Oil Spills

The total spillage of petroleum into the oceans through human activities is estimated to range from about 0.7-1.7 million tons (0.6-1.5 million tons) per year, equivalent to less than 0.1% of the quantity of petroleum transported by tankers. In comparison, the production of hydrocarbons by marine plankton is about 28.7 million tons (26 million tons)/year. These "natural" hydrocarbons contribute to background concentrations in the oceans, but they are well dispersed and not associated with ecological damage or pollution. In addition, natural oil seeps contribute about 6-13%percnt; of the total petroleum input to the oceans, sometimes causing local damage.

The largest, most consequential events of oil pollution involve spills of petroleum or heavy bunker fuel from disabled oceanic tankers or drilling platforms, from barges or ships on inland waters, or from blowouts of wells or damaged pipelines. Damage is also caused by the relatively frequent spills and operational discharges associated with coastal refineries and urban runoff. Large quantities of oil are also spilled when tankers clean out the petroleum residues from their huge storage compartments, often discharging the oily bilge washings directly into the ocean.

Some examples of disastrous oil spills include the following accidents involving oceanic supertankers: (1) the Prestige, which split in half off Galicia, Spain in November 2002, spilling about 67,000 tons (61,000 metric tons) of crude oil; (2) the Torrey Canyon, which ran aground in 1967 off southern England, spilling about 129,000 tons (117,000 metric tons) of crude oil; (3) the Metula, which wrecked in 1973 in the Strait of Magellan and spilled 58,000 tons (53,000 metric tons) of petroleum; (4) the Amoco Cadiz, which went aground in the English Channel in 1978, spilling 253,000 tons (230,000 metric tons) of crude oil; (5) the Exxon Valdez, which ran onto a reef in Prince William Sound in southern Alaska in 1989 and discharged 39,000 tons (35,000 metric tons) of petroleum; and (6) the Braer, which spilled 93,000 tons (84,000 metric tons) of crude oil off the Shetland Islands of Scotland in 1993. All of the tankers involved were of the older single hull design. Such occurrences should decrease with the advent of double-hulled tankers.

Significant oil spills have also occurred from offshore drilling or production platforms as the result of mechanical or operational failure. In 1979 the IXTOC-I exploration well had an uncontrolled blowout that spilled more than 551,000 tons (500,000 metric tons) of petroleum into the Gulf of Mexico. Smaller spills include one that occurred in 1969 off Santa Barbara in southern California, when about 11,000 tons (10,000 metric tons) were discharged, and the Ekofisk blowout in 1977 in the North Sea off Norway, which totaled 33,000 tons (30,000 metric tons) of crude oil.

Enormous quantities of petroleum have also been released during warfare. Because petroleum and its refined products are critically important economic and industrial commodities, enemies have commonly targeted tankers and other petroleum-related facilities during wars. For example, during the World War II German submarines sank 42 tankers off the east coast of the United States, causing a total spillage of about 460,000 tons (417,000 metric tons) of petroleum and refined products. There were 314 attacks on oil tankers during the Iran-Iraq War of 1981–1987, 70% of them by Iraqi forces. The largest individual spill during that war occurred when Iraq damaged five tankers and three production wells at the offshore Nowruz complex, resulting in the spillage of more than 287,000 tons (260,000 metric tons) of petroleum into the Gulf of Arabia.

The largest-ever spill of petroleum into the marine environment occurred during the brief Gulf War of 1991. In that incident Iraqi forces deliberately released an estimated 0.6-2.2 million tons (0.5-2 million tons) of petroleum into the Persian Gulf from several tankers and an offshore tanker-loading facility known as the Sea Island Terminal. An additional, extraordinarily large spill of petroleum to the land and atmosphere also occurred as a result of the Gulf War, when more than 700 production wells in Kuwait were sabotaged and ignited by Iraqi forces in January, 1991. The total spillage of crude oil was an enormous 46-138 million tons (42-126 million tons). Much of the spilled petroleum burned in spectacular atmospheric conflagrations, while additional, massive quantities accumulated locally as lakes of oil, which eventually contained 5.5-23 million tons (5-21 million tons) of crude oil. Enormous quantities of petroleum vapors were dispersed to the atmosphere. About one-half of the free-flowing wells were capped by May, and the last one in November 1991.

After oil is spilled into the environment, it dissipates in a number of ways. Spreading refers to the process by which spilled petroleum moves and disperses itself over the surface of water. The resulting slick can then be transported by currents and winds. The rate and degree of spreading are affected by the viscosity of the oil, wind speed, and turbulence of the water surface. Evaporation is important in the initial reduction of the volume of an oil spillage, especially of relatively light and volatile hydrocarbon fractions. Evaporation typically accounts for almost 100% of spilled gasoline at sea, 30-50% of spilled petroleum, but only 10% of bunker fuel. Solubilization occurs when some fractions of the spilled oil dissolve into the water column, causing a contamination of sub-surface waters in the vicinity of the oil spill. For example, beneath a petroleum slick in the North Sea the concentration of hydrocarbons in water was 4 g/m³ (ppm), compared with about 1 mg/m³ (ppb) in uncontaminated sea-water. Lighter hydrocarbon fractions of petroleum are much more soluble in water than heavier ones, and aromatics are much more soluble than alkanes. (Aromatic hydrocarbons such as benzene and naphthalene have an unsaturated ring structure, while alkanes such as octane have a linear structure.) In addition, some of the spilled hydrocarbons are slowly oxidized by ultraviolet radiation and microorganisms into simpler compounds, ultimately to carbon dioxide and water.

The combined influences of solubilization, evaporation, and oxidation are known as weathering. Weathering preferentially removes the lighter hydrocarbon fractions, leaving a residual material made up of relatively heavy hydrocarbons. Over the shorter term in aquatic environments, this residuum forms a stable water-in-oil emulsion known as "mousse," which is the material that usually impacts shorelines after an offshore spill. The mousse combines with sediment particles on the shore to form sticky patties of oil and sand, which eventually form asphaltic lumps.

At sea, weathering of the mousse eventually results in the formation of a dense, semi-solid, asphaltic residuum known as "tar balls." In the vicinity of frequently traveled tanker routes worldwide, tar balls can be commonly found floating offshore and on beaches. Tar balls are especially common in places where the oceanic circulation resembles a surface vortex. A well known example of this phenomenon is the oceanic gyre known as the Sargasso Sea, famed for its accumulations of natural debris such as floating seaweed, as well as human debris, including tar balls.