Oil spill

An oil spill is the release of a liquid petroleum hydrocarbon into the environment due to human activity, and is a form of pollution. The term often refers to marine oil spills, where oil is released into the ocean or coastal waters. The oil may be a variety of materials, including crude oil, refined petroleum products (such as gasoline or diesel fuel) or by-products, ships' bunkers, oily refuse or oil mixed in waste. Spills take months or even years to clean up.

Oil is also released into the environment from natural geologic seeps on the sea floor. Most man-made oil pollution comes from land-based activity, but public attention and regulation has tended to focus most sharply on seagoing oil tankers.

Environmental effects
Studies of Exxon Valdez oil spill have shown that the environmental damage caused by oil spills can be greater than was previously thought. Petroleum-based hydrocarbons can negatively impact marine life at concentrations as low as one part per billion.

The lighter fractions of oil, such as benzene and toluene, are highly toxic, but are also volatile and evaporate quickly. Heavier components of crude oil, such as polycyclic aromatic hydrocarbons (PAHs) appear to cause the most damage. While they are less toxic than the lighter volatiles, they persist in the environment much longer. A heavy oil spill can also blanket estuaries and shoreline ecosystems such as salt marshes and tidal pools, preventing gas exchange and blocking light. The oil can mix deeply into pebble, shingle or sandy beaches, where it may remain for months or years.

Seabirds are severely affected by spills. The oil penetrates and opens up the structure of their plumage, reducing its insulating ability, and so making the birds more vulnerable to temperature fluctuations and much less buoyant in the water. It also impairs birds' flight abilities, making it difficult or impossible to forage and escape from predators. As they attempt to preen, birds typically ingest oil that coats their feathers, causing kidney damage, altered liver function, and digestive tract irritation. This and the limited foraging ability quickly causes dehydration and metabolic imbalances. Most birds affected by an oil spill die unless there is human intervention.

Marine mammals exposed to oil spills are affected in similar ways as seabirds. Oil coats the fur of Sea otters and seals, reducing its insulation abilities and leading to body temperature fluctuations and hypothermia. Ingestion of the oil causes dehydration and impaired digestion.

Largest oil spills


One tonne of crude oil is roughly equal to 308 US gallons, or 7.33 barrels.

Estimating the volume of a spill
By observing the thickness of the film of oil and its appearance on the surface of the water, it is possible to estimate the quantity of oil spilled. If the surface area of the spill is also known, the total volume of the oil can be calculated.

Methods of cleaning
A sheen is usually dispersed (but not cleaned up) with detergents which makes oil settle to the bottom. Oils that are denser than water, such as Polychlorinated biphenyls (PCBs), can be more difficult to clean as they make the seabed toxic.

Methods for cleaning up include:
 * Bioremediation: use of microorganisms or biological agents to break down or remove oil
 * Controlled burning can effectively reduce the amount of oil in water, if done properly. But it can only be done in low wind, and can cause air pollution.
 * Dispersants act as detergents, clustering around oil globules and allowing them to be carried away in the water.   This improves the surface aesthetically, and mobilises the oil.  Smaller oil droplets, scattered by currents, may cause less harm and may degrade more easily. But the dispersed oil droplets infiltrate into deeper water and can lethally contaminate coral. Recent research indicates that some dispersants are toxic to corals.
 * Watch and wait: in some cases, nautural attentuation of oil may be most appropriate, due to the invasive nature of facilitated methods of remediation, particularly in ecologically sensitive areas.
 * Dredging: for oils dispersed with detergents and other oils denser than water.
 * Skimming: Requires calm waters
 * Solidifying

Equipment used includes :
 * Booms: large floating barriers that round up oil and lift the oil off the water
 * Skimmers: skim the oil
 * Sorbents: large sponges that absorb oil
 * Chemical and biological agents: helps to break down the oil
 * Vacuums: remove oil from beaches and water surface
 * Shovels and other road equipments: typically used to clean up oil on beaches

Prevention

 * Secondary containment - methods to prevent releases of oil or hydrocarbons into environment.
 * Oil Spill Prevention Containment and Countermeasures (SPCC) program by the United States Environmental Protection Agency.
 * Double hulling - build double hulls into vessels, which reduces the risk and severity of a spill in case of a collision or grounding.  Existing single-hull vessels can also be rebuilt to have a double hull.