There are two types of salinity—dryland salinity and irrigation salinity. Dryland salinity occurs when native perennial vegetation is replaced by shallow rooted crops and grazing activities. The amount of rain taken up by plants is dramatically reduced, and so the water table rises, bringing with it salt stored deep in the soil. The same process occurs for irrigation salinity, induced by heavy irrigation, not rainfall.
The salinity crisis currently facing South Australia has been well publicised, with large areas of agricultural land lost to high concentrations of surface and sub-surface salt. Salinity is also an issue for Adelaide's drinking water supplies. Furthermore high salt concentration in water causes the deterioration of pipes and other infrastructure, increasing community costs.
The effect of salinity on the environment is widespread. Individual plants may be replaced by salt-tolerant species, while animals may be lost as their food source disappears; ultimately, entire ecosystems can change. The surface movement of saline water across the landscape increases sediment erosion through the breakdown of the soil structure. Similarly, saline groundwater can seep into rivers affecting water quality. In general, increasing salinity leads to a reduction in biodiversity and an increase in the prevalence of more salt tolerant species.
Salinity in inland waters
Salinity is a measure of how much dissolved salts are in the water. It is also called total dissolved solids or total dissolved salts.
Dissolved salts are usually sodium and chloride ions, although there can also be many others such as potassium and bicarbonate ions. In South Australia, inland waters such as rivers, streams and lakes can naturally have a wide range of salinities due to evaporation and saline groundwater inflows.
Salinity can vary during the year due to rain diluting the salt in the water. Therefore, high salinity is usually recorded in the summer and low salinity in the winter. As a general rule, salinity is relatively low during periods of high flow and vice-versa.
Salinity ranges from fresh to hyper-saline as indicated in the table below.
|up to 1,000||Fresh|
|1,000 to 3,000||Fresh to brackish|
|3,000 to 5,000||Brackish|
|5,000 to 35,000||Saline|
|35,000 and above||Hyper-saline|
The salinity in a watercourse influences aquatic ecosystems and the water's suitability for agricultural uses such as irrigation and livestock drinking water. However, salinity is not classified (as good, moderate or poor) by reference to guideline values because there are confounding issues that can cause misinterpretation. These issues are explained briefly below.
Salt is a natural part of the Australian landscape and a number of plants and animals inhabiting rivers and wetlands are adapted to it. Salt enters aquatic systems dissolved in rain and from a number of other sources such as groundwater, or erosion of sediments (eg weathering, microbial activity). Under natural flow conditions, periods of low flow result in salts being concentrated in wetland and riverine pool habitats. The plants and animals in these ecosystems survive increasing salinity by either tolerating or avoiding it.
It is widely accepted that many of Australia's freshwater ecosystems are becoming degraded by increasing salinity; a result of rising saline groundwater and modifications to the water regime. Available data indicates that aquatic organisms are adversely affected when salinity exceeds 1,000 mg/L. Salinities between 1,000-5,000 mg/L reduce species richness and aquatic plant abundance, zooplankton and macroinvertebrate populations. Freshwater species are generally restricted to salt levels of less than 3,000 mg/L. It is now widely recognised that greater salinity will progressively lead to a reduction in diversity of wetlands and rivers, and see the dominance of saline tolerant animals and plants that can cope with high salt concentrations. >> More
The taste of drinking water is rated according to salinity as follows (Australian Drinking Water Guidelines):
|0 - 600||good|
|600 - 900||fair|
|900 - 1,200||poor|
|> 1,200||unacceptable (unpalatable)|
Note also that water with extremely low salinity may taste flat and insipid.
The salinity tolerance of crops varies from as low as around 360 mg/L (for some sensitive vegetable crops) up to several thousand mg/L (eg barley). More detail on crop salinity tolerances can be obtained from Primary Industries SA.
For livestock drinking a salinity guideline of 2,000 mg/L is recommended for poultry, 2,400 mg/L for dairy cattle and 4,000 mg/L for beef cattle, sheep and horses. More detail on livestock salinity tolerance can be obtained from Primary Industries SA.