Accidental or intentional releases of crude oil or other petroleum products into the environment. Oil spills can occur almost anywhere in the world and cause a great deal of ecological, social, and economic harm, particularly when oil is released into oceans or other waters. Oil can be in the form of crude oil or various petroleum products produced from oil refining. Each type of oil, whether crude oil or refined product, has a unique chemical composition and characteristics. How spilled oil spreads and changes in the environment and the kinds of harm it can cause depend on the type of oil, the environmental conditions in which the spill occurs, and the geographic location. In general, the heavier and thicker an oil is, the more persistent it will be in the environment. Heavier oils tend to be less acutely toxic than lighter oils, but can coat and smother plants and animals (biota) in the spill area. Cleanup of oil spills into water focuses on controlling the source, getting oil off the surface of the water, and cleaning the affected shorelines (Fig. 1). During and after an oil spill response, restoration of the affected area helps to minimize the effects and return the area (and affected animals) to pre-spill conditions over time. See also: Petroleum; Petroleum products; Toxicology
Sources of oil spills
Because of the global use and distribution of crude oil (unrefined oil, as it exists in nature) and refined oil products, spills can occur virtually anywhere humans exist. Oil spills happen where oil production and exploration occur, such as offshore or onshore drilling wells and associated storage facilities. However, the majority of oil spills result from the transport of oil from the production sites to other places, such as refineries or marine ports for export. Thus, widespread sources of oil spills include oil tankers or barges, tanker trucks, pipelines, and, increasingly, railcars. Other major sources of oil spills are accidents involving vehicles, vessels, planes, and other equipment that use petroleum fuels such as gasoline and diesel fuel. See also: Diesel fuel; Gasoline; Marine transportation and the environment; Offshore oil and gas; Oil and gas storage; Oil and gas well drilling; Petroleum processing and refining; Pipeline
Oil properties and environmental fate
In general, all types of oils are made up of hundreds to thousands of different kinds of hydrocarbon compounds, as well as nitrogen, sulfur, oxygen, and a variety of metals. Oil experts divide oil into four groups, based on their properties and environmental fate.
- Group 1: Very light oils, including gasoline and jet fuel, characterized by rapid evaporation
- Group 2: Light oils, including diesel, characterized by moderate evaporation and high dispersion
- Group 3: Medium oils, including most crude oils, characterized by low–moderate evaporation, low dispersion, and long-term contamination
- Group 4: Heavy oils, including fuel oils and heavy crude oils, characterized by very low evaporation and dispersion, and long-term contamination
These groupings help oil spill responders predict where the oil will go in the environment, what harm it might do, and the best response options.
Evaporation, whereby oil goes from a liquid to a gas, is a major mechanism by which the chemical composition of oil changes after a spill, because the heavier components of the oil are left behind. This is particularly true for lighter oil types that partially evaporate. Dispersion of oil from the water surface into the water column, caused largely by wave action, is another important pathway of oil movement. Both evaporation and dispersion vary by oil type. Figure 2 shows the differences in evaporation and dispersion rates for two different oils, using an oil environmental-fate model. Heavy No. 6 fuel oil has very low rates of evaporation and dispersion, and much of the oil remains after five days, often as thick slicks and tarballs. The much lighter diesel oil has a low viscosity and thus spreads into thin slicks that tend to disperse, such that there is hardly any oil remaining after two days. Other natural processes that change spilled oil chemistry include biodegradation (by which microorganisms break down the oil by feeding on it), oxidation (by which oil is degraded by reaction with oxygen), and emulsification (by which oil and water mix to form a suspension, sometimes called mousse). See also: Emulsion; Evaporation; Microbial ecology; Petroleum microbiology; Viscosity
Ecological impacts of oil spills
The severity and types of ecological injuries caused by oil spills depend on the location, type of oil, weather, and other factors. Oil spills on land tend to be relatively less severe for ecosystems than spills to water, because the oil spreads much more slowly and can be cleaned more quickly. Oil spills into water (oceans, lakes, and rivers) are potentially devastating to the animals, plants, and habitats in the spill area. See also: Ecosystem
Because many oils tend to float on water, living organisms at the water surface or associated with shallow water or shorelines often experience a high degree of exposure to oil. Seabirds and other waterfowl, pinnipeds, sea otters, sea turtles, and other wildlife can be severely affected or killed by marine oil spills. Oil that eventually sinks or is dispersed into the water column can affect fish, plankton, and other biota, including organisms on the bottom of a water body. See also: Marine ecology
Oil affects living organisms by two primary mechanisms: (1) physical coating and (2) toxicity when an exposed organism absorbs the oil into its body through ingestion (through grooming or feeding), inhalation, or through the skin. In general, heavier, thicker oils (such as crude oil or heavy fuel oil) tend to have fewer toxic components but cause more problems by physical coating, compared with lighter oil products (such as diesel and gasoline), which are more toxic.
