Pressure and heat are highly valued but rarely acknowledged resources provided by our Great Artesian Basin.
Artesian groundwater is stored under pressure in the sandstone layers that are trapped between impervious rocks or clays. The water can be held under great pressure and if tapped by a water bore will rise above the ground surface. The term ‘artesian’ comes from the former French province of Artois where many wells were dug, starting in the 12th century.
Sub-artesian bores are bores in which the water may rise up above the upper surface of the aquifer, but there is not enough pressure to push it above the land surface.
Only a decade into Australia’s 20th century water bore drilling boom, it became obvious that flows from existing bores were decreasing and that pressure was falling as more bores were installed. It was extremely difficult to measure GAB groundwater pressure at that time and there are few accurate records.
In 1912, the first of many interstate conferences was held to consider solutions. Yet bore drilling continued and flows from the GAB peaked at more than 3,000 megalitres a day in 1915, before falling to about 1,000 megalitres a day in 1998. Pressure head in some bores fell by as much as 80 metres (equivalent to 785 kilopascals) and one-third of bores stopped flowing altogether.
Hopes of an inexhaustible supply of water in the arid interior were dashed. Pastoralists whose bores became sub-artesian had to install expensive pumps to access water.
The drop in pressure was also a problem for some of the GAB springs. Located where the source aquifer expresses at the ground surface, springs are vulnerable to declining pressure and reduced flows. Almost half of the springs became extinct.
Governments began bore rehabilitation work in the 1970s and 1980s. A 15-year cooperative funding scheme called the Great Artesian Basin Sustainability Initiative (GABSI) commenced in 2000. GABSI and its predecessors have been as much about restoring pressure as preventing water wastage and restoring flows to desert springs.
As bores have been rehabilitated and water delivery systems piped and capped, pressure has begun to recover. Over the entire GAB, pressure recovery is on track, similar to the predictions made by GAB groundwater models. There has been significant pressure recovery in northern Queensland, and modelled predictions have already been exceeded over a large part of the GAB in New South Wales. Much work was undertaken in South Australia prior to the GABSI program and there have been gains in pressure in the South Australian part of the GAB.
The effects of pressure recovery on overall spring health are variable and may take many years to be observable, but some gains have been made.
New industries and uses of GAB water are emerging that offer economic development opportunities for the region and additions to the traditional pastoral base. It is important to ensure that the pressure recovery achieved through the good work and investment in GABSI to date is not compromised.
GAB water is warmed by the heat produced by the radioactive decay of uranium, thorium and potassium deep in the Earth’s mantle, and by past volcanic activity.
Groundwater temperature across the GAB is generally highest in the deepest areas. At depth and under pressure, GAB water temperature can exceed 130ºC. The average temperature ranges between 30º and 50ºC at the surface but reaches boiling point in some places.
GAB spring water tends to be cooler than this and ranges between 20ºC and 45ºC. Temperature has a noticeable effect on the life found in and around GAB springs. Thermophilic or heat-loving species are found closest to the springs, with less heat-tolerant plants and animals adapted to cooler waters further away.
The temperature of GAB water has implications for the costs, design and materials used in the closed water delivery systems that have replaced open bore drains in many areas within the GAB. Different types of plastic, thicker-walled pipes, cooling ponds or tanks are being incorporated into the design of water delivery systems in areas where very hot water reaches the surface.
The temperature variation in GAB water has been plotted and mapped as a ‘geothermal gradient’. It is of interest for ‘hot rock’ technology, which aims to use the hot groundwater for the generation of electricity.
For example, the Birdsville geothermal power station derives its energy from a 1,230 metre deep bore. This water emerges from the GAB at close to boiling point (98oC) and also provides the town water supply. Several bores have been sunk to investigate the suitability of areas such as Innamincka in South Australia for this relatively new and low emissions technology.