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A worker looks over an oil boom on April 27, 2010, near the leak that resulted from the explosion and collapse of the Deepwater Horizon oil rig in the Gulf of Mexico. ((Patrick Semansky/Associated Press))

Oil spills happen every day — not to the extent of the 160,000 litres a day of oil that began spewing into the Gulf of Mexico after a drilling rig exploded and sank on April 21, 2010. And not to the extent of the 170,000 litres of oil that was spilled at the Terra Nova oil platform off the southeast coast of Newfoundland on Nov. 21, 2004. It created a slick approximately nine kilometres long and one kilometre wide.

But Environment Canada says there are about 12 reported spills of 4,000 litres of more of oil every day in this country. At least one of them is in navigable waters.

Some are accidents. Others are illegal discharges of bilge oil — a ship's waste oil.

In Canada, polluters are legally responsible for the cleanup of oil spilled in Canadian waters. They pay both for cleanup and for any resulting losses from environmental damage.

Oil companies and ships operating in Canadian waters are required to set up an agreement with a certified, Canadian-based, private-sector response organization that will help them in the event of a spill.

The science of cleaning up oil

No two crude oils are exactly alike, and that plays an important part in how to clean up a slick. The different physical and chemical properties of crude and refined oils influence the physical and biological effects of an oil spill, the behaviour of a slick and the effectiveness of cleanup operations.

The type of oil spilled isn't the only thing that affects cleanup.

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An April 25 satellite photo shows the oil slick from an underwater leak after the sinking of the Deepwater Horizon oil rig. The Mississippi Delta is at the image's centre and the oil slick is a silvery swirl to the right. ((NASA/Associated Press))

Local environmental conditions such as weather, tides and currents, wind speed and direction, the difference between air and sea temperature, and the presence of ice floes, affect the behaviour of spilled oil as well as the ability of crews to work on a spill.

Crews control the movement of an oil slick by containing or diverting the oil.

Floating booms are mechanical barriers that extend above and below the surface of the water to stop the flow of oil. They can be used in three ways:

  • To surround a slick completely and reduce its spread. 
  • To protect harbour entrances or biologically sensitive areas. 
  • To divert oil to an area where it can be recovered.

How effective a boom is depends on the wind and waves at the site.

Sorbent booms and barriers are used to absorb a moving oil slick. They only work well when a slick is thin, because once their surfaces are saturated, they can't absorb anymore.

Once oil is contained by the booms, there are several options to remove it from the water.

Skimmers clean the water without changing the chemical or physical properties of the oil. How well a skimmer works depends on the type of oil spilled, the thickness of the slick and, again, the weather.

There are several types of skimmers, including:

  • Belt skimmers: a stainless steel belt is lowered into the water. It passes through wiper blades which remove the oil from both sides of the belt as it goes through.
  • Drum skimmers: a rotating drum moves through the water depositing the oil into a containment chamber within the skimmer.
  • Mop skimmers: a special type of rope dips into the contaminated liquid, removing both the liquid and the oil. Like wringing a mop, the rope is squeezed to get the oil out. High viscosity oils make mop skimmers less effective: the rope becomes matted down fairly quickly and less absorbent.

Sorbents recover oil either through absorption (the oil is drawn into porous materials) or adsorption (the oil sticks to the surface of the sorbents). They're used in the final mopping-up because they get at trace amounts of oil left after skimming and can be sent into areas where skimmers can't reach.

Sorbents come in two basic types: natural organic materials like peat moss and sawdust; and synthetic sorbents like polypropylene, polyester foam, polystyrene and polyurethane. Both types are usually applied by hand and recovered with nets, rakes, forks and pike poles.

Dispersants are chemical compounds that cluster around oil globules and allow them to be carried away by ocean currents. Smaller chunks of oil may disperse and degrade more easily. However, this method is not very effective on large spills — and it can be toxic in parts of the ocean where there are coral reefs. Some dispersants are toxic to coral.

Manual recovery of oil with buckets and shovels is common, especially when the oil has come close to shore. Viscous oils are more easily removed by manual methods than more fluid oils.

Another option once an oil slick has been contained within a boom is to burn it. Containing the oil makes the slick thick enough to set alight.  The slick needs to be at least two millimetres thick to be successfully burned. That's thick enough to counter the cooling effects of the wind and water.

Choppy waves could extinguish the flames, and if the slick becomes too thin, the fire will go out, leaving lots of oil still on the surface.

What happens to the oil left over can be a major environmental headache. Unburned residue is often thick, resembling the consistency of toffee. It is very difficult to remove from the water — and from land, should it drift to shore.

Some residue may sink, which can wreak havoc on species that make the ocean floor home. That can cause serious problems for the shellfish industry in the area.

Oil slick residue that sinks is far more difficult to recover than oil on the surface of the water.

Another concern when slicks are ignited is the amount of smoke that will be produced by the fire. Studies suggest a smoke plume is an acceptable trade-off — under some circumstances.

When oil makes it to land

Cleaning up a shoreline is usually a hands-on task with many people using rakes, shovels, wheelbarrows and garbage bags to do the job. Hoses are used to attempt to wash off the oil, and sorbent materials are put down to soak up oily residues.

Though these methods can be effective, some experts believe much of the biological damage observed on Alaskan beaches after the Exxon Valdez spill was caused by cleanup activities.

Sometimes chemical cleanup agents are used — but they're not allowed in fresh water or near sensitive areas. High-pressure water hoses wash oil from coarse sediments, rocks and man-made structures. Low-pressure water flushing is used to remove oil from fine sediments, shores with vegetation and marshes.

Sometimes, such as when a beach is hit by a slick, the environment is so contaminated that sand, dirt and plant material are simply removed.

Cleaning wildlife is a lot of work, expensive, and usually ineffective. Even if an oil spill is small, it can have a dramatic impact on bird and animal populations.

Not only does the oil coat the outside of the organisms affected, they often ingest it in water and on their food — effectively poisoning themselves.

Where the oil goes

After the oil is recovered, it's separated from the water, and the oil is disposed of, along with any remaining cleanup materials and other debris.

The disposal of oil and debris in Canada is regulated by local, provincial and federal governments. Relatively fresh oil may be re-refined. In other cases, the recovered oil is burned.

Sometimes cleanup crews can't get to a slick because the weather is too bad. In these cases, waves recirculate the oil in the water, and natural processes eventually break the oil down.

Chemicals can also be used to disperse the oil. This helps some bird species, because it removes the oil from the surface. However, underwater creatures are hit instead.