In many areas across the world groundwater is the only reliable source of water supply for communities, industries and farms.
About 2.5% of the Earth's water is fresh water, of which about 30% is groundwater. A little less than 70% of fresh water on earth is contained in glaciers and ice caps, and less than 2% is surface water.
In some areas of Australia, groundwater is the main source of water for households, urban centres, industry and irrigation, while in other areas, groundwater is a supplementary source of water during drought or when surface water is in short supply. Groundwater accounts for about 17% of Australia's accessible water resources and about 30% of total water use.
Groundwater is water that sits beneath the earth's surface contained in porous soils and fractured rocks called aquifers. Very rarely is groundwater a body of water stored in large cavities in the ground. The level of the saturated portion of ground closest to the soil surface is called the watertable.
Groundwater is a finite resource. It is replenished only when surface water seeps into the aquifers — a process called recharge.
Recharge comes from 2 primary sources. When it rains, water infiltrates the soil until it reaches the aquifer. Water from rivers and streams may also move downwards to an aquifer beneath the bed of the watercourse.
Groundwater is accessed by bores or wells. If there is sufficient pressure in the aquifer, the water will flow to the surface naturally. Many bores however, require a pump to lift the water to the surface. A bore may be tens or hundreds of metres deep, depending of the location of the target aquifer.
Pumping water from a shallow aquifer will lower the watertable and the aquifer will be depleted if the amount of groundwater being taken out is greater than the recharge.
Types of aquifers
Groundwater can be found almost everywhere within the Basin. However aquifers are highly variable in terms of their depth to the watertable, the type of soil or rock they are made of, their rate of recharge and their water quality.
Aquifers may be described by their general form — unconfined or confined; and then by the type of soil or rock they are made of — unconsolidated or consolidated material.
An unconfined aquifer typically has a watertable that is close to the soil surface and follows the topography of the land surface. There is no impermeable soil or rock above an unconfined aquifer to prevent water moving upwards to the surface.
It is easy to extract water from an unconfined aquifer because of its shallow nature; therefore shallow aquifers are frequently used as a water source. Unconfined aquifers are often the water source of springs, streams or wetlands in low-lying areas.
Unconfined aquifers are made up of unconsolidated material such as gravel, sand, silt or clay that has been deposited in river valleys, deltas and basins over millions of years. These aquifers are usually referred to as alluvial aquifers.
Alluvial aquifers occur in most regions of the Australia, including throughout the Basin. About 20% of all bores in Australia access groundwater from alluvial aquifers, accounting for 60% of Australia's groundwater extraction. The water quality of alluvial aquifers is usually ideal for agriculture, and especially irrigation.
A confined aquifer is rock that contains water, deep below the ground surface. A confined aquifer is separated from the upper zones of the earth's surface by impermeable rock or soil. Water enters the aquifer through a recharge zone in an area where the aquifer is not confined and is connected to the ground surface or surface water. The water flows, sometimes over great distance, to the confined section of the aquifer and water pressure builds up. If the pressure builds up sufficiently, water will rise to the surface through a break in the confining layer — this is called artesian flow.
In the Basin, confined aquifers are most commonly sedimentary rock aquifers or fractured rock aquifers.
Sedimentary aquifers occur in rock made of consolidated sediments such as sandstone or limestone. Water is stored in the tiny spaces between units of rock, between sheets of rock, or in zones of weathering. These aquifers are often found in large sedimentary basins, eg. the Great Artesian Basin, and range from tens to hundreds of metres thick. Sedimentary aquifers generally contain vast amounts of water but it is often of poorer quality than water in alluvial aquifers, especially in parts of the sedimentary basin that do not have direct connection with rivers or streams. However, some sedimentary aquifers do provide water of a high enough quality for agricultural use in some parts of the Basin, mainly in south-eastern South Australia and Victoria.
Fractured rock aquifers
Fractured rock aquifers occur in hard rocks that are igneous (eg. basalt or granite) or metamorphic (eg. marble) in origin. Water is stored in cracks and fractures in the rock structure. Fractured rock aquifers can be relatively large but they store much less groundwater than other types of aquifers. Water quality can be variable but some aquifers provide water of a sufficient quality for productive use. About 33% of all bores in Australia are in fractured rock systems, accounting for about 10% of total groundwater extraction.
Groundwater use in the Basin
In the Basin, the majority of groundwater use (approximately 75%) is in Queensland, New South Wales and Victoria. The groundwater is used for:
- irrigation for agriculture
- town drinking supply
- stock and domestic supply
- manufacturing and industries
Within the Basin, groundwater use is greatest in areas with alluvial and shallow sedimentary aquifers as these aquifers have the best quality water and the water is relatively easy to access. Approximately 70–80% of groundwater use in the Basin occurs in the eight groundwater systems shown in the map below. The characteristics of groundwater use and sustainability, vary from one groundwater system to the next.
Groundwater in the Lachlan alluvium is particularly good quality and easily accessed, and accounts for about 45% of water used in the catchment. Consequently groundwater supports a productive irrigated agricultural industry in the lower Lachlan catchment, where corn, cotton, wheat, canola and citrus are produced. There is good connection between streams and alluvial groundwater in the catchment.
Groundwater extraction from the Condamine alluvium is on average 18% of the total water use (surface and groundwater) in the region. Shallow groundwater is used primarily for irrigation of cotton, fodder and grain crops, but also for domestic, stock, intensive livestock, commercial and industrial purposes.
Great Artesian Basin
The Great Artesian Basin (GAB) is the largest groundwater system in the world, at some 1.7 million square kilometres. It is a multi-layered confined aquifer with highly variable water quality. The south-eastern sections of the GAB lie beneath the northern Basin, in Queensland and northern New South Wales. Throughout these areas, artesian water rises to the surface.
There are several areas throughout the northern Basin that are dependent on water from the GAB for urban and household needs, such as Narrabri and the lower Namoi River catchment, and communities throughout the Paroo and Warrego catchments.