The Great Artesian Basin (GAB) is a massive, life-sustaining underground water source underlying a significant portion of arid and semi-arid Australia. It is one of the largest and deepest artesian groundwater basins in the world, holding an immense volume of water beneath the surface. This subterranean reservoir provides the only reliable source of freshwater across much of inland Australia, supporting human settlement, industry, and unique ecosystems. The basin’s formation and the mechanism by which its water is stored and discharged are a testament to geological forces.
Geographic Scope and Scale
The physical dimensions of the Great Artesian Basin span approximately 1.7 million square kilometers. This enormous area covers about 22% of the Australian continent. The basin extends beneath large sections of four major Australian jurisdictions: Queensland, New South Wales, South Australia, and the Northern Territory.
It is not a uniform underground lake but a multi-layered system of rock formations that varies significantly in depth. The GAB reaches depths of up to 3,000 meters in some places. This depth highlights the sheer volume of water stored within its rock layers, estimated to be around 65 million gigalitres.
Geological Formation and Structure
The existence of the GAB is the result of geological processes that took place primarily during the Mesozoic era. This structure is defined by three distinct components: the aquifer, the aquitard, and the overall synclinal shape of the basin. The main water-bearing layers, or aquifers, are composed of porous sandstones laid down during the Triassic, Jurassic, and Cretaceous periods.
These sandstone beds were formed from the sediments of rivers and ancient seas. Crucially, the aquifers are overlain by thick, impermeable layers of fine-grained rock, such as shale and mudstone, which act as aquitards. This capping layer traps the water within the sandstone, preventing it from escaping upward.
The entire structure is gently tilted, forming a broad, saucer-like shape, or syncline, with the edges exposed at higher elevations. This geological arrangement, created by the uplift of the Great Dividing Range in the east, is fundamental to the water pressure within the system.
The Hydrology of Artesian Water
The term “artesian” describes groundwater that is under sufficient hydrostatic pressure to rise to the surface naturally when tapped by a bore. This pressure is generated because the water source, or recharge area, is at a higher elevation than the discharge point. Rainfall enters the system in the uplifted eastern highlands, mainly along the Great Dividing Range in Queensland and New South Wales, where the permeable rock layers are exposed.
Once the rainwater soaks into these intake beds, gravity drives the water deep into the confined aquifer system. Because the confining aquitard layers prevent upward escape, the weight of the water column in the elevated recharge areas pushes the water through the sandstone, creating the pressure.
The water then flows slowly westward and southward across the basin, often taking thousands of years to travel. Flow rates are estimated to be only one to five meters per year. The water found in the deepest parts of the basin can be nearly two million years old, reflecting this immense travel time.
Ecological and Human Importance
The Great Artesian Basin is an irreplaceable resource, providing a reliable, climate-independent water supply across Australia’s arid interior. It has been instrumental in enabling the settlement and development of vast inland areas, supporting remote communities and towns. The water is extensively used for pastoralism, providing essential water for sheep and cattle farming, and for various mining operations across the outback.
Ecologically, the basin is responsible for supporting unique, isolated habitats known as mound springs. These springs occur where the pressurized water naturally finds its way to the surface, often along geological faults or the basin’s margins. The permanent water flow creates small oases in the desert, sustaining specialized plant and animal life that cannot survive elsewhere.
These mound springs are home to a high level of endemic species. These include unique invertebrates, such as hydrobiid snails, as well as endemic fish and plants like the endangered salt pipewort. The springs represent ancient refugia, allowing these species to persist through millions of years of aridification.