- [Voiceover] Beneath the surface of Texas
are extensive water bearing geologic
formations that make up the state's vast
and diverse system of aquifers.
These aquifers provide more than half
of the water Texans use each year.
Aquifers are rarely underground lakes
like you might see in a cave.
They are typically rock formations that are mostly solid
but like sponges they have interconnected fissures
and pore spaces that hold water
and allow it to move within the formation.
An aquifer is best defined as a layer
of saturated rock and the water in its pores.
Aquifers were formed very slowly over vast periods of time.
Water in aquifers comes originally
from the surface of the Earth.
When rain falls, some of the water infiltrates
underground and collects in the pore spaces.
Water in rivers and lakes can also
seep into the subsurface pores.
Eventually, some interconnected
pore spaces become saturated with water.
In other words, they become an aquifer.
- [Voiceover] The ability of an aquifer
to produce water depends on the properties
of the aquifer's rock formation.
The two most important properties
are porosity and permeability.
The porosity is a measure of the volume
of empty space, or pore spaces, inside the rock.
Pore spaces can be openings between grains,
fractures in the rock, and even caverns.
Porosity represents the volume
of water a rock formation can hold.
Porosity itself isn't enough to create an aquifer.
The areas filled with water must connect with each other
so the water can flow from one pore space to another.
That connectivity between pores is known as permeability.
Permeability is a measure of how readily
water can flow within the rock.
Rocks such as pumice and shale can have high porosity
but do not form productive aquifers
because the pores within the rock are not connected.
The permeability of pumice and shale is low.
On the other hand, gravel transmits water quickly
because of large connected pore spaces.
The permeability of gravel is high.
- [Voiceover] Compared to water movement
through rivers or lakes,
the rate of movement through an aquifer is slow.
Of the nine major aquifers in Texas,
six of them, the Ogallala, Gulf Coast,
Carrizo-Wilcox, Pecos Valley, Seymour,
and Hueco-Mesilla Bolsons consist of sedimentary rock
with relativity high porosity and permeability.
Sedimentary rocks are composed
of sand, gravel, silt, and clay.
The three other major aquifers, the Edwards Aquifer,
sometimes referred to as the Balcones Fault Zone,
the Trinity Aquifer, and the Edwards-Trinity Plateau Aquifer
consist either mainly or partly of limestone.
Water in limestone aquifers is held in crevices
and caverns left by the dissolution
of the limestone by ground water.
This subterranean landscape is known as Karst
and it is highly permeable.
Being on a fault zone, the Karst limestone
of the Edwards Aquifer is highly fractured.
Rainwater seeps very quickly
into the Edwards Aquifer through the fault lines,
even while a storm event is happening.
Not all limestone aquifers in Texas recharge this quickly.
- [Voiceover] In nature, aquifers are seldom
separated neatly into geographic areas.
Instead, like a stack of pancakes,
aquifers overlay each other and dip at different angles.
Some parts are exposed to the Earth's surface
while other parts are buried under other aquifers.
The part of an aquifer that is exposed
at land surface is known as the outcrop.
The part that is in the subsurface
is known as the downdip extent.
For example, the outcrop of the Carrizo-Wilcox Aquifer
is a narrow band that lies parallel to the Gulf Coast.
As we move toward the coast the aquifer slopes
beneath the land's surface, forming the downdip.
Aquifers are separated from each other by aquitards.
Aquitards are impermeable layers
that prevent water from easily flowing
from one aquifer to another.
An aquifer is confined when it is buried
beneath other aquifers and/or aquitards.
The combined weight of the rocks
and water above pressurizes the ground water.
This is sometimes referred to as Artesian pressure.
When the aquitard above a confined aquifer
is punctured by drilling a well
Artesian pressure causes the water in the well to rise.
Sometimes this water reaches land surface
resulting in a flowing well.
Springs are water flow into the surface
through natural pathways caused
by the faulting or dissolution of aquifer rocks.
Pumping in a confined aquifer causes
the pressure of the ground water to reduce.
The reduction in pressure lowers water levels
in Artesian wells and springs.
Because confined aquifers are, by definition,
not always well-connected to surface
water sources like streams and lakes,
recharge may be slow, if it happens at all.
- [Voiceover] When an aquifer is exposed to the
Earth's surface, such as at the outcrop, it is unconfined.
The ground water is not pressurized.
When we pump an unconfined aquifer
it lowers water levels in the aquifer itself.
In addition, because the water is not under pressure,
it takes more energy to pump water out of the aquifer.
Unconfined aquifers are usually replenished
more easily than confined aquifers
because rainfall and water from rivers and streams
can flow into the exposed areas of the aquifer.
In addition, the material above an unconfined aquifer is
usually porous so surface water can seep into the aquifer.
- [Voiceover] The better we understand
the geologic properties of aquifers,
the better we can understand their sensitivity
and susceptibility to human use
such as pumping and contamination.