Chapter 4
Groundwater

A VAST HIDDEN RESOURCE

Water lies beneath almost every part of the earth's surface—mountains, plains, and deserts—but 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, http://capp.water.usgs.gov/GIP/gw_gip/gw_a.html) 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, http://ga.water.usgs.gov/edu/earthwherewater.html), 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.

HOW GROUNDWATER OCCURS

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 topsoil—the rich upper layer of soil in which plants have most of their roots—is 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 water—all 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.

AQUIFERS

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, http://www.hiplain.org/states/index.cfm?state=9&c=1&sc=84), 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, http://webserver.cr.usgs.gov/nawqa/hpgw/HPGW_home.html) 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, http://capp.water.usgs.gov/gwa/ch_e/index.html) 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.)

SPRINGS

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, http://montanakids.com/db_engine/presentations/presentation.asp?pid=48&sub=Giant+Springs), 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 (http://www.nps.gov/archive/yell/oldfaithfulcam.htm) so that virtual visitors can see Old Faithful erupt even if they cannot see it in person.

NATURAL CHARACTERISTICS OF GROUNDWATER

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.

Minerals

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.

CURRENT GROUNDWATER USE

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, http://pubs.usgs.gov/circ/2004/circ1268/pdf/circular1268.pdf) 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, http://ga.water.usgs.gov/edu/wugw.html), 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 states—including Idaho, Minnesota, Nebraska, New Mexico, Mississippi, and Florida—drinking 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, http://pubs.usgs.gov/circ/circ1292/pdf/circular1292.pdf), 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.

Overpumping

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.

VULNERABLE RESOURCE—GROUNDWATER QUALITY

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, http://www.gwpc.org/e-Library/Documents/GW_Report_to_Congress.pdf) and 2000 National Water Quality Inventory (August 2002, http://www.epa.gov/305b/2000report/). The worst groundwater contamination is generally in the areas where use is heaviest—towns 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, http://www.epa.gov/safewater/disinfection/gwr/basicinformation.html#six), 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.

FACTORS AFFECTING GROUNDWATER CONTAMINATION

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.

GROUNDWATER CONTAMINATION

Major Types of Groundwater Contaminants

The EPA reports in the National Water Quality Inventory: 1998 Report to Congress (June 2000, http://www.epa.gov/305b/98report/) 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, http://www.epa.gov/owow/monitoring/guide.pdf). 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, 1993–2003 (2006, http://pubs.usgs.gov/sir/2006/5104/pdf/sir2006-5104.pdf), 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, http://www.usgs.gov/newsroom/article.asp?ID=1593), 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

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.

NITRATES

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.

PESTICIDES

Robert J. Gilliom et al. report in The Quality of Our Nation's Water: Pesticides in the Nation's Streams and Ground Water, 1992–2001 (February 15, 2007, http://pubs.usgs.gov/circ/2005/1291/pdf/circ1291.pdf) 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.

VOCs

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.

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.

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.

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

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, http://www.sierraclub.org/pressroom/releases/pr2005-04-19.asp) 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, http://www.gao.gov/new.items/d03529t.pdf), 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, http://www.epa.gov/OUST/pubs/cleanup_brochure.pdf) 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, http://www.gao.gov/new.items/d0645.pdf), 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.

LANDFILLS AND SURFACE IMPOUNDMENTS

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 wastes—including industrial wastes—underground 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.

HAZARDOUS WASTE SITES

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, http://wastec.isproductions.net/webmodules/webarticles/anmviewer.asp?a=1121), 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 carcinogens—cancer-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" (http://www.epa.gov/superfund/sites/query/queryhtm/npltotal.htm) 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.

INJECTION WELLS

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, http://www.epa.gov/safewater/uic/classes.html), 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, http://www.epa.gov/safewater/uic/classv/index.html) 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.

AGRICULTURE

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

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, http://cfpub.epa.gov/owm/septic/faqs.cfm?program_id=70#359), 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.

GROUNDWATER CLEANUP

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, http://www.gao.gov/new.items/d04787r.pdf) 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.

GROUNDWATER PROTECTION

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.

TABLE 4.2

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, http://www.gwpc.org/e-Library/Documents/GW_Report_to_Congress.pdf (accessed January 8, 2007)

CHAPTER 4
GROUNDWATER

A VAST HIDDEN RESOURCE

Water lies beneath almost every part of the Earth's surface—mountains, plains, and deserts—but 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. The amount of groundwater lying within a half mile under the U.S. land surface is approximately four times the amount that fills the Great Lakes.

People have known about the presence of water underground 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. While an estimated one million cubic miles of the Earth's groundwater is located within about one-half 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.

HOW GROUNDWATER OCCURS

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

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

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

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

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

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 areas 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. Stream flows, on the other hand, 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.

Aquifers

An aquifer is a saturated zone that contains enough water to yield significant amounts of water when a well is sunk. 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 (clay or solid granite, for example).

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 very 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 decrease 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 6,000 feet below the surface. Saline (salty) water has been discovered from aquifers that lie more than 30,000 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. Any one aquifer may be a part of a large system of aquifers that feed into each other. Figure 4.4 shows the principal aquifers of the United States.

OGALLALA AQUIFER.

The Ogallala or High Plains Aquifer is one of the world's largest. It is located in the United States and covers 156,000 square miles, stretching from southern South Dakota to the Texas panhandle. Figure 4.5 shows the aquifer's location. The Ogallala's average thickness ranges from 300 feet to more than 1,200 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. Approximately 200,000 wells tap into this vast aquifer and extract huge quantities of water. The Environmental Protection Agency (EPA) has designated the Ogallala Aquifer a sole source aquifer. This means that at least 50% of the population in the area depend on the Ogallala Aquifer for its water supply.

The U.S. Geological Survey (USGS) reported in its 1996 publication Ground Water Atlas of the United States, Oklahoma, Texas, HA 730–E that because the Ogallala Aquifer was being pumped far in excess of recharge, the USGS and the Texas Department of Water Resources projected an increasing shortage of Ogallala aquifer water for future irrigation needs. The projections suggested that the irrigated acreage in the High Plains of Texas (69% of irrigated Texas cropland) would be reduced to one-half of its present acreage by 2030 unless an effective water conservation plan was implemented.

