Underground Water*
Beneath the land surface is another important component of the hydrosphere – underground water. As Table 9-1 shows, the total amount of underground water is many times that contained in lakes and streams. Moreover, underground water is much more widely distributed than surface water. Whereas lakes and rivers are found only in restricted locations, underground water is almost ubiquitous, occurring beneath the land surface throughout the world. Its quantity is sometimes limited, its quality is sometimes poor, and its occurrence is sometimes at great depth, but almost anywhere on Earth one can dig deep enough and find water.
More than half the world’s underground water is found within 800 meters (about half a mile) of the surface. Below that depth, the amount of water generally decreases gradually and erratically. Although water has been found at depths below 10 kilometers (6 miles), it is almost immobilized because the pressure exerted by overlying rocks is so great and openings are so few and so small.
Almost all underground water comes originally from above. Its source is precipitation that either percolates directly into the soil or eventually seeps downward from lakes and streams.
Once the moisture gets underground, any one of several things can happen to it depending largely on the nature of the soil and rocks it infiltrates. The quantity of water that can be held in subsurface material (rock of soil) depends on the porosity of the material, which is the percentage of the total volume of the material that consists of voids (pore spaces or cracks) that can fill with water. The more porous a material is, the greater the amount of open space it contains and the more water it can hold.
Porosity is not the only factor affecting underground water flow. Is water is to move through rock or soil, the pores must be connected to one another and be large enough for the water to move through them. The ability to transmit underground water (as opposed to just hold it) is termed permeability, and this property of subsurface matter is determined by the size of pores and by their degree of interconnectedness. The water moves by twisting and turning through these small, interconnected openings, the smaller and less connected the pore spaces, the less permeable the material and the slower the water moves.
The rate at which water moves through rock depends on both porosity and permeability. For example, clay is usually of high porosity and permeability. For example, clay is usually of high porosity because it has a great many interstices (openings) among the minute flakes that make up the clay, but it generally has low permeability because the interstices are so tiny that the force of the molecular attraction binds the water to the clay flakes and holds it in place. Thus, clay is typically very porous but relatively impermeable and consequently can trap large amounts of water and keep it from draining.
Almost all underground water comes originally from above. Its source is precipitation that either percolates directly into the soil or eventually seeps downward from lakes and streams.
Once the moisture gets underground, any one of several things can happen to it depending largely on the nature of the soil and rocks it infiltrates. The quantity of water that can be held in subsurface material (rock of soil) depends on the porosity of the material, which is the percentage of the total volume of the material that consists of voids (pore spaces or cracks) that can fill with water. The more porous a material is, the greater the amount of open space it contains and the more water it can hold.
Porosity is not the only factor affecting underground water flow. Is water is to move through rock or soil, the pores must be connected to one another and be large enough for the water to move through them. The ability to transmit underground water (as opposed to just hold it) is termed permeability, and this property of subsurface matter is determined by the size of pores and by their degree of interconnectedness. The water moves by twisting and turning through these small, interconnected openings, the smaller and less connected the pore spaces, the less permeable the material and the slower the water moves.
The rate at which water moves through rock depends on both porosity and permeability. For example, clay is usually of high porosity and permeability. For example, clay is usually of high porosity because it has a great many interstices (openings) among the minute flakes that make up the clay, but it generally has low permeability because the interstices are so tiny that the force of the molecular attraction binds the water to the clay flakes and holds it in place. Thus, clay is typically very porous but relatively impermeable and consequently can trap large amounts of water and keep it from draining.
Underground water is stored in, and moves slowly through, moderately to highly permeable rocks called aquifers (from the Latin, aqua, “water,” and ferre, “to bear”). The rate of movement of the water varies with the situation. In some aquifers, the flow rate is only a few centimeters a day; in others, it may be several hundred meters per day. A “rapid” rate of flow would be 12 to 15 meters (40 to 50 feet) per day.
Impermeable materials composed of components such as clay or very dense unfractured rock, which hinder or prevent water movement, are called aquicludes (Figure 9-25). |
The general distribution of underground water can probably be best understood by visualizing a vertical sub-surface cross section. Usually at least three and often four hydrologic zones are arranged one below another. From top to bottom, these layers are called the zone of aeration, the zone of saturation, the zone of confined water, and the waterless zone.
* Hess, Darrel, et al. McKnights Physical Geography. Learning Solutions, 2011.
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