I f you drill a hole into the ground in most places,

its bottom fills with water after a few days. The water appears even if no rain falls and no streams flow nearby. The water that seeps into the hole is part of the vast reservoir of subterranean ground water that saturates the Earth’s crust in a zone between a few meters and a few kilometers below the surface.

Ground water is exploited by digging wells and pump- ing the water to the surface. It provides drinking water for more than half of the population of North America and is

a major source of water for irrigation and industry. However, deep wells and high-speed pumps now extract ground water more rapidly than natural processes replace it in many parts of the central and western United States. In addition, industrial, agricultural, and domestic contami- nants seep into ground water in many parts of the world. Such pollution is often difficult to detect and expensive to clean up.

Over geologic time, seeping ground water dissolves bedrock to create caverns such as this one. (William Palmer/Visuals Unlimited)

260 CHAPTER 15 G RO U N D WAT E R

Well-sorted sediment Poorly sorted sediment

(a)

(b)

Sedimentary rock with cementing material between grains Sand

Pore Space

Pore space

Sand

Figure 15–1 Different materials have different amounts of

grain

open pore space between grains. (a) Well-sorted sediment Sand grain consists of equal-size grains and has a high porosity, about 30

percent in this case. (b) In poorly sorted sediment, small grains fill the spaces among the large ones, and porosity is lower. In

0 0.1 0.2 0.3 mm this drawing it is about 15 percent. (c) Cement partly fills pore

Scale

space in sedimentary rock, lowering the porosity.

(c)

䊳 15.1 CHARACTERISTICS OF

The connections between pores affect permeability

GROUND WATER

because the pores, no matter how large, must be connected for water to flow through rock or soil. Uncemented

POROSITY AND PERMEABILITY sand and gravel are both porous and permeable because their pores are large and well connected. Thus, water

In the upper few kilometers of the Earth, bedrock and flows easily through them. Sedimentary rocks such as soil contain small cracks and voids that are filled with air

sandstone and conglomerate can have high permeabili- or ground water. The proportional volume of these open

ties if cement has not filled the pores and channels. The spaces is called the porosity of rock or soil. The poros-

permeability of many other rocks depends on the density ity of sand and gravel is typically high—40 percent or

of fractures in the rock.

higher. Mud can have a porosity of 90 percent or more If the pores are very small, electrical attractions be- because the tiny clay particles are electrically attracted to

tween water and soil particles slow the passage of water. water. Most rocks have lower porosities than loose sedi-

Clay typically has a high porosity, but because its pores ment. Sandstone and conglomerate can have 5 to 30 per-

are so small it commonly has a very low permeability cent porosity (Fig. 15–1). Shale typically has a porosity

and transmits water slowly.

less than 10 percent. Igneous and metamorphic rocks have very low porosities unless they are fractured.

Porosity indicates the amount of water that rock or

I f you drill a hole into the ground in most places,