Water existing beneath the land surface occurs in two distinct zones.
The first zone, which occurs immediately below the ground surface is the unsaturated zone and contains both air and water within the interconnected void spaces of the soil or rock matrix. This zone is also often referred to as the vadose zone . Below the unsaturated zone, is a zone where all of the interconnected void spaces within the soil or rock matrix are full of water. This lower zone is referred to as the saturated zone. Water within the saturated zone of the subsurface is the only underground water that is available for recovery in supply wells as drinking water, or irrigation water, etc.