Above the saturation zone is the aeration zone. The pore space in this area is filled with water and air, so it is not truly saturated. The soil-moisture belt is the topmost layer of soil. Water in the soil-moisture belt will either be used by plants, evaporate, or continue downward into the zone of aeration.
On top of the zone of saturation, marking the boundary between it and the zone of aeration, is the water table. Often we can see where the water table intersects the surface as it flows from rocks, forming lakes, rivers, or springs.
Groundwater moves slowly at a downward angle, thanks to gravity. Only under extreme pressure will groundwater move uphill. Most groundwater eventually ends up in rivers, lakes or oceans, unless it is used by people first. In fact, many of us get our drinking water from the ground.
We get ground water from from wells or springs. Any water-bearing rock that readily transmits water to the surface through wells or springs is called an aquifer. Wells can be drilled into aquifers, and water can be pumped out. Precipitation will eventually replenish the water in an aquifer. However, you must remember that infiltration is a very slow process. If water is pumped out of an aquifer faster than new water comes into the aquifer, the aquifer could run dry. This has happened in many parts of the United States.
I. Sponge demonstration:
- Place one sponge in a pan, and cover it with a sheet of plastic wrap. The plastic wrap represents an impermeable layer of rock.
- Stack two more sponges on the first one. The top sponge is the soil and zone of aeration. The bottom sponge is the zone of saturation (it won't be truly saturated, but it will be wet enough to complete the activity).
- Slowly pour water onto the sponges, representing a rain storm. Be sure to saturate the top sponge so water can soak through to saturate the bottom sponge.
- Stop for discussion of ground water and infiltration. Wait for about 20-30 minutes.
- Using a clear glass or plastic cup, squeeze the water out of both sponges separately. Mark on the cup how much water came from each sponge. The bottom sponge should have more water than the top sponge due to infiltration and gravity.
II. Sand demonstration:
- Fill a rectangular pan with 2-3 inches of sand.
- Wet the sand.
- Create depressions representing lakes. Make sure the sand is wet enough to have water in the lakes, but not so wet that there is water covering the sand everywhere.
- Ask students to find the water table by drilling "wells" into the sand.
- You should find that the water table in the sand pan is near the same height as the water in the "lake".
What is the zone of saturation? Why is there more water in the zone of saturation than in the zone of aeration? How does gravity influence infiltration? What is a water table? What are two different ways that we can find where the water table is? What might happen if there are pollutants in the soil above the water table? If it takes hundreds of years to refill an aquifer, how long do you think it might take to clean a polluted aquifer? What happens to ground water when landfills and septic tanks leak pollutants into the soil? Many of us drink groundwater; do you think it would be a good idea to be really careful with what we put on and into the ground?
Demonstrate how pollution can affect ground water. Using a pan filled with sand, create topography. Put mountains at one end and a lake at the other. Raise the end of the pan with the mountains about 2-3 inches using blocks. Now create a septic tank near the lake. In the septic tank, pour water dyed with food coloring. Using a watering can, create a rainstorm in the mountains and near the septic tank. Try to pour water slowly enough so that more infiltration happens than runoff. Can you see the colored water move through the sand? Is it in the lake? What if you drill a well downhill from the septic tank? Using an eyedropper, extract water from the well. Is it colored? What if you drill a well uphill from the septic tank? You may need to try this a couple of times to get the right ratio of colored water to rain.