Groundwater is one of our most important and widely available resources, yet people’s perceptions of the subsurface environment from which it comes are often unclear and incorrect. This is because the groundwater environment is hidden from view except in caves and mines, and the impressions people gain from these subsurface openings are often misleading. Observations on the land surface give an impression that Earth is “solid.” This view is not changed very much when we enter a cave and see water flowing in a channel that appears to have been cut into solid rock.

Because of such observations, many people believe that groundwater occurs only in underground “rivers.” However, actual rivers underground are extremely rare. In reality, much of the subsurface environment is not solid at all. Rather, it includes countless tiny pore spaces between grains of soil and sediment plus narrow joints and fractures in bedrock. Together, these spaces add up to an immense volume. Where these subsurface pore spaces are saturated with water, that stored water is called groundwater.

Only a tiny percentage of Earth’s total water occurs underground. Nevertheless, this small percentage, stored in the rocks and sediments beneath Earth’s surface, is a vast quantity. When the oceans are excluded and only sources of freshwater are considered, the significance of groundwater becomes more apparent.

Figure 1 shows an estimate of the distribution of freshwater in the hydrosphere. Clearly, the largest volume occurs as glacial ice. Groundwater is ranked second, with slightly more than  percent of the total. However, when glacial ice is excluded and just liquid water is considered, about 96 percent is groundwater. Without question, groundwater represents the largest reservoir of freshwater that is readily available to humans. Its value in terms of economics and human well-being is incalculable. Worldwide, wells and springs provide water for cities, crops, livestock, and industry. In some areas, however, overuse of this basic resource has caused serious problems, including streamflow depletion, land subsidence, and increased pumping costs. In addition, groundwater contamination resulting from human activities is a real and growing threat in many places.

Geologic Importance of Groundwater

Geologically, groundwater is important as an erosional agent. The dissolving action of groundwater slowly removes soluble rock, such as limestone, causing surface depressions known as sinkholes to form and creating subterranean caverns. Groundwater carries dissolved minerals in solution, depositing them as structures in caves, as giant crystals in rock cavities, and creating sinter deposits of precipitated minerals around hot springs and geysers. The final section of this chapter describes the landforms associated with groundwater.

Groundwater is also an equalizer of streamflow. Much of the water that flows in rivers is not direct runoff from rain and snowmelt. Rather, a large percentage of precipitation soaks into the ground and then moves slowly to stream channels. Groundwater is thus a form of storage that sustains streams during periods when rain does not fall. When we see water flowing in a stream during a dry period, it is water from rain that fell at some earlier time and was stored underground.

Distribution of Groundwater

When rain falls, some of the water runs off, some returns to the atmosphere through evaporation and transpiration, and the remainder soaks into the ground. This last path is the primary source of practically all groundwater. The amount of water that takes each of these paths, however, varies greatly from time to time and place to place. Influential factors include the steepness of the slope, the nature of the surface material, the intensity of the rainfall, and the type and amount of vegetation. Heavy rains falling on steep slopes underlain by impervious materials will obviously result in a high percentage of the water running off. Conversely, if rain falls steadily and gently on more gradual slopes composed of materials that are more easily penetrated by water, a much larger percentage of the water soaks into the ground.

Underground Zones

Some of the water that soaks in does not travel far because it is held by molecular attraction as a surface film on soil particles. This near-surface zone is called the belt of soil moisture. It is crisscrossed by roots, voids left by decayed roots, and animal and worm burrows that enhance the infiltration of rainwater into the soil. Soil water is used by plants for life functions and transpiration. Some of this water also evaporates directly back into the atmosphere.

Water that is not held as soil moisture penetrates downward until it reaches a zone where all the open spaces in sediment and rock are completely filled with water. This is the zone of saturation. Water within it is called groundwater. The upper limit of this zone is known as the water table. The area above the water table where the soil, sediment, and rock are not saturated, is called the unsaturated zone (Figure 2).

Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because it clings too tightly to rock and soil particles. By contrast, below the water table, the water pressure is great enough to allow water to enter wells, thus permitting groundwater to be withdrawn for use. We will examine wells more closely later in the chapter.

Water Table

The water table is rarely level, as we might expect something called a table to be. Instead, its shape is usually a subdued replica of the land’s surface, reaching its highest elevations beneath hills and decreasing in height toward valleys. The water table of a wetland (swamp) is right at the surface. Lakes and streams generally occupy areas low enough that the water table is above the land surface.

Several factors contribute to the irregular surface of the water table. One important influence is the fact that groundwater moves very slowly. Because of this, water tends to “pile up” beneath high areas between stream valleys. If rainfall were to cease completely, these water “hills” would slowly subside and gradually approach the level of the adjacent valleys. However, new supplies of rainwater are usually added often enough to prevent this. Nevertheless, in times of extended drought, the water table may drop enough to dry up shallow wells. Other causes of the uneven water table are variations in rainfall and in the permeability of Earth materials from place to place.

Storage and Movement of Groundwater

The nature of subsurface materials strongly influences the rate of groundwater movement and the amount of groundwater that can be stored. Two factors are especially important: porosity and permeability.

