The reason this bit about economic value is included in the definition is because usage of the term is relative. What is considered an aquifer in one part of the world could be considered an aquitard or aquiclude in other parts, depending on the buildup of the underground.


Lithology

Basically there are three types of rock or soil, namely:
Of these three types of rock formations, the sedimentary deposits are first candidate for aquiferous properties. Igneous rock and metamorphic rock are often, when first formed, solid and impermeable. However, due to secondary events (for example, tectonic disturbances creating cracks, cavities and fissures) and conditions (for example, the carbon dioxide content of local water), or combinations thereof, these types of deposits can be altered so that they become capable of conducting water. One prime example of such secondary permeability of rock formations can be seen in karst, where the morphology of the landscape is shaped by the dissolving action of water on carbonate bedrock (which is a type of metamorphic rock), creating underground cave systems capable of carrying large amounts of water.


Aquifers and the history of subsurface formation

Depending on whether the ground is made up of sedimentary deposits or igneous or metamorphic rock, aquifers differ in the manner in which they are able to contain and carry water. In sedimentary deposits, the permeability and transmissivity depend on porosity and how well the deposits are graded according to grain size. In igneous and metamorphic rock, the abundance and pattern of cracks and fissures is important (especially whether the cracks largely connect with each other, or are made up of isolated cases), as well as the depth of the formation (higher depth equates to higher pressure and smaller cracks). Also important in igneous and metamorphic rock is secondary porosity, caused by dissolving of material by water.

In sedimentary deposits, the manner in which the deposits were formed has an impact on its hydraulic properties. Sedimentary deposits can be:
Alluvial deposits usually consist of fine, round and mostly well sorted material. This means that the porosity is good, but the transmissivity of such deposits can vary due to the grain size, bigger grain sizes resulting in higher transmissivity.

Eolian deposits are very well sorted and often uniform over large areas. Again, depending on grain size the transmissivity can vary. Löss, which is an eolian deposit made up of very fine sand mixed with small amounts of clay and calcium carbonate, is a moderately coherent material, leaving it with very bad aquiferous properties and therefore useless for water extraction.

Marine deposits differ in makeup, depending on a number of parameters. Normally, these deposits are classified according to the zone in which they were deposited, these being the abyssal zone (floors of the oceans), the bathyal zone (slope going down from the continental shelves to the ocean floors) and the neritic zone (continental shelf). Most sediments deposited in the abyssal and bathyal zone are made up of dead organic material, silts and clays. These are mostly not very permeable and therefore yield rock formations that are not good candidates for aquifers.

In the neritic zone, deposits of material depend mostly on the water depth at the time of deposition. Nearly all the sand on beaches is brought there by rivers that carry sediment into sea. The wave action on the beaches grades the sediment according to size, the larger grain sizes deposited near the beach, while finer sand and silt are carried out to sea, where the turbulence gradually weakens and ever finer grained sediments are deposited. The layers of sand deposited nearest the (former) beaches are well graded and have a high porosity, making them prime candidates for aquifers, while the deposits farther from the coast made up of silt and clay particles are much less ideal.

Glacial deposits vary greatly and therefore have varying aquiferous properties, ranging from excellent to very poor, depending on whether the material has been well sorted, whether it has been consolidated, and whether it contains amounts of clay particles.


Types of aquifers

Types of aquifers are:
In the case of confined or semi-confined aquifers it is possible that the hydraulic head in the aquifer (which equates to a virtual water table level2) is higher than the local surface level. This gave rise to so called artesian wells, which were first built in Artois, the northern most province of France. Because the virtual water table in the aquifers in which these wells are drilled is higher than the local surface level, the water flows up freely, without needing the help of pumps.


Fresh water versus salt water

Another boundary to consider in aquifers is that between salt and fresh water. In coastal areas saltwater intrusion is a problem for fresh water extraction from aquifers. Salt water has a higher density than fresh water and therefore always stays beneath fresh water. On the boundary there will be a mixing layer, where the salinity gradually goes from fresh to salt, from top to bottom. The problem is that extraction of fresh water raises this boundary, until the boundary reaches the level of the well and extraction cannot continue. This raising of the salt water boundary can also affect other industries negatively, in particular agriculture. This side effect of water extraction has to be addressed to prevent problems (and lawsuits).

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Sources:
College material - node your homework
http://www.geocities.com/Eureka/Gold/1577/hg_eng.html
Various pages in http://www.aqd.nps.gov/grd/usgsnps/project/home.html
http://www.horne28.freeserve.co.uk/paleco.htm

1 Often this is due to folds in subsurface strata, combined with erosion, opening up part of a confined aquifer
2 That is, if one were to drill a pipe into such a layer, the water level in the pipe would be equal to the hydraulic head in the layer

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