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>> 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.
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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.
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