Key Concepts

After completing this chapter, you will be able to:

1. Outline the most common causes of oil spills on land and at sea.
2. Describe how spilled oil becomes dispersed in the environment.
3. Explain how hydrocarbons cause toxicity to organisms.
4. Explain how petroleum kills birds and how they may be rehabilitated.
5. Describe case studies of the ecological effects of oil spills at sea and on land.
6. Discuss the potential consequences of petroleum resource development in the Arctic.

Introduction

Petroleum (crude oil) is a non-renewable natural resource (Chapter 18) used mainly as a source of energy. It is also used to manufacture a diverse array of petrochemicals, including synthetic materials such as plastics. Petroleum is mined in huge quantities. Pipelines, ships, and trains transport most of this volume, plus its refined products, around the globe. The risks of spillage are always present, and oil spills may cause severe ecological damage. Petroleum accounts for about 33% of the global production of commercial energy (IEA, 2019). The U.S. is the world’s top producer and consumer of petroleum in the world, currently consuming more than it produces (Table 25.1 & Table 25.2). The top ten producing countries account for 72% of the world’s petroleum production, while the top ten consuming countries account for 60% of the world’s petroleum consumption (IEA, 2019).

Table 25.1. Petroleum Production in the Top Ten Producing Countries as of 2020. Data in millions of barrels per day. Source: Data from U.S. Energy Information Administration (2020).

Table 25.2. Petroleum Consumption in the Top Ten Consuming Countries as of 2018. Data in millions of barrels per day. Source: Data from U.S. Energy Information Administration (2020).

The global reserves of petroleum, as of 2019, add up to about 1.65 billion barrels, of which 48% occurs in the Middle East, 14% in North America, and 7% in Africa (BP, 2021). Almost all mining of petroleum occurs far from the places where it is consumed. The Middle East, for example, is a huge exporter of petroleum and its refined products; the amount shipped abroad is about 3.5 times larger than domestic usage in that region. In contrast, Europe (minus the former USSR) produces about 22% of the petroleum it consumes, while Asia produces 28%, and the U.S. produces 54%. Canada produces about 87% more petroleum than it consumes (in 2013; CAPP, 2013). About 42% of the production is conventional crude oil, and the other 58% is synthetic petroleum manufactured from oil-sand bitumen.

Most of the production occurs in remote areas, while most consumption is in densely populated areas of the country. Consequently, enormous quantities of petroleum and its refined products are transported over great distances, mostly by overland pipelines, railroads, and trucks. On the global scale, most petroleum and its refined products are transported by oceanic tankers and overland pipelines. Local distribution systems involve smaller tankers, barges, pipelines, railroads, and trucks. There is a risk of accidental spillage from all of these means of transportation. Some of the spills have been spectacular in their volume and environmental damage. In addition, petroleum is discharged into the environment by many smaller sources, which sum to a large cumulative amount.

In this chapter we examine the causes of oil spills and the ecological damage that can be caused in aquatic and terrestrial environments.

Petroleum and Its Products

Petroleum is a naturally occurring mixture of liquid organic compounds, almost all of which are hydrocarbons (molecules made up only of hydrogen and carbon atoms). Petroleum is a fossil fuel, as are coal, oil-sand (or bitumen-sand), and natural gas. Fossil fuels are derived from ancient plant biomass that became buried in deep sedimentary formations. Over geologically long periods of time, the biomass was subjected to high pressure, high temperature, and anoxia. The resulting chemical reactions eventually produced a rich mixture of gaseous, liquid, and solid compounds. Naturally occurring hydrocarbons range in complexity from gaseous methane, with a weight of only 16 g/mole, to solid substances in coal with molecular weights exceeding 20,000 g/mole. (In chemistry, a mole is a standard quantity of a substance, equivalent to the amount contained in 6.02 × 10²³ atoms or molecules.)

Hydrocarbons can be classified into the following three groups:

• Aliphatic hydrocarbons are compounds in which the carbon atoms are organized in a simple chain. Saturated aliphatics (also called paraffins or alkanes) have a single bond between adjacent carbon atoms, while unsaturated molecules have one or more double or triple bonds. This is illustrated by the two-carbon aliphatic hydrocarbons ethane (H₃C─CH₃), ethylene (H₂C=CH₂), and acetylene (HC≡CH). Unsaturated aliphatics are relatively unstable and do not occur naturally in petroleum. Rather, they are produced during industrial refining, and photochemically in the environment after crude oil is spilled.
• Alicyclic hydrocarbons have some or all of their carbon atoms arranged in a ring structure, which may be saturated or unsaturated. Cyclopropane (C₃H₆) is the simplest alicyclcic hydrocarbon.
• Aromatic hydrocarbons contain one or more five- or six-carbon rings in their molecular structure. Benzene (C₆H₆) is the simplest aromatic hydrocarbons.

Crude petroleums vary greatly in their specific mixtures of hydrocarbons and other chemicals. They typically consist of about 98% liquid hydrocarbons, 1-2% sulfur (or less), and < 1% nitrogen, plus vanadium and nickel up to 0.15%. When petroleum is processed in an industrial refinery, various hydrocarbon fractions are separated by distillation at different temperatures. This is done to produce such products as gasoline, kerosene, heating oil, jet fuel, lubricating oils, waxes, and residual fuel oil (also known as bunker fuel). In addition, a process known as catalytic cracking converts some of the heavier fractions into lighter, more valuable hydrocarbons such as those in gasoline.

