INTRODUCTION - Water Quality Engineering in Natural Systems - page 2

Figure 1.1. river with floating trash.

Figure 1.2. Point source of pollution. Source : South Florida
Water Management District.

most river pollutants originate from terrestrial sources,
surface waters are best managed at the watershed scale
rather than at the scale of individual water bodies. This
is the watershed approach to water-quality management.
The main limitations to implementing the watershed
approach are rooted in our inability to quantify most of
the watershed-scale contaminant-transport processes
that are fundamental to implementing watershed con-
trols. Contaminant inputs into surface waters from the
atmosphere are also considered in water-quality man-
agement plans, and in these cases, the contributing
region is called the airshed . In contrast to surface waters,
the quality of groundwater is influenced primarily by
activities on and below the ground surface, and the
potential sources of groundwater contamination are
influenced by overlying land uses and subsurface
geology. The concept of a watershed is not applicable to
groundwater; however, the management of land overly-
ing groundwater that serves as a source of drinking
water for humans and animals is an essential endeavor.
In many cases, identification of polluted water bodies
are obvious to the casual observer, such as the stream
with floating trash shown in Figure 1.1. However, some
polluted water bodies are not so obvious, such as an
apparently pristine lake that is so contaminated with
acid rain that the existence of aquatic life is extremely
limited.

uted over large areas or are a composite of many point
sources, including runoff from agricultural operations,
fallout from the atmosphere, and urban runoff. Surface
runoff that collects in storm sewers and is discharged
through a pipe into a receiving water is still considered
nonpoint source pollution since it originates as diffuse
runoff from the land surface. Pollution loads from non-
point sources are commonly called diffuse loads . Much
of the pollution in waterways is caused by nonpoint
source pollution as opposed to point source pollution.
Although most pollutant sources can be classified as
point or nonpoint sources, other less common classifica-
tions of pollution sources have also been identified, such
as mobile pollution which is primarily associated with
the marine environment, and in particular is associated
with such ship- and boat-related sources, such as bilge
water, ballast water, and marine accidents (Gürel and
Pehlivanoglu-Mantas, 2010).
Wet weather discharges refer to discharges that result
from precipitation events, such as rainfall and snowmelt.
Wet weather discharges include stormwater runoff,
combined sewer overflows (CSos), and wet-weather
sanitary sewer overflows (SSos). Stormwater runoff
collects pollutants such as oil and grease, nutrients,
metals, bacteria, and other toxic substances as it travels
across land. CSos and wet weather SSos contain a
mixture of raw sewage, industrial wastewater, and
stormwater, and can result in beach closings, shellfish
bed closings, and aesthetic problems.

1.2 SOURCES OF WATER POLLUTION

1.2.1 Point Sources

Sources of water pollution can be broadly grouped into
point sources and nonpoint sources. Point sources are
localized discharges of contaminants that include indus-
trial and municipal wastewater outfalls, septic tank dis-
charges, and hazardous waste spills. Nonpoint sources of
pollution include contaminant sources that are distrib-

The identifying characteristic of point sources is that
they discharge pollutants into receiving waters at iden-
tifiable single- or multiple-point locations. A typical
point source of contamination is shown in Figure 1.2,
where wastewater is being pumped directly into a drain-