Groundwater Glossary
Aquifer
A geological formation, a group of formations, or a part of a formation that is water bearing. A geological formation or structure that stores or transmits water, or both, such as to wells and springs.
Unconfined Aquifer
An unconfined aquifer is a permeable formation which extends from the land surface down to a confining base. It is generally partly filled with water and open to air pressure above.
When penetrated by a bore the water remains in the bore at the same level at which it was struck. This is because the water pressure at the water table is at atmospheric pressure. The water surface in such an aquifer is called the water table.
Confined Aquifer
A confined aquifer exists where an aquifer is overlain by a confining bed. The confining bed prevents upward movement of the groundwater.
These aquifers are usually completely saturated with water. The water is commonly under pressure, therefore when a bore intersects the aquifer, water rises up the bore. The level to which it rises is called the potentiometric surface and it reflects the pressure in the aquifer at that locality.
If the pressure is sufficient to drive the groundwater above the ground level, the bore is called artesian. Confined aquifers occur in all the major Tertiary sedimentary basins in Victoria.
Semi Confined/Semi Unconfined Aquifers
These aquifers are ones in which the upper confining layer is leaky, but still contribute significantly to the flow of the aquifer. Such aquifers normally produce a delayed yield or delayed drainage effect when pumped or through the semi-confining layer only becomes significant only after the head of the aquifer has been altered.
Porous Rock Aquifer
An aquifer where groundwater is stored in and flows through the pore spaces between grains of sediment. These include sands and gravels. They are often highly permeable and can provide large volumes of water to bores which intersect them.
Fractured Rock Aquifers
Aquifers where groundwater is stored and flows through joints, fractures, cavities and other spaces formed by cooling volcanic lavas, by earth movements and limestone dissolved by percolating groundwater.
These can occur in most hard rock such as basalt, granite, sandstone and limestone. They generally yield less water than porous rock aquifers, but in some places provide high yielding bores.
Rock Types
Rocks can be classified in various ways, the first of which is into three different groups based on how they were formed:
- Sedimentary
- Igneous
- Metamorphic
Sand
Sands are sedimentary mineral grains deposited by wind or water action having a particle size of between 1/16 and 2mm diameter. The grains are made up of predominantly quartz and can include other minerals such as feldspars, mica, glauconite and iron oxides.
Because these grains have usually been rolled over or rubbed as part of the deposition process they are partly or wholly rounded.
Sandstone
Sandstone is the result of loose mineral grains of sand having been deposited layer upon layer, compacted by the weight of overlying material and cemented together over millions of years to form a hard rock.
Silt
Silts are sedimentary grains having a particle size of between 1/256 and 1/16mm diameter. It is almost always deposited by water action and usually comprise finely divided particles of quartz, carbonate dust, carbon and iron pyrite minerals. Silt transmits and absorbs water but does not become sticky and is therefore considered to be non-plastic.
Siltstone
Siltstone is the result of grains of silt having been deposited layer upon layer, compacted by the weight of overlying material and cemented together over millions of years to form a hard rock.
Clay
Clays are sedimentary grains having a particle size of less than 1/256mm in diameter. It comprises a similar mineralogy to that of silt but differs in terms of plasticity. Unlike silt, clay becomes sticky when mixed with water and the particles can be moulded or rolled into forms that hold their shape. This feature is called plasticity.
Mudstone
Mudstone is the result of grains of clay having been deposited layer upon layer, compacted by the weight of overlying material and cemented together over millions of years to form a hard rock. They are similar to shales but lack the feature of a layered structure.
Shale
Shale is the result of grains of clay having been deposited layer upon layer, compacted by the weight of overlying material and cemented together over millions of years to form a hard rock. Shales usually show a well marked layering due to the regular orientation of the clay particles parallel to the surface of deposition.
Gravel
In general, gravel refers to sedimentary grains having a particle size of between 2 and 4mm. The term is applied to grains that are larger than coarse sand but finer than pebbles.
Alluvium
Alluvium refers to particles of minerals or rock that has been transported by a river and deposited usually temporarily at points along the flood plain of a river. It is commonly composed of sands and gravels.
Limestone
Limestone is a sedimentary rock formed by biological activity and chemical precipitation. It consists largely of calcite crystals. Most are made up of broken shells and fragments of skeletons, especially those that lived in the sea.
Igneous rocks form by the cooling and solidification of magma.
Granite
Granite is a coarse-grained igneous rock formed by cooling and solidification of magma. The main minerals present in granites are quartz, feldspars and micas. They occur as intrusive bodies that have forced themselves into pre-existing rocks.
Basalt
Basalt is a fine-grained, sometimes glassy igneous rock formed by crystallisation from fluid magmas. They are generally found in lava flows having been extruded from fissures or vents in the earths crust, such as a volcano. The common minerals are calcium-rich feldspars, biotite mica and ferromagnesian minerals.
Metamorphic rocks are igneous or sedimentary rocks which have been subjected to forces of heat and pressure sufficient to cause changes in their texture and composition.
Gneiss
Gneiss refers to a coarse-grained metamorphic rock made up of parallel bands of light and dark minerals. The bands may be highly contorted and form under high grade regional pressure and temperature.
Hornfels
Hornfels is a medium or fine-grained metamorphic rock formed when magma intrudes cooler sedimentary mudstone or shale. The heat and pressure of the intrusion causes a change in composition to form this denser, harder rock.
© The State of Victoria, 1996 - 2008.
This document was last reviewed on 24/09/2008