A great deal of groundwater eventually makes its way to the surface. Often this occurs as discharge into streams, lakes and wetlands, but sometimes, this occurs as a naturally flowing spring or as a spectacularly erupting geyser. We bring much of the groundwater we use to the surface by pumping it from a well. To understand these phenomena, it is necessary to understand Earth’s sometimes complex underground “plumbing.”

Wells and Artesian Systems

According to the National Groundwater Association, there are more than 16 million water wells for all purposes in the United States. Private household wells constitute the largest share—more than 13 million. Another 476,000 wells are used for agricultural irrigation, accounting for almost 70% of all U.S. groundwater consumption. Wells are incredibly important worldwide, as they provide a means of reliable freshwater supply to homes, industry, and agriculture.

Wells

The most common method for removing groundwater is to use a well, a hole drilled into the zone of saturation. Wells serve as small reservoirs into which groundwater migrates and from which it can be piped and pumped to the surface. The water table level can fluctuate considerably during the course of a year, dropping during dry seasons and rising following periods of precipitation. Therefore, to ensure a continuous supply of water, a well must penetrate below the water table.

Whenever a substantial amount of water is withdrawn from a well, the water table around the well is lowered. This effect, termed drawdown, decreases with increasing distance from the well. The result is a depression in the water table, roughly conical in shape, known as a cone of depression (Figure 2). For most small domestic wells, the cone of depression is negligible. However, when wells are used for irrigation or for industrial purposes, the withdrawal of water can be great enough to create a very wide and steep cone of depression that may substantially lower the water table in an area and cause nearby shallow wells to become dry. The lower diagram in Figure 1 illustrates this situation.

Artesian Systems

In most wells, water cannot rise on its own. If water is first encountered at a depth of 30 meters (100 feet), it remains at that level, fluctuating perhaps 1 or 2 meters with seasonal wet and dry periods. However, in some wells, water rises, at times overflowing at the surface.

Artesian system refers to a situation in which groundwater rises in a well above the level where it was initially encountered. For such a situation to occur, two conditions must exist: (1) Water must be confined to an aquifer that is inclined so that one end is exposed at the surface, where it can receive water; and (2) aquitards both above and below the aquifer must be present to prevent the water from escaping. Such an aquifer is called a confined aquifer (Figure 3) because the water is confined between the aquitards, which put pressure on the water. Drilling a well into such a system with natural pressure allows water to rise up and flow out of the aquifer.

In Figure 3, Well 1 is a nonflowing artesian well because, at this location, the pressure surface is below ground level. A pressure surface is an imaginary surface that represents the level to which water in a confined aquifer would rise if not trapped. When the pressure surface is above the ground and a well is drilled into the aquifer, a flowing artesian well is created (Well 2 in Figure 3). Not all artesian systems are wells. Groundwater may reach the surface by rising along a natural fracture, such as a fault, rather than through an artificially produced hole; this is called an artesian spring. In deserts, artesian springs are sometimes responsible for creating oases, a fertile region with a source of freshwater.

Artesian systems act as “natural pipelines,” transmitting water from remote areas of recharge great distances to the points of discharge. In this manner, water that fell in central Wisconsin years ago is now taken from the ground and used by communities many kilometers to the south, in Illinois. In South Dakota, an artesian system of sandstone aquitards brings water from the western Black Hills to artesian wells that extend eastward across the state.

On a different scale, city water systems may be considered examples of artificial artesian systems (Figure 4). A water tower, into which water is pumped, may be considered the area of recharge, the pipes the confined aquifer, and the faucets in homes the flowing artesian wells.

Springs

Springs have aroused the curiosity and wonder of people for thousands of years. The fact that springs were (and to some people still are) rather mysterious phenomena is not difficult to understand because water is flowing freely from the ground in all kinds of weather, in seemingly inexhaustible supply but with no obvious source. Today, we know that the source of springs is water from the zone of saturation and that the ultimate source of this water is precipitation.

Whenever the water table intersects the ground surface, a natural flow of groundwater results, which we call a spring (Figure 5). Many springs form when an aquitard blocks the downward movement of groundwater and forces it to move laterally. Where the permeable bed (aquifer) outcrops in a valley, one or more springs result.

