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Oil Pollution of the Sea

by Stanislav Patin, translation by Elena Cascio
based on "Environmental Impact of the Offshore Oil and Gas Industry"

Below you will find information on oil pollution of the sea, including sources and volumes of oil input into the marine environment. Click on the links at the end of this page if you want to learn more about Environmental Impact of the Offshore Oil and Gas Industry.

Oil pollution of the marine environment

Tables 1 and 2 show the variety of oil pollution sources and give expert estimates of the scales of distribution and impact of each of these sources on the marine environment. Even though these estimates can vary up to 1-2 orders of magnitude (especially in cases of natural oil sources, atmospheric input, and river runoff), many experts agree that the main anthropogenic flows of oil pollution into the marine environment come from land-based sources (refineries, municipal wastes, river runoff, and so on) and transportation activity (tanker oil transportation and shipping). Enough evidence exists to support this opinion [NRC, 1985; GESAMP, 1990; 1993]. Polycyclic aromatic hydrocarbons (PAHs), especially benzo(a)pyrene, enter the marine environment mostly due to atmospheric deposition (Neff, 1979).

Table 1. Sources and scale of oil pollution input into the marine environment

Note: +, -, and ? mean, respectively, presence, absence, and uncertainty of corresponding parameters.

Types and Source of Input
Scale of Distribution and Impact
Hydrosphere Atmosphere Local Regional Global
Natural seeps and erosion of bottom sediments
+ - + ? -
Biosynthesis by marine organisms + - + + +
Marine oil transportation (accidents, operational discharges from tankers, etc.)
+ - + + ?
Marine non-tanker shipping (operational, accidental, and illegal discharges) + - + ? -
Offshore oil production (drilling discharges, accidents, etc.) + + + ? -
Onland sources: sewage waters + - + + ?
Onland sources: oil terminals + - + - -
Onland sources: rivers, land runoff + - + + ?
Incomplete fuel combustion - + + + ?

Table 2. Estimates of global inputs of oil pollution into the marine environment (thousands tons/year of oil hydrocarbons)

Note: * - [NRC, 1985]; ** - [Kornberg, 1981]; *** - [GESAMP, 1993]

Source 1973* 1979** 1981* 1985*** 1990***
Land-based sources:
Urban runoff and discharges





1,175 (50%)

Coastal refineries 200 60 100
- -
Other coastal effluents - 150 50
- -
Oil transportation and shipping:
Operational discharges from tankers





564 (24%)

Tanker accidents 300 300 400
- -
Losses from non-tanker shipping 750 200 320
- -
Offshore production discharges 80 60 50
2% 47 (2%)
Atmospheric fallout 600 600 300
10% 306 (13%)
Natural seeps 600 600 200
8% 259 (11%)
Total discharges 6,110 4,670 3,200 100% 2,351

Table 2 illustrates the general trend of declining total input of oil pollution into the World Ocean over the years. The global situation reflected in this table certainly may differ at the regional level. This depends on natural conditions, degree of coastal urbanization, density of population, industrial development, navigation, oil and gas production, and other activities. For example, in the North Sea, offshore production input reached up to 28% of the total input of oil pollution in 1987 (see GESAMP, 1993) instead of a "modest" 2% on the world scale shown in Table 2. This equaled the annual input of more than 23,000 tons of oil products at the background of their general changeable flow of 120,000-200,000 tons a year in the North Sea [Bruns et al., 1993]. One can expect similar situations in other regions of intensive offshore oil and gas developments, for example, in the Gulf of Mexico, Red Sea, Persian Gulf, or Caspian Sea. Just remember the persistent pollution in oil production areas in the Caspian Sea or the amounts of annual discharges (about 40 million tons of produced waters polluted by oil products) during offshore drilling in the Gulf of Mexico [Anonymous, 1993]. At the same time, no reliable balance estimates exist for these regions.

The continental shelf of the Gulf of Mexico is also distinctive for intense seepage of natural liquid and gaseous hydrocarbons. Some authors [Kennicutt et al., 1992] believe that this can lead to formation of oil slicks and tar balls on the sea surface, which makes assessing and identifying anthropogenic oil pollution more difficult. In any case, the input of oil hydrocarbons from natural sources into the Gulf of Mexico is larger than in many other areas.

In the Baltic Sea, the Sea of Asov, and the Black Sea, the leading role in oil input most likely belongs to land-based sources, which are dominated by river inflow. The Danube River alone annually brings to the Black Sea about 50,000 tons of oil, half of the total oil input of about 100,000 tons [Konovalov, 1995].

