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)