Geology of aquifers in Port Phillip & Western Port
Upper aquifers are thin and occur at shallow depths along the river valleys, near the coast and on the plains west of Melbourne. These aquifers discharge to streams and the bays.
Middle aquifers cover a large part of the region. They are thickest near Koo Wee Rup and are mostly buried, except on the eastern edge of Port Phillip Bay where they occur at the surface.
Lower aquifers extend across the region. They occur at or close to the surface in a small number of areas and are very deep along the coast. Basement rock occurs at the surface in the highlands and are 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 recharge directly from rainfall. They consist of recent sediments, sand and gravel deposits, and basalts. Sand and gravel deposits form localised but important aquifers along the river valleys, flood plains and near the coast. Basalt aquifers formed by volcanic activity are found to the north and west of Melbourne.
Recent sediments aquifers
These aquifers consist of clay, sand and gravel and are found along the river valleys, around Koo Wee Rup and in dune deposits near the coast.
Sand and gravel aquifers
Around Bacchus Marsh and Werribee and near Yarra Glen the upper aquifers underlie productive farmland along the river valleys and flood plains. They formed when soil and rocks eroded from the mountain ranges and were deposited by streams over the past 1 million years. Near Koo Wee Rup the upper aquifer is mostly clayey and acts as an aquitard except in the south-eastern part of the GMU. The Nepean Peninsula west of Boneo is covered by sand aquifers that can be more than 150 metres thick. These aquifers have been formed by deposits from the ocean and wind over the past 4 million years. Sandy dune deposits occur along the coast in the areas around Nepean and Koo Wee Rup and south-east of Melbourne.
These aquifers were formed by numerous basalt flows due to volcanic activity over the past 4 million years and are known as the Newer Volcanics. They are most porous near the volcanic cones and are more clayey and act as confining layers across the plains west of Melbourne. They are thickest near the volcanic cones and where they flowed into and filled old river valleys such as south of Lancefield. They occur at the surface where they are not covered by recent sediments.
The middle aquifers consist of mostly sand and are buried and confined by the upper aquifers except on the eastern side of Port Phillip Bay where they occur at the surface. They are thickest in the Western Port Basin. These aquifers were formed between 4 million and 38 million years ago and are made up of several formations that are connected and act as one aquifer in each basin..
Port Phillip Basin
The middle aquifer in this basin is made up of the Moorabool Viaduct Formation, the Batesford Limestone and the Brighton Group. This aquifer is generally less than 50 metres thick and varies from fine sands and clay to coarse sand and gravel to limestone. It is confined in the west and unconfined in the east.
Western Port Basin
The middle aquifer in this basin is known as the Western Port Group and is made up of the Baxter Sandstone, the Sherwood Formation and the Yallock Formation. It is confined by the upper aquifer and is over 150 metres thick to the south of Koo Wee Rup. The aquifer is thickest and relatively close to the surface around Koo Wee Rup. This aquifer varies from fine sand and clay in the upper western part of the basin to coarse sands, gravel, shells and limestone in the lower eastern part.
The middle aquifers are in the Port Phillip Basin are partially underlain by the middle aquitard which is formed of mainly coal and silt.
Lower aquifers extend across the region and are formed by sand, sandstone and basalt. The lower aquifers occur at the surface only in small areas and the basement occurs at the surface only around the basin margins. 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 the region were formed between 19 and 65 million years ago. They are overlain by the upper and middle aquifers and aquitards and are underlain by the basement which was formed between 65 and 545 million years ago.
The most widespread formation of the lower aquifers is the Werribee Formation. It consists of mainly sand and has clay and coal layers within it that act as aquitards. It occurs at the surface near Bacchus Marsh but in most areas it is deeply buried. It is thickest south-west of Melbourne and on the Nepean Peninsula and is very thin between Moorabbin and Frankston.
The Childers Formation exists within the Western Port Basin. It is mainly sand with some coal and is deeply buried.
The fractured basalt Older Volcanics aquifers occur in scattered highland areas and beneath the Childers Formation in the Western Port Basin. Groundwater does not flow between the separated areas of these aquifers. Where these aquifers occur at or near the surface they receive recharge directly from rainfall. These aquifers are used for groundwater supply mainly near Wandin.
The basement occurs at the surface around the basin margins, forming the Great Dividing Range, Mornington Peninsula Highlands and Strzelecki Ranges. It consists mainly of siltstone, sandstone, claystone and granite. 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 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.