- Courses & careers
- News & media
What is groundwater?
Groundwater is a vast and slow moving resource, that greatly exceeds the volume of other available fresh water sources.
Ground water is contained within the pores (tiny open spaces) of earth materials such as sediment and rock. These sediment and rock formations are commonly referred to as:
Aquifer - a saturated permeable geologic unit that can transmit significant quantities of water under ordinary potential gradients. i.e. normally constitutes medium to coarse sands and gravels.
Aquitard - a saturated geologic unit that is capable of transmitting water under ordinary potential gradients but not it sufficient quantities to allow completion of production wells within them. i.e. normally constitutes clays and silts.
Aquiclude - a saturated geologic unit that is incapable of transmitting significant quantities of water under ordinary potential gradients. i.e. normally constitutes consolidated clays of very low permeability.
|Relationships between components of a groundwater system|
Where is ground water found in Australia ?
The occurrence of groundwater is considerably controlled by the geology of a region. The geology of a groundwater environment can be very complex. The geology of a groundwater environment is therefore simplified into three classifications: Unconfined sedimentary aquifers, Confined aquifers, and Fractured rock aquifers. These classifications are based on the physical storage properties of the host rocks.
Unconfined Sedimentary Aquifers
An unconfined aquifer is defined by the lack of a overlying confining layer. i.e. no low permeability confining layer. These aquifers are typically shallow and normally are comprised of surface sands and sandstones. A common example of an unconfined aquifer system is a coastal dune system. The water table forms the upper boundary of an unconfined aquifer. The water level in a well installed in an unconfined aquifer rests at the water table.
These aquifers are recharged by direct infiltration of rainwater from the ground surface. Typically these geologic units are homogeneous (i.e. only comprised of sand, and that the hydrogeologic properties of sand at one location is the same as the hydrogeologic properties of sand at another location within the aquifer).
Unconfined sedimentary aquifers are also known as water table aquifers.
Unconfined aquifers are, however, typically more complex and invariably contain clay or peat lenses that impede vertical water flow. i.e. they are heterogeneous rather than homogeneous.
In NSW, unconfined aquifers are the most important water bearing formations, especially in coastal drainage basins east of the Great Dividing Range.
Aquifers are typically shallow, and have large ground water yields. NSW unconfined sedimentary aquifers can be subdivided into the following categories:
a. Coastal Drainage Basins - As the Great Dividing Range is close to the NSW coast, these basins are relatively small. This groundwater environment type can be further subdivided into: Fluvial Alluvium, Estuarine Sediments,and Coastal Sandbeds. The most important classification in terms of quantity is Coastal Sandbeds. They are characteristically uniform sands, typically 20 - 30 metres thick. Despite the good yields of such aquifers, there is little demand for their ground water for irrigation. This is because high water use agriculture requires non-sandy soil. Ground water is used for urban water supplies, such are the Tomago Sandbeds aquifer, which supply about 25% of Newcastle's water. Groundwater from sandbeds also serves local centers such as Hat Head and Tea Gardens.
b. Inland Drainage Basins - West of the Great Dividing Ranges are inland drainage basins that provide most of the ground water used in NSW. In the upper reaches of these basins the alluvial fill is relatively shallow. Ground water seeps from streams and other recharge sources into the aquifer, and is typically of good quality. In downstream areas the salinity gradually increases, sometimes making it unsuitable even for stock. Numerous towns depend on inland drainage basin groundwater for all or part of their groundwater supply (e.g.: Mudgee, Wellington, Dubbo and Narromine). Ground water abstracted for irrigation purposes is mostly sourced from well below the groundwater surface from confined aquifer units rather than from unconfined aquifers. Unconfined aquifers, usually the uppermost aquifers, are however typically used for stock or station supply only as represent only a small relative yield.
Confined, Porous Rock Aquifers
A confined aquifer is defined by a overlying and underlying confining layer. i.e. a low permeability layer (aquitards) overlying and underlying the porous geologic unit. These aquifers are generally less porous than unconfined aquifers. Porosity is defined as the volume of fluid / total volume. This is primarily due to homogeneity in grain size distribution (sorting efficiency) in unconfined units.
The water level in a well installed in a confined aquifer lies above the base of the upper confining layer and is generally referred to as the potentiometric surface or piezometric surface. This is due to the water in the confined aquifer being under pressure. The well is called an artesian well. In some cases the water level may rise above ground surface, in which case the well is called a flowing artesian well.
The large sedimentary basins that occur in NSW contain porous rocks that constitute excellent aquifers. They include:
a. The Great Artesian Basin, one of the world's largest artesian basins. It extends over 1.8 million square kilometres. It is in an irregular saucer shape, and the eastern highlands (Great Dividing Range) contain its eastern rim. Its other boundaries extend into Queensland, the Northern Territory and into South Australia.
b. The Murray-Darling Basin covers 14% of Australia's area and accounts for 40% of the nations agricultural production - yet it is threatened by rising water tables and salinity in some areas and over-usage and wastage of ground water in other areas.