In 1999 Texas oil tycoon and corporate raider T. Boone Pickens formed Mesa Water, Inc., to market water from part of the Ogallala Aquifer to large Texas cities for municipal use. About 100 landowners and 200,000 acres of land in the Texas Panhandle were involved in Pickens's plan as of 2005. 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, says that the owner of land that sits upon 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 rule of capture failed to reach a vote in the Texas state senate in May 2005, and in June 2005 Pickens announced that Mesa Water would begin building a system of wells and pipelines to sell the water to major urban centers throughout Texas ("Private Water Group Preparing to Pump the Ogallala Aquifer and Sell Groundwater to Far-Away Texas Cities," http://www.argentco.com/htm/f20050607.614464.htm, June 2005).

Springs

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. Giant Springs removes 7.9 million gallons per hour from underground reserves and maintains a constant temperature of 54°F. Its flow accounts for approximately one-sixth of the water flowing downstream in the Missouri River, and its water has been carbon-dated to be about 3,000 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 very 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.

NATURAL CHARACTERISTICS
OF GROUNDWATER

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.

Minerals

Water is a natural solvent capable of dissolving 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. Water is not considered desirable for drinking if it contains more than 1,000 milligrams per liter (mg/l) of dissolved minerals. However, in areas where less-mineralized water is not available, water with a few thousand mg/l of dissolved minerals is used routinely, although 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 bad 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 very 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 would remain if the water were evaporated. Water is considered soft when it contains 0 to 60 mg/l of hardness constituents, moderately hard with 61 to 120 mg/l, hard with 121 to 180 mg/l, and very hard if more than 180 mg/l are present.

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 since some are beneficial to health. Extremely soft water can corrode metals but is suitable for doing laundry, dishwashing, and bathing. Most communities seek a balance between hard and soft water in their municipal water systems whenever possible.

CURRENT GROUNDWATER USE

The nation's use of groundwater grew dramatically in the last several decades of the twentieth century. According to a report from the USGS (Estimated Use of Water in the United States in 2000), the United States withdrew about 34 billion gallons per day (gpd) in 1950, reached 84 billion gpd in 1980, and dropped off a bit over the next several years before reaching a new high of 84.5 billion gpd in 2000 (the latest data available). Figure 4.6 shows the percentage of groundwater allocated to various uses in the United States.

Human Needs

About half of the U.S. population depends on groundwater as its primary source of drinking water. The percentage is much higher in rural areas, where about 95% of the population depends on groundwater.

Plentiful groundwater is critical to the economy of the United States. According to an October 1999 USGS report (Safe Drinking Water Act, Section 1429 Ground Water Report to Congress), nationally, more than 40% of all water used for crop irrigation and livestock watering and more than 20% of water used by industries comes from a groundwater source. Seventeen states obtain more than 25% of their overall water supply from groundwater. Another seven states depend on groundwater for more than 50% of their total water supply. The thirteen states with the greatest groundwater withdrawals account for 69% of all groundwater used nationally. Of these states, California, Texas, and Nebraska are the three largest users. Groundwater withdrawal is generally the highest in the western United States, where the water is used to sustain important agricultural activities and to support a growing population.

Ecological Needs

Historically, groundwater and surface water have been managed as separate resources. Since the 1970s 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 arid regions, aquifers contribute a large portion of the water found in rivers and streams. Figure 4.7 shows groundwater contributions to surface water.

This source of surface water recharge 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 obliterated the aquatic and semiaquatic life that depended on groundwater flow in surface water streams.

Overpumping

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

If a groundwater system is not overused, the rate of groundwater recharge and discharge balances out; however, in the United States the rate has become uneven. According to a 2000 USGS report (the latest data available), every day in 2000, 84.5 billion gallons of groundwater were withdrawn.

Groundwater is pumped for a multitude of uses, including public (swimming pools, fire fighting, and street washing), domestic (drinking water and other household uses), agricultural (irrigation and livestock), industrial, mining, and other uses. When these withdrawals exceed the rate at which the groundwater source is recharged, they result in 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.
• Permanent loss of aquifer capacity 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 flow of salty water and water of lower quality into a well.
• Lower stream baseflows can dry up wetlands and cause adverse effects on ecological systems that are dependent on groundwater discharge.

When Use Exceeds Replenishment

In 1940 C. V. Theis warned in "The Source of Water from Wells" (Civil Engineering, vol. 10, no. 5, 1940) that pumping from wells profoundly affects aquifers. 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 overpumping.

Groundwater Rights

For some Americans, droughts in the 1990s were a "wake-up call." In Texas, the 1996 drought resulted in federal disaster relief for farmers in 95% of the state's 254 counties and cost the Texas economy an estimated $5 billion. The disaster revived controversy surrounding water rights, including rule of capture, which permits a property owner to pump any amount of water from an aquifer beneath his or her land. Dispute over the control of groundwater prompted innumerable lawsuits, especially regarding the gradually depleted Edwards Aquifer (located in the San Antonio area).

Drought in Texas and other states continued to be a concern in 1998. An August 5, 1998, Federal Emergency Management Administration (FEMA) press release described the strain put on underground water supplies in Texas during the week beginning August 3, 1998. Water flows from the Edwards Aquifer typically amount to 125,000 gallons per minute, but during that week flow was reduced to 80,000 gallons.

EDWARDS AQUIFER: CITIES AND RURAL AREAS IN CONFLICT.

The 160-mile-long Edwards Aquifer, one of the nation's most productive, is located in south-central Texas. Conflict over its use is an example of the growing competition for water. This aquifer has supplied water to farmers since the mid-twentieth century. As the only source of water for San Antonio, the nation's eighth most populous city as of 2004, it has been designated a sole source aquifer by the EPA. Rain and runoff filter through the Edwards Aquifer before emerging in springs. However, increased pumping for irrigation, additional water use for the rapidly growing population of San Antonio, and periodic droughts are steadily depleting the vast underground reservoir. The amount of water taken from this vital aquifer doubled over two decades. The once plentiful and seemingly infinite supply of water that was previously taken for granted is now recognized as an endangered resource.

The Edwards Aquifer has been the subject of bitter dispute between farmers and San Antonio residents as to who has rights to the water. The agricultural population is dependent on the Edwards Aquifer for the production of crops and, therefore, the farmers' livelihood. Officials in San Antonio, however, contend that the water needs of the city must be protected for present and future citizens. The rural and urban interests represent two factions involved in a modern-day version of a classic Western battle over water rights.

