Framework

Salinity

Salts are a natural component of the Australian landscape and are derived from four main sources:

1. water evaporated from the sea includes salt which falls with rain over inland areas. Rainfall may deposit these 'cyclic salts' at a rate of up to 6-8 kg/ha/year in coastal regions and is considered to be a major cause of salt accumulation in soil and groundwater of inland areas;
2. marine sediments derived from inland seas that have retreated over geological time inherently contain large quantities of salts. These 'connate' salts can be subsequently flushed out of the geological material;
3. the continued weathering of rocks in the process of soil formation often releases salts, particularly in arid regions where there is not enough rainfall to allow flushing through the soil profile. The products of rock weathering can accumulate in the soil profile; and
4. aeolian deposits, which are sediments deposited by wind, can contain significant stores of salt. Dust storms originating from the arid interior of Australia can transport and deposit salts.

Salinity is a general term used to describe the presence of elevated levels of different soluble salts such as sodium chloride, magnesium and calcium sulfates and bicarbonates, in soil and water. In some areas it is a natural component of soil formation and water movement. In other areas, increasing salinity is the result of particular land use practices.

As a result of the unique climatic and geological history of the Australian continent, we have many natural saline environments including saline soils and groundwaters. It is estimated that salt has been accumulating in the Australian landscape over the last 30,000-100,000 years, mainly through wind blown accession from salt sources (such as the ocean and salt lakes). In some areas like the south west of Western Australia there is probably 5,000 to 10,000 tonnes of salt stored under every hectare. The natural flora and fauna are adapted to these conditions. However, mobilisation of stored subsurface salts is the process that has resulted in the dramatic expansion of land and water salinisation. Today salinity is one of the most serious causes of land and water degradation facing Australia, by:

1. inducing major changes in the landscape through destruction of natural habitats, both on land and in water, leading to irreparable losses of biodiversity;
2. being closely linked to other soil degradation issues such as soil erosion;
3. having a major impact on agricultural production;
4. causing damage to infrastructure and decreasing the lifespan of road pavements, bitumen, concrete, and brick structures; and
5. most significantly, causing the off-site impact of salinising previously fresh rivers. This adversely affects the quality of water for drinking and irrigation, with serious economic, social and environmental consequences for both rural and urban communities.

Salinity includes three main forms: dryland salinity, irrigation salinity and saltwater intrusion.

Dryland salinity is the build up of salt in the soil surface in non-irrigated areas. Prior to European settlement, groundwater levels in Australia were in long-term equilibrium. The clearing of native vegetation has been a key element of rural development in Australia since the early 1800s. The removal of the deep-rooted native vegetation and its replacement, largely by shallow-rooted annual crops and pastures, has resulted in a significant reduction in plant water use. Infiltration of rainfall past the root zone has increased dramatically and large quantities of water are being added to aquifers. This has produced major changes in the water cycle, causing groundwater levels to rise rapidly. Naturally occurring salts become mobilised by being dissolved in the water and brought towards the surface. The large amount of salts moving to the plants' root zone (within 2m from the surface) affect plant growth, resulting in the dieback of many salt-intolerant species and the encroachment of salt-tolerant species. When the watertable rises to within 1m of the surface, waterlogging and salinisation occurs.

Irrigation salinity occurs on certain irrigated lands resulting from the application of large additional quantities of water, very often without adequate drainage infrastructure to remove excess water. Generally, irrigation areas with inherently low water tables or where lateral drainage is not sufficiently rapid may suffer a rise in groundwater levels resulting in both salinisation and waterlogging. Groundwater within two metres of the soil surface can rise further up the soil profile into the root zone of the crop through capillary action. If groundwater accession continues water will discharge at the soil surface or into surrounding streams, often carrying dissolved salts. Evaporation leaves these salts to accumulate on soil surfaces.

Urban salinity results from a combination of dryland salinity processes and over-watering in towns and urban areas. Many Australian rural towns as well as parts of large cities are experiencing the effects of rising watertables, causing salinity and waterlogging. This is resulting in corrosion damage to buildings, amenities and infrastructure such as roads, paths, pipes and bridges.

Seawater intrusion is the movement of seawater into fresh water coastal aquifers. It is a natural phenomenon that occurs as a consequence of the density contrast between fresh and saline groundwater. If conditions remain unchanged, the saline water in the aquifer will remain stationary unless it moves under tidal influences. However, when there is excessive pumping of fresh groundwater, sea-level change, or changing recharge conditions, the saline water can gradually move landward within the aquifer until a new equilibrium condition is achieved (eg Narayan et al, 2004). The increasing concentration of human settlements along the coast gives rise to excessive pressure on coastal aquifers (groundwater used for irrigation and consumption, recharge conditions changed due to urban developments), resulting in seawater intrusion and related deterioration of water quality. It is extremely difficult and costly to restore the water quality of an aquifer affected by seawater intrusion. Therefore, it is wiser to plan ahead to prevent seawater intrusion.

Management of salinity needs to address the hydrogeological processes controlling salt mobilisation, which requires an integrated understanding of catchment water flows, groundwater dynamics, salt distribution and salt transport processes. Conjunctive water management is the logical approach to salinity mitigation. This can mean incorporating groundwater-surface water connectivity in numerical models used to assess the salinity impacts of new developments or to map management zones around river systems. It can also mean investing in on-ground works such as interception schemes, where bores are operated to pump and remove saline groundwater before it reaches the river.

Relevant Links

National Action Plan for Salinity and Water Quality
National Dryland Salinity Program
National Land and Water Resources Audit
Department of Agriculture Fisheries and Forestry NRM - Salinity
Department of the Environment and Heritage Salinity
Murray-Darling Basin Commission NRM - Salinity
Queensland Department of Natural Resources Mines and Water Salinity Fact Sheets
NSW Department of Natural Resources Salinity Strategy
CRC for Plant-based Management of Dryland Salinity
ABC Salinity - Australia's Silent Flood

References

Narayan KA. Schleeberger C, Charlesworth PB, Bristow KL, 2003. Effects of groundwater pumping on saltwater intrusion in the LowerBurdekin Delta, North Queensland In: Post, D. A. (ed.) MODSIM 2003 International Congress on Modelling and Simulation. Volume 2, pp 212-217. Modelling and Simulation Society of Australia and New Zealand, July 2003.