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

Estuaries in NSW

Estuarine Water Quality

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Suspended Solids

Particulate material (i.e. non-dissolved matter) in the water mass is present as particles of various sizes. Under quiescent conditions, eg in a laboratory test flask, the larger sized particles will settle out. This fraction is referred to as "settle-able solids". However, small-sized particles, typically clays or particles of organic origin, will remain in suspension, even after extended times of settlement. This fraction is referred to as "colloidal solids".

In an estuary, settle-able solids are kept in suspension by the water turbulence generated by tidal and freshwater flows. Some colloidal solids, such as clay particles, tend to settle out in estuarine waters because of the effects of salinity in promoting flocculation. This occurs within the salinity range 0 to 5 parts per thousand (Day et al, 1989).

Suspended solids in estuarine waters can originate from a variety of sources. Settle-able solids can be present as sand and coarse silt particles; colloidal solids can be present in the form of phytoplankton (microscopic aquatic plants); as fine silts and clays transported into an estuary via urban and rural stormwater runoff or in discharges of washings from sandpits, quarries and mines; and as decaying organic matter that originates from leaf litter, untreated sewage discharges, eutrophic conditions in the estuary and from certain industrial discharges.

Suspended solids can have a number of detrimental effects in estuaries. High levels of suspended solids reduce light penetration into the waterbody and hence limit photosynthetic activity and areas colonised by aquatic plants. Suspended solids concentrations as low as 200 mg/L can kill susceptible fish species by abrasive action on their gill tissues; suspended solids that settle can smother plants and benthic organisms. Finally, suspended solids make water look "cloudy", aesthetically unattractive, and dangerous for boating and swimming if submerged obstacles are present.

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