The term groundwater sounds as if it refers to an underground lake or river, but relatively little groundwater is found in this form. Groundwater lies below the surface of the land; in fact, it is almost everywhere underground. Mostly it is found in the tiny pores, or spaces, between rocks and particles of soil and in the cracks of larger rocks.
Where does groundwater come from? When rain falls, some of it flows along the surface of the ground into streams and lakes as runoff. Some of the rain evaporates into the atmosphere, some is taken up by plant roots, and some seeps into the ground to become groundwater.
Aquifers are like big sponges
Underground areas called aquifers collect much of this groundwater. An aquifer is composed of permeable rock, loose material that holds water. Permeable means "having pores that permit a liquid or a gas to pass through." You might think of a groundwater aquifer as a big sponge that soaks up the rain that seeps below the surface.
As water from the surface slowly seeps down, or percolates, through the soil, it eventually hits a solid, or impermeable, layer of rock or soil. The aquifer forms as groundwater collects in the area above this impermeable layer. The water table is the level of the upper surface of the groundwater. If the water table in an area is high, the upper surface of the groundwater is only a short distance below the surface of the ground.
Confined or unconfined?
Groundwater occurs in two conditions: confined and unconfined. A confined aquifer has a layer of impermeable clay or rock above it, and the water is held under pressure greater than the atmospheric pressure. When a well is drilled into a confined aquifer, it penetrates that impermeable, confining layer, allowing the water to rise under pressure. This is called an artesian well. An unconfined aquifer has no impermeable layer above it and is usually shallower than a confined aquifer.
How big is an aquifer?
The size of an aquifer depends on the amount of rainfall and the composition of the underground rock and soil. The world's largest aquifer is in the United States. Called the Ogallala, it spreads under eight western states, from South Dakota to Texas. The Ogallala formed millions of years ago and is still supplying water to cities, businesses, and farms. Unfortunately, people are using water from the Ogallala faster than it can be naturally replenished, and the water table is falling.
Our most precious resource?
Water is a natural but limited resource. Most of the water on Earth is saltwater; 97% of the world's water supply is located in the oceans. That means that only 3% is freshwater, and two-thirds of that is frozen in the polar icecaps, icebergs, and glaciers. Only the remaining 1% is groundwater or surface water in lakes, ponds, and streams.
Today, about three-quarters of the cities in the United States depend on groundwater for part or all of their drinking water. Wells also withdraw groundwater to irrigate crops, keep golf courses green, and meet other recreational needs.
When water is pumped out of an aquifer into a well, the water level drops. If rainfall does not replace that water, the aquifer becomes overdrawn. When water is pumped out faster than it is replaced, the ground may sink, creating sinkholes.
Can aquifers become polluted?
Contamination is another problem. Leaking underground storage tanks may seep petroleum products into groundwater. Inadequate septic systems, sewage treatment plants, fertilizer runoff from farms, salt runoff from highways, and chemicals discharged from factories are other sources of pollution that can make groundwater unsuitable for humans to drink or use.
Pollution can come from specific, identified locations, called point sources, or from scattered areas, called nonpoint sources. Most groundwater pollution comes from nonpoint sources. Once an aquifer is polluted, it may remain that way for years.
Wetlands provide homes for waterfowl and many other animal species. Low-lying wetlands may receive water from an aquifer. If the water is contaminated, it will pollute the wetlands, affecting all the wildlife that depends on these water habitats.
As the human population continues to grow, the demand for fresh, clean water supplies grows too. Careful management and use are essential to maintain the quality of our groundwater and surface water. The following projects will help you understand how aquifers can become contaminated and how dirty water can be cleaned.
Words to Know
- Mixing a gas, like oxygen, with a liquid, like water.
- Underground layer of sand, gravel, or spongy rock that collects water.
- Artesian well
- A well in which water is forced out under pressure.
- A process during which solid particles in a liquid begin to stick together.
- Confined aquifer
- An aquifer with a layer of impermeable rock above it where the water is held under pressure.
- Using chemicals to kill harmful organisms.
- Removing impurities from a liquid with a filter.
- Water that soaks into the ground and is stored in the small spaces between the rocks and soil.
- Not allowing substances to pass through.
- Chemicals or other pollutants in water.
- Nonpoint source
- An unidentified source of pollution, which may actually be a number of sources.
- To pass through a permeable substance.
- Having pores that permit a liquid or a gas to pass through.
- Point source
- An identified source of pollution.
- An opening or space.
- Water that does not soak into the ground or evaporate, but flows across the surface of the ground.
- A process during which gravity pulls particles out of a liquid.
- Surface water
- Water in lakes, rivers, ponds, and streams.
- Unconfined aquifer
- An aquifer under a layer of permeable rock and soil.
- Something that can affect the results of an experiment.
- Water table
- The level of the upper surface of groundwater.
