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Chapter 4


Water lies beneath almost every part of the earth's surfacemountains, plains, and desertsbut underground water is not always easy to find, and, once found, it may not be readily accessible. Groundwater may lie close to the surface, as in a marsh, or it may occur many hundreds of feet below the surface, as in some dry areas of the nation's West.

People have known about the presence of ground-water since ancient times, but it is only recently that geologists have learned how to gauge the quantity of groundwater and have begun to estimate its vast potential for use. The U.S. Geological Survey (USGS) states in "Groundwater" (2006, that even though an estimated one million cubic miles of the earth's groundwater is located within about half a mile of the surface (there are about one billion gallons in a cubic mile), only a small amount of this reservoir of underground water can be tapped and made available for human use through wells and springs. Furthermore, in Where Is Earth's Water Located? (August 28, 2006,, the USGS notes that the total amount of fresh ground-water on Earth at all depths is estimated at about 2.5 million cubic miles.


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 the surface of the ground, some water may run off into lower land areas or lakes and streams. What is left may be absorbed by the soil, seep into deeper layers of soil and rock, or evaporate into the atmosphere.

Below the topsoilthe rich upper layer of soil in which plants have most of their rootsis an area called the unsaturated zone. In times of adequate rainfall the small spaces between rocks and grains of soil in the unsaturated zone contain at least some water, whereas the larger spaces contain mostly air. After a major rain, however, all the open spaces may fill with water temporarily. During a drought, the area may become drained and almost completely dry, although a certain amount of water is held in the soil and rocks by molecular attraction.

Lying beneath the unsaturated zone is the saturated zone. The water table is the level at which the unsaturated zone and the saturated zone meet. (See Figure 4.1.) Water drains through the unsaturated zone to the saturated zone. The saturated zone is full of waterall the spaces between soil and rocks, and within the rocks themselves, contain water. Water from streams, lakes, wetlands, and other water bodies may seep into the saturated zone. 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 porous rock.

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.

Groundwater Flow

Water is always in motion. Groundwater generally moves from recharge areas, where water enters the ground, to discharge areas, where it exits from the ground into a wetland, river, lake, or ocean. Transpiration by plants whose roots extend to a point near the water table is another form of discharge. The path of groundwater movement may be short and simple or incredibly complex, depending on the geology of the area through which the water passes. The complexity of the path also determines the length of time a molecule of water remains in the ground between recharge and discharge points. (See Figure 4.2.)

The velocities of groundwater flow generally are low and are orders of magnitude less than the velocities of stream flow. Groundwater movement normally occurs as slow seepage through the spaces between particles of unconsolidated material or through networks of fractures and openings in consolidated rocks. A velocity of one foot per day or more is a high rate of movement in groundwater. Groundwater velocities can be as low as one foot per decade or one foot per century. By contrast, stream flows are generally measured in feet per second. A velocity of one foot per second is about sixteen miles per day. The low velocities of groundwater flow can have important implications, particularly in relation to the movement of contaminants.

The age of water (time since recharge) varies in different parts of groundwater flow systems. Groundwater gets steadily older along a particular flow path from an area of recharge to an area of discharge. In shallow, local-scale flow systems, groundwater age at areas of discharge can vary from less than a day to a few hundred years. (See Figure 4.2.) In deep, regional flow systems with long flow paths, groundwater age may reach thousands or tens of thousands of years.


An aquifer is a saturated zone that contains enough water to yield significant amounts of water when a well is dug. The zone is actually a path of porous or permeable material through which substantial quantities of water flow relatively easily. The word aquifer comes from the Latin aqua (water) and ferre (to bear or carry). An aquifer can be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubble zone between lava flows, or even a large body of massive rock, such as fractured granite. An aquifer may lie above, below, or in between confining beds that are layers of hard, nonporous material (e.g., clay or solid granite).

There are two types of aquifers: unconfined and confined (artesian). In an unconfined or water table aquifer, precipitation filters down from the land's surface until it hits an impervious layer of rock or clay. The water then accumulates and forms a zone of saturation. Because runoff water can easily seep down to the water table, an unconfined aquifer is susceptible to contamination.

In a confined aquifer the confining beds act more or less like underground boundaries, discouraging water from entering or leaving the aquifer, so that the water is forced to continue its slow movement to its discharge point. Water from precipitation enters the aquifer through a recharge area, where the soil lets the water percolate down to the level of the aquifer. The ability of an aquifer to recharge is dependent on various factors, such as the ease with which water is able to move down through the geological formations (permeability) and the size of the spaces between the rock particles (porosity). Figure 4.3 illustrates natural and artificial aquifer recharge.

Usually, the permeability and porosity of rocks decreases as their depth below the surface increases. How much water can be removed from an aquifer depends on the type of rock. A dense granite, for example, will supply almost no water to a well even though the water is near the surface. A porous sandstone, however, thousands of feet below the surface can yield hundreds of gallons of water per minute. Porous rocks that are capable of supplying freshwater have been found at depths of more than six thousand feet below the surface. Saline (salty) water has been discovered in aquifers that lie more than thirty thousand feet underground.

Aquifers vary from a few feet thick to tens or hundreds of feet thick. They can be located just below the earth's surface or thousands of feet beneath it. An aquifer can cover a few acres of land or many thousands of square miles. Furthermore, any one aquifer may be a part of a large system of aquifers that feed into each other.

Ogallala Aquifer

The Ogallala or High Plains Aquifer in the United States is one of the world's largest aquifers and is the largest in North America. According to the High Plains Aquifer Information Network (2007,, the aquifer stretches from southern South Dakota to the Texas panhandle and covers 174,000 square miles. (See Figure 4.4.) The Ogallala's average thickness ranges from three hundred feet to more than twelve hundred feet in Nebraska. For more than fifty years, the Ogallala has supplied most of the water for irrigation and drinking to the Great Plains states and yields about 30% of the nation's groundwater for irrigation. Robert B. Jackson et al. indicate in "Water in a Changing World" (Issues in Ecology, Spring 2001) that since 1940 approximately 200,000 wells tap into this vast aquifer. Furthermore, the USGS (February 16, 2007, notes that in 2000 these wells extracted about 315 million gallons of water per day for 1.9 million people. The U.S. Environmental Protection Agency (EPA) designates the Ogallala aquifer a sole-source aquifer. This means that at least 50% of the population in the area depends on the aquifer for its water supply.

Paul D. Ryder reports in Ground Water Atlas of the United States, Oklahoma, Texas (1996, that because the Ogallala Aquifer is being pumped far in excess of recharge, the USGS and the Texas Department of Water Resources project an increasing shortage of Ogallala Aquifer water for future irrigation needs. The High Plains Aquifer Information Network agrees that pumping for irrigation from the aquifer has resulted in substantial declines in some of its parts. Projections by the USGS and the Texas Department of Water Resources suggest that the irrigated acreage in the High Plains of Texas (69% of irrigated Texas cropland) will be reduced to half of its present acreage by 2030 unless an effective water conservation plan is implemented.

In 1999 the Texas oil tycoon Boone Pickens formed Mesa Water, Inc., to market water from part of the Ogallala Aquifer to large Texas cities for municipal use. About one hundred landowners and two hundred thousand acres of land in the Texas panhandle were affected by Pickens's plan. The project sparked controversy in Texas, where the hundred-year-old rule of capture was still in effect. The rule of capture, which was at one time standard doctrine in much of the United States, states that the owner of land that sits on an underground water source can pump out unlimited amounts of water regardless of the impact on surrounding property owners.

Concerned that the already rapidly draining water source would become depleted even further, residents of the surrounding panhandle area protested, arguing that their essential source of water should not be pumped hundreds of miles away. Many Texans began campaigning for tighter government regulations on water rights. However, comprehensive water legislation that would have modified the rule of capture failed to reach a vote in the Texas state senate in May 2005. The following month, the article "Private Water Group Preparing to Pump the Ogallala Aquifer and Sell Groundwater to Far-Away Texas Cities" (Western Water Law and Policy Reporter, June 2005) announced that Mesa Water would begin building a system of wells and pipelines to sell the water to major urban centers throughout Texas.

According to the article "Deal Beefs up Authority's Water Rights" (Amarillo Daily News, March 7, 2006), the Canadian River Municipal Water Authority (CRMWA) announced in March 2006 interest in all of Mesa's two hundred thousand acres of water rights. The CRMWA was created over fifty years ago by the Texas legislature to provide a source of municipal and industrial water for its eleven member cities in the Texas panhandle and South Plains via a 322-mile aqueduct system. (The Canadian River is the largest tributary of the Arkansas River and runs through this part of Texas.)


A spring is a natural discharge of water at the earth's surface from a saturated zone that has been filled to overflowing. Springs are classified either according to the amount of water they produce or according to the temperature of the water (hot, warm, or cold). Giant Springs in Great Falls, Montana, is the largest freshwater spring in the United States and is the source of water for the Missouri and Roe rivers. According to Travel Montana (2007,, the springs remove 338 million gallons per day from underground reserves. Furthermore, the water remains a constant temperature of 54°F, and has been carbon-dated to be about three thousand years old.

Thermal springs have water that is warm or, in some places, hot. They are fed by groundwater that is heated by contact with hot rocks deep below the surface. In some areas water can descend slowly to deep levels, getting warmer the farther down it goes. If it rises faster than it descended, it does not have time to cool off before it emerges on the surface. Well-known thermal springs are the Warm Springs in Georgia and the Hot Springs in Arkansas. Geysers are thermal springs that erupt periodically. Old Faithful in Yellowstone National Park is perhaps the most famous and spectacular geyser in the world. It erupts at intervals of thirty to ninety minutes. The park maintains a Webcam ( so that virtual visitors can see Old Faithful erupt even if they cannot see it in person.


As groundwater travels its course from recharge to discharge area, it undergoes chemical and physical changes as it mixes with other groundwater and reacts with the minerals in the sand or rocks through which it flows. These interactions can greatly affect water quality and its suitability or unsuitability for a particular use.


Water is a natural solvent capable of dissolving many other substances. Spring waters may contain dissolved minerals and gases that give them subtle flavors. Without minerals and gases, water tastes flat. The most common dissolved mineral substances are calcium, magnesium, sodium, potassium, chloride, sulfate, and bicarbonate. However, water is not considered desirable for drinking if it contains more than one thousand milligrams per liter (mg/l) of dissolved minerals. In areas where less-mineralized water is not available, water with a few thousand mg/l of dissolved minerals is used routinely, although it is classified as saline.

Some well and spring waters contain such high levels of dissolved minerals that they cannot be tolerated by humans, plants, or animals. In high concentrations, certain minerals can be especially harmful. A large quantity of sodium in drinking water is unhealthy for people with heart disease. Boron, a mineral that is good for some plants in small amounts, is toxic to other plants in only slightly elevated concentrations. Such highly mineralized groundwater usually lies deep below the surface and has limited uses.