Oil Spills

Oil pollution is caused by any spillage of petroleum or its refined products. The largest spills typically involve a discharge of petroleum or bunker fuel to the ocean from a disabled tanker or a drilling platform, to an inland waterway from a barge or ship, or to land or fresh water from a well blowout or broken pipeline. In addition, some enormous oil spills have resulted from deliberate acts of warfare.

Terrestrial Spills

Oil spills onto land are relatively common. Between 1989 and 1995, about thousands of spills per year were reported in the U.S. – most all were relatively small. Many of the spills that occurred were the result of smaller incidents like oil spilling while refueling a ship, pipes disconnecting, or during transport. It was less likely that a spill would occur due to a major tanker incident, such as a leak, malfunction, or explosion.

Most large terrestrial spills involve a ruptured pipeline. The U.S. has over 200,000 miles of pipeline for crude oil, refined products, and natural gas liquids and 300,000 miles of pipeline for gathering and transmitting natural gas, as well as 2.2 million miles for distributing gas to homes, businesses, and other industrial sites (American Geosciences Institute, 2018). Pipeline breaks may be caused by faulty welding, corrosion, or pump malfunctions, as well as by erosion slumps earthquakes, and even armed vandals engaged in target practice. Operator negligence may also be an issue.

The extensive U.S. network of pipelines incorporates three methods of leak detection to identity and mitigate any pipeline mishaps. There are internal systems that use measurement sensors to provide flow and pressure readings and there are external systems that use equipment and sensors to detect escaped fluids. There are also visual inspections of the pipelines from the outside (North American Oil and Gas Pipelines, 2020).

In general, oil spilled on land affects relatively localized areas of terrain because most soils absorb petroleum well. However, much larger areas of aquatic habitat are affected if spilled oil reaches a watercourse, because wind and currents cause slicks to spread and disperse widely.

Consequences of Terrestrial Oil Spills

Oil spills result in severe damage to terrestrial vegetation, but usually relatively local areas are affected (except in the case of extremely large spills). This is because soil, particularly if it is rich in organic matter, has a great absorptive capacity for petroleum. In addition, much of the oil spilled on land tends to accumulate in low spots and does not spread widely. This is particularly true in northern climates, where deep infiltration into the soil may be prevented by impenetrable bedrock or permafrost. The relatively localized impacts of many terrestrial spills of petroleum are very different from the effects in aquatic environments, in which spilled oil spreads widely and can affect an enormous area.

Research has also shown that a wide range of natural, soil-dwelling microorganisms can utilize petroleum residues as a metabolic substrate (as a food). These oil-degrading bacteria, fungi, and other microbes are widespread in soils and waters. After soil becomes polluted by an oil spill, they rapidly proliferate in response to the presence of hydrocarbons that can be used as a source of metabolic energy.

Petroleum is a carbon-rich substrate, but it is highly deficient in key nutrients such as nitrogen and phosphorus. Consequently, the vigor of the microbial response to oiling, and the rate of decomposition of residues, can be greatly increased by adding fertilizer. Microbial decomposition of residues can also be enhanced by tilling the soil to increase the availability of oxygen. In general, fertilizer addition and tilling are relatively inexpensive but effective ways to speed up the biodegradation of petroleum residues, while avoiding the severe damage associated with a physical cleanup. This is particularly true of agricultural areas.

Of course, any spilled oil that reaches groundwater or surface waters will cause severe damage there. Spills into high-energy streams and rivers become extensively dispersed, and some residues will flow into lakes or the ocean. Oil in ponds and lakes can be quite persistent, accumulating around the margins, where vegetation and wildlife habitat are damaged. However, after spilled petroleum has weathered for a year or more, the toxicity of the residues may decrease so that aquatic plants can grow through surface slicks without suffering much damage. The phytoplankton and zooplankton communities are also somewhat resistant to weathered oil. However, any waterfowl that attempt to use oiled waterbodies become fouled with residues, and this is usually fatal to them.

Studies have been made of the effects of petroleum on tundra and boreal forest ecosystems, including experimental spills onto vegetation. These studies found that crude oil behaves as an herbicide to terrestrial vegetation, killing foliage and woody tissue. In some plants, however, the regenerating tissues were not all killed, allowing re-growth after the oiling.

These general observations are illustrated by a study in the western Arctic where the experimental oiling caused a rapid defoliation of plants, reflected by the reduced cover of foliage after the oiling, in contrast to the non-oiled (control) vegetation. Black spruce (Picea mariana) trees are dominant in the boreal forest sites. These did not die immediately after oiling, but did became more vulnerable to physiological stress associated with the hard arctic winter and so eventually died, but only after several years has passed.

After the initial damage, many plants of the forest and tundra began to recover. Black spruce was an exception, as no new seedlings were observed during the five-year study. Lichens and mosses also recovered slowly.

Of course, the environmental consequences of oil development are much broader than the ecological effects of petroleum spills on land or in water. The construction of infrastructure such as roads and pipelines in remote terrain has a variety of environmental consequences. In addition, the influx of large sums of money and wage employment into rural places has huge socio-economic impacts, some of them positive, but others disruptive. As with any industrial development, potential damage to the ecological and socio-economic environments must be identified and, as far as possible, minimized. The residual damage must then be balanced against the economic and social benefits that are expected to be gained from the development of fossil-fuel resources.

