Types of salinity
There are several types of salinity. They are categorised in a number of different ways, depending on how and where salt is mobilised and what the impacts are:
- Dryland salinity is salinity that occurs in non-irrigated area. It usually occurs where deep-rooted perennial vegetation is replaced by crops and pastures that use less water because they have shallow root systems and shorter growth cycles growth. This increases leakage to the groundwater system (recharge) which in some areas, may lead to the mobilisation of salts stored deep in the soil. Saline groundwater may rise to the surface (discharge) in low-lying areas or at the break of slope. Groundwater may also flow underground directly into streams and rivers.
Dryland salinity may also be caused by the exposure of naturally saline soils such as hypersaline clays. Sodic soils (soils that have a high concentration of sodium ions in comparison to other ions like calcium and magnesium) can also cause salinity. When wet, sodic soils disperse causing the soil aggregates to separate and block the soil pores. On drying, sodic soils are often hard and dense, and form a crust on the soil surface. The poor soil structure reduces water infiltration and there is little or no leaching of salts below the root zone. Sodic subsoils can create a perched watertable causing waterlogging of the root zone.
- Irrigation salinity occurs when there is a localised rise in the level of groundwater caused by the application of large volumes of irrigation water. This problem is compounded by the replacement of native vegetation with crops and pastures that use less water. Irrigation salinity is made worse when water used to irrigate is derived from salty rivers or groundwater.
- Urban salinity is the result of a combination of dryland and irrigation salinity processes. Urban development and and problems like over-watering parks and gardens, leaking pipes, drains and tanks, and blocking or changing natural drainage paths can cause the groundwater to rise. Besides naturally occurring salt, in the urban environment there are many other sources of salt that can contribute to urban salinity including salt contained in effluent, building materials, industrial waste water, fertilisers and chemicals, as well as naturally occurring salt.
- Industrial Salinity results from industrial processes that concentrate salt in industrial waste water. Effluent from towns, intensive agriculture and industry can contain high levels of salt. Coal-fired power stations use water for cooling, a process in which water is evaporated and salt concentrated. Mining activities undertaken before the development of strict rehabilitation requirements have led to abandoned mines being a source of salt in some sub-catchments.
- River salinity is caused by saline discharges from areas affected dryland, irrigation and urban salinity flowing into creeks and rivers. Over time, as salinity within catchments worsens, the quality of river water declines.
Salinity rarely occurs in isolation from other natural resource problems such as decreasing soil and water quality, erosion and loss of native vegetation. For example, water coming from areas affected by dryland, irrigation or urban salinity flows into creeks and rivers causing salinity levels to rise. This affects the water quality, which in turn the affects the health of plants and animals. Low water quality affects farm income but may also impact on town water supply, which can have social and economic impacts for both rural and urban dwellers caused by rising council rates and taxes.
Page last updated: 26 February 2011