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Salinisation of land and water is an accumulation of salt in soil and water brought about by physical and chemical processes.
Salinity is traditionally classified as primary or secondary salinity.
Primary salinity refers to naturally occurring salinity and includes playa lakes, saline lakes and the erosion of saline soils. Examples of primary salinity in Australia are Lake Eyre, Lake Amadeus and the playa lakes of Western New South Wales.
Secondary salinity has been proposed to refer to salinity induced by human activity in irrigated and non-irrigated (dryland) areas. Secondary salinity can also occur in urban areas, not just agricultural areas, and salts can cause damage to houses, concrete and roads.
Irrigation salinity is salinity resulting from the application of irrigation water without the possibility of drainage. As a result saline groundwater commonly rises within two metres of the ground surface and salts accumulate in the plant root zone or on the soil surface.
When the application of irrigation water to crops exceeds the plants transpiration and the soils evaporation and drainage capabilities, the excess water percolates through the soil and into the groundwater. This causes water tables to rise. If the water table rises within two metres of the soil surface groundwater is capable of moving up the soil profile via capillary action. Salts build up at the surface by the continuous upward flux of groundwater and evaporation processes.
During times of heavy precipitation events surface runoff exports salts to river systems. Continual application of irrigation water causes saline groundwater to discharge at the soil surface or as saline baseflow into surrounding rivers and creeks.
Dryland salinity is classified as seepages or scalds. Saline seepage results from the mobilisation and accumulation of salts at the surface caused by an increase in groundwater recharge following the clearance of natural vegetation. Salt scalding is the development of a hard impermeable surface on saline or sodic soils as a result of wind or sheet erosion (dry scald) or by surface sealing through deposition of salts and clays following evaporation of surface water (wet scald). Dry scalding does not involve groundwater.
A number of different models have been developed to explain how dryland salinity develops. The fundamental principle of all these models and the current axiom in dryland salinity studies is that the removal of the native vegetation and its replacement with shallow-rooted vegetation has altered natural hydrological regimes. This has resulted in an increase in groundwater recharge and a corresponding increase in groundwater discharge, bringing salts to the surface.
Many dryland salinity studies in recent years have challenged this theory and have shown that many other factors and processes are involved in development of this type of salinity. These studies have shown that there are different modes of dryland salinity resulting from the interaction of different mechanisms and factors. Factors such climate, geology, soils, geomorphology and vegetation can influence the development of salinity.
However, the three basic requirements for the development of any type or form of dryland salinity are:
1. A source of soluble salts
2. A source of water
3. A mechanism for salt redistribution
For instance, if there is no salt in the soil to be mobilised, rising water tables will no cause a salinity problem. Conversely, if there is salt present in the soil but water tables are declining, this salt will not create a dryland salinity problem.
So where does the salt come from?
The sources of salt involved in dryland salinity are: rainfall (cyclic salt), rock weathering, aeolian deposits and ancient seas (connate salts).
Cyclic salts are those salts which are included in water evaporated from oceans and deposited over coastal areas. Rainwater generally contains 10 to 30mg/l of salts. Studies have shown that in Australia the rainfall input of salts is as great as 300kg/ha/yr near the coast, about 30kg/ha/yr 250km inland and about 15kg/ha/yr more than 600km inland.
Salts from rainfall only become a problem when there is not enough precipitation to flush the deposited salts from the soil profile via leaching. The salts will accumulate and if they are subsequently released then they will become an environmental issue.
Connate salt is salt incorporated in marine sediments at the time of deposition. The sediments were deposited by inland seas millions of years ago that naturally contained large quantities of salts. For example, the Wiannamatta Shale Group of the Sydney Basin were deposited by a retreating ocean (marine regression) during the Tertiary. These shales contain salt from seawater trapped during the time of deposition. These shales provide the salts responsible for salinity outbreaks in Western Sydney.
Continued weathering of rocks will release salts. This is generally not a source for large quantities of salt because large amounts of water are required for weathering reactions to occur. This means that if there is a continual flux of water salts will be flushed through the system rather than accumulating.
Dryland salinity occurs on the Great Plains of North America, South Africa, Turkey, Thailand, India, Argentina as well as Australia. In Australia, all states have been afflicted by dryland salinity, with Western Australia the most affected state by far. In 1996, nearly two and half million hectares of the Australian continent were affected by dryland salinity and the potential dryland salinity hazard is far greater if current levels are not treated.
The Australian continent's susceptibility to salinity is a consequence of its variable climate and unique geology. The continent has been relatively stable for the last 60 million years, not suffering the disturbances of glaciation as in North America. This has resulted in old weathered soils and a generally flat topography. Salts have been able to accumulate in the landscape due to flat topography and relatively impermeable soils. As a result groundwater movement is often slow and drainage very poor and salts that have been deposited by various sources have accumulated in the landscape. In countries that are more mountainous and have wetter climates salts do not accumulate but rather are flushed out to sea.
Salinity is also associated with Australia's variable climate. Evaporative concentration of salts occurs when it is very dry and there is a low rainfall. During arid periods of geological history (interglacial periods since about 36,000 years ago), winds transported salt laden dust from western New South Wales to the east. Salt laden dust deposits in the northern Murray-Darling Basin peaked approximately 13,000 years ago in this area (Young et al.2002).
Another form of salinity is coastal salt water intrusion. Salt water intrusion, or encroachment, is the movement of salt water into fresh water aquifers as a result of groundwater development. It can occur in coastal areas where the fresh water aquifers are hydraulically connected with sea water. Under natural conditions, fresh water flows to the sea, limiting the landward movement or encroachment of sea water. However, when groundwater supplies are developed (ie pumped) and the water table or piezometric surface of the fresh water aquifer is lowered, the balance between the sea water/fresh water interface is disturbed and hydraulic gradients allow sea water to encroach the fresh water aquifer.
Salt water intrusion is a problem that affects coastal areas around the world. It has been reported in the Netherlands, Israel, the United States including Long Island, Miami and parts of California. Salt water intrusion is a threat to many islands around the world that only have a thin fresh water lens of groundwater lying above saline groundwater. Fresh water development on small oceanic islands is one of the most challenging problems facing hydrogeologists today.
Salt water intrusion is a problem because the increase in salt concentrations associated with encroachment limits the supply of potable drinking water and can have a detrimental effect on human health and wildlife habitat. Drinking water standards in Australia require that drinking water contain no more than 1000 mg/l of Total Dissolved Solids (TDS). Sea water contains about 30 000mg/l TDS so therefore even a small amount of sea water can cause problems when mixed with fresh water. Saline groundwater used for irrigation can damage and kill crops and destroy soil structure. High chloride concentrations in groundwater can damage infrastructure such as pipes and other plumbing fixtures, cooling systms and corrode agricultural sprinklers.
Methods for controlling salt water intrusion include: