How does ground water occur?
It is difficult to visualize water underground.
Some people believe that ground water collects in underground
lakes or flows in underground rivers. In fact, ground water
is simply the subsurface water that fully saturates pores
or cracks in soils and rocks. Ground water is replenished
by precipitation and, depending on the local climate and geology,
is unevenly distributed in both quantity and quality. When
rain falls or snow melts, some of the water evaporates, some
is transpired by plants, some flows overland and collects
in streams, and some infiltrates into the pores or cracks
of the soil and rocks. The first water that enters the soil
replaces water that has been evaporated or used by plants
during a preceding dry period. Between the land surface and
the aquifer water is a zone that hydrologists call the unsaturated
zone. In this unsaturated zone, there usually is at least
a little water, mostly in smaller openings of the soil and
rock; the larger openings usually contain air instead of water.
After a significant rain, the zone may be almost saturated;
after a long dry spell, it may be almost dry. Some water is
held in the unsaturated zone by molecular attraction, and
it will not flow toward or enter a well. Similar forces hold
enough water in a wet towel to make it feel damp after it
has stopped dripping.
How ground water occurs in rocks.
After the water requirements for plant and
soil are satisfied, any excess water will infiltrate to the
water table--the top of the zone below which the openings
in rocks are saturated. Below the water table, all the openings
in the rocks are full of water that moves through the aquifer
to streams, springs, or wells from which water is being withdrawn.
Natural refilling of aquifers at depth is a slow process because
ground water moves slowly through the unsaturated zone and
the aquifer. The rate of recharge is also an important consideration.
It has been estimated, for example, that if the aquifer that
underlies the High Plains of Texas and New Mexico--an area
of slight precipitation--was emptied, it would take centuries
to refill the aquifer at the present small rate of replenishment.
In contrast, a shallow aquifer in an area of substantial precipitation
may be replenished almost immediately.
Aquifers can be replenished artificially.
For example, large volumes of ground water used for air conditioning
are returned to aquifers through recharge wells on Long Island,
New York. Aquifers may be artificially recharged in two main
ways: One way is to spread water over the land in pits, furrows,
or ditches, or to erect small dams in stream channels to detain
and deflect surface runoff, thereby allowing it to infiltrate
to the aquifer; the other way is to construct recharge wells
and inject water directly into an aquifer. The latter is a
more expensive method but may be justified where the spreading
method is not feasible. Although some artificial-recharge
projects have been successful, others have been disappointments;
there is still much to be learned about different ground-water
environments and their receptivity to artificial-recharge
A well, in simple concept, may be regarded
as nothing more than an extra large pore in the rock. A well
dug or drilled into saturated rocks will fill with water approximately
to the level of the water table. If water is pumped from a
well, gravity will force water to move from the saturated
rocks into the well to replace the pumped water. This leads
to the question: Will water be forced in fast enough under
a pumping stress to assure a continuing water supply? Some
rock, such as clay or solid granite, may have only a few hairline
cracks through which water can move. Obviously, such rocks
transmit only small quantities of water and are poor aquifers.
By comparison, rocks such as fractured sandstones and cavernous
limestone have large connected openings that permit water
to move more freely; such rocks transmit larger quantities
of water and are good aquifers. The amounts of water that
an aquifer will yield to a well may range from a few hundred
gallons a day to as much as several million gallons a day.
An aquifer may be only a few or tens of feet
thick to hundreds of feet thick. It may lie a few feet below
the land surface to thousands of feet below. It may underlie
thousands of square miles to just a few acres. The Dakota
Sandstone, for example, carries water over great distances
beneath many States, including parts of North Dakota, South
Dakota, Montana, Wyoming, Colorado, Nebraska, Kansas, New
Mexico, and Oklahoma. On the other hand, deposits of sand
and gravel along many streams form aquifers of only local
The quantity of water a given type of rock
will hold depends on the rock's porosity--a measure of pore
space between the grains of the rock or of cracks in the rock
that can fill with water. For example, if the grains of a
sand or gravel aquifer are all about the same size, or "well
sorted," the water-filled spaces between the grains account
for a large proportion of the volume of the aquifer. If the
grains, however, are poorly sorted, the spaces between larger
grains may be filled with smaller grains instead of water.
