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? How Much Do We Depend on Groundwater?
According to 1 985 U. S. Figures, group underwater provides an estimated: * 22 percent of all freshwater withdrawals 53 percent of drinking water for the total population and 97 percent of drinking water for the rural population * 40 percent Of public water supply withdrawals * 46 percent of domestic and commercial use * 24 percent of industrial and mining use * 34 percent of agricultural use (mostly for irrigation) How Susceptible Groundwater to Contamination About one-fourth of the average 4. Trillion gallons of precipitation that falls each day on the conterminous United States infiltrates the soil and recharges local aquifers, the sediments and reeks that store and transport groundwater.
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In general, shallow, permeable water table aquifers are the sot susceptible to contamination, but susceptibility of all aquifers to contamination is determined largely by such site-specific characteristics as: ;k distance from the contamination source to the aquifer and residence time of the water in the unsaturated zone; * presence of clay and organic matter in the unsaturated zone materials; * potential of a particular contaminant to biodegrade and decompose; * amount of precipitation, which affects recharge and the rate at whew contact mutants move downward; * photoengraving’s, which in recharge areas may decrease the amount of water that moves downward to the aquifer. What Causes Groundwater Contamination Natural So roes Groundwater contamination can occur in many ways and from many sources, both natural- and human-induced. Groundwater commonly contains one or more naturally occurring chemicals, leached from soil or rocks by percolating water, in concentrations that exceed federal or state drinking water standards or otherwise impair its use.
Dissolved Solids and Chloride 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 MGM/L milliards per liter or approximately equivalent to parts per million) for dissolved solids and 250 MGM/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. Iron and Manganese Although not particularly toxic, iron and manganese in concentrations greater than the limits for federal secondary drinking water standards (0. 3 MGM/L for iron and 0. 05 MGM/L for manganese) can impair the taste of water; stain plumbing fixtures, glassware and laundry; and form encrustations on well screens, thereby reducing well-pumping efficiency.
Nitrate-Nitrogen Most groundwater not affected by human activity contains less than 1 0 MGM/L nitrate-nitrogen, the maximum concentration allowed by federal primary drinking water standards. Nationwide, nitrate-nitrogen concentrations of less than 0. 2 MGM/L generally represent natural conditions, whereas values greater than 3 MGM/L may indicate the effects of human activities. Although relatively nontoxic, nitrate may be reduced by bacteria to nitrite in the intestines of newborn infants and cause the disease methodological. Nitrate also can exact with amines in the human body to form N-intermarries, carcinogenic chemicals known to induce tumors in laboratory animals and thought to be linked to human cancers.
Human Activities 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 Nippon sources. Point sources are localized in areas of an acre or less, whereas Nippon sources are dispersed over broad areas. The most common sources of human-induced groundwater contamination can be grouped into four categories: waste disposal practices; storage and handling f materials and wastes; agricultural activities; and saline water intrusion. Waste Disposal Practices 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: * trinitrotoluene (ETC) * chloroform * benzene * phenolphthalein * tetrarch lorgnette (PACE) * creosote * phenols compounds * I,l,l-theoretician * toluene * Selene Waste disposal can take a number of forms: * septic systems municipal and industrial landfills * surface impoundments * waste-injection wells * direct application of stabilized wastes to the land 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 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 trinitrotoluene that are sold commercially to “clean” the systems.
In 1 980, 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 loud be operating improperly because of poor location, design, construction or maintenance practices. Even when Operating properly, systems mean 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 asses and asses is now exceeded, groundwater contamination caused by septic system failure probably will increase in the future.
Landfills About 150 million tons of municipal solid waste and 240 million tons of industrial solid waste are deposited in 1 6,400 landfills each year. Some hazardous waste material may be deposited in municipal landfills and underlying groundwater may become contaminated. Wastes deposited at industrial landfills aniline a large assortment of trace metals, acids, volatile organic impounds and patricides, which may cause significant local contamination. Surface impoundments Surface impoundments are used to store, treat or dispose of oil and gas rinses, acidic mine wastes, industrial wastes (mainly liquids), animal wastes, municipal treatment plant sludge and cooling water.
For the most part, these impoundments intention Nostradamus 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. Injection Wells In some parts of the country, injection wells dispose of liquid wastes underground. Of patrician concern is the widespread use of drainage wells to dispose of urban stemware runoff and irrigation drainage. Contaminants associated with drainage wells include suspended sediments; dissolved solids; baiter; sodium; chloride; nitrate; phosphate; lead, and organic compounds, including pesticides.
Land Application of Wastes 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 (ERA) 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 1 1 ,900 parcels of land each year. Contamination can occur from improper land-disposal techniques. Storage and Handling of Materials and Wastes 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 transferring of materials on industrial and commercial sites. Aging underground Storage Tanks 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 f 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 Selene contaminants, all of which are organic compounds in diesel and gasoline fuels. Transporting and Stockpiling Many materials and wastes are transported and then temporarily stored in stockpiles before being used or shipped elsewhere.
