THE first important feature of interaction between gaseous traces and marine organisms is the quick fish response to a toxic gas as compared with fish response to other dissolved or suspended toxicants. Gas rapidly penetrates into the organism especially through the gills and disturbs the main functional systems (respiration, nervous system, blood formation, enzyme activity and others). External evidence of these disturbances includes a number of common symptoms mainly of behavioral nature, for example, fish excitement, increased activity, scattering on surface water). Further exposure leads to chronic poisoning. At this point, cumulative effects at the biochemical and physiological level occur. A general effect typical of all fish is gas emboli. These emerge when different gases including the inert ones oversaturate water. The symptoms of gas emboli include the rupture of tissues, enlarging of swim bladder, disturbances of circulatory system, and a number of other pathological changes.
Field and experimental studies support the previously described general pattern of fish response to the presence of methane and its hamologues in the environment. In the Sea of Asov, researchers conducted detailed observations after accidental gas blowouts on drilling platforms during summer-autumn of 1982 and 1985 [ GLABRYBVOD, 1983; AzNIIRKH, 1986]. The results of these observations indicate the existence of a cause-effect relationship between mass fish mortality and large amounts of natural gas input into the water after accidents.
It was found that the fish in the zones of the accidents developed significant pathological changes. In particular, they displayed impaired movement coordination, weakened muscle tone, pathologies of organs and tissues, damaged cell membranes, disturbed blood formation, modifications of protein synthesis, radically increased total peroxidase activity and some other anomalies typical of acute poisoning of fish. These pathological changes were found even in the fish collected at a considerable distance from the place of accident.
Besides the ichthyotoxicological data, studies on gas accidents in the Sea of Asov give some idea about the methane pollution of the water environment and its possible impact on the benthic and pelagic communities. Methane represented over 95 per cent of the released gas. It was present in water in concentrations of 4-6 mg/1 directly near the accidental well and in concentrations of 0.07- 1.4 mg/1 at a distance of 200 metres from the platform. These results suggest that methane and its homologues can stay in the water environment for a rather long period and spread over considerable distances. Similar conclusions were made based on observations in the Gulf of Mexico, where the areas around offshore drilling rigs had extremely high concentrations of methane and ethane in the water [Sackett, Brooks, 1975].
The composition of natural gas varies. It depends on the origin, type, genesis and location of the deposit, geological structure of the region and other factors. Natural gas chiefly consists of saturated aliphatic hydrocarbons, i.e., methane and its homologues. The deeper the location of gas deposit, the higher the number of methane homologues. In gas condensate fields, the content of methane homologues is usually considerably higher than the level of methane. In gases associated with oil, the content of methane homologues is comparable with the content of methane.
Other components commonly found in natural gas are carbon dioxide, hydrogen sulfide, nitrogen and helium. Usually, they constitute an insignificant proportion of natural gas composition. However, in some areas, their concentrations can be considerably higher. The global consequence of all these anthropogenic impacts is the gradual increase of methane concentration in the atmosphere over the last 100 years -- from 0.7x10-4 per cent to 1.7x 10-4 per cent (in volume). Many scientists believe that gases released due to human activities have already begun to affect the earth's overall temperature and the methane anthropogenic emission is responsible for about 30 per cent of the total warming effect. If the concentrations of methane and other greenhouse gases in the atmosphere keep increasing, global changes in climatic conditions on the earth will be noticeable in the near future.
Another component of natural gas -- hydrogen sulfide -- is water soluble in contrast with methane. It can cause hazardous pollution situations in both the atmosphere and the water environment. Its proportion in the composition of natural gas and gas condensate, as previously mentioned, sometimes reaches more than 20 per cent. Pollution by hydrogen sulfide can lead to disturbances in the chemical composition of surface waters. This gas belongs to the group of poisons with acute effects. Its appearance in the atmosphere and hydrosphere can cause serious economic damage and medical problems among local population. For further instance, in Russia, air, soil, and water pollution by hydrogen sulfide and sulfur dioxide has been reported in a number of places.
Especially severe consequences for human health and biota have been observed in the basin of the low Volga River in the zone of development of the Astrakhanskoe gas condensate field [Ecology and Impact of Natural Gas on Organisms, 1989].
Another potential source of gas in the hydrosphere is damaged gas pipeline, both on the seafloor and on land where they cross over rivers and other water bodies. The causes of such damage can vary from corrosion processes to natural disasters ( severe ice conditions, seismic activity and earthquakes), it should be noted that hydrocarbon gases are piped over great distances totaling many thousands of kilometers. These pipelines cross hundreds of water bodies. Possible pipeline damages can lead to hazardous impacts on water ecosystems.
Drilling, transportation and storage accidents are the sources of environmental pollution at all stages of gas or oil exploration or production. Although the causes, scale and severity of the accident consequences are extremely variable. They depend on a concrete combination of many natural, technical and technological factors. To a certain extent, each accidental situation develops in accordance with its unique scenario.
The most typical causes of accidents include equipment failure, personnel mistakes and extreme natural impacts (seismic activity, ice fields, hurricanes and so on). Their main hazard is connected with the spills and blowouts of oil, gas and numerous other chemical substances and compounds. The environmental consequences of accidental episodes are especially severe, sometimes dramatic, when they happen near the shore, in shallow water or in areas with slow water circulation.
Complex and extensive systems of underwater pipelines have a total length of thousands of kilometers. They carry gas or oil, condensate and their mixtures. These pipelines are among the main factors of environmental risk during offshore oil developments, along with tanker transportation and drilling operations. The causes of pipeline damage can differ greatly. They range from material defects and pipe corrosion to ground erosion, tectonic movements on the bottom and encountering ship anchors and bottom trawls. From the statistical data it has been found that the average probability of accidents occurring on the underwater main pipelines of North America and Western Europe are, respectively, 9.3x10-4 and 6.4x10-4. The main causes of these accidents are material and welding defects [Sakhalin-1,1994].
Although the modern technology of pipeline construction and exploitation under different natural conditions, including the extreme ones, achieved indisputable successes, however, pipeline gas or oil transportation does not eliminate the possibility of serious accidents and other consequences.
Altamas Pasha is a freelance journalist and researcher
Source: The Daily Star, Dhaka, February 15, 2002
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