Sudden, massive oil pollution can occur when jumbo tankers break up, on and offshore wells blowout, or pipelines fail. As increasing demand and dwindling supplies necessitate exploiting reserves in increasingly inhospitable arenas, the likelihood of serious accidents increases. But most oil pollution (80 to 90%) occurs during everyday shipping, refining, processing, and burning of hydrocarbons. The most important form of oil pollution is fallout of airborne hydrocarbons. When hydrocarbons reach the oceans, they are diluted and dispersed. Eventually, they disappear through microbial degradation, evaporation, oxidation, and deposition. Along the way, however, great damage may occur to ocean life. Sometimes this damage is obvious, sometimes it is subtle and indirect. Thus, hydrocarbons can destroy vital parts of some food chains directly (for example, aquatic insects); can accumulate and magnify in food chains (for example, in large fish and birds near tops of food chains); and can interfere with the communication systems of organisms (for example, disruption of chemical 'messages' from rivers to fish returning to spawn).
A 1975 report estimated that about 6 million metric tons of petroleum enter the ocean each year. Recent research shows that tanker contributions alone could account for as much as 6 million metric tons. But although about half of the oil transported in the world each year is carried by tankers, most of the oil accumulating in the sea comes from a combination of land-based industrial and municipal sources: 350 million gallons (1.3 billion litres) of used automobile crankcase oil is dumped each year into water drainage systems that reach the sea; and about 600,000 metric tons of oil run into the sea each year as a result of petroleum carried into the atmosphere from poorly tuned automobile engines. 680 million litres of motor oil is sent to landfills or poured down drains in the USA each year (the equivalent of 16 Exxon Valdez spills).
On March 24, 1989, the oil tanker Exxon Valdez ran aground in Bligh Reef in Prince William Sound, Alaska. Its ruptured hull poured nearly 11 million gallons of oil into the water, endangering one of the world's richest fishing grounds, contaminating more than 400 miles of coastline, and killing wildlife. A thick, black coating formed on beaches. Every part of the food chain was affected. As a result of the Exxon Valdez spill, millions of fish, thousands of sea birds, and hundreds of sea otters washed ashore, oil-soaked and dead. Flocks of migrating birds and animals that fed and bred in the spill area could not return safely. Oil fumes damaged their eyes and lungs. Some ingested contaminated oil, which damaged their livers.