Marine Spills

Petroleum spills into the world’s oceans has significantly decreased to around 1.4 million tonnes/year compared to 3-6 million tonnes in the 1970s and early 1980s (National Academy of Sciences 2003; GESAMP 2007; Koons 1984). Figure 25.1 provides a breakdown of the sources of petroleum inputs into the ocean. In addition to petroleum spills, there is a large natural emission to the oceans of hydrocarbons not derived from petroleum. These chemicals are synthesized and released by phytoplankton, at an estimated 26-million tonnes/year. These huge biological releases contribute to the background concentration of hydrocarbons of about 1 ppb (1 µg/L) in seawater. The biogenic emissions are a natural contamination and do not result in known biological damage. There are also natural emissions from underwater seeps, which amount to about 0.6-million tonnes/year and may sometimes cause local ecological damage.

Figure 25.1. Petroleum Inputs to the Oceans by Source. Source: Modified from World Ocean Review (2010) data.

Massive spills associated with wrecked supertankers or well platforms attract a great deal of attention, and deservedly so. On average, they amount to about 170,000 t/y of oil spillage. It is important to recognize, however, that relatively small but frequent discharges are associated with urban runoff, oil refineries, “normal” tanker discharges, and other coastal effluents. Because these discharges are frequent, they account for a large volume of petroleum and are responsible for the local contamination and pollution by hydrocarbons that is typical of many coastal cities and harbors.

Another important source of petroleum inputs to the oceans has been discharges of oily washings from the storage tanks of ships that transport petroleum and liquid fuels. After a tanker delivers a load of petroleum to a refinery, it fills some of its storage tanks with seawater, which acts as stabilizing ballast while the ship travels to get its next load. As the tanker approaches its destination, the ballast may be discharged into the ocean. If the wastewater is not treated, it contains hydrocarbon residues equivalent to about 1.5% of the tanker’s capacity in the case of bunker fuel, less than 1% for petroleum, and about 0.1% for light refined products such as gasoline. For large oil tankers, this could amount to as much as 800 t of hydrocarbons.

This large operational source of marine pollution has decreased greatly since the 1970s due to widespread adoption of two procedures: the load-on-top (LOT) method and the crude oil washing (COW) method. LOT separates and contains most of the oily residues before ballast water is discharged to the marine environment (the residual oil is combined with the next load). If used in calm seas, the LOT technique can recover 99% of the oily residues, although the efficiency may be 90% or less if the tanker has had a turbulent passage.

The COW method is a more recent innovation than LOT. It involves washing the petroleum storage tanks with a spray of crude oil before the new cargo is loaded. The spray dissolves the residual sludge, allowing it to combine with the next load. The advantage of the COW method is that it eliminates the need to rinse the empty tanker compartments with seawater, so there are no bilge washings to discharge to the marine environment.

Thanks to the widespread use of LOT and COW, the operational discharges from tankers has been reduced from about 1.1-million tonnes in 1973 to 19-thousand t in 2007. Although LOT and COW are now widely used, some tankers and other ships continue to illegally discharge oily wastes at sea. This pollution is still an important cause of seabird mortality.

The most disastrous marine spills of petroleum (several of which are described later in this chapter) include the following accidents involving “supertankers” (having a capacity of 500-thousand tonnes or more):

• In 1967, the Torrey Canyon spilled 117-thousand t of petroleum off southern England.
• In 1973, the Metula spilled 53-thousand t in the Strait of Magellan.
• In 1978, the Amoco Cadiz spilled 230-thousand t in the English Channel.
• In 1989, the Exxon Valdez spilled 36-thousand 000 t in southern Alaska.
• In 1993, the Braer spilled 84-thousand t off the Shetland Islands of Scotland.
• In 1996, the Sea Empress spilled 72-thousand t off Wales.
• In 1999, the Erica spilled 20-thousand t into the Bay of Biscay off France and Spain.
• In 2002, the Prestige spilled 63-thousand t into the Atlantic off France and Spain.
• In 2003, the Tasman Spirit spilled 30-thousand t off Pakistan.
• In 2009, the Montera spilled 30-thousand t off northern Australia.

Some enormous accidental spills have occurred from offshore platforms used for exploratory drilling for petroleum:

• In 1979, the blowout of the IXTOC–I exploration well in the Gulf of Mexico spilled about 500-thousand t.
• In 1977, a blowout from the Ekofisk platform in the North Sea off Norway spilled 30-tonnes.
• In 2011, a blowout from the Deepwater Horizon off the Gulf coast of Louisiana, an exploration well that was drilling in extremely deep water (about 1,500 m), resulted in an immense spill of as much as 669-thousand t of petroleum and caused tens of billions of dollars of economic damage.

Oil Spills through Warfare

Huge amounts of petroleum and refined products have been spilled during warfare. During the Second World War, German submarines sank 42 tankers off eastern North America, resulting in the spillage of about 417-thousand tonnes of oil and fuels. During the Iran-Iraq War (1981-1987) there were 314 attacks on oil tankers, 70% of them by Iraqi forces. That war’s largest spill occurred in 1983, when Iraq damaged five tankers and three production wells at the Iranian Nowruz offshore facility, spilling more than 260-thousand t of petroleum into the Gulf of Arabia. The world’s largest-ever marine spill occurred during the brief Gulf War of 1991. Iraqi forces deliberately released huge quantities of petroleum (about 0.8-million tonnes) into the Gulf of Arabia from a Kuwaiti coastal loading facility. In part, this spill was a tactic of warfare – an attempt to make it difficult for Allied forces to execute an amphibious landing during the liberation of Kuwait. Mostly, however, the spill was an act of economic and ecological terrorism. The Iraqis also caused an enormous spillage on land during that war by igniting the more than 700 production wells in Kuwait. An estimated 2-6 million tonnes of petroleum per day were emitted from the burning wells. After the Gulf War was over, it took 11 months to control and cap the blowouts. By that time, an immense 42-126-million tonnes of petroleum had spilled. Most of the crude oil burned in the atmosphere or evaporated, but 5-21 million tonnes accumulated as vast crude-oil lakes in the desert around the blowouts. More recently, during the aftermath of a U.S.-led invasion of Iraq in 2003, insurgent forces routinely attacked oil-exporting pipelines as acts of resistance and economic terrorism. This caused large petroleum spills to occur, although information about the volumes of pollution or environmental damage is not available.

