THE WATER TABLE

THE WATER TABLE

soil can hold; in contrast, permeability is the ability of rock or soil to transmit water (or any other fluid). Water

When rain falls, it usually soaks into the ground. Water can flow rapidly through material with high permeabil-

does not descend into the crust indefinitely, however. ity. Most materials with high porosity also have high per-

Below a depth of a few kilometers, the pressure from meability, but permeability also depends on how well the

overlying rock closes the pores, making bedrock both pores are connected and on pore size.

nonporous and impermeable. Water accumulates on this

Characteristics of Ground Water 261

Soil moisture

Well

Water table

belt

Rock or regolith is moist but not saturated

Porous or fractured rock or

(zone of aeration)

regolith saturated with water (zone of saturation)

Figure 15–2 The water table is the top of the zone of saturation near the Earth’s sur- face. It intersects the land surface at lakes and streams and is the level of standing water in

a well.

impermeable barrier, filling pores in the rock and soil into a well to replenish water that is pumped out. Sand above it. This completely wet layer of soil and bedrock

and gravel, sandstone, limestone, and highly fractured above the barrier is called the zone of saturation. The

bedrock of any kind make excellent aquifers. Shale, clay, water table is the top of the zone of saturation (Fig.

and unfractured igneous and metamorphic rocks are poor 15–2). Above the water table lies the unsaturated zone,

aquifers.

or zone of aeration. In this layer, the rock or soil may

be moist but not saturated. Gravity pulls ground water downward. However, electrical forces can pull water upward through small channels, just as water rises in a paper towel dipped in water (Fig. 15–3). This upward movement of water is called capillary action. Thus, a capillary fringe 30 to

60 centimeters thick rises from the water table. Topsoil usually contains abundant litter and humus, which retain moisture. Thus, in most humid environ- ments, topsoil is wetter than the unsaturated zone be- neath it. This moist surface layer is called the soil mois- ture belt , and it supplies much of the water needed by plants.

If you dig into the unsaturated zone, the hole does not fill with water. However, if you dig below the water table into the zone of saturation, you have dug a well, and the water level in a well is at the level of the water table. During a wet season, rain seeps into the ground to recharge the ground water, and the water table rises. During a dry season, the water table falls. Thus, the wa- ter level in most wells fluctuates with the seasons.

An aquifer is any body of rock or soil that can yield Figure 15–3 Capillary action pulls water upward from the economically significant quantities of water. An aquifer

water table just as it does in this paper towel dipped into must be both porous and permeable so that water flows

dyed water.

262 CHAPTER 15 G RO U N D WAT E R

䊳 15.2 MOVEMENT OF GROUND WATER

Water table

Land surface

Nearly all ground water seeps slowly through bedrock

Gaining stream

and soil. Ground water flows at about 4 centimeters per day (about 15 meters per year), although flow rates may

be much faster or slower depending on permeability. Most aquifers are like sponges through which water seeps, rather than underground pools or streams. However, ground water can flow very rapidly through large fractures in bedrock, and in a few regions under- ground rivers flow through caverns.

Ground water flows from zones where the water

(a)

Saturated zone

table is highest toward areas where it is lowest. In gen- eral, the water table is higher beneath a hill than it is be- neath an adjacent valley (Fig. 15–4). The arrows in Figure 15–5a show that some ground water flows from a hill to

a valley along the sloping surface of the water table.

Losing stream

Much of it, however, flows in curved paths, descending below the valley floor and then rising beneath the lowest part of the valley.

Why does ground water flow upward against the force of gravity? The key to this phenomenon lies in wa- ter pressure. Ground water flows from areas of high wa- ter pressure toward areas of low pressure. The water pressure at any point is proportional to the weight of

Water table

water above that point. Ground water beneath a hill is under greater pressure than water beneath the valley

(b)

because the water table is higher beneath the hill. Thus, Figure 15–5 (a) In a moist climate the water table lies the water pressure beneath the hill forces the water up-

above the stream and ground water seeps into the stream. ward beneath the valley. Because ground water flows

(b) A desert stream lies above the water table. Water seeps from high places to low ones, the water table becomes

from the stream bed to recharge the ground-water reservoir flatter during a dry season.

beneath the desert.

Soil

Streams flow through most valleys. Because ground

moisture

Water table

water rises beneath the valley floor, it continually feeds

belt

Stream

during the

channel

the stream. That is why streams continue to flow even

wet season

when rain has not fallen for weeks or months. A stream that is recharged by ground water is called an effluent (or gaining) stream (Fig. 15–5a). Ground water also seeps into most lakes because lakes occupy low parts of the land.

In a desert, however, the water table commonly lies

Zone of

below a stream bed, and water seeps downward from the

saturation

stream to the water table (Fig. 15–5b). Such a stream is

Water table during

Water table

an influent, or losing, stream. Desert stream channels

prolonged drought

during the

are dry most of the time. When they do run, the water

often flows from nearby mountains where precipitation Figure 15–4 The water table follows topography, rising be-

dry season

is greater, although a desert storm can also fill the chan- neath a hill and sinking beneath a valley. Dashed lines show

nel briefly. Thus, a desert stream feeds the ground-water that it also sinks during drought and rises during the rainy sea-

reservoir, but in temperate climates, ground water feeds son. The arrows show ground-water flow.

the stream.

Use of Ground Water 263

A spring occurs where the water table intersects the sur- face and water flows or seeps onto the land (Fig. 15–6).

Water table

Spring

In some places, a layer of impermeable rock or clay lies above the main water table, creating a locally saturated zone, the top of which is called a perched water table (Fig. 15–6b). Hillside springs often flow from a perched water table.