In 1993 Texas established the Edwards Aquifer Authority (EAA) to limit water withdrawal from this aquifer. Prior to the establishment of the EAA, the governing water-use principle was that "if it's under your land, it's your water." Under the EAA, every Edwards Aquifer user, except for domestic and livestock owners using fewer than twenty-five gallons per day, had to have a permit with a specified annual withdrawal limit. The intent was to ensure fair distribution of water among all users.

In 1998 the Texas legislature adopted the Texas Critical Management Plan (CMP). The CMP allowed the EAA to further reduce withdrawals, despite the levels specified in permits, during low rainfall or critical periods by imposing restrictions on water-use activities and monthly limits on total water use. The purpose was to protect the aquifer's integrity and to conserve water. In addition, the CMP took water management to another level, requiring all Texas water suppliers and users to develop fifty-year water management plans that address both surface and water supplies. These plans were the first step in statewide water source management and conservation.

According to the EAA Web site (www.edwardsaquifer.net), conflict over pumping water from the Edwards Aquifer continues. According to the EAA, the 1993 law that created the Authority contains conflicting provisions. The law mandates that the Authority issue minimum annual pumping rights to those who could prove that they had been using the water during the previous twenty-one years, but it also set a pumping cap of 450,000 acre-feet per year. In February 2003 the Authority had already issued pumping permits for about 482,000 acre-feet, and was considering the granting of another 82,000 acre-feet in annual pumping rights for 184 pending permit applications. In March 2003 the Authority voted to support newly proposed legislation that would raise the pumping cap to 550,000 acre-feet of water per year.

Preservation of the integrity of the Edwards Aquifer and conservation of its water also have implications for endangered species. According to the U.S. Fish & Wild-life Service (USFWS), the Edwards Aquifer is a critical habitat for nine endangered species (three beetles, one harvestman, and five spiders). A critical habitat is defined by the Endangered Species Act as a specific area deemed essential for the conservation of a threatened or endangered species, which may require special management consideration or protection. An April 8, 2003, USFWS press release reported that these endangered species are underground karst-dwelling invertebrates that are found only in karst limestone areas of Bexar County, Texas. According to Dale Hall, director of the Southwest Region of the USFWS, "These species are particularly vulnerable to groundwater pollution and are an indicator species for the health of the Edwards Aquifer, which is the source of drinking water for more than one million people. Designating habitat that is critical to their survival is another step toward ensuring their future and the health of the aquifer itself."

In May 2005 voters in San Antonio approved the second of two tax increases intended to provide funds toward the purchase of lands that contribute to the recharge of the Edwards Aquifer. The goal of the tax propositions is to raise $135 million, at which point the taxes will expire—approximately nine years from their adoption ("Voters Reauthorize Sales Tax for Aquifer Protection Land Purchases," http://www.edwardsaquifer.net/news.html, May2005).

VULNERABLE RESOURCE—GROUNDWATER
QUALITY

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.

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

Once it became apparent that groundwater was being contaminated, the questions of which waters were polluted, the severity of contamination, and what should be done about it had to be addressed. Many government and private organizations are working to find the answers, but it is not an easy task. As with other types of pollution control, problems include lack of accurate data, inadequate reporting and measurement techniques, the difficulty of setting acceptable standards, illegal dumping, designation of cleanup responsibilities, and funding.

The quality of 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 its 2000 National Water Quality Inventory. The worst groundwater contamination is generally in the areas where use is heaviest—towns and cities, industrial complexes, and agricultural regions such as California's Central Valley.

Although positive strides were made in assessing water quality in 2000, groundwater data collection is still too immature to provide a good national assessment. Accurate and representative assessment of groundwater quality requires a well-planned and well-implemented monitoring plan. The states are working to develop and execute these plans as manpower and monetary resources allow. At this time because the aquifer monitoring data reported by states represent different sources and differing monitoring purposes, care must be exercised in drawing conclusions.

Recognizing the need to protect valuable and vulnerable groundwater sources, the states have begun to implement comprehensive groundwater protection programs. These programs are being integrated with surface water protection programs in recognition of the fact that base-flow in the nation's surface waters is dependent on groundwater.

FACTORS AFFECTING GROUNDWATER
CONTAMINATION

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.

POLLUTANTS

Types of Major Contaminants in Groundwater

In the EPA's National Water Quality Inventory—1998 Report to Congress (the latest data available for information on specific contaminants in groundwater) 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.

ARSENIC.

Arsenic is a naturally occurring element in rocks and soils, and is the twentieth most common element in the earth's crust. In groundwater, the presence of arsenic is largely the result of minerals dissolving from naturally weathered rocks and soils over time. The USGS reported that the nation's groundwater typically contains less than one or two parts per billion (ppb) arsenic. A ppb is equal to about one drop in an Olympic-size swimming pool. Moderate to high arsenic levels do occur in some areas throughout the nation in a pattern related 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 has shown that humans need arsenic as a trace element in their diet to survive. Too much arsenic, however, can be harmful. Arsenic can contribute to skin, bladder, and other cancers after prolonged exposure. 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 must comply with the ten ppb level by January 23, 2006.

NITRATES.

Many scientists and geologists consider nitrates to be the most widespread groundwater contaminant. Many states use their presence as an "indicator" of the human impact on groundwater quality. Generally, a level of three ppb or more is considered indicative of human impact.

Nitrate contamination occurs most frequently in shallow groundwater (less than 100 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. For example, the USGS reported in 1997 that groundwater discharge was a significant source of nitrate loading to tidal creeks, coastal estuaries, and the Chesapeake Bay.

PESTICIDES.

Pesticide concentrations in groundwater are generally low and rarely exceed EPA drinking water standards. In 1999 (the latest data available), the USGS reported that pesticide concentrations exceeded standards of guidelines in less than 1% of the wells sampled in the National Water Quality Assessment Program. This assessment, however, may be incomplete with respect to the overall health and environmental risks associated with pesticide presence in shallow groundwater. According to a 1999 report from the USGS (The Quality of Our Nation's Waters: Nutrients and Pesticides. Circular 1225), drinking water standards and guidelines exist for only forty-six of the eighty-three pesticide compounds identified. Of the sites where pesticides were detected, 73% had two or more compounds present. Continued research is needed to help reduce the current uncertainty in estimating risks to human health and to the environment from commonly occurring mixtures of pesticides.

METHYL TERTIARY BUTYL ETHER.