Fractured Rock and Karstic Aquifers
Fractured rock and karstic aquifers can contain ground water due to secondary porosity. Ground water is stored in fractures and partings, which are caused by metamorphism, uplift and weathering. In carbonate rocks like limestone, these fractures may become considerably enlarged due to dissolution of the limestone (calcium carbonate). Waters containing carbon dioxide from overlying soils react and dissolve the rock's calcium carbonate. Such aquifers are called Karstic. Fractures rarely exist beyond 180 metres depth in these aquifers. Apart from carbonate rocks, yields of bores in fractured rocks are usually in the range of only 0.1 to 2 litres per second. This is sufficient for household use and stock watering but inadequate for significant irrigation.
Karstic aquifers such as in limestone regions can contain considerably more groundwater than other fractured rock aquifers. An example is the Jenolan Caves west of Sydney, where large underground caverns store and transmit very large quantities of groundwater.
Where does groundwater come from?
Ground water in Australia has been and is being utilised at very high rates, abstracted yields (maximum amount of ground water that can be extracted from a well) are becoming diminished and contaminated. What was thought to be a renewable resource is in some cases becoming exhausted. Understanding the processes involved, requires understanding the following the groundwater concepts:
a. Groundwater Recharge - Ground water recharge is the infiltration of surface water into an aquifer. A shallow unconfined aquifer can be recharged rapidly by surface rainfall or stream flow. A confined aquifer can be recharged from rainfall or stream flow thousands of kilometers away. Zones of higher elevation and higher rainfall saturate porous rocks, and this water is slowly transported deeper in the sedimentary basin. The greater the height gradient between the recharge zone and the aquifer storage zone, the greater the ground water flow rate. As Australia has very low topographic relief, regional ground water flow rates are generally low.
b. Climate and Paleorecharge - Higher rainfall produces increased groundwater infiltration and recharge. Australia's climate and rainfall has fluctuated considerably over geological time, and these fluctuations have influenced groundwater recharge rates.
c. Ground water Residence Time - Ground water sources represents a far greater volume of water than fresh water sources, however are tempered by long term average residence times. Residence time refers to the time that a water source spends in storage before moving to a different part of the hydrological cycle (i.e. it could be argued it is a rate of replenishment). Whereas atmospheric water stays an average of 10 days in the atmosphere before being precipitated, groundwater takes up to more than 10 000 years before (e.g.) flowing into a coastal discharge zone. Consider the difference between a river's and a confined aquifer's residence time. An over-pumped river system can quickly be recharged when abstraction ceases. An over-pumped groundwater basin does not quickly recover when abstraction ceases.
d. Porosity and Permeability-porosity is the ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment. Primary porosity represents the original pore openings formed when a rock or sediment formed; whilst secondary porosity is that caused by fractures or weathering in a rock or sediment after it has been formed. Up to approximately 70% of a rock or soil's volume (e.g. clays) may contain groundwater. This percentage is dependent on a property called the porosity, which varies according to the rock or soil type. A rock or soil must also be able to transmit this water, this property is called the permeability. An aquifer rock with well-connected pore spaces will have a higher permeability, and the groundwater can be transmitted more readily. Hydraulic conductivity is a more exact term for permeability, and that is the term used by groundwater hydrologists.
e. Specific Yield - Clays and silts may have higher porosities, but when drained they retain most of the water stored within their sediment matrix (i.e. by surface tension effects). Hence, rather than porosity, the more significant figure in relation to the volume of water available is the specific yield. This is the ratio of the volume of water the saturated formation will yield by gravity drainage to the volume of the formation. From this definition a gravel/sand aquifer will have a higher specific yield and therefore a greater volume of water will be available for extraction. Following this definition a clay/silt aquitard will have a relatively low specific yield and will therefore have a much reduced volume of water available for extraction.
How long will groundwater resources last?
With the introduction of such terms as ground water residence time, recharge and specific yield, it is clear that ground water can not be considered an infinite resource. When ground water abstraction exceeds the average rate of recharge then the ground water system, as a whole, is no longer in equilibrium because outputs (abstraction) exceeds inputs (recharge). It is important to understand that this equilibrium is dynamic, in that there is continual flow through the ground water system.
Furthermore, because of the long residence times, it may take a period of years or decades for the ground water system to adjust to the new input/output equilibrium level.
As alluded to above, a further complexity is that in a ground water system with a residence time of 10 000 years paleorecharge may have been much greater than present day and our estimates of present day recharge being used as a yardstick for the amount of ground water we can abstract (safe yield) may be in error.
When ground water abstraction exceeds the rate of recharge this is referred to as ground water mining.
Is groundwater a renewable resource?
The typically long residence time of groundwater can make it a non-renewable resource, considering the time scale of its abstraction and the rate of ground water abstraction. An over-pumped river system can quickly be recharged when abstraction ceases. An over-pumped ground water basin does not quickly recover when abstraction ceases.
Vulnerability of groundwater to pollution
Ground water provides a filtered supply and is less subject to pollution, though if pollution does occur it is difficult to remedy and due to its nature is usually a long-term problem.