Traditional shipping and oil transportation routes are more exposed to the impacts of oil-polluted discharges from tankers and other vessels than other areas. For example, observations in the Caribbean basin [Atwood et al., 1987; Jones, Bacon, 1990; Corbin, 1993], where annually up to 1 million tons of oil enter the marine environment, showed that about 50% of this amount came from tankers and other ships [Hinrichsen, 1990]. In the Bay of Bengal and the Arabian Sea, oil pollution inputs from tanker and other ship discharges equal, respectively, 400,000 tons and 5 million tons of oil a year [Hinrichsen, 1990]. The most intense tanker traffic exists in the Atlantic Ocean and its seas, which accounts for 38% of international maritime oil transportation. In the Indian and Pacific Oceans, this portion is, respectively, 34% and 28% [Monina, 1991].

Enforcing stricter requirements to activities accompanied by oil discharges led to global declining of oil pollution inputs in the marine environment mentioned above [GESAMP, 1993]. In 1981, oil transportation and shipping in general were responsible for discharging about 1.4 million tons of oil products. This amount was reduced to 0.56 million tons in 1990 (see Table 20). The reduction mainly occurred as a result of adopting stricter international regulations concerning transportation operations in the sea (International Convention for Prevention of Pollution from Ships and others). The total oil pollution input into the sea during the same period, according to the estimates given in Table 20, dropped from 3.20 to 2.35 million tons.

Although this tendency of decreasing oil pollution caused by tanker transportation and shipping gives some reason for environmental optimism, two alarming circumstances should not be neglected. One of them has already been mentioned. Strikingly high volumes of oil's input are reported for some regions (e.g., the Caribbean basin, northern part of the Indian Ocean, Mediterranean Sea). These volumes may total hundreds of thousands or even millions of tons of oil [Hinrichsen, 1990]. They are directly connected with highly intensive shipping and tanker transportation in these areas. Some estimates indicate that annual oil pollution input into the marine environment may reach 7.3 million tons [Panov et al., 1986; GESAMP, 1994]. Other researches give even higher figures. For example, data summarized by S.M. Konovalov [Konovalov, 1995] suggest that global oil input into the World Ocean reaches 20 million tons a year, and pollution caused by tankers accounts for 50% of it. Note that annually about 6,500 large tankers transport more than 1.2 billion tons of oil and oil products. In spite of the fact that the latter estimates are considerably higher than the ones based on official statistics (Table 2), they have not been refuted thus far. This raises serious concerns about the actual levels of oil pollution in different marine regions and in the World Ocean in general.

The other circumstance that can affect the tendency of decreasing oil pollution from tankers involves accidental spills. Accidental situations during oil tanker transportation repeatedly happened in the past. Remember, for example, two relatively recent tanker accidents, the Exxon Valdez and the Braer, that spilled 40,000 tons of oil into Alaskan waters in 1989 and 85,000 tons near the shore of the Shetland Islands in 1993, respectively. The probabilistic nature of accidental situations and highly variable volumes of spilled oil do not allow definite conclusions to be made. Although the level of oil pollution has tended to decrease, large volumes of spilled oil could change this situation.

A number of dramatic events show the vulnerability of making optimistic prognosis about decreasing oil pollution at the regional and global levels. For instance, catastrophic large-scale events took place in the Persian Gulf during and after the 1991 Gulf War. Between 0.5 and 1 million tons of oil were released into the coastal waters. Besides, products of combustion of over 70 million tons of oil and oil products were emitted into the atmosphere [Fowler, 1993]. Another large-scale accident occurred in Russia in September-November 1994. About 100,000 tons of oil were spilled on the territory of the Komi Republic. This threatened to cause severe oil pollution for the basin of Pechora River and, possibly, the Pechora Bay.

It must be remembered that catastrophes, in spite of the obvious consequences and all the attention they attract, are inferior to other sources of oil pollution in their scales and degree of environmental hazard. Land-based oil-containing discharges and atmospheric deposition of products of incomplete combustion can accordingly give 50% and 13% of the total volume of oil pollution input into the World Ocean (see Table 2). These diffuse sources continuously create relatively low but persistent chronic contamination over huge areas. Many aspects of chemical composition and biological impacts of these contaminants remain unknown.

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Waste discharges - sources, types and volumes of waste discharges during the offshore oil and gas activity are discussed. Chemical composition of discharged wastes (drilling muds, drilling cuttings and produced waters) is described. Atmospheric emissions and their impact on the marine environment are considered.

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