Geology of aquifers in the South West
Upper aquifers occur at the surface and extend across most of the region. They are thickest near Ballarat and south of Hamilton. These aquifers discharge to streams and the ocean.
Middle aquifers extend between Camperdown, Hamilton, the South Australian border and the coast. They are thickest west of Portland and are mostly buried, except along the coast.
Lower aquifers extend across the southern half of the region. They occur at or close to the surface in small areas and are very deep along the coast. Basement rock occurs at the surface in the highlands and is deeply buried under the lower aquifers along the coast. Discharge from these aquifers occurs offshore into the ocean.
These aquifers occur at the surface and receive recharge directly from rainfall. They consist of sandy sediments and basalts. These are the most recently formed aquifers.
Basalt aquifers formed by volcanic activity are found between Ballarat and Portland. These aquifers were formed by numerous basalt flows due to volcanic activity over the past 5 million years and are known as the Newer Volcanics. They are most porous and thickest near the volcanic cones and are more clayey and act as confining layers across the western plains. They occur at the surface and so receive recharge directly from rainfall.
The Bridgewater Formation is an unconfined sand and limestone aquifer that occurs along the South Australian border. These aquifers are predominantly used for D&S and mineral production because the water quality is generally poor.
These aquifers consist mostly of limestone and are buried and confined by the upper aquifers except along the south coast where they occur at the surface. They are thickest near Warrnambool and Portland. These aquifers were formed between 5 and 40 million years ago. Between Camperdown and the South Australian border, the most significant extents of these aquifers form two horizontal aquifers (upper middle and lower middle aquifers) separated by an aquitard. There are also isolated occurrences of limestone aquifers west of the Grampians, north of Hamilton and around Geelong.
Limestone aquifers (upper middle aquifers)
The upper middle aquifer in this region is made up of the Port Campbell Limestone, Portland Limestone, Fyansford Formation, Batesford Limestone, Upper Gambier Limestone, Bochara Limestone, Duddo Limestone and Winnambool Formation. This aquifer is generally less than 50 metres thick and is cavernous in parts. It is semi-confined where it occurs inland and unconfined along the coast.
The upper middle and lower middle aquifers in this region are separated by the upper middle aquitard – the Gellibrand Marl – which is formed mainly of clay and silt.
Clifton Formation (lower middle aquifer)
The lower middle aquifer in this region is known as the Clifton Formation and is made up sandy limestone and marl. This means that it behaves as an aquifer in some areas but as an aquitard in others. It is older and deeper than the upper middle aquifer. Near Warrnambool and Portland it is confined by the upper middle aquitard and overlying aquifers and is over 500 metres thick. This aquifer occurs relatively close to the surface in the Condah GMU.
These aquifers extend across large parts of the region and are formed mainly of sand. They occur at the surface only in small areas and the basement occurs at the surface around the basin margins west of Ballarat, around the Grampians as well as in the Otway Ranges. Where they occur at the surface they receive recharge directly from rainfall. Elsewhere they receive recharge via downward leakage. The basement acts mainly as an aquitard. The lower aquifers in this region were formed between 40 and 65 million years ago. They are overlain by the upper and middle aquifers and aquitards and are underlain by the basement that was formed between 65 and 545 million years ago.
The most widespread formation of the lower aquifers is the Dilwyn Formation. It consists of mainly sand and has clay layers within it that act as aquitards. It occurs at the surface near Hamilton, near Dartmoor, west of Casterton and in the Otway Ranges, however in most areas it is deeply buried. It is thickest near Warrnambool and Portland but very thin where it occurs at the surface. It is connected to the overlying aquifers in some areas where the lower middle aquitard is thin or absent.
Eastern View Formation
The Eastern View Formation exists in a limited area near Anglesea. It consists mainly of sand with some coal. It occurs at the surface along the eastern flank of the Otway Ranges, becoming deeper towards the coast.
The basement occurs at the surface around the basin margins and forms the Great Dividing Range and the Grampians and Otway Ranges. It consists mainly of siltstone, sandstone, clay and granite. Where it occurs in the ranges the basement acts as a low yielding, fractured rock aquifer and supports some groundwater licences. Where it is buried it acts as an aquitard.
Learn the basics
To find out what aquifers visit our What is groundwater page.
To find out how aquifers receive recharge visit our Where does groundwater come from and where does it go page.