How Dirt Cleans Water
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 appropriate 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.
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.
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.
More to Explore
Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Filtration, aquifers, permeable and impermeable soil, groundwater
Explore Our Science Videos
Make a Water Strider - STEM Activity
Slow Motion Craters - STEM Activity
How to Build a Brushbot