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 exist-
ing methods. The quality of an aquifer depends on two main
quantities, porosity and permeability. Porosity is a measure of
the total amount of open void space in the material. Permeability
is a term that refers to the ease at which a fluid can
move through the open pore spaces, and it depends in part
on the size, the 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 or even aquicludes, which stop the
movement of water.
There are several main types of aquifers. In uniform, permeable
rock and soil masses, aquifers will form as a uniform
layer below the water table. In these simple situations, wells
fill with water simply because they intersect the water table.
However, the rocks below the surface are not always homogeneous
and uniform, which can result in a complex type of
water table known as a perched water table. This results
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 saturated permeable layer, typically sandstone,
is confined between two impermeable beds, creating a
confined aquifer. In these systems, water only enters the system
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 it 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 m) high.
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