Physical coating of birds and mammals compromises feathers and fur, which in aquatic species may lead to reduced waterproofing and buoyancy, such that the animals can no longer swim, eat, or keep warm. Oil also can simply smother smaller animals and plants in its path. Toxic effects in animals include irritated tissues, neurological and immunological problems, hormonal changes, kidney and liver disease, cancer, and developmental defects, all of which can lead ultimately to death. Plants exposed to oil show reduced germination and growth, and death. See also: Environmental toxicology
Because of the interconnectedness of the various living components in an ecosystem, oil spills that affect one or more of these components can have far-reaching and lasting indirect effects. For example, the loss of small organisms living on the bottom of a river removes the food base for spawning salmon, which in turn reduces the salmon population, which affects predators that feed on the salmon, and so on. Another example is the oiling of a species’ breeding habitat, which could cause reproductive failure for that species for a long period of time. See also: Food web
Oil spill cleanup activities can also disrupt animals, plants, and habitats. This is often due to physical destruction, such as when oiled soil and sediment are dug up and removed or surfaces (such as a rocky shore) are washed aggressively. Other factors such as noise, increased human presence, and operation of large equipment can also disturb or harm local biota. See also: Bioremediation; Marine conservation
Human impacts of oil spills
Oil spills can affect humans on several levels. For cleanup workers, exposure to the oil is avoided or minimized by protective equipment and clothing. However, in past oil spills, these workers, as well as fishers and other community members living near the spills, have reported physical symptoms, including respiratory, dermal and eye irritation, headache, nausea, and other problems.
Potential indirect effects of oil spills on human health include contamination of drinking water and seafood. Oil spilled into fresh water can make the water unsafe to drink. In addition, fish, crustaceans, and especially bivalve mollusks can retain chemical components of oil in their tissues. Depending on the nature and concentrations of these chemicals in the affected animals, humans who unknowingly consume them can either encounter a bad smell or taste (referred to as seafood “tainting”) or ingest enough to cause health problems.
Emotional and psychological effects of oil spills can also be significant. Communities near oil spills may suffer increased levels of anxiety and depression, which can be the result of both social and economic impacts caused by the spill. Oil spills and cleanup activities can lead to beach closures, fishing closures, and reduced commercial activity and tourism. These, in turn, can lead to disrupted daily lives, displacement, loss of ability to enjoy and recreate in affected areas, loss of income, and other stresses that can harm mental well-being. See also: Anxiety disorders; Depression
Oil spill response methods
After an oil spill occurs in a waterway, the ensuing response begins with efforts to stabilize and secure the source of the spill. At the same time, response personnel deploy equipment to remove as much of the floating oil as possible and minimize oiling of shorelines and wildlife. Cleanup methods include use of physical or mechanical means as well as chemicals that change the nature of the spilled oil to make it easier to clean up. The selection of cleanup methods depends on the oil type and volume, location, and environmental conditions, such as wind, waves, currents, and water temperature.
Spilled oil in the environment is constantly changing, which may require adjustment of methods over time. In spills to water, response personnel generally remove oil using mechanical methods such as collecting the oil with booms and skimming it from the surface. However, there are conditions where response personnel may choose to use chemical dispersants or in-situ burning (intentionally burning the oil on the water surface). Many studies and reviews of past spills have led to guidelines on selection of the best cleanup options for different spill conditions.
Selection of shoreline cleanup methods depends on the shoreline type and sensitivity, degree of oiling, accessibility, and human use of the shoreline. Beaches with high recreational use are cleaned intensively to remove as much of the oil as possible and minimize human exposure. For marshes, response personnel remove the bulk oil initially, followed by “passive methods,” such as sorbents, to recover oil released by winds and tides. Response personnel use “walking boards” to prevent trampling the oil into the marsh soils and low-pressure flushing and flooding to minimize the risk of soil erosion or disturbance. Responders may also choose to burn an oiled marsh, which can be effective if conditions are safe to do so. Vegetation cutting to remove oil is a controversial method that, in the majority of documented cases, caused more harm than not cutting, or did not improve recovery. See also: Salt marsh
Restoration following an oil spill can help to reduce, shorten, or compensate for the environmental impacts of the spill. It ultimately strives to return the environment to its pre-spill condition. Wildlife rehabilitation is one aspect of restoration involving the capture, cleaning, and care of oiled animals until they can be released back into the wild (Fig. 3). Restoration activities also often include rehabilitating or replacing habitats lost as a result of contamination and/or cleanup activities. Such activities may include placement of clean soil and rocks and replanting lost vegetation in the affected area, or creating or improving similar habitat in other places. Figure 4 illustrates an oiled marsh in which the cleanup resulted in significant habitat destruction, and the resulting appearance after restoration personnel replaced marsh plants and structure to help speed recovery.