- More than half of all groundwater is available within 750 m of the earth’s surface. However, evidences of the presence of groundwater are also found at a depth of more than 11000 m.
- Water below the ground is available in four zones, viz., soil zone, intermediate zone, capillary zone and saturation zone.
Zone of Aeration
The zone where water is available is called the zone of aeration. There are two forces which actively prevent groundwater from moving downward,
- the molecular attraction between water and the rock and earth materials and
- the molecular attraction between water particles.
The zone of aeration is further sub-divided into three layers-
- soil moisture zone
- intermediate zone
- capillary zone
Some amount of water in this zone is used by plants. At the bottom of the intermediate zone lies the capillary fringe (a thin layer of 2 to 3 cm) from where water moves upward. The capillary condition is temporarily destroyed when heavy rain takes place. In such cases the groundwater body is replenished by recharge.
Zone of Saturation
The zone of saturation lies below the zone of aeration and is also called the phreatic zone. The water available in this zone is known as groundwater. The groundwater table or water table segregates the zone of aeration and the zone of saturation. The maximum elevation of water in a well which penetrates the groundwater zone is known as piezometric water table. Generally, the water table follows the irregularities of the earth’s surface; for example, the water table is highest beneath hills and lowest beneath valleys.
