Sand filter experiment
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.
Explore Great Sand Dunes’ web page on hydrology to learn more about the unique natural hydrological system of the dunes.
This activity can be done by the teacher as a demonstration or
more supplies can be brought so several groups of students can do
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
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 bottlelarger
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
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
Afterwards, return the sand to the dunes.
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.