<P> The water of the GAB is held in a sandstone layer laid down by continental erosion of higher ground during the Triassic, Jurassic, and early Cretaceous periods . During a time when much of what is now inland Australia was below sea level, the sandstone was then covered by a layer of marine sedimentary rock shortly afterward, which formed a confining layer, thus trapping water in the sandstone aquifer . The eastern edge of the basin was uplifted when the Great Dividing Range formed . The other side was created from the landforms of the Central Eastern Lowlands and the Great Western Plateau to the west . </P> <P> 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 toward 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 . </P> <P> Discharge water eventually exits through a number of springs and seeps, mostly in the southern part of the basin . The age of the groundwater determined by carbon - 14 and chlorine - 36 measurements combined with hydraulic modelling ranges from several thousand years for the recharge areas in the north to nearly 2 million years in the south - western discharge zones . </P> <P> Prior to European occupation, waters of the GAB discharged through mound springs, many in arid South Australia . These springs supported a variety of endemic invertebrates (molluscs, for example), and supported extensive Aboriginal communities and trade routes . After the arrival of Europeans, they enabled early exploration and faster communications between southeastern Australia and Europe via the Australian Overland Telegraph Line . The Great Artesian Basin became an important water supply for cattle stations, irrigation, and livestock and domestic usage, and is a vital life line for rural Australia . To tap it, water wells are drilled down to a suitable rock layer, where the pressure of the water forces it up, mostly without pumping . </P>

How old is the water in the great artesian basin
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