Reprinted with permission of the Institution of Engineers, Australia
IT IS MY CONSIDERED opinion that there has been over a century of serious error in the management of the underground waters of the Great Artesian Basin of Australia.
At the core of the problem is the mistaken belief that the source of all the groundwater is rainfall on exposed sediments, creating an illusion of perennial recharge. The illusion is based on the idea that the groundwater accumulates by seepage along deep porous aquifers, over very long distances of hundreds of kilometres, and with little or no head loss.
Even though such flow of groundwater can be shown to be physically impossible, the fallacy continues despite disturbing evidence of a great decline in flows. There has been an incredible and reckless waste of water over the past 120 years.
The problem has continued for over a century because the authorities involved, and indeed the professions, cannot comprehend any alternative source of the groundwater other than rainfall.
The misconception about recharge from surface rainfall was highlighted in 2000 when the Queensland government passed legislation to define and proclaim the intake areas along the Great Divide where the water is supposed to enter and seep towards the Great Artesian Basin. It was an interesting example of state sovereignty over water.
The misconception of the Queensland government was then reinforced by the federal government in a large report prepared by the Bureau of Rural Sciences. This report was directed at endorsing the Queensland government, as stated by the federal minister when he released the report. Thus state sensitivity was appeased, but unfortunately both sides were continuing the century of error.
The Bureau of Rural Sciences made the fundamental mistake of assuming that the source of the groundwater must be from surface rainfall, and they then simply reported tests on chemical composition, and age as indicated by isotope proportions. There was no thought that the waters may have never been surface waters. Yet the data firmly indicates that the waters are derived from deep within the earth.
In fairness to the Bureau of Rural Sciences team, it should be mentioned that they are in company with most groundwater specialists worldwide, who are similarly convinced that all major groundwater basins are derived from surface rainfall. The recent disturbing evidence of the very serious decline in groundwater yields, and the profound consequences in some poor countries, indicates otherwise.
It is my view that the groundwaters in the Basin are part of the original constitution of the Earth. They have a similar source to the steam that explodes from volcanoes and the hot acid waters that gush from the deep ocean vents.
The Great Artesian Basin adjoins a long and wide zone of recent volcanism, extending from Torres Strait to Tasmania, which is regarded as one of the most extensive volcanic zones in the world. It is now quiet.But it has been very active in recent geological time, and the eruptions would have been accompanied by the release of great quantities of steam. The most recent active volcano in this zone erupted only 4600 years ago, at Mt Gambier, and there are extensive recent basalt flows in north Queensland adjoining the Great Artesian Basin. The hot sulfurous waters from nearby boreholes in the Basin reveal a recent volcanic origin.
The accompanying map of Australia shows the relationship of the Great Artesian Basin to the zone of volcanism extending from Torres Strait to Tasmania, and the nearby metal mines at Broken Hill, Cobar, Mt Isa and Mt Morgan. These metal mines are based on metal sulfide deposits from hot sulfurous solutions originating from deep within the Earth.
There is a great undersea rift valley immediately off the east coast of Australia, extending from southern Queensland to south of Tasmania, and which is quite extraordinary in world terms. It is remarkable that this great tectonic structure has been relatively quiet seismically since the settlement of Australia. The western scarp of the rift passes near Newcastle, where there was an earthquake in 1989.
One feature on this map is the many and extensive lava flows, and extinct volcanoes, covering an area from Torres Strait to Tasmania. Another feature is the concentration of metal mines adjacent to the outline of the Great Artesian Basin. There is a rift valley, immediately off the east coast. This rift valley has steep sides, a wide floor at a depth of 4500m which is called the Tasman Abysmal Plain. On this plain there are a series of old volcanoes, some over 4000m high, twice the height of Mt Kosciusko. These offshore sea mounts form a line which runs parallel to the line of the on-shore volcanism.
Rising volcanic waters over millions of years were blocked by the impervious strata of the basin, accumulating in a huge resource of groundwater which has been mined for the past 120 years. Future yields of ground water will probably be just a very small fraction of what has been taken.
Rising volcanic waters were trapped under the impervious blanket of sediments of the Great Artesian Basin (see cross-section below), accumulating to form a huge resource of groundwater. I believe that the waters were a relic of the period of volcanism that extended over the past 50 million years. Over the past 120 years these residual waters have simply been mined. Most of the outflow has been wasted. Prospective future yields are small.
The vertical section shown above is drawn along a line from west of the northern edge of Lake Eyre across the Great Artesian Basin, the Great Dividing Range and the Tasman Abysmal Plain to Lord Howe Island. Although the vertical scale is exaggerated, the ground surface from Lake Eyre to the edge of the Great Divide is remarkably flat. The dramatic difference is offshore, where there is a huge rift valley 4500m deep, in which there is a line of old volcanoes 4000m high.
* Emeritus Prof Lance Endersbee was president of the IEAust in 1980.