How Do Aquifers Work?

How Do Aquifers Work?


– [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.

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