Fate of Spilled Oil

Various natural processes affect petroleum and refined products after they are spilled into the environment (Figure 25.2). Depending on their chemical and physical characteristics, various hydrocarbon fractions will selectively evaporate, spread over the surface, dissolve into water, accumulate as persistent residues, or be degraded by microorganisms and solar ultraviolet radiation:

Figure 25.2. Fate of Spilled Petroleum on Water. Source: Modified from Clark and MacLeod (1977).

• Evaporation of vapors is important in reducing the amount of spillage remaining in the aquatic or terrestrial environment. Evaporation typically dissipates almost 100% of gasoline spilled at sea, 30-50% of crude oil, and 10% of bunker fuel. In other words, the relatively light, volatile hydrocarbon fractions are selectively evaporated, leaving heavier residues behind. Rates of evaporation are increased by warm temperatures and vigorous winds.
• Spreading refers to the movement of an oil slick over the surface of water or land. Spreading can occur over extremely large areas on water, but it is much more restricted on land because of the high absorptive capacity of soil. The degree of spreading on water is influenced by the viscosity of the spilled material and by environmental factors such as windspeed, water turbulence and currents, and the presence of surface ice. One experimental spillage of 1 m3 of petroleum onto calm seawater created a slick 0.1 mm thick, with a diameter of 100 m, after 100 minutes. A petroleum slick only 0.3 µm thick or less is visible as a glossy sheen on calm water. In addition, a slick on water is moved about by currents and wind and may eventually wash onto a shore.
• Dissolution causes pollution of the water beneath an oil slick. Lighter hydrocarbons are more soluble in water than heavier ones, while aromatics are much more soluble than alkanes (Table 25.3). After a spill of petroleum at sea, the hydrocarbon concentration in water a few meters beneath the slick may be 4-5 ppm (g/m³), thousands of times greater than the 1 ppb (mg/m³) that normally occurs in ambient seawater.

Table 25.3. Solubility of Alkane and Aromatic Hydrocarbons. Solubility is reported in g/m³ (ppm) in fresh water. Within the aromatics, aqueous solubility decreases with increasing molecular size and with the number of aromatic rings.

• Residual materials remain after the lighter fractions of spilled petroleum have evaporated or dissolved. At sea, residual materials typically form a gelatinous, water-in-oil emulsion known as “mousse” because of its vague resemblance to the whipped chocolate dessert. Oil spilled offshore usually washes onto shorelines as mousse, which may then weather to form a long-lasting tarry residue on rocks. Alternatively, mousse may eventually combine with particles of sediment on the beach to form sticky, tar-like patties that subsequently become buried or may be washed back to sea during a storm. Mousse that does not wash ashore eventually weathers into semisolid, floating asphaltic residues known as “tar balls”.
• Degradation refers to the slow decomposition of spilled materials by microorganisms and by photo-oxidation by solar ultraviolet radiation. Many species of bacteria, fungi, and other microorganisms can utilize hydrocarbons as an energy source. The rate of biodegradation varies greatly, however, depending on ambient temperature, the concentration of oxygen, and the availability of key nutrients such as nitrogen and phosphorus. In general, lighter hydrocarbons are relatively easily decomposed by biological and inorganic oxidations, while heavier fractions resist degradation and can be quite persistent in the environment.

Toxicity

Acute toxicity caused by petroleum, refined products, or pure hydrocarbons is typically associated with the ingestion of the materials, followed by the destruction of cellular membranes, which results in the death of tissues. The toxic effects are influenced by several factors:

• The chemical composition of the spilled material, including its component hydrocarbons
• The intensity of exposure, or the amount or concentration of specific hydrocarbons or type of petroleum
• The frequency of exposure events, such as whether the pollution is a single event, chronic (continuous), or frequent (a series of episodes)
• The timing of the exposure, especially whether it occurs during a critical time for a species or ecosystem
• The condition of the spilled material, including thickness of a slick, nature of the emulsion, degree of weathering, and persistence of residues
• Environmental influences on exposure and toxicity, including weather conditions, oxygen status, and the presence of other pollutants
• Toxicity associated with chemical dispersants or detergents that are used during a cleanup
• The sensitivity of particular species in an affected ecosystem to suffering toxic effects of hydrocarbons

It is important to recognize that severe damage may be caused by methods used during a cleanup, such as the use of dispersants and emulsifiers, hot-water washing, the removal of oiled substrates, burning, and the tilling of oiled soil to improve aeration and decomposition. Ecological effects are also influenced by damage caused to keystone species in the food web, which has disproportionate effects on the community.

