David W. Moody
U.S. Geological Survey
Published by: North Carolina Cooperative Extension Service
Publication Number: AG-441-4
Last Electronic Revision: March 1996 (JWM)
Groundwater is an important source of drinking water for more than half of the nation's population and nearly all its rural population. In recent years, widespread reports of bacteria, nitrate, synthetic organic chemicals and other pollutants in groundwater have increased public concern about the quality of groundwater. What do we know - and what don't we know - about groundwater quality? What causes groundwater contamination, and to what extent are the nation's groundwater supplies at risk?
According to 1985 U.S. figures, groundwater provides an estimated:
About one-fourth of the average 4.2 trillion gallons of precipitation that falls each day on the conterminous United States infiltrates the soil and recharges local aquifers, the sediments and roeks that store and transport groundwater. In general, shallow, permeable water table aquifers are the most susceptible to contamination, but susceptibility of all aquifers to contamination is determined largely by such site-specific characteristics as:
Groundwater contamination can occur in many ways and from many sources, both natural- and human-induced. Groundwater commonly contains one or more naturally ocourring chemicals, leached from soil or rocks by percolating water, in concentrations that exceed federal or state drinking water standards or otherwise impair its use.
One of the most common water quality concerns is the presence of dissolved solids and chloride in concentrations that exceed the recommended maximum limits in federal secondary drinking water standards: 500 mg/L (milligrams per liter or approximately equivalent to parts per million) for dissolved solids and 250 mg/L for chloride. Such concentrations are found at the seaward ends of all coastal aquifers and are quite common in aquifers at depths greater than a few hundred feet below the land surface in many parts of the United States.
Although not partieularly toxic, iron and manganese in concentrations greater than the limits for federal secondary drinking water standards (0.3 mg/L for iron and 0.05 mg/L for manganese) can impair the taste of water; stain plumbing fixtures, glassware and laundry; and form encrustrations on well screens, thereby reducing well-pumping efficiency.
Most groundwater not affected by human activity contains less than 10 mg/L nitrate-nitrogen, the maximum concentration allowed by federal primary drinking water standards. Nationwide, nitrate-nitrogen concentrations of less than 0.2 mg/L generally represent natural conditions, whereas values greater than 3 mg/L may indicate the effects of human activities.
Although relatively nontoxic, nitrate may be reduced by bacteria to nitrite in the intestines of newbom infants and cause the disease methemoglobinemia. Nitrate also can react with amines in the human body to form N-nitrosamines, careinogenic chemicals known to induce tumors in laboratory animals and thought to be linked to human cancers.
Contaminants can enter groundwater from more than 30 different generic sources related to human activities. These sources commonly are referred to as either point or nonpoint sources. Point sources are localized in areas of an acre or less, whereas nonpoint sources are dispersed over broad areas.
The most common sources of human-induced groundwater contamination can be grouped into four categories: waste disposal practiees; storage and handling of materials and wastes; agricultural activities; and saline water intrusion.
Perhaps the best-known sources of groundwater contamination are associated with the storage or disposal of liquid and solid wastes. The organic substances most frequently reported in groundwater as resulting from waste disposal in decreasing order of occurrence, are:
Waste disposal can take a number of forms:
In addition to these regulated forms of disposal, a considerable amount of unregulated disposal, such as illegal dumping and accidental spills, contributes to groundwater contamination.
Septic systems are the largest source by volume of waste discharged to the land. These systems are sources of bacteria, viruses, nitrate, phosphorus, chloride and organic substances, including organic solvents such as trichloroethylene that are sold commercially to "clean" the systems.
In 1980, about 22 million domestic disposal systems were in operation, and about one-half million new systems are installed each year. It is estimated that from one-third to one-half of existing systems eould be operating improperly because of poor location, design, construction or maintenance practices.
Even when Operating properly, systems ean be spaced so densely that their discharge exceeds the capacity of the local soil to assimilate the pollutant loads. Because the 10- to 15-year design life of many septic systems built during the 1960s and 1970s is now exceeded, groundwater contamination caused by septic system failure probably will increase in the future.
About 150 million tons of municipal solid waste and 240 million tons of industrial solid waste are deposited in 16,400 landfills each year. Some hazardous waste material may be deposited in municipal landfills and underlying groundwater may become contaminated. Wastes deposited at industrial landfills inelude a large assortment of trace metals, acids, volatile organie eompounds and peticides, which may eause signifieant local contamination.
