Movement and Storage of Groundwater

N^dSS Groundwater reservoirs provide water to streams and wetlands wherever the water table intersects the surface of the ground.

Real-World Reading Link Have you ever noticed that a stream flows even when it has not rained in a long time? Rainfall contributes to the flow in a stream, but much of the water comes from beneath the ground.

The Hydrosphere

The water on and in Earth's crust makes up the hydrosphere, named after hydros, the Greek word for water. You learned about the hydrosphere in Chapter 1 in the context of Earth's systems, including the geosphere, hydrosphere, atmosphere, and biosphere. About 97 percent of the hydrosphere is contained in the oceans. The water contained by landmasses—nearly all of it freshwater—makes up only about 3 percent of the hydrosphere.

Freshwater is one of Earth's most abundant and important renewable resources. However, of all the freshwater, between 70 and 80 percent is held in polar ice caps and glaciers. All the rivers, streams, and lakes on Earth represent only a small fraction of Earth's liquid freshwater, as shown in Table 10.1. Recall from Chapter 9 that water in the hydrosphere moves through the water cycle.

Table 10.1

WnrlH'i Interactive Table To explore ... . _ . more about Earth's water supply, WaterSupply visit qlencoe.com.

Location

Percentage of Total Water

Water Volume (km3)

Estimated Average Residence Time of Water

Oceans

97.2

1,230,000,000

thousands of years

Ice caps and glaciers

2.15

28,600,000

tens of thousands of years and longer

Groundwater

0.31

4,000,000

hundreds to many thousands of years

Lakes

0.009

123,000

tens of years

Atmosphere

0.001

12,700

nine days

Rivers and streams

0.0001

1200

two weeks

Groundwater and Precipitation

The ultimate source of all water on land is the oceans. Evaporation of seawater cycles water into the atmosphere in the form of invisible water vapor and visible clouds. Winds and weather systems move this atmospheric moisture all over Earth, with much of it concentrated over the continents. Precipitation brings atmospheric moisture back to Earth's surface. Some of this precipitation falls directly into the oceans and some falls on land.

Infiltration is the process by which precipitation that has fallen on land trickles into the ground and becomes groundwater. Only a small portion of precipitation becomes runoff and is returned directly to the oceans through streams and rivers. Groundwater slowly moves through the ground, eventually returns to the surface through springs and seepage into wetlands and streams, and then flows back to the oceans.

^p Reading Check Identify the ultimate source of all water on land.

Groundwater Storage

Puddles of water that are left after it rains quickly disappear, partly by infiltrating the ground. On sandy soils, rain soaks into the ground almost immediately. Where does that water go? The water seeps into small openings within the ground. Although Earth's crust appears solid, it is composed of soil, sediment, and rock that contain countless small openings, called pores spaces.

Pore spaces make up large portions of some of these materials. The amount of pore space in a material is its porosity. The greater the porosity, the easier water can flow through the material. Subsurface materials have porosities ranging from 2 percent to more than 50 percent. For example, the porosity of well-sorted sand is 30 percent; however, in poorly sorted sediment, smaller particles occupy some of the pore spaces and reduce the overall porosity of the sediment, as shown in Figure 10.1. Similarly, the cement that binds the grains of sedimentary rocks together reduces the rocks' porosity. Because of the enormous volume of sediment and rock beneath Earth's surface, enormous quantities of groundwater are stored in the pore spaces.

Figure 10.1 Porosity depends on the size and variety of particles in a material. Compare the porosities shown in each sample.

L 4 '

Well-sorted, large sand grains

Unsorted sand grains

Well-sorted, small sand grains

Zone Saturation
Figure 10.2 The zone of saturation is where groundwater completely fills all the pores of a material below Earth's surface. Describe what is above the zone of saturation.

The Zone of Saturation

The region below Earth's surface in which groundwater completely fills all the pores of a material is called the zone of saturation. The upper boundary of the zone of saturation is the water table, shown in Figure 10.2. Strictly speaking, only the water in the zone of saturation is called groundwater. In the zone of aeration, which is above the water table, materials are moist, but because they are not saturated with water, air occupies much of the pores.

