Groundwater

Groundwater is the water that is located in aquifers beneath the earth's surface. It exists in the cracks and spaces of the soil, sand, and rock that make up the aquifers. Groundwater is a critical source of water that comes from the rain, snow, sleet, and hail that soaks into the ground. It can be found virtually everywhere on the planet and is a vital resource: 98 percent of the water available for human consumption is in the form of groundwater.

Introduction

Groundwater is the water that exists in aquifers beneath the surface layer of the earth. It is the major source of drinking water and water for irrigation, and it exists virtually everywhere on the planet. Groundwater is not a free-flowing underground river, but rather water that exists in the fractures, cracks, and spaces in soil, sand, and rocks that comprise the aquifers.

Gravity causes the water that comes from rain and melted snow, sleet, and ice to seep into the ground. As it penetrates the soil, the water continues to move down through the soil and rock until it reaches what is known as the saturated zone. This zone is bordered on the bottom by an impermeable layer, which does not permit the water to continue its downward movement, and bordered at the top by what is called the water table. Within these borders, the saturation zone is made up of soil, sand, and rock with naturally occurring spaces between them. These spaces are filled with water.

The height of the water table varies depending on how much groundwater exists in the saturated zone; in contrast, the bottom of the saturated zone remains stable against the impermeable layer. The water table moves because the saturated zone fills from the bottom up, with the top surface layer moving accordingly. It is difficult to determine the level of the water table while standing on the ground because it can vary from as little as a foot beneath the surface to several hundred feet beneath the surface. One can, however, see where the water table lies by looking at areas of surface water, such as a lake. The top surface of the lake will be at the level of the water table. This also is true along beaches, where the level of the ocean is the same as the level of the water table. Heavy rains can cause the level of the water table to rise, while heavy pumping of groundwater supplies or drought conditions can cause the level of the water table to fall.

Aquifers

An aquifer is a mass of saturated rock that does not permit the easy movement of water. These rocks can be permeable and porous. They are made up of a variety of rock types that include sandstone, conglomerate, fractured limestone, and unconsolidated materials such as sand, gravel, and silt. Fractured volcanic rocks, particularly the rubble zones between volcanic flows, make excellent aquifers because they are usually porous and permeable. Rocks such as granite and schist are poor aquifers because of their low porosity, and unless they are highly fractured, they cannot create ample space for water. The more permeable the rock, the more quickly water can move through the layers. The more porous the rock, the more space there is for the water to accumulate.

Aquifers occur at various depths. Those closest to the surface are more likely to be used for drinking water and for irrigation. They also are more likely to be filled by local rainfall. Although they “hold” the water in the saturated zone, they are not underground lakes. The water in aquifers moves, but it does not move in an underground river. Rather, the water in an aquifer moves through the spaces between the rock and sediment in the aquifer in both a downward direction until it reaches an impermeable layer and in a “sideways” direction until it finds a way to the surface through a lake or other outlet. Depending on the porosity of the rocks and soil, the movement of the water may be slow or fast. Porosity and permeability are factors in the ability of the aquifer to act as a filter.

Aquifers act as natural filters for the water as it seeps through the layers. The layers of soil and rock trap the sediment and other particles, including bacteria, providing natural purification in the process. The smaller the space the water moves through, the smaller the bacteria that will be filtered. As a result, an aquifer can be filled with clean water as part of a natural process.

An aquifer also can be filled with contaminated water if the contaminants reach the water table. This can happen at landfills or in septic tanks, in areas where high concentrations of an element form naturally, or in areas where chemicals have been dumped. Contamination of aquifers can take place anywhere that an activity creates a pathway that increases the rate at which rain or snowmelt reaches the water table. This occurs because the pathway bypasses the natural purification that takes place when water sources seep into the ground and make their way into the saturated zone naturally, saturating the ground artificially and without any chance for the water to be properly filtered.

Because of gravity, water moves through an aquifer as it makes its way from a higher elevation to a lower elevation. Water also moves from areas of higher pressure to areas of lower pressure. These two forces are the driving force (the hydraulic head) behind groundwater movement. Water in an aquifer can move through four different types of rock: unconsolidated rock (made up of loose sand, silt, and gravel), porous sedimentary rock, porous volcanic rock, and fractured rock. Water is able to move easily between the spaces, which is why gravel and sand are common aquifers. Carbonate rocks are brittle and tend to fracture; the fractures allow some water movement. Volcanic rocks also have fractures that allow water movement.

Aquifer Types

The three types of aquifers are confined, unconfined (which includes the artesian aquifer), and perched aquifers.

The confined aquifer has a water supply sandwiched between two impermeable layers. Because of this physical arrangement, the water cannot easily pass back and forth between the layers once it has made its way into that space. It will stay in the space until it moves to a surface body of water or until the water is pumped from the ground by means of a human-made well. The pressure in a confined aquifer can be quite strong.

