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Estuaries in NSW
Estuarine Water Quality
Salinity
An obvious characteristic of estuarine waters is their saline nature, formed by mixing of seawater and freshwater (see Physical Characteristics - Salinity Behaviour). Seawater consists of a dilute solution of a mixture of salts. Table 1 shows the ionic composition of seawater, which is remarkably constant throughout the oceans and seas of the world. The term "salinity" refers to the total concentration of salts, and has a worldwide average of about 35 kg/m3, or 35 parts per thousand (ppt). The two most common constituents are the chloride ion (55% of total ion mass) and the sodium ion (30% of total ion mass). Salinity levels along an estuary vary in response to both the degree of dilution of seawater by freshwater and the concentration of salts in the freshwater. The loss of water by evaporation from poorly flushed saline marshes and tidal lakes can result in hypersaline conditions (salinities greater than seawater).
Table 1 Average composition of dissolved constituents in seawater Concentration (mg/l) Major Ions chloride Cl- 19,340 sodium Na+ 10,770 sulphate SO42- 2,712 magnesium Mg2+ 1,294 calcium Ca2+ 412 potassium K+ 399 bicarbonate HCO3- 140 bromide Br- 65 strontium Sr+ 9 Trace Elements Boron B 4.5 Silicon Si 5.0 Fluoride F 1.4 Nitrogen N 0.25 Phosphorus P 0.035 Molybdenum Mo 0.011 Zinc Zn 0.005 Iron Fe 0.003 Copper Cu 0.003 Manganese Mn 0.002 Nickel Ni 0.002 Aluminium Al 0.001 Total 35,141 source: Day et al, 1989
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