Water Hardness

Water that contains a lot of calcium and magnesium is said to be hard. The hardness of water can be expressed in terms of the amount of calcium carbonate (the principal constituent of limestone) or equivalent minerals that remain when the water is evaporated. Water is considered soft when it contains 0 to 60 mg/l of hardness constituents, moderately hard from 61 to 120 mg/l, hard between 121 and 180 mg/l, and very hard if more than 180 mg/l.

Very hard water is not desirable for many domestic uses and leaves a scaly deposit on the insides of pipes, boilers, and tanks. Hard water can be made soft at a fairly reasonable cost, although it is not always desirable to remove all the minerals from drinking water because some are beneficial to health. Extremely soft water can corrode metals but is suitable for doing laundry, dishwashing, and bathing. Whenever possible, most communities seek a balance between hard and soft water in their municipal water systems.


Human Needs

The nation's use of groundwater grew dramatically in the last several decades of the twentieth century. Susan S. Hutson et al. report in Estimated Use of Water in the United States in 2000 (2004, that the rate of withdrawal was thirty-four billion gallons per day (Bgal/d) of fresh groundwater in 1950. It increased to 83 Bgal/d in 1980 and dropped off a bit over the next several years before reaching a new high of 83.3 Bgal/d in 2000.

In "Ground Water Use in the United States" (March 27, 2006,, the USGS estimates that approximately 26% of the freshwater used in the United States in 2000 was groundwater. (The rest was surface water.) Figure 4.5 shows the percentage of groundwater allocated to various uses in the United States in 2000. Nearly two-thirds (63%) of all groundwater was used for irrigation. One-fifth (20%) was used for public uses such as drinking, bathing, and cooking. The remaining 17% was used for industry, mining, domestic use (self-supplied water via wells), livestock watering, thermoelectric power plants, and commercial purposes.

Figure 4.6 shows the estimated percent of the population using groundwater as drinking water. In many statesincluding Idaho, Minnesota, Nebraska, New Mexico, Mississippi, and Floridadrinking water is obtained almost exclusively from groundwater sources. According John S. Zogorski et al., in The Quality of Our Nation's Waters: Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells (2006,, overall about 50% of U.S. residents use groundwater as their drinking water source. Rural residents rely heavily on groundwater for this purpose.

Ecological Needs

Historically, groundwater and surface water have been managed as separate resources. Since the 1970s, however, there has been a growing awareness that these two sources are inseparably linked. Groundwater seeps into rivers, streams, lakes, and other water bodies and breaks the surface as springs. In some parts of the United States, especially in arid regions, aquifers contribute a large portion of the water found in rivers and streams. Figure 4.7 shows groundwater contributions to surface water in ten regions across the country.

Groundwater recharge of surface water is particularly important during dry periods. Reductions in surface water can have adverse effects on the ecology of a watershed, stressing fish populations and their food supply, wetlands, and the plants and animals living along the banks of rivers and streams. Groundwater depletion in some areas has resulted in the death of aquatic and semiaquatic species that depended on groundwater flow to surface water streams.


Pumping groundwater from a well always causes a decline in groundwater levels at and near the well, and it always causes a diversion of groundwater that was moving slowly to its natural, possibly distant, area of discharge. Pumping a single well typically has only a local effect on the groundwater flow system. Pumping many wells (sometimes hundreds or thousands of wells) in large areas can have significant regional effects on groundwater systems.

If a groundwater system is not overused, the rate of groundwater recharge and discharge balance one another. However, when the rate of withdrawal exceeds the rate at which the groundwater source is recharged, the result is the lowering of groundwater to levels that may impair the resource.

Overpumping groundwater can have many different effects, including:

  • Neighboring wells can dry up, requiring construction of new, deeper wells or significant changes to existing wells.
  • Aquifer materials can compact, causing the land above the aquifer to sink and leaving gaping holes in the land that cause damage to buildings, roads, canals, pipelines, and other infrastructure.
  • Aquifer capacity may be permanently lost because of compaction of aquifer materials, resulting in higher pumping costs and a decrease in well yields.
  • Changes in the volume and direction of groundwater flow can induce the flow of saltwater and water of lower quality into a well.
  • Wetlands can dry up and cause adverse effects on ecological systems that are dependent on groundwater discharge.

According to the USGS, large withdrawals of groundwater have altered the flow systems and geological and chemical conditions of some of the major aquifers in the United States. Declining groundwater levels can change the location and size of recharge areas and reduce discharge rates. Some aquifers in the West have suffered major losses in aquifer storage because of over-pumping.


Until the mid-twentieth century people believed that soil provided a barrier or protective filter that neutralized the downward migration of contaminants from the land surface and prevented water resources from becoming contaminated. The discovery of pesticides and contaminants in groundwater, however, demonstrated that human activities do influence groundwater quality and that the soil may not be as effective a filter as once thought.

The potential for a contaminant to affect groundwater quality is dependent on its ability to migrate through the overlying soils to the groundwater resource. Figure 4.8 shows sources of groundwater contamination. Contamination can occur as a relatively well-defined localized plume coming from a specific source. It can also occur as a generalized deterioration over a large area because of diffuse nonpoint sources such as fertilizer and pesticide applications.

Once groundwater contamination was recognized, researchers needed to determine which waters were contaminated, the severity of contamination, and what should be done about the contamination. Many government and private organizations began working to find the answers, but it was not an easy task.

The quality of the most available groundwater in the United States is believed to be good, according to the EPA's Safe Drinking Water Act, Section 1429 Groundwater Report to Congress (October 1999, and 2000 National Water Quality Inventory (August 2002, The worst groundwater contamination is generally in the areas where use is heaviesttowns and cities, industrial complexes, and agricultural regions, such as California's Central Valley.

Recognizing the need to protect valuable and vulnerable groundwater sources, the states have begun to implement comprehensive groundwater protection programs. In addition, in October 2006 the EPA finalized the Ground Water Rule (October 25, 2006,, which requires states to conduct sanitary surveys of water provided by public utilities from underground sources, take corrective action if contamination is found, and ensure that disinfection of drinking water is effective.


All pollutants do not cause the same rate of contamination for the same amount of pollutant. Groundwater is affected by many of the following factors:

  • The distance between the land surface where pollution occurs and the depth of the water table. The greater the distance, the greater the chance that the pollutant will biodegrade or react with soil minerals.
  • The mineral composition of the soil and rocks in the unsaturated zone. Heavy soil and organic materials lessen the potential for contamination.
  • The presence or absence of biodegrading microbes in the soil.
  • The amount of rainfall. Less rainfall results in less water entering the saturated zone and, therefore, lower quantities of contaminants.
  • The evapotranspiration rate. (This is the rate at which water is discharged into the atmosphere as a result of evaporation from the soil, surface water, and plants.) High rates reduce the amount of contaminated water reaching the saturated zone.


Major Types of Groundwater Contaminants

The EPA reports in the National Water Quality Inventory: 1998 Report to Congress (June 2000, that thirty-one of the thirty-seven reporting states identified the types of contaminants they found in groundwater. The states said that nitrates, metals, volatile and semivolatile organic compounds, and pesticides were the pollutants found most often. Following the publication of the 2000 National Water Quality Inventory, the EPA entered a transition period in the gathering and analysis of water quality data in nationally consistent, statistically valid assessment reports. Its new reporting schedule is described in "Schedule for Statistically Valid Surveys of the Nation's Waters" (December 5, 2005, The USGS conducts the National Water Quality Assessment Program and publishes a series of reports on water quality issues of regional and national concern.

In Factors Affecting Occurrence and Distribution of Selected Contaminants in Ground Water from Selected Areas in the Piedmont Aquifer System, Eastern United States, 19932003 (2006,, Bruce D. Lindsey et al. discuss the Piedmont Aquifer System (PAS), which is a fingerlike area extending from Pennsylvania and New Jersey in the north to Georgia and Alabama in the south. It is a major aquifer in the eastern United States that follows the eastern foothills of the Appalachian Mountains. Lindsey et al. sampled wells and springs in the PAS as part of the USGS's National Water Quality Assessment Program.

In general, Lindsey et al. provide a positive report concerning groundwater contaminants in this aquifer. In the news release about the report, "Ground Water Meets Most Federal Standards in Major Eastern U.S. Aquifer" (December 20, 2006,, the USGA states:

Many chemicals were detected in ground water from selected areas of the Piedmont Aquifer System (PAS), but concentrations of those chemicals were below drinking-water standards in most cases. The findings in the PAS, based on samples from 255 wells and 19 springs, do not generally imply present human-health risk; however, they are an early warning that land-use activities have an effect on regional water quality. For example, concentrations of nitrate were significantly higher in ground water underlying agricultural land use than in ground water underlying undeveloped or urban land. Herbicides were detected more frequently in agricultural wells, whereas insecticides, VOCs [volatile organic compounds], chloroform, and MTBE [a fuel component in gasoline] were more frequently detected in urban wells.

Findings also show that rock settings can have a great effect on ground-water quality, particularly for radon, a natural product from the radioactive decay of uranium.

A list of drinking water contaminants, their sources, and their health effects is shown in Table 4.1. Some of the more common groundwater contaminants are described in the following sections.


Arsenic is a naturally occurring element in rocks and soils and is the twentieth most common element in the earth's crust. The presence of arsenic in groundwater is largely the result of minerals dissolving from naturally weathered rocks and soils. The USGS reports that the nation's groundwater typically contains less than one or two parts per billion (ppb) of arsenic. One ppb is equal to approximately one teaspoon of powdered arsenic in two Olympic-sized swimming pools. Moderate to high arsenic levels do occur in some areas throughout the nation due to geology, geochemistry, and climate. Elevated arsenic concentrations in groundwater are commonly found in the West and in parts of the Midwest and the Northeast.

Arsenic research shows that humans need arsenic as a trace element in their diet to survive. Too much arsenic, however, can be harmful. Paolo Boffetta and Fredrik Nyberg indicate in "Contribution of Environmental Factors to Cancer Risk" (British Medical Bulletin, 2003) that prolonged exposure to arsenic can contribute to skin, bladder, and other cancers. In January 2001 the EPA proposed lowering the current maximum contaminant level (MCL) for arsenic in drinking water from fifty ppb to ten ppb. The effective date of the new arsenic rule was February 22, 2002. Public water systems had to comply with the ten-ppb level by January 23, 2006.


Many scientists and geologists consider nitrates to be the most widespread groundwater contaminant. Nitrates are simply another form of nitrogen, a plant nutrient. Nitrogen and nitrates, as discussed in Chapter 3, enter bodies of water usually as runoff from fertilized land, leaking septic systems, or sewage discharges. Generally, a level of three ppb or more in groundwater is considered indicative of human impact.

