Page 410 - Environment: The Science Behind the Stories

All water
Oceans (97.5%)

Fresh
water
Groundwater Surface
Ice caps (20%) fresh
and water
glaciers Soil moisture
(79%) Lakes (38%)
Fresh water (2.5%) (52%)

Surface fresh water (1%)
Rivers (1%) Atmospheric
Water within
organisms water vapor
Figure 15.2 Only 2.5% of Earth’s water is fresh water. Of that 2.5%, (1%) (8%)
most is tied up in glaciers and ice caps. Of the 1% that is surface water, most
is in lakes and soil moisture. Data from United Nations Environment Programme (UNEP)
and World Resources Institute.
What percentage of Earth’s water is fresh water
in lakes?

Freshwater Systems in a river or lake) and groundwater is water beneath the surface
held within pores in soil or rock. Any precipitation reaching
“Water, water, everywhere, nor any drop to drink.” The well- Earth’s land surface that does not evaporate, flow into water-
known line from the poem The Rime of the Ancient Mariner ways, or get taken up by organisms infiltrates the surface.
describes the situation on our planet well. Water may seem Groundwater makes up one-fifth of Earth’s fresh water supply
abundant, but water that we can drink is quite rare and lim- and plays a key role in meeting human water needs.
ited (Figure 15.2). About 97.5% of Earth’s water resides in the Groundwater flows downward and from areas of high
oceans and is too salty to drink or to use to water crops. Only pressure to areas of low pressure. However, a typical rate of
2.5% is considered fresh water, water that is relatively pure, flow might be only about 1 m (3 ft) per day, so groundwa-
with few dissolved salts. Because most fresh water is tied up ter can remain underground for a long time. When we pump
in glaciers, icecaps, and underground aquifers, just over 1 part groundwater through wells, we are drawing up ancient water.
in 10,000 of Earth’s water is easily accessible for human use. The average age of groundwater has been estimated at 1400
Water is renewed and recycled as it moves through the years, and some is tens of thousands of years old.
water cycle (pp. 138–139). Precipitation falling from the sky Groundwater is contained within aquifers: porous,
either sinks into the ground or acts as runoff to form rivers, spongelike formations of rock, sand, or gravel that hold
which carry water to the oceans or large inland lakes. As they water (Figure 15.4). An aquifer’s upper layer, or zone of aera-
flow, rivers can interact with ponds, wetlands, and coastal tion, contains pore spaces partly filled with water. In the
aquatic ecosystems. Underground aquifers exchange water lower layer, or zone of saturation, the spaces are completely
with rivers, ponds, lakes, and the ocean through the sediments filled with water. The boundary between these two zones is
on the bottoms of these water bodies. The movement of water the water table. Picture a sponge resting partly submerged
in the water cycle creates a web of interconnected freshwater in a tray of water; the lower part of the sponge is saturated,
and marine aquatic systems (Figure 15.3) that exchange water, whereas the upper portion contains plenty of air in its pores.
organisms, sediments, pollutants, and other dissolved sub- Any area where water infiltrates Earth’s surface and reaches CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
stances. What happens in one system therefore affects other an aquifer below is known as a recharge zone.
systems—even those that are far away. Let’s examine the Like a tray of lasagna, the earth underground consists
freshwater components of the interconnected system, begin- of layers of materials with different textures and densities.
ning with groundwater. Marine and coastal components of the When a porous, water-bearing layer of rock, sand, or gravel
system will be examined subsequently (Chapter 16), but note is trapped between upper and lower layers of less permeable
that all these systems interact extensively. substrate (often clay), we have a confined aquifer, or artesian
aquifer. In such a situation, the water is under great pressure.
In contrast, an unconfined aquifer has no impermeable upper
Groundwater plays key roles layer to confine it, so its water is under less pressure and can
in the hydrologic cycle be readily recharged by surface water.
The largest known aquifer is the Ogallala Aquifer, which
Liquid fresh water occurs either as surface water or groundwa- underlies the Great Plains of the United States. Water from
ter. Surface water is water located atop Earth’s surface (such as this massive aquifer has enabled American farmers to create 409

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