Hydrogeology Analogy Collection

Do you have an analogy you use in your own teaching? Please share your idea.

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Capillary fringe

Submitted by Scott Bair, the Ohio State University

It is very difficult to observe or detect the capillary fringe in the field. One way to demonstrate differences in capillary forces in class is to use two men's ties -- old ties. Get one tie that is made of silk and another that is made of wool and has a distinctly woven appearance to it. (It helps if the ties are light in color with no prominent stripes, patterns, or tweeds.)

Take the two ties and submerge their tips the same depth (4-5 centimeters) into a clear glass beaker of water. Remove the ties after 30 seconds or so. Using masking tape place the ties on the blackboard so that the initial line of saturation (water table) on the two ties is even. With chalk, draw a horizontal line between the two ties connecting the water table. Go on with your lecture or discussion. In a few minutes, bring the students' attention back to the ties and (hopefully) you will see that the larger capillary forces (smaller pore throats) in the silk tie have caused the water to "wick" upwards several more centimeters than on the wool tie, which has larger pore throats and smaller capillary forces. Mark the position on the blackboard of the top of the capillary fringe on each tie. Thus, with two old ties you have made the analogy and a mental picture of the differences in the height of the capillary fringe in a fine-grained material versus a coarser-grained material.

To set up a good punch line, tell the class you purchased the silk tie at K-Mart. Now, ask them if they remember the classic line that Dustin Hoffman (Raymond Babbitt) tells his brother, Tom Cruise (Charlie Babbitt), near the end of the movie 'Rain Man'? If they don't know it, tell 'em "K-Mart sucks."

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Specific discharge IS NOT a measure of groundwater velocity - Fun with shaving cream

Submitted by Scott Bair, the Ohio State University

Understanding that specific discharge (a.k.a. Darcy velocity) is not a true measure of groundwater flow velocity, even though it has the units of velocity, is a difficult concept for some students to grasp. They do not understand why accounting for the pore space through which groundwater flows should cause the actual groundwater velocity to increase.

Here's a demo and an everyday example I use to convey this concept. Ask your students if they have ever been in a shaving cream fight. Most have. Tell them that you can make them the best shaving cream fighter in the dorm by applying the difference between discharge velocity and average linear flow velocity. If you depress the top of the plastic dispenser on a regular shaving cream can, the foam will squirt out an inch or so at a slow rate. This is analogous to water flow without accounting for the restriction in actual (pores only) space created by the minerals grains.

To demonstrate to the class how the limited space through which groundwater can actually flow, through interconnected pores, all you need to do is place a pin in the opening of the plastic spout in the shaving cream dispenser and melt the plastic around the pin using a match. This greatly reduces the space through which the foam can flow resulting in greater velocity, which is equivalent to dividing the specific discharge by the porosity (decimal) causing an increase in the quotient (average linear flow velocity).

To really get the point across, before you explain the analogy to the class, challenge one student to a shaving cream dual (back-to-back, one pace, turn and shoot). Ask the student to leave the room while you explain the analogy. Then, armed with shaving cream cans labeled "Specific Discharge" and "True Velocity," have the dual. [I learned this trick the hard way as a college freshman when I got in a shaving cream fight with my R.A. -- he was a physics major.]

Another analogy you can describe in class is that when you rinse the soap off your car after washing it, you put your thumb over the end of the hose to squirt water further and faster at the suds. The open hose throat is equivalent to specific discharge, which does not account for the mineral grains in rocks and sediments. When you diminish the hose throat with your thumb, you are accounting for the mineral grains, which decreases the available space (interconnected pores) for groundwater to flow, which is why the average linear flow velocity is greater the the specific discharge.