Methyl tertiary butyl ether (MTBE) is a volatile organic chemical that is added to gasoline to increase octane levels and to reduce carbon monoxide and ozone levels in the air. It is more soluble in water, and less likely to be degraded, than other common petroleum constituents. According to an October 2001 USGS report (MTBE and Other VolatileOrganic Compounds—New Findings and Implications on the Quality of Source Waters Used for Drinking-Water Supplies), the EPA tentatively has classified MTBE as a potential human carcinogen. However, because of insufficient toxicity studies, the EPA has not yet instituted a drinking-water health advisory or standard.

In its National Water Quality Assessment Program (NAWQA), the USGS detected MTBE in about 5% of ambient groundwater samples collected across the nation. The concentrations typically were lower than the EPA's drinking-water consumer advisory concentration of twenty to forty micrograms per liter. Less than 1% of the groundwater samples exceeded the EPA consumer advisory concentration of twenty micrograms per liter. MTBE was most often detected in groundwater underlying urban areas (14% of wells tested in urban areas had MTBE). The USGS also studied MTBE concentrations associated with drinking-water supplies from selected communities in twelve states in the Northeast and Mid-Atlantic regions. MTBE was detected in 9% of community water systems. These concentrations were low; less than 1% exceeded the EPA consumer advisory concentration.

Studies of the distribution of MTBE showed that the frequency of MTBE detection increases with its use, typically associated with urban and populated areas. MTBE has been detected in about one out of five wells in MTBE high-use areas. The findings also showed that some areas are more vulnerable to MTBE contamination than others. The frequency of MTBE detection increases with larger community water systems, specifically those serving more than 50,000 people. The difference in vulnerability reflects, in part, natural features, land use, and human activities such as pumping and watercraft use.

What began as an effort to reduce air pollution has become a water-quality concern that has necessitated dozens of costly studies and created a public health risk. In 1999 the EPA appointed the MTBE Blue Ribbon Panel to provide independent advice and counsel on the policy issues associated with the use of MTBE and other oxygenates in gasoline. In July 1999 the panel recommended a package of actions designed to be implemented simultaneously to maintain air quality benefits while enhancing water-quality protection and ensuring a stable fuel supply at reasonable cost. The EPA initiated plans to phase out MTBE, but the Bush Administration cancelled those plans in 2001. As of 2004, more than half the states still allowed MTBE, though California and New York, which together accounted for about 40% of MTBE use, have banned its use since January 1, 2004.

Sources of Pollution

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

UNDERGROUND STORAGE TANKS.

Leaking underground storage tanks (USTs) 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. In general, most 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. One gallon of gasoline can contaminate one million gallons of water, or the amount of water needed for a community of 50,000 people. The fuel additive MTBE is particularly troublesome because it migrates quickly through soils into groundwater, and very small amounts can render groundwater undrinkable. Figure 4.10 shows how groundwater can be contaminated by leaking underground storage tanks.

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.

A March 5, 2003, report from the Government Accountability Office (GAO) (Environmental Protection: Recommendations for Improving the Underground Storage Tank Program) stated that as of December 2002 at least 19 to 26% of states still had problems with leaking underground storage tanks. 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. In order to address the problems, the GAO recommended that Congress provide states more funds from the UST trust fund to ensure improved training, inspections, and enforcement efforts. In addition, the GAO 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.

Underground storage tank 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 have provided financial assurance funds to help UST owners meet the financial requirements. In about 95% of the cases, the EPA or the states have succeeded in getting responsible parties to perform the cleanups. Figure 4.11 and Figure 4.12 contain information about inspection frequency and compliance with underground storage tank regulations.

As of the end of 2004, about 447,000 releases of contaminants from corroded underground storage tanks had been confirmed. The EPA estimated that about half of those releases reached groundwater. According to a February 2005 EPA fact sheet, "Cleaning Up Leaks from Underground Storage Tanks," 92% (more than 412,000) of cleanups had been initiated, and 71% (more than 317,000) had been completed. As of the end of 2004, about 130,000 remained in the national backlog of cleanups required.

LANDFILLS AND SURFACE IMPOUNDMENTS.

Septic systems and landfills were the second- and third-largest sources of groundwater contamination, respectively, in 2000. (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 wastes—including industrial wastes—underground 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 prior to the passage of the Resource Conservation and Recovery Act (RCRA) in 1976 are believed to represent the greatest risk. 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 old 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 are usually composed of man-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 1,000 acres.

Prior to 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 since about 87% of impoundments were located over aquifers that were currently used as sources of drinking water. Aquifers located under nonlined impoundments are vulnerable to contamination. Less than 2% of the surface impoundments were located in areas where there is no groundwater or it is too salty for use. About 70% of the sites were located over thick and very permeable aquifers that would allow any contaminant entering its waters to spread rapidly. Groundwater protection was rarely, if ever, considered during site selection.

Since the passage of 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. (See Figure 4.13.)

HAZARDOUS WASTE SITES.

Hazardous waste is an unavoidable byproduct of an industrial society. Many chemicals are used to manufacture goods. Hazardous waste generators can be large industries, such as automobile manufacturers, or small neighborhood businesses, like 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. The EPA estimates that between 2% and 4% of all waste is hazardous.

Contamination of groundwater with hazardous waste is frequently the result of historic indiscriminate waste disposal in landfills, impoundments, and dumps. Today, sites that handle hazardous waste or a mix of hazardous and nonhazardous waste are subject to very 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 (for example, contamination of groundwater used for drinking with known carcinogens), it is placed on the National Priorities List (NPL) established by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly known as the Superfund). Sites placed on the Superfund list are eligible for federal intervention and cleanup assistance. As of February 11, 2005, 1,238 sites were listed on the NPL. Most were general sites such as industrial and municipal landfills and military bases. About one-fifth of the listed Superfund sites were municipal landfills. A list of contaminants, their sources, and their health effects is shown in Table 4.1.

INJECTION WELLS.

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. The EPA's underground injection control (UIC) program identifies five classes of injection wells:

• 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 has the potential to 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.

According to the EPA, there were more than 650,000 Class V injection wells in the United States in 2001 (http://www.epa.gov/safewater/uic/classvdetermination.html, April 2001). 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. These requirements were scheduled to be phased in over seven years.

AGRICULTURE.

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 application, animal feedlots, irrigation practices, agricultural chemical facilities, and drainage wells. The 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 other metals. This increases the salinity (dissolved solids) of the underlying aquifers. Overpumping can diminish the water in aquifers to the point where salt water 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.

