Key Facts
The Great Artesian Basin (GAB) spans over 1.7 million square kilometers
Underlies 22% of the Australian continent
Up to 3,000 meters deep
Contains approximately 64,900 cubed kilometers of fresh water
Water temperature ranges from 30ºC in shallower areas to 100ºC in the deeper, more pressurized zones
Delineating the GAB
- Defining the GAB’s boundaries is a notoriously difficult and highly contested endeavor. For starters, its name is somewhat of a misnomer, as the term basin traditionally implies that the groundwater flows towards and eventually exits through a singular specific location or area. As we will soon learn, the Great Artesian Basin has 13 primary discharge areas as well as seepage into the Gulf of Carpentaria, so it would be more accurately described as a vast system of interconnected basins. Additionally, it should be noted that the GAB is both the world’s largest and deepest aquifer system, thus making it one of the hardest to study. Given the immense dimensions, complex interconnected nature of its hydrological components, and the differing methodologies used by hydrogeologists to determine flow-paths, perfectly delineating the GAB’s sub-basin boundaries as well as its outer boundaries is an ultimately unachievable task given the current state of knowledge. Although the precise delineation of its sub-basin boundaries is of much debate, there does exist a general consensus that the GAB can be most simply divided into three amalgamated basins based largely on their geologic traits: the northern Carpentaria basin, the central-western Eromanga basin, and the south-eastern Surat Basin.
Formation & Paleogeology
The Great Artesian Basin was formed by continuous sediment deposition throughout the Triassic, Jurassic, and Cretaceous periods, which created a complex web of alternating layers of either permeable sandstone or impermeable siltstones and mudstones.
Late-Triassic (~201 million years ago)
The tectonic shifting of the Earth’s crust in latter stages of the Triassic period resulted in massive geologic uplift along the outer edges of what are now the Carpentaria, Eromanga, and Surat Basins
Jurassic (~200-145 million years ago)
Throughout the Jurassic period, rain events facilitated rivers and streams to erode the slopes of the uplifted areas, thus delivering layers of sand and gravel into the basins. As these uplifted areas continued to flatten, the mode of sediment deposition shifted towards layers of clay and clayey sands that were established by the lakes and expansive floodplains found inside the basins. Sandier sediments deposited throughout this period came to form permeable layers of sandstone and sediments that were richer in clay consolidated to form impermeable layers which are predominantly found on top of the sandstone throughout the basins.
Cretaceous (~100-65 million years ago)
The Cretaceous period saw considerable tectonic activity resulting in down-warping, or the creation of depressions in the Earth’s crust in the area. Additionally, tectonically induced growth of the mid-oceanic ridges displaced sea water around the globe, causing sea levels to rise up to 250 meters. The combination of down-warping and increased sea levels caused a shallow sea to form over the majority of inland Australia, which deposited massive amounts of muddy sediments over the layers of sandstone and impermeable sedimentary rock. Towards the end of of the Cretaceous period, tectonic uplift halted sediment deposition in the area and, in conjunction with erosion, contributed to the exposure of some of the layers of permeable sandstone along the eastern edges of the basins where they meet the western slopes of the Great Dividing Range (shown in red on the map). The Great Artesian Basin began to fill its massive subterranean reserve with rain water as it seeped through this eastern boundary of exposed sandstone.
Recharge in the GAB
Recharge, or how water enters the aquifer, of the Great Artesian Basin’s groundwater occurs in the following two ways:
Rainfall infiltration directly into exposed sandstone
The majority of recharge occurs primarily along the eastern, more elevated boundary of the Basin. Here, as rain water collects down the western slopes of the Great Dividing Range and flows in the direction of the Great Artesian Basin, water begins to seep into the areas of exposed sandstone found along the eastern margins of the Great Artesian Basin (which were created towards the end of the Cretaceous period), thus replenishing the aquifer’s waters.
Seepage through porous sediments covering sandstone aquifers
A lesser amount of recharge occurs primarily via seepage through sandy sediments which overlay the aquifers along the western, more arid margins of the Basin due to decreased precipitation in that region.
In totality, recharge areas constitute just 10% of the entirety of the GAB and only 1-2% of total rainfall contributes to the Basin’s recharge, constituting 1 million megaliters per year on average.
Discharge in the GAB
Discharge, or how water exits the aquifer, occurs in various human-caused (artificial) and natural ways.
