Oil Spill and its adverse effects on marine bio-system and environment:
Oil is the most common pollutant in the oceans. More than 3 million metric tons of oil contaminates the sea every year. The majority of oil pollution in the oceans comes from land. Runoff and waste from cities, industry, and rivers carries oil into the ocean. Ships cause about a third of the oil pollution in the oceans when they wash out their tanks or dump their bilge water. It is an unfortunate by-product of the storage and transportation of oil and petroleum is the occasional spill. Marine oil spill is a serious consequence of off-shore oil drilling and its oceanic transportation. Spill control firms specialize in the prevention, containment and cleanup of industrial oil spills.
A. The major spills of crude oil and its products in the sea occur during their transport by oil tankers, loading and unloading operations, blowouts, etc. When introduced in the marine environment the oil goes through a variety of transformation involving physical, chemical and biological processes. Physical and chemical processes begin to operate soon after petroleum is spilled on the sea. These include evaporation, spreading, emu1sification, dissolution, sea-air exchange and sedimentation. Chemical oxidation of some of the components of petroleum is also induced in the presence of sunlight. The degraded products of these processes include floating tar lumps, dissolved and particulate hydrocarbon materials in the water column and materials deposited on the bed.
Biological processes though slow also act simultaneously with physical and chemical processes. The important biological processes include degradation by microorganisms to carbon dioxide or organic material in intermediate oxidation stages, uptake by large organisms and subsequent metabolism, storage and discharge.
B. Crude oil and its products are highly complex mixtures. Since the fate of petroleum in the marine environment depends on the composition, a preliminary knowledge of major components and types is necessary for understanding the fate of petroleum when spilled on water. The approximate composition of an average crude oil is considered as :
Normal Type -
Gasoline (C5 - C10 ) 30%; kerosene (C10 -C12 ), 10%; light distillate oil (C12 - C 2 0), 15%; heavy distillate oil (C20 C4 0), 25% residium oil ( >C40), 20%,
By molecular type -
Paraffins (alkanes), 30%; naphthenes (cycloalkanes), 50% aromatics, 15% nitrogen, sulphur and oxygen containing compounds (NSO) 5%.
(a) Spreading - Spreading of crude oil on water is probably the most important process following a spill. Apart from chemical nature of oil, the extent of spreading is affected by wind, waves and currents. Under the influence of hydrostatic and surface forces, the oil spreads quickly attaining average thickness of less than 0.03 mm within 24 h. Once a spill has thinned to the point that surface forces begin to play an important role, the oil layer is no longer continuous and uniform but becomes fragmented by wind and waves into islands where thicker layers of oil are in equilibrium with thinner films rich in surface active compounds.
(b) Evaporation - Evaporation and dissolution are the major processes degrading petroleum crude when spilled on water. The composition of oil, its surface area and physical properties, wind velocity, air and sea temperatures, turbulence and intensity of solar radiation, all affect evaporation rates of hydrocarbons. Evaporation alone will remove about 50% of hydrocarbons in an "average" crude oil on the ocean's surface. Loss of volatile hydrocarbons increases the density and the kinematic viscosity of oil. As more volatile hydrocarbons are lost, the viscosity of the resulting oil increases and this results in breakup of slick into smaller patches. Agitation of these patches enhances incorporation of water due to increased surface area.
(c) Photo-oxidation - The natural sunlight in the presence of oxygen can transform several petroleum hydrocarbons into hydroxy compounds such as aldehydes and ketones and ultimately to low molecular weight carboxylic acids, As the products are hydrophilic, they change the solubility behaviour of the spill.
(d) Dispersion - Dispersion is οil-in-water emulsion resulting from the incorporation of small globules of oil into water column. Oil begins dispersing immediately on contact with water and is most significant during the first ten hours or so.
(e) Dissolution - Dissolution is another physical process in which the low molecular weight hydrocarbons as well as polar non-hydrocarbon compounds are partially lost from the oil to the water column.
(f) Degradation – Bio-degradative processes influencing fate of petroleum in aquatic environment include microbial degradation, ingestion by zooplankton, uptake by aquatic invertebrates and vertebrates as well as bio-turbation. Microorganisms capable of oxidising petroleum hydrocarbons and related compounds are widespread in nature. The rate of microbial degradation varies with the chemical complexity of the crude, the microbial populations and many of the environmental conditions.
C. Effects of petroleum crude on marine bio-system: The biological effects of oil include the possibility of
(a) Hazards to man through eating contaminated seafoods,
(b) Decrease of fisheries resources or damage to wild life such as sea birds and marine mammals,
(c) Decrease of aesthetic values due to unsighty slicks or oiled beaches,
(d) Modification of marine ecosystems by elimination of species with an initial decrease in diversity and productivity and
(e) Modification of habitats, delaying or preventing re-colonization.
When an oil spill occurs, many factors determine whether the spill will cause heavy, long lasting biological damage, comparatively little or no damage or some intermediate degree of damage. Thus for instance, if a spill occurs in a small confined area so that the oil is unable to escape, damage will be greater for a given volume and type of oil spilled than if the same volume was released in a relatively open area.
In the open sea the possible impact on biota can be on phytoplankton, zooplankton, benthos, fishery, birds, mammals, etc. whereas in coastal waters the impacts will also be on inter-tidal fauna, aquaculture, seaweeds and mangroves.
D. Oil Spill Control - Oil spills can occur when there is a problem with an oil well, when a pipeline ruptures or leaks or when there is a transportation accident. Since conditions are different with each spill, different methods of spill control may be used.
Some of the tools used to control oil in a spill include booms, which are floating barriers used to clean oil from the surface of water and to prevent slicks from spreading, skimmers which use pumps or vacuums to remove oil as it floats on water and sorbents which absorb oil when they are placed in a spill area.
Sometimes chemicals called dispersants are used to break down oil and move it from the top of the water. Moving the oil in this way keeps it from animals that live at the surface of the water and allows it to eventually be consumed by bacteria.
A process called bioremediation may be used to accelerate the process of biodegradation of the oil after a spill. In this process, bacteria or other microbes are introduced to the environment to help oxidize the oil. Unfortunately, this process can work slowly and is not very useful for large spills.
Occasionally the slick caused by a spill is removed through a controlled burn. Burning only works under certain wind and weather conditions.
Oil spill control on land is often conducted manually. Scooping, cleansing and scraping of the rocks and sand is performed until the oil has been removed.