An aquifer is a body of sand or porous rock capable of storing
and producing significant quantities of water. An aquifer may
be a layer of loose gravel or sand, a layer of porous sandstone,
a limestone layer, or even an igneous or metamorphic body of
rock. An aquifer may be only a few feet to hundreds of feet
thick. Aquifers occur near the surface or buried thousands of
feet below the surface. It may have an aerial extent of thousands
of square miles or a few acres. The key requirements are
that the layer or body has sufficient porosity to store the water,
sufficient permeability to transmit the water, and be at least
partly below the water table. The water table is the elevation
of the top of the completely saturated (phreatic) zone. Above
the water table is the vadose or unsaturated zone where the
pore spaces are only partially saturated and contain a combination
of air and water.
Porosity and permeability are important measures of
producibility in aquifers. Porosity is the ratio of the volume of
voids in a rock or soil to the total volume. Porosity determines
the storage capacity of aquifers. In sand or sedimentary rocks,
porosity is the space between grains and the volume of open
space (per volume) in fractures. In dense rocks such as granite,
porosity is contained largely within the crack and/or fracture
system. Permeability is the capacity of a rock for
transmitting a fluid, and is a measure of the relative ease with
which a fluid can be produced from an aquifer.
A rock that yields large volumes of water at high rates
must have many interconnected pore spaces or cracks. A
dense, low porosity rock such as granite can be an adequate
aquifer only if it contains an extensive enough system of connected
fractures and cracks to be permeable. In the shallow
subsurface, this is common because nearly all (indurate) rocks
are fractured, often heavily. For that reason, caution should be
exercised before assuming a low porosity rock will be an
aquitard (impermeable body) and not an aquifer.
Fluid pressure, measured in pounds per square inch
(psi), in an aquifer depends on whether it is unconfined or confined.
An unconfined aquifer is one that is hydraulically open
or connected to the surface. Examples would include sand
bodies on or near the surface and more deeply buried layers of
rock or sand connected to the surface by fractures and/or
faults. The fluid pressure in unconfined aquifers is equivalent
to what one would measure at a point in a standing body of
water and would increase linearly (at a constant rate) with
depth. The elevation of the top surface of an unconfined
aquifer is free to fluctuate with rainfall.
A confined aquifer is one that is surrounded on all sides
by an aquitard, a formation that does not transmit fluid. The
pressure in a confined aquifer can be different from that of an
unconfined aquifer at the same elevation. A body of sand surrounded
on all sides by a soft, impermeable clay or shale
serves as a typical example.