Geology of aquifers in Gippsland
Upper aquifers are thin and occur at shallow depths along the river valleys and flood plains and in dune deposits near the coast. These aquifers discharge to streams, wetlands and the Gippsland Lakes.
Middle aquifers occur within the central Gippsland area and are generally buried beneath confining layers. In some parts these aquifers are nearly one kilometre thick.
Lower aquifers extend across the region. They occur at or close to the surface near Moe and Leongatha and along the basin margin and are very deep along the coast. Basement rock occurs at the surface in the highlands and is buried under the lower aquifers along the coast. Discharge from these aquifers occurs offshore into Bass Strait.
These aquifers occur at the surface and receive
Sand and gravel aquifers
These aquifers underlie productive farmland along the river valleys and flood plains of the Latrobe, Thomson, Macalister, Avon and Mitchell Rivers. They originate from an alluvial fan that formed at the foot of the mountain ranges between 4 and 10 million years ago. Prior and current streams have eroded the fan into beds of sand and gravel. Other aquifers formed by sand and gravel alluvial deposits occur in the rivers of East Gippsland, most notably the Curlip Gravels along the Snowy River near Orbost. They also occur in the highlands near Licola, Dargo and Swifts Creek.
Sandy dune deposits occur in coastal areas and form aquifers around Sandy Point, Venus Bay and Loch Sport, however they are not significant east of Lakes Entrance. These aquifers provide a very important source of freshwater in an otherwise saline environment.
A clay aquitard known as the Haunted Hills Formation occurs around and under the upper aquifers across nearly the whole sedimentary basin. This separates the upper aquifers from the middle aquifers along with other aquitards such as the Nuntin Clays.
In Gippsland the formation of the middle aquifers is quite complex. To describe them more simply we have separated them into the upper middle aquifers and the lower middle aquifers based on their age and position. The upper middle aquifer was formed more recently and generally occurs closer to the surface than the lower middle aquifers.
Upper middle aquifer (Boisdale Formation)
This aquifer is mostly comprised of the Boisdale Formation, a sand formation that occurs up to 200 metres below the ground surface and is confined by the overlying upper aquifers and a clay aquitard. It is around 50 metres thick across most of its extent and up to 200 metres thick at Sale. Between Sale and Lake Wellington it is under enough pressure to cause groundwater to flow freely from bores.
Upper middle aquitard
A clay marl aquitard extends along the coast where it occurs between the upper middle aquifers and the lower aquifers.
Lower middle aquifers
These aquifers are confined sand aquifers that contain layers of coal. They are made up of a number of formations: the Latrobe Valley Group (Yallourn and Morwell Formations), the Balook Formation and the Lakes Entrance Formation. These aquifers occur close to the surface in the coalfields near Yallourn and Morwell, near Orbost and around the basin margin. They are otherwise buried under several hundred metres of overlying sediments. They are thickest in the Rosedale and Yarram areas.
The Balook Formation is a narrow, vertical aquifer that hydraulically connects the upper middle, lower middle and lower aquifers where they coincide. In other words there is no aquitard directly above or below this formation. Where the Balook Formation is absent the upper middle aquifer is separated from the lower aquifers by thick clay, coal or limestone aquitards.
These aquifers do not occur in the Strzelecki Ranges so there is no connection between the aquifer at Traralgon and the aquifer at Yarram.
Lower middle aquitards
The lower middle aquitard occurs between the middle aquifers and the lower aquifers. It extends from Yarram to Orbost through Sale and Bairnsdale.
The lower aquifers of Gippsland were formed from 65 to 34 million years ago. They are overlain by the upper and middle aquifers and thick clay, coal and limestone aquitards and are underlain by the basement.
The Latrobe Group is a sand and gravel aquifer interspersed with clay, silt and coal. It occurs near the surface around the basin margins but is buried up to one kilometre below the surface at Loch Sport. Where it is buried beneath a thick limestone formation it extends offshore and contains oil and gas fields. This aquifer receives recharge mainly by slow leakage from overlying sediments, however where it occurs near the surface around the basin margin it receives some recharge directly from rainfall.
In the west of the region around Warragul, Thorpdale and Leongatha the lower aquifer is mainly made up of the fractured basalt of the Older Volcanics. This aquifer occurs at or near the surface and receives recharge directly from rainfall.
The basement underlies all other aquifers in the sedimentary basin and rises to the surface in the highlands where it acts as a low yielding fractured rock water table aquifer. Where it is buried the basement acts as an aquitard with confined reservoirs of water, oil and gas. In the north of the region the basement occurs at the surface forming the Great Dividing Range. Here it is made up of mainly siltstone, sandstone, clay and granite.
In the south of the region the basement occurs at the surface forming the Strzelecki Ranges. Here it is made up of sandstone and siltstone.
Learn the basics
To find out what aquifers are 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.