The Great Artesian Basin occupies 1.7 X 10^6 km^2, or about one-fifth of Australia, extending across parts of Queensland, New South Wales, South Australia, and the Northern Territory. It underlies arid and semi-arid regions where surface water is sparse and unreliable. The discovery of the basins groundwater resources around 1880, and their subsequent development, have allowed an important pastoral industry to be established. Pastoral activity and town water supplies are to a very large extent dependent on artesian groundwater. The groundwater basin consist of a multi-layered confined aquifer system, with aquifers occurring in continental quartzose sandstones of Triassic, Jurassic and Cretaceous age. The intervening confining beds consist of siltstone and mudstone; a thick argillaceous sequence of sediments of marine origin and Cretaceous age forms the main confining unit. The basin is, in places, 3000 m thick, and forms a large synclinal structure, uplifted and exposed along its eastern margin and tilted southwest. Recharge occurs mainly in the eastern marginal zone, and large-scale groundwater movement is generally towards the southwestern, western and southern margins. Natural discharge occurs from spring in these areas; most springs are connected with structural features. Minor recharge occurs in the western margin. The potentiometric surfaces of the Triassic, Jurassic and Early Cretaceous aquifers are still above groundlevel in most areas of the basin. Considerable lowering occurred in heavily developed areas; from about 1880 to 1970, regional differences of up to 80 m were recorded, and in some areas waterwells ceased to flow. Water levels of some Cretaceous aquifers are below the groundsurface throughout most of the basin area. Hydraulic gradients of the main aquifers in the Lower Cretaceous-Jurassic sequence are about 1:2000, and of aquifers in the Cretaceous sequence 1:1800. Transmissivity values of the main aquifers in the Lower Cretaceous-Jurassic sequence, from which most flowing artesian wells obtain their water, usually are several tens to several hundreds m^2/day. Hydraulic conductivities range from 0.1 to 10 m/day, with a predominance in the lower part of the range. Storage coefficients, as interpreted from wire-line logs, average about 10^-5. Aquifer thicknesses range from several metres to several hundred metres. Average groundwater velocity in the eastern marginal parts is from 1 to 5 m/year. Environmental isotope analysis shows that the artesian water is of meteoric origin. About 4700 flowing artesian wells have been drilled to depths of up to 2000 m, but average 500 m. Individual flows exceeding 10 000 m^3/day have been recorded. About 3100 wells remained flowing during the early 1970s, when the accumulated artificial discharge was about 1.5 X 10^6 m^3/day, as compared to the maximum flow from the basin of about 2 X 10^6 m^3/day from about 1500 artesian wells around 1918. The high initial discharge in the early years of exploitation, which was caused by the release of pressure in the aquifers, gradually levelled off, and has now approached a steady-state condition, in which total basin discharge is roughly balanced by recharge. Non-flowing artesian water-wells mainly in the higher Cretaceous aquifers number about 20 000, and are generally shallow, up to several hundred metres deep, and are usually equipped with windmill-operated pumps, supplying on average about 10 m^3/day each. Most flowing wells occur in the marginal areas of the basin, as the main aquifers in the Lower Cretaceous-Jurassic sequence which they tap are too deep for economical abstraction in the central part of the basin. In the central part mainly non-flowing shallow wells are found.
