Oil Pollution

Oil Pollution, contamination of any part of the environment with any liquid hydrocarbon. One of the most serious forms of water pollution, it has most often been used in connection with losses of crude oil, or petroleum, to the marine environment. Best known among these are the big tanker wrecks such as the Torrey Canyon and the Exxon Valdez, the former spilling 860,000 barrels (107,000 tonnes) of oil off the coast of Cornwall, England, in 1967, and the latter some 240,000 barrels (30,000 tonnes) into Prince William Sound, Alaska, in March 1989. The largest such spillage was some 2,160,000 barrels (270,000 tonnes) resulting from the collision of two tankers, the Aegean Captain and the Atlantic Empress, near Trinidad and Tobago in 1979. However, only about 10 per cent of the oil entering the sea is derived from tanker accidents. Other sources are atmospheric fallout, natural oil seepage, polluted rivers and urban run-off, coastal oil refineries, and other industries (such as the spillage in the Persian Gulf caused by sabotage during the Gulf War in 1991, estimated at up to 460,000 tonnes), offshore oil production facilities (from which the worst spillage to date has been some 540,000 tonnes, from the Nowruz oilfield in the Persian Gulf in 1983), and operational discharges from tankers. At 22 per cent, such tanker discharges are the largest single contribution.

These operational discharges arise from the washing of tanks at sea and the dumping of oily ballast water prior to loading. It is these operations that have caused the chronic pollution of amenity beaches with tar-like deposits. Such pollution has been significantly reduced since the mid-1970s due to the guidance of the Inter-governmental Maritime Consultative Organization and the requirements of the International Convention on the Prevention of Pollution from Ships (MARPOL 73/78). Improvements include the load on top system which avoids the need for oily discharges, the provision of port facilities for the reception and treatment of oily ballast water and other effluents, the provision of oil/water separators and oil-in-water monitoring equipment on ships, and the requirement for dedicated ballast tanks on new tankers.

Oil discharged to the marine environment will be degraded by physical, chemical, and biological processes. Initially, an oil spill will spread quickly across the sea surface and will break up into a series of “windrows” parallel to the direction of the prevailing wind. Evaporation of the oil occurs rapidly, with the volatile components evaporating off within about 24 hours; light fuel-oil slicks can lose about 50 per cent within a matter of hours. The remaining heavier fractions of the oil will be dispersed in the water as droplets, which will eventually be broken down by bacteria and other micro-organisms. In some cases a water-in-oil emulsion will form, creating what is known as “chocolate mousse” at the surface.

The rates of the above processes will be determined by the weather, sea state, temperature, and oil type. Thus, when the tanker Braer was wrecked on the Shetland coast in January 1993 losing 680,000 barrels (85,000 tonnes) of oil, damage was restricted to local fish farms and seabird populations because the sea was so rough, the wind was offshore, and the oil was relatively light.

At sea, oil pollution is most damaging to surface-living animals, particularly seabirds, but also marine mammals and reptiles. The plumage of seabirds is damaged by oil and they may also ingest oil in an attempt to clean themselves. On shore, certain habitats are both vulnerable and sensitive to oil pollution. These include corals, saltmarshes, and mangroves. Oil pollution can also be very damaging to coastal fish farms (particularly salmon cages and oyster racks), and to coastal amenities such as beaches and marinas.

In the past, oil slicks were usually sprayed with dispersants. However, experience has shown that dispersants themselves, or the emulsions that they form, can be more toxic than the oil itself. The current philosophy is to contain oil with floating booms and recover it using various types of skimmers. Only if oil is in imminent danger of coming ashore is it sprayed. Similarly, beached oil is best left where it is to degrade naturally, unless it is on an amenity beach. Even then, physical removal is preferable to the use of dispersants, which may drive the oil deeper into the sand. Dispersants are reserved for the cleaning of essential facilities like slipways.

Some experiments have been conducted on the spraying of oil slicks with cultures of bacteria selected to digest the oil. However, nutrients must also be added to encourage bacterial growth and these may themselves be damaging to coastal water quality.

Most coastal oil refineries and oil terminals now have an oil pollution contingency plan. This lists the actions to be taken in the event of a spill, the equipment to be used, and the protection or treatment to be given to defined sensitive areas.

Oil pollution due to oil exploration and production on land can also be very damaging to the environment. In most cases, oil pollution is due to poor engineering, maintenance, and management. For example, in Ecuadorean Amazonia extensive pollution of soils and water courses has been caused by “blowouts”, careless disposal of waste oil, and the malfunctioning of oil-water separators. In Russia, large-scale pollution has been caused by the corrosion and neglect of oil pipelines. In October 1994, an estimated 60,000 to 80,000 tonnes of oil was spilled from a fractured pipeline near Usinsk, just south of the Arctic Circle. In such northern latitudes the tundra and taiga ecosystems are highly sensitive to oil pollution and the natural physical and biological processes of oil degradation are extremely slow. Damage also occurs in the tropics: in the Niger delta region of Nigeria, poorly constructed and maintained pipelines laid across the surface of agricultural land regularly leak; attempts to burn off the residue often leave a lifeless crust of earth up to 2 m (6 ft) deep, rendering the land unusable for the foreseeable future. The effects of such oil pollution will therefore be apparent for many decades to come.