What is an aquifer?

    An aquifer is an underground water reservoir, but it isn't like an empty crater as surface reservoirs can be.  Rain water sinks into the ground and fills spaces between rocks, sand, and gravel.   It keeps on sinking deeper with gravity until it is stopped by a layer of ground that won't let it through.  They call this layer an impermeable layer.
    This stopped water fills the area of sand, gravel, clay, and rock where it is stored.  It doesn't move very fast while it's underground.  It will move from one to three inches a day.  The water does not just soak into this layer and sit.  Gravity pulls it so that it flows slowly until it reaches another body of water or another impermeable layer. 
    We use water from aquifers when we drill for wells.  Towns can tap into aquifers for water or it might flow into a stream that goes into a reservoir.  A really important thing to remember when wells are being drilled is that a well can't take out more water than the aquifer can replace. Wells run out of water when more water is used than can be replaced.  Wells run out of water when more water is used than can be replaced.

Water that finds its way into this area fills up and can be under pressure.  It cannot move downhill as it wants to and has no way to get out.  Sometimes the pressure in this kind of aquifer gets so strong that when a well is drilled into it, the water will push up by itself.  In a normal well, pumps pull the water to the surface.  With a well drilled into a confined aquifer, a pump sometimes isn't needed because the pressure makes the water rush up to the top of the well all by itself.  This is called an Artesian well.