Ocean Pollution and Oil Spills

Oil spills resulting from human error often affect marine and coastal areas. Past oil spills in different areas of the world demonstrate that environmental damage depends on the toxicity and the persistence of the oil, both of which vary widely depending on a variety of factors.

Why Oil Spills Occur

Oil is the lifeblood of modern societies, especially in the most economically developed nations of the world. According to the United States Energy Information Administration (EIA), in 2021 the United States consumed approximately 21 percent of the world’s total petroleum production. This included crude oil, all other petroleum liquids, and biofuels. The United States, however, comprises less than 10 percent of the world’s population. The EIA reported that in 2020, the United States consumed a total of about 6.6 billion barrels of petroleum products, a number that had decreased to its smallest level since the 1990s (a total of approximately 7.5 billion barrels were consumed in 2019) likely because of the coronavirus disease 2019 (COVID-19) pandemic. This number increased by 2022, with the EIA reporting a consumption of 7.4 billion barrels of petroleum products. The percentage of imported oil that Americans consumed grew steadily after 1982. This largely negated the lessons learned about cartel politics and energy conservation in the late 1970s. The amount of oil imported into the United States continued to grow even during periods when its trade deficit was in decline. Americans have also demonstrated they are willing to go to war for oil. In the 1991 Persian Gulf War with Iraq, the United States paid a huge cost to protect oil fields in Kuwaiti and world oil markets though little oil produced in the Persian Gulf was imported to the United States. Most of the nation’s imported oil in the twenty-first century comes from Canada as well as Central and South America. While domestic production had declined for several decades, by the end of the 2010s, it had drastically increased through the use of hydraulic fracturing techniques and new infrastructure such as the Keystone and Colonial Pipelines.

Almost all imported and Alaskan oil is transported to US refineries and consumers by ocean-going bulk shipping vessels, or tankers. Most oil spills result from marine transportation accidents, with human error often being the cause or at least playing a major role. Navigation errors, equipment malfunctions, bad judgment, and the inability of crew members to speak a common language have all been major contributing factors in the largest and most environmentally damaging oil spills. The running aground of a large Japanese carrier vessel and the subsequent spilling of oil into the Indian Ocean off the coast of Mauritius in 2020 only increased concerns about related standards, particularly as this incident involved a type of low-sulfur fuel considered experimental. In 2021, a ruptured underwater pipeline off the southern California coast led to a spill of approximately 126,000 gallons of crude oil into the Pacific Ocean. When initial investigations introduced a theory that a ship's anchor might have caused the damage to the pipeline months to even a year earlier, further questions were raised about the state of cleanup technologies and techniques as well as operating companies' accountability and levels of monitoring. Spilled oil can decimate plant and animal populations by a combination of mechanical and chemical toxicity effects resulting from an organism’s physiological reaction to the chemicals present in oil.

Environmental Damage

The most common sight during an oil spill is dark, gelatinous masses of “mousse,” an oil and water emulsion that floats on top of the water, sticking to everything with which it comes into contact. Mousse usually causes the majority of the environmental damage during an oil spill by processes of mechanical toxicity, as it suffocates and smothers organisms that ingest it or are covered by it. Seabirds and furry marine mammals are highly susceptible to this process, succumbing to exposure, dehydration, or starvation.

Crude oil is a complex mixture of thousands of different chemicals called hydrocarbons, named after their molecular structures that consist solely of hydrogen and carbon atoms. Different hydrocarbons vary in their chemical properties, toxicity, and behavior during an oil spill. The major groups are classified by molecular geometry and weight. The simplest and lowest-molecular-weight molecules (aliphatics) are generally single-bonded, chain-shaped molecules, such as those that make up gasoline. They are the most volatile and are acutely toxic. Such compounds tend to evaporate or burn easily during an oil spill and therefore do not persist in the environment for long periods. The aromatics are ring-shaped molecules, such as benzene. They also tend to be highly volatile and are more reactive than simple hydrocarbons, and so can cause biological impacts because of both acute and chronic toxicity. Aromatic hydrocarbon compounds and aliphatic hydrocarbons of higher molecular weight are more environmentally persistent than the simple aliphatics. Because many are carcinogenic, they can also cause different forms of biological damage, disease, and death even after a long time period and in low doses. The highest-molecular-weight oil compounds include polycyclic aromatic hydrocarbons. These are composed of ring-shaped structures that are bonded together that also include very large aliphatic and aromatic hydrocarbons whose molecular structures consist of long chains of carbon-hydrogen units. These typically have the consistency of heavy lubricating oils, greases, waxes, and even the tar used in road paving. Although they are generally not very chemically reactive under environmental conditions and do not dissolve well in water, many are carcinogenic, and they tend to be very environmentally persistent.