Effects on Birds

Seabirds are extremely vulnerable to oil spills. These include cormorants, sea ducks (eiders, mergansers, scaup, scoters), alcids (auklets, murres, puffins, razorbills), and penguins. During the non-breeding season, a spill can cause enormous mortality to these birds because they may congregate in large, seasonal flocks. Moreover, since alcids and penguins have low reproductive rates, their abundance can take a long time to recover from an event of mass mortality caused by an oil spill. Murres, for example, do not begin to breed until they are five years old, lay a one-egg clutch, and raise only about 0.5 young per pair of breeding adults per year.

The most common cause of death of seabirds results from their feathers becoming oiled when they dive through or swim in oil-polluted water (Image 25.1). This causes the birds to lose critical insulation and buoyancy, and they die from excessive heat loss leading to hypothermia or by drowning. They also ingest toxic oil while attempting to clean their feathers by preening. In addition, bird embryos can be killed by even a light oiling of the egg by the feathers of a contaminated parent.

Image 25.1. A Dead Bird Covered in Oil. The Deepwater Horizon oil spill in the Gulf of Mexico had devastating consequences for marine wildlife. Source: “Deepwater Horizon oil spill: avian casualty” by Encyclopædia Britannica.

The size of a petroleum spill is not an accurate indicator of its potential to damage bird populations. The ecological context is also critically important, because even a small spill in a sensitive habitat can wreak havoc. For example, in 1981, a relatively small discharge of oily bilge water from the tanker Stylis off Norway killed about 30-thousand seabirds. This happened because the spill affected a critical habitat where seabirds are abundant during the winter. In another case, more than 16-thousand oiled Magellanic penguins (Spheniscus magellanicus) were discovered on beaches in Argentina in 1991, even though no offshore slick could be found. The oil likely came from the bilge washings of a passing tanker. Similar damage has occurred off Newfoundland and Nova Scotia because of illegal discharges of oily bilge water by tankers. In January 1997, about 30-thousand murres and other seabirds were killed in this way near Cape St. Mary’s in southern Newfoundland.

Ecological Effects

In the following sections we will examine case studies of oil spills to understand the kinds of ecological damage that are caused by oil pollution and cleanup methods. We will examine spills from wrecked supertankers and offshore drilling platforms, chronic emissions near petroleum refineries, and oiling of terrestrial vegetation.

Oil Spills from Wrecked Tankers

The Torrey Canyon wreck in 1967 was the first oil spill involving a supertanker. The ship was bound for a refinery in Wales with 117-thousand tonnes of crude oil when it ran aground, spilling its entire cargo and polluting hundreds of kilometres of coast. Seabirds were among the most tragic victims of this spill, with at least 30-thousand killed. Although almost 8-thousand oiled birds were captured and cleaned, the rehabilitation methods of the time were not very successful and only a few of the birds survived long enough to be released (see In Detail 25.1).

Immediately after the wreck occurred, an intensive cleanup of oiled beaches began. This effort used large amounts of detergent and dispersant to create oil-in-water emulsions on polluted shorelines, which were then rinsed to shore waters using pressurized water streams from hoses. Unfortunately, the chemicals used as emulsifiers were extremely toxic and their enthusiastic use greatly increased the damage already caused by petroleum to the flora and fauna of coastal habitats.

However, emulsifiers were not used during the cleanup of rocky beaches. There, the marine algae, although damaged by oily residues, preserved some of their regenerative tissue and then regrew relatively quickly. Some species of intertidal invertebrates also proved rather tolerant to oiling. Many limpets (Patella spp.), for example, survived and were later able to graze on algae on oiled rocks.

The unanticipated damage caused by toxic emulsifiers was an important lesson from the cleanup of the Torrey Canyon disaster. Soon after, less-toxic dispersants were developed for use in oil-spill emergencies. Techniques improved too, so these chemicals could be used more judiciously, mainly to clean sites of high value for industrial or recreational purposes and to treat offshore locations where ecological damage would be less.

A post-oiling succession occurred after the Torrey Canyon spill, which eventually restored ecosystems that are typical of the region. Oiled habitat in the rocky intertidal zone was initially colonized by the opportunistic green alga Enteromorpha. As invertebrate herbivores recovered, this alga was grazed and replaced by species of perennial seaweeds, which are the typical algae of rocky intertidal habitats. Except for lingering effects on seabird populations, ecological damage caused by the Torrey Canyon spill turned out to be relatively short term because the recovery was vigorous.

In habitats that were cleaned with emulsifiers, however, the recovery was much slower. Some areas took up to 10 years to recover communities similar to those present before the spill.

The Amoco Cadiz supertanker accident occurred in 1978, about a decade after the Torrey Canyon and in the same general area, but closer to France. The wreck of the Amoco Cadiz spilled 233-thousand t of petroleum and fouled about 360 km of shoreline, of which 140 km were heavily oiled. The cleanup of some beaches involved digging up and removing oily sand and sediment. Detergent and low-toxicity dispersants were used only to remove fouling residues in harbors and to disperse floating slicks of mousse in offshore waters. Because emulsifiers were used judiciously, many of the ecological damages caused by oil pollution and the cleanup were much less severe than after the Torrey Canyon spill. Recovery from the Amoco Cadiz spill was substantially complete within several years. However, some effects on benthic invertebrates lasted for a decade, and there was lingering damage to local colonies of alcid seabirds.