Another situation that can produce a spring is illustrated in Figure 6. Here, an aquitard is situated above the main water table. As water percolates downward, a portion accumulates above the aquitard to create a localized zone of saturation and a perched water table. Springs, however, are not confined to places where a perched water table creates a flow at the surface. Many geologic situations lead to the formation of springs because subsurface conditions vary greatly from place to place. Even in areas underlain by impermeable crystalline rocks, permeable zones may exist in the form of fractures or solution channels. If these openings fill with water and intersect the ground surface along a slope, a spring results.

Hot Springs

There is no universally accepted definition of hot spring. One frequently used definition is that the water in a hot spring is 6°C to 9°C (10°F to 15°F) warmer than the average annual air temperature for the locality where it occurs. In the United States alone, there are well over 100 such springs.

Within the Earth, temperatures generally rise with increasing depth, at an average of about 2°C per 100 meters (1°F per 100 feet), a figure known as the geothermal gradient. Therefore, when groundwater circulates at great depths, it becomes heated. If the hot water rises rapidly to the surface, it may emerge as a hot spring. The water of some hot springs in the eastern United States is heated in this manner. The springs at Hot Springs National Park in Arkansas are one example. Water temperatures of these springs average about 60°C (140°F) and originate from groundwater rising rapidly through fractured rock from depths of up to 2400 meters (8000 feet).

The great majority (more than 95 percent) of the hot springs (and geysers) in the United States are found in the West. The reason for this distribution is that the sources of heat for most hot springs are magma bodies and hot igneous rocks, and it is in the West that igneous activity has occurred most recently. In these areas, the geothermal gradient is higher and groundwater warms up closer to the surface. The hot springs and geysers of the Yellowstone region are well-known examples.

Geysers

Intermittent fountains in which columns of hot water and steam are ejected with great force, often rising 30 to 60 meters (100 to 200 feet) into the air, are called geysers. After the jet of water ceases, a column of steam rushes out, often with a thunderous roar. Perhaps the most famous geyser in the world is Old Faithful in Yellowstone National Park (Figure 7). The great abundance, diversity, and spectacular nature of Yellowstone’s geysers and other thermal features undoubtedly was the primary reason for it becoming the first national park in the United States. Geysers are also found in other parts of the world, notably New Zealand and Iceland. In fact, the Icelandic word geysa, meaning “to gush,” gives us the name geyser.

Geysers occur where extensive underground chambers exist within hot igneous rocks. As relatively cool groundwater enters the chambers, it is heated by the surrounding rock. At the bottom of the chamber, the water is under great pressure because of the weight of the overlying water. This great pressure prevents the water from boiling at the normal surface temperature of 100°C (212°F). For example, at the bottom of a 300-meter (1000-foot) water-filled chamber, water must attain a temperature of nearly 230°C (450°F) before it will boil. Heating causes the water to expand, and as a result, some of the water is forced out at the surface. This loss of water reduces the pressure on the remaining water in the chamber, which lowers the boiling point. As a result, a portion of the water deep within the chamber quickly turns to an expanding mass of steam, which causes the geyser to erupt. Following the eruption, cool groundwater again seeps into the chamber, and the cycle begins anew.

• Wells, which are openings bored into the zone of saturation, withdraw groundwater and may create roughly conical depressions in the water table known as cones of depression.

• Artesian wells tap into inclined aquifers bounded above and below by aquitards. For a system to qualify as artesian, the water in the well must be under sufficient pressure for the water to rise above the top of the confined aquifer. Artesian wells may be flowing or nonflowing, depending on whether the pressure surface is above or below the ground surface.

• Springs occur where the water table intersects the land surface and a natural flow of groundwater results. When groundwater circulates deep below the surface, it may become heated and emerge at the surface as a hot spring. Geysers occur when groundwater is heated in underground chambers and expands, with some water quickly changing to steam and causing the geyser to erupt. The source of heat for most hot springs and geysers is hot igneous rock.

• artesian system: A system in which groundwater under pressure rises above the level of the aquifer.

• cone of depression: A cone-shaped depression in the water table immediately surrounding a well where groundwater levels have been reduced.

• confined aquifer: An aquifer that has impermeable layers (aquitards) both above and below.

• drawdown: The difference in height between the bottom of a cone of depression and the original height of the water table.

• geysers: Hot springs that periodically eject fountains of hot water and steam.

• perched water table: A localized zone of saturation above the main water table created by an impermeable layer.

• spring: A flow of groundwater that emerges naturally at the ground surface.

• well: A hole bored into the zone of saturation to obtain water stored as groundwater.