Lab / experiment Grade 7 Science
Kelby Flats Aquifer Materials: A Grade 7 Porosity and Flow Lab
A grade 7 lab where students measure porosity and soak-in time for gravel, sand, and a mix, then use a boring log to compare a sand-gravel aquifer with clay.
Objectives
- Students will measure the porosity of gravel, sand, and a sand-gravel mix and time how quickly water soaks into each.
- Students will compute porosity and percent decrease from volume data.
- Students will use porosity and drainage data from a boring log to explain why some ground supplies far more usable groundwater than nearby ground.
Vocabulary
- Porosity
- Porosity measures how much empty space a rock or soil has between its grains, which tells you how much water it can hold. A sponge has high porosity, while a piece of glass has almost none.
- Permeability
- Permeability describes how easily water can move through a material. Water flows quickly through gravel but very slowly through clay, even though both can hold water.
- 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.
Also: pore space, sorting
Teacher overview
Groundwater is not stored in underground lakes. It sits in the open spaces, called pores, between grains of gravel, sand, silt, and clay. The share of a material that is open pore space is its porosity (porosity % = void volume ÷ total volume × 100). How easily water moves through those pores is a different property, called permeability. A good aquifer needs both: enough pore space to store water and pores large and connected enough that water can flow to a well.
In this lab, students fill cups with equal volumes of gravel, sand, and a mix of the two, time how fast water soaks in, and measure how much water fills the pores. Most groups find two patterns: gravel and sand have similar porosity even though their grains are very different in size, and mixing them lowers the porosity because sand grains fill the spaces between the gravel. Geologists describe this with the term sorting: a well-sorted material has grains of about the same size, while a poorly sorted material mixes many sizes and usually has less pore space. Students then read a boring log from the fictional town of Kelby Flats, where a clay layer holds more water than a sand-and-gravel layer yet gives up very little of it to a well.
Teacher note on soil. This lab does not use packed soil, and the reason is worth sharing with students. Fine-grained soils and clay often have as much pore space as sand or more, yet water enters them so slowly that it pools on top long before the pores fill. A pour-until-level test on packed soil therefore measures slow soak-in, not pore space, and gives a falsely low porosity. The boring log in Data & Analysis presents clay through lab values instead.
Procedure
Step 1. Mark the fill lines. Label three cups Gravel, Sand, and Mix. Using the graduated cylinder, pour 200 mL of water into each empty cup and draw a line at the water level with the marker. Pour the water out and dry each cup completely with a paper towel, because any water left in the cup would be counted as pore space later.
Step 2. Make the mix. In the bowl, stir together roughly equal amounts of dry gravel and dry sand (about 1 cup of each) until the sand is spread evenly through the gravel.
Step 3. Fill the cups. Fill the Gravel cup with dry gravel up to the 200 mL line, the Sand cup with dry sand, and the Mix cup with the sand-gravel mix. Level the top of each by tapping the cup gently on the table two or three times. Do not press the material down, and use the same tapping for all three cups so the comparison is fair.
Step 4. Time the soak-in. Fill the graduated cylinder to 100 mL. Set the Gravel cup on the catch tray. Start the stopwatch as you pour 30 mL of water evenly onto the surface of the gravel, and stop it when no standing water is visible on top. Record the time in seconds.
Step 5. Fill the pores. Continue pouring slowly from the graduated cylinder until the water level just reaches the top of the gravel and the surface glistens. If you need more than 100 mL, note how much you poured, refill the cylinder to 100 mL, and keep going. Add up all the water you poured, including the first 30 mL, and record that total as the water added to fill the pores. Pour slowly, because pouring fast traps air bubbles, and trapped air takes up space that water should fill.
Step 6. Repeat for sand and the mix. Repeat Steps 4 and 5 for the Sand cup and then the Mix cup, using a fresh 100 mL fill of the cylinder to start each cup.
Step 7. Calculate. For each cup, calculate porosity as water added ÷ 200 mL × 100, and record it in the table.
Safety note. Wipe up spilled water right away so the floor does not get slippery, and keep the catch tray under the cups. Do not taste any water or material, and wash your hands after the lab.
Data & Analysis
Record your group's results here. The total volume is 200 mL for every cup.
| Material | Total volume (mL) | Soak-in time for the first 30 mL (s) | Water added to fill the pores (mL) | Porosity (%) |
|---|---|---|---|---|
| Gravel | 200 | |||
| Sand | 200 | |||
| Sand-gravel mix | 200 |
Sample data from another class. A grade 7 class at Kelby Flats Middle School ran the same lab and recorded these results.
| Material | Total volume (mL) | Soak-in time for the first 30 mL (s) | Water added to fill the pores (mL) |
|---|---|---|---|
| Gravel | 200 | 1 | 80 |
| Sand | 200 | 9 | 70 |
| Sand-gravel mix | 200 | 14 | 48 |
Kelby Flats boring log. Before choosing a site for a new town well, the fictional Kelby County water office drilled two test borings 2 km apart. At both sites the water table is 3 m below the ground, and the ground from 3 m to 15 m deep is completely filled with water. Core samples from that water-filled layer were tested in a lab.
| Boring | Location | Water-filled layer | Material in that layer | Porosity (%) | Share of total volume that drains out to a well (%) | Time for 500 mL of water to pass through a 30 cm lab test column |
|---|---|---|---|---|---|---|
| KF-1 | Kelby Flats, near Tamsin Creek | 3 m to 15 m deep | Sand and gravel | 30 | 24 | 40 seconds |
| KF-2 | Orrin Bench, 2 km east | 3 m to 15 m deep | Clay | 50 | 3 | About 3 weeks |
- Using your own data, rank the three materials from highest to lowest porosity, and then from fastest to slowest soak-in. Did the material with the highest porosity also have the fastest soak-in? Describe what your two rankings show.
