Great Artesian Basin (GAB)

The GAB and Drought Relief

Some thoughts from FOMS Patron, Lynn Brake

Over the past few months the media has been filled with information about the effect of the current drought on primary producers, especially in the drier parts of Australia. In most of the country underlain by the GAB the effect of the dry weather has been exceptionally severe. Water from the GAB is the only water to sustain life and supports all human activity in much arid and semi-arid parts of Australia. There is no alternative source. More than 120 towns, hundreds of pastoral stations, mining and petroleum industries and visitors rely all on GAB water. There are more than 34,000 water bores in the GAB. They support more than $12bil in production and have a replacement cost of more than $4bil. There are more than 6000 GAB springs. These and the groundwater dependent ecosystems that they support are sites of immense cultural and natural value. The GAB is one of the largest artesian Basins in the world and unquestionably Australia’s most important groundwater resource.

Recent research by Geoscience Australia demonstrates that recharge rates into the GAB from rain along the dividing range is as much as 90% less than indicated by previous studies. This means that we are effectively ‘mining ‘water from the GAB. Pressure is naturally falling; this is being accelerated by water extraction through bores. As a result, the judicious use of GAB water is essential to eliminate any waste and sustain the benefits that we value from the GAB. The springs will be affected and many even dry completely with just a small pressure drop. Pressure is not the only issue however. As islands of wet in an otherwise sea of dry springs attract animals from a great distance to water and feed on the vegetation in and around the spring vents. The impact of these animals risks the natural and cultural values that springs support.

So far the risk to overexploitation of the GAB is being quite well managed. If we are judicious and diligent and work cooperatively together to manage the risks to artesian pressure and maintain the surface structures around springs and the Ecosystems that they support the GAB will continue to provide water and benefits for people, industries and spring fed ecosystems for many years to come.

GAB spring on Finniss Springs (L) and Bore control infrastructure (R)

The Great Artesian Basin and Its Springs

M.A. Habermehl

Canberra-based Dr MA (Rien) Habermehl is an internationally recognised hydrogeologist who has decades of experience in researching the Great Artesian Basin and its associated mound springs. Dr Habermehl is a FOMS member and accompanied the group on its field trip in August 2015. Here Dr Habermehl provides an overview of the GAB and its springs.

Great Artesian Basin map
Great Artesian Basin map

The Great Artesian Basin is a confined groundwater basin, which underlies arid and semi-arid regions across 1.7 million km2 or one- fifth of Australia. The basin’s groundwater resources were discovered around 1880, and their development allowed the establishment of an important pastoral industry. Pastoral activity, town water supplies, mining and petroleum ventures are all totally dependent on artesian groundwater. The Great Artesian Basin is a multi- layered confined aquifer system, with artesian aquifers in Triassic, Jurassic and Cretaceous continental quartzose sandstones. Intervening confining beds (aquitards) consist of siltstone and mudstone; Cretaceous marine sediments form the main confining unit. The Basin is up to 3000 m thick, and is a large synclinal structure, uplifted and exposed along its eastern margin and tilted southwest.

Recharge occurs in the relative high rainfall eastern margin, and the western margin in the arid centre of the continent receives minor recharge. Regional groundwater movement is towards the southern, southwestern, western and northern margins, where artesian springs discharge and produce carbonate mounds. Average groundwater flow rates in the eastern and western marginal parts range from less than 1 to 5 m/year based on hydraulic data and 14C, 36Cl and 4He measurements. Chlorine-36 and carbon-14 isochrones show residence times from recent to several thousand years near the marginal recharge areas, to more than one million years near the centre of the Basin. Environmental (stable and radio-active) isotopes and hydrochemical studies confirmed the source and origin of artesian groundwater as meteoric water, which recharged from geological to modern times.

Great Artesian Basin diagramPotentiometric surfaces (pressure levels) of the Triassic, Jurassic and Early Cretaceous aquifers are still above ground level, but pressure drawdowns of up to 100 m were recorded from 1880 to the 1990s in some relatively closely developed areas. Artesian groundwater extraction by the pastoral industry and for homestead and town water supplies, which peaked at about 2000 ML/day around 1918, caused this drawdown. About 4700 flowing artesian water bores were drilled in the main Lower Cretaceous-Jurassic aquifers at depths of up to 2000 m, but average 500 m. About 3100 controlled and uncontrolled artesian waterbores remain flowing with an accumulated discharge of 1500 ML/day. About 25000 non-flowing artesian water bores, generally using windmill operated pumps, tap shallower Cretaceous aquifers. Some flowing artesian water bores ceased to flow, necessitating groundwater to be pumped and spring discharges declined, caused by water bore development and resultant lowered artesian pressures in many parts of the basin during the last 130 years and in some areas springs have ceased to flowGroundwater quality of the Lower Cretaceous-Jurassic aquifers is good at 500 to 1500 mg/L total dissolved solids. Groundwater is suitable for domestic, town water supply and stock use, though unsuitable for irrigation in most areas.