Nitrate contamination occurs most frequently in shallow groundwater (less than one hundred feet below the surface) and in aquifers that allow the rapid movement of water. Regional differences in nitrate levels are related to soil drainage properties, other geologic characteristics, and agricultural practices. Nitrate in groundwater is generally highest in areas with well-drained soils and intensive cultivation of row crops, particularly corn, cotton, and vegetables. Low nitrate concentrations are found in areas of poorly drained soil and where pasture and woodland are intermixed with cropland. Crop fertilization is the most important agricultural practice for introducing nitrogen into groundwater. The primary source of nitrates is fertilizers used in agriculture and, in some areas, feedlot operations.

Nitrates are important because they affect both human and ecological health. They can cause a public health risk to infants and young livestock. In some areas of the country substantial amounts of nitrates in surface water are contributed by groundwater sources.


Robert J. Gilliom et al. report in The Quality of Our Nation's Water: Pesticides in the Nation's Streams and Ground Water, 19922001 (February 15, 2007, that pesticides are found less frequently in ground-water than in surface water. Nonetheless, pesticides and their broken-down products are found frequently in shallow groundwater, especially in residential and agricultural areas. However, the pesticides are rarely found in concentrations exceeding water quality benchmarks for human health.


Volatile organic compounds (VOCs) are those that contain the element carbon and tend to evaporate more quickly than water. Examples of substances that contain a variety of volatile organic compounds are gasoline, diesel fuel, paint, glue, spot removers, and cleaning solutions. VOCs are used extensively in industry to manufacture such products as cars, electronics, computers, adhesives, dyes, and plastics; they are also used in dry cleaning and refrigeration.

Drinking water contaminants, their sources, and potential health effects, 2003
Type Contaminant MCL or TTa (mg/L)b Potential health effects from exposure above the MCL Common sources of contaminant in drinking water Public health goal
OC Acrylamide TTh Nervous system or blood problems; increased risk of cancer treatment Added to water during sewage/wastewater zero
OC Alachlor 0.002 Eye, liver, kidney or spleen problems; anemia; increased risk of cancer Runoff from herbicide used on row crops zero
R Alpha particles 15 picocuries per liter (pCi/L) Increased risk of cancer Erosion of natural deposits of certain minerals that are radioactive and may emit a form of radiation known as alpha radiation zero
IOC Antimony 0.006 Increase in blood cholesterol; decrease in blood sugar Discharge from petroleum refineries; fire retardants; ceramics; electronics; solder 0.006
IOC Arsenic 0.010 as of 1/23/06 Skin damage or problems with circulatory systems, and may have increased risk of getting cancer Erosion of natural deposits; runoff from orchards, runoff from glass & electronics production wastes C
IOC Asbestos (fibers >10 micrometers) 7 million fibers per liter (MFL) Increased risk of developing benign intestinal polyps Decay of asbestos cement in water mains; erosion of natural deposits 7 MFL
OC Atrazine 0.003 Cardiovascular system or reproductive problems Runoff from herbicide used on row crops 0.003
IOC Barium 2 Increase in blood pressure Discharge of drilling wastes; discharge from metal refineries; erosion of natural deposits 2
OC Benzene 0.005 Anemia; decrease in blood platelets; increased risk of cancer Discharge from factories; leaching from gas storage tanks and landfills zero
OC Benzo(a)pyrene (PAHs) 0.0002 Reproductive difficulties; increased risk of cancer Leaching from linings of water storage tanks and distribution lines zero
IOC Beryllium 0.004 Intestinal lesions Discharge from metal refineries and coal-burning factories; discharge from electrical, aerospace, and defense industries 0.004
R Beta particles and photon emitters 4 millirems per year Increased risk of cancer Decay of natural and man-made deposits of certain minerals that are radioactive and may emit forms of radiation known as photons and beta radiation zero
DBP Bromate 0.010 Increased risk of cancer By-product of drinking water disinfection zero
IOC Cadmium 0.005 Kidney damage Corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints 0.005
OC Carbofuran 0.04 Problems with blood, nervous system, or reproductive system Leaching of soil fumigant used on rice and alfalfa 0.04
OC Carbon tetrachloride 0.005 Liver problems; increased risk of cancer Discharge from chemical plants and other industrial activities zero
D Chloramines (as Cl2) MRDL=4.0a Eye/nose irritation; stomach discomfort, anemia Water additive used to control microbes MRDLG=4a
OC Chlordane 0.002 Liver or nervous system problems; increased risk of cancer Residue of banned termiticide zero
D Chlorine (as CI2) MRDL=4.0a Eye/nose irritation; stomach discomfort Water additive used to control microbes MRDLG=4a
D Chlorine dioxide (as CIO2) MRDL = 0.8a Anemia; infants & young children: nervous system effects Water additive used to control microbes MRDLG = 0.8a
DBP Chlorite 1.0 Anemia; infants & young children: nervous system effects By-product of drinking water disinfection 0.8
OC Chlorobenzene 0.1 Liver or kidney problems Discharge from chemical and agricultural chemical factories 0.1
IOC Chromium (total) 0.1 Allergic dermatitis Discharge from steel and pulp mills; erosion of natural deposits 0.1
IOC Copper TTg; action level=1.3 Short-term exposure: gastrointestinal distress. Long-term exposure: liver or kidney damage. People with Wilson's Disease should consult their personal doctor if the amount of copper in their water exceeds the action level Corrosion of household plumbing systems; erosion of natural deposits 1.3
M Cryptosporidium TTc Gastrointestinal illness (e.g., diarrhea, vomiting, cramps) Human and animal fecal waste zero
IOC Cyanide (as free cyanide) 0.2 Nerve damage or thyroid problems Discharge from steel/metal factories; discharge from plastic and fertilizer factories
OC 2,4-D 0.07 Kidney, liver, or adrenal gland problems Runoff from herbicide used on row crops
OC Dalapon 0.2 Minor kidney changes Runoff from herbicide used on rights of way

VOCs can cause cancer, have adverse effects on various body organs and systems, and affect the brain, ears, eyes, skin, and throat. Groundwater contamination can occur from landfills, hazardous waste facilities, and septic systems into which VOCs have been discarded, or from sources such as leaking underground storage tanks. They can be released into the environment from industry, enter the atmosphere, and fall to the ground as atmospheric deposition. Some VOCs do not degrade quickly and can remain in groundwater for years and even decades. VOCs are of concern not only because they contaminate groundwater but also because their presence in groundwater signals that soil and other conditions favor VOCs reaching the groundwater.

Drinking water contaminants, their sources, and potential health effects, 2003 [continued]
Type Contaminant MCL or TTa (mg/L)b Potential health effects from exposure above the MCL Common sources of contaminant in drinking water Public health goal
OC 1,2-Dibromo-3-chloropropane (DBCP) 0.0002 Reproductive difficulties; increased risk of cancer Runoff/leaching from soil fumigant used on soybeans, cotton, pineapples, and orchards zero
OC o-Dichlorobenzene 0.6 Liver, kidney, or circulatory system problems Discharge from industrial chemical factories 0.6
OC p-Dichlorobenzene 0.075 Anemia; liver, kidney or spleen damage; changes in blood Discharge from industrial chemical factories 0.075
OC 1,2-Dichloroethane 0.005 Increased risk of cancer Discharge from industrial chemical factories zero
OC 1,1-Dichloroethylene 0.007 Liver problems Discharge from industrial chemical factories 0.007
OC cis-1,2-Dichloroethylene 0.07 Liver problems Discharge from industrial chemical factories 0.07
OC trans-1,2-Dichloroethylene 0.1 Liver problems Discharge from industrial chemical factories 0.1
OC Dichloromethane 0.005 Liver problems; increased risk of cancer Discharge from drug and chemical factories zero
OC 1,2-Dichloropropane 0.005 Increased risk of cancer Discharge from industrial chemical factories zero
OC Di(2-ethylhexyl) adipate 0.4 Weight loss, liver problems, or possible reproductive difficulties Discharge from chemical factories 0.4
OC Di(2-ethylhexyl) phthalate 0.006 Reproductive difficulties; liver problems; increased risk of cancer Discharge from rubber and chemical factories zero
OC Dinoseb 0.007 Reproductive difficulties Runoff from herbicide used on soybeans and vegetables 0.007
OC Dioxin (2,3,7,8-TCDD) 0.00000003 Reproductive difficulties; increased risk of cancer Emissions from waste incineration and other combustion; discharge from chemical factories zero
OC Diquat 0.02 Cataracts Runoff from herbicide use 0.02
OC Endothall 0.1 Stomach and intestinal problems Runoff from herbicide use 0.1
OC Endrin 0.002 Liver problems Residue of banned insecticide 0.002
OC Epichlorohydrin TTh Increased cancer risk, and over a long period of time, stomach problems Discharge from industrial chemical factories; an impurity of some water treatment chemicals zero
OC Ethylbenzene 0.7 Liver or kidneys problems Discharge from petroleum refineries 0.7
OC Ethylene dibromide 0.00005 Problems with liver, stomach, reproductive system, or kidneys; increased risk of cancer Discharge from petroleum refineries zero
IOC Fluoride 4.0 Bone disease (pain and tenderness of the bones); children may get mottled teeth Water additive which promotes strong teeth; erosion of natural deposits; discharge from fertilizer and aluminum factories 4.0
M Giardia lamblia TTc Gastrointestinal illness (e.g., diarrhea, vomiting, cramps) Human and animal fecal waste zero
OC Glyphosate 0.7 Kidney problems; reproductive difficulties Runoff from herbicide use 0.7
DBP Haloacetic acids (HAA5) 0.060 Increased risk of cancer By-product of drinking water disinfection n/af
OC Heptachlor 0.0004 Liver damage; increased risk of cancer Residue of banned termiticide zero
OC Heptachlor epoxide 0.0002 Liver damage; increased risk of cancer Breakdown of heptachlor zero
M Heterotrophic plate count (HPC) TTc HPC has no health effects; it is an analytic method used to measure the variety of bacteria that are common in water. The lower the concentration of bacteria in drinking water, the better maintained the water system is. HPC measures a range of bacteria that are naturally present in the environment n/a
OC Hexachlorobenzene 0.001 Liver or kidney problems; reproductive difficulties; increased risk of cancer Discharge from metal refineries and agricultural chemical factories zero
OC Hexachlorocyclopentadiene 0.05 Kidney or stomach problems Discharge from chemical factories 0.05
IOC Lead TTg; action level=0.015   Infants and children: delays in physical or mental development; children could show slight deficits in attention span and learning abilities; adults: kidney problems; high blood pressure   Corrosion of household plumbing systems; erosion of natural deposits zero
M Legionella TTc Legionnaire's Disease, a type of pneumonia Found naturally in water; multiplies in heating systems zero
OC Lindane 0.0002 Liver or kidney problems Runoff/leaching from insecticide used on cattle, lumber, gardens 0.0002
IOC Mercury (inorganic) 0.002 Kidney damage Erosion of natural deposits; discharge from refineries and factories; runoff from landfills and croplands 0.002
OC Methoxychlor 0.04 Reproductive difficulties Runoff/leaching from insecticide used on fruits, vegetables, alfalfa, livestock 0.04
Drinking water contaminants, their sources, and potential health effects, 2003 [continued]
Type Contaminant MCL or TTa (mg/L)b Potential health effects from exposure above the MCL Common sources of contaminant in drinking water Public health goal
IOC Nitrate (measured as nitrogen) 10 Infants below the age of six months who drink water containing nitrate in excess of the MCL could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue-baby syndrome. Runoff from fertilizer use; leaching from septic tanks, sewage; erosion of natural deposits 10
IOC Nitrite (measured as nitrogen) 1 Infants below the age of six months who drink water containing nitrite in excess of the MCL could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue-baby syndrome. Runoff from fertilizer use; leaching from septic tanks, sewage; erosion of natural deposits 1
OC Oxamyl (vydate) 0.2 Slight nervous system effects Runoff/leaching from insecticide used on apples, potatoes, and tomatoes 0.2
OC Pentachlorophenol 0.001 Liver or kidney problems; increased cancer risk Discharge from wood preserving factories zero
OC Picloram 0.5 Liver problems Herbicide runoff 0.5
OC Polychlorinated biphenyls (PCBs) 0.0005 Skin changes; thymus gland problems; immune deficiencies; reproductive or nervous system difficulties; increased risk of cancer Runoff from landfills; discharge of waste chemicals zero
R Radium 226 and radium 228 (combined) 5 pCi/L Increased risk of cancer Erosion of natural deposits zero
IOC Selenium 0.05 Hair or fingernail loss; numbness in fingers or toes; circulatory problems Discharge from petroleum refineries; erosion of natural deposits; discharge from mines 0.05
OC Simazine 0.004 Problems with blood Herbicide runoff 0.004
OC Styrene 0.1 Liver, kidney, or circulatory system problems Discharge from rubber and plastic factories; leaching from landfills 0.1
OC Tetrachloroethylene 0.005 Liver problems; increased risk of cancer Discharge from factories and dry cleaners zero
IOC Thallium 0.002 Hair loss; changes in blood; kidney, intestine, or liver problems Leaching from ore-processing sites; discharge from electronics, glass, and drug factories 0.0005
OC Toluene 1 Nervous system, kidney, or liver problems Discharge from petroleum factories 1
M Total coliforms (including fecal coliform and E. coli) 5.0%d Not a health threat in itself; it is used to indicate whether other potentially harmful bacteria may be presente Coliforms are naturally present in the environment as well as feces; fecal coliforms and E. coli only come from human and animal fecal waste. zero
DBP Total trihalomethanes (TTHMs) 0.10 0.080 after 12/31/03 Liver, kidney, or central nervous system problems; increased risk of cancer By-product of drinking water disinfection n/af
OC Toxaphene 0.003 Kidney, liver, or thyroid problems; increased risk of cancer Runoff/leaching from insecticide used on cotton and cattle zero
OC 2,4,5-TP (silvex) 0.05 Liver problems Residue of banned herbicide 0.05
OC 1,2,4-Trichlorobenzene 0.07 Changes in adrenal glands Discharge from textile finishing factories 0.07
OC 1,1,1-Trichloroethane 0.2 Liver, nervous system, or circulatory problems Discharge from metal degreasing sites and other factories 0.20
OC 1,1,2-Trichloroethane 0.005 Liver, kidney, or immune system problems Discharge from industrial chemical factories 0.003
OC Trichloroethylene 0.005 Liver problems; increased risk of cancer Discharge from metal degreasing sites and other factories zero
M Turbidity TTc Turbidity is a measure of the cloudiness of water. It is used to indicate water quality and filtration effectiveness (e.g., whether diseasecausing organisms are present). Higher turbidity levels are often associated with higher levels of disease-causing micro-organisms such as viruses, parasites, and some bacteria. These organisms can cause symptoms such as nausea, cramps, diarrhea, and associated headaches. Soil runoff n/a
R Uranium 30 ug/L as of 12/08/03 Increased risk of cancer, kidney toxicity Erosion of natural deposits zero
OC Vinyl chloride 0.002 Increased risk of cancer Leaching from PVC pipes; discharge from plastic factories zero