Porosity

Water soaks into the ground because bedrock, sediment, and soil contain countless voids, or openings. These openings, which are similar to those of a sponge, are called pore spaces. The quantity of groundwater that can be stored depends on the porosity of the material, which is the percentage of the total volume of rock or sediment that consists of pore spaces (Figure 3). Voids most often are spaces between sedimentary particles; also common are joints, faults, cavities formed by the dissolving of soluble rock such as limestone and vesicles (voids left by gases escaping from lava).

Variations in porosity can be great. Sediment is commonly quite porous, and pore space may occupy 10 percent to 50 percent of the sediment’s total volume. Pore space depends on the size and shape of the grains, how they are packed together, the degree of sorting, and, in sedimentary rocks, the amount of cementing material. Most igneous and metamorphic rocks, as well as some sedimentary rocks, are composed of tightly interlocking crystals, so the voids between grains may be negligible. In this case, void space may be provided by fractures within the rock.

Permeability

Porosity alone cannot measure a material’s capacity to yield groundwater. Rock or sediment may be very porous and still prohibit water from moving through it. The permeability of a material indicates its ability to transmit a fluid. Groundwater moves by twisting and turning through interconnected small openings. The smaller the pore spaces, the slower the groundwater moves.

If the spaces between particles are too small, or if these spaces are not connected to each other, water cannot move at all. For example, clay’s ability to store water can be great due to its high porosity, but its pore spaces are so small that water is unable to move through it. Thus, we say that clay is impermeable. A layer of clay underground that hinders or prevents water movement is termed an aquitard (aqua = water, tard = slow).

Aquifers

In contrast to clay, larger particles, such as sand or gravel, have larger pore spaces. Therefore, water moves with relative ease. Permeable rock strata or sediments and fractured rocks that transmit groundwater freely are called aquifers (“water carriers”). Aquifers are not free-flowing underground rivers or lakes, however, and must be recharged continuously by rain or snow to maintain saturation.

Aquifers vary widely in size and depth. The Great Artesian Basin aquifer in Australia, for example, covers 1.7 million square kilometers (~656,000 square miles), is 3000 meters (~9800 feet) deep in places, and contains 64,900 cubic kilometers (~15,600 cubic miles) of groundwater. By contrast, the Floridan Aquifer underlying Florida and parts of Alabama, Georgia, and South Carolina covers an area of about 260,000 square kilometers (~100,000 square miles) and ranges in thickness from about 75 meters (~250 feet) in parts of Georgia to more than 900 meters (3000 feet) thick in South Florida.

Because they are the water-bearing layers sought out for human water consumption, aquifers are critical to human survival. The Floridan Aquifer is the source of drinking water for more than  million people, and nearly 50 percent of the withdrawals from the aquifer are used for agricultural irrigation. We will discuss the environmental problems associated with excessive groundwater withdrawal later in this chapter.

Groundwater Movement

The movement of most groundwater is exceedingly slow, from pore to pore. A typical rate is a few centimeters per day. The energy that makes the water move is provided by the force of gravity. In response to gravity, water moves from areas where the water table is high to zones where the water table is lower. This means that water usually gravitates toward a stream channel, lake, or spring. Although some water takes the most direct path down the slope of the water table, much of the water follows long, curving paths toward the area where it is discharged.

Figure 4 shows how water percolates into a stream from all possible directions. Some paths clearly turn upward, apparently against the force of gravity, and enter through the bottom of the channel. This is easily explained: The deeper you go into the zone of saturation, the greater the water pressure. Thus, the looping curves followed by water in the saturated zone may be thought of as a compromise between the downward pull of gravity and the tendency of water to move toward areas of reduced pressure.

The residence time of water in an aquifer varies depending on its size, depth, permeability, and rates of recharge and discharge. Shallow, fast-moving aquifers may have residence times of days to weeks, but the majority of large aquifers have residence times of hundreds to thousands of years. Understanding an aquifer’s residence time is important for sustainable use of groundwater resources.

• Groundwater represents the largest reservoir of freshwater that is readily available to humans. Geologically, groundwater is an equalizer of streamflow, and the dissolving action of groundwater produces caverns and sinkholes.

• Groundwater is water that occupies the pore spaces in sediment and rock in a zone beneath the surface called the zone of saturation. The upper limit of this zone is called the water table. The zone above the water table where the material is not saturated is called the unsaturated zone.

• The quantity of water that can be stored in the open spaces in rock or sediment is termed porosity. Permeability, the ability of a material to transmit a fluid through interconnected pore spaces, is a key factor affecting the movement of groundwater. Aquifers are permeable materials that transmit groundwater freely, whereas aquitards are impermeable materials.

• aquifer: Rock or soil through which groundwater moves easily.

• aquitard: Impermeable beds that hinder or prevent groundwater movement.

• groundwater: Water held underground in the zone of saturation.

• permeability: A measure of a material’s ability to transmit water.

• porosity: The volume of open spaces in rock or soil.

• unsaturated zone: The underground area above the water table where openings in soil, sediment, and rock are not saturated but filled mainly with air.

• water table: The upper level of the saturated zone of groundwater.

• zone of saturation: The underground zone where all open spaces in sediment and rock are completely filled with water.