Septic systems were cited as the second most common source of groundwater contamination by thirty-one reporting states. 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 prior to the distribution of liquid wastes into permeable leach beds for absorption into soil. Wastewater is attacked in the leach beds by biological organisms in the soil and broken down over time. According to information collected by the National Small Flows Clearinghouse (1999), approximately twenty-three million Americans living mostly in rural areas use individual sewage disposal systems. Millions of commercial and industrial facilities also use these systems. American households dispose of 3.5 billion gallons of liquid waste into septic systems each day, and between 820 and 1,450 billion gallons of waste into the ground each year.

Improperly constructed and poorly maintained septic systems may cause substantial and widespread nutrient and microbial contamination to groundwater. This belief is based on some studies done in different parts of the country, which have shown nitrogen levels elevated above background nitrogen levels in areas with concentrated septic systems. There has been no systematic study done nationwide. Septic systems have also been blamed for some outbreaks of bacterial and viral disease associated with drinking groundwater.

GROUNDWATER CLEANUP

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. The Water Protection Council in 1996 estimated $14 billion annually is needed to remediate groundwater and soil at Superfund sites, hazardous waste sites with corrective actions, leaking UST sites, Departments of Defense and Energy waste sites, and other contaminated sites. The GAO in 1999 reported that Super-fund cleanups had already cost $14 billion, and projected another $9.9 billion was needed to complete cleanup at remaining sites. In 2004 the GAO reported 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 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.

GROUNDWATER PROTECTION

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 level. 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 SDWA requirements for state programs that address protection of wellheads used as drinking water sources, coupled with other federal and state source water protection programs, should lead to better, more coordinated management of potential drinking water contaminant sources within drinking water source protection areas.

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

Lack of funding targeted directly to groundwater is the reason most commonly cited by states for their limited efforts in undertaking a more comprehensive approach to groundwater issues. Groundwater is not a high priority for funding. In most states the funding is directed toward state or federally mandated programs. Mandated programs are generally specific as to how and for what purposes money can be spent, often precluding the state from using the funds to address nonmandated groundwater protection priorities even when the result would prevent groundwater pollution in the future. Funding targeted to groundwater concerns would allow the states to better address the problems of program fragmentation within the state and to provide basic program needs such as monitoring, resource characterization, and the development and implementation of protection programs.

Ultimately, surface water and groundwater protection takes place at the state and local level, where the costs and benefits are recognized initially. This financial burden is often beyond the resources of state and local governments, which struggle to meet federal, state, and local mandates in many widely differing arenas with limited resources.

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 the federal laws 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 cleanup of contaminated groundwater at Superfund, RCRA corrective action facilities, and leaking USTs.

Until this time, the emphasis in federal and state programs has been on cleaning up existing contamination, not on pollution prevention. The cleanup programs have been very costly. These costs have become considerable incentives for the private, state, local, and federal sectors to investigate and implement preventive measures.

Certain federal and state programs are designed to prevent groundwater contamination. The problem is that these programs tend to focus on a narrow set of contaminants or contaminant sources. For example, the Federal Insecticide, Fungicide, and Rodenticide Act established regulations to prevent groundwater contamination resulting from pesticide use.

FUTURE GROUNDWATER MANAGEMENT

The nation's groundwater management is highly fragmented, with responsibilities spread over many federal, state, and local programs. Unique legal authorities at each government level allow for controlling one or more of the groundwater threats discussed here. For groundwater management to be successful in protecting and conserving this resource, these various authorities need to complement one another to allow for comprehensive management. Programs such as the EPA's groundwater protection strategy, which places greater emphasis on comprehensive state management of groundwater resources, the SDWA-mandated wellhead protection, and source water assessment and protection programs are a good beginning.

Groundwater

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 ashallow 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 point—in 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 groundwater—for 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.

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.

Subsidence.

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 Houston–Galveston area of Texas (1 to 2 meters, or 3.3–6.6 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

Bibliography

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 <http://www.ohd.hr.state.or.us/dwp/gwater.htm>.

GROUNDWATER AND LAND USE

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.

AGATES, GEODES AND PETRIFIED WOOD

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.

Groundwater

Introduction

Groundwater is water that has accumulated deep in the ground in permeable rocks or soils known as an aquifer. These water-holding formations are located beneath most areas of Earth. Rain filters down through the surface layers of rock and soil and is stored in the aquifer. This process acts as a natural filtration system, removing harmful substances from the water.

Groundwater has long been an important natural resource for humankind. By using groundwater, many civilizations have been able to avoid disease spread by surface water that is contaminated with sewage. Groundwater may also be present in areas that have little surface water, allowing civilization to flourish in areas it would otherwise not. Today, groundwater is still a valuable resource that people rely on. However, dangerous pollutants have contaminated some groundwater, making it unsafe to drink. In places where water treatment is available, this is less of a problem, but in undeveloped areas, some people are exposed to dangerous substances through groundwater.

Historical Background and Scientific Foundations

Groundwater begins to form when rain falls to the ground or snow melts. The water sinks into the ground and some of it evaporates or is absorbed by the roots of plants. As it is sinking down, the water is known as “recharge water,” because it will soon refill the aquifer. Drawn by gravity, the water continues downward until it reaches an impermeable layer. It rests there, usually in cracks in rocks or voids in sand and gravel. These saturated layers are known as the aquifer, and the top level they reach is called the water table. On the shores of some lakes, the water table may reach to the surface and water can be seen filling holes dug on the shoreline.

The water table may rise or fall, depending on weather conditions and the amount of water drawn from the aquifer. During summer droughts, more water might be pumped out of an aquifer than is replaced by rainfall and the water table will fall. If there is an adequate amount of snow or rain in the winter, the aquifer will be recharged. Problems can arise when more water is drawn from an aquifer than is quickly replaced. In parts of Texas, Oklahoma, Kansas, and New Mexico, water tables have dropped more than 100 ft (30 m) from historical measurements. This raises the possibility that the aquifer could be pumped dry, causing water supplies to run out. The same thing can happen when water is recharged into the aquifer very slowly. Enough water may be replenished by rainfall and snowmelt each year, but it takes a very long time to reach the aquifer. As people use the water from the aquifer, it can become depleted, even though enough recharge water will eventually reach it. This situation could occur if Las Vegas, Nevada, which gets most of its water from the Colorado River, starts pumping additional supplies from its aquifer.

