Types of Aquifer
An aquifer is a geological unit capable of storing and transmitting useful quantities of groundwater.
Aquifers may be
confined or unconfined and are often classified according to the three types of water storage within them:
- Fractured rock aquifers.
- Intergranular aquifers.
- Dual porosity aquifers.
Karst Aquifer
A fourth type,
karst aquifers, are a special type of fractured rock aquifer where the fractures have been dissolved to form larger solution cavitities.
Tasmanian Aquifer Framework
The first version of the Tasmanian Aquifer Framework has been developed as a part of the
"Assistance for development of Hydrogeological Units - Tasmania Project". It has been carried out as subproject of BOM's
"National Aquifer Framework Project (NAF)" by the Groundwater Section of the Department of Natural Resources and Environment Tasmania. The main initial objective of this project was to assist in the development of the National Hydrogeological Units (HGUs) that can be used by the State as a starting point for population of BOM's National Groundwater Information System (NGIS) Core Database Model and for future development of more detailed Tasmanian HGUs and aquifer classifications.
A common understanding of the definition of aquifers and aquitards is critical to the on-going discussion about the sustainable management of groundwater resources. A unified aquifer framework allows a consistent approach to groundwater resource investigation and management and provides a common communication language. The development of a system of hydrogeological units, hydrogeological complexes and an associated aquifer listing will contribute towards the overall groundwater management and resource assessments.
You can access the framework document here:
Tasmanian Aquifer Framework (15Mb)
Please note that for full interactive options it is advised that you download the document to your PC.
Confined and Unconfined Aquifers
Confined Aquifers
Confined aquifers are permeable rock units that are usually deeper under the ground than unconfined aquifers. They are overlain by relatively impermeable rock or clay that limits groundwater movement into, or out of, the confined aquifer.
Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the potentiometric surface. An artesian flow is where water flows out of the borehole under natural pressure.
Confined aquifers may be replenished, or recharged by rain or streamwater infilitrating the rock at some considerable distance away from the confined aquifer. Groundwater in these aquifers can sometimes be thousands of years old.
Unconfined Aquifers
Where groundwater is in direct contact with the atmosphere through the open pore spaces of the overlying soil or rock, then the aquifer is said to be unconfined. The upper groundwater surface in an unconfined aquifer is called the water table. The depth to the water table varies according to factors such as the topography, geology, season and tidal effects, and the quantities of water being pumped from the aquifer.
Unconfined aquifers are usually recharged by rain or streamwater infiltrating directly through the overlying soil. Typical examples of unconfined aquifers include many areas of coastal sands and alluvial deposits in river valleys.
In dual porosity aquifers, the main storage of water occurs in the intergranular pore spaces, and the main groundwater transmission system is the fracture system in the rock.
These are known as dual porosity aquifers, and their permeability reflects the frequency, openness and interconnectivity of the fractures. These aquifers are often the sedimentary rocks in Tasmania.
In fractured rock aquifers, groundwater is stored in the fractures, joints, bedding planes and cavities of the rock mass. Water availability is largely dependent on the nature of the fractures and their interconnection.
In Tasmania, most fractured rocks are of pre-Tertiary age (> 65 million years old) and include a wide range of sedimentary, igneous and metamorphic rocks where there are essentially no primary pore spaces.
The wide distribution of fractured rock aquifers in Tasmania makes them a valuable source of water for livestock and domestic requirements, and in some areas irrigation supplies.
In unconsolidated sediments (silts, sands and gravels) groundwater is stored in the pore spaces between loose grains of sediment.
These aquifers can often provide very reliable supplies where there is a reasonable thickness of saturated sediment present. In some places, the aquifer may be perched above a clay layer which is not extensive enough to provide enough storage for a good supply.
Karst aquifers are a special type of fractured rock aquifers (limestone, dolomite or magnesite) in which the water in the fractures has dissolved the relatively soft rock, thereby significantly increasing the size of the fractures. In some places, these solution cavities can be tens of metres wide and form underground cave systems.
Groundwater flow in karst aquifers is more concentrated and more rapid than in the other aquifer types. Obtaining a successful borehole supply depends on striking a large, well connected solution cavity system.
Springs occur where groundwater discharges naturally at the ground surface.
They often form at low points in the topography where the water table in an
unconfined aquifer intersects the ground surface, or they may be the result of subsurface joints, faults or differences in permeability that direct water towards the ground surface under pressure from deeper confined aquifer.
More Information
For an easy to follow guide to groundwater processes, a suggested read is
Introducing Groundwater by M. Price (publishers George Allen and Unwin).