In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. 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.