Water and Water Pollution - Environmental Science - page 255

Science: Point and Nonpoint Sources
of Water Pollution
Water pollution can come from single sources or from
a variety of dispersed sources.
Point sources discharge pollutants at specific locations
through drain pipes, ditches, or sewer lines into bodies
of surface water. Examples include factories, sewage
treatment plants (which remove some but not all pol-
lutants), underground mines, and oil tankers.
Because point sources are located at specific
places, they are fairly easy to identify, monitor, and
regulate. Most developed countries control point-
source discharges of harmful chemicals into aquatic
systems. Unfortunately, there is little control of such
discharges in most developing countries.
Nonpoint sources are scattered and diffuse and
cannot be traced to any single site of discharge.
Examples include deposition from the atmosphere
and runoff of chemicals into surface water from crop-
land, livestock feedlots, logged forests, urban streets,
lawns, golf courses, and parking lots. We have made
little progress in controlling nonpoint water pollution
because of the difficulty and expense of identifying
and

11-6 POLLUTION OF FRESHWATER
STREAMS, LAKES, AND AQUIFERS

Science: Water Pollution Problems
of Streams
Flowing streams can recover from a moderate level of
degradable water pollutants if their flows are not
reduced.
Rivers and other flowing streams can recover rapidly
from moderate levels of degradable, oxygen-demand-
ing wastes and excess heat through a combination of
dilution and biodegradation of such wastes by bacte-
ria. But this natural recovery process does not work
if streams become overloaded with pollutants or
when drought, damming, or water diversion for agri-
culture and industry reduces their flows. Likewise,
these natural dilution and biodegradation processes
do not eliminate slowly degradable and nondegrad-
able pollutants.
In a flowing stream, the breakdown of degradable
wastes by bacteria depletes dissolved oxygen and cre-
ates an oxygen sag curve (Figure 11-24, p. 256). This
process reduces or eliminates populations of organ-
isms with high oxygen requirements, cleansing the
stream of such wastes. Similar oxygen sag curves can
be plotted when heated water from industrial and
power plants is discharged into streams.

controlling discharges from so many diffuse

sources.
The leading sources of water pollution are agricul-
ture, industries, and mining. Agricultural activities are
by far the leading cause of water pollution. Sediment
eroded from agricultural lands and overgrazed range-
land is the largest source. Other major agricultural pol-
lutants include fertilizers and pesticides, bacteria from
livestock and food processing wastes, and excess salt
from soils of irrigated cropland.
Industrial facilities are another major source of wa-
ter pollution. Mining is a third source. Surface mining
disturbs the earth's surface, creating a major source of
eroded sediments and runoff of toxic chemicals.

Learn more about how pollution affects the water in a
stream and the creatures living there at Environmental
ScienceNow.

Science: Stream Pollution
in Developed Countries
Most developed countries have sharply reduced
point-source pollution, but toxic chemicals and
pollution from nonpoint sources remain problems.
Wa ter pollution control laws enacted in the 1970s have
greatly increased the number and quality of waste-
water treatment plants in the United States and most
other developed countries. Such laws also require in-
dustries to reduce or eliminate their point-source dis-
charges into surface waters.
These efforts have enabled the United States to
hold the line against increased pollution by disease-
causing agents and oxygen-demanding wastes in most
of its streams. This is an impressive accomplishment
given the country's increased economic activity, re-
source consumption, and population since passage of
these laws.
One success story is the cleanup of Ohio's Cuya-
hoga River. It was so polluted that, in both 1959 and
1969, it caught fire and burned for several days as

Global Outlook: Is the Water Safe to Drink?
One of every five people in the world does not have
access to safe drinking water.
About 95% of the people in developed countries have
access to safe drinking water. However, according to
the WHO, 1.4 billion people—about one in five—in
developing countries do not have access to clean
drinking water. As a result, each day about 9,300
people—most of them children younger than age 5—
die prematurely from infectious diseases spread by
contaminated water or lack of water for adequate
hygiene.
The United Nations estimates that it would cost
$23 billion per year over 8-10 years to bring low-cost,
safe water and sanitation to the 1.4 billion people who
do not have it.