What are these faucets tapping into?
This certainly is an odd sight! We expect to see water at Earth's surface, but coming out of solid rock? Amazingly, much of Earth's fresh water is hidden from sight. It is present in rock and soil. In fact, 20 times more of Earth’s liquid freshwater is found below the surface than on the surface.
Freshwater below Earth’s surface is called groundwater. The water infiltrates, or seeps down into, the ground from the surface. How does this happen? And where does the water go?
Porous and Impermeable Rock
Water infiltrates the ground because soil and rock may have air spaces between the grains. These pores, or tiny holes, result in the rock's porosity. If water can move through a rock, the rock is permeable. Eventually, the water reaches a layer of rock that is not porous. This rock is impermeable. Water stops moving downward when it reaches an impermeable layer.
This animation shows porosity and permeability: http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/POROS_3.MPG. The water droplets are found in the pores between the sediment grains. This is porosity. When the water can travel between pores, that’s permeability.
The diagram below (Figure below) shows two layers of porous rock. The top layer is not saturated; it is not full of water. The next layer is saturated. The water in this layer has nowhere else to go. It cannot seep any deeper into the ground because the rock below it is impermeable.
Water seeps into the ground through permeable material. The water stops when it reaches an impermeable rock. Predict the purpose of the well in the diagram.
The Water Table
The top of the saturated rock layer above (Figure above) is called the water table. The water table isn’t like a real table. It doesn’t remain firmly in one place. Instead, it rises or falls, depending on how much water seeps down from the surface. The water table is higher when there is a lot of rain, and it is lower when the weather is dry.
- Water infiltrates into the ground.
- There is much more fresh water underground than on the surface.
- A rock layer must be porous and permeable to be a good aquifer. An impermeable layer makes up the bottom of an aquifer.
- The water table rises and falls with additions or subtractions to the groundwater system.
Use the resource below to answer the questions that follow.
- What is groundwater?
- How does groundwater begin?
- What is the water table?
- What is an aquifer?
- What is the cone of depression? How is it created?
- Why does rock or soil need to be permeable for there to be groundwater?
- How does water become groundwater?
- What happens if there is an impermeable rock layer underground?
- What can make the water table move up or down?