Page 67 - Water Engineering Hydraulics, Distribution and Treatment

Chapter3
Water Sources: Groundwater
As a source of a permanent and reliable water supply,
Groundwater from wells and springs has served as a source
of domestic water supply since antiquity. Table 3.1 shows only that portion of the subsurface water that is in the zone
that in the United States more water systems have ground- of saturation need be considered. In this zone almost all the
water than surface water as a source—but more people drink interstices are completely filled with water under hydrostatic
from a surface-water system. Thirty-five percent (107 mil- pressure (atmospheric pressure or greater). That water is free
lion) of the total population of 306 million served by public to move in accordance with the laws of saturated flow from
water systems depend on groundwater. Groundwater works places where it enters the zone of saturation (recharge areas)
are 10 times more numerous than surface-water installations to places where it is discharged. The main features of the
(about 141,000 to 14,000); the average capacity of groundwa- groundwater phase of the hydrologic cycle are depicted in
ter facilities is, however, much smaller. Contributions from Fig. 3.1.
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. 3.1 POROSITY AND EFFECTIVE POROSITY
Groundwater is more widely distributed than surface
The amount of groundwater stored in saturated materials
water. Its nearly universal, albeit uneven, occurrence and
depends on the material’s porosity, the ratio of the aggregate
other desirable characteristics make it an attractive source of
volume of interstices in a rock or soil to its total volume. It is
water supply. Groundwater offers a naturally purer, cheaper,
usually expressed as a percentage. The concept of porosity
and more satisfactory supply than does surface water. It is
involves all types of interstices, both primary (original) and
generally available at the point of use and obviates the need
secondary. Primary interstices were created at the time of
to incur substantial transmission costs. It occurs as an under-
the rock’s origin. In granular unconsolidated sediments, they
ground reservoir, thus eliminating the necessity of impound-
coincide with intergranular spaces. In volcanic rocks, they
ment works. It is economical even when produced in small
include tubular and vesicular openings. Secondary interstices
quantities.
result from the action of geologic, mechanical, and chemical
To an increasing degree, engineers are being called on
forces on the original rock. They include joints, faults, fis-
to investigate the possibility of developing groundwater as a
sures, solution channels, and bedding planes in hard rocks.
usable resource. The following factors need to be considered:
The extent of fracturing and intensity of weathering exert a
profound influence on the distribution of larger interstices.
1. The effective water content, that is, the maximum vol-
The importance of secondary porosity in determining the
ume of water that can be withdrawn from a body of
amount of water that can be obtained from a formation is
groundwater through engineering works. The effec-
often great in those hard rocks that lack intergrain porosity.
tive porosity and storage coefficient of the water-
This type of porosity is dependent on local conditions and
bearing material control the useful storage.
gives water-bearing formations a heterogeneous character.
2. The ability of the aquifer to transmit water in requisite
The distribution of secondary porosity varies markedly with
quantities to wells or other engineering installations.
depth.
Permeability and transmissivity are the indicators of
Porosity is a static quality of rocks and soils. It is not
this capability.
itself a measure of perviousness or permeability, which are
3. The suitability of the quality of water for the intended dynamic quantities controlling the flow. Not all the water
use, after treatment if necessary. stored in a saturated material is available for movement;
4. The reliability and permanence of the available sup- only the interconnected interstices can participate in flow.
ply with respect to both the quantity and the quality Water in isolated openings is held immobile. Furthermore,
of water. water in a part of the interconnected pore space is held in
Water Engineering: Hydraulics, Distribution and Treatment, First Edition. Nazih K. Shammas and Lawrence K. Wang.
© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.
45