Groundwater
Groundwater is derived from rain which percolates down through soil or fractures in rock, 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, containing 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). There are also deeper, confined (sometimes called artesian) aquifers where the water is confined under pressure between relatively impervious layers.
At any given point the water table shows an annual 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 metres) 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.
The Department of Water assesses and reviews the State's groundwater resources to ensure sustainable management of aquifer systems and dependent ecosystems. We advise on resource development and licensing, coordination of groundwater drilling exploration and monitoring, and all aspects of groundwater resources management.
We have a groundwater systems modelling group, providing drilling contracts management and hydrogeological advice for major water use sectors, including mining, irrigated agriculture, and public water supply.
To provide data on groundwater water quantity and quality, there are approximately 3000 monitoring bores in the coastal plain around Perth, between Moore River and Mandurah, and a similar number distributed throughout the rest of the State. Water levels and samples for chemical analysis are taken at regular intervals, usually monthly or quarterly, providing a growing historical record of the state of the aquifers. This is the basic information from which policies are developed to protect our groundwater supplies and the wetlands and vegetation which rely on groundwater.
Groundwater in Western Australia
Groundwater is a critically important resource in Western Australia. As it supplies much of the water we need for irrigation, drinking and other uses, the allocation and conservation functions of the Department are closely related to groundwater.
In the Perth region, part of the Swan Coastal Plain, the superficial aquifer averages about 50 metres thick. The Gnangara mound occurs to the north of city (between the ocean, the Swan River, Ellenbrook and Moore River, centered about 15 kilometres 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.
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 kilometer 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.