Surface impoundments are used to store, treat or dispose of oil and gas brines, acidic mine wastes, industrial wastes (mainly liquids), animal wastes, municipal treatment plant sludges and cooling water. For the most part, these impoundments eontain nonhazardous wastes; however, hazardous wastes are known to be treated, stored and disposed of by 400 facilities involving about 3,200 impoundments. Some of these impoundments have significant potential for contaminating groundwater.
In some parts of the country, injection wells dispose of liquid wastes underground. Of paticular concern is the widespread use of drainage wells to dispose of urban stormwater runoff and irrigation drainage. Contaminants associated with drainage wells include suspended sediments; dissolved solids; baeteria; sodium; chloride; nitrate; phosphate; lead, and organic compounds, including pesticides.
In many places, solid and liquid wastes are placed or sprayed on the land, commonly after treatment and stabilization. The U.S. Environmental Protection Agency (EPA) has estimated that more than 7 million dry tons of sludge from at least 2,463 publicly owned waste treatment plants are applied to about 11,900 parcels of land each year. Contamination can occur from improper land-disposal techniques.
Groundwater contamination as the result of storage and handling of materials includes leaks from both above-ground and underground storage tanks, as well as unintentional spills or poor housekeeping practices in the handling and transfering of materials on industrial and commercial sites.
Possibly as many as 7 million steel tanks are used to store petroleum products, acids, chemicals, industrial solvents and other types of waste underground. The potential of these tanks to leak increases with age. About 20 percent of existing steel tanks are more than 16 years old, and estimates of the total number that presently leak petroleum products range from 25 to 30 percent. Underground storage tanks appear to be a leading source of benzene, toluene and xylene contaminants, all of which are organic compounds in diesel and gasoline fuels.
Many materials and wastes are transported and then temporarily stored in stockpiles before being used or shipped elsewhere. Precipitation can leaeh potential contaminants from such stockpile; storage containers can eorrode and leak; and accidental spills ean oecur - as many as 10,000 to 16,000 per year, according to EPA estimates.
Mining of coal, uranium and other substances and the related mine spoil can lead to groundwater contamination in several ways:
Since the 1800s, hundreds of thousands of exploratory and production wells have been drilled for oil and gas in the United States. During production, oil wells produce brines that are separated from the oil and stored in surface impoundments. EPA estimates that 125,100 brine-disposal impoundments exist that might affect local groundwater supplies.
Agriculture is one of the most widespread human activities that affects the quality of groundwater. In 1987, about 330 million acres were used for growing crops in the United States, of which 45 million acres were irrigated.
During the 1960s and 1970s, nitrogen, phosphorus and potassium fertilizer use steadily increased to a peak of 23 million tons in 1981. By 1987, however, fertilizer use had declined to 19.2 million tons, reflecting the large number of acres withdrawn from production as part of the Conservation Reserve Program and other government programs.
If nitrogen supply exceeds nitrogen uptake by crops, excess nitrogen ean be leached to groundwater. In such areas, local nitrate-nitrogen concentrations may exceed the federal drinking water standard of 10 mg/L
Pesticides have been used since the 1940s to combat a variety of agricultural pests. Between 1964 and 1982, the amount of active ingredients applied to croplands increased 170 pereent. Herbicide usage peaked in 1982, and since then has declined from about 500 million pounds of active ingredients per year to about 430 million pounds in 1987.
In addition to crop applications, infiltration of spilled pesticides can cause contamination in locations where pesticides are stored, and where sprayers and other equipment used to apply pesticides are loaded and washed.
Pesticides most frequently detected in groundwater are the fumigants ethylene dibromide (EDB) and 1,2-dichloropropane; the insecticides aldicarb, carbofuran and chlordane; and the herbieides alachlor and atrazine.
Feedlots confine livestock and poultry and create problems of animal-waste disposal. Feedlot wastes often are collected in impoundments from which they might infiltrate to groundwater and raise nitrate concentrations. Runoff from farmyards may also directly enter an aquifer along the outside of a poorly sealed well easing.
Percolation of irrigation water into soils dissolves soil salts and transports them downward. Evapotranspiration of applied water from the root zone concentrates salts in the soil and increases the salt load to the groundwater.