The Exxon Valdez suffered an accidental grounding in 1989 in southern Alaska, and this caused the most damaging tanker accident ever to occur in North American waters. A significant amount of the petroleum extracted in the U.S. is mined in northern Alaska, from where a 1,280 km pipeline carries it south to the port of Valdez. The oil is then transported to markets in the western U.S. by a fleet of supertankers. The first part of the oceanic passage runs through a narrow shipping channel in Prince William Sound.

Before the Exxon Valdez accident in March 1989, tankers had navigated that passage about 16-thousand times. However, the Exxon Valdez, the newest tanker in the Exxon fleet, was incompetently steered onto a submerged reef, resulting in a spill of 36-thousand tonnes of its 176-thousand t load of petroleum. About 40% of the spill washed onto shoreline habitat of Prince William Sound, while 25% was carried out of the sound by currents, and 35% evaporated at sea. Less than 10% was recovered or burned at sea.

This accident could have been avoided by more sensible operation of the tanker. At the time that the ship went aground, its bridge was under the command of an unqualified mate. Unaccountably, the captain was in his cabin. Only some 10 minutes after assuming control of the ship, the mate, who was not well familiar with the shipping channel and its aids to navigation, had run the supertanker onto the unforgiving reef.

The environmental damage was compounded by a lack of preparedness by industry and government for dealing with an oil-spill emergency. Essential equipment for containment and oil recovery was not immediately available, and it took too long to mobilize trained personnel. Consequently, despite favorable sea conditions during the first critical days after the grounding, few effective oil-spill countermeasures were mounted. Not until the second day of the spill was it possible to off-load unspilled petroleum from the Exxon Valdez to another tanker, and not until the third day were floating booms deployed to contain part of the spill. Unfortunately, a gale developed on the fourth day, making it impossible to contain or recover spilled petroleum, which then became widely dispersed.

The region around Prince William Sound is famous for its spectacular scenery and large populations of wildlife. Some ecological communities and species were severely damaged by the oil spill. However, controversies have arisen about both the poor understanding of some ecological effects, and the role of science and scientists in sorting out legal and political aspects of the disaster (Holloway, 1996; Weins, 1996). For a long time, some scientists were prohibited from sharing their information because of legal needs for confidentiality. Controversies arose among scientists, environmental advocates, and other interest groups about the scale and intensity of some of the reported damages.

About 1,900 km of shoreline habitat was oiled to some degree. A survey found that 140 km was “heavily oiled,” meaning there was at least a 6 m wide oiled zone. Another 93 km were “moderately oiled” (3-6 m wide zone), 323 km were “lightly oiled” (3 m wide), and the rest “very lightly oiled” (< 10% cover of oil). Overall, about 20% of the shoreline of the Sound, plus 14% of beaches on the nearby Kenai Peninsula and Kodiak Island, suffered some degree of oiling.

A heroic and extremely expensive effort was undertaken to clean up some of the pollution from oiled beaches. About 11-thousand people were involved, costing the Exxon corporation about $2.5 billion. The U.S. government spent an additional $154 million. Residues were removed from heavily oiled beaches by machines and people wielding shovels and bags. Other places were cleaned by pressurized streams of hot or cold water. On some beaches, people actually wiped oiled rocks with absorbent cloths, a procedure that was ironically referred to as “rock polishing”.

These cleanup efforts helped greatly, and they were aided by natural processes, especially winter storms and microbial degradation of residues. Consequently, the amount of residues on beaches declined rapidly in the years following the spill. One survey of 28 polluted sites found an average of 37% surface oil cover in the first post-spill summer of 1989, but less than 2% in 1990. Another survey in 1991, after two post-spill winters and three summers, found that fewer than 2% of the beaches still had visible surface residues, compared with 20% in the first summer after the spill. However, subsurface residues still existed in many places.

Initially, severe damage was caused to the seaweed-dominated intertidal zone of affected coastlines. These effects were made worse by certain cleanup methods, particularly washing with pressurized hot water. Fortunately, much of this damage proved to be short term, and by the end of 1991 a substantial recovery of seaweeds and invertebrates had begun. However, there were lingering effects on community structure, and vestiges of oil were still present 15 years later at some sites.

Prince William Sound supports large fisheries for salmon and herring. In 1988, before the spill, the catch had a value of about $90 million. The fishery was closed in 1989 because of the spill, and Exxon paid compensation of $302 million to displaced fishers and processors (many of whom were also employed in the cleanup, earning $105 million in wages and vessel charters).

In 1990, the harvest of pink salmon (Oncorhynchus gorbuscha) in the Sound was 44-million fish, larger than the previous record-high catch of 29-million fish. These were two-year-old fish, about one-quarter of which would have passed through the Sound during their migration from rivers to the sea in 1989, the year of the spill. The rest had been released from hatcheries. The 1991 catch of pink salmon was also large, at 37-million fish. There was also a large harvest of herring (Clupea harengus pallasi) in 1990, when 7,500 t were landed. The largest catch in a decade was made in 1991, at 10,800 t. Clearly, the fishery landings were not devastated by the oil spill.

Sea otters (Enhydra lutris) were the hardest-hit marine mammals. More than 3,500 otters were killed by oiling, out of a population of 5-10-thousand. A total of 357 oiled sea otters were captured and treated, of which 223 survived and were released or placed in zoos.

Seabirds are very abundant in the region, particularly so in the autumn when certain species aggregate there during their southern migration. At that time, about 10-million seabirds may inhabit the Sound. Fortunately, the Exxon Valdez disaster happened in late winter, but there were still about 600-thousand seabirds present. About 36-thousand dead birds were found, but many additional corpses sank or drifted out to sea, and the total mortality may have been 375-435-thousand seabirds.

