IT is one of the world's largest underground water reservoirs, covering an area bigger than Iran. But a new report has found that the Great Artesian Basin's pumping power comes from an area smaller than Tasmania.
A scientific review has raised questions about the basin's capacity to withstand water extraction necessary for coal-seam gas mining.
The concern is not the impact on the basin's volume, but the pressure that keeps bores flowing from Cape York to Coober Pedy.
The report, to be presented at today's meeting of the NSW Great Artesian Basin Advisory Group, has found the reservoir's "recharge" area is about a third as big as previously thought, covering less than 10 per cent of the 1.7 million sq km basin. The area where the basin is topped up by more than 5mm a year — the minimum needed to keep the basin pressurised — is about a quarter of this.
The report says just 0.2 per cent of the basin provides recharge waters in excess of 30mm a year. Most of this is in north-western NSW's Pilliga region, where energy giant Santos is conducting exploratory drilling for a controversial CSG project. "The significance of the recharge zones is not so much as an immediate water supply, but that they provide the pressure head required to drive the water to the surface," says the report, by soil scientist Robert Banks.
The Great Artesian Basin (GAB) of Australia extends over 22% of the Australian continent where it is the only reliable groundwater or surface water source. The GAB contains 65 000 km3 (or 115 658 Sydney Harbours) of groundwater which is released under pressure to the surface through natural springs and artesian bores across its extent (QDNRM 2012).
Much of the groundwater held in the GAB is very old, having taken thousands to many hundreds of thousands of years to reach its current position in the basin from the recharge beds which are predominantly around the margins of the basin. Modern recharge is not thought to add significantly to the volume stored in the basin however it provides the crucial pressure head to keep the artesian waters flowing to the surface across this massive expanse of land. In most areas, the bulk of the GAB has a recharge value of less than 0.1 mm/yr…..
The following description of the Great Artesian Basin (GAB) is given in Ransley and Smerdon (2012).
The GAB contains an extensive and complex groundwater system. It encompasses several geological basins that were deposited at different times in Earth's history, from 200 to 65 million years ago in the Jurassic and Cretaceous periods. These geological basins sit on top of deeper, older geological basins and in turn, have newer surface drainage divisions situated on top of them (e.g. the Lake Eyre and Murray-Darling river basins). In this context – as a groundwater basin – the GAB is a vast groundwater entity underlying one-fifth of Australia.
Discharge from the GAB aquifers occurs naturally in the form of concentrated outflow from artesian springs, vertical diffuse leakage from the Lower Cretaceous-Jurassic aquifers towards the Cretaceous aquifers and upwards to the regional watertable and as artificial discharge by means of free or controlled artesian flow and pumped abstraction from water bores drilled into the aquifers.
For the GAB, like many other semi-arid to arid zone aquifers around the world, the current rate of recharge is significantly less than discharge. Groundwater currently stored in the Cadna-owie – Hooray Aquifer and equivalents is a legacy from higher recharge rates that occurred during much wetter periods in the early Holocene and Pleistocene age (essentially the last 2.6 million years).
The significance of the recharge zones to the GAB is not so much as an immediate water supply to central parts of the basin and natural discharge areas, but that they provide the pressure head required to drive the water to the surface. Removal of this pressure through water abstraction associated particularly with Coal Seam Gas (where local drawdown of in excess of 1000 m can be experienced around gas fields) risks removing the driving force of many of the free flowing artesian bores and springs in the GAB…..
Concern regarding CSG extraction is raised in Ransley and Smerdon (2012) in the following quote. "CSG production in the Surat Basin targets the Jurassic Walloon Coal Measures. The main CSG producing fields are located in the northern Surat Basin in a broad arc extending from Dalby to Roma. For gas to be harvested, the coal seams need to be depressurised by pumping groundwater from tens of thousands of wells intersecting the Walloon Coal Measures. Drawdowns of several hundred metres will be generated by the depressurisation and significant volumes of groundwater are to be pumped from the Walloon Coal Measures –averaging about 75 to 98 GL/year over the next 60 years (RPS Australia East Pty Ltd, 2011). This process will induce drawdown in overlying and underlying GAB aquifers, the amount of which will depend on the leakiness of the system."…..
In NSW the recharge areas of higher than 5 mm/yr are almost entirely contained within the east Pilliga area……
The above results show that:
Recharge along the Jurassic to Cretaceous margins of the GAB is crucial to providing hydraulic head which drives the whole system.
Significant recharge to the bulk of the GAB is much more limited in area than
previously thought.
Although approximately 30% of the GAB is mapped as recharge, only 9 – 10% of the GAB is effective recharge which maintains the pressure head on the bulk of the GAB (excluding the Carpentaria basin).
Only 2.3% of the GAB has effective recharge of greater than 5 mm/yr.
Only 0.2% of the GAB has effective recharge of 30 – 79 mm/yr.
In NSW, the main occurrence of recharge >30 mm is in the east Pilliga between
Coonabarabran and Narrabri.
Draw down of many hundreds of metres is reported in Ransley and Smerdon (2012) for the northern Surat basin coal seam gas fields where coal seams are being
dewatered to release gas.
Draw down of in excess of 1000 m is proposed in the Pilliga in the south eastern Surat Basin (ICSG Forum, 2014).
Both of the Pilliga and the northern Surat gas fields or license areas occur in the very limited high recharge (>30 mm) areas of the GAB.
Excessive draw down of pressure heads in the recharge zone of the GAB associated with gas extraction, has the potential to reduced pressure heads on artesian waters across much of the GAB, and potentially stopping the free flow of waters to the surface at springs and bores.
Gas and petroleum exploration and production licenses cover 80% of the entire GAB.
Gas and petroleum exploration and production licenses cover 79% of the critical
higher recharge areas of the GAB……
Consideration should be given to a basin wide approach to the management of the GAB with respect to minerals and natural resources, particularly with respect to potentially wide ranging activities such as gas and petroleum production where groundwater from below the GAB is drawn down and produced as an excess or waste byproduct of such development. In particular, serious thought needs to be given to the management of the few high recharge zones within the GAB and how these might interact with future water supplies…...
The East Pilliga area between Narrabri and Coonabarabran in NSW has Soil and Land Capability Classification (SLC) of between 4 and 6, meaning that there are no contiguous areas of Biophysical Agricultural Land (BSAL) in the area. BSAL is defined as Classes 1 to 3. This means that currently no special consideration which includes landscape function is given with regard to CSG and Mining applications in the high recharge zone areas of theGAB within the East Pilliga……
