Understanding Salinity
Changes in landuse, seasonal variations in our weather and longer-term changes to climate can all affect surface water, groundwater, the flows between them, and the amounts of salt that they contain.
The term "salinity" refers to the concentrations of salts in water or soils. Salinity can take three forms, classified by their causes: primary salinity (also called natural salinity); secondary salinity (also called dryland salinity), and tertiary salinity (also called irrigation salinity).
Small amounts of dissolved salts in natural waters are vital for the life of aquatic plants and animals; higher levels of salinity alter the way the water can be used {see Salinity Classification Table}, yet even the most hypersaline water can be used for some purposes. However, high levels of salinity and acidity (if present) are harmful to many plants and animals.
Where does the salt come from?
Salt in our water resources is generally derived from three sources. Firstly, small amounts of salt (primarily sodium chloride) are evaporated from ocean water and are carried in rainclouds and deposited across the landscape with rainfall.
Secondly, some landscapes may also contain salt that have been released from rocks during weathering (gradual breakdown), and thirdly, salt may remain in sediments left behind by retreating seas after periods where ocean levels were much higher or the land surface much lower.
Salt concentrations in rainfall are higher near the coast, and decrease as one moves inland. Depending on rainfall and other factors, between about 3 and 360 kg of salt per hectare are deposited each year across Western Australia (Hingston FJ & Gailitis 1976 'The geographic variation of salt precipitated over Western Australia' in Australian Journal of Soil Research, Vol 14, pp 319–335).
Primary salinity (also called natural salinity)
Primary salinity is caused by natural processes such the accumulation of salt from rainfall over many thousands of years or from the weathering of rocks.
When rain falls on a landscape, some evaporates from soil, vegetation surfaces and water bodies, some infiltrates into the soil and the ground water, and some enters streams and rivers and flows into lakes or oceans. The small amounts of salt brought by the rain can build up in soils over time (especially clayey soils), and can also move into the groundwater.
In areas that receive a lot of rain, the large amounts of water infiltrating soils, entering and discharging from groundwater, and leaving the catchment through streams and rivers provide a flushing effect, such that the soil and groundwater salinities stay relatively fresh.
However, in drier areas with natural vegetation, there is not so much flushing and a larger proportion of the water that falls on a landscape is lost through evaporation and transpiration from plants. Here, the salts tend to build up in the soil and groundwater and can accumulate over long time periods to reach high levels. Groundwater salinities can also be very high, particularly if salts have also been released in weathering of the bedrock.
Lake Johnson, a naturally-formed salt lake in the Great Western Woodlands of South-Western Australia. The salt lake formed by saline groundwater approaching the ground surface, and is dry for most of the year, except following rains or when groundwater levels rise above the lake's ground level. Photograph by Keren G. Raiter.
Secondary or dryland salinity
Secondary salinity is caused where groundwater levels rise, bringing salt accumulated through 'primary' salinity processes to the surface. This is caused by clearing of perennial (long-lived) vegetation in drier areas; i.e. areas that tend to accumulate salt in the soil profile and groundwater over time. When vegetation is cleared, as happened extensively in the Western Australian wheatbelt, the amount of water lost from the landscape through plants is drastically reduced. Instead, more water enters the groundwater and groundwater levels rise.
As groundwater levels rise, they bring with them the salt that is in the groundwater, and also dissolve the salt in the previously unsaturated part of the soil profile. Eventually, low lying areas of valley floors may become fully saturated (especially during winter) and the amount and duration of flow in streams and rivers increases. The discharging saline groundwater mixes with the fresher surface water to cause flows which vary between marginal to brine. As these saturated areas dry out after the wet season, salt crystals can be left behind, causing a salt scald. The state government has conducted experiments to understand the impacts of land use change and undertaken engineering works in controlled conditions to develop feasible solutions to manage the effects of stream salinity.
Increased salinity and flow in streams and wetlands is likely to make an issue of the salt tolerance of vegetation. Many plants tolerate higher salinities for short periods, but cannot survive long periods of inundation as well (Barrett-Lennard EG 2003 'The interaction between waterlogging and salinity in higher plants: causes, consequences and implications'. Plant and Soil Vol 253, pp 35-54).
Salinisation of streams and rivers can threaten ecosystems and their constituent species, and may render the water unusable for human users. The salinity classification table below shows the thresholds for which water is considered fit for public drinking water supply, irrigation and industry.
Doradine Creek (2003), a naturally-formed creek in the Dumbleyung Catchment of South-Western Australia. Crusted salt is seen on the banks and flows are from deep drains upstream intersecting saline groundwater.
Photograph by DoW.
Salinity status classifications, by total salt concentration
Salinity status | Salinity (milligrams of salt per litre) | Description and use |
Fresh | < 500 | Drinking and all irrigation |
Marginal | 500 –1 000 | Most irrigation, adverse effects on ecosystems become apparent |
Brackish | 1 000 – 2 000 | Irrigation certain crops only; useful for most stock |
Saline | 2 000 – 10 000 | Useful for most livestock |
Highly saline | 10 000–35 000 | Very saline groundwater, limited use for certain livestock |
Brine | >35 000 | Seawater; some mining and industrial uses exist |
Classifications from Mayer, XM, Ruprecht, JK & Bari, MA 2005, Stream salinity status and trends in south-west Western Australia, Department of Environment, Salinity and land use impacts series, Report No. SLUI 38
Lake Eganu (2001), a naturally-formed salt lake in the Moore Catchment of South-Western Australia. The salt lake formed in low part of valley floor and receiving runoff from a mainly cleared Marchagee catchment and saline groundwater discharge.
Photograph by Peter Muirden.
Tertiary or irrigated salinity
Tertiary salinity occurs when water is reapplied to crops or horticulture over many cycles, either directly or by allowing it to filter into the groundwater before pumping it out for re-application. Each time the water is applied, some of it will evaporate and the salts in the water remaining will become more concentrated; very high salt concentrations can result from multiple cycles of reuse.