According to Zogorski et al., the most frequently detected groups of VOCs in aquifers are the trihalomethanes and organic solvents. Trihalomethanes are VOCs that are used as solvents and in refrigeration. Organic solvents are substances containing carbon that dissolve other substances. They include chloroform and alcohol but do not include water.

Drinking water contaminants, their sources, and potential health effects, 2003 [continued]
Type Contaminant MCL or TTa (mg/L)b Potential health effects from exposure above the MCL Common sources of contaminant in drinking water Public health goal
Type Legend:
D = Disinfectant
DBP = Disinfection by product
OC = Inorganic chemical
M = Microorganism
OC = Organic chemical
R = Radionuclides
  • Maximum Contaminant Level Goal (MCLG)The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety and are non-enforceable public health goals.
  • Maximum Contaminant Level (MCL)The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to MCLGs as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable standards.
  • Maximum Residual Disinfectant Level Goal (MRDLG)The level of a drinking water disinfectant below which there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control microbial contaminants.
  • Maximum Residual Disinfectant Level (MRDL)The highest level of a disinfectant allowed in drinking water. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.
  • Treatment Technique (TT)A required process intended to reduce the level of a contaminant in drinking water.
bUnits are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million (ppm).
cEPA's surface water treatment rules require systems using surface water or ground water under the direct influence of surface water to (1) disinfect their water, and (2) filter their water or meet criteria for avoiding filtration so that the following contaminants are controlled at the following levels:
  • Cryptosporidium (as of 1/1/02 for systems serving >10,000 and 1/14/05 for systems serving <10,000) 99% removal.
  • Giardia lamblia: 99.9% removal/inactivation.
  • Viruses: 99.99% removal/inactivation.
  • Legionella: No limit, but EPA believes that if Giardia and viruses are removed/inactivated, Legionella will also be controlled.
  • Turbidity: At no time can turbidity (cloudiness of water) go above 5 nephelolometric turbidity units (NTU); systems that filter must ensure that the turbidity go no higher than 1 NTU (0.5 NTU for conventional or direct filtration) in at least 95% of the daily samples in any month. As of January 1, 2002, for systems servicing >10,000, and January 14, 2005, for systems servicing <10,000, turbidity may never exceed 1 NTU, and must not exceed 0.3 NTU in 95% of daily samples in any month.
  • HPC: No more than 500 bacterial colonies per milliliter.
  • Long Term 1 Enhanced Surface Water Treatment (Effective Date: January 14, 2005); Surface water systems or (GWUDI) systems serving fewer than 10,000 people must comply with the applicable Long Term 1 Enhanced Surface Water Treatment Rule provisions (e.g. turbidity standards, individual filter monitoring, Cryptosporidium removal requirements, updated watershed control requirements for unfiltered systems).
  • Filter Backwash Recycling: The Filter Backwash Recycling Rule requires systems that recycle to return specific recycle flows through all processes of the system's existing conventional or direct filtration system or at an alternate location approved by the state.
dNo more than 5.0% samples total coliform-positive in a month. (For water systems that collect fewer than 40 routine samples per month, no more than one sample can be total coliform-positive per month.) Every sample that has total coliform must be analyzed for either fecal coliforms or E. coli if two consecutive TC-positive samples, and one is also positive for E. coli fecal coliforms, system has an acute MCL violation.
eFecal coliform and E. coli are bacteria whose presence indicates that the water may be contaminated with human or animal wastes. Disease-causing microbes (pathogens) in these wastes can cause diarrhea, cramps, nausea, headaches, or other symptoms. These pathogens may pose a special health risk for infants, young children, and people with severely compromised immune systems.
fAlthough there is no collective MCLG for this contaminant group, there are individual MCLGs for some of the individual contaminants
  • Haloacetic acids: dichloroacetic acid (zero); trichloroacetic acid (0.3 mg/L)
  • Trihalomethanes: bromodichloromethane (zero); bromoform (zero); dibromochloromethane (0.06 mg/L)
gLead and copper are regulated by a Treatment Technique that requires systems to control the corrosiveness of their water. If more than 10% of tap water samples exceed the action level, water systems must take additional steps. For copper, the action level is 1.3 mg/L, and for lead is 0.015 mg/L.
hEach water system must certify, in writing, to the state (using third-party or manufacturers certification) that when it uses acrylamide and/or epichlorohydrin to treat water, the combination (or product) of dose and monomer level does not exceed the levels specified, as follows: Acrylamide=0.05% dosed at 1 mg/L (or equivalent); Epichlorohydrin=0.01% dosed at 20 mg/L (or equivalent).
Source: "EPA National Primary Drinking Water Standards," U.S. Environmental Protection Agency, Office of Water, June 2003, (accessed January 5, 2007)
M Viruses (enteric) TTc Gastrointestinal illness (e.g., diarrhea, vomiting, cramps) Human and animal fecal waste zero
OC Xylenes (total) 10 Nervous system damage Discharge from petroleum factories; discharge from chemical factories 10

Zogorski et al. note that VOCs are detected frequently in domestic and public wells, but that only 1% to 2% of the samples taken from these wells had VOC concentrations of potential human-health concern.

Sources of Groundwater Contaminants

In 2000 the EPA requested that states identify the major sources that potentially threaten groundwater in each state. Figure 4.9 shows the results of that survey. Thirty-nine states rated USTs as the most serious threat to their groundwater quality. Septic systems, landfills, industrial facilities, agriculture, and pesticides were also important contamination sources.


Leaking underground storage tanks (LUSTs) have been identified by the EPA as the leading source of groundwater contamination since the mid-1990s and were cited as such in the 2000 National Water Quality Inventory and as a source of VOC groundwater contamination by Zogorski et al.

In general, most underground storage tanks (USTs) are found at commercial and industrial facilities in the more heavily developed urban and suburban areas. USTs are used to store gasoline, hazardous and toxic chemicals, and diluted wastes. Gasoline leaking from UST systems at service stations is one of the most common causes of groundwater contamination. The primary causes of leakage in USTs are faulty installation and corrosion of tanks and pipelines.

At one time, USTs were made of steel, which eventually rusted and disintegrated, releasing their contents into the soil. This led to the discovery that a contaminant in the ground is likely to become a contaminant of groundwater. The Sierra Club reports in the news release "Leaking Underground Storage Tanks Continue to Contaminate Groundwater" (April 19, 2005, that one gallon of gasoline can contaminate one million gallons of water. The fuel additive methyl tertiary butyl ether (MTBE), which is a VOC, is particularly troublesome because it migrates quickly through soils into groundwater, and small amounts can render groundwater undrinkable. Figure 4.10 shows how groundwater can be contaminated by LUSTs.

