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.
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:
<;ul>
Unconfined aquifer; a water bearing layer with a water table that is the upper boundary of the aquifer itself.
Confined aquifer; a water bearing layer that is confined between two impermeable strata, often with a hydraulic head that lies above the upper boundary of the aquifer. A confined aquifer can be fully confined or semi-confined, depending on the lay of its confining layers1.
Perched aquifer; a body of groundwater that is separated from the main groundwater by an impermeable stratum of small dimensions, lying above the main level of groundwater.
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).
.
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