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Ground-water aquifers


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.

                       
When a water-bearing rock readily transmits water to wells and springs, they are  called aquifers. 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.

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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