The water is of the Na-HCO3-Cl type, and these ions contribute more than 90 % of the total ionic strength of solutes in the main Basin area.  In the southwestern part of the Basin the groundwater is characterised by Na-Cl-SO4 type water, and the two regional groundwater flow directions show different hydrochemical characteristics, with westward flowing water being of the Na-HCO3-Cl type and eastward flowing water being of the Na-Cl-SO4 type.

Water quality is better in the lower aquifers than in the higher aquifers in the Lower Cretaceous- Jurassic sequence. Groundwater temperatures at the bore-heads range from 300 to 1000 C, and are a potential geothermal energy source. Spring temperatures range from 200 to 450 C.

Free-flowing open earth bore drain
Free-flowing open earth bore drain

Prior to the 1960s, artesian water use was mainly for pastoral, homestead and town supplies, but since then the development of petroleum resources in the basin area also used its groundwater. The basin (and underlying rocks) comprises abundant hydrocarbon reservoir (and some source) rocks, and commercial and sub-commercial oil and gas is produced from Jurassic and Cretaceous sandstones, contradicting earlier beliefs that the basin-wide groundwater throughflow had flushed out hydrocarbons.  Since the 1980s, mineral mining in and near the basin started using the basin’s artesian groundwater, in particular from the Olympic Dam bore-fields in South Australia and mines in north-western Queensland and north- eastern New South Wales. Present day and future additional oil and gas, mineral mining, coal seam gas and geothermal developments could affect artesian groundwater flow to water bores and springs.

Environmental geology issues relate to the development of the artesian groundwater resources of the Great Artesian Basin and include aspects of sustainable groundwater use and groundwater and rangelands management. Extraction of artesian groundwater during the last 130 years has affected the Basin to varying degrees through large scale drawdowns, which reduced artesian pressures and reduced discharges from artesian water-bores and springs.

The Great Artesian Basin Bore Rehabilitation Program (GABBRP, 1989-1999) and the subsequent Great Artesian Basin Sustainability Initiative (GABSI, 1999-2014) which provides a substantial level of Australian Government and State Government assistance to landholders to rehabilitate flowing artesian bores and replace the open earth bore drains of free-flowing artesian bores with piped water distribution systems throughout the GAB.

Great Artesian Basin Bore Rehabilitation Program
Great Artesian Basin Bore Rehabilitation Program

GABSI assists the implementation of the GAB Strategic Management Plan prepared by the Great Artesian Basin Coordinating Committee (GABC 2000). he Plan provides for the restoration of the environmental assets of the Great Artesian Basin with an emphasis on springs (GABC 2000).

Following the implementation of GABSI and rehabilitation of bores and reduction of their outflows, potentiometric surfaces have become stable and increased in some areas and may increase flows from springs. Securing groundwater flows to springs, particularly those with high conservation values, could be an addition to the criteria used to select bores for rehabilitation and piping to maximise opportunities to sustain or re-activate springs. The GABSI program has recently (2015) been extended for a period of three years. These programs aim to rehabilitate waterbores in poor condition and equip bores with control valves.  The replacement of the inefficient open earth drain distribution system, which causes up to 95 percent wastage of the water, with a piping system is encouraged. These measures will benefit groundwater and rangeland management.

In addition, the Environment Protection and Biodiversity Act (EPBC) 1999 provides environmental protection for the GAB springs, and is the main regulation tool for larger developments. Groundwater use by the petroleum and mining industries during the last 20 – 35 years affect some parts of the Basin. The South Australian part of the Basin is an example where the combination of groundwater exploitation for the pastoral industry, town and homestead water supplies and petroleum and mining industries impact on the Basin’s groundwater conditions and on the artesian springs, the natural outflow points of the Basin.

