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Understanding groundwater

Groundwater is a critically important resource in Western Australia. The allocation and conservation functions of the Department of Water are closely related to groundwater as it supplies much of the water we need for irrigation, drinking and other uses.

In the Perth region, part of the Swan Coastal Plain, the superficial aquifer averages about 50 metres in thickness.

The Gnangara mound occurs to the north of the city (between the ocean, the Swan River, Ellenbrook and Moore River, and is centred about 15 kilometres north-east of Wanneroo). 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.

Water is lost naturally from the superficial aquifer by evaporation from the ground surface and lakes (which generally occur in a depression where the water table rises above the ground level), by transpiration from plants, especially trees, and by the slow movement of groundwater to springs, rivers and the ocean.

The water within the shallow aquifer is extracted not only by the Water Corporation for scheme water, but also by householders for their gardens, by industry, by local authorities, institutions and golf courses to water lawns and gardens, and by market gardeners and horticulturists.

A smaller volume is drawn from the confined aquifers, mostly by the Water Corporation, since the required bores are much more expensive than those tapping the superficial aquifer.

Where the water table is close to the surface in agricultural or housing areas, drains are often installed to control the maximum height of the water table, with the excess water being directed via pipes or open channels to lakes, rivers or the ocean.

Drainage in the region around Perth removes more water than all the bores in the superficial aquifer in the same area.

In areas close to the coast and the Swan River estuary, the fresh superficial groundwater overlies a wedge of salty water extending up to a kilometre  inland.

Excessive pumping from bores in these areas can increase the size of the salt water wedge at the expense of the fresh water, so that the bores start to pump salty water.

Once this occurs it can take decades with no extraction before the system can recover.

Similar potential problems occur at Carnarvon and other country areas.

State Groundwater Investigation Program

The hydrogeological atlas is available here and the Perth Groundwater Atlas is available here.

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