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A

FIGURE 24.8 An artesian aquifer has groundwater that is
under pressure because the groundwater is confined between two
impermeable layers and has a recharge area at a higher elevation.
The pressure will cause the water to rise in a well drilled into the
B aquifer, becoming a flowing well if the pressure is sufficiently high.
FIGURE 24.6 (A) Sand and gravel have large, irregular particles
with large pore spaces, so they have a high porosity. Water can at a rate that ranges from kilometers (miles) per day to meters
move from one pore space to the next, so they also have a high
(feet) per year. Surface streams, on the other hand, move much
permeability. (B) Clay has small, flat particles, so it is practically
impermeable because water cannot move from one pore to the next faster at rates up to about 30 km per hour (about 20 mi/h).
(not drawn to scale). An aquifer is a layer of sand, gravel, sandstone, or other
highly permeable material beneath the surface that is capable
of producing water in usable quantities. In some places, an
and joints. Above this impermeable layer, it collects in all avail- aquifer carries water from a higher elevation, resulting in a
able spaces to form a zone of saturation. Water from the zone pressure on water trapped by impermeable layers at lower
of saturation is considered to be groundwater. Water from the elevations. Groundwater that is under such a confining pressure
zone above is not considered to be groundwater. The surface is in an artesian aquifer. “Artesian” refers to the pressure, and
of the boundary between the zone of saturation and the zone groundwater from an artesian well rises above the top of the
above is called the water table. The surface of a water table is aquifer but not necessarily to the surface. Some artesian wells
not necessarily horizontal, but it tends to follow the topography are under sufficient pressure to produce a fountainlike flow or
of the surface in a humid climate. A hole that is dug or drilled spring (Figure 24.8). Some people call groundwater from any
through Earth to the water table is called a well. The part of the deep well “artesian water,” which is technically incorrect.
well that is below the water table will fill with groundwater.
Precipitation falls on the land and percolates down to the
zone of saturation, then begins to move laterally, or sideways, CONCEPTS Applied
to lower and lower elevations until it finds its way back to the
surface. This surface outflowing could take place at a stream, Deep Wells?
pond, lake, swamp, or spring (Figure 24.7). Groundwater flows After doing some research, make a drawing to show the
gradually and very slowly through the tiny pore spaces, moving depth of the water table in your area and its relationship to
local streams, lakes, swamps, and water wells.

FRESHWATER AS A RESOURCE
Water is an essential resource, not only because it is required
for life processes but also because of its role in a modern in-
dustrialized society. Water is used in the home for drinking,
cooking, and cleaning; as a carrier to remove wastes; and for
maintaining lawns and gardens. These domestic uses lead to
an equivalent consumption of about 570 L per person each day
(about 150 gal/person/day), but this is only about 10 percent of
the total consumed. Average daily use of water in the United States
amounts to some 5,700 L per person each day (about 1,500 gal/
FIGURE 24.7 Groundwater from below the water table seeps
into lakes, streams, and swamps and returns to the surface natu- person/day), or about enough water to fill a small swimming
rally at a spring. Groundwater eventually returns to the ocean, but pool once a week. The bulk of the water is used by agriculture
the trip may take hundreds of years. (about 40 percent), for the production of electricity (about

24-7 CHAPTER 24 Earth’s Waters 603