a half percent of total spills, and spills above fifty thousand metric tons occur on average once a year. The average spill size is around seven thousand metric tons (Smets 1982).

If left alone, oil spills will eventually break up naturally. The natural degradation is influenced by temperature, wind, wave action, the thickness of the oil, the degree of dispersion, and the oil's tendency to form emulsions with water (Smith 1968). There are eight primary processes in the natural degradation of oil. The first step is spreading and motion. This step can be broken down into three smaller steps: gravity, viscous forces, and surface tension. Gravity initially spreads the oil into an even layer across the surface. Viscous forces then take over and account for even more spreading. The oil is finally spread into a monomolecular slick by the surface tension of the water.

The second step of natural degradation is evaporation. The amount of evaporation that occurs is dependent on how far the oil slick has spread. There is a higher amount of evaporation when the slick is spread out to a greater degree. Fifty percent of hydrocarbons found in oil are removed from a light spill within ten days, by evaporation (Beer 1983). With crude oil, twenty-five percent of its volume can be evaporated during the first few days. Evaporation continues at a diminishing rate for several weeks (Smith 1968).

The third step in natural degradation occurs when the wind and the waves cause the hydrocarbons to enter into a solution with the ocean water. Typically, the most soluble hydrocarbons are the most toxic.

Emulsification is the fourth step in the natural degradation process. Insoluble components of the oil emulsify with the ocean water and are described as oil in water or water in oil emulsions, depending on the ratio of one to another (Beer 1983). Emulsions can contain up to eighty percent water. As the mass of the oil in the emulsions decreases through evaporation, the specific gravity or density increases, which makes the emulsions more likely to sink (Smith 1968).

The next step is direct sea to air exchange. Wave-produced spray transfers hydrocarbons to the air in the same way salt is transferred (Beer 1983). There is a constant cycle of hydrocarbons between the sea and the air, but these are more localized along tanker routes, and coastal areas (Garrett 1975).

The sixth part of this process is photochemical oxidation, which occurs when sunlight induces chemical changes in the oil and it begins to harden.

The seventh step in the natural degradation of oil is sedimentation. As the density continues to increase, the oil and its other components become heavier than water, so they eventually sink to the bottom of the ocean (Beer 1983).

The final step in the process is bacterial degradation. The remaining sediments on the ocean bottom are broken up by bacteria. If an oil spill stays at sea for longer than three months, only fifteen percent of the original amount should theoretically remain.

The impact of oil on a marine ecosystem is not the same on different organisms. Organisms living at or near the surface are impacted more than others in the area. These organisms include those in intertidal areas, heuston, seabirds, and bottom dwellers. The type of oil spilled influences the extent of damage that is done to the organism.

Crude oil is the most common petroleum product that is found in oil spills. Crude oil is a complex mix of hydrocarbons with small amounts of sulfur, oxygen, and nitrogen and traces of inorganic and organometalic compounds. There are three classes of hydrocarbons: alkanes, alkenes, and aromatics. Alkanes, in low concentrations, cause anesthesia and narcosis, and in high concentrations, cell damage and death, in marine invertebrates. Alkenes, which are not typically found in crude oil but in refined products such as gasoline, are more toxic than alkanes, but less toxic than aromatics. Aromatics are not common in nature but they can kill ocean organisms, not only at full strength, but in diluted concentrations as well. Aromatics are the most immediately toxic component found in oil (Boesch, Hershmer, et. al. 1974).

Some dangerous hydrocarbons include benzene, toluene, naphthalene, and phenanthrene. Benzene inhibits blood cell formation, and they all cause local irritation of the respiratory system, excitation or depression of the central nervous system, and are mutagenic, carcinogenic, and teratogenic. The effects of oil on organisms can be broken down into two subdivisions, chemical and physical. Chemical effects are caused directly by the involved components and usually take place immediately. Physical effects such as oil coating and reduction of the photosynthetic rate from oil films, are indirect and can take longer to occur (Duffus 1980).

In the intertidal area, physical affects of oil include smothering, abrasion, and removal and alteration of the substrate. Organisms with mucus, such as macroalgae and anemones are often unaffected by the physical effects of oil. Direct toxicity is rare in the intertidal area. Most organisms are harmed by the non-toxic results of the oil being in the environment. Snails often retract into their shells to avoid the oil, lose their grip on a solid surface, and are washed away. Mollusks can become so encrusted with oil that they are swept away. Mobile animals sometimes are cemented to rocks with the thick oil. Sessile animals such as barnacles can become covered in oil and suffocate (Boesch, Hershner, et al 1974).

Bird death tolls from oil spills are recorded for the animals that reach the shore. It is believed that this accounts for roughly five to fifteen percent of the actual bird mortalities from oil spills. Diving sea birds have inordinate mortalities from oil spills. Other birds, such as seagulls and shearwaters,