Water movement Water in the zone of saturation and zone of aeration can be classified as either gravitational water or capillary water. Gravitational water is water that trickles downward as a result of gravity. Capillary water is water that is drawn upward through capillary action above the water table and is held in the pore spaces of rocks and sediment because of surface tension. Capillary action can be seen when the tip of a paper towel is dipped into water and the water seems to climb up through the fibers of the paper towel.

The water table The depth of the water table often varies depending on local conditions. For example, in stream valleys, groundwater is relatively close to Earth's surface, and thus the water table can be only a few meters deep. In swampy areas, the water table is at Earth's surface, whereas on hilltops or in arid regions, the water table can be tens to hundreds of meters or more beneath the surface. As shown in Figure 10.2, the topography of the water table generally follows the topography of the land above it. For example, the slope of the water table corresponds to the shape of valleys and hills on the surface above.

Because of its dependence on precipitation, the water table fluctuates with seasonal and other weather conditions. It rises during wet seasons, usually in spring, and drops during dry seasons, often in late summer.

Groundwater Movement

Groundwater flows downhill in the direction of the slope of the water table. Usually, this downhill movement is slow because the water has to flow through numerous tiny pores in the subsurface material. The tendency of a material to let water pass through it is its permeability. Materials with large, connected pores, such as sand and gravel, have high permeability and permit relatively high flow velocities up to hundreds of meters per hour. Other permeable subsurface materials include highly fractured bedrock, sandstone, and limestone.

Permeability Groundwater flows through permeable sediment and rock, called aquifers, such as the one shown in Figure 10.3. In aquifers, the pore spaces are large and connected. Fine-grained materials have low permeabilities because their pores are small. These materials are said to be impermeable. Groundwater flows so slowly through impermeable materials that the flow is often measured in millimeters per day. Some examples of impermeable materials include silt, clay, and shale. Clay is so impermeable that a clay-lined depression will hold water. For this reason, clay is often used to line artificial ponds and landfills. Impermeable layers, called aquicludes, are barriers to groundwater flow.

Flow velocity The flow velocity of groundwater depends on the slope of the water table and the permeability of the material through which the groundwater is moving. The force of gravity pulling the water downward is greater when the slope of the water table surface is steeper. Water also flows faster through a large opening than through a small opening. The flow velocity of groundwater is proportional to both the slope of the water table and the permeability of the material through which the water flows.

Figure 10.3 An aquifer is a layer of permeable subsurface material that is saturated with water. This aquifer is located between two impermeable layers called aquicludes.

Precipitation and infiltration

Aquicludes'

Aquifer

Aquifer

Springs

Groundwater moves slowly but continuously through aquifers and eventually returns to Earth's surface. In most cases, groundwater emerges wherever the water table intersects Earth's surface. Such intersections commonly occur in areas that have sloping surface topography. The exact places where groundwater emerges depend on the arrangement of aquifers and aquicludes in an area.

^ Reading Check Explain how the slope of the land can affect where groundwater emerges.

As you learned on the previous page, aquifers are permeable underground layers through which groundwater flows easily, and aquicludes are impermeable layers. Aquifers are commonly composed of layers of sand and gravel, sandstone, and limestone. In contrast, aquicludes, such as layers of clay or shale, block groundwater movement. As a result, groundwater tends to discharge at Earth's surface where an aquifer and an aquiclude are in contact, as shown in Figure 10.4. These natural discharges of groundwater are called springs.

Emergence of springs The volume of water that is discharged by a spring might be a mere trickle or it might form a stream. In some regions called karst regions, an entire river might emerge from the ground. Such a superspring is called a karst spring. Karst springs occur in limestone regions where springs discharge water from underground pathways. In regions of nearly horizontal sedimentary rocks, springs often emerge on the sides of valleys at about the same elevation, at the bases of aquifers, as shown in Figure 10.5. Springs might also emerge at the edges of perched water tables. In a perched water table, a zone of saturation that overlies an aquiclude separates it from the main water table below. Other areas where springs tend to emerge are along faults, which are huge fractures along which large masses of rock have moved, and sometimes block aquifers. In limestone regions, springs discharge water from underground pathways as karst springs.

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Responses

  • Klaudia
    What is the zone of saturation?
    8 years ago
  • teija
    What is movment of groundwater?
    6 years ago
  • George
    What is the storage of ground water?
    6 years ago
  • Kimberly
    How geology affect the movement & chemistry of groundwater?
    4 years ago

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