Lab / experiment Grade 6 Science
Which Ground Soaks Up Rain? Tamworth Field Infiltration
A grade 6 lab where students time how fast water passes through sand, loam, and clay, then compute infiltration rates and connect the results to runoff and groundwater.
Objectives
- Students will measure how long water takes to pass through three kinds of soil.
- Students will calculate infiltration rates in milliliters per second and compare them.
- Students will connect soil infiltration to surface runoff and to groundwater recharge.
Vocabulary
- Infiltration
- Infiltration is when rain or melted snow soaks down into the soil instead of flowing away across the top of it. Water that infiltrates can feed plant roots or keep sinking to become groundwater.
- Recharge
- Recharge is water soaking down through the ground to refill an aquifer, the way rain refills an underground sponge. Aquifers stay healthy only if recharge keeps up with pumping.
- Aquifer
- An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
Also: runoff
Teacher overview
When rain falls, some water soaks into the ground and some runs off across the top. The word for water soaking in is infiltration. The kind of soil makes a big difference. This lab uses three soils from the fictional Tamworth Field, and students time how fast water passes through each one. They then find each soil's infiltration rate and use it to explain why some ground floods and other ground helps refill the water underground. A common misconception this lab addresses is that all soil soaks up water at the same speed; the timing data shows how different they really are.
Procedure
Work in groups of three or four. Each group tests three soils: sand, loam, and clay.
Step 1. Put a coffee filter inside each funnel. Set each funnel on top of a clear cup. Line up all three over a tray.
Step 2. Add about 1 cup of sand to the first funnel, 1 cup of loam to the second, and 1 cup of packed clay to the third. Press the clay down so it is firm.
Step 3. Measure 100 mL of water. Pour it into the sand funnel all at once. Start the timer the moment you pour.
Step 4. Stop the timer when the drips into the cup come more than 10 seconds apart, or at 300 seconds (5 minutes), whichever comes first. Write the time in seconds in your data table. If water is still dripping at 300 seconds, write "300+" so the table shows the soil was still draining. This is trial 1 for sand.
Step 5. Do Step 3 and Step 4 again for the loam, then for the clay. Use 100 mL of water each time.
Step 6. Run a second trial for all three soils so you have two times for each. Add each soil's two times and divide by 2 to get the average.
Step 7. Safety. Do not drink any water used in this lab. Wipe up spills right away, because wet floors are slippery. Wash your hands when you finish.
Data & Analysis
| Soil | Trial 1 time (s) | Trial 2 time (s) | Average time (s) |
|---|---|---|---|
| Sand | |||
| Loam | |||
| Clay |
- What was your average infiltration time for the sand?
- What was your average infiltration time for the clay?
- Which soil let the water pass through fastest, and which was slowest?
- Suppose 100 mL of water passed through sand in 20 seconds. What is the infiltration rate in milliliters per second?
- Suppose the same 100 mL took 200 seconds to pass through clay. What is the clay's infiltration rate in milliliters per second?
- Using your answers to questions 4 and 5, how many times faster did water pass through the sand than the clay?
- In a heavy storm, which ground is more likely to make water run off the top: sandy soil or packed clay? Explain using your times.
- Which soil best lets rain soak down to refill the aquifer below the ground? Explain using your times.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: Accept the student's own average, which should be a small number of seconds because sand drains fast.
- Answer: Accept the student's own average, which should be a much larger number of seconds because clay drains slowly.
- Answer: Sand was fastest and clay was slowest for almost every group. Accept the student's result as long as it matches their own recorded times.
- Answer: 5 mL/s.
100 mL divided by 20 seconds equals 5 milliliters per second. - Answer: 0.5 mL/s.
100 mL divided by 200 seconds equals 0.5 milliliters per second. - Answer: 10 times.
5 milliliters per second divided by 0.5 milliliters per second equals 10, so water passed through the sand 10 times faster. - Answer: Packed clay is more likely to make water run off. Water passes through clay slowly, so during a heavy storm it cannot soak in fast enough and pools or runs across the surface. Accept answers that cite the student's own longer clay times.
- Answer: Sand best lets rain soak down to the aquifer. Water passes through sand quickly, so more of it moves down through the ground to refill the water below instead of running off. Accept answers that cite the student's own shorter sand times.
Differentiation
Support
Pre-build one funnel setup as a class demonstration before groups begin, and give a data table that asks for only one trial per soil. Provide a calculator for questions 4, 5, and 6.
Extension
Have students test a fourth sample, such as sand mixed with clay, and predict where its infiltration rate will fall compared with the pure sand and pure clay, then check their prediction against the data.
ELL supports
Provide picture cards labeled "sand," "loam," and "clay," and a labeled drawing of water soaking in versus running off. Allow students to record results as numbers and short phrases rather than full sentences.
Extensions
Take the class outside to pour a measured cup of water on a paved spot and on a grassy spot, and have students describe which one soaked in and which ran off, linking what they see to their lab data.