Precipitation can lea potential contaminants from such stockpile; storage containers can eroded and leak; and accidental spills mean occur – as many as 10,000 to 16,000 per year, according to EPA estimates. Mining Practices Mining of coal, uranium and other substances and the related mine spoil can lead to groundwater contamination in several ways: * Shafts and tunnels can intersect aquifers. * Exposing coal to oxygen can form sulfuric acid, which can degrade water * Contaminants from tailings can leach into groundwater. Oil-Well quality. Bribes Since the asses, hundreds of thousands of exploratory and production wells have been drilled for oil and gas in the United States.
During production, Oil wells produce bribes that are separated from the oil and stored in surface impoundments. EPA estimates that 1 25, 100 brine-disposal impoundments exist that might affect local groundwater supplies. Agricultural Activities 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. Fertilizers During the 1 9605 and asses, 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. 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 mean be leached to groundwater. In such areas, local nitrate-nitrogen concentrations may exceed the federal drinking water standard of 10 MGM/L Pesticides Pesticides have been used since the asses to combat a variety of agricultural pests. Between 1 964 and 1 982, the amount of active ingredients applied to croplands increased 1 70 percent. 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 debrief (DB) and 1 disproportional; the insecticides Aldrich, carbons and chlordane; and the herbicides alcohol and trainee. Feedlots 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 room farmyards may also directly enter an aquifer along the outside of a poorly sealed well easing. Irrigation Percolation of irrigation water into soils dissolves soil salts and transports them downward. Photoengraving’s of applied water from the root zone concentrates salts in the soil and increases the salt load to the groundwater. Checkmating, the practice of mixing and distributing pesticides and fertilizers with irrigation water, may cause contamination if more chemicals are applied than crops can use. It may also cause local contamination if chemicals back- siphon from the holding tank directly into the aquifer through an irrigation ell.
Saline Water Intrusion 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. How Extensive is Groundwater Contamination Assessment of the extent of groundwater contamination is difficult, due to such factors as limited and inconsistent access to the water (usually pendent 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 Sunny studies have made the following assessments: * The United States has large amounts of potable water available for use. Locally, however, high concentrations of a variety of toxic metals, organic chemicals and petroleum products form plumes around such point sources as leaking underground storage tanks, waste disposal sites and chemical or waste handling areas. These types of problems generally occur in urban or industrialized areas, although they are found occasionally in rural areas. Large regions have been identified in which contaminants, derived from Nippon sources and often at minimum detectable levels, are present in many shallow wells throughout a given area. In a small percentage of wells, such contaminants as titrate may exceed drinking water standards or health advisories. Generally, such Nippon source contamination is associated with densely populated urban areas, agricultural land uses and concentrations of septic systems. Furthermore, such contamination commonly affects only the shallowest aquifers. * 20 percent of 124,000 wells sampled over the past 25 years contained a maximum nitrate-nitrogen concentration greater than 3 MGM/L, suggesting the effects of human activities. 6 percent of the samples exceeded the federal drinking water standard for nitrate-nitrogen of 10 MGM/L. Although 44 state summaries in the U. S. Geological Surrey’s 1 986 National Water Summary on groundwater quality mention detection of pesticides in groundwater, data are insufficient to draw conclusions about the extent of contamination. The state summaries do, however, express widespread concern that the frequency of detections and the concentrations of pesticides will increase over time. The U. S. EPA has compiled reports on the occurrence of 46 pesticides in groundwater. In 26 states, one or more pesticides have been tweeted in groundwater that mean be attributed to normal agricultural use. The most commonly detected pesticides are trainee and Aldrich.
EPA currently is conducting its National Pesticide Survey to provide a nationwide estimate of the occurrence 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 ample thus far, samples from 8 wells had nitrate-nitrogen concentrations that exceeded the 10 MGM/L drinking water standard. All 8 samples were from private wells.
Statistically reliable estimates of the percentage of wells contaminated will be available when the survey results are released in late 1990. Summary: How Contaminated is Groundwater? Although little systematic information exists on the extent and severity of groundwater contamination, available evidence suggests that * The shallowest aquifers generally are at greatest risk of contamination, especially those where the overlying unsaturated zone is thin and permeable. Contamination of shallow aquifers by nitrates and synthetic organic chemicals is widespread in many areas; * Shallow groundwater contamination can be related to land use. * As yet, deeper as fifers, which commonly are used for public drinking water supplies, are relatively free from contamination.
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 re taken before improvements in water quality are obsessed. 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 with long-term exposure to very small concentrations of contaminants, and improvements in the computer models used to predict contaminant behavior.