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Groundwater - How Groundwater Occurs

water zone saturated unsaturated

Groundwater is not in underground lakes, nor is it water flowing in underground rivers. It is simply water that fills pores or cracks in subsurface rocks. When rain falls or snow melts on a ground surface, water may run off into lower land areas or lakes and streams. What is left is absorbed into the soil where it can be used by vegetation, seeps into deeper layers of soil and rock, or evaporates back into the atmosphere.

Below the topsoil is an area called the unsaturated zone, where, in times of adequate rainfall, the small spaces between rocks and grains of soil contain at least some water, while the larger spaces contain mostly air. After a major rain, the zone may become saturated, that is, all the open spaces fill with water. During a drought, the area may become drained and almost completely dry. In the unsaturated zone, a certain amount of water is held in the soil and rocks by molecular attraction, making the soil moist but not wet. This is similar to having enough water in a wet towel to make it feel damp after it has stopped dripping. A well dug into the unsaturated zone will not fill with water because the water in the unsaturated zone is at atmospheric pressure.

With continued rainfall, excess water will drain through the unsaturated zone (which now has absorbed as much water as it can hold) to the saturated zone to recharge the aquifer. (See Figure 4.1.) The saturated zone is full of water—all the spaces between soil and rocks, and the rocks themselves, contain water. Water from streams, lakes, wetlands, and other water bodies may seep into the saturated zones. Streams are commonly a significant source of recharge to groundwater downstream from mountain fronts and steep hillsides in arid and semiarid areas, and in areas underlaid by limestone and other soluble rocks.

In the saturated zone, water is under pressure that is higher than atmospheric pressure. When a well is dug into the saturated zone, water flows from the area of higher pressure (in the ground) to the area of lower pressure (in the hollow well), and the well fills with water to the level of the existing groundwater. In some cases, the pressure is high enough to force the water to flow to the surface without pumping.

The water table is the level at which the unsaturated zone and the saturated zone meet. The water table is not fixed, but may rise or fall, depending on water availability. In areas where the climate is fairly consistent, the level of the water table may vary little; in areas subject to extreme flooding and drought, it may rise and fall substantially.

FIGURE 4.1
Groundwater
SOURCE: "Figure 1.1. Ground Water," in National Water Quality Inventory: 1998 Report to Congress, U.S. Environmental Protection Agency, 2000

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