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.
Read more on the background and details of the Gnangara Mound.