Lab / experiment Grade 7 Science
Which Ground Soaks Up Rain? A Schoolyard Infiltration Lab
A grade 7 lab where students pour water through gravel, sand, and packed soil, time the drain, and compute infiltration rates for runoff and recharge.
Objectives
- Students will measure how fast water drains through gravel, sand, and packed soil.
- Students will compute an infiltration rate in milliliters per second.
- Students will connect infiltration results to runoff and aquifer 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.
Also: runoff, permeable, rate
Teacher overview
When rain falls, some of it soaks into the ground, a process called infiltration, and the rest runs off the surface. The kind of ground makes a large difference: loose gravel lets water pass quickly, while packed soil or pavement sheds most of it. This lab has students pour a measured amount of water through three ground materials, time how long it takes to drain, and compute an infiltration rate. The results model why a paved parking lot produces so much runoff and why loose, permeable ground helps refill the aquifer below.
Procedure
Set up the three columns. Fill one holed cup with gravel, one with sand, and one with firmly pressed potting soil. Label each cup. Hold or rest each cup over the catch tray so drained water can be seen and measured.
Measure a fixed pour. Using the graduated cylinder, measure 200 mL of water for each test. Every column gets the same 200 mL so the comparison is fair.
Run the gravel test. Pour the 200 mL of water steadily onto the gravel and start the timer as you begin pouring. Stop the timer when water stops draining from the bottom. Record the time in seconds.
Repeat for a second trial. Run the gravel test a second time and record that time too, so you can average the two trials.
Test sand, then soil. Repeat the same two-trial process for the sand column and then the packed-soil column, keeping the pour at 200 mL each time.
Safety note. Wipe up spilled water right away so the floor does not become slippery, and keep the catch tray under each cup to contain the water.
Data & Analysis
Record the seconds until water stops draining from the bottom of each column.
| Material | Trial 1 (s) | Trial 2 (s) | Average (s) |
|---|---|---|---|
| Gravel | |||
| Sand | |||
| Packed soil |
- Based on your data, which material let the water drain fastest?
- Based on your data, which material was slowest to drain?
- Suppose one column drained all 200 mL in 20 seconds. What is its infiltration rate in milliliters per second?
- Suppose another column drained 200 mL in 50 seconds. What is its rate in milliliters per second, and which of the two columns, this one or the item 3 column, drains faster?
- A paved parking lot acts like an even more tightly sealed version of the packed-soil column. Using your results, explain what happens to rainwater that lands on pavement instead of loose ground.
- Aquifers refill when water soaks down into the ground. Based on your data, which of your three materials would let an aquifer recharge fastest, and why?
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: Gravel (in most trials).
Accept the material with the shortest average drain time from the student's own data; gravel's large, connected gaps usually drain fastest. - Answer: Packed soil (in most trials).
Accept the material with the longest average drain time from the student's data; pressed soil has the smallest pore spaces and usually drains slowest. - Answer: 10 mL/sec.
200 mL divided by 20 seconds equals 10 milliliters per second. - Answer: 4 mL/sec.
200 mL divided by 50 seconds equals 4 milliliters per second. The item 3 column, at 10 mL/sec, drains faster than this 4 mL/sec column. - Answer: It runs off instead of soaking in.
Accept any answer explaining that pavement lets almost no water soak in, and packed soil lets water in only slowly through its small pores, so most of the rain flows across the surface as runoff rather than infiltrating. - Answer: The fastest-draining material.
Accept the student's fastest material (usually gravel) with reasoning that water which infiltrates quickly can travel down to refill the aquifer, while slow ground sends more water to runoff.
Differentiation
Support
Pre-build one demonstration column and run a sample trial as a class so students see where to start and stop the timer. Provide a data table with only one trial per material to reduce the recording load.
Extension
Have students test a fourth column that mixes sand and gravel, predict where its rate will fall relative to the pure materials, and check their prediction against the measured rate.
ELL supports
Provide picture cards for gravel, sand, and soil labeled with their names, and a labeled diagram of the timer start and stop points. Allow students to report which material was fastest or slowest by pointing to the labeled cups.