HUMAN BIOLOGY

480 Chapter 25

other city in the world. Since then, dwindling supplies have

forced the city to launch a program of water conservation.

In coastal areas, overuse of groundwater can cause salt­

water intrusion into human water supplies. Much of the

United States is experiencing water problems (Figure 25.13).

In many regions, agricultural runoff pollutes public

water sources with sediments, pesticides, and fertilizers.

Power plants and factories pollute water with excess heat as

well as chemicals (including carcinogens) and radioactive

materials. Such pollutants may accumulate in lakes, rivers,

and bays before reaching their ultimate destination, the

oceans (Figure 25.14). Contaminants from human activities

have begun to turn up even in supposedly “pure” water in

underground aquifers.

Many people view the oceans as convenient refuse

dumps. Cities throughout the world dump untreated sew­

age, garbage, and other noxious debris into coastal waters.

Cities along rivers and harbors maintain shipping channels

by dredging the polluted muck and barging it out to sea.

We don’t yet know the full impact such practices may have

on fisheries that provide human food.

Managing solid wastes is another challenge

In 2008, the United States generated 505 million tons of

garbage. In natural ecosystems, solid wastes are recycled,

but we humans bury them in landfills or incinerate them.

problems with Water and Wastes

Figure 25.12 Large-scale irrigation is common in many
U.S. agricultural areas. A center-pivot sprinkler system
is about 70 to 80 percent efficient. (Craig Aurness/Comet/Corbis)

n Three of every four humans do not have enough clean

water to meet basic needs. Most of Earth’s water is salty

(in oceans). Of every million liters of water on our planet,

only 6 liters are readily usable for human activities.

Water issues affect 75 percent of humans

As the human population grows exponentially, so do
the demands and impacts on Earth’s limited supply of
fresh water.
About a third of the world’s food grows on land that
is irrigated with water piped in from groundwater, lakes,
or rivers. Irrigation water often contains large amounts
of mineral salts. Where soil drainage is poor, evaporation
may cause salt buildup, or salinization. Globally, saliniza­
tion is estimated to have reduced yields on 25 percent of
all irrigated cropland. Large­scale irrigation (Figure 25.12)
is depleting groundwater stored in the Ogallala aquifer,
which extends from South Dakota to Texas and has been
providing about 30 percent of the groundwater used for
irrigation in the United States. In some areas the farmers
are working to reduce their water use by switching to more
efficient irrigation systems.
Communities located in deserts have been notorious
water wasters. In a 1999 estimate, Las Vegas, Nevada—
desert home of golf courses, swimming pools, and lush
lawns—was said to use more water per resident than any

25.6

Figure 25.13 Many areas of the United States have groundwater troubles.

High

Moderate

Insignificant Saltwater intrusion from nearby seas

Significant groundwater contamination

Groundwater

overdrafts:

Alaska

Hawaiian

Islands

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