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Approximately 60% of the water supplied to Perth comes from groundwater supplies.

Groundwater is derived from rain which percolates down through the soil or fractures in rock, so filling up the pores between sand grains or the fissures in rocks. Anything from none to half of the rainfall in a given area may reach the water table and thus recharge the groundwater. Geological formations such as those composed of sand, sandstone and limestone which contain useable quantities of groundwater are called aquifers. The aquifer closest to the ground surface is called the shallow, or unconfined, aquifer (its upper surface is the water table) but there are also deeper confined (sometimes called artesian) aquifers where the water is confined under pressure between relatively impervious layers.

The water table at any given point shows an annual variation variation related to the seasonal pattern of rainfall and evaporation. Other factors being equal, the changes are likely to be greatest in clayey soils (several meters) and least in coarse sandy soils (typically less than one metre). The water table tends to follow the ground surface, but is constrained by the ocean and rivers and streams, so groundwater mounds tend to develop in regions where the topography is higher than surrounding areas. There is a slow (10 to 100 metres per year) horizontal flow of water outwards from the mounds under the influence of gravity.

In the Perth region, part of the Swan Coastal Plain, the superficial aquifer is on average about 50 metres thick. The Gnangara mound occurs to the north of city (between the ocean, the Swan River, Ellenbrook and Moore River and centred about 15 kilometers north-east of Wanneroo), and the smaller Jandakot mound occurs to the south (between the ocean and the Swan, Canning and Serpentine rivers). Below the superficial aquifer there are a number of confined aquifers, the largest and most extensive of which are the Leederville, which is typically several hundred metres thick, and the Yarragadee, which is often greater than 1000 metres thick.

• Groundwater publications - Technical papers relating to the Gnangara Mound groundwater source
• Department of Water - General information on Perth's groundwater can be found at the Department of Water website

• Waterwise Ways for WA - Click here to find out more about Waterwise Ways for WA