Physical Geology – Groundwater


>>Today we will discuss groundwater. We’ve
already discussed the hydrologic cycle, and groundwater is a very important part of that
hydrologic cycle. But why is groundwater important? Well, there’s several reasons. One, we’re depleting our supplies of groundwater,
principally for irrigation. That’s a major issue. Contamination is another problem. We
sometimes contaminate groundwater. You can’t drink contaminated groundwater. We also have
an issue with radioactive waste. Where are we going to put all of our radioactive waste?
Other reasons groundwater is important in terms of geothermal energy. Groundwater can
produce ore deposits, and it can also cause caves to form, which we’ll talk about a little
later in this video. Now when water occurs in the ground there
are two zones. There’s the vadose zone, which is above what is called the water table. And
then there’s the phreatic zone, which is below the water table. The vadose zone, the pores
are filled mostly with air, maybe a little bit of water. In the phreatic zone below the
water table, the pores are filled with water. And there are a couple terms we need to review
when we think about groundwater. One of those terms is porosity. What is the
porosity? It’s the volume of pore space in the rock. Another term in permeability, which
is the ease of flow through the rocks. Porosity and permeability are often related. For example,
if the porosity is connected, you will probably have good permeability. Now underground, if
we have a rock that has a lot of water, we call it an aquifer. That’s a highly permeable
or porous zone that can hold water. It’s commonly associated with shales, which are impermeable
and are called aquitards, either above the aquifer or below it. And these are a very
important water source in many parts of the country. We need to be concerned about depletion and
contamination of our groundwater supplies. And we also have to be a bit concerned, dependent
on where you are, about caves and sinkholes. So what are caves and where do they form?
Well, most caves form at the top of the water table if they form in a limestone terrain.
Remember water is a weak acid, and it accumulates at the top of the water table. And it can
dissolve limestones, and that can create a cave. Now let’s assume a cave forms, all right?
And there’s been some erosion on the land’s surface. And let’s a cow walks on top of that
cave. What can happen? A cow can fall in and the whole cave can collapse into itself. And
that is called a sinkhole. Sinkholes are very common in limestone terrains.
Almost every year you will here about a city or a house in Florida where they’ve had a
sinkhole and somebody’s lost a house or lost a car. It is a big problem wherever you have
limestone terrains as the bedrock.