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Lesson plan Grade 6 Science

The Dunmere Aquifer Model: Porosity and Groundwater Storage

A 50-minute grade 6 science lesson where students model an aquifer with gravel and sand, then calculate porosity to explain why some ground stores groundwater well.

Objectives

  1. Students will model how water fills the pore spaces in gravel and sand.
  2. Students will calculate porosity from given void and total volumes as a percentage.
  3. Students will explain why sand-and-gravel ground yields groundwater to wells better than clay.

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.
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.
Confining layer
A confining layer is a layer of tightly packed clay or solid rock that water cannot easily pass through. It acts like a lid or a floor for the water stored underground.
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.
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

Teacher overview

The fictional town of Dunmere gets its drinking water from an aquifer, a layer of underground sand and gravel that holds water in the tiny spaces between grains. This lesson has students build a simple aquifer model, then measure and calculate porosity, which is the percent of a material that is open space. A common misconception is that groundwater sits in large underground lakes or rivers; the model helps students see that most groundwater fills small pore spaces instead. A second misconception is that the material with the most pore space always makes the best aquifer. Clay holds a lot of water, but it lets that water flow only very slowly.

Procedure

Hook (5 min): Ask students where they think Dunmere's underground water is stored, and take a few guesses. Show a clear cup of dry gravel and ask how much water they think could fit inside it. Record two or three guesses on the board without saying which is right.

Direct instruction (12 min): Define aquifer, pore space, and porosity. Explain that porosity is the percent of a material that is open space, found with the formula porosity (%) = void volume divided by total volume, times 100. Model one example on the board that is not used on the handout: a cup holding 200 mL of gravel that takes 60 mL of water to fill its spaces has a porosity of 60 divided by 200, times 100, which equals 30%. Introduce the idea of a confining layer of clay that water cannot easily pass through. Clay can have high porosity, yet its pores are too small and poorly connected for water to flow through easily; how easily water flows through a material is called permeability.

Model building (13 min): Give each group three cups, gravel, sand, and a measuring cup. Groups fill one cup with a measured amount of gravel and a second with the same amount of sand. They slowly pour measured water into each until it just reaches the top of the grains, tracking how much water each material takes. Remind students to pour over the tray and to add water slowly so they can read the amount used.

Guided practice (10 min): Walk the class through calculating porosity for the gravel cup using their own measured numbers, writing the division and the percent step by step. Circulate and check that each group divides the water volume by the total volume before multiplying by 100.

Independent practice (7 min): Students complete the Student Handout, which gives set numbers so every student practices the same calculations and can check their work against a partner.

Closure (3 min): Poll the class: thumbs up if sand had a higher porosity than gravel in the handout, thumbs down if not. Confirm understanding when at least 80% of the class shows thumbs up, and clear up any confusion before dismissal.

The Dunmere Aquifer Model: Porosity and Groundwater Storage — Grade 6 Science
Name: ____________________________ Date: ______________

Student Handout

Use the porosity formula for every question: porosity (%) = void volume divided by total volume, times 100.

  1. A cup holds 300 mL of gravel. It takes 90 mL of water to fill all the spaces between the grains. What is the porosity of the gravel?

  2. A second cup holds 300 mL of sand. It takes 105 mL of water to fill all the spaces. What is the porosity of the sand?

  3. Which material has more pore space, the gravel or the sand? Use your answers from questions 1 and 2.

  4. A third cup holds 250 mL of packed clay. It takes 110 mL of water to fill the spaces, but the water soaks in very slowly. What is the porosity of the clay?

  5. Rank the three materials from the most pore space to the least, using your three porosity answers.

  6. The aquifer under Dunmere is mostly sand and gravel. Explain why that is good for storing groundwater and pumping it from wells, using the word porosity.

  7. A layer of clay sits below the sand and gravel. Explain what that clay layer does to water that tries to move down through it.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: 30%.
    Porosity is 90 divided by 300, times 100. 90 / 300 = 0.30, and 0.30 x 100 = 30%.

  2. Answer: 35%.
    Porosity is 105 divided by 300, times 100. 105 / 300 = 0.35, and 0.35 x 100 = 35%.

  3. Answer: The sand.
    The sand's porosity is 35%, which is higher than the gravel's 30%, so the sand has more pore space in this model.

  4. Answer: 44%.
    Porosity is 110 divided by 250, times 100. 110 / 250 = 0.44, and 0.44 x 100 = 44%.

  5. Answer: Clay (44%), then sand (35%), then gravel (30%).
    Ranking the three porosity values from largest to smallest gives clay first, sand second, and gravel last.

  6. Answer: Sand and gravel have high porosity, so they have many open spaces to hold groundwater, and their larger, connected pores let water flow to a well.
    Accept any answer that says the pore space gives water room to be stored and that water can flow through those spaces to a well.

  7. Answer: The clay layer blocks or slows water moving down. Clay's pores are so tiny and poorly connected that water moves through it very slowly, even though clay holds a lot of water.
    Accept any answer describing the clay as a confining layer that keeps water from passing through easily.

Differentiation

Support

Provide the porosity formula on a printed card with one fully worked example, and let students use a calculator for the division. Pre-fill the "total volume" column on a small table so students only need to enter the water amount and divide.

Extension

Ask students to design a fourth sample with a porosity between the clay's and the gravel's, stating its total volume and the water it would take, then showing the calculation that proves the porosity value. Have them explain how a driller would use porosity and permeability to pick a good spot for a well.

ELL supports

Provide a labeled picture of an aquifer showing grains with water in the spaces between them, and a visual glossary card for aquifer, pore space, porosity, and confining layer. Allow students to answer questions 6 and 7 with a labeled drawing plus a short phrase.