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Great Artesian Basin.

The Great Artesian Basin lies under approximately one-fifth of Australia over an area of 1,711,000 square kilometres. It extends beneath arid and semi-arid regions of Queensland, New South Wales, South Australia and the Northern Territory. The Great Artesian Basin was formed between 100 and 250 million years ago, beginning with sediments laid down in three large depressions, the Carpentaria Basin, the Eromanga Basin and the Surat Basin. During the deposition process alternating layers of sand and gravel, and clays and clayey sands, were laid down in floodplains and lakes. Sandy sediments consolidated to form permeable sandstone layers, and the clayey sediments became impermeable layers of mudstone and siltsones preventing water escaping from the permeable sandstone. After sedimentation ceased, uplift and erosion at the edges of the Basin exposed the uplifted permeable sandstone and rainwater was able to enter the sandstone and slowly percolate through the sandstone to fill the aquifer. Water moves through the sandstone at a rate of between one and five metres a year. Thickness of this sequence varies from less than 100 metres on the Basin extremities to over 3,000 metres in the deeper parts of the Basin. Water continues to infiltrate into the outcropping sandstone aquifers mainly along the eastern margins of the Basin, especially along the western slopes of the Great Dividing Range, and is constrained in the aquifers by the impermeable layers as it flows generally westward to the south-west over most of the Basin but to the north-west and north in the northern section. Natural discharge from the Basin occurs in mound springs where groundwater flows at the surface; these natural water sources are a valuable resource for wildlife. Recharge water for the aquifers slowly moves along the permeable sandstone of the aquifer from the recharge area in the uplifted and exposed sandstones at the edge of the Basin and fills the aquifer to the level of the intake area. As the aquifer is confined by the overlying impermeable layer the water becomes pressurised. When a bore is drilled into an aquifer, the water rises up the bore due to this pressure to a level called the potentiometric surface. If this surface is above ground level, then water will flow from the bore without assistance and the bore is described as an 'artesian' bore. If the water does not naturally flow from the bore but has to be pumped to the surface the bore is described as 'subartesian' Dating of artesian waters has given ages of almost 2 million years for the oldest water. Most water is believed to have entered the basin when there was a mor etropical, and generally wetter climate, at least one million years ago. Following an intensive campaign over the last decade to cap artesian bores and control the flow of water form the basin, it is estimated that the quanity of water being extracted at present is about the same as the quantity of water entering the system from the recharge areas along the Great Dividing Range. However, the pressure is considerably less than when the first bores were drilled. Water quality is generally good. In some places high sodium levels make the water unsuitable for irrigation; other bores produce water tainted by a smell of sulphur but which is still suitable for human consumption. Temperature of water from shallower bores is about 30° C to about 100° C from deeper bores. Bore depths vary up to 2,000 metres with the average being 500 metres. Sometimes oil and gas is encountered in bores. Different bores tap into different aquifers at different depths and can produce different qualities of water. This windmill at Boulia pumped artesian water to supply the township until it was replaced by an electric pump; the windmill was relocated to a pastoral property where it continued pumping water. It has now been erected in Boulia where it was first installed. European use of the Great Artesian Basin began when a first shallow bore at Bourke in 1878 produced flowing water. Many bores were soon drilled in New South Wales and South Australia around the edges of the Basin. In 1885 the Queensland Government decided to drill a deep bore at Blackall to prove the existence of deep artesian water supplies during the drought; before this well was drilled the first artesian flow in Queensland was obtained near Cunnamulla in 1887. In 1889 there were 34 bores in Queensland; this increased to 524 bores in ten years to 1899, and by 1915 there were more than 1,500 flowing artesian bores in Queensland taking over 2,000 megalitres a day from the Basin. Often this water was distributed in open channels called bore drains which ran for many kilometres (one is recorded as 190 kilometres long) providing water for sheep and cattle. Availability of reliable water for stock and settlers meant the development of the valuable sheep and cattle industry and artesian water continues to be essential for some towns and properties sometimes being the only water source for towns. During this period the windmill was widely used to pump water from subartesian bores for watering stock, supplying properties and keeping townships supplied with water, and windmills became a prominent feature of the outback landscape. In the 1920s internal combustion engines began replacing windmills and, more recently, electrical power has been available to power pumps at many bores. But many remote watering points still rely on a whirring, clanking windmill to keep the water flowing. Since 1915 artesian pressure and water discharge rates from the Great Artesian Basin have declined; total flow from the Basin in 2000 was about 1,500 megalitres per day. Tighter control of bore cap quality and projects to refurbish many older bores with uncontrolled flow are being undertaken to reduce water wastage from the Basin. New bores are required to be controlled and distribution to be via tanks and pipes, not open channels. The Great Artesian Basin Consultative Council estimated in 1998 that there were about 3,000 free-flowing artesian bores and about 35,000 subartesian bores tapping into the reserves in the Great Artesian Basin. Water from the Great Artesian Basin remains a valuable national resource, since without this water there would be little primary production and many fewer townships in much of western Queensland. Measures aimed at ensuring that water continues to be available from the Great Artesian Basin include;-   -Constantly monitoring the quantity of water extracted and the rate of draw-down.   -Continuing the bore-capping programme, thereby controlling outflow and minimising wastage.   -Using water efficiently and responsibly.   -Publicising the importance of protecting the water supply and restoring the pressure so that the intake areas are appropriately managed.

Left. Uncontrolled flow of water from a long-established artesian bore at the Blackall Woolscour at Blackall. Right. Town Bore No 4 at Blackall showing a capped artesian bore feeding water into the town reticulation.

Information. ¶   http://www.nrm.qld.gov.au/water/gab/basin.html ¶   The Miles Historical Village at Miles has an excellent and comprehensive section dealing with the Great Artesian Basin ¶  Desert Uplands Strategic Land Resource Assessment  by Environmental Protection Agency/Queensland Parks and Wildlife Service at http://www.epa.qld.gov.au

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