Contents preface (VII) introduction 1—37

4.3. OCCURRENCE OF GROUND WATER
The subsurface medium within which ground water occurs is either porous or fractured or both. The subsurface occurrence of ground water can be divided into two zones (Fig. 4.1): (i) the vadose zone or unsaturated zone or zone of aeration, and (ii) the phreatic zone or saturated zone or zone of saturation. In the saturated zone, all pores or voids are filled with water whereas in the unsaturated zone, pores contain gases (mainly air and water vapours) in addition to water.

Soil water	Unsaturated zone
	or
	Zone of aeration
Intermediate zone	or
	Vadose zone
Capillary water
	Water table
	Zone of saturation
Phreatic water	or
(Ground water)	Phreatic zone
	or
	Saturated zone

Fig. 4.1 Vertical distribution of subsurface water

124 IRRIGATION AND WATER RESOURCES ENGINEERING
The water table is defined as the upper limit of the saturated zone. However, it should be noted that all the pores near the base of the capillary water zone (which itself may range from practically nothing in coarse material to about 2.5 m or more in clay materials) may be completely saturated. The number of pores filled with water decreases in the upward direction of the capillary water zone. One can, therefore, expect the upper limit of actual saturation to be an irregular surface. Water table should, therefore, be redefined as the upper limit of saturation at atmospheric pressure.
The saturated zone containing interconnected pores may exceed depths penetrated by oil wells (more than 12,000 m). However, freshwater (part of the hydrologic cycle) is found only up to depths of about 800 m (2).
A saturated geologic formation capable of yielding water economically in sufficient quantity is known as an aquifer (or water-bearing formation or ground water reservoir). Ground water constantly moves through an aquifer under local hydraulic gradients. Thus, aquifers perform storage as well as conduit functions. Ground water may exist in aquifers in two different manners: (i) unconfined, and (ii) confined. The unconfined condition occurs when the water table is under atmospheric pressure and is free to rise or fall with changes in the volume of the stored water. An aquifer with unconfined conditions is referred to as an unconfined or water table aquifer. An aquifer which is separated from the unsaturated zone by an impermeable or very less permeable formation is known as confined aquifer (or artesian aquifer or pressure aquifer). Ground water in a confined aquifer is under pressure which is greater than the atmospheric pressure. The water level in a well penetrating a confined aquifer indicates the piezometric pressure at that point and will be above the bottom of the upper confining formation. Such wells are known as artesian wells and if the water level rises above the land surface, a flowing well results (Fig. 4.2).

Recharge area

Flowing

well

Artesian well

Piezometric surface
Water table

Water table

^well Unconfined

aquifer
Confining stratum

Confined aquifer
Impermeable

stratum