Salinity is a measure of the content of salts in soil or water. Salts are highly soluble in surface and groundwater and can be transported with water movement. Large salt deposits are a natural feature of vast areas of the Australian landscape, stored deep in soils or as surface salt deposits and salt lakes. This natural distribution of salt in the landscape is referred to as ‘primary salinity’.
Photo by Arthur Mostead
In normal circumstances, the deep roots of native plants absorb most water entering the soil before it reaches the salt contained in groundwater below the plant root zone. However, widespread vegetation clearance, poor land use, irrigation and industrial practices have made it easier for salt to be transported to the soil surface or to waterways. The additional salt from these altered land use and management practices is referred to as ‘secondary salinity’.
Excessive amounts of dissolved salt in water can affect agriculture, drinking water supplies and ecosystem health. Salinity is a significant issue in south-western Australia and in some Murray-Darling Basin regions within New South Wales, Victoria and South Australia.
Causes of salinity
Primary salinity is produced by natural processes such as weathering of rocks and wind and rain depositing salt over thousands of years. Salt deposits are unevenly distributed throughout Australia and the patterns and impacts of salinity vary in eastern and western parts of the country because of different topography and the age of the landscapes: salinity in the west tends to be more pervasively spread across the landscape, whereas salinity in eastern regions is more localised.
Secondary salinity has occurred with widespread land clearing and altered land use, and may take the form of ‘dryland salinity’ or ‘irrigation-induced salinity’.
Dryland salinity occurs when deep-rooted native plants are removed or replaced with shallow-rooted plants that use less water. As a result of this vegetation imbalance, more water passes through soil to groundwater, raising the water table and bringing salt to the surface where it can be left behind as the water evaporates. Irrigation-induced salinity occurs when excess water applied to crops travels past the root zone to groundwater, raising the water table and salt to the surface. Salt may also be transported across groundwater systems.
Effects of salinity
High concentrations of salt pose hazards for the environment as well as affecting agriculture and infrastructure and, therefore, the wider economy.
High levels of salinity in water and soil may cause native vegetation to become unhealthy or die and lead to a decline in biodiversity through dominance of salt-resistant species, potentially altering ecosystem structures.
Reduced groundcover also makes soil more prone to erosion, which can pollute water with increased sediment, making it unsuitable for both human and animal consumption and threatening high value ecosystems and the plant and animal species they support.
Despite the negative effects of salinity, some aquatic environments have adapted to a range of salt concentrations.
Increased salinity can reduce crop yields when it impairs the growth and health of salt-intolerant crops and may result in corrosion of machinery and infrastructure such as fences, roads and bridges. These impacts of salinity can be extremely costly—ranging from impaired agricultural production and additional water treatment costs to the replacement of corroded civil and agricultural infrastructure.
Management of salinity
Due to the complex nature and scale of salinity, a mixture of management responses is usually required. Salinity has been a major land management concern in Australia and various measures have been introduced, including maintaining the health of wetlands, restoring vegetation cover with appropriate native species to control the surfacing of salt, planting crops that reduce drainage and establishing salt interception schemes to divert saline water to evaporation basins.
Affected jurisdictions have developed their own salinity management plans or strategies with identified objectives and priority areas for rehabilitation. Management plans specify the best treatments for the circumstances, which are then monitored and reviewed for effectiveness. Agricultural industries may benefit from employing more efficient farming, irrigation and drainage techniques, and redesigning the timing, volumes and locations of irrigation.
Australian Government action on salinity
National Water Quality Management Strategy includes guidance on salinity trigger values and outlines ways to reduce salinity and adapt irrigation practices, while recognising the difficulty of treating this extensive problem. All states and territories were involved in developing this national water quality framework and are committed to its ongoing implementation.
The Murray-Darling Basin Plan, being prepared by the Murray-Darling Basin Authority, will help ensure that salt concentration and load targets are met and that water quality will remain fit for purpose by flushing out salt with adequate water flows and modifying land management practices. These measures will have benefits for aquatic ecosystems, drinking water supplies and irrigation.
As part of Murray-Darling Basin reforms, the Australian Government is acquiring water entitlements with the objective of returning more water to the environment. These entitlements become part of the Commonwealth environmental water holdings and are managed so that increased flows are provided to rivers and wetlands. Environmental watering also helps to achieve more natural wetting and drying cycles, flushing out toxicants, improving water quality, and minimising exposure of soil to oxygen.
Photo by Dragi Markovic
Biodiversity: the range of interrelated plant and animal taxa and the habitat in which they live.
Ecosystem: a specific composition of animals and plants which interact with one another and their environment.
Groundwater: deposits of water below the land which fill spaces in soil and rock and often contribute to the health of surface water ecosystems.
Primary and secondary salinity: primary salinity occurs naturally and is the result of rainfall interacting with geographical features over thousands of years. Secondary salinity is the result of human land use and either produces more salt or causes primary salinity to rise to the surface of the land.
Salinity: a measure of the concentration of total dissolved salts in soil or water, mainly sodium chloride (common table salt).