AUSTRALIA: Lack of knowledge threatens groundwater resources

More water is being extracted than is naturally replenished by rural producers and water engineers because they do not know how fast groundwater sourced from fractured rock aquifers moves and the rate at which it recharges, says CSIRO Land & Water’s Dr Peter Cook.

Fractured rock groundwater systems extend over 40 per cent of Australia’s land area. They are being used increasingly to supply irrigation and domestic water, chiefly in regions where population or agriculture is growing and where existing sand aquifers are already fully exploited or over-exploited.

Dr Cook says a key challenge for water managers is to determine the amount of rainfall required to recharge a fractured rock aquifer. This can vary from one percent to 30 per cent of annual rainfall, depending on the location, soil type and other environmental factors. “If we don’t know, then no realistic allocation rate can be set for its extraction,” says Dr Cook.

They also need to understand how the alignment of the cracks in the rock affects the water’s flow and speed. This will help in siting bores correctly and drilling to the right depth.

Understanding the character of rock fractures can also help reduce groundwater contamination, because high water speed through fractures can move contaminants long distances very quickly. Dr Cook says many water engineers mistakenly believe water storage and transport in fractured rock aquifers is similar to sand aquifers, which are less complex and much easier to manage.

“Very little is known about fractured rock aquifers. This means people often use the wrong management methods, even though they don’t work,” Dr Cook says. “As a result there are areas of Australia where people are using three to four times more water than is being recharged, so the water is running out. This highlights the need to develop proper tools to manage and use groundwater sustainably.”

Three CSIRO-led research projects are pioneering systems to measure the movement and recharge rates for fractured rock aquifers across Australia.

In Queensland’s Atherton Tablelands, where fractured rock groundwater is used to irrigate horticultural crops during the dry season, there are concerns that over-extraction may imperil Heritage-listed rainforest further downstream.

In the NSW city of Wagga Wagga, water movement and direction in fractured rock aquifers is being harnessed to maximise efforts to combat dryland salinity.

“Understanding the behaviour and direction of underground water flow will help determine the effectiveness of pumping groundwater to lower the watertable,” Dr Cook explains.

In South Australia’s premier wine-growing district, the Clare Valley, growers want to know how much more water they can use to increase vineyard capacity, without overexploiting the resource.

Dr Cook says use of water from fractured rock aquifers is becoming more widespread around Australia as many sand aquifers are now fully exploited.

“We need to give people the ability to use groundwater sustainably. In many areas people are watching the water levels falling and know they are using too much but it is very hard after the fact to do anything about it. “So as we make more and more use of this water source, it’s vital we quickly gain an understanding about it and how to manage it.”