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Not all of the water that falls to the Earth as rain, snow, sleet, or hail runs off into lakes, ponds, rivers, and streams. Some of the water soaks into the ground. Water contained in the ground is one of the Earth's most important natural resources. There is more fresh water below the surface of the land than in all the lakes and reservoirs on the Earth's surface.

GROUND WATER

If you live in a rural, or country, area, you probably do not get your water from a reservoir or river. More likely, your water is pumped from a well in the ground. As you learned in the previous section, the water stored in the ground is known as groundwater. In many areas, groundwater provides a continuous supply of fresh water.

Groundwater is present because the various forms of precipitation-rain, snow, sleet, and hail-do not stop traveling when they hit the ground. Instead, the precipitation continues to move slowly downward through pores, or spaces, in the rocks and soil. If the rocks and soil have many pores between their particles, they can hold large quantities of groundwater. Sand and gravel are two types of soil that contain many pores.

As the water seeps down, it passes through layers of rocks and soil that allow it to move quickly. Material through which water can move quickly is described as permeable (PER-mee-uh-buhl). Sandstone is a rock that is very permeable. But clay, which has small pores between its particles, is not as permeable. Clay is sometimes described as impermeable.

Underground Zones

Groundwater continues to move downward through permeable rock and soil until it reaches an impermeable layer of rock. When it reaches an impermeable layer, it can go no farther. So the groundwater begins to fill up all the pores above the impermeable layer. This underground region in which all the pores are filled with water is called the zone of saturation (sach-uh-RAY-shuhn).

An example from the kitchen may help you to understand what happens when spaces in the ground become filled with water. You may never have looked closely at the sponge on a kitchen sink. When a sponge is barely moist, only some of the spaces in the sponge are filled with water. Most of the spaces hold air. When you place the sponge in water, it swells. Eventually, all the spaces are filled and the sponge cannot take up any more water. The ground acts in much the same way as the sponge. Once the spaces in the ground are filled, the ground is saturated. It cannot hold any more water.

Above the water-filled zone, the ground is not as wet. Pores in the soil and rocks are filled mostly with air. This drier region in which the pores are filled mostly with air is called the zone of aeration.

A cross section of the zones of underground water is shown here. What separates the zone of aeration from the zone of saturation

The surface between the zone of saturation and the zone of aeration is an important boundary. It marks the level below which the ground is saturated, or soaked, with water. This level is called the water table. .

At the seashore, the water table is easy to find. After you dig down 10 or 20 centimeters, you may notice that the hole you are digging fills with water. At this point, you have located the water table. In general, the water-table is not very deep near a large body of water.

In areas near hills or mountains, the water table may be deep within the ground. In low-lying areas such as valleys with swamps and marshes, the water table may be close to or at the surface. The depth of the water table also varies with the climate of an area. It may be deep in very dry areas, such as deserts. It may be close to the surface in wet, low-lying forest areas. In very moist climate regions, the water table may come right to the surface and form a swamp, lake, or spring. Why do you think low-lying areas have a water table that is close to the surface?

Even in the same area, the depth of the water table may change. Heavy rains and melting snows will make the water table rise. If there is a long, dry period, the water table will fall. The depth of the water table will also change if wells are overused or if many wells are located in a small area. Wells are holes drilled or dug to the water table to bring water to the surface. The use of several wells in an area may draw so much water from the water table that only very deep wells are able to pump water to the surface.

The depth of the water table may have other effects. In order to provide a proper foundation for a tall building, a builder must dig a deep hole. In some places in New York City, the water table is very high, and water rapidly fills the foundation hole. This water must be pumped out in order for construction to proceed. This extra work adds to the cost of a building. In certain areas, wells are dug to provide a source of household water. It is relatively inexpensive to dig a well in areas where the water table is high. In areas where the water table is deep, however, it can be very expensive to dig a well. Remember-a water table is always present, no matter where you live. And you will always reach it if you dig deep enough!

Aquifers

As groundwater moves through a permeable rock layer, it often reaches an impermeable rock layer or the water table. At this point, the groundwater may move sideways through a layer of rock or sediment that allows it to pass freely. Such a layer is called an aquifer (AK-wuh-fer). Aquifers are usually layers of sandstone, gravel, sand, or cracked limestone.

Because rocks form in layers, a layer of permeable rock may become trapped between two layers of impermeable rock. Sandstone (permeable rock) trapped between two layers of shale (impermeable rock) is an example. If the layer of sandstone contains water, an aquifer forms. An aquifer may also form when soil saturated with groundwater is located above an impermeable rock layer.

An aquifer is a source of groundwater. To reach this water, a well is often dug or drilled into the aquifer. Groundwater moves into the well hole and forms a pool. Each time water is pumped from the well, more water moves through the aquifer into the well hole. Nassau and Suffolk counties in New York State pump much of the water used by their inhabitants from huge aquifers.

Groundwater can be obtained from an aquifer by means of an ordinary well or artesian well. The amount of water pressure in an artesian well depends on hww close the well is to the water table

Because water often moves great distances through aquifers, these underground water sources are extremely vulnerable to pollution. Any pollutants added to an aquifer may spread through the aquifer, endangering water sources far from the pollutants' point of origin.

In some places where the underground rock layers slope, an aquifer carries water from a higher altitude to a lower altitude. If the aquifer is trapped between two layers of impermeable rock, pressure may build up at the lower altitude. A well drilled into the aquifer at this point will provide water without pumping. A well from which water flows on its own without pumping is called an artesian (ahr-TEE-zhuhn) well.

GROUND WATER FORMATIONS

In some areas, the underlying rock is limestone. Because limestone is affected by groundwater in a particular way, underground caverns (KAv-ernz) often form in these areas. As water moves down through the soil, it combines with carbon dioxide to form a weak acid that can dissolve limestone. This acid, called carbonic acid, is the weak acid found in seltzer water and other carbonated beverages. You are probably familiar with this weak acid as the "fizz" in a carbonated beverage.

Stalactites (hanging) and Stalagmites (standing) are limestone formations found in caverns throughout the world. It is a federal offense to remove formations from caves in the United States

When groundwater enters cracks in limestone, the carbonic acid it contains causes the cracks to become wider. If this process continues long enough, underground passages large enough to walk through may be formed.

Sometimes large underground caverns with many passages are formed. If you walk through these caverns, you will see what looks like long stone icicles hanging from the ceilings. These icicles are called stalactites (stuh-LAK-tights). Stalagmites (stuh-LAG-mights) look like stone icicles built up from the floors of the caverns. Stalactites and stalagmites are formed when dissolved substances in groundwater are deposited.

In many caverns, underground lakes are formed as groundwater moves through limestone. This lake is found in Hamss Caves, Spain.