Sand and gravel aquifers having well-sorted grains, therefore,
hold and transmit larger quantities of water than such aquifers
with poorly sorted grains.
Natural and artificial recharge of an aquifer.
Artesian aquifer. Both wells are artesian
wells, although only one flows.
If water is to move through rock, the pores
must be connected to one another. If the pore spaces are connected
and large enough that water can move freely through them,
the rock is said to be permeable. A rock that will yield large
volumes of water to wells or springs must have many interconnected
pore spaces or cracks. A compact rock almost without pore
spaces, such as granite, may be permeable if it contains enough
sizable and interconnected cracks or fractures. Nearly all
consolidated rock formations are broken by parallel systems
of cracks, called joints. These joints are caused by stresses
in the Earth's crust. At first many joints are hairline cracks,
but they tend to enlarge through the action of many physical
and chemical processes. Ice crystals formed by water that
freezes in rock crevices near the land surface will cause
the rocks to split open. Heating by the Sun and cooling at
night cause expansion and contraction that produce the same
result. Water will enter the joints and may gradually dissolve
the rock or erode weathered rock and thereby enlarge the openings.
A relationship does not necessarily exist
between the water-bearing capacity of rocks and the depth
at which they are found. A very dense granite that will yield
little or no water to a well may be exposed at the land surface.
Conversely, a porous sandstone, such as the Dakota Sandstone
mentioned previously, may lie hundreds or thousands of feet
below the land surface and may yield hundreds of gallons per
minute of water. Rocks that yield fresh water have been found
at depths of more than 6,000 feet, and salty water has come
from oil wells at depths of more than 30,000 feet. On the
average, however, the porosity and permeability of rocks decrease
as their depth below land surface increases; the pores and
cracks in rocks at great depths are closed or greatly reduced
in size because of the weight of overlying rocks.
After entering an aquifer, water moves slowly
toward lower lying places and eventually is discharged from
the aquifer from springs, seeps into streams, or is intercepted
by wells. Ground water in aquifers between layers of poorly
permeable rock, such as clay or shale, may be confined under
pressure. If such a confined aquifer is tapped by a well,
water will rise above the top of the aquifer and may even
flow from the well onto the land surface. Water confined in
this way is said to be under artesian pressure, and the aquifer
is called an artesian aquifer. The word artesian comes from
the town of Artois in France, the old Roman city of Artesium,
where the best known flowing artesian wells were drilled in
the Middle Ages. The level to which water will rise in tightly
cased wells in artesian aquifers is called the potentiometric
Deep wells drilled into rock to intersect
the water table and reaching far below it are often called
artesian wells in ordinary conversation, but this is not necessarily
a correct use of the term. Such deep wells may be just like
ordinary, shallower wells; great depth alone does not automatically
make them artesian wells. The word artesian, properly used,
refers to situations where the water is confined under pressure
below layers of relatively impermeable rock.
Where ground water is not confined under
pressure, it is described as being under water-table conditions.
Water-table aquifers generally are recharged locally, and
water tables in shallow aquifers may fluctuate up and down
directly in unison with precipitation or streamflow.
A spring is the result of an aquifer being
filled to the point that the water overflows onto the land
surface. There are different kinds of springs and they may
be classified according to the geologic formation from which
they obtain their water, such as limestone springs or lava-rock
springs; or according to the amount of water they discharge-large
or small; or according to the temperature of the water-hot,
warm, or cold; or by the forces causing the spring-gravity
or artesian flow.
Thermal springs are ordinary springs except
that the water is warm and, in some places, hot. Many thermal
springs occur in regions of recent volcanic activity and are
fed by water heated by contact with hot rocks far below the
surface. Such are the thermal springs in Yellowstone National
Park. Even where there has been no recent volcanic action,
rocks become warmer with increasing depth. In some such areas
water may migrate slowly to considerable depth, warming as
it descends through rocks deep in the Earth. If it then reaches
a large crevice that offers a path of less resistance, it
may rise more quickly than it descended. Water that does not
have time to cool before it emerges forms a thermal spring.
The famous Warm Springs of Georgia and Hot Springs of Arkansas
are of this type. Geysers are thermal springs that erupt intermittently
and to differing heights above the land surface. Some geysers
are spectacular and world famous, such as Old Faithful in
Yellowstone National Park.