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

Estuaries in NSW

Estuarine Water Quality

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pH

The acidity or alkalinity of estuarine waters is measured in terms of pH on a scale of 0 to 14. Neutral waters, i.e. waters in which acidity and alkalinity are in balance, have a pH of 7. Acid waters have a pH of less than 7, the lower the pH the stronger the acidity. Alkaline waters have a pH of greater than 7, the higher the pH the stronger the alkalinity. The range of pH in natural waters varies from about 4.5 for acidic, peaty, upland freshwaters to around 10 for waters supporting intense photo-synthetic activity. Most natural waters have a pH in the range 6.5 to 8.5.

Seawater contains a small but significant amount of dissolved bicarbonate ions and dissolved carbon dioxide (see Table C1). This material forms a "buffering" system which tends to maintain the pH of seawater within the slightly alkaline range 7.8 to 8.3. The dilution of seawater in estuaries reduces its buffering capacity, but pH still generally remains in the range 7.5 to 8.8 (Day et al, 1989).

Abnormal pH in an estuary can have a number of detrimental effects on the estuarine ecosystems, both direct and indirect. As pH levels increasingly depart from "normal" values, fish suffer a number of ill effects, which eventually lead to death. A range of pH values of 5.0 to 9.0 is considered allowable for fish, but a range of 6.5 to 8.5 is preferable.

The pH level also affects the concentration and activity of a variety of chemical substances detrimental to wildlife, eg cyanide, ammonia, heavy metals and chlorine. Cyanide toxicity to fish increases as pH falls (i.e. as the water becomes more acid), whereas ammonia becomes more toxic as pH increases.

Acid sulphate soils, which are discussed elsewhere, are one prominent source of the acid pollution of estuarine waters in New South Wales (SPCC, 1990).

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