Gnangara groundwater system – a unique water resource
The Gnangara groundwater system is Perth's largest source of groundwater, stretching from Gingin in the north to the Swan River in the south and to the Darling Scarp in the east.
The resources of the Gnangara groundwater system are managed through the Gnangara groundwater areas allocation plan which sets out how much water is available from the resources of the system, how much water needs to be left in the system and what conditions will be applied to licences in the local area. The Gnangara groundwater areas allocation plan cover an area of approximately 2 200 km2.
The Gnangara groundwater system comprises four main aquifers:
- The shallow unconfined Superficial (water table) aquifer
- The deeper semi-confined Mirrabooka aquifer
- The deep, mostly confined Leederville and Yarragadee aquifers.
What is the Gnangara Mound?
The Gnangara Mound is one part of the Gnangara groundwater system. A groundwater mound refers to mounding of the water table within a superficial aquifer. The Gnangara Mound extends across most of the Superficial aquifer. It is called a mound because the water table forms a mound shape. The mound develops in an area where the rate at which rainwater infiltrates vertically is greater than the rate at which groundwater flows horizontally.
Which part of the Gnangara groundwater system is most water taken from?
Most private abstraction (including garden bores) from the Gnangara groundwater system is from the Superficial aquifer or Gnangara Mound, with some from the Leederville. Abstraction for the public water supply is from all aquifers and is apportioned between the aquifers (more from the deeper aquifers, less from the Superficial aquifer) to minimise the impact on groundwater dependant ecosystems. The amount abstracted for public water supply varies from year to year depending on the requested allocation from the Water Corporation.
When people refer to groundwater abstraction for the public water supply (i.e. the Water Corporation allocation) they are generally referring to the total from all the aquifers and from both the Gnangara and Jandakot groundwater systems (the Jandakot groundwater system is located south of the Swan River). Abstraction for the public water supply from the Jandakot groundwater system is relatively small (about 10 GL) compared to abstraction for the public water supply from the Gnangara groundwater system. The department provides a media statement each year after the Water Corporation allocation is finalised.
Groundwater levels in the Superficial aquifer have been under considerable pressure in recent years due to a combination of:
- A drying climate (less rainfall and recharge);
- Dense vegetation (native and pine plantations) limiting recharge; and
- Abstraction for irrigation and public water supply.
Limiting environmental impacts of abstraction
The Superficial aquifer supports groundwater ecosystems such as wetlands, caves and springs. Environmental impacts from abstraction occur where ecosystems are in direct connection to the Superficial aquifer and groundwater levels in the Superficial aquifer fall. Impacts on the environment can occur from abstraction within the Superficial aquifer itself and also where abstraction from deeper aquifers in locations where the deeper aquifer is connected to the Superficial aquifer. The Department of Water manages the environmental impacts of abstraction by limiting abstraction from areas where pumping affects groundwater dependant ecosystems supported by the Superficial aquifer.
Falls in groundwater levels of the Superficial aquifer have impacted a number of different groundwater dependent ecosystems on the Gnangara Mound such as Loch McNess and the Yanchep caves system.
In a few areas, where soils, sediments or organic material containing iron sulfide minerals occur, decreasing superficial groundwater levels also have the potential to expose acid sulfate soils, which can significantly reduce water quality.
The Gnangara groundwater system is important for the urban water needs of Perth. In the context of a drying climate, it is important that abstraction is managed to limit the impact on the Superficial aquifer and the groundwater dependent ecosystems it supports. The Department of Water is working with Water Corporation and other water users to ensure that this abstraction has the least possible impact on important groundwater dependant ecosystems.
Learn more about the Gnangara Sustainability Strategy.
Learn more about planning for the Gnangara groundwater areas.
Learn more about metering of irrigation water abstracted from the Superficial aquifer.
View the Review of ministerial conditions on the groundwater resources of the Gnangara groundwater system.
For licensing information, please contact the Swan-Avon Regional Office on 08 6250 8000 and ask to speak to a Licensing Officer, or visit the Water regions website.
