World wide, groundwater is a huge source of water, with some estimates saying underground freshwater supplies total as much as eight million cubic kilometres – or 97 per cent of all freshwater (not frozen) found on the planet. In a dry country such as Australia, the challenge is to understand how groundwater systems and groundwater dependent ecosystems operate, how big they are, and how best to manage them in a sustainable and innovative way. It's not always easy: the science can be complicated, and there can be differing theories and unknown variables. The general view is that aquifers in Australia are topped up by rainfall, with the water slowly percolating through the aquifers and rock structures. In the case of the Great Artesian Basin, it's commonly believed that it is an open system which is fed by the rains falling on the eastern boundary of the GAB, along the western slopes of the Great Dividing Range. Dating of groundwater Radioactive isotopes such as tritium, carbon-14 and chlorine-36 can be used to date groundwater by estimating the time it has been in the aquifer. One study involving chlorine-36 dated water in the GAB to be 1.5 million years old, possibly some of the oldest flowing water on the planet. Isotope testing indicates the oldest water is in the centre of the basin, and the youngest waters come from the eastern side of the basin. Other types of tests identify the waters of the GAB as originating from rainwater. Using chlorine-36 dating, researchers discovered that there are some shallow aquifers in arid areas that appear to have recharged entirely during wetter periods in Australia's geological history. Because they are not currently being recharged, the use of such groundwater sources depletes the aquifer. This has important implications for communities who rely on old groundwater sources, including allowing sufficient time for infrequent, very large storms to recharge these aquifers. More recently, Australian researchers have used the isotope S35 to date groundwater. The shorter half-life of S35 – 87 days – means that it can be used to measure recent aquifer recharge, and to compare current recharge rates from, for example, heavy versus light rainfall events. Groundwater and surface water are connected There is a growing awareness of the interconnections between groundwater and surface water resources. Other isotopes can be used to measure evaporation and the extent of mixing of river waters and groundwater. This can be helpful in determining what's called 'conjunctive water management' – basically developing integrated water management plans which combine groundwater and above ground supplies in modelling and planning. The individual characteristics of each aquifer, determined by the rainfall, evaporation, river flow, rock types, vegetation cover, extraction and recharge rates can all impact on the sustainability of aquifer use. Because of these variables, extraction of water from each aquifer should be considered on a case-by-case basis. With increasing knowledge about groundwater systems, sustainable yields are being revised. With changing rainfall patterns being experienced across Australia due to global warming, re-evaluation of existing information is required. For example, a 10 per cent decrease in rainfall due to climate change does not simply mean a 10 percent decrease in surface runoff or aquifer recharge. In times of drought much of the water wets the parched soil, providing little runoff or recharge. Related sites:
Other boxes Box 2. Our outback oases – groundwater dependent ecosystems Box 3. Research into sustainable groundwater use
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Posted February 2007.
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