- Areas that are wet or covered with water for at least part of the year.
How to Experiment Safely
Do not drink the water you are using in this project.
- Tape the straw vertically inside the plastic container along one side, as illustrated. Do not let the bottom end of the straw touch the bottom of the container. This will be the "well."
- Pour a 1.5-inch (3.8-centimeter) layer of sand on the bottom of the container.
- Pour water into the sand, wetting it completely without creating puddles. The water will be absorbed into the sand, surrounding the particles, much as it is stored in an aquifer.
- Flatten the clay into a thin layer and cover half the sand with it, pressing the clay into three sides of the container. The clay represents the confining or impermeable layer that keeps water from passing through.
- Pour a small amount of water onto the clay. Most should remain on top of the clay, with some flowing into the uncovered sand.
- Cover the whole surface of the sand and clay with the aquarium rocks. On one side, slope the rocks to form a hill and a valley.
- Fill the container with water until it is nearly even with the top of your hill. See how the water is stored around the rocks in the aquifer. Also notice a surface supply of water (a small lake). This model represents groundwater and surface water, both of which can be used for drinking.
- Put a few drops of red food coloring into the straw to represent pollution. People often use old wells to dispose of farm chemicals, trash, and used motor oils. The food coloring will color the sand. This demonstrates one way that pollution can spread into and through an aquifer.
- Place the green felt on the hill. Use a little clay to fasten it to the sides of the container.
- Sprinkle some cocoa on the hill, representing the improper use of materials such as lawn chemicals or fertilizers.
- Fill the spray bottle with water. Make it rain on the hill and over the aquifer. The cocoa will seep through the felt and wash into the surface water. This is another way that pollution reaches aquifers.
- Check the area around the straw. The pollution has probably spread farther. Remove the top of the spray bottle and insert the stem into the straw. Depress the trigger to pull up water from the well. Note its appearance. This is the same water that people would drink. It also is contaminated.
Summary of Results
From your model, you can easily see how pollution spread into the surface water and the aquifer, contaminating the water supply. Write a paragraph about what you observed.
Here is a problem that might arise, a possible cause, and a way to remedy the problem.
Problem: The straw is clogged with sand.
Possible cause: The straw is too close to the bottom of the container. Make sure you put the straw in first and leave a small space between it and the bottom of the container. Then pour in the sand. If sand still clogs the straw, gently blow through the straw to unclog it.
How to Experiment Safely
Do not drink the water you are using in this project. Be careful using the scissors when you cut the tops and bottoms off the soda bottles.
- Pour about 1.5 quart (1.5 liter) of the swamp water into the uncut soft-drink bottle. On a data sheet, describe the look and smell of the water.
- To aerate the water, place the cap on the bottle and shake it vigorously for 30 seconds. The shaking allows gases trapped in the water to escape and adds oxygen to the water. Then pour the water back and forth between the bottle with the cap and the cut-off bottle ten times. Describe any changes in the water. Pour the aerated water into the large beaker or bottle bottom.
- To coagulate solid impurities in the water so they can be removed, add the alum crystals to the water. Slowly stir for five minutes.
- To allow sedimentation, let the water stand undisturbed for 20 minutes. Observe it at five-minute intervals and write your observations about the changes in the water's appearance.
- Construct a filter from the bottle with its bottom removed. First, attach the coffee filter to the outside of the neck of the bottle with a rubber band. Turn the bottle top upside down and pour in a layer of pebbles. The filter will prevent the pebbles from falling out. Pour the coarse sand on top of the pebbles. Pour the fine sand on top of the coarse sand. Clean the filter by slowly and carefully pouring through 10.5 pints (5 liters), or more, of clean tap water. Try not to disturb the top layer of sand as you pour.
- To filter the swamp water, wait until a large amount of sediment has settled on the bottom of the bottle of swamp water. Then carefully--without disturbing the sediment--pour the top two-thirds of the swamp water through the filter. Collect the filtered water in a beaker or other container.
- Compare the smell and appearance of the treated and untreated water.
Note: The final step in water treatment is disinfection by adding chemicals to kill any harmful organisms. Because disinfectants must be handled carefully, this process is not included here. Do remember that the water you have treated is NOT safe to drink.
Summary of Results
Write a report of your observations of the smell and look of the water before and after treatment. Include the amount of time that it took for the sediments to form.
Here is a problem that might arise during this project, a possible cause, and a way to remedy the problem.
Problem: During sedimentation, the sediments mixed into the water that was being filtered.
Possible cause: You might have poured the swamp water too quickly. Pour the contaminated water back into the sedimentation bottle and let it sit undisturbed again. Or pour it through the coffee filter and see if the sediment makes the water flow more slowly. The filter may not take all the sediment out, or it may become clogged with sediment, one of the many problems that occur during the actual water treatment process.