adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Groundwater is water that exists in the pore spaces and fractures in rock
and sediment beneath the Earth's surface. It originates as rainfall or
snow, and then moves through the soil into the groundwater system, where
it eventually makes its way back to surface streams, lakes, or oceans.
Groundwater makes up about 1% of the water on Earth (most water is
But, groundwater makes up about 35 times the amount of water in
lakes and streams.
Groundwater occurs everywhere beneath the Earth's surface, but is
usually restricted to depths less that about 750 meters.
The volume of groundwater is a equivalent to a 55 meter thick layer
spread out over the entire surface of the Earth.
The surface below which all rocks are saturated with groundwater is
the water table.
The Water Table
Rain that falls on the surface seeps down through the soil and into a zone
called the zone of aeration or unsaturated zone where most of the pore
spaces are filled with air. As it penetrates deeper it eventually enters a
zone where all pore spaces and fractures are filled with water. This zone
is called the saturated zone. The surface below which all openings in the
rock are filled with water (the top of the saturated zone) is called the
The water table occurs everywhere beneath the Earth's surface. In desert
regions it is always present, but rarely intersects the surface.
In more humid regions it reaches the surface at streams and lakes, and
generally tends to follow surface topography. The depth to the water table
may change, however, as the amount of water flowing into and out of the
saturated zone changes. During dry seasons, the depth to the water table
increases. During wet seasons, the depth to the water table decreases.
Movement of Groundwater
Groundwater is in constant motion, although the rate at which it moves is
generally slower than it would move in a stream because it must pass
through the intricate passageways between free space in the rock. First
the groundwater moves downward due to the pull of gravity. But it can also
move upward because it will flow from higher pressure areas to lower
pressure areas, as can be seen by a simple experiment illustrated below.
Imagine that we have a "U"-shaped tube, filled with water. If we put
pressure on one side of the tube, the water level on the other side rises,
thus the water moves from high pressure zones to low pressure zones.
The same thing happens beneath the surface of the Earth, where pressure is
higher beneath the hills and lower beneath the valleys
The rate of groundwater flow is controlled by two properties
of the rock: porosity and permeability.
is the percentage of the volume of the rock that is open space (pore
space). This determines the amount of water that a rock can contain.
In sediments or sedimentary rocks the porosity depends on grain size, the
shapes of the grains, and
the degree of sorting, and
the degree of cementation.
Well-rounded coarse-grained sediments usually have higher porosity than
fine-grained sediments, because the grains do not fit together well.
Poorly sorted sediments usually have lower porosity because the
fine-grained fragments tend to fill in the open space.
Since cements tend to fill in the pore space, highly cemented sedimentary
rocks have lower porosity.
In igneous and metamorphic rocks porosity is usually low because the
minerals tend to be intergrown, leaving little free space. Highly
fractured igneous and metamorphic rocks, however, could have high porosity
is a measure of the degree to which the pore spaces are
interconnected, and the size of the interconnections. Low porosity usually
results in low permeability, but high porosity does not necessarily imply
high permeability. It is possible to have a highly porous rock with little
or no interconnections between pores. A good example of a rock with high
porosity and low permeability is a vesicular volcanic rock, where the
bubbles that once contained gas give the rock a high porosity, but since
these holes are not connected to one another the rock has low
A thin layer of water will always be attracted to mineral grains due to
the unsatisfied ionic charge on the surface. This is called the force of
molecular attraction. If the size of interconnections is not as large as
the zone of molecular attraction, the water can't move. Thus,
coarse-grained rocks are usually more permeable than fine-grained rocks,
and sands are more permeable than clays.
Movement in the Zone of Aeration
Rainwater soaks into the soil where some of it is evaporated, some of it
adheres to grains in the soil by molecular attraction, some is absorbed by
plant roots, and some seeps down into the saturated zone. During long
periods without rain the zone of aeration may remain dry.
Movement in the Saturated Zone
In the saturated zone (below the water table) water percolates through the
interconnected pore spaces, moving downward by the force of gravity, and
upward toward zones of lower pressure. Where the water table intersects
the surface, such as at a surface stream, lake, or swamp, the groundwater
returns to the surface.
Recharge Areas and Discharge Areas
The Earth's surface can be divided into areas where some of the water
falling on the surface seeps into the saturated zone and other areas where
water flows out of the saturated zone onto the surface. Areas where water
enters the saturated zone are called recharge areas, because the saturated
zone is recharged with groundwater beneath these areas. Areas where
groundwater reaches the surface (lakes, streams, swamps, & springs)
are called discharge areas, because the water is discharged from the
saturated zone. Generally, recharge areas are greater than discharge
Discharge and Velocity
The rate at which groundwater moves through the saturated zone depends on
the permeability of the rock and the hydraulic gradient. The hydraulic
gradient is defined as the difference in elevation divided by the distance
between two points on the water table. Velocity, V, is then:
V = K(h2 - h1)/L
where K is the coefficient of permeability.
If we multiply this expression by the area, A, through which the water is
moving, then we get the discharge, Q.
Q = AK(h2 - h1)/L, which is Darcy's Law.
Springs and Wells
A spring is an area on the surface of the Earth where the water table
intersects the surface and water flows out of the ground. Springs occur
when an impermeable rock (called an aquiclude) intersects an permeable
rock that contains groundwater (an aquifer). Such juxtaposition between
permeable and impermeable rock can occur along geological contacts
(surfaces separating two bodies of rock), and fault zones.
