National Ground Water Association

Students should have completed the introductory lesson and the activity that created their own aquifer model prior to beginning this activity. Conducting these lessons over a few different classes allows the students to actually learn the vocabulary prior to working on the activities.

Students will determine the type of material that best contributes to aquifer water storage.

Students must have a complete understanding of the terms permeability and porosity.

1. Review and restate that porosity is not equal to permeability. Porosity is the percentage of the total volume of the rock consisting of voids. Permeability is the capacity of a rock to transmit fluids.
   
   The different types of porosity in rocks include:
   • Porosity resulting from well-sorted sedimentary grains (sand and gravel with pore size = 12 percent to 45 percent volume)
   • Porosity in poorly sorted sedimentary deposits, where much of the space between large grains is filled with smaller grains
   • Porosity in cemented sedimentary rocks (sandstone), where the cementing material fills the space between the grains
   • Porosity due to fractures
   • Porosity due to fractures enlarged by dissolving activity.
   Porosity changes with depth. When there are two miles thick of substrate resting on top of the material, the overlying rock compresses the voids and porosity diminishes.
   
   Permeability of rock depends on the connections between the voids. As a rule, shale, dense limestone, unfractured igneous rock, and quartzite have low permeability. Conglomerates, sandstones, fractured igneous rock, and fractured or dissolved limestone exhibit high permeability. 
   
2. Have students compare the ability to store water by the different substrates and the availability of water in the different substrates by completing the following experiment. (Note: Storage ability of the different substrates is compared by pouring a measured amount of water from a graduated cylinder into the substrate containers. Availability of stored water in different substrates is compared by measuring the amount of water that can be drained from the containers.) Ask students to predict which material will hold the most water.
   1. Fill tall clear container three-fourths full of the same substrate used in their model for the water-bearing portion of the aquifer.
   2. Pour a measured amount of water into the container. (You might want to allow students to add a small amount of red or blue food coloring so that they can see the water movements.)
   3. Continue pouring water until the water table is at the surface of the material, or the zone of saturation is at ground level. Be sure to keep a running total of the amount of water poured into each container.
      
3. Ask the students to predict which material will have the most water availability. Will it be the material with the most storage ability? Have students compare the availability of water in the different substrates using the following steps.
   1. Place a hand pump from a soap dispenser into the container, with the intake protected by pantyhose or a screen. Place the pump all the way to the bottom.
   2. Pump the water into a cup. After a set amount of time, all water removal should stop. This is to differentiate the substrate porosity, which allows water movement.
   3. Measure the amount of water removed from the container. Is it equal to the amount that entered the substrate? Where is the remaining water?
4. Have students record all of their findings and predictions to submit at the end of class for assessment. It is helpful to discuss these with the entire class prior to collecting to ensure that students have all of the required information in order.