How Dirt Cleans Water

Summary

Key Concepts

Filtration, aquifers, permeable and impermeable soil, groundwater

Credits

Sabine De Brabandere, PhD, Science Buddies

Introduction

Did you know that more than half of the bottled water sold in America is labeled as “spring water,” but only a fraction of this water naturally flowed from a spring? The FDA allows the sale of groundwater that is sucked up by hydraulic pumps that are installed close to a spring as “spring water.” In the year 2014, this was about six billion gallons of water, a number that increases each year. That is a lot of water! You might wonder how water is stored under the ground, 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 is. Will your “groundwater” be as tasty as spring water? Do the activity to find out!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

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

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. Ground that is saturated with water has all its holes or pores filled with water. These layers are called aquifers; they can be small or huge. The largest aquifer in North America (the Ogallala) runs from South Dakota, covers all Nebraska and runs south all the way to Texas.

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

Materials

About 500 ml of water
Pot that can hold a little over 500 ml
Red food coloring, preferably liquid. Red, flavored-drink powder can be used as well.
Cacao powder
Coarse ground black pepper
Spoon
Three empty 500 ml disposable water bottles with lids
Scissors
Popping corn
Cornmeal
Cornstarch
Three tall glasses; preferably glasses with a rather narrow opening, so an upside-down water bottle can rest in the glass opening
1/4 cup measuring cup
Workspace that can tolerate some liquid splashes

Preparation

In this science activity, you will create “dirty water” and observe how it makes its way through different soil types soil.
To make the dirty water, pour about 500 ml 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.
To create your three soil containers, 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 1-inch layer of cornmeal on top of the layer of popping corn in one soil container, which represents coarse sand in nature.
Add a 1-inch layer of cornstarch on top of the layer of popping corn in another soil container, which represents clay-like soil in nature.
Add a 1-inch layer of popping corn to the last container. All three containers should now have the same level of “soil.”

Instructions

In a moment, you will let dirty water make its way through the soil in each container. Which “soil” do you expect to allow the water to seep through fastest?
Stir your dirty water well.
First, wet the surface of the soils by pouring 4 tablespoons of dirty water over the soil in all 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 1/4 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 another soil container and then for the third soil container. 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 with water.” 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 ask 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?
Ask an adult to unscrew the bottle cap at the bottom of a soil container. Pay attention; your aquifer will instantly deplete. Ask the adult to collect the water as best as she/he can 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.
Ask the adult 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 to 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! Your sand might not be clean!

Observations and Results

Did you notice how the aquifer in the container with only corn kernels (pebbles) filled almost instantly, while the one with a layer of cornmeal (sand) filled slower and the last 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 these materials.

Did you also notice that the kernels only filtered out black pepper, while the cornmeal filtered out most of the cacao powder and a little bit of the food coloring? 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 and expensive to clean an aquifer.