What is salinity?
Salinity is the presence of soluble salts in soils or waters. It is a general term used to describe the presence of elevated levels of different salts such as sodium chloride, magnesium and calcium sulfates and bicarbonates, in soil and water. It usually results from water tables rising to, or close to, the ground surface.
- Types of salinity
- Contributing factors
- Salinity indicators
- Adverse effects
- Preventing salinity problems
- Managing salinity
Types of salinity
The three main types of salinity are:
- dryland salinity, which is caused when saline seeps scald the surface of non-irrigated lands, in turn affecting plant growth and degrading soil structure
- irrigation salinity, which results when overuse of water for irrigation causes rising watertables
- sea water intrusion, which occurs in coastal aquifer systems where sea water replaces groundwater that has been over-exploited.
Contributing factors
The occurrence of salinity depends on several factors, the most important of which are:
- the characteristics of the landscape
- the climate
- the effects of human activities.
An understanding of water movement associated with the salt-affected area is needed to determine the likely extent of any problem.
The sources of these salts are:
- rainfall, which carries low concentrations of salts that have accumulated in our landscapes over thousands of years
- weathering and erosion of surface rocks
- groundwater that has soaked through sediments and sedimentary rocks that originally formed in salty marine environments.
The amount of salts that accumulate in soil depends on the properties of the soil and rainfall, with clay soils (which occur extensively in inland Queensland) generally having the highest levels.
Salinity indicators
Signs to look out for include:
- a ground surface that is becoming permanently or seasonally damp or waterlogged, or remains damp after extended rain
- intermittent streams that flow for longer periods
- dieback of vegetation in low lying areas, or failure of plants to germinate or grow
- areas of bare soil or an increase in salt-tolerant plants in an area
- changing pasture composition and reduced diversity (with couch grass and other salt tolerant plants dominating)
- rising damp in buildings
- deterioration in the quality of surface or groundwater
- road deterioration and crumbling
- rising groundwater levels in bores.
The National Land and Water Resources Audit through the National Coordination Committee for Salinity Information has developed a set of four indicators and protocols to assist in evaluating changes in land salinity. Further information on the indicators and protocols is located on the National Land and Water Resources Audit site.
Adverse effects
Some land-use activities cause the watertable to rise and thus carry salts closer to the surface or into surface water systems. These include excessive irrigation, and clearing of deep-rooted vegetation such as native trees and grasses.
This can:
- retard or kill crops and vegetation
- increase soil erosion
- increase salt pollution of rivers and dams, harming water supplies for drinking and irrigation.
Rising salty watertables can damage roads, fences, railways and buildings and harm natural ecosystems.
Preventing salinity problems
In areas identifies a high level of hazard:
- monitor groundwater levels and the amount of salt in the land and water
- encourage preventative actions by landholders to stop salt moving towards the surface
- stop the further loss of deep-rooted native vegetation both in high-hazard areas and in those areas that contribute groundwater to them.
Specific methods of prevention include:
- retaining and regenerating native vegetation
- avoiding building dams at sites where the watertable is high
- locating roads along ridges where disruption to water tables will be minimal
- siting houses (and septic systems) away from areas with high watertables
- maintaining good pasture cover through conservative stocking rates
- adopting high water efficiency cropping practices which minimise deep drainage.
Managing salinity
Managing salinity involves striking a balance between the volume of water entering the groundwater system (recharge) and the volume of water leaving it (discharge).
The watertable can be lowered by:
- planting, regenerating and maintaining native vegetation and good ground cover in recharge, transmission and discharge zones, where possible
- increasing groundwater use in recharge areas by pumping water from bores and by using drainage to redirect water to other storages
- installing bores and interceptor drains in discharge areas—water of suitable quality can be used to irrigate adjacent areas
- installing sub-surface drainage
- maximising cropping opportunities and minimising fallow land.
Last updated: 20 October 2008