The water down under
This topic is sponsored by the Australian Research Council Linkage Learned Academies Special Project Grant.
You might not think it, but in a country as dry as Australia many people are literally walking on water.
Most people think of rain water, rivers and dams as the major sources of water that quench Australia's thirst. But the reality is that groundwater provides more than 20 per cent of the water used in Australia each year and is set to become even more important due to overallocation of surface water and the recent drought.
Groundwater is the water that exists underground. While it can be present as underground lakes beneath the Earth's surface, it's more commonly the water that lies in the tiny spaces between grains of sand or bits of fractured rock. It's a bit like the effect you'd get if you poured water into a jar of sand or pebbles the water wouldn't float to the top, but instead would settle in the spaces between grains, filling the spaces between sand or stones.
Use of groundwater resources
Across Australia, groundwater plays a critical role as a source of water for drinking, irrigation and industrial purposes. It some cases, it is also the source for spring water and mineral water companies, and provides the water for making soft drinks and beer. Bores utilise groundwater to water suburban gardens and to irrigate parks, golf courses and crops. It is also a vital part of the ecosystems that support wetlands and some unique plant and animal species. Groundwater also keeps many of our rivers flowing even when there are long periods without rain.
Out of sight, out of mind
Groundwater is a major resource, but one that has been taken for granted for decades. In the past, groundwater supplies were treated as an infinite resource, and subject to an 'out of sight, out of mind' attitude. But that's changing. There's now an enormous interest in the way our groundwater resources are measured, managed and utilised. There are also concerns over issues such as over-extraction of water, pollution, wastage, allocation and licensing issues, water pricing and groundwater salinisation.
The water source beneath one-fifth of Australia
The most well-known and important groundwater source in Australia is the Great Artesian Basin, or GAB. This is a vast groundwater source that underlies 22 per cent of Australia extending beneath the arid and semi-arid regions of Queensland, the Northern Territory, South Australia and New South Wales. It covers about 1.7 million square kilometres, and contains an estimated 8700 million megalitres of water. Not surprisingly, it's one of the largest artesian water basins in the world.
Its other statistics are equally amazing. The water-bearing aquifers of the GAB extend down to a depth of three kilometres, and some of the water is believed to be up to two million years old (Box 1: Using science to measure and manage groundwater). The average water temperature is between 30 and 50 degrees, but it can be as hot as 100 degrees Celsius.
History of the GAB
The GAB has always been an important source of water for many outback communities, especially during drought. Indigenous people relied on its natural springs for thousands of years and the waters form part of their culture. More recently, the springs helped the development of the pastoral industry, with the drovers watering their stock as they pushed further across the outback (Box 2: Our outback oases – groundwater dependent ecosystems). The waters of the GAB are stored under natural pressure. Across much of the basin you can dig a bore and the water will come to the surface without the need for a pump. Such bores are called artesian bores.
Hundreds of bores have been sunk into the GAB since the first in 1878 near Bourke in New South Wales. By 1915, there were 1500 bores and a peak flow of 2000 megalitres a day. Over the decades as more and more bores were sunk, water pressure and flow rates dropped across much of the GAB.
Much of the water that is extracted is wasted. It is estimated that up to 80 per cent of the total outflow from the GAB is wasted because of inefficient water delivery systems. Thousands of kilometres of open drains dug across properties to water animals lose water to evaporation and seepage.
Bore owners, as well as State and Australian Governments have undertaken the Great Artesian Basin Sustainability Initiative to counter waste and help restore pressure throughout the basin.
The GAB plan capping the bores
The Great Artesian Basin Consultative Council released a 15-year strategic management plan to cap bores so there is no further decrease in pressure and to reduce wastage. Part of the plan involves rehabilitating 880 bores and replacing 34,000 kilometres of bore drains with poly piping and troughs.
In another attempt to maintain pressure in the basin, in 2005 the Queensland Government declared a moratorium on the issuing of new licences to extract water from most GAB aquifers.
Sustainable use of groundwater or water mining?
Although the Great Artesian Basin is the best-known source of groundwater in Australia, large groundwater sources are found across the country and form an important part of future water strategies. A key consideration is ensuring groundwater supplies are managed and used sustainably.
Groundwater already plays a big part in irrigating many of Australia's crops, and water managers have plans for future domestic schemes to tap into groundwater supplies. In most areas, human use is currently in excess of the sustainable yield (Box 3: Research into sustainable groundwater use).
Getting the balance right
The sustainable yield of a groundwater source depends on balancing the use or discharge against recharge rates. Normally discharge of groundwater occurs through vegetation, into streams and lakes, or through evaporation into the atmosphere. Sustainable yield cannot simply be determined by a measure of the recharge rate. If water is extracted for human use at the recharge rate, discharge to other areas can be affected.
'Drought-proofing' urban areas
Groundwater is also being used to provide more flexibility in water planning, particularly in times of drought. In New South Wales, for instance, a controversial desalination plant has been put on hold, and instead two aquifers to the west and south of Sydney are being investigated as potential sources of water for Sydney. The plan is to provide 30 gigalitres of water a year from the aquifers for several years during drought, and then allow the aquifers to replenish.
However, the proposed scheme has some difficulties. Extraction of water from the Hawkesbury sandstone is proving a problem due to low flow rates. One estimate suggests that Sydney’s aquifers can supply about 10 gigalitres a year of the 630 gigalitres a year consumed by Sydneysiders. Repeat use of the aquifers depends on how quickly they are replenished. In the Sydney Basin, the apparent (uncorrected) age of the water can be thousands of years old. Corrections need to be applied to get a true measure of the age of the water, due to the presence of siderite on the water.
Extraction of groundwater can also lead to salinity problems and have a negative impact on native vegetation with roots that tap into groundwater, as well as wetlands, rivers and streams. The full impact of using these aquifers as planned is not known, but is likely to reduce the rate of water flowing to support rivers and wetlands and other groundwater dependent ecosystems.
In Western Australia, there is a plan to draw 45 gigalitres a year from the Yarragadee aquifer in the State's south-west. The aquifer is a large source of good quality groundwater, but it will only be developed as a source if proven to be sustainable. Changing land use in the area of Gnangara Mound north of Perth which provides up to 60 per cent of Perth's water supplies is also being investigated in an attempt to conserve water and increase recharge rates to the aquifer.
Some residents of Sydney have another alarming groundwater to deal with: pollution. In August 2006, thousands of Sydney residents in several suburbs were banned from using groundwater bores due to industrial contamination of the Botany Sands aquifer. The residents can't use bore water for garden use, swimming pools or even washing their cars due to the presence of contaminants from eight industrial sites.
For the residents involved, the precautionary groundwater ban is a very down-to-earth reminder of the importance of the water beneath our feet.
Posted February 2007.