Using Darcy Flow Tubes to Teach Concepts of Ground-Water Geology

The concepts of porosity and fluid flow through porous media can be explored by using a simple apparatus called a Darcy tube. The Darcy tube consists of a ten-cm-diameter, clear, plexiglas cylinder one-meter long filled with a porous medium such as dime sand or river gravel. Water is delivered to one end of the tube, and the drop in hydrostatic pressure along the tube is recorded by a series of piezometers. By adjusting the hydraulic gradient and measuring discharge, students can confirm (or discover) the essential basis for Darcy's Law. The Darcy tube can also be used in a vertical orientation to measure the porosity of various sediments by measuring the volume of water required to saturate the sediment fully. Specific yield can be measured by simply measuring the volume of water freely drained by gravity from the tube. Other laboratory experiments are also possible, such as measuring the rate at which contaminants migrate through various porous media.