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>> Geology is the study of planet earth, its history,
and all of 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're standing on top of the Edward's 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.
Hydro geologists use two terms when investigating aquifers.
Porosity and permeability.
Porosity is all the empty pore space inside a rock given in a percent volume.
Porosity represents the volume of water a rock formation can potentially hold.
Permeability is how well a flute 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 aquitard in ground water flow systems.
Aquifers that are bounded on the top and bottom by aquitards 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 aquitard.
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.
Rain water may infiltrate the shall soil and into the aquifer.
Plant roots may extend down into the aquifer and use the water to survive.
Grand water close to the earth's surface may also evaporate.
Hydro geologists use the terms recharge and discharge
to describe how water enters and levers an aquifer.
[ Running Water ]
One way an aquifer can discharge is through a spring.
Springs occur when the water table intersects the land surface.
The Barton Springs pool is fed by this spring here.
Notice the large fracture which is water flows out of.
Here at upper Barton Spring, water's flowing out of the ground from,
of course, aquifer, or small caves.
To understand better how aquifers work, many hydro geologists
and scientists develop physical models.
To see the physical model in action, continue on to part two.
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Welcome back.
Thanks for joining me for the second segment covering unconfined
and confined aquifers using a physical model.
For our first demonstration, we'll look at an unconfined aquifer composed of gravel and sand.
In the model, you can see the aquitard at the base of the unconfined aquifer
and open to the surface at the top.
We will use dye to show how water flows through the aquifer.
On the right side of the tank, we have higher elevations
and to the left, we have a lake and a river.
As we start to inject the dye, where do you think it will go?
Let's watch.
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Did you figure it out?
The dye flows from high head to low head.
Head essentially means the height of the water surface.
There is more horizontal movement than vertical movement.
You can also see that the low permeability aquitard is restricting the dye
from flowing into deeper aquifers.
Eventually, ground water flow paths discharge the dye into the river.
For the confined aquifer, you can see it is kept by an aquitard and an aquiclude,
a type of aquitard that allows no water to flow through.
Watch what happens when we inject the dye.
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The dye follows a similar path of the unconfined aquifer.
There is little vertical movement, yet lots of horizontal movement.
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As the dye reaches the artesian wells, the dye flows up and discharges on to the land surface.
Again, this is because confined aquifers are under pressure.
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In a fractured bedrock aquifer,
a type of confined aquifer commonly called a karst aquifer,
the surrounding rock has very low if any permeability.
Thus, it is kept by an aquiclude, a layer of rock which will not transmit water.
As we inject the dye, where do you think it will go?
Think about the speed of the water, too.
Will it flow faster or slower than the other aquifers?
Let's watch.
Look at that!
It dye flows only through the fractures, not through the rock matrix.
And it flows much more quickly than the other aquifers.
Again, once it reaches an artesian well, the dye discharges
onto the land surface and flows into the river.
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In this third and final installment, we will be looking at recharge
and surface water interaction using a physical model.
For this demonstration, we will pour blue dye into the model which represents precipitation
as it drips onto the land surface.
Once the blue water infiltrates the land surface
and enters the aquifer, it is considered recharged.
Watch carefully where the blue water goes.
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On the right, you can see the dye soaking through the surface
and flowing deep into the ground.
It also moves horizontally with the flow of water.
Same in this area.
The blue dye can be seen moving deeper and to the left where the head is lower.
Let's rewatch this at a faster Swede so that you can see the dye move more quickly.
Notice how the recharge shows up in the confined and karst aquifers as well.
If there is any contamination spread over the land surface such as from fertilizers
or pesticides, the precipitation would pick it up and follow the same path
as the blue dye into our aquifers.
This is called nonpoint source pollution.
For point source pollution, contaminants are concentrated in a smaller area
such as the septic tank, underground storage tank, or an oil spill.
Let's see where the dye goes if these tanks have a leak.
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Not only does the contamination follow ground water flow, it can easily discharge
into nearby lakes and rivers, too.
For both point source and nonpoint source pollution, some dye remains in the ground just
like actual contaminant leaving behind the residue.
Cleaning up this residue can be costly and, in most cases, it can never be completely cleaned.
Be on the lookout for potential pollution and do your best to prevent ground water contamination.
Thanks for joining me.
The next time you're out escaping the Texas heat in a cool spring,
think about how that water got to you.
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