About 400 people, 140 boats, and 5 aircraft were hired by Exxon to capture and rehabilitate oiled birds. They managed to treat 1,600 birds of 71 species, but half of them died of their injuries. The rest were treated and released to the wild, but the lingering effects of hydrocarbon poisoning likely prevented most of them from surviving for long.

Although severe ecological damage was caused by the Exxon Valdez spill, the recovery was rapid. Waves and winter storms quickly removed most of the spill residues. Even bird and mammal populations that suffered large mortality recovered to their natural abundance within a decade or less. From a strictly environmental perspective, the habitats affected by the disaster showed an impressive amount of resilience (Image 25.2). However, people and local communities were also affected by this calamity, and surveys have shown that their bad memories are deeply ingrained.

Image 25.2. Recovery of Green Island, Prince William Sound after Oil Spill. The top photo shows a heavily oiled beach on Green Island, Prince William Sound, soon after the Exxon Valdez disaster in 1989. The site was cleaned with warm-water washing in 1989, and then manually in 1990. In 1990 and 1991, it was treated with fertilizer to enhance microbial breakdown of the petroleum residues. The bottom photo shows the improved condition of the same beach in 1992, as a result of the natural and managed cleanups. Although little visible damage occurs on the surface, there are hydrocarbon residues deeper in the substrate. Source: Exxon Corporation.

Spills from Offshore Platforms

The Deepwater Horizon spill off the Gulf coast of Louisiana in 2011 was the largest accidental blowout (an uncontrolled discharge from a wellhead) in history (Image 25.3). The Deepwater Horizon was a drilling and exploration platform working in extremely deep water (about 1,500 m). The blowout was apparently caused by a failure of the casing of the borehole and also of the fail-safe blowout preventer. These likely occurred because of an unwise engineering decision to use an insufficient cementing regime for the borehole despite encountering extremely high geological pressure during the drilling. This resulted in a fire and explosion on the drilling platform, which sank during the fire-fighting action because of the enormous amounts of water poured into it. The blowout lasted for 87 days and the immense spill was as much as 669-thousand tonnes of crude oil, which spread over as much as 176-thousand km² of water, affected beached from western Florida to Texas, and caused tens of billions of dollars of economic damage.

The spill engendered a massive effort to staunch the blowout, burn the soil at sea or recover it for disposal on land, to protect coastal habitats from fouling, and to capture and rehabilitate oiled wildlife. About 7,000 m³ of dispersant was used to help protect coastal infrastructure and habitats, and also to disperse the petroleum as it issued from the subsea blowout itself. Despite the enormous effort, considerable damage was done to natural habitats, recreational beaches, the commercial fishery, and harbors, with some effects of residues lingering years later. There were extensive deaths of marine mammals, birds, fish, and other marine life.

Subsequent legal actions found that British Petroleum (BP), the operator of the drilling project, bore primary responsibility for the disaster. Eventually, BP paid more than $42-billion in criminal and civil settlements,

Image 25.3. View of the Deepwater Horizon Spill. The coast of Louisiana and Alabama are shown, in the greater region of the delta of the Mississippi River. The floating slick of petroleum from the Deepwater Horizon blowout shows as bright zones, due to the spilled oil calming the surface water and affecting its reflectance properties. Source: NASA image file via Wikimedia Commons.

The IXTOC-I spill in 1979 was one of the world’s largest accidental spills. This was a Mexican drilling platform being used for petroleum exploration in the Gulf of Mexico. The blowout remained uncontrolled for more than nine months, resulting in a spillage estimated at 476-thousand tonnes of petroleum. About 50% of the spill is thought to have evaporated into the atmosphere, while 25% sank to the bottom, 12% was degraded photochemically or by microorganisms, 6% was burned at sea or recovered near the spill site, and 7% fouled about 600 km of shoreline in Mexico and Texas.

This enormous blowout caused great economic damage. It fouled beaches important to tourism, and affected the fishing industry by oiling boats and gear, preventing fishing near slicks, and tainting valuable fish and invertebrates with foul-tasting hydrocarbons. Many birds, sea mammals, turtles, and other wildlife were oiled and died, although these and other ecological damages were not well documented.

The Santa Barbara offshore blowout occurred in 1969 off southern California. This spill involved about 10-thousand t of petroleum and fouled 230 km of coastline. Birds were the most obvious victims, with about 9-thousand killed, or half of the population occurring at the time of the spill. About 60% of the dead birds were grebes and loons, which winter in the area. Attempts were made to capture and clean oiled birds, but the efforts were not very successful. Coastal ecosystems were also severely damaged, especially in rocky intertidal habitats, but recovery was fairly rapid. Within one year, barnacles began to re-colonize intertidal habitat, even on rocks still covered with asphaltic residue. Beaches used for recreation were cleaned by the removal of oily sand, blasting with water or steam, or spraying with solvent to wash residues back to sea. As with the Torrey Canyon cleanup, these methods using highly toxic dispersants greatly worsened the ecological damage.

Spills in the Arctic Ocean

Large but still poorly known reserves of oil and gas occur in Arctic regions of Canada and Alaska. Exploratory drilling is widespread, and there are land-based production wells in the western Arctic near Norman Wells and on the north slope of Alaska. The exploration, production, and transport of hydrocarbons from the Arctic carries the risk of accidental spillage in terrestrial or marine environments. The consequences of a petroleum spill in the Arctic Ocean are potentially catastrophic. Such a spill could result from a tanker accident in ice-choked waters or from an offshore well.