- Using the sample data, calculate the porosity of the gravel, the sand, and the sand-gravel mix. Use porosity % = void volume ÷ total volume × 100.
- By what percent is the mix's porosity lower than the gravel's porosity? Use percent decrease = (gravel porosity − mix porosity) ÷ gravel porosity × 100. Then explain why mixing sand into gravel lowers the porosity.
- In the sample data, the gravel's porosity is only 5 percentage points higher than the sand's, yet the first 30 mL took 9 times as long to soak into the sand. What does this show about the difference between porosity and permeability?
- Use the boring log. Under each 1 square meter of ground, the 12 m thick water-filled layer has a volume of 12 cubic meters, which is 12,000 L. How many liters of water are stored in the pores of that volume at KF-1, and how many at KF-2? Which layer stores more water?
- Using the "drains out to a well" column, how many liters of water could drain from the same 12,000 L of layer at KF-1, and how many at KF-2? How many times as much water could a well draw from KF-1 as from KF-2?
- KF-1 and KF-2 are only 2 km apart and sit above the same volume of water-filled ground. Using your answers to items 5 and 6 and the test-column times, explain which site the town should choose for its well and why groundwater that is usable by people is spread so unevenly underground.
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: Rankings from the student's own data.
Accept rankings that match the student's table. Most groups find gravel and sand within a few percentage points of each other in porosity, the mix lowest, and soak-in fastest in gravel and slowest in the mix. A strong answer notes that the porosity ranking and the soak-in ranking do not have to match, because how much pore space a material has and how fast water moves through it are separate properties. If a group's sand porosity is much lower than its gravel porosity, trapped air from fast pouring is the likely cause. - Answer: Gravel 40%, sand 35%, sand-gravel mix 24%.
Gravel: 80 mL ÷ 200 mL = 0.40, and 0.40 × 100 = 40%. Sand: 70 mL ÷ 200 mL = 0.35, and 0.35 × 100 = 35%. Mix: 48 mL ÷ 200 mL = 0.24, and 0.24 × 100 = 24%. - Answer: 40%.
40 − 24 = 16 percentage points, and 16 ÷ 40 = 0.40, so the mix's porosity is 40% lower than the gravel's. Accept explanations that the small sand grains settle into the open spaces between the larger gravel pieces, so less of the cup is left as empty space. A material with grains of many sizes (poorly sorted) usually has lower porosity than a material whose grains are all about the same size (well sorted). - Answer: Porosity and permeability are different properties.
The two materials have nearly the same amount of pore space, yet water moved into the gravel much faster. Accept answers explaining that gravel's pores are larger and more open, so water flows through them easily (high permeability), while sand's pores are smaller, so water moves through them more slowly even though the total pore space is similar. How fast water moves depends mostly on the size of the pores and how well they connect, not only on how much pore space there is. - Answer: KF-1 stores 3,600 L, KF-2 stores 6,000 L, and the clay layer at KF-2 stores more.
KF-1: 30% of 12,000 L = 0.30 × 12,000 = 3,600 L. KF-2: 50% of 12,000 L = 0.50 × 12,000 = 6,000 L. Students should notice that the clay holds 2,400 L more water than the sand and gravel. - Answer: 2,880 L from KF-1, 360 L from KF-2, which is 8 times as much from KF-1.
KF-1: 24% of 12,000 L = 0.24 × 12,000 = 2,880 L. KF-2: 3% of 12,000 L = 0.03 × 12,000 = 360 L. 2,880 ÷ 360 = 8. - Answer: KF-1, because its sand and gravel release far more water, and release it quickly.
Accept answers that use the numbers: the clay at KF-2 stores more water (6,000 L compared with 3,600 L), but most of that water clings to the tiny clay particles, so only 360 L could drain to a well, and water takes about 3 weeks to pass through a test column that the sand and gravel lets through in 40 seconds. KF-1 could supply 2,880 L, 8 times as much, and water would flow toward the well fast enough to keep it supplied. Strong answers explain that usable groundwater depends on the type of material underground, which changes from place to place because of how layers of sediment were laid down, so two nearby sites can have very different groundwater supplies. Students may also note that the sample class's sand and gravel (35% to 40% porosity) measured higher than the KF-1 layer (30%), which fits item 3, because natural sand-and-gravel layers are usually mixed and packed together by the weight of the ground above them.
Differentiation
Support
Mark the 200 mL fill lines and prepare the sand-gravel mix before class so groups can start at Step 3. Provide the porosity formula with one worked example that uses different numbers (for example, 60 mL of water in a 150 mL cup gives 60 ÷ 150 × 100 = 40%) before students try item 2, and give a percent-decrease example before item 3. For items 5 and 6, remind students that 30% of a number is 0.30 times that number.
Extension
Have students fill a fourth cup to the line with potting soil or clay-rich soil and repeat Steps 4 and 5. Ask them to describe what happens to the water on the surface, calculate the porosity the test gives, and explain why this number is probably much lower than the soil's real pore space. They should compare their result with the clay in the boring log and propose a better way to measure the porosity of fine soil, such as letting a soil sample sit submerged in water overnight before measuring.
ELL supports
Provide labeled picture cards for gravel, sand, mix, clay, pore space, and well, and a diagram showing water filling the spaces between grains. Pre-teach the difference between "holds water" (porosity) and "lets water through" (permeability) with two gestures, such as cupped hands and a hand moving forward. Allow students to show their item 1 rankings by ordering the cups before writing, and offer the sentence frame "The clay stores more water, but ___."