Aquifer
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An aquifer is an underground layer where the material contains water.
That can be less solid material like sand, gravel, clay or silt, but it can be rock as well as long as the rock allows water to get in (that means that it is water-bearing). From such layers or groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology.
[change] Exploitation
Aquifers can occur at various depths. Those closer to the surface are not only more likely to be exploited for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them which can be exploited as groundwater resources. If more water is taken out than get in again one speeks of over exploitation. Along the coastlines of certain countries, such as Libya and Israel, population growth has led to over-population which has caused the lowering of water table and the subsequent contamination of the groundwater with saltwater from the sea (saline intrusions). In these cases the aquifer contains brackish water.
The diagram shows typical flow directions in a cross-sectional view of a simple confined/unconfined aquifer system (two aquifers with one impermeable layer, between them. The water table and unsaturated zone are also illustrated.
