Aquifers are any body of permeable rock or regolith saturated with water through which groundwater moves. The term aquifer is usually reserved for rock or soil bodies that contain economical quantities of water that are extractable by existing methods. The quality of an aquifer depends on two main qualities, porosity and permeability. Porosity is a measure of the total amount of open void space in the material. Permeability refers to the ease at which a fluid can move through the open-pore spaces, and depends in part on the size, shape, and how connected individual pore spaces are in the material. Gravels and sandstone make good aquifers, as do fractured rock bodies. Clay is so impermeable that it makes bad aquifers and typically forms aquicludes, which stop the movement of water.
There are several main types of aquifers. In uniform, permeable rock and soil masses aquifers form as a uniform layer below the water table. In these simple situations wells fill with water simply because they intersect the water table. But the rocks below the surface are not always homogeneous and uniform, which can result in a complex type of water table know as a perched water table. These result from discontinuous impermeable rock or soil bodies in the subsurface, which create domed pockets of water at elevations higher than the main water table, resting on top of the impermeable layer.
When the upper boundary of the groundwater in an aquifer is the water table, the aquifer is said to be unconfined. In many regions, a permeable layer, typically a sandstone, is confined between two impermeable beds, creating a confined aquifer. In these systems water enters the system only in a small recharge area, and if this is in the mountains, then the aquifer may be under considerable pressure. This is known as an artesian system. Water that escapes the system from the fracture or well reflects the pressure difference between the elevation of the source area and the discharge area (hydraulic gradient), and rises above the aquifer as an artesian spring or artesian well. some of these wells have made fountains that have spewed water 200 feet (60 meters) high. one example of an artesian system is that in Florida, where water enters in the recharge area, and is released near miami about 19,000 years later.
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