Encyclopedia of Environmental Science and Engineering, Volume I and II

GROUNDWATER RESOURCES 441

system, among others. Second, a number of solute-transport

models have become available, such as the USGS MOC

(U.S. Geological Survey method of characteristics) series

and the USGS MODFLOW-2000 (modular finite-difference

flow model), to name a few. As a result, analysis, planning,

and development of large-scale problems can be achieved.

Thus far, studies of groundwater resources have made tre-

mendous strides. Major developments include economical

use, environmental considerations, and man’s effect on the

groundwater resource.

WELL HYDRAULICS

Well hydraulics is the most important single element in

groundwater-resources engineering. Wells are drilled either

for abstraction or observation purposes. Abstraction wells,

or pumping wells, are drilled and developed to achieve the

optimum output quantity of groundwater, while observa-

tion wells are for evaluation of the aquifer characteristics.

Equations of well hydraulics under steady and unsteady

conditions with specific geologic settings (i.e., confined,

unconfined, leaky, and fractured) and their specific bound-

ary conditions can be formulated by fluid-dynamics equa-

tions when used in conjunction with Darcy’s Law. For

practical purposes, most solutions have been reduced to

graphical form (Lohman, 1972), tables (Hantush, 1964), or

computer programs (Appel and Reilly, 1994). Here, the case

of unsteady flow to a well in a confined aquifer (unsteady

artesian well) is presented because of its fundamental char-

acteristic and importance in practical applications. A variety

of solutions to well hydraulics for other aquifer-pumping

and well conditions are available in standard texts (e.g.,

Todd and Mays, 2005; Bear, 1979).^

In the derivation of the nonequilibrium drawdown equa-

tion for an aquifer, it is assumed that the release of water occurs

instantaneously and that any compaction occurs only in the

aquifer. If the aquifer is a limestone formation, such compac-

tion is not too likely to occur. On the other hand, if the aquifer

is a clay formation, it will compact quite readily when it loses

its moisture content. Thus, before controlled withdrawals are

put into effect, the ground levels can subside substantially. For

example, the subsidence of Mexico City was caused by the

compaction of the clays overlying the limestone formation

from which water was drawn. In any case, if compaction does

occur, the storage coefficient will vary. Since the variation of

the storage coefficient is rather slow, it can be considered a

basic property of the soil for most purposes. Assumptions are

summarized as follows (see Figure 3):

1. The aquifer is horizontal and infinite in extent in
   all directions.


2. Pumping starts instantaneously, at a rate Q.


3. Darcy’s Law is valid.

The discharge of groundwater flow can be evaluated by

Darcy’s equation:

Q = KiA (1)

where Q is the discharge flow rate, ft^3 /d; K is the hydraulic

conductivity, ft/d; i is the hydraulic gradient; and A is the

Aquiclude Imper-

vious Rock or Clay

Impervious

Aquiclude Clay

Confined Aquifer

Unconfined

Aquifer

Piezometric Level

Spring

Groundwater Table

Artesian or Pressure Well

Free Surface or Gravity Well

FIGURE 2 Types of groundwater withdrawal.

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