444 GROUNDWATER RESOURCES
STt
r^03 O
2. (8)
In order to account for some of the different situations
encountered in the field, modification to the basic equations
may be necessary by incorporating the well friction losses in
the formwell friction losses = kQ n (9)when n = 2 is generally used. The total loss can be expressed assskQ′′ww′(^2) (10)
where
sCsww′^ (11)
and
s
Q
T
Tt
w rSw
23
4
03
10 2
.
log
.
p
(12)
where coefficient C is a gross measurement of the degree of
well screen penetration, flow curvature, the radius of the well
itself, and the anisotropy of the permeability of the aquifer. It
can be evaluated by making field tests at the site.
True equilibrium between the rate of withdrawal and the
rate of replenishment is seldom, if ever, achieved. However,
a state of quasi-equilibrium can sometimes be attained
through several means. For example, replenishment may be
by a stream located near the point of withdrawal or, in the
case of artesian aquifers, by rain falling on an exposed por-
tion of the aquifer miles away or by rainwater infiltrating to
the groundwater in the near vicinity of the well.
QUALITY CONSIDERATION
While the quantity of available groundwater and its movement
are still of increasing interest to engineers and scientists, the
solutions for well hydraulics represent the quantitative aspect
of groundwater resources. It is now recognized that the quality
of groundwater is equally important. A major effort has been
made by environmental engineers and scientists throughout
the world on water quality. In this and the following sec-
tions, the quality aspect and remediation considerations are
introduced. From a practical standpoint, the quality of any
water is a relative term. Water that is of good quality for an
industrial-cooling purpose may be of unacceptable quality as
a drinking -water source. Generally, water-quality consider-
ations are related to chemical, biological, and radiological
content and temperature, and are based on the intended use.
As water moves through the hydrologic cycle, its quality is
altered. Rainwater, which was considered a “pure” substance,
is not pure at all. When rainfall reaches the ground surface, it
contains dissolved gases (CO 2 , O 2 , N 2 , NO 2 , NH 3 , CO, SO 2 ,
H 2 S, etc.) as well as other dissolved materials. The expression
“acid rain,” for example, is familiar to most people.
As water reaches the soil, its quality changes further. As
water percolates through the biologically active soil mantle,
organic material, both suspended and dissolved, is removed
due to aerobic bacterial action. Filtration occurs, as do ion
exchange and adsorption. Thus, by the time water has reached
the level of rock, a great deal of the organic material, effec-
tively all of the suspended material, and some of the dissolved
inorganic material have been removed. However, inorganic
material may also have been added to the water, due to ion-
exchange reactions and chemical equilibrium. For example,
ammonia dissolved in the rainwater or present in the soil will
be oxidized by soil bacteria to nitrate, and the nitrate will be
released into the water and carried downward to the water
table. As the water moves through rock strata, it picks up
inorganic matter as part of dissolution action. This dissolving
action is responsible for the relative hardness of most ground-
water. It is also responsible for the formation of caves and
caverns, especially in limestone (CaCO 3 ) strata. The natural
changes in groundwater quality may be seen in Figure 6.
In accordance with their genesis, therefore, there are two
different sources of pollutants in the groundwater system:
(1) the dissolved chemicals from the geologic formation, and
(2) the man-made wastes. The movement of groundwater, as
a carrier of the dissolved chemical mass, gives rise to many of
the reaction-transport processes that occur in the subsurface.
In many geologic processes, groundwater plays a critical role
in the formation and dissolution of certain ores and hydrocar-
bon deposits. For a detailed discussion, the reader is referred
to Phillips (1991) and Ingebritsen and Sanford (1998). On
the other hand, the dissolved chemicals from man-made
waste become a primary source of pollutants in the ground-
water system. Groundwater hydrologists are interested in the
quality of existing groundwater resources and the effect that
human influence will have on the quality of groundwater.
Human influence on the quality of groundwater results
primarily from activities that generate wastes. (There are
some exceptions, such as nitrates in water due to fertilizers,
and saltwater intrusion due to pumping of aquifers.) There
are many potential sources of environmental contamina-
tion, including agrochemicals, industrial effluents, storage-
tank leaks, seepage from disposal sites for toxic substances,
and petroleum-product spills. It has been estimated that the
amount of hazardous waste generated annually is 264 mil-
lion metric tons in the United States alone. The enormous
amount of hazardous waste that has accumulated over the
years makes this figure even more astounding. It is reported
that only 10% of the waste generated prior to 1980 was dis-
posed of by practices that would be considered adequate
according to current standards. Thus, as much as 90% of
the hazardous waste was disposed of at unregulated facili-
ties. These irresponsible disposal practices have created
over 22,000 sites containing unregulated hazardous waste in
United States. The improper disposal of hazardous waste has
caused a number of serious problems that not only result in
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