The natural flowing artesian springs originating from the Jurassic-Lower Cretaceous aquifers occur throughout the GAB, though mainly in the marginal areas, within twelve large supergroups, including the Barcaldine, Springsure, Bogan River, Bourke, Eulo, Lake Frome, Lake Eyre, Dalhousie, Mulligan River, Springvale, Flinders River and Cape York. Most springs are concentrated in relatively small areas, with rates of discharge of individual springs ranging from less than 1 L/s to about 150 L/s (from a spring at Dalhousie Springs, northern South Australia).  Flowing artesian springs within the Basin and in the discharge margins of the Basin are generally associated with structural features, such as faults, folds, monoclines and intersecting  lineaments.

Upwards groundwater flow along faults is the source of many springs, as well as the abutment of aquifers against impervious bedrock and pressure water breaking through confining beds near the discharge margins of the Basin. Diffuse discharge occurs from the artesian aquifers near the margins where the overlying confining beds are thin and waterlevels high.  Springs are quite  common in the recharge areas along the eastern margins, but most of these springs are the  result of “overflow” or the “rejection” of recharge into the aquifers, or result from the intersection of the local topography and aquifers. Many springs have built up conical mounds consisting of deposits of clayey and/or sandy sediments and carbonates, which are several metres to several tens of metres in diameter, and up to several metres high. The mounds are formed by deposition of particles brought up from the aquifers and the confining beds, by accumulation of aeolian material and by the chemical and biological precipitation of solids dissolved in the artesian groundwater.

Some mounds consist of mud, but many mounds, particularly those built by springs in the western and southwestern margin of the Great Artesian Basin are built up of carbonates, which are dominated by tufa, travertine and very fine-grained limestone or crystalline carbonate, mainly calcite and dolomite. The carbonates originate from the combined chemical precipitation of calcium carbonate out of the artesian groundwater, and precipitation by algae and bacteria. Terraced mounds and waterfall or cascade deposits produced by algae are common, though most accumulations consist of steeply sloping mounds. These springs are terminal evaporitic systems, usually comprising a central carbonate mound, with outer zones dominated by sulphate and chloride salts. Mound morphology is controlled by several factors, including groundwater discharge rates, hydrochemistry, evaporation, influence of inorganic versus organic carbonate precipitation and local subsidence of the mound.

The natural flowing artesian springs and their spring mound deposits in South Australia are of particular significance and interest as the deposits are geological (and hydrogeological) records because of their fossils of fauna and flora, their ages, and their relationship to wetter and drier times (palaeoclimates) in central and eastern Australia during the last several 100 000s years and changes in groundwater discharges and spring deposits, their Aboriginal and early European history and culture, and the presence of unique flora and fauna.

Artesian springs and their deposits in the Lake Eyre region in the southwestern part of the Basin range from topographically high springs to younger, topographically low springs as a result of the lowering of the land surface and spring outlet levels in Quaternary times (deposits of extinct pre-Quaternary springs occur more than 40 m above the present springs). Fossil carbonate spring deposits of Pleistocene ages, consisting of very fine- grained carbonates with abundant reed casts, gastropod shells and algal structures, rise several tens of metres above the present land surface where the present active springs (and Recent spring deposits) occur. The higher, older spring carbonates cap circular mesas and hills and overlie pedestals of Cretaceous mudstones (Hamilton Hill, Beresford Hill and Strangways Springs).

This also indicates that the potentiometric surface in the Lake Eyre region has declined considerably during recent geological time. Morphological diversity and lithofacies patterns indicate that the spring complexes have developed over several climatic cycles. The dated ages of spring deposits range up to 740 000 ±120 000 years, with some spring deposits probably being older. The ages of several basal spring deposits, as determined from thermoluminescence, 14C, U/Th and palaeomagnetic studies suggest that some springs might have been (re-) activated as a result of major climatic changes.

An increased awareness and understanding of the springs and environmental aspects since the 1980s has helped to preserve the springs, despite increased groundwater development by pastoral and resources industries and increased tourism and visitor numbers, and through the creation of the Witjira (Dalhousie Springs) National Park and the Wabma Kadarbu Mound Springs Conservation Park.

The provision of access to some good examples of springs in these Parks, fencing and explanatory signs and other guidance, has and should offer regulated access and limit environmental damage to the accessible springs and to other springs within fenced areas. The cooperation between landholders with springs on their properties and government authorities has resulted in successful management, though further activities, including fencing, should be undertaken to exclude grazing and trampling by livestock and feral animals.

A large number of scientific studies on the springs, by Australian Government and State Government organisations, universities and resources industries, as well as long term monitoring of their flora and fauna, environmental aspects, nature and origin, including the deposition and composition of the spring mound sediments, their ages and the groundwater flow and hydrochemistry, has significantly increased knowledge about the springs since the 1980s, though continuing monitoring and additional studies should be carried out.