by: Cynthia House

Demonstration Materials:

• 125 ml graduated cylinder or similar item
• ~100 ml of pea gravel or small marbles
• kitchen sponge
• tap  water

Experiment Materials:

• preforms and racks (three preforms/student or group)
• fine gravel  such as aquarium gravel (~ 30 ml/student or group)
• coarse sand* (~ 30 ml/student or group)
• fine sand* (~ 30 ml/student or group)
• small plastic cups ~ 100 ml capacity
• squares of tulle (“bridal illusion”) and organza, ~ 10 cm x 10 cm
• rubber bands
• electronic balance (capacity at least 100 gm)
• one pound margarine tub or similarly sized plastic cup per balance**
• stopwatch or count-up timer (MyChron Student Timer)
• 125 ml graduated cylinder or similar item
• calculators
• tap water

* Home centers sell sand for sand boxes, landscaping, paving, mortar etc.  Beaches are another source, although you may encounter undesirable contamination. Sifting non-homogeneous sand with a fine kitchen strainer may yield two usable grades of sand. 

** secondary containment to prevent accidental spillage of water onto the balance

Background Vocabulary:

Porosity is the measure of how much groundwater a soil can hold, permeability is the measure of how quickly water passes through a soil, while retention is the measure of how much water stays behind.  Even elementary students can relate these concepts to their everyday lives. They observe that some areas in their yards or school grounds form puddles while others drain quickly after a rainstorm. They may wonder why one neighbor’s garden and yard remains lush and green although a sprinkler is rarely used. Children in communities dependent upon well water can understand the importance of replenishing the water table. In most rural and many suburban areas, homes use septic tanks and drain fields to process household wastewater. The “water cycle” is a topic in elementary science curricula. There are many excellent age-appropriate online sources for information on these topics including the United States Geological Survey (USGS) and the GLOBE program.

The commercially available kits I investigated for porosity/permeability/retention experiments were designed for high school students, requiring a level of dexterity not yet developed in many younger students. They also required much larger quantities of test material. One can expect larger samples to provide more accurate results, however, we achieved acceptable accuracy and precision with the method described here.

Procedure:

Set-up: Using the 125 ml graduated cylinder, measure 25 ml of gravel or sand into each preform tube.  Prepare one sample of each material for each student or team of students.  If you have enough material and preforms, prepare some extra samples in the event a group spoils a test and has time to repeat it. The caps that come with the preforms are particularly useful in that filled tubes need not be stored upright for convenient storage or transport. Provide each student or team of students with their samples placed in racks, one piece of tulle and one piece of damp organza fabric, a small cup, several rubber bands, a timer, pencil, and copies of “Procedure” and “Data Table” sheets.

Demonstration:

Model the procedure by following the instructions on the “Procedure” sheet as the students follow along.  Use the 125 ml graduated cylinder in place of a preform tube, and ~100 ml of pea gravel or small marbles. The students will be able to visualize the concepts of “voids” within the sample, and how measuring the water that fills those voids allows one to determine their overall volume.  Although it will move very quickly, point out the movement of the waterfront, and when to start and stop the timer. In particular, point out when to stop adding water, and that adding water very slowly is important with the finer materials so as not to overshoot the mark. I believe that overfilling the tube was the cause of most errors.

Students can easily visualize the concept of retention. Let the students handle a dry kitchen sponge. Soak the sponge in a container of water until it is saturated.  Wring out the sponge, then allow the students to handle the still-damp sponge.

Experiment:

I allowed the students to proceed at their own pace. I work with first through fifth grade students in a science club; for the purposes of this experiment I paired first and second grade students with older children, primarily because of the reading, and the math involved later on. Adults manned the balances both to speed up the weighing process and to reduce spillage. I used two balances for twelve teams of students; if you have more balances available, and somewhat older students, the students could handle the weighing process themselves. They may need to be taught how to tare the balance using an empty cup.

Analysis:

Students fill in the remaining columns in the data table, rounding porosity and retention values to the nearest five percent. They should calculate permeability to two significant figures.  Plot porosity and retention results as histograms, one for each material type.  This experiment lends itself to elementary statistical analysis, i.e. mean, mode, and average. I have included histograms from one of my sessions. The presence of a few errant results, both too high and too low, sparked conversation among the students as to possible sources of error in the experiment.

Clean-up:

The sand and gravel can be rinsed out of the preforms, filtered, then spread out on layers of newspaper to dry for reuse.

Cynthia House is the Science Club Adviser at Olive-Mary Stitt Elementary School