Oil Pollution

Oil pollution of the sea attracts public attention because it is visible.  Many people will encounter oil pollution when they visit coastal areas and others will watch the devastating effects of oil spills on television, or read about them in the press.  But accidents at sea are not the only way that oil enters the ocean environment: there are a number of other routes by which an estimated 2.5 million tons of petroleum hydrocarbons contaminate our seas every year.

About three billion tons of crude oil is produced every year and half of this is transported by sea-going oil tankers.  After a tanker has unloaded its cargo of oil it has to take on seawater as ballast for stability, for the return journey to the oil fields.  The ballast water is usually stored in cargo compartments which previously contained oil.  Ballast water inevitably becomes contaminated by oil residue in the cargo holds.  In a 200 000 ton tanker, residue oil may amount as much as 8000 tons.  In the past a significant amount of this dirty ballast water was discharged into the sea and was responsible for much of the oil pollution in the world`s oceans.  Today transportation companies have formulated ways of reducing the amount of oil that is discharged in ballast water.

All ships, including tankers, visit dry docks for servicing and repairs.  In order to eliminate fire hazards, oil has to be removed from cargo compartments and fuel tanks before the ship may be repaired.  Today, ship yards are compelled to provide “slop reception” facilities and as a result the amount of oil discharged by dry docks have been drastically reduced.

Oil tankers load and discharge their cargoes of oil at marine terminals around the world.  Accidents at marine terminals as a result of human error or pipeline failure, are an inevitable occurrence.  For instance, in Cape Town, in 1998, an estimated 150 tons of bunker oil leaked into Table Bay Harbour when a pipe, that led to an oil storage facility, burst.

All ships need to pump out bilge water which usually contains oil from the ship`s engines.  The quantity of oil that is released by each ship may be small, but since all shipping contributes, the total amount of oil entering the sea is considerable.

Oil spillage in Knysa 16 August 2013
Picture credit: environment.co.za

A large number of accidents involving oil tankers take place each year.  Most shipping accidents occur where the density of shipping is high, usually close to shore.  As a result, coastal pollution is the almost inevitable result of major shipping disasters. In 1994 the iron ore carrier, Apollo Sea, broke up in heavy seas close to Cape Town, spilling 2400 tonnes of heavy bunker fuel oil.  The consequences were devastating:  oil contaminated beaches took months to clean and some 7500 seabirds had to be rescued and cleaned by SANCCOB following the disaster.  At least 1500 birds are known to have died from oil contamination.  The cost of the clean-up operation was R27 million.  In June 2000 the ill-fated ore carrier Treasure sank in 50m water some 10km off Melkbosstrand, spewing hundreds of tons of fuel oil into the sea.  Over 20 000 oiled penguins were rescued and cleaned and a further 20 000 unoiled penguins were transported to Port Elizabeth.

When liquid oil spills into the sea, it spreads over the surface of the water and forms a thin film – an oil slick.  An oil slick may have a devastating effect on the environment.  If it contaminates rocky shores, oil can cause severe damage to the inter-tidal area, smothering invertebrates and killing algae.  On sandy beaches, liquid oil penetrates into the substratum where low oxygen levels slow down the degradation process.  In some cases, oil leaches from the sediments sometime after the spill, causing renewed contamination.  Estuarine environments, such as salt marshes are particularly vulnerable to oil spills, because these are calm environments where oil is likely to be trapped.  Oiled plants may be inhibited from flowering or germinating and in many cases they are simply killed.

Oiled penguins in Cape Town after Treasure spill in 2000
Picture credit: John Hrusal/FAW

Whatever other effects oil pollution may have, its effect on sea birds is the most insidious.  It is not the toxicity of oil that kills seabirds, but rather the fact that oil effects the water repellent properties of a seabird`s plumage.  Oiled birds rapidly become water-logged and some will sink and drown.  Others will lose their thermal insulation, become exhausted and die.  Penguins are particularly vulnerable because they cannot fly and must enter the water to feed.

Source: SA Coast – Claire Attwood