An artesian aquifer is a confined aquifer accompanied by pressure. The water in an artesian well experiences upward force so that when an artesian well is dug, its water is pushed to the surface during the drilling process. Often the pressure will force the water above the level of the water table, to the surface and even higher, where it will then flow back into the ground, all without a pump.

An unconfined aquifer (a type of artesian aquifer) is a water supply with an impermeable layer below it and a permeable layer above it. The water table in such an aquifer is free to fluctuate up and down with the recharge and discharge of groundwater. The water from the surface is directly available through this top level of the saturated zone. Because these aquifers have readily permeable layers above them and are close to the water table, they are particularly vulnerable to contamination. They are also subject to seasonal variations in rain and snowmelt.

A perched aquifer has an impermeable layer below it but exists above the water table. It is not unusual to find a perched aquifer in an area of glacial sediment or sitting atop a layer of clay. A perched aquifer is essentially a form of unconfined aquifer but is distinctly smaller in size. A perched aquifer usually can be found at elevations higher than extensive regional aquifers.

Groundwater Discharge

Water can be discharged from an aquifer in three ways: by movement of the water through the aquifer and into a body of surface water, by the drilling of a well, and by a natural spring. A well is a hole that is drilled or dug deep enough to intersect with the water table. A hole dug in an unconfined aquifer will be deep enough to reach beneath the water table, and the water level in the well will match that of the water table. The water is then hauled or pumped to the surface. Also, the well must be dug deep enough to allow for seasonal variations in the water table. If too many wells are dug or too much water is taken from existing wells during a time when there is little or no snowmelt or rainfall, the wells can deplete the aquifer, and the well will run dry.

In a confined aquifer, the pressure from beneath the ground will result in an artesian well in which no pump will be required. The hydraulic pressure will cause the water to move up the sides of the well without the need for a pump. Usually, the water will rise above the level of the ground. Springs are naturally occurring sources of groundwater discharge. They occur where the water table intersects the surface and water flows out of the ground and where impermeable rock meets an aquifer. In the case of a confined aquifer, the spring is known as an artesian spring.

Groundwater in an aquifer is not static, so it is of increasing importance to understand what the use of water in one area has to do with the availability of water in another. Groundwater sources affect other water sources, so management of a single aquifer may not be adequate in regions with large amounts of groundwater discharge.

Groundwater Recharge

Among other uses, aquifers supply drinking water and water to irrigate crops. Water pumped or otherwise taken from an aquifer leads to less water in that aquifer. The aquifer is recharged, or refilled, naturally as part of a hydrologic cycle in which water moves downward from the surface to the saturated zone. The water comes almost exclusively from rain and snowmelt.

In areas with little or no human activity, in which vast amounts of land remain undeveloped or unpaved, water can find its way into the ground and into the aquifers beneath. In areas with much human activity, such as development and logging, a reduced amount of water makes its way into the saturated zone; rainfall and snowmelt lead to runoff rather than seepage. Three other factors that influence the amount of groundwater and rate of recharge are evaporation of water from surface water such as lakes, transpiration from existing vegetation, and infiltration, which is determined by the amount of unpaved surface and the permeability of the surface soil.

Inadequate recharge of the groundwater leads to a depletion of underground resources. Once this happens, a quick, natural recharge of the aquifer is unlikely, requiring instead the costly artificial recharge of the saturated zone. Understanding the relationship between the discharge and recharge of an aquifer is essential to the proper management of that aquifer.

Many models are available to estimate groundwater recharge. These models take into account the amount of paved surfaces and development and an area's logging activity. The models also factor in the permeability of the subsurface material and other variables, such as the density of the materials. From the usage side of the equation, groundwater recharge models are used to decide the number of wells or projects needed to tap into the groundwater supply. Considered on the source side are the amount of annual rainfall and snowmelt, any unusual weather patterns, and indications that the typical patterns will be disrupted. In total, these models provide good data for groundwater management.

Issues

The heavy reliance of humans on groundwater for drinking, irrigation, and other purposes has caused many problems for the natural hydrological cycle. In the United States alone, approximately half of the population uses groundwater as the chief source of drinking water, and more than 50 billion gallons are used for agriculture every day. This heavy use, along with decreased recharge, has led to many instances of groundwater depletion, in which the water table is significantly lowered. Depletion has many negative effects, including the disruption of wells that are no longer deep enough to reach the water table. If a well goes dry, it must be deepened or replaced—often an expensive project—and new pumps may be required that are more costly and need more energy to draw water. Meanwhile, the property above the aquifer may experience land subsidence, in which the removal of water causes the underlying soil and rock to compress and collapse. This can cause major damage to buildings and other structures in the affected area.