Chemigation, the practice of mixing and distributing pesticides and fertilizers with irrigation water, may cause contamination if more chemicals are applied than crops can use. lt may also cause local contamination if chemicals back-siphon from the holding tank directly into the aquifer through an irrigation well.
The encroachment of saline water into the freshwater part of an aquifer is an ever-present threat when water supplies are developed from the highly productive coastal plain aquifers of the United States, or from aquifers underlain by saline water in the interior of the country. Local incidents of saline water intrusion have occurred on all coasts of the United States.
Assessment of the extent of groundwater contamination is difficult, due to such factors as limited and inconsistent access to the water (usually dependent on wells and springs); the potential for bias in existing data (if originally collected to explore a particular water quality problem); incomplete information about the well (did the well draw from more than one aquifer?); and inconsistent methods of sampling and analysis.
It is also important to keep in mind that the trend of increasing reports of detections of contaminants in groundwater is largely due to the intensive search for contaminants now under way by many state agencies, as well as continued improvements in the sensitivity of analytical methods used to measure the concentration of contaminants.
The volume of groundwater within 2500 feet of the surface has been estimated at 100 quadrillion gallons, or about 16 times the volume of the Great Lakes. Of this amount, at least half is too saline from natural causes to use for drinking water, although some of it may be suitable for other uses. The total amount of the remaining groundwater that is contaminated is unknown, although EPA estimates the amount contaminated by point sources to be 2-3 percent.
Recent U.S. Geological Suney studies have made the following assessments:
The U.S. EPA has compiled reports on the ocourrence of 46 pesticides in groundwater. In 26 states, one or more pesticides have been deteeted in groundwater that ean be attributed to normal agricultural use. The most commonly detected pesticides are atrazine and aldicarb.
EPA currently is conducting its National Pesticide Survey to provide a nationwide estimate of the occurrenee of pesticides in drinking water wells. The survey includes the collection of water samples from a statistically representative sample of community water system wells (600) and private wells (750).
Interim results show that 6 of 180 community well samples collected thus far and 9 of 115 private well samples had detectable pesticide residues. Of the 15 wells that had detectable levels of pesticides,3 had concentrations that exceeded lifetime health advisory levels established by EPA. Of 295 wells sampled thus far, samples from 8 wells had nitrate-nitrogen concentrations that exceeded the 10 mg/L drinking water standard. All 8 samples were from private wells. Statistically reliable estimates of the percentage of wells containated will be available when the survey results are released in late 1990.
Although little systematic information exists on the extent and severity of groundwater contamination, available evidence suggests that:
Additional reports of groundwater contamination may be expected in the coming years, as federal, state and local agencies expand their groundwater quality monitoring programs using sophisticated analytical methods that can measure very small concentrations of contaminants. Groundwater moves very slowly, and it may be years after remedial actions are taken before improvements in water quality are obsened. For this reason, the enhancement of the quality of the nation's groundwater requires a long-term commitment.
More definitive assessments of groundwater quality will have to await the expansion of data-collection programs, the use of standard sampling and analytical procedures, research on the health risks associated wtih long-term exposure to very small concentrations of contaminants, and improvements in the computer models used to predict contaminant behavior.
Although current assessments of groundwater quality are far from definitive, they do suggest the widespread presence of shallow groundwater contamination. While we have much yet to learn about the sources and extent of contamination, the general principles and steps needed to protect groundwater from future contamination are well understood. Reductions of wastes, control of contamination sources and improved land management practices can significantly reduce the risk of contamination in the future.
The unedited version of the paper on which this leaflet is based appears in the March-April issue (Volume 45, Number 2) of the Jourrzal of Soil and Water Conservation.
Groundwater Protection - Groundwater, Saving the Unseen Resource, and a Guide to Groundwater Pollution Problems, Causes and Government Responses.1987. The Conservation Foundation, Washington, D.C.
National Water Quality Inventory: 1986 Report to Congress. 1987. U.S. Environmental Protection Agency. Report Number EPA-440/87/008.
National Water Summary 1986: Hydrologic Events and Groundwater Quality. 1988. U.S. Geological Survey Water Supply Paper 2325.
Charles Abdalla, Penn State University
David Allee, Cornell University
Leon Danielson, North Carolina State University
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.