Many ancient civilizations relied heavily on groundwater and used it as a resource to help them expand. The ancient Romans are famous for their extensive and complex waterworks. Unlike many cities that came before and after Rome, each resident could rely on a plentiful supply of clean water, easily accessible at neighborhood fountains. The greater supply of water simply allowed more people to live in the city than otherwise could. The purity of the water permitted Rome to grow and flourish relatively free from water-borne diseases that spread very easily in crowded cities.

The most obvious large water source in Rome is the river Tiber. It flows through the city and would have been easily accessible in ancient times. However, the river was a poor source of drinking water because of the large amount of silt in the water. As the city grew, most of the sewers emptied into the river, making it unsuitable for drinking. There are some small springs that erupt within the city, but they did not provide enough water to slake a city of a million people. Today, engineers try to sink wells and pump water from the local aquifer, but the ancient Romans did not have the technology to do this on a large scale. Instead, Romans built aqueducts to access the pure groundwater of the surrounding hills, which flowed to the surface in artesian springs. Most aqueducts were subterranean tunnels, carved out of the local rock. When aqueducts had to cross a valley, they were raised onto the stone arches we commonly associate with Roman aqueducts.

Rome, of course, was not the only ancient city to successfully utilize groundwater, nor was it even among the first. The Minoan settlement of Knossos, flourishing on the island of Crete around 2000 BC, supplied its water from nearby springs using a system of pipes. In Egypt, people accessed groundwater with wells as early as 3000 BC. By the fifth century BC, the Greek physician Alcmaeon of Croton had observed that water quality affected a person’s health. The Romans realized that mine waste could contaminate groundwater, making it poisonous. After the fall of the Roman Empire, their water system fell into disrepair. During the Middle Ages, the size of the city of Rome decreased dramatically, partially due to the lack of clean water.

The use of groundwater experienced a resurgence in the nineteenth century with the invention of better pumping technology. At this time, the city of London relied heavily on both deep and shallow wells to draw drinking water. In 1854, an outbreak of cholera ravaged a few neighborhoods in the city. It was eventually linked to a shallow well that had been contaminated by sewage. Dr. John Snow (1813–1858) determined that those who utilized pumped groundwater were not exposed to the disease. Not only was this an important episode in the development of germ theory, but it illustrates one reason why groundwater supplies became so desirable and heavily used.

Issues and Impacts

When groundwater is polluted, serious public health problems can occur. Because many cities and towns distribute groundwater as drinking water, contamination in the groundwater is efficiently spread to many people. There are many kinds of contamination that can affect groundwater. Some wells are especially vulnerable to fecal contamination, usually from farm animals. Aquifers can be contaminated with saltwater, either from the ocean or from underground sources. Contamination with industrial chemicals is a serious problem in some areas. Additionally, there is growing concern that trace

WORDS TO KNOW

AQUIFER: Rock, soil, or sand that is able to hold and transmit water.

ARTESIAN SPRING OR WELL: Groundwater that flows from the aquifer to the surface without the need for a pump.

WATER TABLE: The level of the aquifer under the ground.

amounts of pharmaceuticals, illicit drugs, and personal care products are making their way into the water supply.

There are many ways in which groundwater can become polluted with dangerous substances. Tainted soil, improperly disposed chemicals, and leaky septic systems can all leach dangerous substances into the aquifer. Poorly maintained sewer and storm-drain systems can leak sewage or tainted runoff, which makes its way into groundwater. Runoff from livestock or poultry farms can sometimes infiltrate groundwater, particularly if it runs down wells. Contaminated surface water can also taint groundwater. The water from some streams, rivers, and lakes seeps into the ground to the aquifer. This effect can be great during floods or times of heavy precipitation when the ground is saturated, the water table rises, and streams rise in their banks, capturing soil that it normally doesn’t overflow. If the bodies of water are contaminated or pick up tainted soil, the pollution can be spread to the aquifer. Many kinds of contaminants can be spread in these ways, from industrial chemicals and salt to dangerous bacteria.

Groundwater tainted with human or animal feces can cause a public health emergency. Many animals, especially cattle, shed dangerous E. coli bacteria in their feces. When humans ingest the bacteria, the first symptoms of illness are usually gastrointestinal: vomiting, intestinal cramps, fever, and bloody diarrhea. The disease can progress to kidney failure, circulation collapse, loss of limbs, and death. The small town of Walkerton in Ontario, Canada, was the site of a serious outbreak of E. coli in the spring of 2000, leading to the deaths of seven people. On May 12, heavy rain swept cattle feces into one of the central wells that served the town, contaminating the aquifer with E. coli. Unbeknownst to the town, the water manager had not been treating the well water properly with chlorine, allowing the bacteria to grow. In all, 500 people complained of E. coli symptoms, several children died, and the town, residents, and government spent millions of dollars decontaminating the water system.

Any water that contains raw or treated sewage has the potential to contain pharmaceuticals and personal care products (PPCPs). These substances enter the water supply through human and veterinary use and waste from manufacturing. PPCPs include grooming products like shampoo, toothpaste, and shaving cream; pharmaceuticals such as hormones, steroids, and antibiotics; and illegal drugs and the substances the body metabolizes them into. Since many of the pharmaceutical products and illegal drugs are used specifically for their ability to affect cell receptors, they can have powerful effects even in small amounts. Though PPCPs are a larger problem in surface water bodies, the interactions between groundwater and surface water mean that PPCPs have been found in almost every source that has been examined. PPCP contamination in groundwater sources is of special concern because the inaccessibility of most aquifers makes any pollution difficult to remove.

Chemicals from industrial sources, farms, homes, and businesses can pollute groundwater with leaks or due to improper disposal. Underground tanks are a large source of groundwater pollution because these tanks can leak for a long time before the spill is discovered. Sometimes, underground tanks are abandoned or forgotten, with no one taking responsibility for their maintenance. Underground tanks are responsible for a great number of gasoline, fuel oil, and diesel leaks. In the past, landfills were not as heavily regulated as they are today. Hazardous chemicals like paint, oil, solvents, pesticides, and wastes containing heavy metals were thrown out with regular household trash. As a result, many old or abandoned landfills have the potential to pollute groundwater when rain falls on them and drains into the underlying aquifer.

