One of our most valuable resources is the water beneath our feet - something you can't see and may not even know is there! As you may have read, most of the void spaces in the rocks below the water table are filled with water. But rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks.
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When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer. Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water (recharge) into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry. In fact, pumping your well too fast can even cause your neighbor's well to run dry if you both are pumping from the same aquifer. |
In the diagram below, you can see how the ground below the water table (the blue area) is saturated with water. The "unsaturated zone" above the water table (the greenish area) still contains water (after all, plants' roots live in this area), but it is not totally saturated with water. You can see this in the two drawings at the bottom of the diagram, which show a close-up of how water is stored in between underground rock particles.
Sometimes the porous rock layers become tilted in the earth. There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. If a well is drilled into this "pressurized" aquifer, the internal pressure might (depending on the ability of the rock to transport water) be enough to push the water up the well and up to the surface without the aid of a pump, sometimes completely out of the well. This type of well is called artesian. The pressure of water from an artesian well can be quite dramatic.
Here's a little experiment to show you how artesian pressure works. Fill a plastic baggie with water, put a straw in through the opening, tape the opening around the straw closed, DON'T point the straw towards your teacher, and then squeeze the baggie. Artesian water is pushed out through the straw.
Some information on this page is from Waller, Roger M., Ground Water and the Rural Homeowner, Pamphlet, U.S. Geolgoical Survey, 1982
Sources and more information
• Ground Water and the Rural Homeowner, USGS General Information publication, 1982
• Ground Water and Surface Water A Single Resource, USGS Circular 1139
• The Nature of Water: Environment Canada
Wells
Ground-water quality
Sinkholes
Land subsidence
Ground-water decline
How hydrologists find ground water
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/earthgwaquifer.html
Page Contact Information: Howard Perlman
Page Last Modified: Monday, 28-Aug-2006 14:56:21 EDT
The water cycle: Ground-water