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Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates. Salinity is the measure of total dissolved solids (TDS) or salts in water and is reported as milligrams per litre (mg/L). On average, seawater in the world’s oceans has a salinity of about 35,000 mg/L TDS.
The salinity of groundwater in Western Australia varies considerably. The Department of Water and Environmental Regulation categorises the groundwater salinity according to the salt content and its application for public drinking, irrigation, stock water etc.
The range of salinity of natural water is:
Category Salinity range
Fresh 0-500 mg/L TDS
Marginal 500-1000 mg/L TDS
Brackish 1000-3000 mg/L TDS
Saline 3000-35,000 mg/L TDS
Hypersaline >35,000 mg/L TDS
Fresh and marginal groundwater, of a salinity of less than 1000 mg/L TDS, is used for public water supply and selected agriculture. Brackish groundwater of salinity 1000-3000 mg/L TDS is used for parkland irrigation, and the more saline groundwater with salinity greater than 3000 mg/L is used for industry and stock watering (up to 10,000 mg/L TDS).
It is unsafe for people to drink untreated water drawn from surface water bodies and shallow groundwater. Please refer to Department of Water’s Water quality protection note 41 (available as a publication on our website) for more details.
The Geological Survey of Western Australia produced a state map at 1:2 500 000 scale in 1989 together with a state groundwater salinity map that is very generalised. The Department of Water and Environmental Regulation has produced a large number of more detailed groundwater salinity maps. The most detailed maps cover the Perth region.
The salinity contours (or isohalines) are based on the best available groundwater salinity data from bores. The salinity of groundwater is highly variable and depends on the precise location, seasonal influence, depth of the measuring bore, proximity to a saltwater interface, abstraction, and level of groundwater contamination.
The Department of Water and Environmental Regulation advises against drilling garden bores in the areas underlain by a saltwater interface. If garden bores are sunk near the saltwater interface wedge, these bores should be as shallow as possible to maintain an adequate supply without drawing up brackish to saline groundwater. If bores located in these areas are too deep or excessively pumped, the groundwater may become progressively more saline and unsuitable for garden irrigation.