Groundwater depletion also impacts the water level in nearby bodies of surface water, such as lakes, streams, and wetlands. Declining water levels can disrupt ecosystems, reducing the habitat for plants dependent on reaching the water level and, therefore, that of various animals as well. Many formerly lush riparian ecosystems in dry areas of the United States and elsewhere have been reduced by excessive groundwater pumping. Another potential consequence of groundwater depletion is a drop in water quality. Often this comes in the form of saltwater intrusion, a process in which saltwater rushes in to replace pumped-out fresh groundwater, contaminating the water supply for humans and other organisms. Saltwater intrusion is common in coastal areas due to the salty water found beneath oceans, but some very deep inland groundwater is also saline and can intrude and contaminate aquifers far from the sea.

Even in areas that have yet to experience groundwater depletion, human activity presents threats to water quality, especially in the form of groundwater pollution. Groundwater can be contaminated in many ways, including seepage of surface pollutants that also threaten surface water. Substances such as oil and gas, salt used on roads, fertilizers and pesticides, toxic waste from mines or manufacturers, and waste from landfills or septic tanks can all enter the hydrologic cycle and make their way into groundwater. Improper disposal of household chemicals and pharmaceutical and personal care products (PPCPs) is another growing source of groundwater pollution, and studies continue to investigate the potential health effects of small amounts of such substances ingested through drinking water. Contaminated groundwater poses a serious environmental and public health problem; drinking polluted water can cause serious diseases ranging from dysentery to cancer.

Conclusions

Groundwater is a vital natural resource and one that is in great demand. As groundwater in one aquifer is used to meet the needs of a community, that same aquifer may have a role in the function of other nearby aquifers and surface-water sources, thereby stretching its capabilities. As aquifers come under increased demand because of development and industrialization, this demand heightens the need for aquifer management to promote sustainable use and avoid groundwater pollution. The depletion of aquifers around the world due to overdraft for human water use has become a prominent issue connected to broader trends in environmental management and water rights. Falling water tables force the drilling of deeper wells and may cause challenges such as land subsidence and saltwater intrusion, along with potential shortages of drinking water. The management models used today are sophisticated systems that account for soil conditions, weather patterns, population and industrial density, and changes in weather patterns caused by global climate change. Organizations such as The Groundwater Foundation exist to bring attention to threats to groundwater and educate the public on the need to protect it.

Principal Terms

aquifer: the water-bearing stratum of permeable rock, sediment, or soil

artesian aquifer: an aquifer that is bounded above and below by impermeable beds and that experiences hydraulic pressure; a type of confined aquifer

artesian pressure: a confining internal pressure of groundwater in an artesian aquifer; it is significantly greater than atmospheric pressure and causes groundwater to rise above its natural level in the aquifer, often above the surface of the ground, without the use of a pump

bedrock: the solid rock underlying soil or any other unconsolidated superficial cover

groundwater: the water that exists in the pore spaces and fractures in rock and sediment (aquifers) beneath the earth's surface

hydrologic cycle: the complete cycle through which water passes from the oceans, through the atmosphere, to the land, and back to the ocean; a water cycle

infiltration: the penetration of water through the ground surface into subsurface soil, or the penetration of water from the soil into sewer or other pipes through defective joints, connections, or utility hole walls

subsurface water: the water in the root zone of the soil and groundwater flowing or stored in the rock mantle of the earth

surface water: the water flowing in stream channels

transpiration: the process by which water vapor is lost to the atmosphere by living plants

water cycle: a hydrologic cycle

water table: the upper surface of the zone of saturation in permeable rocks not confined by impermeable rocks; it rises and falls with the amount of groundwater

Bibliography

Anderson, Mary P., ed. Groundwater. Oxford: Intl. Assn. of Hydrological Sciences, 2008. Print.

Bear, Jacob. Dynamics of Fluids in Porous Media. Mineola: Dover, 1988. Print.

Bear, Jacob. Hydraulics of Groundwater. Mineola: Dover, 2007. Print.

"Groundwater Depletion." USGS Water Science School. US Geological Survey, 9 Dec. 2015. Web. 2 Feb. 2016.

Groundwater Foundation. Groundwater Foundation, 2016. Web. 2 Feb. 2016.

Healy, Richard, and Bridget Scanlon. Estimating Groundwater Recharge. New York: Cambridge UP, 2013. Print.

Mays, Larry W. Ground and Surface Water Hydrology. Hoboken: Wiley, 2012. Print.

Moore, John E. Field Hydrogeology: A Guide for Site Investigations and Report Preparation. Boca Raton: CRC, 2012. Print.

Odell, Lee H. Treatment Technologies for Groundwater. Denver: American Water Works Assn., 2010. Print.

Strassberg, Gil, Norman L. Jones, and David R Maidment. Arc Hydro Groundwater: GIS for Hydrogeology. Redlands: ESRI, 2011. Print.

"What is Groundwater?" The Groundwater Foundation, 2022, groundwater.org/what-is-groundwater/. Accessed 15 Apr. 2023.

Woods, Andrew W. Flow in Porous Rocks: Energy and Environmental Applications. Cambridge: Cambridge UP, 2015. Print.