For hundreds of millions of years before human beings evolved, oil “spilled” naturally into the world’s oceans from natural oil seeps, which are fractures in the earth that tap deep, oil-bearing rocks. A variety of natural processes act to reduce the environmental impacts of this oil, and these same processes also take place during a human-caused oil spill. Oil is dispersed from the oil slick and into the larger environment by five basic processes. Evaporation of the low-molecular-weight hydrocarbon compounds removes much of the oil relatively quickly. Sunlight can degrade additional oil in a process called photodegradation if the oil is exposed for enough time. Because oil is an organic substance, additional oil is removed by natural biodegradation thanks to “oil-eating” microorganisms. Most of the rest of the oil either washes up onto a coastal area or breaks up into heavy “tar balls” rich in high-molecular-weight hydrocarbons that eventually sink.

Some oil spills put so much oil into the environment that these processes cannot respond quickly enough to prevent environmental damage. Other factors can also enhance environmental damage from oil spills. Some types of oil or refined petroleum products are more toxic than others. Oil spills in cold climates generally cause more damage because cold temperatures hinder evaporation and the microbial metabolic rates necessary for rapid oil removal. Furthermore, sunlight is often of low intensity, which slows photodegradation. Wave conditions and tidal currents can affect how much oil washes up onto a coastal area and how rapidly it is moved elsewhere or removed. Finally, the amount of environmental damage from an oil spill is highly dependent on the type of coastal environment affected by the spill, as coastal environments vary in density, or biomass, and varieties of wildlife. Coastlines also vary in the degree to which they are sheltered from natural oil-removal processes. In general, rocky headlands, wave-cut rock platforms, and reefs exposed to high wave activity suffer far less damage during an oil spill than do sheltered marshes, tidal flats, and mangrove forests. The damage on beaches is related to the grain size of the beach sediment. Fine-sand beaches are relatively flat and hard-packed, and oil does not soak into the sediment or persist for long. Oil will soak deeply into coarse sand, gravel, and shell beaches, causing more damage over a longer period.

Most of what has been learned about oil spill behavior, environmental damage, and oil spill cleanup techniques comes from studying past spills. In most cases, spill prevention is far cheaper and infinitely more effective than spill response, and cleanup efforts usually capture very little of the spilled oil.

In 2018, an oil spill in the East China Sea inspired great concern from scientists regarding how different types of oil affect ocean life. On January 6, the Iranian tanker Sanchi, reportedly carrying approximately one million barrels of condensate, crashed into a Chinese freighter. After catching fire, the tanker burned for several days before ultimately sinking. While accidents involving ships carrying condensate had occurred previously, this incident was the first of such a great magnitude, and less is generally known about the impact of the chemicals that compose condensate on marine ecology than that of crude oil. Though condensate, which is a light petroleum by-product of natural gas production, does not cause the same type of thick oil slicks that spread and linger in the spills of crude oil, it is believed that it causes a deep underwater plume to form that could be extremely harmful to marine life; therefore, scientists are particularly concerned about possibly severe effects occurring over a short-term period versus the long-term effects typical of a crude oil spill. In the weeks after the spill, Chinese officials reported that traces of petroleum were found in some fish taken from the area. Because condensate is lighter and highly flammable, the usual cleanup tactics for crude oil largely could not be used.

Gulf of Mexico Oil Spills

The Ixtoc I spill of June 3, 1979, was the result of an explosion, or “blowout,” of an offshore oil well that was drilling into a subsurface oil reservoir. Although human error was definitely a factor, the cause of the blowout remains unresolved. It has been blamed on the use of drilling mud that was not dense enough to counteract the pressure of the oil and gas at depth, as well as on the improper installation of the blowout preventer, a fail-safe device used on drilling rigs to prevent just this type of disaster. The result was a continuous 290-day oil spill, during which an estimated 475,000 metric tons of crude oil (one metric ton equals approximately five barrels) was released into the environment. In addition to doing considerable environmental damage on the coast of Mexico, oil fouled much of the barrier island coast of Texas. However, most of the oil did not make it to shore, and the final accounting for this spill gives a good indication of the long-term fate of spilled oil in offshore areas: 1 percent burned at the spill site, 50 percent evaporated, 13 percent photodegraded or biodegraded, 7 percent washed up on the coast (6 percent in Mexico, 1 percent in Texas), 5 percent was mechanically removed by skimmers and booms, and 24 percent sank to the sea floor (assumed by mass balance).