In 1986 Subtitle I of the Solid Waste Disposal Act created the Underground Storage Tank (UST) Program under the management of the EPA. In 1988 the EPA issued "comprehensive and stringent" rules that required devices to detect leaks, modification of tanks to prevent corrosion, regular monitoring, and immediate cleanup of leaks and spills. By December 1998 existing tanks had to be upgraded to meet those standards, replaced with new tanks, or closed. Existing tanks were to be replaced with expensive tanks made of durable, noncorrosive materials.

In testimony before the Subcommittee on Environment and Hazardous Materials, Committee on Energy and Commerce, U.S. House of Representatives, John Stephenson (March 5, 2003,, the director of the Natural Resources and Environment, stated that as of December 2002 at least 19% to 26% of states still had problems with LUSTs. Although 89% of the 693,107 tanks subject to UST rules had leak prevention and detection equipment installed, more than 200,000 tanks were not being operated or maintained properly. The states reported that because of inadequate operation and maintenance of the leak detection equipment, even those tanks with the new equipment continued to leak. To address the problems, Stephenson recommended that Congress provide states more funds from the UST federal trust fund that was created in 1986 to ensure improved training, inspections, and enforcement efforts. In addition, Stephenson suggested that Congress require the states to inspect tanks at least every three years and provide the EPA and the states with additional enforcement authorities.

UST owners and operators must also meet financial responsibility requirements that ensure they will have the resources to pay for costs associated with cleaning up releases and compensating third parties. Many states provide financial assurance funds that help UST owners meet the financial requirements.

As of the end of 2004, 447,233 releases of contaminants from corroded USTs had been confirmed. (See Figure 4.11.) The EPA estimates in the fact sheet Cleaning Up Leaks from Underground Storage Tanks (February 2005, that about half of these releases reached groundwater. According to the EPA, 92% (412,567) of cleanups had been initiated, 71% (317,405) had been completed, and 29% (129,827) remained in the national backlog of cleanups in 2004.

In Environmental Protection: More Complete Data on Continued Emphasis on Leak Prevention Could Improve EPA's Underground Storage Tank Program (November 2005,, the U.S. Government Accountability Office (GAO) notes that as of early 2005 cleanup had not yet begun on over thirty-two thousand LUSTs. The GAO suggests that the EPA's UST Program could benefit from more specific data on abandoned tanks and that the EPA obtain data on USTs that the states compile.

On August 8, 2005, President George W. Bush signed the Energy Policy Act of 2005, which contained amendments to Subtitle I of the Solid Waste Disposal Act, the original legislation that created the UST Program. These amendments were titled the Underground Storage Tank Compliance Act of 2005, required major changes to the UST Program, and were aimed at preventing releases from USTs. The new legislation also expanded eligible uses of the LUST Trust Fund and included provisions regarding inspections, containment, financial responsibility, and cleanup.


In 2000 septic systems and landfills were the second and third largest sources of groundwater contamination, respectively. (See Figure 4.9.) Landfills are areas set aside for disposal of garbage, trash, and other municipal wastes. Early environmental regulation aimed at reducing air and surface water pollution called for disposing of solid wastesincluding industrial wastesunderground and gave little consideration to the potential for groundwater contamination. Landfills were generally situated on land considered to have no other use. Many of the disposal sites were nothing more than large holes in the ground, abandoned gravel pits, old strip mines, marshlands, and sinkholes.

The leachate (the liquid that percolates through the waste materials) from landfills contains contaminants that can easily pollute groundwater when disposal areas are not properly lined. Landfills built and operated before the passage of the 1976 Resource Conservation and Recovery Act (RCRA; also known as the Solid Waste Disposal Act) are believed to represent the greatest risk. The RCRA was enacted to protect human health and the environment by establishing a regulatory framework to investigate and address past, present, and future environmental contamination of groundwater and other media. The adoption of these new standards in 1976 forced many landfills to close, as they could not meet the RCRA's safety standards, but in many cases the garbage dumped in them while in operation remains in place and is a threat to groundwater.

Surface impoundments are the industrial equivalent of landfills for liquids and usually consist of human-made pits, lagoons, and ponds that receive treated or untreated wastes directly from the discharge point. They may also be used to store chemicals for later use, to wash or treat ores, or to treat water for further use. Most are small, less than one acre, but some industrial and mining impoundments may be as large as a thousand acres.

Before the RCRA most impoundments were not lined with a synthetic or impermeable natural material, such as clay, to prevent liquids from leaching into the ground. This is particularly important because impoundments are often located over aquifers that are used as sources of drinking water and that may discharge into nearby surface water. Aquifers located under nonlined impoundments are vulnerable to contamination.

Since the passage of the RCRA, landfills and surface impoundments have been required to adhere to increasingly stringent regulations for site selection, construction, operation, and groundwater monitoring to avoid contaminating groundwater. Prevention of groundwater contamination is largely the responsibility of state and local government. Examples of the more stringent requirements are landfill liners and groundwater monitoring.

Figure 4.12 shows the positioning of a lined landfill in the unsaturated zone. Groundwater monitoring is accomplished by sampling the water in an upgradient well to assess the quality of the groundwater before it passes the landfill site. Downgradient wells are used to assess the quality of groundwater at various levels after it flows past the landfill site.


In general, hazardous wastes are substances with the potential to harm human health and the environment. Hazardous waste is an unavoidable byproduct of an industrial society, because many chemicals are used to manufacture goods. Hazardous waste generators can be large industries, such as automobile manufacturers, or small neighborhood businesses, such as the local photo shop. Although the quantity of hazardous waste can be reduced through innovation and good management, it is impossible to eliminate all hazardous residue because of the demand for goods.

According to the National Solid Wastes Management Association (November 8, 2006,, 30.2 million tons of RCRA hazardous waste were generated in 2003. Texas generated the most at 22% of the total hazardous waste, followed by Louisiana (15%) and Kentucky (8%).

Contamination of groundwater with hazardous waste is frequently the result of historic indiscriminate waste disposal in landfills, impoundments, and dumps. Sites that handle hazardous waste or a mix of hazardous and nonhazardous waste are subject to strict controls.

When a waste site is found to be so badly contaminated with hazardous waste that it represents a serious threat to human health (e.g., contamination of groundwater used for drinking with known carcinogenscancer-causing agents), it is placed on the National Priorities List, which was established by the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CER-CLA; commonly known as the Superfund). Sites placed on the Superfund list are eligible for federal intervention and cleanup assistance. The EPA reports in "NPL Site Totals by Status and Milestone" ( that as of March 2007, 1,243 sites were listed on the National Priorities List. Most were general sites such as industrial and municipal landfills and military bases.


An injection well is any bored, drilled, driven shaft, or dug hole that is deeper than it is wide that is used for the disposal of waste underground. In "Classes of Injection Wells" (February 28, 2006,, the EPA identifies five classes of injection wells of the Underground Injection Control (UIC) Program:

  • Class I wells are used to inject hazardous and nonhazardous waste beneath the lowest formation containing an underground source of drinking water (USDW) within a quarter mile of the well bore.
  • Class II wells are used to inject fluids associated with oil and natural gas recovery and storage of liquid hydrocarbons.
  • Class III wells are used in connection with the solution mining of minerals that are not conventionally mined.
  • Class IV wells are used to inject hazardous or radioactive waste into or above a formation that is within a quarter mile of a USDW.
  • Class V wells are injection wells not included in Classes I through IV.

Each well class can contaminate groundwater. Classes I through IV have specific regulations and are closely monitored. Class V wells are typically shallow wells used to place a variety of fluids underground.

The EPA reports in "Shallow Injection Wells (Class V)" (November 24, 2006, that in 2006 there were more than five hundred thousand Class V injection wells in the United States. These wells are found in every state, especially in unsewered areas. There are many types of Class V wells, including large-capacity cesspools, motor vehicle waste disposal systems, storm water drainage wells, large-capacity septic systems, aquifer remediation wells, and many others. The waste entering these wells is not treated. Certain types of these wells have great potential to have high concentrations of contaminants that might endanger groundwater.

Class V injection wells are regulated by the UIC Program under the authority of the Safe Drinking Water Act (SDWA). Class V wells are "authorized by rule," which means that they do not require a permit if they comply with UIC Program requirements and do not endanger underground sources of drinking water. In December 1999 the EPA adopted regulations addressing Class V wells that were large-capacity cesspools and motor vehicle waste disposal wells. Under these regulations:

  • New cesspools were prohibited as of April 2000.
  • Existing cesspools had to be phased out by April 2005.
  • New motor vehicle waste disposal wells were prohibited.
  • Existing wells in regulated areas were to be phased out in groundwater protection areas identified in state source water assessment programs.


As in surface water contamination, agricultural practices play a major role in groundwater contamination. Agricultural practices that have the potential to contaminate groundwater include fertilizer and pesticide applications, animal feedlots, irrigation practices, agricultural chemical facilities, and drainage wells. Contamination can result from routine applications, spillage or misuse of pesticides and fertilizers during handling and storage, manure storage and spreading, improper storage of chemicals, irrigation practices, and irrigation return drains serving as direct conduits to groundwater. Fields with overapplied or misapplied fertilizer and pesticides can introduce nitrogen, pesticides, and other contaminants into groundwater. Animal feedlots often have impoundments from which wastes (bacteria, nitrates, and total and dissolved solids) may infiltrate groundwater.

Human-induced salinity in groundwater also occurs in agricultural regions where irrigation is used extensively. Irrigation water continually flushes nitrate-related compounds from fertilizers into the shallow aquifers along with high levels of chloride, sodium, and several types of metals. This increases the salinity (dissolved solids) of the underlying aquifers. Overpumping can diminish the water in aquifers to the point where saltwater from nearby coastal areas will intrude into the aquifer. Salinas Valley, California, is an example of the occurrence of saltwater intrusion. Eleven states identified saltwater intrusion as a major source of groundwater contamination in their 2000 305(b) reports to the EPA. (See Figure 4.9.)


Septic systems were cited as the second most common source of groundwater contamination by thirty-one reporting states. (See Figure 4.9.) Septic systems are on-site waste disposal systems that are used where public sewerage is not available. Septic tanks are used to detain domestic wastes to allow the settling and digestion of solids before the distribution of liquid wastes into permeable leach beds for absorption into soil. Wastewater is digested in the leach beds by organisms in the soil and broken down over time.

According to the EPA (September 20, 2006,, the U.S. Census Bureau reports that approximately twenty-six million Americans use individual sewage disposal systems. The EPA notes that the use of septic systems varies across the country from about 55% of the population in Vermont to around 10% in California. More than sixty million people nationwide live in homes with septic systems. Improperly constructed and poorly maintained septic systems may cause substantial and widespread contamination to groundwater of nitrogen and disease-causing microbes.


Cleaning up the nation's groundwater is expensive. The costs associated with alternative water supplies, water treatment, and contaminant source removal or remediation are in the millions per site. However, the GAO (May 14, 2004, reports that "the net Superfund program appropriations decreased from $1,757 million to $1,242 million, in constant 2003 dollars, from fiscal year 1993 to fiscal year 2004."

