Earth

Immense reserves

The phreatic zone in a karst system can grow in size and depth depending on the geological structure. At times the phreatic zone may be quite thin or absent as in karst systems that are perched above the base level, at times the phreatic zone may be hundreds of metres deep and thus provide an immense and precious water reserve.
The more superficial area of the phreatic zone, known as the epiphreatic zone, varies during seasons, and can rise various tens of metres during rainy periods. This is a very important zone for the formation of caves because, as a result of Boegli's effect, due to mixing of waters with different chemical compositions (meteoric water and groundwater), most of the largest galleries are formed in these zones.
Below the epiphreatic zone, the water in the deep phreatic zone moves very slowly and may remain inside the karst aquifer for tens or hundreds of years before it returns outside in a spring.
For this reason karst water is very vulnerable to pollution (a polluting agent may remain for decades inside a phreatic zone) and excessive exploiting (the emptying of a phreatic zone may require decades in order to reach the original water level again). Therefore karst aquifers are precious reserves that must be protected and exploited very cautiously.