The dwindling river: Salinity: the white plague

Geological processes have contributed to the naturally saline environment of the Murray Basin. The region was covered by ocean several million years ago and as the sea retreated to its present level about one million years ago, salt residues were deposited in the soils and groundwater. The low-lying terrain, minimal rainfall and high evaporation of the region combined to concentrate salt in the soil and groundwater. The causes of the River Murray's current salinity issues can be traced to the commencement of river regulation and dry-land irrigation along the river, which aggravated the naturally saline conditions by disrupting natural hydrological processes. This salt enters the River in three main ways:

• Natural surface run-off and inflow from tributaries
  Surface water run-off from rainfall dissolves and carries salts accumulated on the land surface into waterways. The streams and rivers that join the Murray also carry these salts and salt from groundwater into the main tributary.
• Drainage from dry-land irrigation
  Clearing of deep-rooted native vegetation and dry-land irrigation practices cause groundwater levels to rise. Capillary action then draws salt upward to the plant root-zone causing soil salinisation. Irrigation water that drains through this soil and into the river carries and deposits high salt loads.
• Groundwater Inflow
  Saline groundwater from rising water tables can enter the river as a result of natural gradients or increased gradients caused by weirs adjacent to the river. As in point 2, salt from groundwater also enters the river through the discharge of irrigation drainage.

Water users in the southern downstream parts of the Murray basin are more affected by increased river salinity as evaporation, aridity, slower flows and human activities concentrate the salt levels. There are three sectors of the South Australian community chiefly affected by the economic and environmental impact of salinity.

• Agricultural users in irrigation districts find that crops watered with saline river water yield less fruit. High concentrations of salt at the plant root-zone over long periods of time can retard or kill certain trees and perennial crops, therefore limiting the choice of produce grown.
• Industrial users face problems with the corrosion of machinery. Action can be taken to install softening and demineralisation processes to deal with saline water, but they are expensive and time-consuming.
• Domestic users are affected by water 'hardness', as more soap and detergents are needed and some households require the installation of water softening units. Domestic appliances wear out more quickly as pipes corrode and mechanisms fail. Gardens are affected as saline water stunts growth and kills plants. Urban water and sewer pipes are corroded by affected water.

The Murray-Darling Basin Commission contends that the greatest risk to the Murray will be the release of dry-land salt into the river system as the underground water table rises. The Commission predicts that by 2050 the salinity levels in the lower Murray will exceed World Health Organisation drinking water limits whose recommended limit for safe drinking water is 800 EC units. The Electrical Conductivity (EC) unit is a measure of electrical conductivity, commonly used to indicate the salinity of water. 1EC = 1 micro-Siemen per centimetre measured at 25° Celsius.

It is expected that the average salinity level in the Murray River at Morgan will rise from the 1998 level of 570 EC units to 900 EC units over the next century.

However, Dr Jennifer Marohasy, director of the Institute of Public Affairs' environmental unit, contends that some claims about the Murray's water quality do not compare with available data. In her research, detailed in her 2003 paper 'Received Evidence for Deterioration in Water Quality in the River Murray'. Marohasy cites official figures from the Murray-Darling Basin Commission which show that salinity at Morgan (considered a key indicator site for water quality in the Murray-Darling Basin as it is just upstream from the pipeline off-takes for Adelaide's water supply) is nearing its lowest point in 20 years. She writes that large quantities of salt have always entered the Murray River naturally and that given the current drought situation across the Basin, the figures are even more noteworthy.

There are numerous projects underway to combat salinity. In South Australia the South Australian River Murray Salinity Strategy 2001- 2015 and the State Dryland Salinity Strategy initially aim to maintain salinity at present levels, and ultimately to 'reverse the trend' of rising salinity in order to preserve water quality, safeguard the environment and protect our built infrastructure including roads, bridges sewers and railways.

One suggested approach is the interception of shallow saline groundwater. Sub-surface drains are sunk to a depth of 90-25 metres at suitable sites along the river to intercept salty groundwater as it moves towards the river, then it is pumped to a disposal basin to evaporate. This system has been used at sites between Waikerie and Overland Corner.

Saline irrigation drainage is also harvested and pumped into evaporation basins. An example of this strategy is the Noora Evaporation Basin, located 20 kilometres east of Loxton, taking drainage from Renmark, Loxton and Berri.

Another strategy is improving the efficiency of water conveyancing systems. Open channel irrigation such as earth ditches and concrete channels lead to leakage and overflows, resulting in salt flows into the river. Utilising pressurised pipes that connect to modern systems such as drip irrigation significantly reduces water wastage and seepage, thereby reducing the need for more drainage systems.

Other measures being used to combat salinity are:

• using stored water to dilute saline water
• replanting and protecting native vegetation
• planting trees and perennial plants to lower the saline water table
• whole of farm management
• coping and living with saline water, for example, growing salt-tolerant crops, investment in desalinisation plants

Unprecedented water restrictions have also been implemented in South Australia in the hope of reducing the amount of water taken from the Murray and thereby reducing salinity.

Salinity is the major environmental concern for the Murray-Darling Basin and the numerous groups that rely on the river for economic, domestic and recreational use. The salt is a natural part of the environment due to geological processes and we now have enough evidence to know that we need to manage the river resource correctly to minimise the impact of salinity.

Salinity issues and policies and projects to control salinity in South Australia are detailed in the following websites.

Further reading

Pigram, JJ. Issues in the management of Australia's water resources, Melbourne: Longman Cheshire, 1986

Walker, Glen. Groundwater flow systems framework : essential tools for planning salinity management, Canberra : CSIRO Land and Water, 2003 

Links

Atlas of South Australia 1986 See: Environment resources: Environmental change

Australian Natural Resources Atlas See: Natural resource topics: Dryland salinity

Department of Water, Land and Biodiversity [South Australia] See: River Murray: Salinity and water quality

Murray-Darling Basin Authority See: Water: River Information: River Salinity

NOVA [Australian Academy of Science] See: Environment: Sustainability: Salinity - the awakening monster from the deep and Monitoring the white death - soil salinity

Salinity: Australia's silent flood [ABC online]