Origins of the GAB’s Water

Since 1900 some of Australia’s and the world’s most respected scientists have argued about the origins of the water in the Great Artesian Basin. The common assumption was that any water taken out was replenished by rainfall.

Traditionally, the recharge for the GAB has been thought to be 30% of the basin, and that it was significant. The recently published information contained in this report shows clearly that effective recharge (>1 mm/yr) is only 6% of the GAB. (SoilFutures Report, p. 23)

Recent studies reveal that three theories apply to different sections of the basin and although it is replenished by rainfall it is at an alarmingly slow rate compared to the amount of water being taken out. The following are the three theories of the origins of the water in the GAB -

Meteoric Theory: Artesian water is derived from rainwater absorbed

1. By intake beds/recharge areas along the western slopes of New South Wales and Queensland.

2. Where creeks, rivers and alluvial fan systems cross the intake beds. (An alluvial fan system is when rainwater flows fast down a mountain range to a point then fans out when reaching the plains. This is very important within the sandy East Pilliga section of the Coonamble Embayment of the Surat Basin. The alluvial fan disappears after high rainfall.)

3. By aquifers above the GAB.

Plutonic Theory: Artesian water is derived from rocks in the deeper layers of the earth's crust. This water may be referred to as juvenile as it appears on the earth’s surface for the first time.

Connate Theory: Water was entrapped within the structural basins or hollows when the porous strata and overlaying clays and shales were deposited.

I asked geomorphologist, Robert Banks about the origins of the water in each of the four basins, which make up the GAB.

Surat Basin – The most easterly basin is recharged by rainwater (meteoric) from the higher eastern border and from overlying aquifers. This basin has a proven ten percent connection with underlying petrochemical rich basins.

Central Eromanga Basin – The big basin in the centre of the GAB is fed from the sides from recharge areas and from water overflowing from the Surat Basin (meteoric). About ten percent of the water in the Central Eromanga basin comes up from underneath (plutonic). It has up to 50 percent connection with underlying petrochemical rich basins.

Western Eromanga Basin – The most western basin contains some plutonic water, is fed from the Central Eromanga basin and has low recharge areas on its western border.

Carpentaria Basin – The most northern basin has a high rainfall and its own recharge areas from overlying regional aquifers (meteoric) on its eastern side. It discharges against the coast of New Guinea. This basin was not included in Rob Banks’ study as it does not affect the rest of the Basin.

‘Some of the water in the GAB is almost certainly still there from the time when the rocks were formed, which is connate,’ said Rob. ‘So it’s a mixture of all three. Recharge water can also come from rain falling outside the basin’s surface area. River systems, such as the Murray Darling, cross recharge areas and contribute to the GAB too.’

‘Most of the groundwater in the GAB has taken thousands to many hundreds of thousands of years to reach its current position. Some of the water, in the southern and central parts of the basin, is over two million years old. That’s how long it has taken to get from somewhere like The Pilliga to South Australia. Only six percent of the area of the GAB has effective recharge zones providing more than one millimetre per year. Modern recharge is not thought to add significantly to the volume of water but it provides the pressure head (or height and weight of water) needed to keep the bores flowing to the surface without the aid of pumps.’

Recently Rob did a soil survey in The Pilliga. He found that some of the creek beds have water below the surface within a few metres at all times. He also found water in sand monkeys away from the creeks.  Both are responsible for contributing to recharge of the GAB.

‘Sand monkeys are low rises of a metre or less in the terrain that are dominated by very deep sand. They are like sandy stream beds, with not much in the way of sub soil underneath them. We dug up sand monkeys and found really good water within six metres whereas areas in between had no water at all. These deep sand bodies are where the rough barked apple or apple box trees grow. These trees need lots of water and their roots can go three times deeper than their canopy, accessing water 60 metres below the surface. Lines of them grow in the Pilliga in these sand bodies and along creeks.’

Ref: Great Artesian BasinRecharge Systems and Extent of Petroleum and Gas Leases, 2011, SoilFutures Consulting Pty Ltd for the Artesian Bore Water Users Association. 

     Map of GAB and different Basins on page 5

     Recharge maps see pages 9 - 15