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Great Artesian BasinThe Great Artesian Basin provides the only reliable source of water through much of inland Australia and is the largest artesian basin in the world. It underlies more than 20% of the continent, including most of Queensland, the south-east corner of the Northern Territory, the north-east part of South Australia, and northern New South Wales. The basin is 3000 metres deep in places and contains an estimated 8700 megalitres of groundwater. The aquifers that make up the Great Artesian Basin are composed of layers of quartzose sandstone laid down by continental erosion of higher ground during the Triassic, Jurassic, and early Cretaceous periods, and covered by a layer of marine sedimentary rock laid down shortly afterwards, duing a time when much of what is now inland Australia was below sea level. The eastern edge of the basin was uplifted when the Great Dividing Range formed. Most recharge water enters the rock formations from relatively high ground near the eastern edge of the basin (in Queensland and New South Wales) and very gradually flows towards the south and west. (A much smaller amount enters along the western margin in arid central Australia, flowing to the south and east.) Because the sandstones are permeable, water gradually makes its way through the pores between the sand grains, flowing at a rate of one to five metres per year. Discharge water eventually exits through a number of springs and seeps, mostly in the southern part of the basin. It takes up to two million years for water to travel to the springs in the Lake Eyre area. It is the source of most of the water used in these areas. Whilst unsuitable for irrigation, it is adequate for stock and domestic usage (with treatment) and is thus vital to human activity. To tap it, bores are drilled down to a suitable rock layer, where the pressure of the water forces it up, mostly without pumping. Overusage of the basin's water has led to a decline in the water pressure available, a source of great concern to water users as, if left unchecked, would mean that bores might go dry (or, at best, require water to be actively pumped out). It is commonplace for bores to be tapping water that is two million years old. Various governments are discussing cooperation to reduce usage, but little has been done.
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