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Science and Society

Water Quality
hat do you think of when you see a Indeed, water fit for use in a power plant as carbon dioxide and nitrogen. Examples
Wstream? Do you wonder how deep it might not be suitable for irrigation. Water of persistent pollutants include some pesti-
is? Do you think of something to do with quality is determined by the kinds and cides, petroleum and petroleum products,
the water, such as swimming, fishing, or amounts of substances dissolved and sus- plastic materials, leached chemicals from
having a good time? As you might imagine, pended in the water and the consequences landfill sites, oil-based paints, heavy metals
not all people look at a stream and think to users. Whether a source of water can be and metal compounds such as lead, mercury,
about the same thing. A city engineer, for used for drinking water or not, for example, and cadmium, and certain radioactive ma-
example, might wonder if the stream has is regulated by stringent rules and guidelines terials. The damage they cause is either irre-
enough water to serve as a source to supple- about what cannot be in the water. These versible or reparable only over long periods.
ment the city water supply. A rancher or rules are designed to protect human health
farmer might wonder how the stream could but do not call for absolutely pure water.
be easily diverted to serve as a source of The water of even the healthiest stream
QUESTIONS TO DISCUSS
water for irrigation. An electric utility plan- is not absolutely pure. All water contains
ner, on the other hand, might wonder if the many naturally occurring substances such Discuss with your group the following
stream could serve as a source of power. as ions of bicarbonates, calcium, and magne- questions concerning water quality:
Water in a stream is a resource that sium. A pollutant is not naturally occurring;
1. Which water use requires the purest wa-
can be used many different ways, but using it is usually a waste material that contami-
ter? Which use requires the least pure?
it requires knowing about the water qual- nates air, soil, or water. There are basically
ity as well as quantity. We need to know if two types of water pollutants: degradable 2. Create a hierarchy of possible water
the quality of the water is good enough for and persistent. Examples of degradable pol- uses; for example, can water used in a
the intended use—and different uses have lutants include sewage, fertilizers, and some power plant later be recycled for agri-
different requirements. Water fit for use in industrial wastes. As the term implies, culture? Recycled for domestic use?
an electric power plant, for example, might degradable pollutants can be broken down 3. What can an individual do to improve
not be suitable for use as a city water supply. into simple, nonpolluting substances such water quality?

SURFACE WATER stream is known as the stream’s drainage basin, or watershed.
If you could follow the smallest of streams downhill, you Each stream has its own watershed, but the watershed of a large
would find that it eventually merges with other streams until river includes all the watersheds of the smaller streams that feed
they merge to form a major river. The land area drained by a into the larger river. Figure 24.5 shows the watersheds of the
Columbia River, the Colorado River, and the Mississippi River.
Note that the water from the Columbia River and the Colorado
River watersheds empties into the Pacific Ocean. The Missis-
sippi River watershed drains into the Gulf of Mexico, which is
part of the Atlantic Ocean.

Steam

Runoff
Runoff

Groundwater

FIGURE 24.3 On average, more water is evaporated from the FIGURE 24.4 Some of the precipitation soaks into the ground
ocean than is returned by precipitation. More water is precipitated to become groundwater. Groundwater slowly moves underground,
over the land than evaporates. The difference is returned to the and some of it emerges in streambeds, keeping the streams running
ocean by rivers and streams. during dry spells.

600 CHAPTER 24 Earth’s Waters 24-4