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ANSTO scientists are using nuclear techniques to
investigate groundwater trapped beneath the Sydney Basin
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Search for water goes underground |
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With Sydney's dam levels dropping below 40% of capacity, and the rest of Australia preparing for a long dry summer, talk about a solution revolves around desalination and water-recycling plants. ANSTO scientists are also looking for answers - underground.
Dr Chris Waring's team from the Australian Nuclear Science and Technology Organisation (ANSTO) is using nuclear techniques to assess and date groundwater trapped in Sydney's aquifers.
The Sydney Basin extends from Newcastle to Jervis Bay and west to Lithgow, and contains an enormous amount of water. The problem? The water is trapped deep in aquifers in the sandstone.
Extracting groundwater from the region's famous Hawkesbury sandstone proving to be as hard as squeezing blood from a stone. This is because the rock isn't very permeable - less than half the bores drilled are reaching aquifers where the water's flow rate is high enough to make extraction viable.
Chris's team utilises nuclear techniques to assess bores to understand the detailed hydrogeology. "We use californium-251, to produce neutrons which activate elements surrounding the bore hole and emit gamma ray," Chris explained.
"Gamma spectral analysis detects the major elements including hydrogen, silicon, chlorine, and iron. Measuring hydrogen and silicon is important because this tells us the porosity of the sandstone."
The next step involves plain old stable salt, a tracer which is into the bore hole. By calculating how far the salt tracer has moved, Chris can measure hydraulic conductivity (how easily water moves throughout rocks) at 20cm increments up the bore hole.
"The key to making groundwater viable as an emergency supply for Sydney is extracting it at a reasonable rate. Water pumped out of the rock can flow into dams to top them up in an emergency - effectively increasing the storage."
The other side of the coin is sustainability - having defined the resource, we don't want to overexploit it. Crucial to effective management is knowing how quickly an aquifer is replenished, or 'recharged'. If water is pumped out at a rate that matches recharge, the aquifer is sustainable. Short term draw-down in groundwater reserves may be acceptable if replenished by a later recharge.
A clue to recharge is the age of the groundwater. Chris said, "Age-dating tells us residence times and the flow rates. In the Sydney Basin, water in the top 200 metres of the aquifer may have apparent (uncorrected) ages of a thousand years. Deeper water with longer flow paths to river or ocean discharge could be much older."
'Young' groundwater suggests rapid recharge from recent rainfall. 'Old' groundwater indicates that recharge is slow, so usage must be managed with particular care.
To determine the age of the water, the scientists analyse the carbon-14 in dissolved carbon dioxide and the tritium radioisotopes in water. Then it gets tricky. They have to correct the apparent age, because of potential interaction with carbonates in the rock, such as siderite. Some siderite may dissolve, mixing and diluting the carbon dioxide from near surface soil water giving an apparently older age than is the true age. By analysing the carbon-13 in siderite, they can help determine the true age of the water.
In one study at Mangrove Mountain on the NSW Central Coast, some groundwater had an apparent age of some thousands of years, much older than groundwater model studies had predicted without measurement.
Chris explained, "We will have to apply a correction and see if we can reconcile these results. If the apparent ages don't change very much a closer eye will have to be kept on its usage - it already supplies local residents, farmers and industries, and is bottled as spring water."
ANSTO anticipates its specialist services will be called upon in the search for sustainable water in other parts of the Sydney Basin.
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