Aquifers are defined as water-bearing porous soil or rock strata that yield significant amounts of water to wells. Inherent in the definition are two points:
- the aquifer formation must be porous (e.g., sand and gravel or cracks and fractures in more solid rock) and
- water must be able to flow through and out of the formation in quantities large enough to be significant.
Underground formations like clay do not permit water to flow through them and prevent upward and downward movement of groundwater. These formations are known as aquitards or confining layers.
Aquifers are broadly classified into two categories, unconfined and confined.
Unconfined aquifers are the saturated portions of the upper soil profile located above a confining layer. Their upper surface is in direct contact with the atmosphere through porous materials. This upper surface is known as the "water table."
Confined aquifers are separated from the atmosphere by a very slowly permeable or rock (aquitard) called a confining layer. Water in these aquifers is under pressure and, in a well, will rise above the top of the aquifer.