In allocating limited resources, cleanup decisions are based on a cost-benefit analysis that considers such factors as the extent of the problem, the potential health effects, and the alternatives, if any. If the pollution is localized, it may be more practical to simply shut down the contaminated wells and find water elsewhere. Cleanup options range from capping a section of an aquifer with a layer of impermeable clay to prevent more pollution, to more complex (and expensive) methods, such as pumping out and treating the water and then returning it to the aquifer.


States' Role

Prevention of groundwater contamination is largely the responsibility of state and local government. In 1991 the EPA established a national groundwater protection strategy to place greater emphasis on comprehensive state management of groundwater resources. The EPA recognized that the wide range of land-use practices that can adversely affect groundwater quality is most effectively managed at the state and local levels. The states use three basic approaches to protect groundwater and address the problems of contaminants and contamination sources:

  • Nondegradation policies that are designed to protect groundwater quality at its existing level.
  • Limited degradation policies that involve setting up water quality standards to protect groundwater. These standards set maximum contamination levels for chemicals and bacteria and establish guidelines for taste, odor, and color of the water.
  • Groundwater classification systems that are similar to the classification systems for surface waters established under the Clean Water Act and its amendments.

These classification systems are used by state officials to determine which aquifers should receive higher or lower priorities for protection and cleanup. High-priority areas include recharge areas, which affect large quantities of water, or public water supplies, where pollution affects drinking water.

The most important benefit derived from comprehensive groundwater management approaches is the ability to establish coordinated priorities among the many groups involved in groundwater management. The following key components are common to successful state programs:

  • Enacting legislation
  • Publicly announcing protection regulations
  • Establishing interagency coordination with surface water and other programs
  • Performing groundwater mapping and classification
  • Monitoring groundwater quality
  • Developing comprehensive data management systems
  • Adopting and implementing prevention and remediation programs

Federal Role

Federal laws, regulations, and programs since the 1970s have reflected the growing recognition of the need to protect the nation's groundwater and use it wisely. Table 4.2 summarizes major federal legislation affecting groundwater. The Federal Water Pollution Control Act (Clean Water Act) in 1972 and the SDWA in 1974 began the federal role in groundwater protection. The passage of the RCRA in 1976 and CERCLA (or Superfund) in 1980 cemented the federal government's current focus on groundwater remediation. Since the passage of these acts, the federal government has directed billions of dollars in private and public money and resources toward the cleanup of contaminated groundwater at Superfund, RCRA corrective action facilities, and LUSTs.


Federal laws administered by the EPA affecting groundwater

Clean Water Act (CWA)

Groundwater protection is addressed in Section 102 of the CWA, providing for the development of federal, state, and local comprehensive programs for reducing, eliminating, and preventing ground water contamination.

Safe Drinking Water Act (SDWA)

Under the SDWA, EPA is authorized to ensure that water is safe for human consumption. To support this effort, SDWA gives EPA the authority to promulgate Maximum Contaminant Levels (MCLs) that define safe levels for some contaminants in public drinking water supplies. One of the most fundamental ways to ensure consistently safe drinking water is to protect the source of that water (i.e., groundwater). Source water protection is achieved through four programs: the Wellhead Protection Program (WHP), the Sole Source Aquifer Program, the Underground Injection Control (UIC) Program, and, under the 1996 Amendments, the Source Water Assessment Program.

Resource Conservation and Recovery Act (RCRA)

The intent of RCRA is to protect human health and the environment by establishing a comprehensive regulatory framework for investigating and addressing past, present, and future environmental contamination or groundwater and other environmental media. In addition, management of underground storage tanks is also addressed under RCRA.

Comprehensive Environmental, Response, Compensation, and Liability Act (CERCLA)

CERCLA provides a federal "superfund" to clean-up soil and ground water contaminated by uncontrolled or abandoned hazardous waste sites as well as accidents, spill, and other emergency releases of pollutants and contaminants into the environment. Through the Act, EPA was given power to seek out those parties responsible for any release and assure their cooperation in the clean-up. The program is designed to recover costs, when possible, from financially viable individuals and companies when the clean-up is complete.

Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA)

FIFRA protects human health and the environment from the risks of pesticide use by requiring the testing and registration of all chemicals used as active ingredients of pesticides and pesticide products. Under the Pesticide Management Program, states and tribes wishing to continue use of chemicals of concern are required to prepare a prevention plan that targets specific areas vulnerable to ground water contamination.

Superfund Amendments and Reauthorization Act (SARA)

SARA reauthorized CERCLA in 1986 to continue cleanup activities around the country Several site-specific amendments, definitions clarifications, and technical requirements were added to the legislation, including additional enforcement authorities.

source: Adapted from "Federal Laws Administered by EPA Affecting Ground Water," in Safe Drinking Water Act, Section 1429 Ground Water Report to Congress, U.S. Environmental Protection Agency, Office of Water, October 1999, (accessed January 8, 2007)

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Water that occurs below the ground and is brought to the land surface by wells or springs is referred to as groundwater. Groundwater is a significant part of the hydrologic cycle, containing 21 percent of Earth's freshwater. Groundwater comprises 97 percent of fresh water not tied up as ice and snow in polar ice sheets, glaciers , and snowfields. This greatly exceeds the amount of water in streams, rivers, and lakes.

Groundwater is critically important in supplying water to streams and wetlands, and in providing water for irrigation , manufacturing, and other uses. In the United States, 80 to 90 percent of available fresh water comes from groundwater. Dependence on this resource will continue to grow, particularly in areas where surface water is limited or has already been fully allocated for existing uses, including instream requirements (and particularly ecosystem needs).

Groundwater once was thought to be of unlimited quantity and naturally protected by the soils above it. It is, however, significantly vulnerable to overuse and the improper use and disposal of chemicals at the land surface. The proper use and protection of this resource requires an understanding of how the groundwater system works. In particular, water managers need to know what groundwater is, where it comes from, how it occurs in the subsurface, and how it moves below the ground.

Groundwater Origin and Occurrence

Groundwater is water that occurs below the Earth's surface at depths where all the pore (open) spaces in the soil, sediment , or rock are completely filled with water (see Figure 1). All groundwater, whether from a shallow well or a deep well , originates and is replenished (recharged) by precipitation.

Groundwater is part of the hydrologic cycle, originating when part of the precipitation that falls on the Earth's surface sinks (infiltrates) through the soil and percolates (seeps) downward to become groundwater. Groundwater will eventually come back to the surface, discharging to streams, springs, lakes, or the oceans, to complete the hydrologic cycle.

Groundwater Zones.

A well that is drilled will first pass through a zone called the unsaturated (vadose) zone where the openings in the soil, sediment, or rock are primarily filled with air (Figure 1). Water exists here only in transit downward. The thickness of this zone depends on such factors as climate, elevation, season of the year, and area-wide groundwater withdrawals through pumping. In the rainy season of humid areas, the unsaturated zone may be a fairly thin layer, extending from the land surface to only a few meters (10 feet or so) below the surface. But in drier months of the year, the unsaturated zone may extend deeper as recharge to the aquifer declines and withdrawals increase. In arid regions, the unsaturated zone may be a thick layer, extending from the land surface to 300 meters (1,000 feet) or more below it.

Further drilling will reach a zone called the saturated zone where all of the openings are filled with water, and where the water is known as groundwater. If the saturated zone is permeable enough to supply a well with water under normal hydraulic gradients, this saturated zone is called an aquifer. Importantly, an aquifer is not an underground river, lake, or pool. Rather it consists of geologic materials whose open spaces (pore spaces) are filled with water that moves down a pressure gradient, and which can be tapped productively by wells. The top of the saturated zone is called either the water table (if the aquifer is unconfined) or the potentiometric surface (if the aquifer is confined): see Figure 2.

To visualize the zones, imagine a bucket filled with gravel. Ample pore space exists between the individual pieces of gravel. If water is poured on top of the gravel, the water will percolate down through the pore spaces and begin to fill these spaces from the bottom up. The water in pore spaces at the bottom of the bucket represents groundwater; that is, all the pore spaces are filled with water. If holes were punched in the bottom of the bucket, water would flow out. Using this analogy, the bucket of gravel is like an aquifer: water is stored within in it and will move through it toward a discharge pointin this case, the hole in the bucket.

Permeability is determined by the size of pores and the degree to which they are interconnected, and hence, the ease by which water can flow through the material. Highly permeable aquifers, such as those comprised primarily of coarse sand and gravel, can supply more water than less permeable aquifers, such as those comprised of silts or clays. In this example, the pores in sand and gravel are larger than those in silt and clay, so water moves through sand and gravel more quickly. In some aquifers, especially in sedimentary bedrock, water occurs in fractures (cracks) instead of pore spaces in sediments. The yield from a fractured rock aquifer can vary from less than 1 liter per minute, or about 0.3 gallons per minute (if the well encounters few fractures) to large quantities of groundwaterfor example, more than 300 liters per minute, or about 100 gallons per minute (if the fractures are numerous and large).

Aquifer and Well Types.

Aquifers are divided into two types: unconfined and confined. An unconfined aquifer is often shallow, and the vadose zone above it primarily contains permeable material. The top of the aquifer is the water table. The water table moves up and down on a seasonal basis. It is highest during the wet season owing to higher recharge and lower pumping rates (e.g., no irrigation), and lowest during the dry season because of limited recharge and higher use (e.g., a high rate of irrigation).

Confined aquifers may be shallow or deep, and are characterized by being separated from the surface by low-permeability strata (e.g., geologic layers) that confines the groundwater below it. In a confined aquifer, groundwater is generally under pressure. This water pressure may vary seasonally, similar to the water table in an unconfined aquifer.

Because groundwater in a confined aquifer is under pressure, it will rise in a well bore above the level of the aquifer penetrated by that well. One way to visualize this is to squeeze a milk or juice pouch that is punctured at the top by a straw. If the straw fits firmly into the squeezed pouch, the liquid will rise up into the straw, above its level inside the pouch.

Artesian and flowing artesian wells are typical of wells drilled into confined aquifers. An artesian well is one in which the groundwater rises above the level of the penetrated aquifer. The water in an artesian well will rise to an elevation at which the pressure of the water in the aquifer is matched by the pressure reflected by the elevation of the water in the well; this level is known as the hydrostatic level. If groundwater reaches all the way to the surface under its own pressure, the well is called a flowing artesian well (see Figure 2).

Groundwater Movement

The water table (or the potentiometric surface of a confined aquifer) is not a flat surface: rather, there are high areas and low areas just like the hills and valleys found on land. Just as surface water tends to flow downhill, groundwater tends to move downgradient from water-table areas (or potentiometric regions) of higher elevation to water-table areas (or potentiometric regions) of lower elevation.

