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areas, treated city wastewater is already being recycled for use
in power plants and for watering parks. A practically limitless
supply of freshwater could be available by desalting ocean wa-
ter, something which occurs naturally in the hydrologic cycle.
The treatment of seawater to obtain a new supply of fresh -
water is presently too expensive because of the cost of energy
to accomplish the task. New technologies, perhaps ones that
use solar energy, may make this more practical in the future.
In the meantime, the best sources of extending the supply of
fresh water appear to be the control of pollution, the recycling of

wastewater, and conservation of the existing supply.

CONCEPTS Applied

Who Uses How Much?

Find out how much water is used in the industrial processes
in your location. Compare this to the amount of water used
in the home for drinking, cooking, cleaning, and so on.

24.2 SEAWATER
More than 70 percent of the surface of Earth is covered by sea-
water, with an average depth of 3,800 m (about 12,500 ft). The
FIGURE 24.9 The filtering beds of a city water treatment land areas cover 30 percent with an average elevation of only
facility. Surface water contains more sediments, bacteria, and other about 830 m (about 2,700 ft). With this comparison, you can see
suspended materials because it is on the surface and is exposed to that humans live on and fulfill most of their needs by drawing
the atmosphere. This means that surface water must be filtered and from a small part of the total Earth. As populations continue
treated when used as a domestic resource. Such processing is not
to grow and as resources of the land continue to diminish, the
required when groundwater is used as the resource.
ocean will be looked at more as a resource than a convenient
place for dumping wastes. The ocean already provides some
food and is a source of some minerals, but it can possibly pro-
vide freshwater, new sources of food, new sources of important
minerals, and new energy sources in the future. There are vast
deposits of phosphorite and manganese nodules on the ocean
bottom, for example, that can provide valuable minerals. Phos-
phate is an important fertilizer needed in agriculture, and the
land supplies are becoming depleted. Manganese nodules, which
occur in great abundance on the ocean bottom, can be a source
of manganese, iron, copper, cobalt, and nickel. Seawater contains
enough deuterium to make it a feasible source of energy. One
gallon of seawater contains about a spoonful of deuterium, with
the energy equivalent of 300 gal of gasoline. It has been estimated
there is sufficient deuterium in the oceans to supply power at
100 times the present consumption for the next 10 billion years.
The development of controlled nuclear fusion is needed, how-
ever, to utilize this potential energy source. The sea may provide
new sources of food through aquaculture, the farming of the sea
as the land is presently farmed. Some aquaculture projects have
already started with the farming of oysters, lobsters, shrimp,
clams, and certain fishes, but these projects have barely begun to
utilize the full resources that are possible (Figure 24.11).
FIGURE 24.10 This is groundwater pumped from the ground
for irrigation. In some areas, groundwater is being removed from the Part of the problem of utilizing the ocean is that the ocean
has remained mostly unexplored and a mystery until recent
ground faster than it is being replaced by precipitation, resulting in
a water table that is falling. It is thus possible that the groundwater times. Only now are scientists beginning to understand the
resource will soon become depleted in some areas. complex patterns of the circulation of ocean waters, the nature

606 CHAPTER 24 Earth’s Waters 24-10