Climatic conditions in the Arctic are severe – a factor that greatly increases the likelihood of spills from offshore oil wells through equipment failure or human error. Furthermore, the icy conditions of the long winter would make it difficult to quickly drill an offshore relief well, a necessary step in controlling a blowout. Containing or cleaning up a spill in Arctic seas would also be a daunting task. Because of entrapment under sea ice and the cold, nutrient-poor conditions, spilled oil would not evaporate or dissolve into seawater as effectively as under warmer conditions, and microbial biodegradation would be extremely slow. Consequently, the amount of spilled oil would not decrease much over time, and most of the initial toxicity would persist. (Note that the spill from the Exxon Valdez occurred in boreal waters of southern Alaska, which are subject to much less severe temperature and ice conditions than occur in the Arctic Ocean.)

Arctic marine wildlife, particularly migratory seabirds and mammals, are extremely vulnerable to the effects of an oil spill. When they return to their northern breeding habitat in the early summer, marine birds and mammals often aggregate in dense populations in patches of ice-free water, known as leads and polynyas. These open-water habitats are places where spilled petroleum would accumulate. Enormous mortality of migrating sea ducks, murres, seals, whales, polar bears and other species would result as they became oiled by sticky residues. Because of the persistence of residues in the cold ocean, this threat would persist for years, and long-term debilitating damage to these animals would result. Potential damage to fish, zooplankton, and other components of the marine ecosystem are little known, but might be less intensive than the effects on marine birds and mammals.

A number of exploration wells have been drilled on the continental shelf of the Arctic Ocean off northern Canada and Alaska (and also in boreal and temperate waters off Newfoundland and Nova Scotia, where production wells now operate). Fortunately, there have not been any large spills of petroleum from the offshore drilling activities in the Arctic Ocean of North America (although there have been several blowouts involving natural gas). However, in spite of the adoption of the most modern spill-prevention technologies, a severe spill may be inevitable during offshore exploration and production activity in the Arctic. Such an accident would cause enormous ecological damage, from which recovery would be very slow.

Chronic Oil Pollution

Environments around tanker terminals and coastal petroleum refineries are chronically exposed to small but frequent oil spills, discharges of contaminated wastewater, and airborne contaminants from industrial sources. Similarly, coastal ecosystems near cities and towns, both marine and freshwater, are chronically affected by oil and fuel that are dumped into sewers, which often discharge these wastes directly into the aquatic environment. Chronic exposures such as these are much less intense than the severe pollution associated with wrecked tankers, but environmental damage still results.

Chronic exposure to hydrocarbons and other pollutants has been blamed for unusually high frequencies of cancers and other diseases in fish and shellfish. Although the exact causes of many of these wildlife diseases have not been determined, many scientists believe they are somehow caused by chronic pollution. One study of a river near Detroit, Michigan, found an unusually large incidence of gonadal tumors in fish (up to 100% in older males). However, epidemics of wildlife diseases are not always observed in chronically polluted environments. Ecological damage at the community level has been observed near effluent discharges from some coastal petroleum refineries. Studies in Britain, for example, have shown a deterioration of salt-marsh vegetation near oil refineries. Exposed bare mud was found where well-vegetated, grassy salt marshes had occurred previously. However, in places where industry made serious efforts to reduce the emission of pollutants, new vegetation was able to re-colonize the mud and re-develop a salt marsh.

Conclusions

Petroleum is a vital natural resource that is transported over long distances from places where it is extracted to those where it is consumed. Refined products, such as gasoline and kerosene, are also transported widely. There is always a risk of accidental spills and even deliberate ones (such as acts of war or terrorism). When they occur, they may cause extreme damage to the environment. There have been some spectacularly large petroleum spills, particularly as a result of shipping accidents involving large tankers, as well as incidents during war. These large spills have had devastating effects on affected ecosystems. In some cases, the natural recovery can be aided by massive cleanup efforts and wildlife rehabilitation. It is important to recognize, however, that most large spills are accidents that can be prevented. This can be done if tankers, pipelines, and other equipment are designed and maintained to a high standard of reliability, if effective spill-containment measures are in place, and if personnel work diligently to prevent these disasters. It is always best to avoid oil spills and other environmental emergencies than to engage in very expensive post-spill actions to clean them up.

Questions for Review

1. What are the causes of petroleum spills to the oceans?
2. Why were the ecological effects of the Amoco Cadiz spill fewer and shorter-lasting than those of the Torrey Canyon?
3. Explain why the addition of fertilizer can be an effective way of treating the residues of oil spills.
4. Why does oil spilled on water affect a much larger area than a comparable volume spilled on land?

Questions for Discussion

1. Considering the poor survival of aquatic birds after they have been “rehabilitated” from oiling and returned to the ocean, do you think that it is worthwhile to treat these victims of oil spills?
2. In view of the ecological risks, do you think that oil exploration and extraction should be allowed in the Arctic?
3. Why is it not possible to prevent all spills of petroleum?

Exploring Issues

1. A proposal has been made to build an oil refinery on the coast (choose whichever one you live closest to). The crude oil will be brought to the refinery by tanker ships, and the refined products will be distributed by ship, train, and truck. You are working as an environmental consultant and have been asked to recommend spill-prevention and countermeasure tactics to protect the marine and terrestrial environments around the refinery. Provide a list of practices that would provide this function of spill prevention and countermeasures.

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