Encyclopedia of Environmental Science and Engineering, Volume I and II

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GROUNDWATER RESOURCES

Water plays a vital role in sustaining the existence of all

living things. The importance of managing the different

aspects of the water, including its quantity, quality, trans-

port processes, utilization, and management becomes appar-

ent when the supply of water, especially potable water, is

limited. This article is intended to be an introduction of the

subject of groundwater. It includes a historical perspective

of development, well hydraulics, the quality aspect, and a

number of remediation technologies.

It has been estimated that approximately 300 million

cubic miles of water exist on the earth at different locations

and in different states phases. From Table 1, it can be seen

that the ocean forms about 97% of the earth’s water as saline

water. The remaining 3% is freshwater. This freshwater

supply would be quickly depleted if it were not for its end-

less interaction and exchange with the ocean. This move-

ment of the earth’s waters, known as the hydrologic cycle,

is shown schematically in Figure 1. Due to the complex-

ity of the hydrologic cycle, not all of the components can

be determined easily. A detailed discussion on the subject

is presented by the United Nations Educational, Scientific,

and Cultural Organization (UNESCO) (USSR National

Committee for the International Hydrological Decade,

1978).^ On a long-term average basis, the total freshwater

supply is to be considered constant. Therefore, the budget

equation for freshwater can be expressed as:

[Total precipitation] 
 [Evaporation]  [Transpiration]

Here, the freshwater is found in the atmosphere as water

vapor; on the land as water, dew, snow, and ice; and in the

ground usually in the form of water. Approximately 99.5%

of all the Earth’s water is in a form or location unavailable

for use (see Table 1), leaving only about 0.5% of the earth’s

water remaining for consumption. The largest source of avail-

able water is groundwater, which forms an important part of

the water supply for municipalities, agriculture, and industry.

For instance, in the United States, approximately 50% of the

total water usage is from groundwater, while over 90% of

the world’s potable water supply is derived from groundwa-

ter. Thus, groundwater is one of the most important natural

resources. Groundwater occurs in the saturated zone of water-

bearing soils. However, it may also be found in sedimentary

deposits, fissures in igneous rocks, limestone caverns, and

other water-bearing sources, which may lie near the sur-

face or hundreds of feet beneath the surface. The subject of

groundwater is addressed by a variety of disciplines, including

hydrology, geology, and hydraulics.

In general, hydrologists and engineers are concerned with

the long-range yield from watersheds and aquifers. Yield is a

function of watershed storage capacity and the supply, which

fluctuates with time. Typical means of groundwater removal

are shown in Figure 2. It is of paramount importance to ensure

enough supply to last through drought periods. Currently,

accurate prediction of drought phenomena is difficult and

unavailable, as indicated by the National Research Council.

The subject is still an active research in water-resource engi-

neering. Because of the cost of well drilling, it is desirable to

have some assurance that the water-supply well will reach a

satisfactory aquifer. In order for the hydrologist or engineer

to obtain an appreciation of groundwater flow, he/she must

understand that the notion of “safe yield” in groundwater is

the quantity of groundwater that can be withdrawn without

impairing its quality.

HISTORICAL NOTES

A long time passed in the history of man before the concept

of the hydrologic cycle was established. Groundwater has

been used since ancient times. Plato wrote about rainfall,

runoff, and the infiltration of source water as the source for

springs. Aristotle recognized the hydrologic cycle, includ-

ing evaporation, condensation, streamflow, infiltration, and

the reappearance of groundwater as springs. Among the

early investigations of the hydrologic cycles and ground-

water flow, the contributions of Leonardo da Vinci, Bernard

Palissy, Claude Perrault, Edme Mariotte, Edmond Halley,

and La Metherie are of particular note.

In 1856, studying the movement of water through sand,

Henry Darcy developed the celebrated Darcy’s Law, that

is, the velocity of percolation as a function of permeability

and hydraulic gradient. Darcy’s Law provides the basis for

a quantitative method used in groundwater evaluation. Most

of the developments centered in this period on the problems

of well hydraulics and well-field design. A detailed account of

Darcy’s contributions and life history is presented in a recent

publication (Brown, Garbrecht, and Hager, 2003). During the

late nineteenth and twentieth centuries, Boussinesq, Dupuit,

Theim, Forchheimer, and many others laid an analytical

foundation for the subject. Their contributions encompassed

a large number of analytical solutions for which equations

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