Sand Filters

Background

The dunefield at Great Sand Dunes, as well as other areas along the Sangre de Cristo mountain front, are considered aquifer recharge zones. Because few non-porous layers of sediment, such as clay, exist here, water that seeps into the ground in these areas flows directly into the deep aquifer. Elsewhere in the San Luis Valley, layers of clay inhibit the movement of water vertically into the confined aquifer.

As water seeps through sand and rocks on its way to the aquifer, it is filtered and made pure. Many human-created water filtration systems are based on the same principles that we see when water flows vertically into an aquifer.

Procedure

This activity can be done by the teacher as a demonstration or more supplies can be brought so several groups of students can do it together.

Sand dunes are not only made of sand but also of water. You don't have to dig very far to see the moisture in the sand. One of the unique features of sand is that it helps filter water that passes through it.

Go out to the sand and have the students dig into the sand and describe what they find. Talk about why they think it is cool and wet. Ask the students where they think the water came from and where they think it goes. Doing Castles in the Sand prior to this activity helps students understand how sand and water work together through a process called capillary action.

Ask students where water they drink comes from. Discuss the various answers. Ask them if they would like to drink some of the muddy water you brought. Since no one should want to, tell them that even local water we drink must go through several steps in a treatment process in order to be safe and clean and that sand (and the geological landscape in general) plays an important part in filtering water to remove impurities.

If you have a sand sifter, let several students sift sand to separate coarse and fine sand. If not, the experiment will still work if pebbles and larger sand grains are placed in the neck of the two-liter bottle. Place the screen on the end where the cap used to be and secure it with string or tape. Place the materials into the bottle—larger materials at the bottom and smaller materials at the top. The charcoal, if available, should be placed on the top.

Pour clean water through the filter to flush the filtration system of bits of dirt that gathered in the neck of the bottle. Next, pour some dirty water into the top of the filter and watch what comes out.

After the water has filtered thoroughly, have the students compare what came out with what went in. Is there any difference? If the water does not come out clean, ask the students what they think happened and how they could change the experiment to make it better. How would the organize materials for the best filtration system?

Discuss the function of sand and soil on the ground around us and why it is important. Explain to the students that the Great Sand Dunes is an aquifer recharge zone, where water is able to seep down into the confined aquifer. Also, discuss the water they drink at home. Does their water come from an aquifer, a river, a lake, or elsewhere?

Afterwards, return the sand to the dunes.

Extension

Spread a layer of clay into your filtration system to demonstrate how clay acts as a non-porous boundary between aquifers.

Adapted from Educator's Guide to Great Sand Dunes, by Lori Cooper, Friends of the Dunes.