A well is human-made hole that is dug or drilled deep enough to intersect
the water table. Wells are usually used as a source for groundwater. If
the well is dug beneath the water table, water will fill the open space to
the level of the water table, and can be drawn out by a bucket or by
pumping. Fracture systems and perched water bodies can often make it
difficult to locate the best site for a well.
An aquifer is a large body of permeable material where groundwater is
present in the saturated zone. Good aquifers are those with high
permeability such as poorly cemented sands, gravels, and sandstones or
highly fractured rock. Large aquifers can be excellent sources of water
for human usage such as the High Plains Aquifer (in sands and gravels) or
the Floridian Aquifer (in porous limestones) as outlined in your text.
Aquifers can be of two types:
Unconfined Aquifers - the most common type of aquifer, where the
water table is exposed to the Earth's atmosphere through the zone of
aeration. Most of the aquifers depicted in the drawings so far have
been unconfined aquifers.
Confined Aquifers - these are less common, but occur when an aquifer
is confined between layers of impermeable strata. A special kind of
confined aquifer is an artesian system, shown below. Artesian systems
are desirable because they result in free flowing artesian springs and
Changes in the Groundwater System
When discharge of groundwater exceeds recharge of the system, several
adverse effects can occur. Most common is lowering of the water table,
resulting in springs drying up and wells having to be dug to deeper
levels. If water is pumped out of an aquifer, pore pressure can be reduced
in the aquifer that could result in compaction of the now dry aquifer and
result in land subsidence. In some cases withdrawal of groundwater exceeds
recharge by natural processes, and thus groundwater should be considered a
non-renewable natural resource.
Water Quality and Groundwater Contamination
Water quality refers to such things as the temperature of the water, the
amount of dissolved solids, and lack of toxic and biological pollutants.
Water that contains a high amount of dissolved material through the action
of chemical weathering can have a bitter taste, and is commonly referred
to as hard water. Hot water can occur if water comes from a deep source or
encounters a cooling magma body on its traverse through the groundwater
system. Such hot water may desirable for bath houses or geothermal energy,
but is not usually desirable for human consumption or agricultural
purposes. Most pollution of groundwater is the result of biological
activity, much of it human. Among the sources of contamination are:
Sewers and septic tanks
Waste dumps (both industrial and residential)
Gasoline Tanks (like occur beneath all service stations)
Biological waste products - Biological contaminants can be removed
from the groundwater by natural processes if the aquifer has
interconnections between pores that are smaller than the microbes. For
example a sandy aquifer may act as a filter for biological
Agricultural pollutants such as fertilizers and pesticides.
Salt water contamination - results from excessive discharge of fresh
groundwater in coastal areas.
Geologic Activity of Groundwater
Dissolution - Recall that water is the main agent of chemical
weathering. Groundwater is an active weathering agent and can leach
ions from rock, and, in the case of carbonate rocks like limestone,
can completely dissolve the rock.
Chemical Cementation and Replacement - Water is also the main agent
acting during diagenesis. It carries in dissolved ions which can
precipitate to form chemical cements that hold sedimentary rocks
together. Groundwater can also replace other molecules in matter on a
molecule by molecule basis, often preserving the original structure
such as in fossilization or petrified wood
Caves and Caverns
- If large areas of limestone underground are dissolved by the action of
groundwater these cavities can become caves or caverns (caves with many
interconnected chambers) once the water table is lowered. Once a cave
forms, it is open to the atmosphere and water percolating in can
precipitate new material such as the common cave decorations like
stalagtites (hang from the ceiling), stalagmites (grow from the floor
upward), and dripstones, and flowstones.
Sinkholes - If the roof of a cave or cavern collapses, this results in a
sinkhole. Sinkholes, likes caves, are common in areas underlain by
limestones. For example, in Florida, which is underlain by limestones, a
new sinkhole forms about once each year, gobbling up cars and houses in
Karst Topography - In an area where the main type of weathering is
dissolution (like in limestone terrains), the formation of caves and
sinkholes, and their collapse and coalescence may result in a highly
irregular topography called karst topography
Factories and underground
are a source of groundwater pollutants. If a tank with a water soluble
liquid leaks the liquid travels down to the water table. It then dissolves
in the groundwater. These pollutants flow as a plume along with the
groundwater. They can pollute wells and surface water fed by the
groundwater along the plume path.
If the liquid that
leaks is less dense than water, it floats on top of the groundwater
table. Some of the liquid will evaporate, traveling upwards to the
surface in the form of vapor fumes. Some of the liquid will dissolve
and travel as a plume in the groundwater.
If the chemical is not very soluble
in water the major part of the liquid will float on the groundwater
and flow along with the groundwater.
Liquids that are denser than
water sink through the groundwater until they reach an aquitard. They
then move along the top of the aquitard along with the groundwater.
are another source of pollution for groundwater. They are often located at
the top of hills, and are thus upgradient from the local groundwater
flow. Decomposing bodies within the cemetery release bacteria, breakdown
products from decay and chemicals used for embalming into the local
Areas surrounded by the saltwater
, need to be concerned about contamination by sea water. Since the
specific gravity of fresh water is less than that of salt water (1.0 vs.
1.025), fresh water floats on top of sea water, forming a fresh water
For every foot of fresh water above the sea level, there are 40 feet of
freshwater below sea level. Below the fresh water salt water saturates the
aquifer. When you pump out fresh water rapidly, you lower the height of
the freshwater in the aquifer forming a cone of depression. The salt water
rises 40 feet for ever 1 foot of freshwater depression and forms a cone of
ascension. Seawater may eventually enter a well that was once pumping
fresh water, making it unusable.