How does soil act as a filter

Key concepts
Filtration
Aquifers
Permeable and impermeable soil
Groundwater

Introduction
Have you ever noticed the claim on a bottle of water that it contains "spring water"? More than half of the bottled water sold in the U.S. is labeled this way, but only a fraction of this water actually flowed naturally from a spring. Most is from groundwater that is sucked up by pumps (which are installed near a spring). In 2014 this was about 22.7 billion liters of water. That is a lot of water! You might wonder how water is stored underground—and what replenishes these reservoirs. In this activity you will create a model, fill up three reservoirs and evaluate how clean the water in these reservoirs becomes. Will your “groundwater” be as tasty as spring water? Try this activity to find out!

Background
Imagine a rainstorm has just come through; some water runs down the pavement and into a patch of grass where some of it soaks into the ground. Where does that water go? When water soaks—or infiltrates—into a permeable soil, it makes its way through the spaces between the particles in the soil. Soil with larger particles has larger holes, resulting in fast-draining water. Soil with small particles drains water more slowly. Some soils, such as clay, make it very hard for water to seep through and are almost impermeable. Ground such as granite is impermeable. Water flows over the particles into cracks but cannot get through the particles.

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As water seeps deeper into the ground, it will eventually reach an impermeable layer and either collect or flow sideways. This creates underground layers of permeable soil that are saturated with water. Saturated ground has all its holes or pores filled with water. These layers are called aquifers, and they can be small or massive. The largest aquifer in North America (the Ogallala) runs from South Dakota all the way south to Texas.

Unlike surface water collected in rivers and lakes, groundwater is often clean and ready to drink. This is because the soil actually filters the water. The soil can hold onto pollutants—such as living organisms, harmful chemicals and minerals—and only let the clean water through.

• About 500 milliliters (one pint) of water
• Pot that can hold a little more than 500 milliliters
• Red food coloring, preferably liquid (Red, flavored-drink powder also can be used.)
• Cacao powder (one tablespoon)
• Coarse ground black pepper (one half tablespoon)
• Spoon
• Three empty 500-milliliter disposable water bottles with lids
• Scissors
• Popping corn kernels (unpopped)
• Cornmeal
• Cornstarch
• Three tall glasses (Preferably they should be glasses with a rather narrow opening so that an upside-down water bottle can rest in each glass’s opening.)
• One-quarter cup measuring cup
• Workspace that can tolerate some liquid splashes
• An adult helper

Preparation

• Pour about 500 milliliters of water into a pot. Add two drops of red food coloring or some of the red, flavored-drink powder, one tablespoon of cacao powder and half a tablespoon of coarse ground black pepper. Mix well. This will serve as your “dirty” water.
• To create your three soil containers, have an adult carefully help you cut off and discard the bottoms of three empty water bottles. Turn the tops upside down so they look like elongated funnels. Leave the bottle caps on and let the funnels rest in the glass openings.
• Fill the funnel part of each soil container with popping corn kernels. The kernels represent pebbles in nature.
• Add a 2.5-centimeter (one-inch) layer of cornmeal on top of the layer of popping corn in one soil container, which represents coarse sand in nature.
• Add a 2.5-centimeter layer of cornstarch on top of the layer of popping corn in the second soil container, which represents claylike soil in nature.
• Add a 2.5-centimeter layer of popping corn to the third container. All three containers should now have the same level of “soil”. Which “soil” do you expect will allow the water to seep through fastest?

• Stir your dirty water well.

• First, wet the surface of the soils by pouring four tablespoons of dirty water over the soil in each of the three containers. Look at the side of the soil containers. Can you see how high this water reaches? Is it the same for all three containers?

• Scoop out one-quarter cup of dirty water and pour it over the soil in one container. Watch what happens. Does water collect at the bottom of the funnel? Does it run fast or slowly through this soil?
• Repeat the previous step for the second soil container and then for the third one. Remember you predicted which “soil” would allow water to seep through fastest. Was your prediction correct? Why do you think this soil drains water fastest?
• The water will collect at the bottom of the funnel, where it is stopped by the bottle cap (an impermeable layer). This is like an aquifer, an underground layer of soil saturated with water. Soil in which all the holes between particles are filled with water is called saturated. Compare how fast the aquifer grows in the three containers.
• While you wait for the water to seep through the “soil,” reflect on the following questions: Which of the three “soil” types has bigger holes between the particles, the corn kernels (pebbles), the cornmeal (sand) or the cornstarch (clay)? Could that explain why some “soils” allow water to seep through more quickly than others?
• You poured the same amount of water on the three soil containers. Do you think that if you wait long enough, the three aquifers will eventually hold the same amount of water or will one hold more than another? Why do you think this is the case?
• In a moment you will (or a helper an adult to) unscrew the bottles and collect the water gathered in the aquifer in the glasses. What do you expect the dirty water to be like after it runs through each type of “soil”? Would any pollutants get stuck in the dirt and thus be filtered out? Would the different “soils” have different abilities in filtering your dirty water?
• Now, unscrew (or ask an adult helper to unscrew) the bottle cap at the bottom of the corn kernel only soil container. Pay attention; your aquifer will instantly deplete. Ask the adult to collect as much of the water as possible in the tall glass on which the container was resting. When done, replace the unscrewed container on the tall glass so more water can drip out. Note some corn kernels might drip out; this is fine.
• Now repeat (or ask an adult helper to repeat) the previous step for the other two containers.
• Examine the water that ran through the “soils.” Did running the dirty water through “soil” remove the red food coloring?
• Taste the original dirty water and the post-filtered water. Does it taste different? Did any of the “soil” filters remove the cacao powder?
• Look carefully at the bottom of the glasses for signs of black pepper. Did any of the “soil” filters remove the black pepper?

• Extra: Let your bottles drain over a longer period of time. Did more water drain through one “soil” type compared with another?Would this imply that some types of soils retain more water than others?
• Extra: Try a thicker layer of soil. Would a thicker layer of soil be able to filter out more pollutants?
• Extra: Repeat the activity with gravel, sand and clay. Wash the gravel before you start, but definitively do not taste the water collected in the glasses!

Observations and results
Did you notice how the aquifer in the container with only corn kernels (pebbles) filled almost instantly whereas the one with a layer of cornmeal (sand) filled slower and the one with a layer of cornstarch (clay) took a long time?

This is to be expected. The larger holes between the kernels (or pebbles) allow water to seep through quickly—the water drains fast. Cornmeal has smaller particles, just like sand. These particles pack close together and leave little holes in between. The water can still seep through but takes a little longer. Cornstarch is similar to clay. It consists of very small particles packed closely together. The water has a very hard time getting through this material.

Did you also notice the kernels only filtered out black pepper whereas the cornmeal filtered out most of the cacao powder and a little bit of the food coloring, too? If you were patient, you could see that the cornstarch filtered out all the cacao power and more food coloring. This is similar to what happens when dirty rainwater seeps through the soil and gathers in an aquifer. The soil filters the dirty water. Contaminants get stuck in the soil, and clean water reaches the aquifer.

Although groundwater is usually clean, soils are not perfect filters. Some contaminants still make their way through the soil and contaminate the groundwater. This is a serious problem; once polluted, it is hard to clean an aquifer.

Cleanup
The content of the soil containers can go into the composting bin. The plastic bottles can be recycled.

More to explore Capable Carbon Filter, from Scientific American Aquifer, from National Geographic Society
Pollution of Ground Water, from Water Encyclopedia

This activity brought to you in partnership with Science Buddies