Today, known hazardous wastes are disposed of in special landfills that are fitted with impermeable liners and systems to collect leaching chemicals. In the past, disposal sites have been less secure or the containment was breached sometime after the chemicals were deposited. This can lead to groundwater contamination by allowing the stored chemicals to escape. Many industrial companies use stabilization ponds or lagoons to store and treat chemical waste before disposal. Farms also utilize them to break down animal waste. These ponds use light, bacteria, chemical reactions, or a combination of these factors to break down the wastes. If the ponds are not properly lined, or if they break, leak, or overflow, organic chemicals or disease-causing bacteria can escape the ponds and taint groundwater.

Polluted aquifers have a limited ability to restore themselves to purity, depending on the type and extent of pollution. The U.S. Geological Survey (USGS) has been studying an aquifer under a former military base on Cape Cod, in Massachusetts. Treated wastewater that still contained dissolved contaminants was allowed to seep into the aquifer for more than 60 years. The scientists have concluded that while the aquifer will be able to recover, it will take more than 30 years from the time the dumping ceased. With greater demand on limited water resources, this recovery period restricts access to necessary water.

New processes designed to treat the whole aquifer and accelerate the restoration of clean groundwater are being used in many places. The State of California has studied the use of the chemical potassium permanganate

(KMn0₄) to break down volatile organic compounds (VOCs) that are polluting aquifers. By injecting the chemical into the aquifer in strategic places, the contaminants are decomposed, leaving clean water to be pumped to the surface. Special bacteria are also being used to remove chemicals like benzene (C₆H₆) and toluene (C₇H₈) from groundwater. Under the right conditions, bacteria can be introduced into the polluted aquifer and decompose the dangerous chemicals in place. Oxygen (O) from regular air has the ability to break down hazardous chemicals before they reach sensitive groundwater. Injecting air under leaking tanks encourages the growth of natural microorganisms that degrade dangerous chemicals. A new, more extreme method of protecting aquifers is known as in situ vitrification. Large electrodes are used to heat contaminated soil to extreme temperatures. The soil melts and cools into a sold block of glass, trapping the dangerous chemicals so they cannot escape into the groundwater. This method is particularly useful because it deals with all kinds of contaminants, rather than just a specific one. This process also minimizes chemical exposure to workers and the outside environment.

With growing population, pollution, and natural scarcity in some areas, clean water is an increasingly valuable resource. Since 50% of drinking water in the United States is groundwater, it is necessary to protect aquifers from pollution and carefully use their water. Water is also necessary to operate power plants, irrigate crops, water livestock, and dispose of waste. In the United States, a complicated system of laws, rules, and precedents governs the rights to use groundwater. One of the earliest rules governing the use of groundwater allowed landowners the right to use the water pumped from their land. Another similar rule gave rights to whoever pumped the groundwater to the surface. These systems ignored the limited nature of groundwater as a resource and the fact that an aquifer generally extends beneath the property of many people.

In the eastern part of the United States, where groundwater is relatively plentiful and there are fewer large-scale agricultural demands, the legal precept known as “reasonable use” dominates, meaning everyone is entitled to enough groundwater within reasonable limits. In the drier, more agricultural western areas, water rights are allocated according to who has the oldest claim on the resource. This has created some problems, with some sources having more claims than available resources.

The long-term sustainability of groundwater as a vital resource depends on many factors. Pollution of groundwater sources must be controlled if humans are to make use of the water. Similarly, contaminated sources will need to be restored to purity to meet the increasing water demands of a growing population. There are also some groundwater sources that are naturally undrinkable because of large amounts of salt or other minerals. The ability to treat the water from these aquifers may become necessary in the future. In some cases, alternate sources of water will need to be found for areas that rely on aquifers that cannot be replenished, or are replenished so slowly that replacements cannot meet demand. In all cases, conservation of groundwater resources will become increasingly important as human demand for water grows.

See Also Groundwater Quality

BIBLIOGRAPHY

Periodicals

Vuorinen, H. S., P. S. Juuti, and T. S. Katko. “History of Water and Health from Ancient Civilizations to Modern Times.” Water Science and Technology: Water Supply 7, 1 (2007): 49-57.

Web Sites

CTV Television Network. “Walkerton Chronology.” December 20, 2004. http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/1103559265883_98968465 (accessed April 19, 2008).

Hu, G. Jeffrey, and Rebecca Chou. California Regional Water Quality Control Board “In Situ Application of Potassium Permanganate Solution for VOCs-Impacted Groundwater Cleanup—The Regulatory Perspective.” http://www.containment.fsu.edu/cd/content/pdf/344.pdf (accessed April 19, 2008).

National Ground Water Association. “Ground Water Sustainability.” February 24, 2004. http://www.ngwa.org/PROGRAMS/government/issues/possustain.aspx (accessed April 19, 2008).

Popular Mechanics. “Las Vegas Tries to Prevent a Water Shortage.” February 2007. http://www.popularmechanics.com/science/earth/4210244.html (accessed April 19, 2008).

U.S. Environmental Protection Agency. “Ground Water Rule.” November 21, 2006. http://www.epa.gov/safewater/disinfection/gwr/index.html (accessed April 19, 2008).

U.S. Environmental Protection Agency. “Pharmaceuticals and Personal Care Products (PPCPs).” December 14, 2007. http://epa.gov/ppcp/faq.html (accessed April 19, 2008).

U.S. Geological Survey. “Decades Required for Natural Processes to Clean Wastewater-Contaminated Ground Water.” December 3, 2007. http://toxics.usgs.gov//highlights/gw_cessation.html (accessed April 19, 2008).

U.S. Geological Survey. “Ground Water and Surface Water a Single Resource.”September 1, 2005. http://pubs.usgs.gov/circ/circ1139 (accessed April 19, 2008).

U.S. Geological Survey. “Sustainability of Ground-Water Resources.”October 30, 2007. http://pubs.usgs.gov/circ/circ1186 (accessed April 19, 2008).

Virginia Tech Civil Engineering Department. “Groundwater Primer.” June 7, 1998. http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/gwprimer.html (accessed April 19,2008).

Kenneth T. LaPensee

Groundwater

Many of Earth's groundwater supplies are threatened, mainly by human population growth and contamination. Groundwater is freshwater that resides underground; it collects following the movement of water from the surface down through soil and rock. As the water moves downward, a zone or line is formed underground, above which the spaces in the soil and rock are filled with air and water (called the zone of infiltration) and below which water occupies every available opening (called the zone of saturation). The water table is at the top of the zone of saturation, and groundwater lies beneath the water table.

