Part I: Aquifers Explained

Part I: Aquifers Explained


[ Music ]>>Geology is the study of planet Earth, its
history, and all the processes that act on it. Hydrogeology is the branch of
geology which studies how water and rocks interact underground
mainly in aquifers. An aquifer is a rock unit that can hold
enough water to supply water to wells. Aquifers can be found in many types of
rocks including sandstone, conglomerate, unconsolidated sand and gravel, and fractured
rocks composed of limestone or igneous rocks. Here at Barton Springs in Austin,
Texas, we are standing on top of the Edwards aquifer composed
mostly of fractured limestone. Over time, these fractured rocks
can dissolve to create large cave like systems called karst aquifers. So, when you hear the word karst, think cave. Some of these caves are big. Some of them are small. Karst aquifers are different from sedimentary
aquifers where water mostly flows through gravel and sand grains similar to a sponge. Hydrogeologists use two terms when investigating
aquifers, porosity and permeability. Porosity is call the empty pore space
inside a rock given in a percent volume. Porosity represents the volume of the water
a rock formation can potentially hold. Permeability is how well a fluid can flow within
the pore spaces of the rock within the aquifer. For water, we describe this
property as hydraulic conductivity. For example, clay and rocks like
pumice often have high porosity. But because their pores do not connect well with
each other, they are often low in permeability. Low permeability materials such as clay
and shale, typically act as barriers to ground water flow and may often function
as an aquatard in ground water flow systems. Aquifers that are bounded and the top and bottom
by aquatards are called confined aquifers. In confined aquifers, the
ground water is under pressure. If penetrated by a well, the water level in a confined aquifer will rise
above the top of the aquifer. In some cases, the water will reach the surface
resulting in a flowing or artesian well. Confined aquifers function
differently than unconfined aquifers which have no overlying aquatard. Unconfined aquifers occur
near the Earth’s surface. So, they may interact better to the processes
that occur near or above land surface. Because unconfined aquifers
are exposed to the atmosphere, they may interact with surface water
features such as rivers or lakes. If the water level in the aquifer is
higher than in the adjacent rivers or lakes, the rivers or lakes gain water. In other words, water from the aquifer
flows into the rivers or lakes. If the water level in the aquifer is
lower than in the rivers or lakes, then the rivers or lakes lose water. That’s saying that water flows from
the rivers or lakes into the aquifer. Rainwater may infiltrate the
shallow soil and into the aquifer. Plant roots may extend down into the
aquifer and use the water to survive. Ground water close to the Earth’s
surface may also evaporate. Hydrogeologists use the terms
recharge and discharge to describe how water enters
and leaves and aquifer. [ Water Sounds ] One way an aquifer can discharge
is through a spring. Springs occur when the water table
intersects the land’s surface. The Barton Springs pool is
fed by this spring here. Notice the large fracture
which the water flows out of. Here at upper Barton Spring,
water is flowing out of the ground from a karst aquifer or small saves. To understand better how aquifers
work, many hydrogeologists and scientists develop physical models. To see the physical model in
action, continue on to Part 2. [ Music ]

4 thoughts on “Part I: Aquifers Explained

  1. Very nice the three videos, very clear and useful in nay language, cause the subtitles work reasonably well. I am showing and recommending these videos in my classes in Spain (Master of Environmental Engineering). More like these, developing cases of GW pollution and contaminant transport, would be great.

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