Normally, but not exclusively, the higher water-table areas of uncon-fined aquifers coincide with higher elevation at the land surface, and the lower water-table areas coincide with low areas. As a result, groundwater in unconfined aquifers tends to flow towards, and discharge to, streams, lakes, and wetlands, because these waterbodies often occur in low points of the watershed. Even groundwater from confined aquifers tends to discharge to larger area-wide rivers.

Two other common discharge areas for groundwater are springs and wells. A spring is an area where groundwater has access to the land surface. In some cases, precipitation infiltrating downward from the ground surface encounters a relatively impermeable rock or sediment layer as it moves down toward the underlying aquifer. The groundwater, which cannot pass through the low-permeability layer, moves along the top of the layer until the layer is exposed at the ground surface and the water can emerge as a spring. In this typical "gravity spring," the most common form of spring, gravity is the driving force for water movement. Such springs commonly occur at the side of a hill, or at an outcrop such as a bluff or canyon wall (see the small figure below). In other cases, fractures allow groundwater to move from the aquifer to the surface. Groundwater from a spring can issue onto the land surface, or directly into a stream, lake, or ocean.

A well also provides a connection between groundwater and the land surface. In general, a pump is used to draw the groundwater up to the land surface where it can then be used.

Flow Rate.

When referring to how fast surface water moves, hydrologists generally talk in terms of either meters or feet per second. Groundwater moves much more slowly than water in streams, often at rates of only a few centimeters (inches) per day. Groundwater velocity is controlled by the permeability of the aquifer and steepness of the water table (or potentiometric surface). The more permeable the aquifer and the steeper the the slope of the water table or potentiometric surface (i.e., the pressure gradient), the faster groundwater moves. In highly permeable gravels or in fractures, groundwater may move 10 meters (33 feet) per day or more.


Recharge, or replenishment, of an unconfined aquifer occurs at the ground surface directly above the aquifer. In contrast, recharge to a confined aquifer may occur many miles away, typically at a higher elevation where the aquifer is no longer confined; that is, where the overlying materials are permeable and allow percolating rainfall to reach the confined aquifer (See "Recharge to Confined Aquifer" in Figure 2). Once recharged, the groundwater flows downgradient to where the aquifer is confined.

Pumping and Overpumping

Wells supply water by being drilled to a depth below the water table of an unconfined aquifer or into a confined aquifer. When a well is pumping, it lowers the water table (or the potentiometric surface) around it. This causes groundwater to move towards the well, supplying the water that is being pumped out of the well. As long as the pump intake is below the water level in the well, water can be pumped.

Well Interference.

Well interference occurs when the water table (or potentiometric level) in one well is significantly lowered as a result of the pumping of a nearby well. Sometimes the interference can be so great that the water table (or potentiometric level) in the first well is lowered below the pump intakes or even below the bottom of the well itself.

How can wells interfere with one another? When pumping begins, the water table (or potentiometric surface) around the well begins to drop as the aquifer supplies water to the well. This drop in water levels is called drawdown. The area of drawdown is shaped like a three-dimensional, funnel-shaped cone, centered on the well. Pumping rate and aquifer characteristic determine the depth of the drawdown cone and the areal extent (horizontal) of the drawdown.

The following illustration represents the water table as the top of the lower cube; the aquifer as the lower cube itself; and the well as a pipe extending from the land surface (top of the upper cube) down to the aquifer, below the water table. The drawdown cone, also known as a cone of depression, is indicated.

How deep and how wide the drawdown cone is depends on aquifer characteristics and how much water is being pumped. Interference occurs when the drawdown cone from one well reaches another well and lowers the water level in that well. Consider the following two examples of well interference in a water-table aquifer.

First, suppose a high-capacity irrigation well (Well 1 on page 155) is drilled near two shallow private drinking-water wells (Well 2 and Well 3). High pumping volumes from the irrigation well may lower the water table from its original level to a new (lower) level, which is at a depth below the private wells. In this example, Well 2 would be unable to pump water while the irrigation well (Well 1) is pumping. Well 3 barely penetrates the lowered water table, and would likely experience intermittent pumping failures if the irrigation well were also pumping. In other words, the irrigation well pumpage interferes with the water production from both Well 2 and Well 3.

As another example, suppose a city wants to increase its water supply. A new well (Well B below) is drilled very close to an existing well (Well A) that is highly productive. When both wells are pumping, they mutually interfere with one another, and the production of both wells is lowered. Because of this mutual interference, the combined production of the two wells may be only slightly greater than the production of each well by itself.

In the diagram below, the drawdown associated with Well A by itself is shown as a heavy dashed line. Well B pumping by itself would produce a similar drawdown around itself. Note that the amount of water available to Well A (from the bottom of the cone to the bottom of the well) has been substantially reduced by the placement of Well B too close to Well A. Neither Well A or Well B can now produce as much as if they were pumping by themselves.

In most states, laws prohibit significant well interference. A well cannot be operated in such a manner to significantly reduce the production from previously existing wells. The potential of well interference can be calculated if hydrologists know the permeability, thickness and storage characteristics of the aquifer and at what rate the well(s) in question will be pumping.

Groundwater Mining.

Excessive pumping of water from an aquifer may result in an areawide lowering of the water table. This will eventually occur anywhere more water is pumped than is recharged by infiltrating precipitation. Overdrafting an aquifer can result in changes in groundwater quality, a reduction in groundwater availability (and hence the loss of water supply to current and future wells), and perhaps even a permanent loss of the aquifer's capacity to store water. Many states use the water right process to manage groundwater quantity and to ensure that overdrafting does not occur.


There is a limit to how much groundwater can be pumped out of an aquifer without causing depletion of the resource. If more groundwater is pumped out than is naturally recharged by precipitation, the amount of water stored in the aquifer will decline. In some areas, pumping has resulted in subsidence (sinking) of the land surface. Similar conditions may arise from the pumping of petroleum.

Groundwater occupies volume in an aquifer by filling pore spaces between the mineral grains. Because water is essentially incompressible, that water helps support the weight of the overlying rock and soil. When the water is pumped out, the pore spaces may collapse under the load and the volume of the rock and soil decreases. In many areas, that pore space is forever lost; that is, water cannot reenter the aquifer.

Significant subsidence as the result of excessive pumping has been recorded in areas such as:

  • the HoustonGalveston area of Texas (1 to 2 meters, or 3.36.6 feet);
  • New Orleans (2 meters, or 7 feet);
  • Venice, Italy (3 meters, or 10 feet);
  • Mexico City (more than 7 meters, or 26 feet); and
  • the San Joaquin Valley of California (8 meters, or 26 feet).

Unlike the subsidence caused by cave-ins that result from the collapse of underground mines, sinkholes caused by dissolution of underground carbonate rocks, or human-made sinkholes caused by broken water mains, subsidence as a result of aquifer overdrafting occurs slowly and over a large area. Residents in the region are unlikely to even notice it.

see also Aquifer Characteristics; Groundwater Supplies, Exploration For; Hydrologic Cycle; Land Use and Water Quality; Modeling Groundwater Flow and Transport; Springs; Stream Hydrology; Stream, Hyporheic Zone of a; Wells and Well Drilling.

Dennis O. Nelson


Heath, R. C. Basic Ground-Water Hydrology. U.S. Geological Survey Water-Supply Paper 2220 (1983).

Todd, David Keith. Groundwater Hydrology, 2nd ed. New York: John Wiley & Sons, 1980.

Internet Resources

Drinking Water Program. Groundwater. Portland, OR: Oregon Department of Human Services, 1995. Available online at <>.


Knowing where groundwater comes from provides a better perspective of how groundwater can become contaminated from land uses. Because groundwater originates as precipitation sinking down from the land surface, downward-infiltrating water must pass through whatever is at or below the surface.

Unconfined aquifers are more vulnerable to contamination than confined aquifers. Hence, it would be unwise to locate a facility that uses hazardous chemicals, for example, above a shallow, un-confined aquifer. Understanding groundwater movement allows a determination of whether a specific land-use activity (e.g., a landfill or septic system) overlies part of the aquifer carrying groundwater that moves towards and supplies a well.


Whether in a nature shop, at a rock and mineral show, or in a museum, many people have marveled at the beauty of natural mineral formations such as agates, geodes, and petrified wood. Agates, having exotic names such as blue lace, plume, sagenite, pom pom, crazy lace, fire, snakeskin, sweetwater, and thunder egg, all are variations of a multicolored and banded form of silica (SiO2), called chalcedony, a microcrystalline form of quartz. Geodes also consist of chalcedony, but commonly are cut in the shape of a bowl to reveal well-formed quartz crystals projecting into the center of the "bowl." In petrified wood, again the dominant material is chalcedony, however in many cases, the original structure of the wood is preserved.

All agates, geodes, and petrified wood share a common origin. They are formed as the result of groundwater. In agates and geodes, the silica is deposited in open spaces in existing rock. Typically, these openings represent "frozen" gas bubbles in lava flows or ash deposits; the rock solidified before the gas bubble was able to escape. The surrounding volcanic deposits, particularly the ash, readily yield silica to circulating groundwater. When the silica-rich groundwater enters larger open spaces, the silica is precipitated as chalcedony, often in layer form. The different colors represent variations in concentrations of elements such as iron, chromium, and titanium. Agates represent cases where the entire open space is filled, whereas geodes are the case where the open space remained filled with silica-rich groundwater and conditions were favorable for slow growth of quartz crystals into the solution.

Naturally occurring agates and geodes are rounded masses or nodules that reflect the geometry of the hole in which they formed. They often are found loose on the ground because the hardness of silica is generally greater than that of the surrounding rock. With time, the agate and geodes are released as the softer surrounding rock is weathered away. The beauty of the agate and geode are revealed only by cutting them open to reveal the internal structure.

Petrified wood is also the result of silica being deposited by groundwater. Though not a completely understood process, the groundwater is able to remove the original organic matter of the wood and deposit silica in a manner that replicates the original structure of the wood. Examples of petrified wood vary from isolated limbs to entire forests such as in the National Petrified Forest Monument in Arizona.

Agates, petrified wood, and geodes may be enhanced by sawing with a diamond-blade, then polishing the cut surfaces.

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Groundwater occupies the void space in geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle .

Almost all groundwater originates as surface water . Some portion of rain hitting the earth runs off into streams and lakes , and another portion soaks into the soil , where it is available for use by plants and subject to evaporation back into the atmosphere. The third portion soaks below the root zone and continues moving downward until it enters the groundwater. Precipitation is the major source of groundwater. Other sources include the movement of water from lakes or streams and contributions from such activities as excess irrigation and seepage from canals. Water has also been purposely applied to increase the available supply of groundwater. Water-bearing formations called aquifers act as reservoirs for storage and conduits for transmission back to the surface.

