How does petroleum (oil products) become a pollutant in the coastal and marine environment?

Accidental or deliberate, operational discharges and spills of oil from ships, especially tankers, offshore platforms and pipelines, is the most obvious and visible cause of oil pollution of the marine environment. As summarized by NOAA: "The kind of oil spill we usually think about is the accidental or intentional release of petroleum products into the environment as result of human activity (drilling, manufacturing, storing, transporting, waste management). Examples would be things like well blowouts, pipeline breaks, ship collisions or groundings, overfilling of gas tanks and bilge pumping from ships, leaking underground storage tanks, and oil-contaminated water runoff from streets and parking lots during rain storms".

However, oils enter the ocean from a variety of sources, and both natural sources (large quantities) and land-based sources account for a large part of the total annual input of oil to the marine environment.

Also, hydrocarbons enter the ocean not merely as "wet" oil products but also as gaseous air pollutants. Hydrocarbons from vapours deriving from the loading and unloading of oil at different stages from extraction to consumption, in the form of non-methane volatile organic compounds (nmVOCs), is one example. Polycyclic aromatic hydrocarbons (PAHs) from incomplete combustion (exhaust gases and flue gases) is another category of gaseous hydrocarbons that enter the marine environment as oil pollution.


In a report published in 2002 by the National Research Council (NRC) of the U.S. National Academy of Sciences, the average total worldwide annual release of petroleum (oils) from all known sources to the sea has been estimated at 1.3 million tonnes. However, the range is wide, from a possible 470,000 tonnes to a possible 8.4 million tonnes per year. According to the report, the main categories of sources contribute to the total input as follows:

  • natural seeps: 46%
  • discharges from consumption of oils (operational discharges from ships and discharges from land-based sources): 37%
  • accidental spills from ships; 12%
  • extraction of oil: 3%

The Australian Petroleum Production and Exploration Association (APPEA) claims the following distribution of the inputs from different sources:

  • Land-based sources (urban runoff and discharges from industry): 37%
  • Natural seeps: 7%
  • The oil industry - tanker accidents and offshore oil extraction: 14%
  • Operational discharges from ships not within the oil industry: 33%
  • Airborne hydrocarbons: 9%

In a report in 1993, the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) estimated a total input of oils at 2.3 million tonnes per year and ranked the sources like this:

  • Land-based sources (urban runoff, coastal refineries): 50%
  • Oil transporting and shipping (operational discharges, tanker accidents): 24%
  • Offshore production discharges: 2%
  • Atmospheric fallout: 13%
  • Natural seeps: 11%
In a report published in 1980, the total input of oil to the ocean was estimated at 3,2 million tonnes. Half of that amount (1.5 million tonnes) was estimated to come from vessels (about 1.2 million tonnes of which from operational, deliberate discharges). The source "discarded lubricants" (from both sea-based and land-based sources?) was estimated to account for about 1.3 million tonnes. As pointed out in a recent comment to these figures, "oil pollution from ships probably reached its peak in 1979.  Despite the publicity that oil spills always attract, even in 1979 only a small fraction of the oil entering the sea came from tanker accidents. Most came from routine operations, and discarded lubricants – such as engine oil poured into drains – accounted for a much higher percentage of the total. Since 1979, the amount of sea getting into the sea as a result of shipping operations has declined dramatically."

Sources of oil input to the marine environment are often divided into natural, sea-based and land-based sources. In the NRC report, the perspective of "following the oil" is used, with four main categories of sources: discharges through natural seeps, discharges during the extraction of oil, discharges during the transportation of oil, and discharges during the consumption of oil (including both sea-based and land-based sources). There are also other ways of placing accidental or operational/deliberate discharges of oils into different main categories.

NATURAL • Natural seeps

SEA-BASED • [Operational discharges] • [Accidental discharges] • [Air pollution]

  • Accidental oil spills from tankers; other commercial vessels; grounded and abandoned vessels; oil platforms (blowouts); pipelines.
  • Deliberate, operational discharges of oil from all kinds of commercial vessels (ship- or cargo-related discharges);oil platforms; pipelines.
  • Emissions of nmVOCs and PAHs from tankers and pleasure craft, and from oil extraction.
  • Other ship-related activities (dry docking, scrapping).
  • Other activities (dumping of oily waste, etc.)


  • Discharges of untreated or insufficiently treated municipal sewage and storm water (urban runoff).
  • Discharges with rivers.
  • Discharges of untreated or insufficiently treated waste water from coastal industries.
  • Accidental or operational discharges of oil from coastal refineries, oil storage facilities, oil terminals, and reception facilities.
  • Emissions of gaseous hydrocarbons from oil-handling onshore facilities (terminals, refineries, filling stations) and from vehicles exhausts (traffic).

  • U.S. National Academy of Sciences: Oil in the sea III: Inputs, fates and effects. Report 2002 by the National Research Council (NRC) Committee on Oil in the Sea: Inputs, Fates, and Effects. • See also U.S. National Academies press release about the conclusions in the NRC Report. See also U.S. National Academy's Web Extra on Oil (including summary of sources of oil). • See also references to the figures published in the 1985 NRC report: on the Ocean Planet Exhibition web site, and on the web page Oil in the sea: About offshore oil and gas. (U.S.) National Ocean Industries Association.
  • U.S. NOAA: General oil spill questions (FAQs). U.S. National Oceanic and Atmospheric Administration (NOAA) Office of Response and Restoration.
  • APPEA: Discovery: Explore the world of oil and gas: Oceans and oil spills. Australian Petroleum Production and Exploration Association (APPEA).
  • GESAMP: "Impact of oil and related chemicals and wastes on the marine environment". GESAMP Report 50, 1993. Not available online, but the figures referred to can also be found in the online article "Oil pollution of the sea" in the book Environmental Impact of the Offshore Oil and Gas Industry).
  • UN Atlas of the Oceans: "The impact of marine pollution". Report (1980) by Douglas J. Cuisine and John P. Grant. Table published on the UN Atlas of the Oceans web site.