Classroom activity Grade 3 Science
Build a Mini Model of an Underground Water Supply: A Grade 3 Activity
Grade 3 teams pack two soil cups differently, pour equal water on each, and time which soil lets the water soak in faster in a fair test.
Objectives
- Students will build two model ground columns, one with loose soil and one with packed soil, and test how quickly water passes through each.
- Students will identify the independent and dependent variables in a fair test comparing two soil conditions.
- Students will explain why loosely packed soil usually lets water reach an underground water supply faster than tightly packed soil.
Vocabulary
- 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.
- Groundwater
- Groundwater is water that has soaked into the ground and fills the spaces between soil particles and cracks in rock underground. It is a large underground source of fresh water that wells and springs draw from.
- 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: packed, fair test
Teacher overview
This is a small-team building and testing activity for pairs or trios of students. Underground, rain soaks down through soil until it reaches a zone that is already full of water, called an aquifer, which is where many towns pump their drinking water from wells before it goes to a treatment plant. Each team builds two identical clear cups of soil, one left loose and one packed down firmly, then pours the same amount of water on each and times how long it takes to fully soak in, physically modeling how ground conditions change the speed of recharge.
Activity Directions
The investigation. Before building, tell students the testable question this activity answers: does loose soil or tightly packed soil let water soak in faster to reach a model underground water supply? The independent variable, the one thing changed on purpose, is whether the soil is loose or packed. The dependent variable, what is measured to find the effect, is the time in seconds for the poured water to fully soak in. The controlled variables, everything kept the same in both cups, are the cup size, the depth of soil, the amount of water poured, and the way it is poured.
Fair-testing rules. Use identical cups for both tests. Fill both cups to the same soil depth, about 3 inches. Pour exactly 1/2 cup of water into each cup, poured steadily rather than dumped all at once. Change only whether the soil is loose or packed.
- Label one cup "Cup A: Loose" and the other "Cup B: Packed" with tape and a marker.
- Fill Cup A with potting soil, poured in gently and left loose and crumbly, without pressing it down. Use a ruler or the cup's markings to check it reaches about 3 inches deep.
- Fill Cup B with the same potting soil, pressing it down firmly with your hand or the bottom of a cup as you add it. Keep adding and pressing until it also reaches about 3 inches deep. Cup B will need more soil than Cup A.
- Set both cups side by side on the tray. Get the timer and the measured 1/2 cup of colored water ready for Cup A.
- Pour the 1/2 cup of colored water onto the surface of Cup A in one steady pour. Start the timer the moment the pour begins.
- Watch the surface of Cup A. Stop the timer the moment no standing water is visible on top, meaning it has fully soaked in, or after 2 minutes (120 seconds) passes, whichever happens first.
- Record Cup A's time on the data recording sheet. If water is still standing after 120 seconds, record "over 120 seconds."
- Refill the measuring cup with a fresh 1/2 cup of colored water and repeat steps 5 through 7 for Cup B.
- Everyone washes their hands after handling the soil. The activity ends once every team has recorded a time for both cups and completed the Student Handout questions below. Gather as a class to compare results and discuss the debrief questions together.
Student Handout
Record your team's results in this table.
| Cup | Soil condition | Time for water to soak in (seconds) |
|---|---|---|
| A | Loose | |
| B | Packed |
- Before testing, which cup did your team predict would soak in water faster: Cup A (loose) or Cup B (packed)? Why?
- What is the independent variable in this test?
- What is the dependent variable in this test?
- Why did every team use exactly 1/2 cup of water and the same 3-inch soil depth for both cups?
- Explain why loose soil usually lets water pass through faster than tightly packed soil.
- Real wells pump water up from an aquifer for a town's drinking supply, which is then treated before it reaches a tap. Based on your test, which soil condition would probably let an aquifer refill, or recharge, faster after a storm?
- Example problem. Suppose one team recorded that Cup A's water fully soaked in at 45 seconds, while Cup B's water was still standing after the full 120 seconds. How many more seconds, at least, did Cup B take compared to Cup A?
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: Predictions vary
Predictions will vary; accept any prediction with a brief reason. Many students will guess Cup A (loose) because it looks like it has more open space for water to move through.
Answer: The soil condition, whether the soil in the cup is loose or tightly packed.
Answer: The time, in seconds, for the poured water to fully soak into the soil.
Answer: To make it a fair test
Using the same water amount and soil depth for both cups makes it a fair test, so any difference in soak time is caused only by whether the soil was loose or packed, not by unequal amounts of water or soil.
Answer: Loose soil has bigger air gaps
Loose soil has larger gaps of air between the clumps of dirt, giving water more open paths to move downward quickly. Packing the soil down firmly squeezes those gaps mostly shut, so water has to push through much tighter spaces and moves more slowly, or pools on top instead.
Answer: Loose soil
Loose soil would likely let an aquifer recharge faster, because it let water soak through fastest in the test, meaning more rain could reach the underground water supply more quickly after a storm.
Answer: 75 seconds
At least 75 seconds. 120 − 45 = 75 seconds.
Differentiation
Support (below grade level)
Pre-fill both cups with soil before the activity begins (one loose, one already packed), so teams start directly at step 4 with pouring and timing. Round times to the nearest 10 seconds and provide a simple sentence frame: "Cup ___ soaked in faster because it took ___ seconds instead of ___ seconds."
Extension (above grade level)
Have advanced teams add a third cup with soil packed even harder than Cup B (using extra pressing and a heavier object left on top for a minute before testing), then compare all three times and describe the pattern they see as soil gets more and more compacted.
ELL supports
Demonstrate the difference between "loose" and "packed" with a visible squeeze of a handful of dry soil before the activity starts, so the vocabulary is tied to something students can see and feel. Provide picture cards showing Cup A and Cup B side by side, labeled in both English and students' home languages where possible.
Extensions
Ask students where they have seen very packed, hard ground near their own homes or school, such as a well-used dirt path or a spot where cars or heavy equipment often sit, and connect that observation to why some areas soak up rain more slowly than a garden or lawn.