The occurrence of groundwater is usually discussed by distinguishing between a zone of saturation and a zone of aeration. In the zone of saturation, the pores are entirely filled with water, while the zone of aeration has pores that are at least partially filled by air. Suspended water does occur in this zone. This water is called vadose, and the zone of aeration is also known as the vadose zone. In the zone of aeration, water moves downward due to gravity , but in the zone of saturation it moves in a direction determined by the relative heights of water at different locations.

Water that occurs in the zone of saturation is termed groundwater. This zone can be thought of as a natural storage area or reservoir whose capacity is the total volume of the pores of openings in rocks.

An important exception to the distinction between these zones is the presence of ancient seawater in some sedimentary formations. The pore spaces of materials that have accumulated on an ocean floor, which has then been raised through later geological processes, can sometimes contain salt water. This is called connate water.

Formations or strata within the saturated zone from which water can be obtained are called aquifers. Aquifers must yield water through wells or springs at a rate that can serve as a practical source of water supply. To be considered an aquifer the geological formation must contain pores or open spaces filled with water, and the openings must be large enough to permit water to move through them at a measurable rate. Both the size of pores and the total pore volume depends on the type of material. Individual pores in fine-grained materials such as clay , for example, can be extremely small, but the total volume is large. Conversely, in coarse material such as sand , individual pores may be quite large but total volume is less. The rate of movement for fine-grained materials, such as clay, will be slow due to the small pore size, and it may not yield sufficient water to wells to be considered an aquifer. However, the sand is considered an aquifer, even though they yield a smaller volume of water, because they will yield water to a well.

The water table is not stationary, but moves up or down depending on surface conditions such as excess precipitation, drought , or heavy use. Formations where the top of the saturated zone or water table define the upper limit of the aquifer are called unconfined aquifers. The hydraulic pressure at any level with an aquifer is equal to the depth from the water table, and there is a type known as a water-table aquifer, where a well drilled produces a static water level which stands at the same level as the water table.

A local zone of saturation occurring in an aerated zone separated from the main water table is called a perched water table. These most often occur when there is an impervious strata or significant particle-size change in the zone of aeration, which causes the water to accumulate. A confined aquifer is found between impermeable layers. Because of the confining upper layer, the water in the aquifer exists within the pores at pressures greater than the atmosphere. This is termed an artesian condition and gives rise to an artesian well.

Groundwater can be pumped from any aquifer that can be reached by modern well-drilling apparatus. Once a well is constructed, hydraulic pumps pull the water up to the surface through pipes. As water from the aquifer is pulled up to the surface, water moves through the aquifer towards the well. Because water is usually pumped out of an aquifer more quickly than new water can flow to replace what has been withdrawn, the level of the aquifer surrounding the well drops, and a cone of depression is formed in the immediate area around the well.

Groundwater can be polluted by the spilling or dumping of contaminants. As surface water percolates downward, contaminants can be carried into the aquifer. The most prevalent sources of contamination are waste disposal , the storage, transportation and handling of commercial materials, mining operations, and nonpoint sources such as agricultural activities. Two other forms of groundwater pollution are the result of pumping too much water too quickly, so that the rate of water withdrawal from the aquifer exceeds the rate of aquifer recharge. In coastal areas, salty water may migrate towards the well, replacing the fresh water that has been withdrawn. This is called salt-water intrusion. Eventually, the well will begin pulling this salt water to the surface; once this happens, the well will have to be abandoned. A similar phenomenon, called connate ascension, occurs when a freshwater aquifer overlies a layer of sedimentary rocks containing connate water. In some cases, over pumping will cause the connate water to migrate out of the sedimentary rocks and into the freshwater aquifer. This results in a brackish, briney contamination similar to the effects of a salt-water intrusion. Unlike salt water intrusion, however, connate ascension is not particularly associated with coastal areas.

Groundwater has always been an important resource, and it will become more so in the future as the need for good quality water increases due to urbanization and agricultural production. It has recently been estimated that 50% of the drinking water in the United States comes from groundwater; 75% of the nation's cities obtain all or part of their supplies from groundwater, and rural areas are 95% dependent upon it. For these reasons every precaution should be taken to protect groundwater purity. Once contaminated, groundwater is difficult, expensive, and sometimes impossible to clean up.

See also Freshwater; Hydrogeology

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An estimated 100 million Americans rely on groundwater for their source of drinking water. Approximately one-third of all public supplies and 95 percent of all rural domestic supplies use groundwater sources. In Asia, groundwater provides half of the drinking water, and in Europe the percentage is even much higher, as much as 98 percent in Denmark and 96 percent in Austria.

An aquifer is an underground formation of permeable rock or loose material that can produce useful quantities of water when tapped by a well. Groundwater is held within the tiny pores of the surrounding aquifer material. Aquifers vary in size from a few hectares to thousands of square kilometers of the earth's surface. The rate of groundwater flow is very slow compared to the flow of water on the surfaceusually in the range of several inches per year to several feet per year. More than 96 percent of all available fresh water supplies occur in the form of groundwater, which is usually cleaner and more pure than most surface water sources.

Groundwater only partially fills unconfined aquifers. The upper surface of the groundwater, known as the water table, is thus free to rise and fall. The height of the water table will be the same as the water level in a well drilled in an unconfined aquifer. Unconfined aquifers can be vulnerable to contamination, especially if they are close to the surface. In these unconfined aquifers, gravity drives the movement of groundwater. Groundwater can leave the aquifer through the process of discharge, either when it reaches the land surface at a spring or other surface water body, or through the pumping of a well. Discharge can lead to contaminants in groundwater flowing into surface water bodies.

A confined aquifer (also known as an artesian aquifer) occurs between confining beds, which are layers of impermeable materials, such as clay, that impede the movement of water in and out of the aquifer. The groundwater in these artesian aquifers is under high pressure due to the confining beds. A recharge zone occurs where the confined aquifer is exposed to the surface. The confined aquifer is actually unconfined at the recharge zone. Confining beds serve two purposes. The first is to obstruct the movement of water into and out of the aquifer. The second is to bar the entry of contaminants from the overlying unconfined aquifers.

Aquifers are replenished with water from the surface through a process called "recharge." This occurs as a part of the hydrologic cycle when water from rainfall percolates into underlying aquifers. The rate of recharge can be influenced by different factors, such as soil, plant cover, water content of surface materials, and rainfall intensity. Groundwater recharge may also occur from surface water bodies in arid areas. Overwithdrawal of groundwater occurs when the discharge of groundwater in an aquifer exceeds the recharge rate over a period of time.

Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. This pollution poses a great risk to public health since the majority of the fresh water supply occurs as groundwater. Many of the groundwater pollutants are colorless, odorless, and tasteless. Degradation of groundwater supplies also occurs as a result of poor waste-disposal practices or poor land management.

Mark G. Robson

(see also: Ambient Water Quality; Drinking Water; Groundwater Contamination; Water Quality )


Groundwater Foundation. Groundwater and Aquifers, 2000. Available at

Groundwater Basics, 2000. Available at

Koren, H., and Bisesi, M. (1996). Handbook of Environmental Health and Safety, 3rd edition, Vol. 2. Boca Raton, FL: Lewis Publishers.

Nadakavukaren, A. (2000). Our Global Environment. Prospect Heights, IL: Waveland Press.

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Groundwater is the water that exists below the land surface and fills the spaces between sediment grains and fractures in rocks. A geologic formation saturated with groundwater is considered to be an aquifer if it is sufficiently permeable as to allow the groundwater to be economically extracted. It is replenished naturally through the infiltration of rainfall and artificially through the irrigation of crops. Soluble chemicals in rainwater (like NOx in acid rain) or at the land surface (like pesticides) can be transported downward with percolating water to reach groundwater. Underground petroleum storage tanks (USTs) or buried pipelines also pose threats if they should leak. Over 400,000 leaking USTs have been identified in the United States as of 2001. Dissolved chemicals are transported with the flowing groundwater. Once groundwater is contaminated, remediation can be expensive and time-consuming; billions of dollars are spent annually in the United States on the remediation of contaminated sites and some of the groundwater contamination cannot be reversed. Groundwater discharges naturally into lakes, rivers, oceans, and springs. It is also extracted via pumping wells.

Approximately 80 percent of municipal water systems and close to 99 percent of rural residents in the United States rely on groundwater. In total, approximately 51 percent of the U.S. population depends on it for their water supply. The 1986 amendments to the Safe Drinking Water Act requires that well head protection plans be developed by each state to protect the land around municipal water supply wells from contamination. Individuals can help protect groundwater by disposing of household chemicals properly and fertilizing plants in limited quantities and can help conserve groundwater by limiting water use at home by taking shorter showers, not running water while brushing teeth, running dish and clothes washers with full loads, fixing leaky faucets and pipes, and limiting plant watering in the garden.

If withdrawals exceed recharge over a long period, groundwater levels fall and aquifiers can become depleted. This results in decreased groundwater discharge and may adversely impact on ecosystems dependent on an aquatic habitat. Excess lowering of groundwater levels may result in land subsidence . In central California, for instance, groundwater withdrawals from 1930 to 1955 for crop irrigation caused approximately three meters of subsidence. In some arid regions (like the Middle East), little groundwater recharge occurs because of low amounts of rainfall and high amounts of evaporation. Ancient groundwater that infiltrated thousands of years ago during climates wetter than those of the present is being extracted via pumping. This practice is termed groundwater mining because groundwater at this location is a nonrenewable resource that is being depleted.

see also Drinking Water; Pesticides; Superfund; Underground Storage Tanks; Water Pollution.


U.S. Environmental Protection Agency (1990). Citizen's Guide to Ground-Water Protection. EPA 440/6-90-004. Washington: U.S. Environmental Protection Agency Office of Water.

Alley, William M., Richard W. Healy, James W. Labaugh, and Thomas E. Reilly (2002). "Flow and Storage in Groundwater Systems." In Science Magazine, 296:19851990.

Internet Resources

U.S. Environmental Protection Agency Office of Ground Water and Drinking Water Information Page. Available from

U.S. Geological Survey Ground Water Information Page. Available from

Karen M. Salvage

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groundwater Water that occurs below the Earth's surface, contained in pore spaces within regolith and bedrock. It is either passing through or standing in the soil and underlying strata, and is free to move under the influence of gravity. Most groundwater derives from surface sources (meteoric water); the remainder is either introduced by magmatic processes (juvenile water) or is connate water. See also vadose zone.

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groundwater Water that lies beneath the surface of the Earth. It comes chiefly from rain, although some is of volcanic or sedimentary origin. It moves through porous rocks and soil and can be collected in wells. Ground water can dissolve minerals and leave deposits, creating structures such as caves, stalagmites, and stalactites. See also water table

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groundwater All the water contained in the void space within rocks. The term is generally taken to exclude vadose water (water travelling between the surface and the water-table). Most groundwater derives from surface sources (meteoric water); the remainder is either introduced by magmatic processes (juvenile water) or is connate water.

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ground·wa·ter / ˈgroundˌwôtər; -ˌwätər/ • n. water held underground in the soil or in pores and crevices in rock.

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