Groundwater – Always on the move
Permeable
Impermeable
Porous media
Fractured aquifers
Unconfined aquifers
Piezometric surface
Aquifers and wells
Permeable
material contains interconnected cracks or spaces that are both numerous
enough and large enough to allow water to move freely. In some permeable
materials groundwater may move several meters in a day; in other places,
it moves only a few centimeters in a century. Groundwater moves very
slowly through relatively impermeable materials such as clay
and shale.
Groundwater scientists
generally distinguish between two types of aquifers in terms of the
physical attributes of the aquifer: porous media and fractured aquifers.
Porous media
are those aquifers consisting of aggregates of individual particles such
as sand or gravel. The groundwater occurs in and moves through the
openings between the individual grains. Porous media where the grains
are not connected to each other are considered unconsolidated.
If the grains are cemented together, such aquifers are called
consolidated. Sandstones are examples of consolidated porous media.
Fractured
aquifers are rocks in which the groundwater moves through cracks,
joints or fractures in otherwise solid rock. Examples of fractured
aquifers include granite and basalt. Limestones are often fractured
aquifers, but here the cracks and fractures may be enlarged by solution,
forming large channels or even caverns. Limestone terrain where solution
has been very active is termed karst. Porous media such as
sandstone may become so highly cemented or recrystallized that all of
the original space is filled. In this case, the rock is no longer a
porous medium. However, if it contains cracks it can still act as a
fractured aquifer.
Most of the aquifers of
importance to us are unconsolidated porous media such as sand and
gravel. Some very porous materials are not permeable. Clay, for
instance, has many spaces between its grains, but the spaces are not
large enough to permit free movement of water.
Groundwater usually flows
downhill with the slope of the water table. Like surface water,
groundwater flows toward, and eventually drains into streams, rivers,
lakes and the oceans. Groundwater flow in the aquifers underlying
surface drainage basins, however, does not always mirror the flow of
water on the surface. Therefore, groundwater may move in different
directions below the ground than the water flowing on the surface.
Unconfined
aquifers are those that are bounded by the water table. Some
aquifers, however, lie beneath layers of impermeable materials. These
are called confined aquifers, or sometimes artesian
aquifers. A well in such an aquifer is called an artesian well.
The water in these wells rises higher than the top of the aquifer
because of confining pressure. If the water level rises above the ground
surface a flowing artesian well occurs. The piezometric
surface is the level to which the water in an artesian aquifer will
rise.
 Groundwater
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