After a certain deepness, the ground is waterlogged. The layer of ground that is saturated with water is called the phreatic zone. Wells are constructed to access the water in the phreatic zone. The layers of soil over the phreatic zone function as water purification filter that stops surface contaminants from reaching the groundwater.
The deepness at which the phreatic can be located vary according to the geology of an area. Some places have shallow phreatic zone while others have phreatic zones that are so deep that special drilling machines are needed to build the well. Generally, places near large bodies of water like lakes and also rivers have shallow phreatic zones. The phreatic zones in these places are very easy to access. The problem with shallow phreatic zone is that the closeness of the water to the surface makes it more likely to be contaminated by surface contaminants.
Deep phreatic zones generally have better water quality. In lots of cases, water purification is not needed. The water is safe for consumption due to the thick layer of soil and rocks above it that serve as sieve.
Although phreatic zones contain large amount of water, this water is not unlimited. Pumping out so much water from a well can dry up the phreatic zone. The phreatic zone can be replenished by rain and melted snow.