A similar accident occurred in 2010 when the blowout preventer on BP’s Deepwater Horizon well site failed, causing an explosion at the drilling platform. In the resulting spill, oil jetted continuously from the wellhead on the sea bottom for a prolonged period of time, as various efforts to recap the well failed. Massive amounts of crude oil expanded over a large portion of the water’s surface and washed up along the coastline between Louisiana and Florida. The incident decimated the fisheries in the Gulf of Mexico, creating an extensive “dead zone” where natural water currents had localized much of the toxic spill materials and created areas that could no longer support plant or animal life. The disruption of the beaches in the area also had a great negative impact on the tourist industry upon which the region depends. While data for deepwater species remained largely insufficient, scientists who had been studying the impact of the spill argued in 2020 that there were several kinds of coral and animal species that seemed to still be showing negative long-term symptoms of the spill.

The Exxon Valdez

The Exxon Valdez oil spill—which occurred in Prince William Sound, Alaska, on March 24, 1989—is a good example of how environmental damage follows human error and inadequate response. After departing Port Valdez with a full cargo, the Exxon Valdez oil tanker struck a well-charted submerged rock reef located 1.6 kilometers outside the shipping lane. The ship was under the command of an unlicensed third mate in calm seas and left the shipping lane with permission from the Coast Guard to avoid ice. However, it strayed too close to the reef before evasive action was attempted. The captain, who had a history of drunk-driving convictions, was in his cabin under the influence of alcohol during events leading to the accident. His blood alcohol level nine hours after the grounding was measured at 0.06 percent; the estimate at the time of the accident was 0.19 percent. Convicted of negligence and stripped of his commander’s license, he was subsequently employed as an instructor to teach others to operate supertankers.

Leaking oil was observed immediately. Oil-spill response crews funded by Exxon and the Alyeska Pipeline Consortium, oil companies that used the Port Valdez terminal, were poorly prepared and reacted too slowly and with inadequate equipment. The first response arrived ten hours after the accident with insufficient booms and skimmers. Chemical dispersants applied to break up the oil slick were ineffective in the calm seas and caused the oil slick to thin and spread more rapidly. Four days later, the weather changed: 114-kilometer-per-hour winds mixed the oil with seawater, creating a frothy mousse. More than 65,000 metric tons of oil spilled out of the stricken vessel over the next several weeks. About 15,600 square kilometers of ocean and 1,300 kilometers of shoreline were affected. Federal estimates of wildlife mortality include 3,500 to 5,500 otters; 580,000 seabirds; and 300 deer poisoned by eating oiled kelp. Economic damages totaled more than $5 billion. The long-term effects on commercial marine organisms, larval organisms, and bottom-dwelling life are not known.

Exxon promised to clean nearly 500 kilometers of shoreline by September 1989, but completed only 2 kilometers during the first month after the spill. Exxon and its contractors used a variety of cleanup techniques, including placing booms and skimmers, sopping up oil with absorbent materials, scraping oil by hand from rocks, stimulating the growth of oil-eating bacteria cultures, and washing coastal areas with cold water, hot water, and steam. The use of hot water and steam was effective at cosmetically removing surface oil, but it did not remove oil that had soaked into the sediment; the technique subsequently killed most of the organisms that had escaped the oil. The oil washed from the beach was to be collected by booms and skimmers offshore, but this process was so inefficient that much of the oil migrated to tide pools that had not been affected by the spill directly. Ironically, only eighteen months after the spill, life had returned to oiled coasts that had received little or no cleanup, while beaches that had been cleaned with hot water were still relatively sterile and required several years to repopulate. Exxon announced that it would not return to clean more shoreline in 1990 but relented under threat of a court order from the Coast Guard to enforce federal cleanup requirements. During the summer of 1990, shoreline cleanup resumed, including application of fertilizer to stimulate growth of naturally occurring oil-eating bacteria, a technique that is not very efficient in the cold waters of southern Alaska.

The tale of the Exxon Valdez is not complete without mentioning that the Port Valdez Coast Guard did not have state-of-the-art radar equipment for monitoring ship movement in this heavily used and environmentally sensitive area. In the early 1980s, federal and state funds for monitoring the Port Valdez oil companies’ compliance with oil-spill preparedness legislation had been cut by more than 50 percent. The original environmental impact statement for oil-handling activity in Prince William Sound included an agreement that defines cleanup responsibility for oil spills. Exxon, as the company responsible for the spill, was to pay the first $14 million of cleanup costs, with $86 million in additional cleanup funds from the Alyeska contingency fund. Thus, the maximum financial responsibility to oil companies from a spill was $100 million unless the spill was judged to be caused by negligence. Cleanup activities ceased eighteen months after the spill with total expenditure of $2.2 billion—most of this at taxpayer expense. In 1994, a federal court unanimously awarded $5.3 billion in punitive and compensatory damages, the largest-ever jury award, to some 35,000 people impacted by the spill. By June 1999, Exxon had yet to pay a single dollar as the case continued through the legal process. It is interesting to note that Exxon’s estimate of cleanup costs in late 1989 were $500 million, and it carried $400 million of oil spill liability insurance. Exxon saved $22 million by not building the Exxon Valdez with a double hull. Its 1988 annual profits were $5,300 million.

