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Chapter 3

Water Sources: Groundwater

Groundwater from wells and springs has served as a source of domestic water supply
since antiquity. Table 3.1 shows that in the United States more water systems have ground-
water than surface water as a source—but more people drink from a surface-water system.
Thirty-five percent (107 million) of the total population of 306 million served by public
water systems depend on groundwater. Groundwater works are 10 times more numerous
than surface-water installations (about 141,000 to 14,000); the average capacity of ground-
water facilities is, however, much smaller. Contributions from groundwater also play a
major role in the supplies depending on surface sources. It is the discharge of groundwater
that sustains the dry-season flow of most streams.
Groundwater is more widely distributed than surface water. Its nearly universal, albeit
uneven, occurrence and other desirable characteristics make it an attractive source of water
supply. Groundwater offers a naturally purer, cheaper, and more satisfactory supply than
does surface water. It is generally available at the point of use and obviates the need to
incur substantial transmission costs. It occurs as an underground reservoir, thus eliminat-
ing the necessity of impoundment works. It is economical even when produced in small
quantities.
To an increasing degree, engineers are being called on to investigate the possibility of
developing groundwater as a usable resource. The following factors need to be considered:
1. The effective water content, that is, the maximum volume of water that can be
withdrawn from a body of groundwater through engineering works. The effective
porosity and storage coefficient of the water-bearing material control the useful
storage.
2. The ability of the aquifer to transmit water in requisite quantities to wells or other
engineering installations. Permeability and transmissivity are the indicators of this
capability.
3. The suitability of the quality of water for the intended use, after treatment if
necessary.
4. The reliability and permanence of the available supply with respect to both the
quantity and the quality of water.
As a source of a permanent and reliable water supply, only that portion of the sub-
surface water that is in the zone of saturation need be considered. In this zone almost all
the interstices are completely filled with water under hydrostatic pressure (atmospheric
pressure or greater). That water is free to move in accordance with the laws of saturated
flow from places where it enters the zone of saturation (recharge areas) to places where
it is discharged. The main features of the groundwater phase of the hydrologic cycle are
depicted in Fig. 3.1.

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