Artificial Discharge
Artificial discharge in the Great Artesian Basin is caused by extraction via either flowing or pumped boreholes for agricultural, livestock, and domestic use. Boreholes, also known as waterbores, are a product of drilling narrow shafts deep into the ground until it reaches an aquifer. Given that the pressure of the groundwater is fairly high throughout much of the Basin, most of the boreholes were initially flowing sources, meaning that the pressure alone brings water to the surface and they don’t require to be pumped. However, due to the increased prevalence of extraction which causes lowering pressures, some boreholes which used to flow freely now require to be pumped to continue to serve as a viable water source. Currently, there are roughly 4,700 flowing boreholes and 30,000 non-flowing boreholes throughout the Great Artesian Basin that account for an average of 570,000 megaliters to be discharged per year.
Natural Discharge
01 / 01
1
Lake Eyre
Modes of natural discharge include outflow via vertical leakage through semi-permeable confining layers into the surrounding watertable, subterranean outflow into other basins and the Gulf of Carpentaria, and through clustered springs found throughout the basin. Vertical leakage mainly occurs along the Basin’s margins, where the watertables are shallower, the confining beds are thinner, and the pressures are higher. Though generally occurring at a very slow rate, it can account for a substantial volume of discharge. In the Basin’s primary discharge area, the area surrounding Lake Eyre, this vertical leakage makes up around 45% of the total discharge in the area.
Artesian Springs
Spring outflow is fairly abundant throughout the Basin’s marginal areas, and especially in the southern, south-western, north-western and northern marginal regions. These springs are generally found in clusters atop either geologic faults where groundwater flows upwards, where the aquifers flow into impervious bedrock, and where highly pressurized groundwater breaks through areas where the confining layers are much thinner. Thirteen groups of these springs have been identified throughout the Basin. Twelve of these spring groups are noted to have very low rates of discharge, ranging from less than 1 liter per second (L/s) to 150 L/s. However, one particular group of springs, located near the Cape York Peninsula (Group L on the map), has a much higher discharge rate peaking at 20,000 L/s. These Cape York Peninsula springs contribute significantly to the baseflow, or “normal” flow, of the Dalhunty, Mitchell, Wenlock, Archer, and Jardine Rivers, the latter of which receives 77% of its baseflow from these springs.
Biotic Communities of the GAB
The GAB’s artesian water, aside from providing a constant, clean supply of water for humans and local fauna, also plays a vital role in nourishing micro-wetlands at spring sites and providing additional support for keystone tree species, namely the river red gum (Eucalyptus camaldulensis). The river red gum is the most abundant eucalyptus tree in Australia due in large part to its ability to thrive in arid climates. The river red gum’s adaptability is attributed to the fact that it can survive by selectively obtaining water from groundwater, surface water and soil moisture depending on availability. Additionally, the fact that river red gum will tend to prefer groundwater over in-stream water in situations where both are readily available underscores the vital role the GAB plays in local biotic communities.
Pre-Colonial Use
Archaeological evidence and oral histories indicate that Aboriginal and Indigenous peoples were aware of and frequented the artesian springs located towards the outer edges of the GAB thousands of years before the first European settlement in 1788. Across the Basin, the springs were of massive cultural significance as they were a reliable source of water during droughts, nourished the wildlife that was hunted for food, and were crucial in establishing travel and trade routes. For the peoples inhabiting the more arid deserts of the Basin’s southern and western regions, mound springs were a highly respected resource. Even though they produced a constant flow of fresh water, they were primarily only used during the most severe of droughts, when all their other resources had become too scarce. Additionally, though none of the springs serve as ceremonial or ritual centers, the springs are heavily included in various myths and stories of Ancestors.
Colonial to Present-Day Use
01 / 01
1
Bourke, New South Wales
Colonial settlers first began extracting water from the GAB in 1878 when a relatively shallow bore hole was drilled near the town of Bourke. Many more wells were quickly drilled as they not only facilitated easier access to drinking water in areas far away from lakes or streams, but they also allowed for agricultural and livestock operations to expand and thus generate more income. (Image from Powell et al. 2015)
Vanishing Springs
By 1915, there were over 1,500 flowing bores excavated in the GAB, many of which flowed at rates of over 10 megaliters per day (ML/d). Due to continued extraction decreasing the volume of water in the aquifer, the amount of underground pressure has dropped, causing one third of all previously flowing bores to require to be pumped. Currently, the majority of existing flowing bores discharge at rates between 0.01 and 6 ML/d.
One of the more extreme examples of the unforeseen consequences of such uncontrolled boring in the GAB is found in Wee Wallah Springs, which immediately stopped flowing as soon as a bore was drilled and tapped nearby. This sudden redirection of water not only eliminated any chance of the spring flowing naturally again, but it also engendered the demise of the ~120 m^2 micro-wetland it nourished.