According to National Oceanic and Atmospheric Administration (NOAA) estimates, less than 1 percent of Exxon Valdez’s oil burned at the site, 20 percent evaporated, and 8 percent was mechanically removed. Nearly 72 percent was deposited on the sea floor. According to Exxon’s estimates, 7 percent of the oil burned at the site, 32 percent evaporated, 9 percent photodegraded or biodegraded, 15 percent was mechanically removed, and 37 percent was assumed deposited on the sea floor.

The Gulf War Oil Spill

In January 1991, the Persian Gulf War oil spill, the largest oil spill in history at that time, occurred when the Iraqi military opened valves and pumps at Sea Island Terminal, a tanker loading dock located 16 kilometers off the coast of Kuwait. This facility had a production capacity of 100,000 barrels per day, about three Exxon Valdez loads each week. The Iraqis also opened plugs on five Kuwaiti tankers, spilling an additional 60,000 barrels. The estimate for the entire spill is 6 million barrels, or roughly 30 times the volume of the Exxon Valdez. About 650 square kilometers of coast was heavily contaminated.

Three days after starting the spill, the Iraqis ignited the oil leaking from the terminal. This was the best thing to happen from an environmental perspective. During most spills, more oil is removed by natural evaporation than by any cleanup technique; igniting the oil merely speeds up this process. Burning can be an important mechanism for removing oil from the sea and avoiding environmental damage, and tests have shown purposeful ignition in open water away from the coast to be an excellent oil-slick fighting strategy. However, this must be done within the first few hours of the spill. In order to maintain the fire, the slick must be more than 1 millimeter thick and must contain relatively little emulsified water. To maintain thickness, the slick is best surrounded with fireproof booms. However, at the time of the Gulf War oil spill, almost all the fireproof boom in the world was in Prince William Sound. Saudi Arabia also used dispersants on portions of the slick, but this effort was too late to be effective before a thick mousse had formed.

The prime objectives of causing the spill were to hamper an amphibious military landing by oiling the beaches and to disrupt desalination of drinking water at Khafji and Jubail, the two primary sources of potable water for Saudi Arabia. The Saudis used booms to protect the plant intakes with great success. The retreating Iraqis also ignited more than seven hundred of about one thousand inland wells, resulting in an additional 6 million barrels per day burned. This volume eventually made the marine spill insignificant, and the burning created 3 percent of total global carbon emissions during the time period of the event.

The Persian Gulf is an unusual body of water. It is very shallow (average 33 meters) and is nearly enclosed as a marine basin. Because it is also microtidal (the tidal range is less than 0.6 meter), it flushes out slowly (once every two hundred years, compared with once every few days for Prince William Sound). It is also important to remember that this is not a pristine marine environment. Natural oil seeps are very common, there is a general lack of environmental standards and poor cooperation among Persian Gulf nations, and virtually no oil spill preparations or equipment were present in this part of the world. Earlier spills had occurred in the region, but they typically were associated with ongoing wars; the hostile environment made it difficult to utilize spill abatement specialists and equipment. For example, during the Iran-Iraq War, Iraq attacked an Iranian offshore platform (Nowruz) in 1982, spilling more than 2 million barrels of oil, a volume nearly half as large as the Gulf War spill. Losses of marine mammals and birds were great.

During the Gulf War oil spill, about 180 kilometers of Saudi Arabian coastline was oiled (65 kilometers was severely damaged), and oil reached south as far as the United Arab Emirates and Bahrain. Much of the southern Kuwaiti coast, made up of sea-grass beds, marsh, and mangroves, was severely damaged, and about 25 percent of the Saudi shrimp industry was lost. Although some twenty thousand wading birds were killed, no deaths of dolphins or dugongs were reported. However, these animals suffered greatly during the Iran-Iraq War. Estimates of the time required for ecological renewal of the Persian Gulf following the Gulf War spill (one to four years) were relatively short for two reasons: The high water temperature results in high microbial activity and biodegradation of the oil, and much of the oil was burned.

Principal Terms

boom: a floating oil fence made of a weighted flexible sheet protruding vertically above and below the sea surface; used to contain or move floating oil during a spill, they are most effective during calm seas

environmental persistence: the relative length of time that oil remains in the environment with the possibility of causing negative environmental effects

mechanical toxicity: the process by which most organisms are impacted or killed by spilled oil, as ingesting or being coated by oil may lead to death by suffocation or exposure

mousse: a gelatinous oil-water emulsion resembling chocolate pudding that is created when crude oil is spilled in churning seawater

skimmer: a specialized oil-spill response vessel that picks up floating oil with an absorbent conveyor belt; skimmers are most effective during calm seas

toxicity: a measure of the dose required to produce a negative health effect

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