In a desert, groundwater is a vital source of life to plants, which can produce roots that are dozens of feet (meters) long, to reach down from the dry surface to the water below. Groundwater is also important to humans for drinking water, growing crops (irrigation), and other uses. For example, in the year 2000, about 21% of the 408 billion gallons of water used in the United States each day came from underground sources. Over the course of a year, that adds up to about 32 trillion gallons of groundwater!

Threats to groundwater

The main reason that groundwater is threatened has to do with the chemistry of water. Many other compounds (substances formed by the joining of two or more elements) can easily dissolve into the liquid form of water. Liquids into which other compounds dissolve in are called solvents, and water is known as the "universal solvent." If there is a toxic (poisonous) chemical near to groundwater, chances are good that it will also be able to dissolve into and contaminate the groundwater. Toxins (poisonous substances) that cannot dissolve in water (such as oils), can still ruin the quality of groundwater if exposed to an underground water source.

"Out of sight, out of mind." Because many areas have a plentiful supply of groundwater, this resource can easily be over-looked. In Canada for example, before 1990, water scientists found that although 30% of Canadians relied on groundwater for their drinking water, the government had not formed laws to help protect groundwater quality. Since that time, the number of Canadians depending upon groundwater for drinking water has increased, but Canada still has no groundwater protection policy.

Individual versus community groundwater use. A well that supplies one house does not usually harm a groundwater source. This is because the amount of water that is taken from the ground is more than balanced out by the amount of water that goes back into the ground as rainfall, melting snow, floodwater or other sources. However, when a community relies on one or a series of wells for its water supply, the total amount of water that is withdrawn from the groundwater source can be large. If an area experiences a drought (prolonged shortage of rainfall), water consumption rises, or wells are placed too close together, then more groundwater can be taken than is replenished. Rather than drawing water from different parts of the geographical area where water from the region collects (the aquifer), wells very near one another can draw water from the same part of the aquifer, depleting it faster than if the wells were further apart.

These imbalances cannot continue indefinitely or the groundwater supply will run out. The city of Las Vegas, Nevada is an example of a community whose demand for water is causing problems to the underground supply of water. So much groundwater has been taken from the ground around Las Vegas due to a growing population's demand for water, that areas of the land have dropped by over five feet (1.5 meters) in the past century.

Other sources of groundwater depletion. Some industries use large amounts of groundwater. In the oil industry, for example, groundwater (along with surface water) is forced down into oil wells to help bring the hard-to-pump oil up to the surface. While much of this water is eventually returned to the ground, some is lost, mainly through evaporation, or is so polluted that it cannot be put back into the environment. Mines that are made by digging huge holes in the ground (quarries) also help deplete groundwater. Groundwater will often leak out of the walls of the quarry and pool in the bottom of the hole. The quarry acts as a sponge, drawing water out of the surrounding ground. Even though the water can be pumped out and returned to the surrounding ground, the rate of water loss from the ground is sometimes higher than the rate at which the water returns to the ground.

Nature is another source of groundwater depletion, in the form of a drought. Without rainfall to replenish the groundwater, an underground water source can quickly run dry with continued use.

Groundwater contamination

Most of the chemicals and microorganisms in water will be removed as it seeps down through layers of soil and rock to reach the water table. Soil and rocks filter the chemicals and microorganisms in water by sticking to the contaminants or because they are too large to pass through the tiny cracks in the rock. This cleaning effect is not, however, always foolproof. There can be breaks in the soil-rock barrier that allow chemicals and microorganisms to rapidly move down to the groundwater, where they collect and contaminate the groundwater.

Pesticides in groundwater. Only a few generations ago, the contamination of groundwater by chemicals designed to kill weeds and other pests (pesticides) was not a concern. But as the population of the United States has grown, and with the demand to produce more crops from each acre of land, scientists began to develop chemical pesticides. The use of pesticides has increased the amount of food that can be grown. Indeed, the United States has become the largest producer of food in the world, partially due to the fact that crops are protected from destruction by insects and other pests with the use of pesticides.

This increase in production has come with a big disadvantage, the contamination of groundwater (along with surface bodies of water and soil) by the pesticides. Because pesticides can take decades to get from the surface to groundwater, the problem with pesticides in groundwater could become worse in the future. Organizations such as the U.S. Environmental Protection Agency are working to understand pesticide contamination of groundwater and to try to set standards for how much pesticide can be safely applied to the land.

Other sources of groundwater contamination. Another example of how groundwater can be contaminated is via a sinkhole; a crater formed on the surface when the rook of a cavern located directly below collapses. The sinkhole creates a kind of water freeway; water can move easily down through the many open spaces in the rock to the groundwater zone. Sinkholes often occur in limestone rock. In states enriched in limestone, such as Iowa, sinkhole-related contamination of groundwater is a concern. In just the northeast part of Iowa, almost 13,000 sinkholes are known to exist.

Groundwater can also be contaminated when underground storage tanks, such as the tanks located underneath gas stations, develop a leak. As the toxin leaks out of the tank, it can make its way to the groundwater. When the chemicals disperse (spread) in the groundwater, a wide area (many city blocks) around the tank can become contaminated. As this type of groundwater contamination is harder to detect, most communities have systems in place to check underground storage tanks for leaks on a regular basis.

Since water moves downward to the groundwater, any contaminant that is on the surface may also be carried down. Along with pesticides, surface contaminants can also include wells in fields that are designed to help drain excess water, water containing salt that is applied to roads in northern climates to prevent ice from forming, old uncovered wells that contain animal or other waste, and the burial of waste material in landfills.

Brian Hoyle, Ph.D.

For More Information

Books

Glennon, Robert. Water Follies: Groundwater Pumping and the Fate of America's Fresh Waters. Washington, DC: Island Press, 2004.

Websites

Environment Canada. "Groundwater." The Green Lane.http://www.ec.gc.ca/water/en/nature/grdwtr/e_gdwtr.htm (accessed on September 1, 2004).

"Get Informed." The Groundwater Foundation.http://www.groundwater.org/gi/gi.html (accessed on September 1, 2004).

U.S. Geological Survey. "Pesticides in Ground Water." Water Science for Schools.http://ga.water.usgs.gov/edu/pesticidesgw.html (accessed on September 1, 2004).