Oil is the greatest pollutant of the Earth’s oceans and river estuaries – and the giant tankers that transport it are the world’s worst culprits.
Nearly ten years after the Libyan-owned tanker Amoco Cadiz was wrecked off Brittany in March 1978, scientists reported that fish were still not breeding properly along that section of coast.
Plaice had abnormal and defective reproductive organs and local oysters were contaminated.
In 1989, a giant oil slick in Prince William Sound, Alaska, contaminated the breeding grounds of seals, sea lions and birds. Ten million gallons (45 million litres) of oil was split when the 216,000 ton tanker Exxon Valdez hit a reef.
No one knows precisely how much oil is accidentally split or deliberately jettisoned in routine ballast-dumping into the world’s waters.
Contamination is often caused when tankers wash out their empty tanks with sea water after a delivery.
The residue, which is pumped into the sea, can be considerable.
Left to itself, an oil spill will eventually disperse, breaking down to harmless residues – but not before it wreaks havoc on marine wildlife. The breakdown can be speeded up by spraying the oil with chemical dispersal agents, which are basically detergents.
But these too, can have undesirable effects. They destroy the natural oils on seabirds’ feathers, which give the birds their buoyancy.
After the supertanker Torrey Canyon went aground off the Scilly Isles in 1967, releasing about 21,000 gallons (95,000 litres) of oil, scientists believed the chemicals used to disperse the oil itself. Other, less damaging, chemicals have been developed for dispersing oil but even they have to be used with great care.
If possible, it is better to trap an oil slick before it spreads, and then pump it from the surface of the sea.
To do this, the slick is surrounded by a long, floating boom of inflatable tubes. Because oil floats on water, the boom stops it from spreading. One of the largest boom systems developed in the 1980s was British Petroleum’s weir boom. The ends of the boom are attached to supply ships. These move it slowly through the water, trapping the oil. One of the ships pumps the oil through a floating pipeline to a nearby tanker. The system can collect 15,000 tons of oil-and-water mixture a day.
Several other devices have been developed to suck up the oil slick once it has been trapped. These include sorption skimmers and weir skimmers.
Sorption skimmers use rollers, belts or mops, whose surfaces are treated with synthetic chemical materials to which oil sticks but water does not. A drum or belt rotates in the slick, picking up the oil and carrying it to a container, where it is scraped off with a blade that resembles a windscreen wiper.
Weir skimmers work by placing a weir just below the surface so that the oil flows over it. The level on the other side is kept lower by pump, and as the oil flows over the weir, it is pumped into a reservoir.
The simplest form of this manoeuvre uses open-topped oil drums weighted with stones, placed in water so shallow that their rims are just beneath the surface. The floating oil flows into the drums and can then be pumped out.
Spills on land or oil washed ashore can be difficult to clean up. Sometimes earth-moving equipment is used, or drainage trenches are dug. Straw, sawdust or peat can be used for a final clean up.
Picture Credit : Google