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Home > Coastal and floodplains > Estuaries > Factsheets > Water Quality

Estuaries in NSW

Estuarine Water Quality

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Oils

Petroleum products can enter estuarine waters in urban runoff (from road surfaces), by wind and tidal action following a spill at sea or within an estuary, and as oil leaks occur from the motors of commercial and leisure boats. In addition, raw sewage discharge contains small, but significant levels of fats and oils (see Physical Characteristics - Movement of Surface Pollutants).

Oil spills in the estuarine environment are of concern because of the physical smothering of seagrass beds, mangroves, waterbirds and other animals, and because of the general detrimental impact of oil on estuarine use and amenity. In addition, about 1% of an oil mass consists of water-soluble fractions that are quite toxic. These fractions dissolve into the water mass as the slick spreads, but tend to be concentrated at the surface (Chapman, 1985). Finally, many of the dispersants used to "break up" oil slicks can create significant environmental problems in their own right.

Oil contains a number of different compounds which vary in boiling point, molecular weight and toxicity. Generally, those compounds with low molecular weights and low boiling points are the most toxic. These evaporate first so the toxicity of the oil mass itself tends to decrease noticeably a day or so after the oil has entered the receiving waters. However, these low molecular weight fractions are also the most soluble in water and so may become dispersed in the water surrounding the point of discharge. Oils from different sources have quite distinctive properties. For instance, refined petroleum products contain a higher proportion of the more volatile substances. The different characteristics of the various oil fractions will result in some fractions remaining on the surface as a slick, some forming emulsions (water-in-oil or oil-in-water), and some dissolving in the water.

Oil slicks can interfere with the respiration of aquatic insects. Oil and grease may also interfere with the transfer of oxygen from the air to the water body. Oil coatings damage aquatic plants and reduce the amount of food available for aquatic fauna. Surface oil adversely affects water birds, either directly causing death through smothering or loss of flight, or indirectly by destroying feeding beds.

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